Electrochemical recovery of sodium and sulfur species from ... · investigated. A two-compartment...

Faculty of Bioscience Engineering

Academic year 2014 – 2015

Electrochemical recovery of sodium and sulfur species

from spent caustic streams

Thomas Provijn

Promotors: Prof. dr. ir. Korneel Rabaey and Dr. Eleni Vaiopoulou

Tutors: Dr. Eleni Vaiopoulou and Dr. Antonin Prévoteau

Thesis submitted in fulfilment of the requirements for the degree of

Master of Science in Bioscience Engineering: Environmental technology

Copy right

“The author and promotors give the permission to use this thesis for consultation and to copy parts

of it for personal use. Every other use is subject to the copyright laws, more specifically the source

must be extensively specified when using results from this thesis.”

“De auteur en promotors geven de toelating deze scriptie voor consultatie beschikbaar te stellen en

delen ervan te kopiëren voor persoonlijk gebruik. Elk ander gebruik valt onder de beperkingen van

het auteursrecht, in het bijzonder met betrekking tot de verplichting uitdrukkelijk de bron te

vermelden bij het aanhalen van resultaten uit deze scriptie.”

Ghent, June 2015

The promotors, The tutors, The author,

Prof. dr. ir. Korneel Rabaey Dr. Eleni Vaiopoulou Thomas Provijn

Dr. Eleni Vaiopoulou Dr. Antonin Prévoteau

Acknowledgements

Hier is hij dan, mijn thesis. Het orgelpunt van mijn opleiding, in letters gegoten en samengeperst in

een bundeltje, dat nu, beste lezer, nog een beetje stilletjes ligt te beven in uw handen. Het was een

werk van lange adem, een Sisyphus opdracht. Vele uren van labo-, schrijf-, opzoek- en denkwerk

waren nodig om het voor mekaar te krijgen. Maar dat niet alleen. Er waren heel wat helpende en

ondersteunende handen nodig, die ik hier wil bedanken.

First of all, I want to mention the magnificent team that was directing and supporting me during this

thesis. Professor Rabaey, Eleni and Antonin, it was a real pleasure and honor to have you all in this

team. It gave some nice perspectives and insights to have a professor who has a broad vision on the

current industrial technologies, the economics of those techniques, or the “macro electrochemistry”

as I would like to call it. Uw enthousiasme en kennis omtrent resource recovery heeft zeker bij mij

een positieve invloed achter gelaten, niet alleen tijdens deze thesis, maar ook tijdens mijn opleiding.

For the level below that, the “micro electrochemistry”, Eleni had the specific know-how of

electrochemical systems, how to operate these in practice and how to design the experiments. Eleni,

you were a great supervisor during my thesis. You’ve given me not only wisdom in the area of

science, but also on a personal level. Your joy and courage seems to be inexhaustible, and that has

kept my head up at some difficult moments during this thesis year. I will never forget your quotes

such as “Step by step, Thomas, step by step.” and “How is Jesus doing?”.

Antonin, a.k.a. the specialist on the “nano electrochemistry”, was the right man to give me insights

on all the information you can get from electrochemical techniques such as his precious cyclic

voltammetry technique. Comme Eleni, tu es une personne bien vivante, Antonin, et ça rend le travail

au labo beaucoup moins ennuyant. Merci pour tout!

Verder wil ik ook Greet & Jan bedanken voor het helpen opzetten van een methode om met de

ionenchromatograaf zwavel componenten te meten. Het heeft ons heel wat tijd en moeite gekost,

maar uiteindelijk is het ons wel gelukt. Ook bedankt Greet om zorgvuldig de vele IC stalen te

analyseren.

Stephen, a.k.a. Mr. Anderson, thank you for helping me in designing the reactor and making

calculations for the experiments. The CREO cluster meetings you organized were interesting to get

some nice feedback on my experiments, so thanks for that as well. I hope we can play some squash

in the near future.

Way, thanks for helping me when Pisa was leaning too much or when Jesus needed to be resurrected

after good Friday. You were a great neighbor, and I hope one day you will be able to make some

grass candy lollypops.

In general, I would like to thank the whole LabMET group, including all the students, PhDs, post-

doc’s, professors and the technical staff. The year I spent at LabMET was a year I won’t forget easily.

It felt as I was adopted by a warmhearted family. I enjoyed the interesting presentations during the

seminars and, even more, the events organized by LabMET, with the barbecue as the icing on the

cake. I also want to thank the coffee machine of LabMET, for comforting and boosting me with his

delicious blackish liquid when times were hard.

Verder wil ik mijn ouders en broer bedanken voor hun steun en luisterend oor. Ook mijn vrienden,

die me dit jaar wat minder hebben gezien door mijn thesis, bedankt voor jullie steun en toeverlaat.

Het gemis zal ik tijdens de zomervakantie hopelijk ruimschoots kunnen compenseren.

En als laatste wil ik mijn urfje Marthe bedanken, die mij door dik en dun gesupporterd en gesteund

heeft tijdens dit jaar. We hebben ons samen door dit lastige thesisjaar kunnen spartellen, waar slaap

en vrije tijd vaak afwezig waren. Een maandje naar Zweden zal ons veel deugd doen, nietwaar?

Thomas Provijn

Gent, 5 juni 2015

The best way to predict your future is to create it.

Abraham Lincoln

Abstract

Today, sulfide from raw fuel gasses is removed mostly via absorption in a sodium hydroxide stream

in a wet scrubber. This creates a spent caustic stream (SCS), which is treated via various

approaches. However, these treatments have some disadvantages such as a high operating cost, a

high chemical usage or low robustness. Moreover, these treatments often convert sulfide to sulfate,

so no recovery of other products from sulfide oxidation can be achieved.

In this thesis, the possibility of recovery of sodium hydroxide and sulfur compounds from a SCS was

investigated. A two-compartment electrochemical cell was used. An iridium tantalum mixed metal

oxide coated titanium anode was placed in the anode compartment to oxidize sulfide to its sulfur

oxidation products, such as elemental sulfur, sulfate, thiosulfate, sulfite and polysulfides. A cation

exchange membrane was used in order to selectively transfer sodium ions to the cathode

compartment in an attempt to create a concentrated sodium hydroxide stream with the hydroxide

electrochemically produced at the stainless steel mesh cathode. An artificial SCS (4 wt% sodium

hydroxide, 1 wt% sulfide-S) and deionized water were continuously fed to the anode and cathode

compartment respectively.

In a first part, a stable sulfide removal (68 ± 1 %) and cell voltage (2.74 ± 0.10 V) were observed

during a two and a half months operation at 100 A m-2 and 46.9 ± 2.3 g S L-1 d-1. Sodium hydroxide

was efficiently recovered in the catholyte (4.96 ± 0.36 wt%) with a coulombic efficiency of 96 ± 2 %.

In a next step, the effect of an increasing sulfide loading rate was investigated. Higher sulfide loading

rates (i.e. higher anolyte flowrate) resulted in lower sulfide removal efficiencies, but coulombic

efficiencies for sulfide increased. Increasing the sulfide loading rate also resulted in less sulfate

production.

The effect of current density was investigated in a third set of experiments. Increasing the current

density increased the sulfate and thiosulfate concentration and decreased polysulfides and

elemental sulfur in the anode effluent. 200 A m-2 current density and sulfide loading rate of 40.2 g ±

3.9 S L-1 d-1 were found to be limiting for the system, as the cell voltage increased due to a built up of

elemental sulfur on the anode.

The highest sulfide removal efficiency (86 ± 3 %) was recorded at 150 A m-2 and 42.9 ± 5.2 g S L-1 d-1

. At 50 A m2- and 45.3 ± 5.8 g S L-1 d-1, a high removal efficiency (73 ± 1 %) combined with high

polysulfide and elemental sulfur production (2.7 ± 1.4 g S L-1) were observed. These operational

conditions are recommended to harvest high value calcium polysulfide that can be used as a

fungicide or for remediation of heavy metals polluted soils. Further research on the performance

and stability of the electrochemical system with real SCS will help in determining the viability of the

process as an alternative for treating SCS.

Samenvatting

Sulfide uit ruwe brandstofgassen wordt vandaag de dag meestal geabsorbeerd in een

natriumhydroxide stroom in een natte gaswasser. Dit vormt een toxische bijtende stroom (spent

caustic stream, SCS) die behandeld wordt via verschillende processen. Deze processen hebben

echter een aantal nadelen zoals een hoge operationele kost, een hoog verbruik van chemicaliën of

een lage robuustheid. Deze behandelingen zetten daarenboven sulfide om in sulfaat, waardoor er

geen herwinning kan plaatsvinden van andere zwavel producten.

In deze thesis wordt de mogelijkheid onderzocht om natriumhydroxide en zwavel componenten te

herwinnen uit een SCS. Hiervoor werd een elektrochemische cel gebruikt, bestaande uit 2

compartimenten. In het anode compartiment werd een iridium tantalum gemengd metaal oxide

gecoate titanium anode geplaatst om sulfides te oxideren tot sulfide-oxidatieproducten, zoals

elementair zwavel, sulfaat, thiosulfaat, sulfiet en polysulfides. Een kationuitwisselingsmembraan

tussen de twee compartimenten werd gebruikt om een selectieve overdracht van natrium ionen

naar het kathode compartiment te bekomen. Zo werd getracht een geconcentreerde natrium-

hydroxide stroom te verkrijgen met het hydroxide dat elektrochemisch aan de roestvrij stalen

kathode geproduceerd werd. Een kunstmatige SCS (4 wt% natriumhydroxide en 1 wt% sulfide-S) en

geïoniseerd water werden continu toegevoerd naar respectievelijk het anode en kathode

compartiment.

In een eerste deel werd een stabiele sulfide verwijdering (68 ± 1 %) en cel voltage (2.74 ± 0.10 V)

geobserveerd tijdens een twee en een half maand operatie met een stroomdichtheid van 100 A m-2

en een sulfide belasting van 46.9 ± 2.3 g S L-1 d-1. Natrium hydroxide werd efficiënt gerecupereerd

(4.96 ± 0.36 wt%) met een coulombische efficiëntie van 96 ± 2 %.

In een volgende stap werd het effect van een stijgende sulfide belasting bekeken. Hoger sulfide

belasting (i.e. hogere inkomende SCS stroomsnelheid) resulteerde in lagere sulfide verwijderings-

efficiënties, maar de coulombische efficiënties voor sulfide stegen. Stijgende sulfide belasting

resulteerde ook in lagere sulfaat productie.

Het effect van stroom dichtheid werd bekeken in een derde set van experimenten. Hogere

stroomdichtheden zorgden voor hogere productie van sulfaat en thiosulfaat en lagere productie van

polysulfiden en elementair zwavel. De combinatie van 200 A m-2 en of 40.2 g ± 3.9 S L-1 d-1 bleek

limiterend te zijn voor het system, aangezien het cel voltage continu steeg door een opbouw van

lagen elementair zwavel die de anode passiveerden.

De hoogste verwijderingsefficiëntie (86 ± 3 %) werd gevonden bij 150 A m-2 en 42.9 ± 5.2 g S L-1 d-1.

Bij 50 A m2- en 45.3 ± 5.8 g S L-1 d-1 werd reeds een hoge verwijderingsefficiëntie gezien (73 ± 1 %),

alsook een hoge polysulfide en elementair zwavel productie (2.7 ± 1.4 g S L-1). Deze laatste

operationele condities worden voorgesteld om hoogwaardig polysulfide te verkrijgen, dat dienst

kan doen als fungicide of gebruikt kan worden voor de remediatie van met zware metalen

verontreinigde bodems. Verder onderzoek is nodig op de prestaties en stabiliteit van dit

elektrochemisch systeem met een echte SCS om de haalbaarheid te bepalen van dit proces als

alternatieve manier om SCS te behandelen.

