Accepted Manuscript

Degradation kinetics of tetracycline in aqueous solutions usingperoxydisulfate activated by ultrasound irradiation: Effect ofradical scavenger and water matrix

Simin Nasseri, Amir Hossein Mahvi, Mehdi Seyedsalehi, KamyarYaghmaeian, Ramin Nabizadeh, Mahmood Alimohammadi,Gholam Hossein Safari

PII: S0167-7322(16)33791-6DOI: doi: 10.1016/j.molliq.2017.05.137Reference: MOLLIQ 7429

To appear in: Journal of Molecular Liquids

Received date: 27 November 2016Revised date: 20 May 2017Accepted date: 22 May 2017

Please cite this article as: Simin Nasseri, Amir Hossein Mahvi, Mehdi Seyedsalehi,Kamyar Yaghmaeian, Ramin Nabizadeh, Mahmood Alimohammadi, Gholam HosseinSafari , Degradation kinetics of tetracycline in aqueous solutions using peroxydisulfateactivated by ultrasound irradiation: Effect of radical scavenger and water matrix, Journalof Molecular Liquids (2017), doi: 10.1016/j.molliq.2017.05.137

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Degradation kinetics of tetracycline in aqueous solutions using Peroxydisulfate

activated by ultrasound irradiation: Effect of radical scavenger and water matrix

Simin Nasseri1, 2

, Amir Hossein Mahvi 1, 3

, Mehdi Seyedsalehi4, Kamyar

Yaghmaeian1, 3

Ramin Nabizadeh1,5

, Mahmood Alimohammadi1, Gholam Hossein

Safari6, 7

* 1Department of Environmental Health Engineering, School of Public Health, Tehran

University of Medical Sciences, Tehran, Iran. 2Center for Water Quality Research, Institute for Environmental Research, Tehran

University of Medical Sciences, Tehran, Iran. 3Center for Solid Waste Research, Institute for Environmental Research, Tehran

University of Medical Sciences, Tehran, Iran. 4 Department of Environmental Engineering, Science and Research Branch, Islamic

Azad University, Tehran, Iran 5Center for Air Pollution Research, Institute for Environmental Research, Tehran

University of Medical Sciences, Tehran, Iran. 6Department of Environmental Health Engineering, School of Public Health, Tabriz

University of Medical Sciences, Tabriz, Iran. 7 Health and Environmental Research Center, Tabriz University of Medical Sciences,

Tabriz, Iran.

* Corresponding Author; hsafari13@yahoo.com

Abstract

Degradation kinetics of the tetracycline antibiotic in aqueous solution was

investigated using sulphate radicals under ultrasound irradiation. The effect of various

operational parameters including initial tetracycline concentration, initial

peroxydisulfate concentration, solution pH, reaction time, temperature, ultrasound

power, the presence of natural organic matter, radical scavenger (tert-butyl alcohol

and methanol), as well as the chemical composition of water using ultrapure water,

drinking water, and secondary effluent on the degradation efficiency of tetracycline

were studied. The preliminary studies were performed using only peroxydisulfate,

ultrasound and ultrasound activated peroxydisulfate. The results indicated that

tetracycline degradation rate increased with the increase of initial peroxydisulfate

concentration, temperature and ultrasonic power, but decreased with the increase of

initial tetracycline concentration. The tetracycline degradation rate was highly

dependent of initial pH of the solution. The degradation of tetracycline followed the

first-order kinetics. The addition of humic acid in concentrations above 10 mg. L-1

decreased the degradation rate of tetracycline, although the effect could be

compensated using higher concentrations of peroxydisulfate. The role of active

radicals (sulfate and hydroxyl radicals) was investigated using radical scavengers of

methanol and tert-butyl alcohol. Under optimum operational conditions, 96.5% of

tetracycline removal was achieved with chemical oxygen demand and total organic

carbon removal of about 74% and 61.2%, respectively. The degradation rate of

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tetracycline was higher in drinking water compared to ultrapure water. Finally, the

intermediates were identified and the possible degradation pathway of tetracycline

was investigated. Overall, ultrasound activated peroxydisulfate was found to be a

promising process for tetracycline degradation in aqueous solutions.