Table of contents

I LITERATURE REVIEW ................................................................................................................................................ 1

1 Sulfur species and cycle ............................................................................................................................................. 1

2 Sulfur issues .................................................................................................................................................................... 3

2.1 General issues.......................................................................................................................................................... 3

2.2 Spent caustic streams .......................................................................................................................................... 4

3 Conventional spent caustic treatment ................................................................................................................. 5

3.1 Chemical treatment ............................................................................................................................................... 5

3.2 Thermal treatment ................................................................................................................................................ 5

3.3 Biological treatment ............................................................................................................................................. 5

3.4 Advantages and disadvantages ........................................................................................................................ 5

4 Electrochemical spent caustic treatment ........................................................................................................... 7

4.1 Electrochemical sulfide removal ..................................................................................................................... 7

4.1.1 Fuel cells .......................................................................................................................................................... 7

4.1.2 Indirect electrolysis..................................................................................................................................... 7

4.1.3 Direct electrolysis ........................................................................................................................................ 8

4.2 Electrochemical sodium recovery .................................................................................................................. 9

4.3 Combined sulfur and sodium recovery ..................................................................................................... 11

II OBJECTIVES ................................................................................................................................................................. 12

III MATERIAL AND METHODS ................................................................................................................................... 13

1 Reactor set-up ............................................................................................................................................................. 13

2 Experimental procedure ......................................................................................................................................... 14

2.1 Feed .......................................................................................................................................................................... 14

2.2 Experiments .......................................................................................................................................................... 15

3 Analysis and measurements ................................................................................................................................. 17

3.1 Sulfide anti-oxidant buffer .............................................................................................................................. 17

3.2 Sulfide, thiosulfate and sulfite analysis ..................................................................................................... 17

3.3 Sulfate analysis .................................................................................................................................................... 18

3.4 Polysulfide and elemental sulfur analysis ................................................................................................ 18

3.5 Hydroxide analysis ............................................................................................................................................. 18

3.6 Flowrate measurement & sulfide loading rate calculation ............................................................... 19

3.7 Setting up method for sulfur species .......................................................................................................... 19

3.7.1 Standard stock solutions ........................................................................................................................ 19

3.7.2 Interference effect of SAOB ................................................................................................................... 20

3.7.3 Effect of time on sample storage boundaries ................................................................................ 21

4 Electrochemical techniques .................................................................................................................................. 21

4.1 Chronopotentiometry (CP) ............................................................................................................................. 21

4.2 Membrane resistance ........................................................................................................................................ 21

4.3 Cyclic Voltammetry (CV) ................................................................................................................................. 23

5 Calculations .................................................................................................................................................................. 23

5.1 Efficiencies ............................................................................................................................................................. 23

5.2 Electron balances ................................................................................................................................................ 24

IV RESULTS ........................................................................................................................................................................ 26

1 Ion Chromatography method for sulfur species ........................................................................................... 26

1.1 Calibration ............................................................................................................................................................. 26

1.2 Interference effect of SAOB ............................................................................................................................ 27

1.3 Sample preservation time ............................................................................................................................... 28

2 Electrochemical sulfide removal ......................................................................................................................... 28

2.1 Control experiments .......................................................................................................................................... 28

2.1.1 Open circuit ................................................................................................................................................. 28

2.1.2 No sulfide Feed ........................................................................................................................................... 29

2.2 Long term operation experiment ................................................................................................................. 30

2.2.1 Cell voltage ................................................................................................................................................... 30

2.2.2 Sulfur species .............................................................................................................................................. 30

2.2.3 Sodium hydroxide ..................................................................................................................................... 32

2.2.4 Cyclic voltammetry ................................................................................................................................... 33

2.2.5 Membrane resistance .............................................................................................................................. 33

2.2.6 Reproducibility of performance .......................................................................................................... 34

2.3 Effect of current density .................................................................................................................................. 35

2.3.1 Cell voltage ................................................................................................................................................... 35

2.3.2 Sulfur species .............................................................................................................................................. 36



2.4 Effect of sulfide loading rate........................................................................................................................... 40

2.4.1 Cell voltage ................................................................................................................................................... 40

2.4.2 Sulfur species .............................................................................................................................................. 40



V DISCUSSION ................................................................................................................................................................. 44

1 Analysis .......................................................................................................................................................................... 44

2 Control experiments ................................................................................................................................................. 44

3 Long term operation................................................................................................................................................. 45

4 Effect of current density and sulfide loading rate ........................................................................................ 45

5 Evaluation of proposed technique ...................................................................................................................... 48

6 Valorisation of sulfur species................................................................................................................................ 48

7 Economic analysis ..................................................................................................................................................... 50

8 Future Perspective .................................................................................................................................................... 52

VI CONCLUSION ............................................................................................................................................................... 53

VII BIBLIOGRAPHY........................................................................................................................................................... 54

VIII ADDENDUM: SUSTAINABILITY ........................................................................................................................... 58

List of figures

Figure 1: Indirect electrolysis set-up ................................................................................................................................ 8

Figure 2 : Spent caustic treatment with an Electro-electrodialysis system ................................................... 10

Figure 3: Spent caustic treatment with a Bipolar membrane electrodialysis system ............................... 10

Figure 4: The different parts of the two-compartment electrochemical cell. ............................................... 13

Figure 5: Electrochemical cell set-up schematically. ............................................................................................... 14

Figure 6: Electrochemical cell set-up. ............................................................................................................................ 15

Figure 7 : Scheme of the long term experiment. ....................................................................................................... 16

Figure 8 : Scheme of the current density effect experiment. ............................................................................... 16

Figure 9: Scheme of the sulfide loading rate effect experiment. ........................................................................ 17

Figure 10: Scheme for the calculation of the membrane resistance.. ............................................................... 22

Figure 11: Eh-pH diagram for the sulfur-water system .......................................................................................... 25

Figure 12: Calibration curves of sulfide, thiosulfate and sulfite. ........................................................................ 26

Figure 13: Chromatograms of the calibration standards.. .................................................................................... 27

Figure 14: SAOB interference............................................................................................................................................ 27

Figure 15: Sample perseveration time with and without SAOB addition.. ..................................................... 28

Figure 16: Sulfide, sulfate and thiosulfate concentrations without electrode polarization.. .................. 29

Figure 17: Cyclic voltammogram with no sulfide in the anolyte.. ...................................................................... 29

Figure 18 : Cell voltage during the long term experiment.. .................................................................................. 30

Figure 19: Sulfur compounds concentrations during the long term run ........................................................ 30

Figure 20: Coulombic efficiency and removal efficiency of sulfide during long term experiment ...... 31

Figure 21: The sulfide loading rate from day 105 till 132 for the long term experiment ........................ 31

Figure 22: Electrons generated per sulfur species for the long term experiment. ..................................... 32

Figure 23: Hydroxide concentration in the cathode effluent for the long term experiment. ................. 32

Figure 24: Coulombic efficiency of sodium hydroxide during long term experiment ............................. 33

Figure 25: Cyclic voltammograms for different recirculation flowrates. ....................................................... 33

Figure 26: The working electrode and counter electrode potential ................................................................. 34

Figure 27: Effect of no anode feeding. ........................................................................................................................... 34

Figure 28: Cyclic voltammograms for a steady state anolyte and for an interfered state. ...................... 35

Figure 29: Cell voltage over time during different current densities . ............................................................. 36

Figure 30: Sulfur species in the anode effluent during the current density effect test. ............................ 36

Figure 31: Sulfur species concentrations for the different applied current densities. .............................. 37

Figure 32: The sulfide removal efficiency and coulombic efficiency of sulfide.. .......................................... 37

Figure 33: Average sulfide loading rate for each current density. .................................................................... 38

Figure 34: Electrons generated per sulfur species for each current density. ............................................... 38

Figure 35: Sodium hydroxide concentration for each current density.. ......................................................... 39

Figure 36: Cyclic voltammograms of different current densities. ..................................................................... 39

Figure 37: Cell voltage over time recorded for different sulfide loading rate. ............................................. 40

Figure 38: Concentration of sulfur species in the anode effluent over time.. ............................................... 40

Figure 39: Mean sulfur species concentrations for the different applied sulfide loading rates. ........... 41

Figure 40: Removal and coulombic efficiency of sulfide for different sulfide loading rates................... 41

Figure 41: Measured sulfide loading rate for every loading rate run.. ............................................................ 42

Figure 42: Electrons generated per sulfur species for each sulfide loading rate run. ............................... 42

Figure 43: Sodium hydroxide concentration for each sulfide loading rate run. .......................................... 43

Figure 44: Cyclic voltammograms at the steady state conditions of different flowrates. ........................ 43

Figure 45: Estimated economic prospect..................................................................................................................... 51

List of tables

Table 1: Sulfur species occurring in water ..................................................................................................................... 2

Table 2: Key processes and prokaryotes in the sulfur cycle ................................................................................... 2

Table 3: Health effects of sulfide gases ............................................................................................................................ 3

Table 4: Characteristics of typical refinery caustic streams ................................................................................... 4

Table 5: Advantages and disadvantages of conventional techniques to treat spent caustics. .................. 6

Table 6: Possible anode reactions during direct electrolysis. ................................................................................ 9

Table 7: Possible cathode reaction .................................................................................................................................... 9

Table 8: Standards used for IC calibration and their concentrations ............................................................... 20

Table 9: Composition of the IC samples with different strengths towards SAOB. ...................................... 20

Table 10: Reactions at the anode for the electron balance calculation. .......................................................... 24

Table 11: Properties of the calibration curves of sulfide, thiosulfate and sulfite. ....................................... 26

Table 12: Comparison between different electrochemical treatments. .......................................................... 47

Table 13: Estimation of prices for operational cost calculation. ........................................................................ 50

Table 14: Calculated consumption and production per day. ............................................................................... 50

Table 15: Economic margin ............................................................................................................................................... 51

List of abbreviations

2-SCS-ES 2-compartment electrochemical cell

3-SCS-ES 3-compartment electrochemical cell

BMED Bipolar membrane electrodialysis

CAPEX Capital expenditures

CE Coulombic efficiency

CP Chronopotentiometry

CV Cyclic voltammetry

EED Electro-electrodialysis

HA-SOB Haloalkaliphilic sulfide oxidizing bacteria

MMO Mixed metal oxide

OPEX Operating expenditures

RE Removal efficiency

REF Reference electrode

SAOB Sulfide anti-oxidant buffer

SCS Spent Caustic stream

WE Working electrode

1

I LITERATURE REVIEW 1 SULFUR SPECIES AND CYCLE

Sulfur is omnipresent on earth and essential to life. In aqueous systems, it can be present under

many oxidation states and chemicals forms (Table 1). The most common forms are sulfide, sulfate,

elemental sulfur, polysulfides, thiosulfate and sulfite.

Sulfide

Sulfide is known to be present in natural gas, waste gases and wastewaters (Rabaey et al. 2010). It is

toxic and corrosive. Sulfide gas is a colourless gas that has the smell of rotten eggs. The threshold

odour concentration in clear water lies between 0.025 and 0.25 µg L-1 (APHA, 1992). Sulfide is well

soluble in water, with a solubility comprised between 3,000 and 4,000 mg L-1 at atmospheric

pressure and normal temperature (Rabaey et al. 2010).

Elemental sulfur

The stable molecules of elemental sulfur (S0) at 25 °C and 1 atm are S6 and S8 rings and Sn chains. At

the same conditions, the most stable allotrope form of sulfur is rhombic sulfur which contains

stacked S8 rings and has a bright yellow colour (Zumdahl 2009).

Polysulfide

Polysulfide anions (Sn2-) are unbranched chains of (n-1) zerovalent sulfur atoms and one sulfide

atom in divalent state. The pH, temperature and the total sulfur concentration will determine the

chain length of the polysulfide anion. Polysulfides are not stable under acidic conditions (Rabaey et

al. 2010).

2

Table 1: Sulfur species occurring in water (adapted from Keller-Lehmann et al. (2006)).

Sulfur species Name Oxidation state

pKa Oxidation products

SxO62- (x≥3) polythionates III 1/3 to 0

S2O82- peroxodisulfate VII 0, 0.9 SO4

2-

S2O72- disulfate VI SO4

2-

SO42- sulfate VI 1.98, -3 very stable

S2O62- dithionate V

S2O52- disulfite IV SO3

2-, SO42-

SO32- sulfite IV 1.89, 7.21 SO4

2-

S2O42- dithionite III 0.35, 2.45 S2O3

2-, SO32-, SO4

2-

S4O62- tetrathionite II 1/2 SO4

2-

Mmx+(S2O3)y

(mx-2y) metal thiosulfate complexes II

S2O32- thiosulfate II 0.6, 1.72 SO4

2-

S° & S8 elemental sulfur 0

CH3SxCH3 dimethylpolysulfide (DMPS) 0,I

RSH sulfhydryl thiols 0

SCN- thiocyanate 0 -1.8

Sx2- (x≥2) polysulfides 0 to -I S0, S2O3

2-

HS- hydrogen sulfide -II 7, 12.9 S2O32-, SO3

2-, SO42-

Microbial sulfur cycle

The microbial sulfur cycle consists of oxidation and reduction steps mostly between elemental

sulfur, sulfide and sulfate (Table 2).

Table 2: Key processes and prokaryotes in the sulfur cycle (adapted from Madigan et al. (2008)).

Process Chemical conversion Bacteria

Aerobic and anoxic sulfide/sulfur oxidation

H2S S0 SO42-

Sulfur chemolitotrophs (Thiobacillus, Begiatoa, many others)

Anaerobic sulfide/sulfur oxidation

H2S S0 SO42-

Purple and green phototrophic bacteria, some chemolitotrophs

Sulfate reduction (anaerobic) SO42- H2S Desulfovibrio, Desulfobacter

Sulfur reduction (aerobic) S0 H2S Desulfuromonas, many hyperthermophylic Archaea

Thiosulfate disproportionation S2O32- H2S + SO4

2- Desulfovibrio, and others

Organic sulfur compound oxidation or reduction

CH3SH CO2 + H2S DMSO1 DMS2

Wide variety of microbial species

Desulfurylation Organic-S H2S Wide variety of microbial species

1 DMSO = Dimethyl sulfoxide , 2 DMS = Dimethyl sulfide

3

Anthropogenic sulfur emissions

The sulfur emissions in 2006 were estimated to be 68,700 kT sulfur on a global scale (Huang et al.

2009). More stringent government actions regarding reducing emissions from cars, power plants,

industry and households have been applied. In this way the global emissions have fallen at an

average rate of 2.7 % per annum since 1990 (Stern 2005). The emissions of SO2 by international sea

transport are expected to exceed the emissions on land by 2020 (Commision of the European

Communities 2005). Hence, directive 2012/33/EU, that was brought into force in January 2015,

brought new limits to sulfur emission by ships (European Commission 2012).

2 SULFUR ISSUES

2.1 GENERAL ISSUES

Sulfur species are known to cause problems in the environment as for human beings. Sulfide gas has

already a large toxic impact on humans at low concentrations (Table 3). This is why hydrogen

sulfide gases caused death of numerous workers in sewers (Knight & Presnell 2005).