Keyword: Tetracycline; peroxydisulfate; Ultrasound; Sonochemical degradation;

Advanced oxidation processes; Sulfate radical; kinetics

Abbreviation

AOPs Advanced oxidation processes

BW Bottled water

COD Chemical oxygen demand

DW deionized water

HA Humic acid

HPLC high-performance liquid chromatography

LC-MS Liquid Chromatography Mass

MA Methanol

MTBE Methyl tert-butyl ether

NOM Natural organic matter

PMS peroxymonosulfate

PDS

PS

Peroxydisulfate

Persulfate

SE Secondary effluent

SR-AOPs Sulfate radical-based advanced oxidation processes

STP Sewage treatment plants

TBA Tert-butyl alcohol

TC Tetracycline

TCs Tetracyclines

TCA Trichloroethane

TOC Total organic carbon

TW Tap water

US Ultrasound

UV Ultraviolet

UV–Vis Ultraviolet Visible

UW Ultrapure water

WWPTFE water-wettable polytetrafluoroethylene

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1. Introduction

Pharmaceutical compounds are one of the major classes of emerging contaminants due

to their excessive use, their chemical and physical features and pseudo-persistence in

the national aquatic environment [1-4]. Among these compounds, antibiotics require a

special attention because of their intensive application in human and veterinary

medicine [5-6]. The presence of antibiotics in the environment can lead to the spread

of antibiotic-resistant genes, considerable adverse effects on soil microbial

communities, reduced water quality, problems in biota and ultimately harm to human

health and ecosystem safety [6- 7]. The antibiotics residues and their metabolites have

frequently been detected in various environmental matrices including soils, municipal

sewage effluents, sediments, surface water and groundwater at trace levels (ng/L to

µg/L) [8-9]. These compounds are not perfectly eliminated during conventional water

and wastewater treatment processes, with removal nearly 12-80 %, resulting in

contamination of water resources [9-10].

Tetracyclines (TCs) are a group of broad spectrum antibiotics that are widely used in

aquaculture, human and veterinary medicine for prevention and treatment of bacterial

infections, and also as growth promoters in livestock-farming [11-14]. TCs cannot be

effectively metabolized and absorbed, about 25–75 % or even 70–90 % of tetracycline

antibiotics are excreted as active metabolites from the body through feces and urine,

and eventually enter to sewage treatment plants (STP) [12-15]. Moreover, TCs have

relatively long half-life, up to 180 days, based on the environmental conditions [16].

Advanced oxidation processes (AOPs) are efficient alternatives to degradation of non-

biodegradable substances such as antibiotics [17-18]. Recently, sulfate radical (SO4-•)-

based advanced oxidation processes (SR-AOPs) have been successfully used to

degradation of non-biodegradable organic compounds including antibiotics [19-21].

Peroxydisulfate (PDS, S2O82-

) and peroxymonosulfate (PMS, HSO5-

) are typically

used as oxidants in these processes. The primary mechanism of SR-AOPs is

production of sulfate radical (SO4-•

) with high redox potential (E0=2.5–3.1 V) [22-23].

Persulfate (PS) salts are dissociated in aquatic environment to PS anion (S2O82-

) which

is kinetically slow to react with many organic substances including antibiotics, despite

having strong oxidation potential (E0 = 2.01 V) [24-26]. The PS anion can be activated

using ultraviolet light [27], heat [28], microwave [29], sono [30], base [31], granular

activated carbon [32], quinones [33], phenols [34], soil minerals [35], radiolysis [36]

and transition metals [37], to generate sulfate free radicals with higher oxidative

potential (E0= 2.5–3.1 V) than the PS anion.

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Moreover, sulfate radicals (SO4-•

) have an even more oxidative potential than

hydroxyl radical in degradation of many organic compounds. They have higher redox

potentials, longer half-life and higher selectivity in oxidation of organic compounds

(SO4-•

, E

0 =2.5-3.1 V, half-life = 30-40 µs) than hydroxyl radical (OH

•, E

0 =1.89–2.72

V, half-life = 10-3

µs) [38-42]. Ultrasound may accelerate the production of sulphate

and hydroxyl radicals by activating persulfate, as illustrated in the following reactions

[43-45]:

H2O ))))→ OH• + H• (1)

S2O8−2 +

))))→ 2SO4

−• (2)

SO4−• + H2O → SO4

2− + OH• + H+ (3)