Table 3: Health effects of sulfide gases (adapted from Pikaar (2011) and Yalamanchili & Smith (2008))

H2S concentration (ppm) Health effects

0.02-1 Rotten egg smell, odour complaints

10 Occupational exposure limit for 8 hours (EPA)

20 Self-contained breathing apparatus required

100 May cause headache/nausea, sense of smell lost in 2 to 15 minutes

200 Rapid loss of smell, burning eyes and throat

500 Loss of reasoning and balance, respiratory distress

700 Immediate unconsciousness, seizures

Another problem is that hydrogen sulfide in wastewater causes directly and indirectly serious

corrosion in concrete sewers because it is oxidized to H2SO4 on the sewer walls. Only minor

problems in sewers have been reported in the sulfide concentration range of 0.1-0.5 mg S L-1.

However, at a concentration of 2.0 mg S L-1 more severe corrosion in sewers may occur. This

corrosion leads to frequent replacing of sewers. In fact, up to 10 % of the total cost for wastewater

collection and treatment in Flanders is accounted to the corrosion by sulfur in sewers (Zhang et al.

2008). Sulfate aerosol emissions have a cooling effect due to their absorption and reflection of

4

radiation, so they have an impact on the global climate change (Stern 2005). Dong et al. 2014

showed that the sulfur dioxide aerosol emission contribute to a decreasing Sahel precipitation.

Sulfur emissions are also the perpetrators of acid rain (Stern 2005). Acid rain can cause irritation to

the skin and eyes and is also a potential threat for safe drinking water (Heggie 2009).

2.2 SPENT CAUSTIC STREAMS

Origin

Crude fossil oils and gases naturally contain a high amount of sulfur. The sulfur needs to be removed

to obtain a stream that conforms with regulations concerning sulfur emissions. Manufacturing

plants for gasoline, diesel, liquid petroleum gas, natural gas and ethylene gas use several methods to

lower the sulfur content. This is also referred to as the “sweetening” process. One among these is a

liquid or gas/liquid contact with a sodium hydroxide stream. This generates a toxic spent caustic

waste streams containing sulfide compounds in a high concentration.

Properties

Spent caustic stream contaminants vary in composition and concentration depending on the treated

stream. This stream has a black, brown or yellow colour, has a high alkalinity (pH >12), salinity (5-

12 wt%) and a high sulfide concentration (Ben Hariz et al. 2013). Spent caustic streams can be

divided in 3 groups: sulfidic, cresylic and naphthenic (Alnaizy 2008). These groups have their own

typical components and concentration ranges (Table 4).

Table 4: Characteristics of typical refinery caustic streams (adopted from Alnaizy (2008)).

Naphthenic Cresylic Sulfidic

Jet fuel Diesel Strong caustic

operation Dilute caustic

operation

NaOH [wt%] 1-4 1-4 10-15 1-4 2-10

Sulfide as S-[wt%] 0-0.1 Trace 0-1 0-0.2 0.5-4

Mercaptide as S+ [wt%] 0-0.5 0-0.5 0-4 0-0.5 0.1-4

Naphthenic acids [wt%] 2-10 2-15

Carbonates as CO3 [wt%] 0-0.5 0-0.1

Cresylic [wt%] 1-3 0-1 10-25 2-5 0-4

pH 12-14 12-14 12-14 12-14 13-14

5

3 CONVENTIONAL SPENT CAUSTIC TREATMENT

3.1 CHEMICAL TREATMENT

In the chemical oxidation treatment of spent caustics, sulfide is oxidized to sulfate. Chemicals that

are used are hydrogen peroxide, sodium hypochlorite, potassium permanganate, chlorine and

ferrate (VI) ions (Paulino & Alfonso 2012). A Fenton reagent, which contains an iron catalyst and

hydrogen peroxide, can be used in order to remove organics as well (Veerabhadraiah et al. 2011).

Sulfides can also be precipitated by percolating the SCS over solid iron or manganese hydroxides.

(Paulino & Alfonso 2012). A third option is neutralization or acidification of the spent caustic

stream. This treatment converts the spent caustic components into their original forms (sulfide,

mercaptans, phenol & naphthenic acid) (Veerabhadraiah et al. 2011).

3.2 THERMAL TREATMENT

In wet air oxidation, aqueous sulfide reacts with oxygen at elevated temperatures (around 200 °C)

and pressures (around 28 bars) in a liquid-phase reaction (Luck 1999; Sheu & Weng 2001). By

doing so, sulfide is converted to oxidation products such as sulfate. Another thermal treatment that

occurs at much higher temperatures is incineration. The process converts the inorganic fraction to

molten forms and decomposes the organic fraction into its most stable states (Veerabhadraiah et al.

2011).

3.3 BIOLOGICAL TREATMENT

A spent caustic stream can be treated biologically through bacterial adsorption, respiration and

synthesis mechanisms (Veerabhadraiah et al. 2011). Small amounts of spent caustic stream can be

treated in an on-site conventional biological waste water treatment plant (de Graaff et al. 2011).

Higher amounts of spent caustic can be treated for example by Haloalkaliphilic sulfide oxidizing

bacteria (HA-SOB). De Graaff et al. (2012) describes a two-step process where in a first step a spent

caustic stream was detoxified by converting the sulfide to thiosulfate. In the second step sulfate was

formed by HA-SOB out of the remaining sulfide and thiosulfate.

3.4 ADVANTAGES AND DISADVANTAGES

Each chemical, thermal and biological treatment described above has his own advantages and

disadvantages (Table 5).

6

Table 5: Advantages and disadvantages of conventional techniques to treat spent caustics (adapted from Veerabhadraiah et al. (2011) and de Graaff et al. (2011)).

Treatment Advantages Disadvantages

Chemical oxidation Complete oxidation of sulfides Low CAPEX

High chemical consumption (high OPEX)

Fenton oxidation Oxidation of organics Low CAPEX

High chemical consumption Unsuitable for sulfide removal Handling of corrosive sulfuric

acid Generates chemical sludge

Chemical precipitation

Complete removal of sulfides Also removes emulsified oil (EO)

and TSS Can be applied in existing

flotation units Low CAPEX

Need for in situ generation of chemicals

High chemicals consumption Large chemical sludge

generation Handling of corrosive chemicals Occupation risk of chlorine gas

leaks

Neutralization Recovers valuable phenol/organic acids

High CAPEX/OPEX for sulfides removal with add-on stripping and acid gas handling systems

Handling of corrosive sulfuric acid

Odor issues

Wet oxidation Complete oxidation of sulfides

High CAPEX and OPEX

Incineration Complete oxidation of sulfides and organics

Can use waste oil/vent gases as fuels

May allow direct disposal

High OPEX, if fresh-grade fuels are used

Waste fuels may need special injector/atomizer

Biological Safer

Can easily be disturbed by fluctuating pH conditions, increasing salt concentrations and the accumulation of toxic compounds

7

4 ELECTROCHEMICAL SPENT CAUSTIC TREATMENT

Almost all of the conventional SCS treatments have some disadvantages such as a high CAPEX or

OPEX, a high chemical usage, a low stress resistant or no recovery (Table 5). Regarding these

drawbacks, electrochemical spent caustic treatment has been investigated as an alternative

technology that could potentially recover sulfur and sodium.

4.1 ELECTROCHEMICAL SULFIDE REMOVAL

4.1.1 FUEL CELLS

Fuel cells can be used to remove sulfide and generate electricity at the same time. Fuel cells are

generally composed out of a cathode, at which oxygen is provided as an electron acceptor, an

electrolyte and an anode. At the anode, sulfide can be oxidized. Different types of fuel cells exist.

Solid oxide fuel cells (Aguilar et al. 2004; Liu et al. 2001), proton exchange membrane fuel cells

(Slavov et al. 1998) and direct alkaline solid fuel cells (Kim & Han 2014) were investigated for

sulfide removal. The difference between these cells are the type of ions that are transferred across

the electrolyte. Solid oxide fuel cells, proton exchange membrane fuel cells and direct alkaline solid

fuel cells transfer respectively oxygen anions, protons and hydroxides across the electrolyte. Kim &

Han (2014) performed experiments on a direct alkaline solid fuel cells where H2S is oxidized to

sulfur oxyanions and where at the cathode side a 99 % pure liquid oxygen is converted to H2O. This

fuel cell can generate 25.86 mW cm-2 when a stream of 1 M sulfide and 3 M NaOH is introduced at 80

°C. Carbon supported platinum nanoparticles (Pt/C) were used to enhance the process.

Microbial fuel cells are also becoming a promising technology to remove sulfide. Dutta et al. (2008)

showed a 98 % removal of sulfide by coupling a microbial fuel cell with an anaerobic sludge blanket

reactor. Zhang et al. (2013) studied the simultaneous removal of sulfide with corn stover biomass

using a microbial fuel cell. After 72h, 91 % removal of sulfide could be achieved, with a maximum

power production of 0.744 mW cm-².

4.1.2 INDIRECT ELECTROLYSIS

In indirect electrolysis, an intermediate chemical is first reduced while oxidizing sulfide. The

reduced intermediate chemical is then again oxidized at the anode of an electrolysis cell. Different

kinds of intermediates were examined for sulfide removal, such as soluble tri-iodide (I3-) (Kalinka &

Maas 1985, part I), soluble iodate (IO3-) (Kalina & Maas 1985, part II) and iron complexes (Olson

1984). Huang et al. (2009) used vanadium dioxide (VO2+) to absorb sulfide gas in an absorption

8

reactor followed by a separation step where elemental sulfur was yielded. In an electrochemical cell,

the vanadium dioxide was regenerated with additional hydrogen gas formation (Figure 1). This

system removes sulfide by absorption at an efficiency greater than 90 % at 50 °C. At this

temperature sulfur forms particles size of 2-4 µm size. Lower temperatures resulted in a gummy

sulfur which adhered to the reactor wall, causing clogging.

Figure 1: Indirect electrolysis set-up (adopted from Huang et al. 2009).

4.1.3 DIRECT ELECTROLYSIS

In direct electrolysis, the sulfide is directly oxidized at the anode of an electrochemical cell. Mao et

al. (1991) described the different reactions that can occur at the anode (Table 6). Sulfide ions are

readily oxidized to elemental sulfur (S0), polysulfides (Sn2-), sulfates (SO42-), thiosulfates (S2O32-),

sulfites (SO32-) or dithionates (S2O42-).

Direct electrolysis of sulfide has being investigated since Bolmer introduced the principle in 1968

(Bolmer 1968). Different anode materials have been suggested, such as graphite, nickel, porous

nickel-chromium alloy (Anani et al. 1990), activated glassy carbon (Nygren et al. 1989) and Ir/Ta

mixed metal oxide (MMO) coated titanium (Pikaar et al. 2011). An advantage of direct electrolysis is

the lower input of power compared to the power needed for regeneration of the reduced oxidant in

indirect systems. In addition, direct electrolysis tends to produce the crystalline sulfur compared to

the colloidal sulfur produced in indirect electrolysis system (Anani et al. 1990). On the other hand,

one of the main disadvantages is that the insulating elemental sulfur formed on the anode may

9

cause its passivation (Dutta et al. 2008). Another disadvantage is the oxygen evolution at the anode

which creates an additional cost (Ateya et al. 2003).

Table 6 : Possible anode reactions during direct electrolysis of aqueous sulfide solutions, with corresponding standard potentials (E°) (adopted from Mao et al. (1991)).

Possible anode reactions E° [V]1

HS− + OH− ⇌ S0 + H2O + 2e− 0.478 (1)

S2− ⇌ S0 + 2e− -0.508 (2)

S2− + Sn−12− ⇌ Sn

2− + 2e− , with 2 ≤ n ≤ 5 -0.466 to -0.520 (3)

S2− + 6 OH− → SO32− + 3H2O + 6e− -0.597 (4)

Sn2− + 6 OH− → S2O3

2− + 3 H2O + (n − 2)S + 6e− , with 2 ≤ n ≤ 5 -0.647 to -0.667 (5)

2 HS− + 8 OH− → S2O32− + 5 H2O + 8e− -1.131 (6)

SO32− + 2 OH− → SO4

2− + H2O + 2 e− -0.930 (7)

1 Standard electrode potential (E°) at 25°C and pH 0, with respect to SHE

4.2 ELECTROCHEMICAL SODIUM RECOVERY

At the cathode side of an electrochemical cell, sodium hydroxide can be formed by evolution of

hydrogen at the cathode (Anani et al. 1990; Table 7) and sodium ions crossing over the membrane.

Table 7: Possible cathode reaction (adapted from Anani et al. (1990)).

Possible cathode reaction E° [V]1

2 H2O + 2 e− → H2 + 2 OH− -0.83 (8)

1 Standard electrode potential (E°) at 25°C and pH 14

Cathode materials, such as nickel, graphite (Anani et al. 1990) and stainless steel (Moran & Jackson

2000), have been investigated. Wei et al. (2013) used a titanium cathode coated with ruthenium in

an electro-electrodialysis (EED) to recover NaOH from spent caustics. A SCS was fed to the anode

compartment. Sodium crosses over a cation exchange membrane to the cathode compartment

where it forms caustic soda with hydroxide ions produced at the cathode (Figure 2).