S2O82− + OH• → HSO4

− + SO4−• +

1

2 O2 (4)

S2O82− + H• → HSO4

− + SO4−• (5)

S2O82− +

pyrolysis→ 2SO4

−• (6)

Furthermore, during ultrasound irradiation, the collapse of cavitation bubbles lead to

high temperatures and pressures, that produce free radicals and other reactive species

and would also increase the number of collisions between free radicals and

contaminants. Thus, the US/S2O82-

process could be an efficient method for

degradation of recalcitrant organic contaminants due to its high oxidizing ability [44-

47]. The combination of ultrasound and persulfate (US+S2O82-

) has been effective for

removal of methyl tert-butyl ether (MTBE) [48], nitric oxide [49], 1,4-dioxane [50],

arsenic(III) [51], amoxicillin [52], tetracycline [53] and dinitrotoluenes [54].

Specific objectives of this study included: (1) to investigate degradation of

tetracycline in aqueous solution using PS activated by ultrasound (US, S2O82-

, and

US/S2O82-

processes); (2) to survey the effect of various operational parameters, such

as initial TC concentration, initial S2O82-

concentration, solution pH, reaction time,

temperature, ultrasound power, ultrasonic frequency, natural organic matter (NOM)

and the chemical composition of water using ultrapure water (UW), tap water (TW),

bottled water (BW) and secondary effluent (SE) on the degradation efficiency of

tetracycline; (3) to evaluate the kinetics of TC degradation by US/S2O82-

process; (4)

to investigate the mineralization efficiency of tetracycline by analyzing

concentrations of total organic carbon (TOC) and chemical oxygen demand (COD) ;

(7) to survey the dominant species of radical oxidant responsible in the US/S2O82-

process.

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2. Materials and methods

2.1 Materials

The tetracycline hydrochloride [C22H25N2O8Cl] (AR, 99%), Sodium persulfate

(Na2S2O8, 98%), Humic acid (HA), Acetonitrile and Oxalic acid (the mobile phase in

HPLC) were purchased from Sigma–Aldrich. Tert-butyl alcohol (t-BuOH) and

methanol (MA) were obtained from Merck (Darmstadt, Germany). All chemicals were

high purity analytical grade reagents and used without further purification. All

solvents were HPLC grade. All the solutions were prepared with Milli-Q water. The

main chemical properties of TC are illustrated in Table 1 [5, 55].

Table 1

The main chemical properties of Tetracycline hydrochloride

Parameter Character

Molecular formula C22H24O8N2HCl

Molecular weight (g/mol) 480.9

Solubility (mol/L) 0.041

Dissociation constant (pKa) 3.3, 7.7, 9.7 and 12

Chemical structure

2.2. Procedure

Sonochemical treatment of TC solution was done in batch mode using an ultrasonic

bath with a dimension of 15 cm × 24 cm × 13 cm and a capacity of 7.3 L (Elma,

Singen, Germany). The liquid depth in the bath was kept at 2/3rd

of its total depth. The

stock solution of TC (0.208 mM) was prepared daily with Milli-Q water and diluted to

reach an initial concentration as required. All the experiments were carried out in a

reactor vessel of 200 mL with a fixed volume of 100 mL of TC solution with a

specified concentration. The reactor was covered with a tinfoil to avoid any

photochemical effects. The vessel solution completely immersed into the aqueous tub

media. The location of the vessel in the aqueous tub was always kept the same. All

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experiments were done at a specific temperature using a cooling water jacket around

the reactor. The solution pH was adjusted using 1M NaOH or 1M HCL. At

determined intervals, 2 mL sample was drawn out, filtered through syringe filter

(WWPTFE membrane, diam. 13 mm, pore size 0.2 μm) and subsequently added the

same volume of methanol to quench the reaction immediately prior to analysis. All the

experiments were done in triplicate and the mean and standard deviation were

calculated.