10

Figure 2 : Spent caustic treatment with an Electro-electrodialysis system (adopted from Wei et al. 2013). C = cation exchange membrane

Also a bipolar membrane electrodialysis (BMED) cell has been used to produce NaOH from SCS

(Wei et al. 2012). It consists of a three-compartment cell with an anode, middle and cathode

compartment separated by a cation exchange membrane and a bipolar membrane (Figure 3). The

SCS is fed to the anode compartment, of which the sodium ions cross over the cation exchange

membrane to the middle compartment. EED and BMED had both high current efficiencies (96%) at

high current densities (80 mA cm-2). However, the estimated process cost of this BMED set-up was

higher than for the EED set-up: 0.97 $ kg-1 NaOH for BMED, 0.86 $ kg-1 NaOH for EED (Wei et al.

2013).

Figure 3: Spent caustic treatment with a Bipolar membrane electrodialysis system (adopted from Wei et al. 2012). BP= bipolar membrane , C = cation exchange membrane.

11

4.3 COMBINED SULFUR AND SODIUM RECOVERY

Suetens (2014) examined the possibility of a combined removal of sulfide and recovery of sodium

hydroxide from a spent caustic stream in a two-compartment electrochemical cell (2-SCS-ES) as

well as a three-compartment cell (3-SCS-ES), operating in batch mode. A higher sulfide removal

efficiency for the 2-SCS-ES (84 ± 4 %) than for the 3-SCS-ES (55 ± 7 %) was observed. In both

reactors there was an efficient sodium hydroxide recovery as the corresponding coulombic

efficiencies were 91 ± 5 % and 82 ± 2 % for the 2-SCS-ES and the 3-SCS-ES respectively.

12

II OBJECTIVES Spent caustic streams are commonly treated with physicochemical processes, such as wet air

oxidation, chemical oxidation and precipitation. Although these techniques are quite robust and

effective, they require chemicals consumption, heating at high temperatures and large chemical

sludge production. On the other hand, biological sulfide removal, although possible, exhibits

limitations concerning robustness and toxicity.

Electrochemical treatment of spent caustic streams is investigated in this thesis as an alternative to

conventional widely used technologies. Hereby the aim is to overcome their limitations, recover

sodium hydroxide and investigate the possibility to recover sulfur oxidation products.

In a first stage, the robustness and reproducibility of the proposed treatment is examined during a

long term experiment. Next, the cell operation is optimized by assessing the effect of different

current densities and sulfide loading rates. Particular interest is placed on efficient operation of the

cell, efficiency of sulfide removal and sodium hydroxide recovery. Moreover, the possibility of sulfur

recovery is also assessed and linked to the operational conditions of the cell.

13

III MATERIAL AND METHODS 1 REACTOR SET-UP

A two-compartment electrochemical cell (Figure 4) consisted of two outer and two inner

rectangular Perspex plates (dimensions: 10 x 5 cm; Vlaeminck Metaalbewerking, Belgium), bolted

together with butterfly bolts. The two inner Perspex plates have a volume of 26 mL each. These two

were both equipped with influent and effluent connectors (Serto, Switserland) and ports for a

reference electrode. Rubber sheets were used to make the cell gas and liquid leak-proof. Spacers

(ElectroCell Europe A/S, Denmark) were used to avoid any contact of the electrodes with the cation

exchange membrane.

Figure 4: The different parts of the two-compartment electrochemical cell for spent caustic treatment. It consists of 2 outer Perspex frames (a), 2 inner Perspex frames with influent and effluent connectors (b), 6 rubber sheets (c), an iridium tantalum mixed metal oxide coated titanium anode (d), 2 spacers (e), a cation exchange membrane (f) and a stainlesss steel mesh cathode (g).

Electrode material

The anode (Figure 4 d) was a flattened mesh shaped iridium tantalum mixed metal oxide

(IrO2/TaO2: 0.65/0.35) coated titanium electrode ( 5 x 2 cm, 1 mm thickness) and functioned as a

working electrode (Magneto Anodes BV, The Netherlands). The anode was connected to the

potentiostat with a stainless steel current collector. As a cathode (Figure 4 g) , a stainless steel mesh

was used (projected area : 5 x 2 cm; mesh width : 44 micron; wire thickness: 33 micron; Solana,

Belgium). In order to perform cyclic voltammetry (section III4.3), an Ag/AgCl reference electrode

was temporally placed in the anode compartment port.

14

Ion selective membrane

A cation exchange membrane (Figure 4 f) (Fumasep® FKB-PK-130, Fumatech GmbH, Germany)

worked as a separator of the liquid in the anode and cathode compartment, and at the same time

only permitted the transfer of sodium ions. No pretreatment was necessary.

Data acquisition

Data acquisition was performed by a Data acquisition/Switch unit type 34970 A (Agilent, USA). The

cell voltage was measured every 5 min.

2 EXPERIMENTAL PROCEDURE

2.1 FEED

The electrochemical cell was run in a continuous mode (Figure 5 and Figure 6). Peristaltic pumps of

the type WM520s (Watson-Marlow Inc., USA) and Masterflex L/S (Cole-Parmer, USA) were used to

respectively feed and recirculate the catholyte and anolyte. The recirculation pumps were set at a

recycle flowrate of 2 L h-1. Norprene® and Tygon® chemical Masterflex tubing were used (Cole-

Parmer, USA).

Figure 5: Electrochemical cell set-up schematically.

15

Figure 6: Electrochemical cell set-up.

The anode influent consisted of 1 wt% sodium sulfide hydrate (Na2S. x H2O; 60-63 % pure; Acros

organics, Belgium), 4 wt% NaOH (VWR Chemical, Belgium) and degassed deionized water. An argon

gasbag was attached to the sealed anode influent bottle to keep it anoxic without having a pressure

build-up. The cathode influent consisted of deionized water.

2.2 EXPERIMENTS

Long term experiment

In the long term experiment, the current was kept constant at 100 A m-2 . The sulfide loading rate

was kept constant at 50 g S L-1 d-1 except for the period of day 25 till day 34, when the sulfide loading

rate was lowered to 25 g L-1 d-1. From day 49 till 105, there was a resting period for 55 days with no

sulfide loading and only a low current of 0.05 A m-2 (Figure 7). The 100 A m-2 and 50 g S L-1 d-1

conditions would give us a theoretical removal efficiency of about 100% if the only process

occurring is the two electron oxidation of sulfide to elemental sulfur.

16

Figure 7 : Scheme of the long term experiment.

Effect of current density

In this experiment, the current is built up from 50, 100, 150 to 200 A m-2. The current is then again

decreased by a step of 50 A m2- downwards (Figure 8). The sulfide loading rate is kept at

50 g S L-1 d-1.

Figure 8 : Scheme of the current density effect experiment.

Effect of sulfide loading rate

In this experiment, the sulfide loading rate is built up from 50, 100, 150 to 200 g L-1 d-1 by increasing

the anolyte flowrate . The current is then again decreased in the same steps down to 50 g S L-1 d-1

(Figure 9). The current density is kept at 100 A m-2.

0

30

60

90

120

150

0

25

50

75

100

0 25 50 75 100 125 150

Cu

rre

nt

de

nsi

ty (

A m

-2)

Sulf

ide

load

ing

rate

(g

S L-1

d-1

)

Time(days)

Sulfide loading rate

Current density

0

50

100

150

200

250

0 5 10 15 20 25 30 35

Cu

rre

nt

de

nsi

ty (

A m

- ²)

Time(days)

17

Figure 9: Scheme of the sulfide loading rate effect experiment.

3 ANALYSIS AND MEASUREMENTS

3.1 SULFIDE ANTI-OXIDANT BUFFER

A Sulfide Anti-Oxidant Buffer (SAOB) was used in order to preserve the sulfur species in the

samples. The preparation method of Keller-Lehmann et al. (2006) was used, with some minor

modifications. A bottle of 100 mL deionized water was sparged with Argon for 20 minutes, and 0.32

g NaOH (VWR Chemical, Belgium) and 0.27 g L-ascorbic acid (Sigma-Aldrich co., USA) were added.

The solution continued to be sparged until all had dissolved. The bottle was then closed with a

rubber sealing, allowing the use of a needle to take out the buffer, wrapped in aluminium foil and

stored shielded from light at 4 °C for no longer than 4 days. For every sample to be injected in one of

the two IC’s for sulfur species, 0.5 mL of SAOB was added. Also back-up long term preservation

samples were made, containing 1.1 mL sample and 1.1 mL SAOB.

3.2 SULFIDE, THIOSULFATE AND SULFITE ANALYSIS

Sulfide (HS-), thiosulfate (S2O32-) and sulfite (SO32-) were analysed using a 930 Compact Metrohm IC

system, according to Keller-Lehmann et al. (2006). The eluent, consisting of 3.5 mM Na2CO3 and 3

mM NaHCO3, had a flowrate of 0.8 mL.min-1. A 0.1 M NaOH solution was used to produce a gradient

needed for thiosulfate measurement. The IC samples were made by filling vials with 0.5 mL SAOB,

followed by 40 µL of sample and 1.66 mL of degassed Milli-Q water (55x dilution). The total sample

volume of 2.2 mL was chosen to avoid as much as possible an air headspace in the vial. As the pH

was higher than 10 in every sample, a negligible part of the sulfide was present in unionized form

and no significant loss in the gas phase could occur.

0

50

100

150

200

250

0 5 10 15 20 25 30 35

Sulf

ide

load

ing

rate

(g

S L-1

d-1

)

Time(days)

18

3.3 SULFATE ANALYSIS

Sulfate (SO42-) was analysed using a 930 Compact Metrohm IC system, according to SOP ANA-006,

LabMET. This anion IC was equipped with a Metrosep A Supp 5-150 column and a conductivity

detector. The eluent, consisting of 3.2 mM Na2CO3 and 1 mM NaHCO3, had a flowrate of 0.7 mL min-1.

The IC samples were made by filling vials with 0.5 mL SAOB, 0.1 mL sample and 12.7 mL of

degassed Milli-Q water (133 × dilution). The total sample volume of 13.3 mL was chosen to

minimize the air headspace in the vial.

3.4 POLYSULFIDE AND ELEMENTAL SULFUR ANALYSIS

In order to measure the polysulfide and elemental sulfur in the anode effluent, an oxidation of all

sulfur species to sulfate was performed by hydrogen peroxide (H2O2) addition (Dutta et al. 2010;

reaction (9)). 1 mL of sample was added in 49 mL of milli-Q water. 2.6 mL of 30 % H2O2 (Merck

Schuchardt OHG, Hohenbrunn, Germany) was added slowly. Samples for the analysis of SO4 in the

anion IC were prepared by adding 1 mL of this solution in vials filled with 9 mL of milli-Q water.

Also samples for the IC analysis of sulfide, thiosulfate and sulfite were prepared to check if there

were any remaining unconverted sulfur species. The difference between the sulfate concentration

after H2O2 oxidation and the species present in the sample before the oxidation was regarded as the

sum of the concentrations of polysulfide and soluble elemental sulfur (equation (11)).

[HS−]eff + [S°]eff + [Sn2−]eff + [S2O3

2−]eff + [SO32−]eff + H2O2 → [SO4

2−]gen (9)

[S]eff,tot = [SO42−]eff + [SO4

2−]gen (10)

[S°]eff + [Sn2−]eff = [S]eff,tot − [HS−]eff − [SO4

2−]eff − [S2O32−]eff − [SO3

2−]eff (11)

with [HS-]eff, [S°]eff, [Sn2-]eff, [S2O32-]eff, [SO32-]eff and [SO42-]eff the effluent concentrations (g S L-1) of

respectively sulfide, elemental sulfur, polysulfide, thiosulfate, sulfite and sulfate. [SO42-]gen is the

sulfate generated by H2O2 destruction. [S]eff,tot is the total effluent sulfur.

3.5 HYDROXIDE ANALYSIS

When the pH was below 13, pH was measured with a Metrohm 744 pH meter (Metrohm,

Switzerland). It was regularly calibrated with calibration solutions of pH 4, 7 and 10. As the pH was

above 13 most of the time, no pH meter could be used. 20 mL catholyte samples were titrated

instead with 1 M HCl (Sigma-Aldrich, USA) to pH 4-5. The molar hydroxide concentration was

assumed to be equal to the molar sodium hydroxide concentration, as the only cation present in

anolyte and catholyte was sodium.

19

3.6 FLOWRATE MEASUREMENT & SULFIDE LOADING RATE CALCULATION

The flowrate was measured by weighting the begin and end weight of the effluent bottles. The

sulfide loading rate (SLR, g S L-1 d-1) was calculated with the incoming sulfide concentration ([HS-] in,

g S-HS- L-1), the anode flowrate (Qan, L d-1) and the total anode compartment volume, comprising the

compartment space and the recirculation part, of 0.031 L (Van) (equation (12)).

SLR =

[HS−]in × Qan

Van

(12)

3.7 SETTING UP METHOD FOR SULFUR SPECIES

Standard solutions of sulfide, thiosulfate and sulfite were made according to Keller-Lehmann et al.

(2006), with some minor changes indicated below.

3.7.1 STANDARD STOCK SOLUTIONS

Sulfide stock solution (1,000 mg S L-1)

For the sulfide standard , 0.5 L Milli-Q water was bubbled for 20 min with Argon gas. 0.5 mL was

removed and replaced by 0.5 mL 50-52 % pure NaOH (Sigma-Aldrich, USA) to reach a 20 mM

concentration. Finally 1.98 g Na2S.xH2O (60-63% pure; Acros organics, Belgium) was added.

Meanwhile, the headspace was flushed with argon-gas.

Thiosulfate stock solution (1,000 mg S L-1)

For the thiosulfate standard, 0.5 L Milli-Q water was bubbled for 20 min with Argon gas.