2.3 Analysis methods

The pH was determined at room temperature using an S-20 pH meter, which was

calibrated by pH 4.0 and 7 reference buffer solutions. The TC concentration was

measured using high-performance liquid chromatography (HPLC), consisting of a LC-

20 AB pump (Shimadzu, Kyoto, Japan), a reversed-phase column (VP-ODS-C18 4.6

mm × 250 mm, 5 µm, Shim-Pack, Kyoto, Japan), and a UV detector (Shimadzu UV-

1600 spectrophotometer). The injection volume, retention time and detection

wavelength of TC were 20 µL, 2.38 min and 360 nm, respectively. The mobile phase

was acetonitrile and 0.01 M oxalic acid solution (31:69, v/v) with a constant flow rate

of 1.0 mL min-1

. UV–Visible absorption spectra were also conducted using a

Shimatzu UV-1800 spectrophotometer. TOC was measured using a Shimadzu TOC-V

CPH analyzer. COD was determined using open reflux titrimetric method based on

the standard methods.

3. Results and discussions

3.1. Degradation of TC under various oxidation processes

The degradation of TC by various processes, including the control, only PDS, only US

and the combination of US and PS (US+PS) are shown in Fig. 1. After 120 min of

reaction, 9.7%, 26.88%, 57.34% and 88.51% of the TC were removed by the control,

US, PS and (US+PS) processes, respectively. In the control experiments (pH=10), TC

was only slightly removed due to the stability of the tetracycline hydrochloride in

acidic and neutral mediums compared to alkaline mediums [12, 55]. The

concentration of TC slowly decreased under ultrasound irradiation because only a

limited amount of radicals are produced in exposure to ultrasound alone. These results

are in agreement with the results obtained by other researchers [56].

Fig. 1. Degradation of TC under different oxidation systems ([TC] = 0.104 mM;

[S2O82-

] = 4 mM; pH = 10; T = 25 0C; US Power: 500 W, 35 KHz).

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Oxidation of S2O82-

is usually low and not effective at ambient temperature. Hence,

S2O82-

activation is essential to accelerate its reaction with organic pollutants [41, 50].

Activation of S2O82-

can be done using alkaline conditions (pH ≥10) to form SO4-•

, the

superoxide (O2-•

) and OH• radicals as presented in Eqs. (7) to (9) [57-58]. Therefore,

more degradation of TC by S2O82-

alone compared to US alone was attributed to the

alkaline S2O82-

activation. So that, at neutral and acidic pH, the removal efficiency of

S2O82-

alone was lower than US alone (data not shown).

S2O82− + 2H2O

OH−

→ HO2− + 2SO4

2− + 3H+ (7)

HO2− + S2O8

2− → SO4−• + SO4

2− + H+ + O2−• (8)

SO4−• + OH− → SO4

2− + OH• k= 6.5× 107 M

-1S

-1 (9)

When S2O82-

was combined with US at ambient temperature, much faster degradation

of TC was achieved. The results suggest that S2O82-

could be properly activated by

ultrasound irradiation under alkaline pH. These results may be explained by the

exacerbated cavitation under low temperatures, which leads to enhanced degradation

and activation of S2O82-

. Moreover, US can elevate degradation of S2O82-

to produce

SO4-•

radical [53].

3.2. Kinetics of tetracycline degradation

Kinetics of TC degradation was investigated to optimize the conditions sonochemical

degradation process of TC. The sonochemical degradation process typically follows

first-order kinetics as shown in Eqs. (10) and (11). Many studies have suggested that

oxidation of organic pollutants by ultrasound follows first-order kinetics [47, 52, 59].

−d[TC]

dt= k𝑜[TC] (10)

lnCt

C0= −kot (11)

Where Co is the initial concentration of TC, Ct is the concentration of TC at time t, ko

is the first order reaction rate constant (min−1

) and t is the reaction time (min). The

reaction rate constant (k) could be calculated from the slope of a plot of ln (Co/Ct)

versus (t). To determine the rate of TC degradation by US/S2O82-

process, the data

obtained was studied using the first order kinetics. The effects of different parameters

such as initial TC concentration, initial PS concentration, solution pH, ultrasound

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power, ultrasound frequency, temperature, HA concentration and water matrix on the

kinetic of TC degradation were evaluated. In all the experiments, TC degradation

well-fitted the first-order kinetics model with high correlation coefficients (R2). The

values of kinetic rate constants (k0) related to the different parameters, and their

regression coefficients R2

are illustrated in Table 2.