Subsequently, 1.94 g of Na2S2O3.5H2O (VEL n.v., Belgium) was added while having the bottle

headspace flushed with argon-gas.

Sulfite standard stock solution (1,000 mg S L-1)

For the sulfite standard, 0.5 L Milli-Q water was bubbled for 20 min with argon gas. A 0.1%

formaldehyde (VWR Prolabo, France) concentration was created in order to stabilize the sulfite in

the stock solution. Subsequently, 1.97 g Na2SO3 (VEL n.v., Belgium) was added while having the

bottle headspace flushed with argon-gas.

20

Blank standard stock solution

A blank standard stock solution was made by bubbling 0.5 L Milli-Q water for 20 min with argon

gas. A 0.1% formaldehyde (VWR Prolabo, France) concentration and a 20 mM NaOH (Sigma-Aldrich,

USA) concentration were created.

Mixed standard stock solution

A mixed standard solution of sulfide (20 mg S L-1), sulfite (200 mg S L-1) and thiosulfate (80 mg S L-1)

was made by adding 35 mL, 1 mL, 10 mL and 4 mL of respectively the blank, sulfide, sulfite and

thiosulfate standard stock solutions in a 50 mL Scott bottle. 2.2 mL of this mixed standard solution

was put in IC vials and were injected in the IC at different injection volumes (50, 25, 10, 5 and 1 µL)

to obtain different standards (Table 8).

Table 8: Standards used for IC calibration and their concentrations.

Concentrations (mg S L-1)

Standard 1 Standard 5 Standard 10 Standard 25 Standard 50

Sulfide 0.4 2 4 10 20

Sulfite 4 20 40 100 200

Thiosulfate 1.6 8 16 40 80

3.7.2 INTERFERENCE EFFECT OF SAOB

To see if there were any interferences of SAOB on the chromatograms, a SAOB without ascorbic acid

and a SAOB without sodium hydroxide were prepared. These were injected in the IC in different

strengths (Table 9).

Table 9: Composition of the IC samples with different strengths towards SAOB to examine the interference effect.

Strength degree SAOB [mL] Milli-Q [mL]

4 x 2.00 0.20

2 x 1.00 1.20

1 x 0.50 1.70

0.5 x 0.25 1.95

21

3.7.3 EFFECT OF TIME ON SAMPLE STORAGE BOUNDARIES

To test how long IC samples could be stored in the fridge without sulfide oxidation, a preservation

test was performed. Samples of an anode influent solution (10 g Na2S-S L-1 and 1 M NaOH) were

prepared as IC samples (55x dilution, final concentration: 182 mg L-1) with and without 0.5 mL

SAOB. These samples were put in the fridge. Each day, three samples were taken out and analysed in

the IC.

4 ELECTROCHEMICAL TECHNIQUES

4.1 CHRONOPOTENTIOMETRY (CP)

In chronopotentiometry, a constant electrical current flows across the electrochemical cell while at

the same time the voltage of the cell and/or the potential of the working electrode (WE) versus a

reference electrode (REF) are recorded. The high pH and sulfide concentration made it impossible

to continuously use a reference electrode so the cell voltage, which provide information on the

energetic efficiency of the system, was generally recorded.

4.2 MEMBRANE RESISTANCE

To estimate the membrane resistance, an Ag/AgCl reference electrode was put in the anode

compartment port and a chronopotentiometry for both working and counter electrode was run at a

constant current. Once stable potentials were observed between the WE and the REF (Ewe1) and

between the counter electrode and the REF (Ece1), the Ag/AgCl reference electrode was moved in the

cathode compartment port and an identical chronopotentiometry was recorded until again stable

potentials were observed (Figure 10).

The operating (under current) cell voltage between the anode and cathode (Ecell) consists of the

open circuit voltage (OCV, difference between anode and cathode open circuit potential OCP), the

overpotentials driving the anodic and cathodic reactions (ηan and ηcath) and the ohmic drops for the

membrane (EΩ,mem) and the anolyte and catholyte (EΩ,an and EΩ,cath) (equation (13)). As d1 and d4 (0.5

cm) are much larger than d2 and d3 (0.05 cm), the ohmic drop of the liquids at d2 and d3 were

neglected. In equations (14) to (17), the formulas of the different potentials are given.

22

Figure 10: Scheme for the calculation of the membrane resistance. Ewe1 and Ece1 are respectively the working electrode (anode) potential and the counter electrode (cathode) potential with respect to reference 1 (REF1). Same for Ewe2, Ece2 and REF2. Distances between the electrodes or between an electrode and the membrane are indicated as d1,d2,d3 and d4.

Ecell = OCV + ηan + ηcath + EΩ,mem (13)

Ewe1 = OCPan + ηan + EΩ,an (14)

Ece1 = OCPcath + ηcath + EΩ,an + EΩ,mem (15)

Ewe2 = OCPan + ηan + EΩ,cath + EΩ,mem (16)

Ece2 = OCPcath + ηcath + EΩ,cath (17)

The ionic resistance of the anolyte and catholyte (Ran and Rcath) are determined by measuring the

conductivity of the anolyte effluent σan and catholyte effluent σcath (equation (18) and (19))

EΩ,an = Ran × I =

d1

σan × S× I

(18)

ANOD E

CA THODE

-

-

---

-

-

-

-

-

R E F 1

R E F 2

Ewe1

Ece1

Ece2

Ewe2

d1 d

2 d

3 d

4

23

EΩ,cath =

d4

σcath × S× I

(19)

With S the section for ionic current pathway, assumed to be the common projected surface area of

the electrodes and membrane (10 cm²) and I the current.

The membrane ohmic drop can then be calculated with Ewe2 and Ewe1 (equation (20)) or with Ece1

and Ece2 (equation (21))

Ewe2 − Ewe1 = EΩ,cath − EΩ,an + EΩ,mem (20)

Ece1 − Ece2 = EΩ,an − EΩ,cath + EΩ,mem (21)

Finally, the membrane resistance (Rmem) can be calculated in 2 ways(equations (22) and (23)).

Rmem =

EΩ,mem

I=

Ewe2 − Ewe1 − EΩ,cath + EΩ,an

I

(22)

Rmem =

Ece1 − Ece2 − EΩ,an + EΩ,cath

I

(23)

4.3 CYCLIC VOLTAMMETRY (CV)

In cyclic voltammetry, the potential of the WE is ramped linearly towards a set potential and then

forward until the initial potential. By doing so, information can be gathered about the

electrochemical reactions on the working electrode. Cyclic voltammetry was recorded at the end of

each long term chronopotentiometry to investigate the electrochemical reactions occurring on the

anode for each operating steady state. The feed pumps were stopped, while the recirculation pumps

still ran. CV’s were recorded at a specific scan rate of 50 mV.s-1 unless otherwise specified.

5 CALCULATIONS

5.1 EFFICIENCIES

The removal efficiency of sulfide (RE sulfide) was calculated by dividing the removed sulfide by the

sulfide concentration in the anode influent (equation (24)).

RE sulfide =

[HS−]an,in − [HS−]an,eff

[HS−]an,in

(24)

24

The coulombic efficiency of sulfide (CE sulfide) was calculated by dividing the removed sulfide by

the amount of sulfide that can be theoretically removed ([HS-]rem,theor) by the applied amount of

electrons, assuming all sulfide is converted to elemental sulfur in a 2 e- oxidation(equations (25) &

(26)).

[HS−]rem,theor =

I × 86400

r1 × F × Q

(25)

CE sulfide =

[HS−]an,eff − [HS−]an,in

[HS−]rem,theor

(26)

With I the applied current (A), F the faraday constant (96485.3 C per mol e-), Q the flowrate (L d-1),

r1 the amount of electrons involved in the oxidation of one HS- (2 mol e- per mol HS-)

The CE for sodium hydroxide (NaOH) was calculated by dividing the amount of sodium hydroxide

generated in the catholyte ([NaOH]cath,eff) by the amount of sodium hydroxide that can be

theoretically formed by the applied amount of electrons ([NaOH]form,theor) (equation (28)).

[NaOH]form,theor =

I × 86400

r2 × F × Q

(27)

CE sulfide =

[NaOH]cath,eff

[NaOH]form,theor

(28)

With r2 the amount of electrons involved in the reduction of one OH- (1 mol e- per mol OH-)

5.2 ELECTRON BALANCES

Electron balances were made considering the reactions (29) to (32) to be the only reactions

occurring in the system.

Table 10: Reactions at the anode for the electron balance calculation (adapted from Mao (1991)).

Reactions Mol e- needed per mol HS-

HS− ⇌ S° + H+ + 2e− 2 (29)

HS− + (n − 1) S° ⇌ Sn2− + H+ with 2≤n≤5 1 to 1.6 (30)

HS− + 9 OH− → SO42− + 5 H2O + 8 e− 8 (31)

2 HS− + 8 OH− → S2O32− + 5 H2O + 8e− 4 (32)

25

It was assumed that no dissolved elemental sulfur was present in the anode effluent, which means

that the remaining fraction only contained polysulfide (section III3.4). As the polysulfide chain

length (n) can be lying between 2 and 5 at a higher pH (Figure 11), it was assumed to be 3.5. This

polysulfide needs 1.43 mol e- per mol HS- to oxidize.

Figure 11: Eh-pH diagram for the sulfur-water system at 25°C with concentrations of 2*10-4 mol.L-1 for sulfur(- II) species and 10-6 mol. L-1 for polysulfide species (adopted from (Buckley & Hamilton 1987).

The amount of electrons used per sulfur component was then divided by the theoretical amount of

electrons that were retrieved from the anode (equation (33)) to calculate the relative electron usage

for each sulfur species (i.e. the electron usage for each sulfur component of the total available

electrons) (equation (34))

total electrons available =

I × 86400

F × Q

(33)

relative electron usage =

electrons used

total electrons available

(34)

26

IV RESULTS 1 ION CHROMATOGRAPHY METHOD FOR SULFUR SPECIES

1.1 CALIBRATION

After optimization, calibration curves for sulfide, thiosulfate and sulfite were created (Figure 12). A

quadratic regression was made for sulfide. For sulfite and thiosulfate a linear regression was made.

Correlation coefficients higher than 0.999 were found for all sulfur species (Table 11). The

chromatograms show a good peak separation between the sulfur compounds (Figure 13).

Figure 12: Calibration curves of sulfide, thiosulfate and sulfite.

Table 11: Properties of the calibration curves of sulfide, thiosulfate and sulfite.

Sulfide Thiosulfate Sulfite

Curve type quadratic linear linear

Relative standard deviation 0.48% 3.51% 2.57%

Correlation coefficient 0.9999 0.9996 0.9998

27

Figure 13: Chromatograms of the calibration standards. The blue, yellow, green, red and black lines represent respectivelly standard 50, 25, 10, 5 and 1. The sulfide, sulfite and thiosulfate peaks are observed at retention times of 4, 12 and 15.5 minutes respectivelly. A good seperation between these peaks is observed.

1.2 INTERFERENCE EFFECT OF SAOB

The SAOB made an interference due to the present ascorbic acid for the 4× and 2× strength samples

(Section III3.7.2; Figure 14). That is why in this thesis 0.5 mL SAOB was added to a 1.7 mL sample.

Figure 14: SAOB interference. The yellow, green, red and black lines are the chromatograms of respectively the 2 mL, 1 mL, 0.5 mL and 0.25 mL SAOB samples for the samples with only NaOH (A). The green, red and black lines are the chromatograms of respectively the 2 mL, 1 mL, 0.5 mL SAOB samples for the samples with only ascorbic acid (B). The ascorbic acid gives a large interference for the 2 mL and 1 mL SAOB samples.

28

1.3 SAMPLE PRESERVATION TIME

The preservation time of the samples for sulfide analysis seem to increase by using SAOB buffer

(Figure 15). During the storage in the fridge for 14 days, the sulfide concentration in the samples

with 0.5 mL SAOB did not seem to have decreased. The sulfide concentration in the samples without

SAOB started to decrease from day 5. After 14 days, the sulfide concentration decreased by 39 %.

This proves that SAOB was needed for the storage of the samples in the freezer, and that the

samples could be stored for at least 14 days without sulfide oxidation.

Figure 15: Sample perseveration time with and without SAOB addition. "no SAOB" samples consisted of 40 µL sample and 2.16 mL degassed Milli-Q. "SAOB" samples consisted of 0.5 mL SAOB, 40 µL sample and 1.66 mL degassed milli-Q. The sulfide concentration in the samples with SAOB didn’t seem to have decreased after 14 days, whereas it decreased in the samples without SAOB with 39%.

2 ELECTROCHEMICAL SULFIDE REMOVAL

2.1 CONTROL EXPERIMENTS

2.1.1 OPEN CIRCUIT

The electrochemical cell was run with no current (open circuit) for 8 days. The sulfur species

concentrations in anode influent and effluent stayed the same (Figure 16). No sulfite was detected in

the anode influent nor in the anode effluent.

0

2

4

6

8

10

12

0 3 6 9 12 15

Sulf

ide

co

nce

ntr

atio

n (

g SL

-1)

Time (days)

no SAOB

SAOB

29

Figure 16: Sulfide, sulfate and thiosulfate concentrations of the anode influent and effluent without electrode polarization. No sulfite was observed. Same concentrations of sulfide, sulfate and thiosulfate were found in the anode in and effluent.