3.3. The effect of S2O82-

concentration on the degradation of tetracycline

The effects of initial S2O82-

concentration on the TC degradation rate with different

initial concentrations of S2O82-

(1 to 6 mM) are shown in Fig. 2. It was observed that

with an increase in S2O82-

concentration from 1 to 4 mM, the TC degradation rate

increased markedly from 55.76% to 88.51% at alkaline pH and ambient temperature

within 120 min. These results suggest that with an increase in S2O82-

concentration,

more sulfate radicals are produced which leads to faster TC degradation [61-62].

However, further increase of S2O82-

concentration over 4 mM didn’t have significant

effect on the TC degradation rate. To avoid adding excess content of oxidant, the

subsequent experiments were performed using the optimum S2O82-

concentration (4

mM). Also, further increase of S2O82-

concentration had not negative effect on the TC

degradation rate. This may be due to the highest concentration of S2O82-

(6 mM) used

in this study, which did not achieve the critical level that began to reduce the TC

degradation rate. These results are in agreement with the results obtained by other

researchers [41, 52]. However, it could be noted that excessive addition of S2O82-

may

generate sulfate anions without generation of active SO4-•

. In addition, radical–radical

reactions can be formed prior to the radical-organic reactions and SO4-• can also be

scavenged by S2O82-

as shown in the following Eqs. (11) to (12) [45, 53].

SO4−• + SO4

−• → S2O82− k= 4× 10

8 M

-1S

-1 (11)

SO4−• + S2O8

2− → SO42− + S2O8

−• k= 6.1 × 108 M

-1S

-1 (12)

Table 2

Effect of different parameters on the kinetics of TC degradation

parameter Value Equation k0 (min−1

) R2

t 1/2 (min)*

0.052 y = 0.027 x - 0.0148 2.7 × 10-2

0.9934 25.6

TC concentration 0.104 y = 0.0175 x + 0.0255 1.75 × 10-2

0.9952 39.6

(mM)

0.156 y = 0.0121 x + 0.0388 1.21× 10-2

0.9957 57.2

1 y = 0.0067 x + 0.0497 0.67× 10-2

0.9843 103.4

2 y = 0.0084 x + 0.0690 0.84 × 10-2

0.9811 82.5

PS concentration 3 y = 0.0114 x + 0.0624 1.14 × 10-2

0.9916 60.7

(mM) 4 y = 0.0175 x + 0.0255 1.75 × 10-2

0.9952 38.0

5 y = 0.0182 x + 0.0233 1.82 × 10-2

0.9952 36.6

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6 y = 0.0189 x + 0.0219 1.89 × 10-2

0.9941

37.0

4 y = 0.0122 x + 0.0501 1.22 × 10-2

0.9937 58.7

pH 7 y = 0.0081 x + 0.0310 0.81 × 10-2

0.9942 88.8

10 y = 0.0175 x + 0.0255 1.75 × 10-2

0.9952 39.6

100 y = 0.0072 x - 0.0158 0.72 × 10-2

0.9920 99.2

200 y = 0.0106 x - 0.0043 1.06 × 10-2

0.9910 65.3

US power 300 y = 0.0134 x + 0.0154 1.34 × 10-2

0.9937 51.7

(W) 400 y = 0.0159 x + 0.0218 1.59 × 10-2

0.9958 43.5

500 y = 0.0175 x + 0.0255 1.75 × 10-2

0.9952 39.6

25 y = 0.0175 x + 0.0225 1.75 × 10-2

0.9952 39.6

Temperature 45 y = 0.0385 x - 0.16896 3.85 × 10-2

0.9784 18.0

(0C) 55 y = 0.0563 x - 0.1595 5.63 × 10

-2 0.9953 12.3

65

0

y = 0.1573 x – 1.019

y = 0.0175 x + 0.0255

15.7 × 10-2

1.75 × 10-2

0.9290

0. 9952

4.4

39.6

5 y = 0.0237 x - 0.0580 2.37 × 10-2

0.9859 29.2

HA concentration 10 y = 0.0127 x + 0.059 1.27 × 10-2

0.9956 54.5

(mg/L) 20 y = 0.0087 x + 0.0364 0.87 × 10-2

0.9960 79.6

DW y = 0.0175 x + 0.0255 1.75× 10-2

0.9952 39.8

Water matrix BW y = 0.0222 x - 0.0746 2.22 × 10-2

0.9854 31.2

TW y = 0.0190 x - 0.0075 1.90 × 10-2

0.9969 36.4

WE y = 0.0125 x - 0.0320 1.25 × 10-2

0.9960 55.4

Fig. 2. Effect of various concentrations of S2O82-

on the TC degradation ([TC] = 0.104

mM; [S2O82-

] = 4 mM; pH = 10; T = 25 0C; US Power: 500 W, 35 KHz)