2.1.2 NO SULFIDE FEED

A CV was run when there was only a 1 M NaOH solution in the anolyte (Figure 17). The water

oxidation (oxygen evolution) started around + 0.5 V vs. Ag/AgCl.

Figure 17: Cyclic voltammogram with no sulfide in the anolyte. The only peak observed is a water oxidation peak.

-50

0

50

100

150

200

250

300

350

400

450

-0.75 -0.5 -0.25 -1E-15 0.25 0.5 0.75 1

Cu

rre

nt

de

nsi

ty (

A m

--2 )

Ewe (V vs Ag/AgCl)

30

2.2 LONG TERM OPERATION EXPERIMENT

2.2.1 CELL VOLTAGE

A stable cell voltage of 2.74 ± 0.10 V was recorded during the 77 operational days of the system

(Figure 18). After the period of a decreased sulfide loading rate (day 25 till 34), the cell voltage was

again restored to the previous cell voltage. The same was seen after the incidents (day 47 and 124).

Figure 18 : Cell voltage during the long term experiment. Without considering the period during the interruption period (day 49 till day 104), the decrease in loading rate (day 25 till 34), and the incidents (day 47 and day 124), a stable cell voltage of 2.74 ± 0.10 V was recorded over 77 days of operational time at a current density of 100 A m-2.

2.2.2 SULFUR SPECIES

Figure 19: Sulfide, sulfate, thiosulfate and elemental sulfur/polysulfide concentrations of anode influent and effluent during the long term run from day 105 till day 132. No sulfite was observed.

31

Quite stable effluent concentrations were recorded for sulfate, sulfide and thiosulfate (Figure 19).

For polysulfide and elemental sulfur less stable anode effluent concentrations were calculated

(section III3.4).

The sulfide removal efficiency and coulombic efficiency stayed constant over time (Figure 20). The

removal efficiency was 68 ± 1 % and the coulombic efficiency was 69 ± 3 % on average.

Figure 20: Coulombic efficiency and removal efficiency of sulfide during long term experiment from day 105 till day 132. The coulombic efficiency was 69 ± 3 % on average and the removal efficiency was 68 ± 1 %.

The sulfide loading rate stayed rather constant (Figure 21). The average sulfide loading rate was

46.9 ± 2.3 g S L-1 d-1, which was close to the aimed 50 g S L-1 d-1.

Figure 21: The sulfide loading rate from day 105 till 132. The sulfide loading rate was close to the aimed 50 g S L-1 d-1.

The electron balance could be closed (Figure 22). The electrons generated during sulfate, thiosulfate

and polysulfide production were found to be 46 ± 8 %, 56 ± 11 % and 3 % respectively. Oxidation of

sulfide to sulfate and thiosulfate provided most of the electrons.

0%

20%

40%

60%

80%

100%

105 110 115 120 125 130 135

Effi

cie

ncy

Time (days)

CE Sulfide

RE Sulfide

0

10

20

30

40

50

60

105 110 115 120 125 130 135

Sulf

ide

load

ing

rate

(g

S L-1

d-1

)

Time (days)

32

Figure 22: Electrons generated per sulfur species divided by the total supplied amount of electrons. The balance could more or less be closed.

2.2.3 SODIUM HYDROXIDE

The sodium hydroxide concentration obtained in the cathode effluent was close to the expected

concentration for every sample (Figure 23). Before and after the switch in catholyte feed pumps, the

sodium concentration was respectively 37.7 ± 1.0 g L-1 and 49.6 ± 3.6 g L-1. The coulombic efficiency

was 96 ± 2% on average (Figure 24).

Figure 23: Hydroxide concentration in the cathode effluent and the theoretical (CE of 100%) hydroxide concentration in the cathode effluent for the long term SCS-ES-2 system from day 108 till 132. At day 116 the hydroxide concentration changed due to a switch in cathode feed pumps. The hydroxide concentration was close to the expected concentration.

0%

20%

40%

60%

80%

100%

120%

Ele

ctro

ns

use

d p

er

sulf

ur

spe

cie

s

Polysulfide

Thiosulfate

Sulfate

0

10

20

30

40

50

60

105 110 115 120 125 130

NaO

H c

on

cen

trat

ion

(g

S L-1

)

Time (days)

Cathode effluent

Theoretical

33

Figure 24: Coulombic efficiency of sodium hydroxide during long term experiment from day 105 till day 132. The coulombic efficiency was 96 ± 2% on average.

2.2.4 CYCLIC VOLTAMMETRY

A clear sulfide oxidation peak at 0.2 V vs Ag/AgCl and elemental sulfur reduction peak at -0.5 V vs

Ag/AgCl can be recorded on the cyclic voltammograms (Figure 25). The sulfide oxidation peak

seems to increase at higher recirculation flowrates, suggesting that mass transfer is an important

parameter that could limit the process at this current density.

Figure 25: Cyclic voltammograms for different recirculation flowrates at a steady state condition of the anolyte (100 A m2- and 50 g S L-1 d-1). A scan rate of 20 mV s-1 was used. The sulfide oxidation peak at 0.2 V vs Ag/AgCl seems to increase using higher recirculation flowrates.

2.2.5 MEMBRANE RESISTANCE

The working electrode (Ewe) and counter electrode potential (Ece) were recorded with an Ag/AgCl

reference electrode both in the anode and cathode compartment (Figure 26). The anolyte and the

catholyte conductivity were respectively 66.2 ± 0.9 and 251 ± 1.6 mS cm-1. The mean membrane

resistance was calculated (following the steps indicated in section III4.2) to be 4.5 ± 0.6 Ω (or 0.45

Ω.cm2 when related to the membrane surface area). The corresponding ohmic drop EΩ for the

0%

20%

40%

60%

80%

100%

105 110 115 120 125 130 135C

E N

aOH

Time (days)

-50

0

50

100

150

200

250

300

350

400

450

-0.75 -0.5 -0.25 -1E-15 0.25 0.5 0.75 1 1.25

Cu

rre

nt

de

nsi

ty (

A m

-2)

Potential (V vs Ag/AgCl)

0 L per h

1 L per h

2 L per h

S°→ HS-

HS-→ S°

34

process running at 100 A m-2 (i.e. 0.1 A) is estimated around 0.45 V, and therefore accounted for

about 16 % of the total operating voltage.

Figure 26: The working electrode (Ewe) and counter electrode (Ece) potential recorded with an Ag/AgCl reference electrode in anode and cathode compartment, in order to calculate the membrane resistance.

2.2.6 REPRODUCIBILITY OF PERFORMANCE

The electrochemical cell was never opened during the long term run, in spite of some incidents.

Breakage of anode feed pump tubing

The anode feed pump tubing broke at day 47 and so no influent was entering the anode

compartment. This resulted in a depletion of sulfide and a subsequent increase in cell voltage

(Figure 27). The cell was flushed with new anode influent until all the formed elemental sulfur

flocks had left the cell. The cell was then restarted at 10 A m-2, followed by 100 A m-2. The cell

efficiency was then recovered in terms of sulfide removal and cell voltage from day 49 on.

Figure 27: Effect of no anode feeding due to breakage of anode feed pump tubing at day 47 on the cell voltage over time. An increase in cell voltage was recorded, but it was restored to the pre-incident cell voltage after flushing the anolyte and a restart at 10 A m-2 and 100 A m-2.

-3

-2

-1

0

1

2

3

0 5 10 15 20 25 30

Po

ten

tial

(V

)

Time (min)

Ewe

Ece

Reference in anode compartment

Reference in cathode compartment

0

2

4

6

8

10

12

14

45 46 47 48 49

Ce

ll vo

ltag

e (

V)

Time (days)

Breakage of anode feed pump tubing

Restart at 10 A m-2

Restart at 100 A m-2

35

Headspace in anode compartment

At day 124, the anode compartment was not completely full (headspace of approximately 20 %).

This caused a steep increase (2V h-1) in cell voltage. Current supply stopped when the cell voltage

surpassed the 12 V threshold. The anode compartment most likely ran out of sulfide and the

stainless steel of the screws was oxidized instead, causing greenish flocks which can be iron sulfate

formation. A CV was run at this incident condition (day 124) and compared to a CV at a normal

steady state condition (day 122) (Figure 28). In the altered condition CV, no sulfide oxidation peak

was observed. This means that all sulfide was oxidized in the anolyte. Also no reduction of elemental

sulfur towards sulfide was found. This can be an indication that elemental sulfur was completely

oxidized, which supports the possibility that iron sulfate had formed. After a restart at 10 A m2- and

100 A m-2, the same steady state cell voltage was recorded showing the reproducible behavior of the

cell (Figure 28).

Figure 28: Cyclic voltammograms for a normal steady state anolyte (normal condition, at day 122) and for an interfered state (incident condition, at day 124). The sulfide oxidation and sulfur reduction peak are clearly visible for the normal steady state anolyte at a working electrode potential of 0.2 V vs Ag/AgCl reference electrode, whereas they are lacking for the interfered state.

2.3 EFFECT OF CURRENT DENSITY

2.3.1 CELL VOLTAGE

The cell voltage stayed constant during each current density run respectively, except for the run at

200 A m-2 where the voltage increased over time (Figure 29). At 200 A m-2 run, the cell voltage went

over 12 V creating an overload to the potentiostat that stopped current supply. The cell voltage was

-50

0

50

100

150

200

250

300

350

400

450

-0.75 -0.5 -0.25 -1E-15 0.25 0.5 0.75 1 1.25

Cu

rre

nt

de

nsi

ty (

A m

-2)

Ewe (V vs Ag/AgCl)

Normal condition

Incident condition

S°→ HS-

HS-→ S°

36

decreased on day 19 to 2.6 V due low influent (headspace formation) in the anode compartment,

which was restored when the anolyte was replaced.

Figure 29: Cell voltage over time during different current densities . Cell voltage stayed stable during each run, except at 200 A m-2. At day 19 the anolyte was replaced by fresh influent due to an incomplete filled anode compartment.


As the current density increases, more thiosulfate and sulfate were produced in the anode effluent

whereas polysulfide and elemental sulfur concentration decreased by (Figure 30 and Figure 31).

Figure 30: Sulfur species in the anode effluent during the current density effect test. At low applied current density (50 A m-2), almost no sulfate was produced in the effluent, but more polysulfide and thiosulfate. At a current density of 100 A m-2 and higher, sulfate was produced the anode effluent. At higher current density, the anode effluent contained more sulfate and thiosulfate

0

50

100

150

200

250

0

1

2

3

4

5

6

7

8

9

0 5 10 15 20 25 30 35 40 45 50

Cu

rre

nt

de

nsi

ty (

A m

- ²)

Ce

ll vo

ltag

e (

V)

Time (days)

Cell voltage

Current density

0

2

4

6

8

10

12

14

16

1 6 11 16 21 26 31 36 41 46

Sulf

ur

spe

cie

s (g

L-1

)

Time (days)

PolyS/S°

Sulfate

Thiosulfate

Sulfide

100 A m-² 50 A m-² 100 A m-² 150 A m-² 200 A m-² 150 A m-²

50 A m-²

37

Figure 31: Mean sulfur species concentrations in the anode effluent for the different applied current densities. At 50 A m-2 almost no sulfate was produced. As the current density increases, more thiosulfate and sulfate are produced whereas polyS/S° concentration decreases.

The sulfide removal efficiency increases from 50 to 150 A m-2 from 73 ± 1 % to 86 ± 3 %

respectively (Figure 32). The coulombic efficiency of sulfide decreases with increasing current

density.

Figure 32: The sulfide removal efficiency (RE Sulfide) and coulombic efficiency (CE Sulfide) of sulfide. The RE of sulfide increases towards 150 A m-2 . The CE of sulfide decreases with increasing current density.

The average sulfide loading rate for each current density was lower (42.3 ± 3.8 %) than the aimed

sulfide loading rate of 50 g S L-1 d-1.

0

1

2

3

4

5

6

7

8

9

50 100 150 200

S sp

eci

es

con

cen

trat

ion

(g

S L-1

)

Current density (A m-²)

Sulfide

Thiosulfate

Sulfate

PolyS/S°

0%

20%

40%

60%

80%

100%

120%

140%

160%

180%

50 100 150 200

Effi

cie

ncy

(%

)


RE Sulfide

CE Sulfide

38

Figure 33: Average sulfide loading rate for each current density.

The origin of the electrons transferred to the anode shifted from the production of thiosulfate and

polysulfide towards the production of sulfate (Figure 34). A remaining 24 ± 15 % of electrons at 200

A m-2 were not coming from the production of sulfur oxidation species in the anode effluent. These

electrons might have been coming from the electrons that were generated during the continuous

elemental sulfur formation that passivated the electrode.

Figure 34: Electrons generated per sulfur species divided by the total amount of electrons given to the cell for each current density. The origin of the electrons transferred to the anode shifted from the production of thiosulfate and polysulfide towards the production of sulfate.

0

10

20

30

40

50

60

50 100 150 200

Sulf

ide

load

ing

rate

(g

S L-1

d-1

)


50 100 150 200

Polysulfide 34% 25% 3% 0%

Thiosulfate 106% 72% 52% 34%

Sulfate 8% 34% 52% 42%

0%

20%

40%

60%

80%

100%

120%

140%

160%

Ele

ctro

ns

use

d p

er

sulf

ur

spe

cie

s

Current density (A m-2)

39


The sodium hydroxide concentration in the cathode effluent increases with increasing current

density (Figure 35). The expected (theoretical) NaOH concentrations match with the analyzed NaOH

concentrations in the cathode effluent.

Figure 35: Sodium hydroxide (NaOH) concentration in the cathode effluent and the theoretical (CE of 100%) concentration for each current density. The NaOH concentration increases with increasing current density. The calculated NaOH concentrations match with the NaOH concentrations in the cathode effluent.

The coulombic efficiency of NaOH stays constant for an increasing current density. On average the

coulombic efficiency of NaOH is 97 ± 1 %.


The current “peak” of the oxidation of sulfide to elemental sulfur seems to be higher when recorded

after a steady state chronopotentiometry was reached at the lowest current density (50 A m-2,

Figure 36).

Figure 36: Cyclic voltammograms at the steady state conditions of different current densities. The peak of the oxidation of sulfide to elemental sulfur seems to increase at 150 A m-2.

0

20

40

60

80

100

120

50 100 150 200 150 100 50

NaO

H c

on

cen

trat

ion

(g

L-1)


Cathode effluent

Theoretical

-50

0

50

100

150

200

250

300

350

400

450

-0.75 -0.5 -0.25 -1E-15 0.25 0.5 0.75 1 1.25

Cu

rre

nt

de

nsi

ty (

A m

-2)

Ewe (V vs Ag/AgCl)

50 A/m²

100 A/m²

150 A/m²

HS-→ S°

S°→ HS-

40

2.4 EFFECT OF SULFIDE LOADING RATE

2.4.1 CELL VOLTAGE

The cell voltage reached a quasi-steady state for each sulfide loading rate tested (Figure 37). A

higher loading rate decreases the cell voltage in general.

Figure 37: Cell voltage over time recorded for different sulfide loading rate at 100 A m2. The cell voltage stayed constant during each sulfide loading rate run-.


As the sulfide loading rate increases, less thiosulfate and more sulfide remains unconverted in the

anode effluent (Figure 38 and Figure 39). Sulfate was produced only at 50 g S L-1 d-1.

Figure 38: Concentration of sulfur species in the anode effluent over time. Thiosulfate concentration decreases and remaining sulfide increases by increasing sulfide loading rate (i.e. increasing the anolyte flowrate). Sulfate was only found at 50 g S L-1 d-1.

0

50

100

150

200

250

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 5 10 15 20 25 30 35 40 45 50

Sulf

ide

load

ing

rate

(g

S L-1

d-1

)

Ce

ll vo

ltag

e (

V)

Time (days)

Cell voltage

Sulfide loading rate

0

2

4

6

8

10

12

14

16

0 5 10 15 20 25 30 35 40

Co

nce

ntr

atio

n s

ulf

ur

spe

cie

s (g

S L

-1)

Time (days)

polyS/S°

Sulfate

Thiosulfate

Sulfide

50 g S L-1 d-1

100 g S L-1 d-1

150 g S L-1 d-1

200 g S L-1 d-1

150 g S L-1 d-1

100 g S L-1 d-1

50 g S L-1 d-1

41

Figure 39: Sulfur species mean concentrations in the anode effluent for the different applied sulfide loading rates. Thiosulfate concentration decreases and remaining sulfide increases by increasing sulfide loading rate. Sulfate was only found at 50 g S L-1 d-1.

The sulfide removal efficiency decreases by increasing sulfide loading rates (Figure 40). The

coulombic efficiency increases by increasing sulfide loading rates until 150 g S L-1 d-1. CE higher than

100 % can be observed because the maximal theoretical removal has been assumed for a 2-electron

process per S, while polysulfide formation requires less than 2 electrons per S (section III5.1).

Figure 40: Removal efficiency (RE) and coulombic efficiency (CE) of sulfide for different sulfide loading rates. The removal efficiency decreases with increasing sulfide loading rates. The coulombic efficiency increases with increasing sulfide loading rates until 150 g S L-1 d-1.

The measured loading rate was close to the aimed loading rate for each loading rate run (Figure 41).

0

2

4

6

8

10

50 100 150 200

Me

an c

on

cen

trat

ion

(g

S L-1

)

Sulfide loading rate ( g S L-1 d-1)

Sulfide

Thiosulfate

Sulfate

PolyS/S°

0%

40%

80%

120%

160%

200%

50 100 150 200

Effi

cie

ncy

Sulfide loading rate (g S L-1 d-1)

RE Sulfide

CE Sulfide

42

Figure 41: Measured sulfide loading rate for every loading rate run. The measured loading rate was close to the aimed loading rate for each loading rate run.

The largest fraction of the electrons transferred to the anode was coming from the production of

thiosulfate (Figure 42).

Figure 42: Electrons generated per sulfur species divided by the total amount of electrons given to the electrode for each sulfide loading rate run. The largest fraction of the electrons transferred to the anode was coming from the production of thiosulfate.


The sodium hydroxide concentration stays quite constant for each sulfide loading rate (Figure 43).

The calculated NaOH concentrations match with the NaOH concentration found in the cathode

effluent. The coulombic efficiency of NaOH was 100 ± 7 % on average.

0

50

100

150

200

250

50 100 150 200

me

asu

red

load

ing

rate

(g

S L-1

d-1

)

Expected loading rate (g S L-1 d-1)

50 100 150 200

Polysulfide 8% 25% 38% 8%

Thiosulfate 71% 135% 100% 133%

Sulfate 43% 0% 0% 0%

0%

20%

40%

60%

80%

100%

120%

140%

160%

180%

200%

Ele

ctro

ns

use

d p

er

sulf

ur

spe

cie

Sulfide loading rate g S L-1 d-1

43

Figure 43: Sodium hydroxide (NaOH) concentration in the cathode effluent and the theoretical one for each sulfide loading rate run. The NaOH concentration stays quite constant for each sulfide loading rate. The NaOH concentration in the cathode effluent matches with the theoretical maximal NaOH concentrations electrochemically generated.


The peak of the oxidation of sulfide to elemental sulfur increased at higher sulfide loading rates but

recorded at the same potential (Figure 44).

Figure 44: Cyclic voltammograms at the steady state conditions of different flowrates. The sulfide oxidation peak is more elevated at higher sulfide loading rate.

0

10

20

30

40

50

60

70

80

50 100 150 200 150 100 50

NaO

H c

on

cen

trat

ion

(g

L-1)

Sulfide loading rate (g S L-1 d-1)

Cathode effluent

Theoretical

-50

0

50

100

150

200

250

300

350

400

450

-0.75 -0.5 -0.25 -1E-15 0.25 0.5 0.75 1 1.25

Cu

rre

nt

de

nsi

ty (

A m

- ²)

Ewe (V vs Ag/AgCl)

50 g S/L/d

100 g S/L/d

150 g S/L/d

S°→ HS-

HS-→ S°

44

V DISCUSSION 1 ANALYSIS

No sulfite was detected during analysis of the influent and effluent of samples. This means either

that (1) there is no sulfite formation, (2) the sulfite that is formed directly reacts at the anode

towards other oxidation species or (3) the sulfite was already oxidized before analysis. Sulfite is

known to be even faster than oxidation of sulfide by air or oxygen (Keller-Lehmann et al. 2006).To

prevent oxidation, the samples were purged with argon. However, purging samples with argon or

nitrogen cannot completely prevent the oxidation of sulfite (O’Reilly et al. 2001). Oxygen can also

penetrate the PTFE tubing used in IC’s (Hansen et al. 1979). Pre-analysis derivatization methods can

be used to prevent sulfite oxidation, such as the formation of oxidation resistant

hydroxymethanesulfonate by addition of formaldehyde (reaction(35));(Moses et al. 1984) ) .

H2C(OH)2 + HSO3− → H2C(OH)(SO3)− + H2O (35)

As reaction (35) is reversed at high pH, which is the case for the samples used in these experiments,

this method could not be used. EDTA and L-ascorbic acid can also be added to prevent sulfite

oxidation. Hassan (1995) showed a sulfite oxidation to negligible levels by addition of these

chemicals. Although the SAOB that was added to the IC samples contained L-ascorbic acid, there

might have been sulfite oxidation by oxygen in the anode effluent bottle before sampling.

2 CONTROL EXPERIMENTS

The sulfur species concentrations in anode influent and effluent stayed the same after an eight days

run at open circuit (Figure 16, pg. 29). This means that there was no oxidation of sulfide during the

entire hydraulic retention time in the electrochemical cell, as well as the time the anolyte remained

in the anode effluent bottle before sampling. The set-up was thus gas tight, as sulfide is known for

being rapidly oxidised by oxygen or air, especially when exposed to light (Keller-Lehmann et al.

2006). Other conclusions that can be drawn from these results are that there was any volatilization,

adsorption or cross-membrane diffusion of sulfide.

45

3 LONG TERM OPERATION

During the long term operation period, a stable cell voltage was recorded (Figure 18, pg. 30). This

indicates that the membrane as well as electrodes remained unaffected by the harsh operation

conditions and exposure to high pH and high sulfide concentration. In particular, it strongly suggest

that there is hardly any passivation of the anode surface by elemental sulfur. Elemental sulfur layers

seems to build up at the anode surface as the cell voltage initially increases, until a certain steady

state is reached and the cell voltage stays constant. This suggests that the anode surface chemistry

reached a steady state and that no further accumulation of insulating elemental sulfur occurs, likely

because the formation of S0 on the surface is strictly compensated by its consumption for dissolved

polysulfide formation. The stable voltage also indicates that same electrochemical reactions occur at

the anode over time, which was also seen in the stable anode effluent sulfur species concentrations

(Figure 19, pg. 30).

Furthermore, after each intended (sulfide loading rate decrease) or unintended (breakage of the

anode feed tubing and loss of anolyte) parameter modification, the cell voltage and anodic effluent

composition were recovered once the initial parameters were applied and reached steady state

conditions. This suggests reproducibility of the process outcomes. Even though some unintended

incidents occurred, the electrochemical cell was never disassembled throughout the entire

operation period of 132 days. A back flush was sufficient to restore the cell to its normal operation.

These results show that the electrochemical system can most probably be implemented in full scale

applications efficiently.

4 EFFECT OF CURRENT DENSITY AND SULFIDE LOADING RATE

An increase in concentrations of more oxidized sulfur species, e.g. thiosulfate and sulfate, in the

anode effluent was observed with higher current density and lower sulfide loading rate (Figure 31,

pg. 37 and Figure 39 pg. 41). On the other hand, more polysulfide and elemental sulfur were

produced at lower current density. These findings are attributed to the electrons that are needed

per reaction. Polysulfide and S° formation need less electrons (1 to 2 e- per HS-, reaction (36) and

(37) ) compared to thiosulfate (4 e- per HS- reaction (38)) and sulfate (8 e- per HS- reaction (39)).

46

HS− ⇌ S° + H+ + 2e− (36)

HS− + (n − 1) S° ⇌ Sn2− + H+ with 2 ≤ n ≤ 5 (37)

2 HS− + 8 OH− → S2O32− + 5 H2O + 8e− (38)

HS− + 9 OH− → SO42− + 5 H2O + 8 e− (39)

*Reactions adapted from (Mao 1991)

These results are in agreement with the above sequence of reactions. In a first phase, polysulfide

and elemental sulfur are formed, the First Phase Sulfur Oxidation Species. Next, thiosulfate is

formed, the Second Phase Sulfur Oxidation Species, which was also observed by Behm & Simonsson

1997 (part II). Last species in the chain is sulfate, the Third Phase Sulfur Oxidation Species.

200 A m-2 and 50 g S L-1d-1 are limiting conditions of the process, since the cell voltage kept rising

(Figure 29, pg. 36). This can be caused by a built up of elemental sulfur layers on the anode, which

was also seen in the electron balance (Figure 34, pg. 38). There was a part of the electrons that

couldn’t be addressed to the sulfur compound production in the effluent, so these electrons might

have been used to create elemental sulfur deposition on the anode. These growing elemental sulfur

layers on the anode will have likely increased diffusion layer thickness, i.e. sulfide ions had more

obstruction to reach the anode. This increased the cell resistance and thus the cell voltage.

The sulfide oxidation peak on the CV increased when the CV was recorded after a CP at low current

density or at higher sulfide loading rates (Figure 36, pg. 39 and Figure 44, pg. 43). This is logical

since more sulfide is available for getting oxidized on the anode surface before reaching mass

transfer limitation.

The RE for sulfide was 68 ± 1% during the long term operation and 77 ± 5% during the current

effect experiment, although both were run at 100 A m-2. This is attributed to the sulfide loading rate,

which was 46.9 ± 2.3 g S L-1 d-1 in the long term operation and 40.9 ± 0.4 g S L-1 d- 1 in the current

effect experiment.

47

Table 12: Comparison between different electrochemical treatments.

Reference Waste type Treatment Anode type pH

Sulfide

influent

(g S L-1)

NaOH

influent

(wt%)

Current

density

(A m-2)

RE (%)

Sulfide

CE (%)

Sulfide

removal

CE (%)

NaOH

recovery

This thesis synthetic

SCS

Electrolysis

in two-

compartment cell

MMO coated

titanium 14.3 10 4

50 73 ± 1 141 ± 25 99 ± 1

100 77 ± 5 72 ± 1 97 ± 2

150 86 ± 3 72 ± 7 97 ± 3

Wei et al.

2012 SCS

Bipolar membrane

electrodialysis

Ruthenium

coated

titanium

13.5 ND 0.4 800 ND ND 98

Wei et al.

2013 SCS

Electro-

electrodialysis

Ruthenium

coated

titanium

13.5 ND 0.4 800 ND ND 97

Ben Hariz

et al. 2013 SCS

Electrocoagulation

(sacrificial Fe

anode) Fe

13

(corrected

to 9)

34.5 7.5 212 84 ND 0

Waterston

et al. 2007

aqueous

waste

Electrolysis to

sulfate in a single

compartment cell

Boron-doped

diamond 11 1 0.001 33,300 ND 90 0

Pikaar et al.

2012 Sewage

Electrolysis in

two-compartment

cell

MMO coated

titanium 7 0.009 0 100 88 ± 9 76 ± 30 ND

* ND: not determined

48

5 EVALUATION OF PROPOSED TECHNIQUE

A coulombic efficiency for NaOH recovery around 98 % was also observed in experiments where

bipolar membrane electrodialysis and an electro-electrodialysis were applied (Table 12). In these

experiments, a ruthenium coated titanium was used as a cathode, which is more expensive than the

stainless steel cathode used in the proposed system. It is thus more advisable to use a stainless steel

cathode in this application.

When the proposed technique is compared to the electrocoagulation technique, the sulfide removal

efficiency was higher at 150 A m2- but lower at 50 and 100 A m2-. Nevertheless, electrocoagulation

has the disadvantages of a depleting iron anode which has to be regularly replaced and the lack of

the possibility towards recovery of sulfur compounds or sodium hydroxide.

Pikaar et al. (2012) examined a similar electrolysis cell as the proposed system, treating however an

incoming sewage waste stream with a neutral pH and 1000 times lower sulfide concentration

(Table 12). Quite similar coulombic and removal efficiencies were observed compared to the

operation of the proposed technique at 150 A m-2, suggesting that the proposed technique can be

applied for the treatment of a broad range of sulfide concentrations and thus a broad spectrum of

sulfide containing waste streams.

6 VALORISATION OF SULFUR SPECIES

Polysulfide

At 50 A m2- and 50 g S L-1 d-1, a 2.7 ± 1.4 g S L-1 polysulfide and elemental sulfur concentration was

found in the anode effluent. Most of this fraction was probably polysulfide as the pH of the anode

was high (more than 13), which favours polysulfide formation and stability (Steudel 2000). This

polysulfide could potentially be harvested from the effluent as calcium polysulfide by adding low

cost calcium oxide (CaO) or calcium chloride (CaCl). Polysulfide can be an interesting product since

it can be added during the kraft process to increase the yield of wood pulp (Borchardt & Easty

1984). Calcium polysulfides can also be used as a fungicide on citrus fruits (Smilanick & Sorenson

2001) or to detoxify chlorobenzene in fly ash (Tabata et al. 2013). Another use of calcium

polysulfide is the remediation of soils contaminated with heavy metals (Aratani et al. 1979), such as

chromium (Cr(VI)) (Graham et al. 2006).

49

At 50 A m-2, still a large amount of thiosulfate was formed (Figure 31, pg. 37). Thiosulfate, which is

Second Phase Sulfur Oxidation Species as described above, might likely be less produced if lower

current densities are applied, e.g. 30 A m2-. Thereafter, the concentration of polysulfide in the anode

effluent might increase as well.

Electrochemical production of polysulfide has already been investigated. Behm & Simonsson (1997,

part II) found that when temperature and sulfide concentration are increased, the rate of

electrochemical polysulfide formation increases. In the experiments run, the temperature was

around 20°C. To get more polysulfide, heating the electrochemical cell can be option. The

temperatures of SCS can already be elevated to some extent, as during the gas-liquid contact of, e.g.

a fuel gas and the sodium hydroxide stream, heat can be transferred from the gas to the SCS.

Thiosulfate

During the long term experiment, a thiosulfate concentration of 3.4 ± 0.4 g S L-1 was produced. This

concentration was even increased to 4.8 ± 0.9 g S L-1 at 150 A m2-. Thiosulfate has been used to leach

gold as an non-toxic alternative to gold leaching with cyanide (reaction (40); Aylmore & Muir 2001).

4 Au + 8 S2O32− + O2 + 2 H2O → 4[Au(S2O3)2]3− + OH− (40)

Thiosulfate can also neutralize chlorine compounds in the paper industry, such as hypochlorite

(reaction (43) (Dhawale 1993))

HOCl + 2 S2O32− → Cl− + S4O6

2− + OH− (41)

A medical application for thiosulfate is to use it as an antidote for cyanide poisoning (reaction (42);

(Dhawale 1993)).

CN− + S2O32− → CNS− + SO3

2− + OH− (42)

50

7 ECONOMIC ANALYSIS

Making an economic analysis can be interesting to see whether this technique can be profitable and,

if so, at which specific conditions. We assume 1 m³ of SCS (10 g S L-1) to be treated per day in the

proposed electrochemical system at an applied current density of 50, 100 or 150 A m-2. After a

rough upscale calculation1, the anode surface area would need to be about 8 m². This could be

realized by having eight 1 m² anodes in parallel in the anode compartment. Considering a cost of an

electrochemical system of about € 6,000 m-2, the investment for this cell would be € 48,000. The

operational cost would consist out of the cost of electricity and the input of calcium oxide, that could

be used to precipitate calcium polysulfide. The gains from this process would be the production of

sodium hydroxide and calcium polysulfide. Another potential gain that is not included is hydrogen

gas cathodically produced, which is a valuable feedstock in heavy oil upgrading and refining

operations (Huang et al. 2009). A drawback is the special storage and handling needed of hydrogen

to be conform to workplace health & safety.

Table 13: Estimation of prices for operational cost calculation.

Price

Electricity 0.108 € kWh-1 Based on data of 2013 for industries, ec.europa.eu/Eurostat

CaO 0.012 € kg-1 Based on data of 2010, indexmundi.com

NaOH 0.356 € kg-1 Based on data of 2014, Meishan Jiayuan Chemical

Ca polysulfide 3.07 € kg-1 Based on data of 2012 for Fungicide, AG (Shanghai) Agriculture Technology Co., Ltd.

Table 14: Calculated consumption and production per day.

Conditions

Consumption

50 A m-2 100 A m-2 150 A m-2

Electricity kWh 15.03 55.25 115.07

CaO1 kg d-1 4.73 1.35 0.42

Production

NaOH kg d-1 12.74 28.24 40.41

Ca polysulfide kg d-1 12.83 3.66 1.14 1 1 mol CaO needed to precipitate 1 mol Ca polysulfide.

1 41 g S L-1 d -1was fed during the current effect experiment in the case of 100 A m2-, which is 1.27 g S d-1 when we know the anode compartment had a volume of 0.031 L. Feeding 10,000 g S d-1 instead of 1.27 g S d-1 needs a scale up of 7,900 times. This means that the anode (10 cm²) should be 7,900 times bigger , i.e. an anode surface of about 8 m²

51

With the prices estimated for these units (Table 13) and the calculated amounts that were produced

or consumed per day (Table 14), an economic margin could be calculated (Table 15) to make an

economic prospect over 25 years after installation of the electrochemical cell (Figure 45). The pay

back time for the investment costs at the operation conditions of 50, 100 and 150 A m-2 are

estimated to be respectively 3 year, 9 years and 23 years.

Table 15: Economic margin for the 50, 100 and 150 A m2-

Conditions

Consumption 50 A m-2 100 A m-2 150 A m-2

Electricity € d-1 1.62 5.97 12.43

CaO € d-1 0.45 0.13 0.04

Production

NaOH € d-1 4.54 10.05 14.39

Ca polysulfide € d-1 39.36 11.22 3.50

Figure 45: Estimated economic prospect between the different operations conditions at a current density of 50 A m2-, 100 A m2- and 150 A m2-. The pay back times for the conditions of 50, 100 and 150 A m-2 are respectively 3 year, 9 years and 23 years.

This economic prospect calculation does not take into account some cost factors, such as

engineering cost of design and operation, salaries cost, and the CAPEX and OPEX of pumps and

control devices. These costs increase the pay back time. Moreover, this device has only been tested

on a two and a half months small scale operation. It should be tested on a longer period to see how

long the anode and membrane remain intact. By up-scaling more implications might come up, such

as scaling effects and discrepancies in outcome results. Thereafter, testing a pilot scale

electrochemical cell should be the first step taken before full scale applications.

-50

0

50

100

150

200

250

300

350

400

0 5 10 15 20 25

Eco

nco

mic

pro

spe

ct (

k€)

Time (days)

50 A m-²

100 A m-²

150 A m-²

52

Nevertheless, the electrochemical treatment of SCS has some interesting economic products. This is

in contrast to existing treatment techniques such as chemical precipitation with iron(III)chloride

(FeCl3), where no recovery occurs. Assuming a removal efficiency of 70% and 2 mol FeCl3 needed

for the removal of 3 mol sulfide (reaction (43)), the daily treatment of 1 m³ SCS would cost €10 per

day2.

2 FeCl3 + 3 HS− → 2 FeS(s) + S0 + 3 H+ (43)

The treatment is thus even more expensive compared to the cost for the 50 and 100 A m2- runs

(Table 15), without even considering the potential economic benefits of the electrochemical cell.

This suggests that the electrochemical treatment can be very interesting alternative for SCS

treatment in an economic way.

8 FUTURE PERSPECTIVE

The results of lab scale testing and preliminary economic balance suggest that this electrochemical

process could be an alternative to conventional spent caustic treatments. Further research is needed

on a longer term (more than 6 months) operation of this electrochemical cell fed with a real spent

caustic stream. The aim would be to see whether there are implications introduced in the process by

other components of SCS, such as mercaptides, naphthenic acids and cresylic acids. The idea of

converting polysulfide produced by the process into a valuable product has to be further tested

using calcium oxide or calcium chloride. Based on the outcome of real SCS and product harvesting

tests, the next step would be to upscale the process.

2 Assuming a price of FeCl3 of €0.45 kg-1 , based on data of 2010, Dalian Future International Co., China

53

VI CONCLUSION Spent caustic streams are nowadays treated either chemically, thermically or biologically. These

treatments have some disadvantages such as high chemical dosage, high operating cost, low stability

or no recovery of chemicals. In this thesis, a spent caustic treatment in a two-compartment

electrochemical cell was investigated as an alternative for conventional spent caustic treatments.

A continuous reactor was run for 2.5 months at 100 A m2- and 46.9 ± 2.3 g S L-1 d-1. The operating

cell voltage was stable at 2.74 ± 0.10 V. Sulfide removal efficiency was 68 ± 1% at a coulombic

efficiency of 69 ± 3 % and NaOH production was 49.6 ± 3.6 g L-1 at a coulombic efficiency of 96 ± 2%.

Long term operation of the cell did not have negative effects on electrodes and membrane despite

the harsh operational conditions of high sulfide concentration and high pH. Additionally, no

limitation of the anode surface by elemental sulfur built up was observed.

In a next step, the effect of an increasing sulfide loading rate was investigated. Higher loading rates

(100, 150 and 200 g S L-1 d-1 ) resulted in lower sulfide removal efficiencies (respectively 58 ± 1 %,

47 ± 2 % and 34 ± 2 %), but coulombic efficiencies for sulfide removal increased (respectively 125 ±

7 %, 155 ± 19 % and 153 ± 18 %). No sulfate production was observed at sulfide loading rates

higher than 50 g S L-1 d-1, since the production of less oxidized sulphur-based compounds was

favoured.

The effect of current density was investigated in a third step. The conditions of a 200 A m-2 current

density and a sulfide loading rate of 40.2 g ± 3.9 S L-1 d-1 were found to be limiting for the system, as

the cell voltage continuously increased due to a built up of elemental sulfur on the anode. The

highest sulfide removal efficiency (86 ± 3%) was determined at 150 A m2- and 42.9 ± 5.2 g S L-1 d-1.

At these conditions also the highest thiosulfate (4.8 ± 0.9 g S L-1) and sulfate (2.4 ± 1.0 g S L-1)

production was observed. The operation at 50 A m2- and 45.3 ± 5.8 g S L-1 d-1 had a high removal

efficiency (73 ± 1 %) combined with the highest polysulfide and elemental sulfur (2.7 ± 1.4 g S L-1)

production. This last operation type can be interesting to harvest high value polysulfide that can be

used as a fungicide or used for the remediation of heavy metals polluted soils.

54

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VIII ADDENDUM: SUSTAINABILITY Although the general sustainability concern at LabMET seems to be alright, there might be still some

potential in enhancing it.

A first suggestion is to reuse certain plastic materials, such as tips and weighing boats. Tips and

weighing boats in the analytical lab are wasted in the chemical waste bins, whereas they could be

collected and washed in e.g. the dishwasher, and subsequently reused for applications that don’t

require high purity. A next point is that it might be a good idea to have a workshop for new students

regarding prevention of waste. During the design of experiments, a balance should be made on how

many samples that are needed to get sufficient statically sound results, and how much waste will be

made with it. A standard action plan can be made with the different basic steps of designing a

sustainable experiment, which students could easily use to get some nice sustainable experiments.

Another important part in sustainability is the social pillar, which is in this case the wellbeing of the

lab workers. During this thesis year, some lab workers got urine infections. This might be caused by

dehydration in the lab. Working in hot labs, for example in the K32 or the analytical lab when the

sun is shining intensively, combined with the long periods as some lab workers need to stay in the

lab several hours at a stretch, these urine infections could indeed have been caused by dehydration

during the lab work. A solution for this might be to have some water fountains installed in these

labs, in a way no contamination can occur and conform to regulations. A water fountain in the coffee

room might be an easier solution, as it could also stimulate water drinking.

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