3.4. The effect of initial pH on the degradation of tetracycline

The initial pH is one of the most effective factors in sonochemical processes. The

effect of initial pH on the TC degradation rate is shown in Fig. 3. After 120 min of

reaction, the TC degradation rates were 77.35%, 62.46% and 88.51% at pH 4, 7 and

10, respectively. To prevent potential problems to the reaction, the effect of initial pH

was investigated without adding any buffer solution (such as NaHCO3/KHSO4,

Na2HPO4/KH2PO4). These buffers may act as scavengers of SO4-• as presented in Eqs.

(13) and (14). Carboxyl acids such as acetic acid and sulfuric acid are generated

during the process [53]. Moreover, sulfate anion (SO42-

) was generated by the reaction

of SO4-• with water (Eq (3)), as an indirect support to the formation of SO4

-•. Hence, it

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is clear that pH values decreased during the process in the absence of buffer solution

[51, 53]. In all these experiments, the solution pH decreased from 4.0 to 3.4, from 7.0

to 5.8, and from 10 to 7.3, respectively.

SO4−• + HCO3

− → SO42− + HCO3

− (13)

HCO3 → H+ + CO3

− (14)

TC is an amphoteric molecule with pKa values of 3.3, 7.7 and 9.7 (Fig. 4). TC

molecules are predominantly neutral and positively charged at pH= 4 and negatively

charged at pH= 9. Jiao et al. studied aqueous photolysis of tetracycline and reported

that the TC molecules degradation was dramatically prevented in TC+

form and

enhanced in TC-

form. The negative charged TC molecules tend to attract reactive

species, such as OH•, because of the high electrical density on ring system, which

resulted in accelerating the degradation of TC [55].

Fig. 3. Effect of initial pH on the TC degradation ([TC] = 0.104 mM; [S2O82-

] = 4

mM; US Power: 500 W, 35 KHz; T = 25 0C).

Moreover, at alkaline pH values (pH ≥10), alkaline-activated persulfate is primarily

responsible for the production of SO4-•, O2

-• and OH

• radicals as Eqs. (7) to (9)

present. Also, under alkaline conditions, sulfate radicals can react with hydroxyl

anions to generate hydroxyl radicals (OH•) as shown in Eq. (3). Therefore, with

increasing pH, the rate of S2O82-

degradation into OH• and SO4

•- increased. [60]. SO4

-•

is the predominant radical responsible for TC degradation at pH 4, whereas both SO4-

•and OH

• are contribute to TC degradation at pH 7. Thus, three reactions compete with

each other in pH 7: the reaction between SO4-• and

OH

•, the reaction between SO4

-•

and TC, the reaction between OH• and TC, the simultaneous occurrence of these

reactions may reduce the TC degradation rate [40, 63].

Fig.4. Structure, ionization states and state transition of TC

3.5. The effect of initial TC concentration on the degradation of tetracycline

Fig. 5 shows TC degradation rate at various initial concentrations from 0.052 to 0.156

mM. It is clear that degradation rate of TC reduced as the initial TC concentration

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increased. In constant conditions, along with an increase in initial TC concentration,

possibility of reaction between TC molecules and reactive species decreased.

Moreover, higher concentrations of TC may suggest higher scavenging effect and the

concentration of sulfate radical decreased as a result. With an increase in initial TC

concentration from 0.052 to 0.156 mM, the degradation rate of TC decreased from

96.48% to 76.66% whereas, the total amount of degraded TC increased from 24.12 to

57.5 mg L-1

. These finding were in agreement with the results obtained by other

researchers [53, 64].

Fig. 5. Effect of initial TC concentration on TC degradation ([S2O82-

] = 4 mM; pH =

10; US Power: 500 W, 35 KHz; T = 25 0C).

3.6. The effect of ultrasonic power on the degradation of tetracycline

To investigate the effect of ultrasound power on the TC degradation rate, experiments

were done applying various levels of ultrasound power from100 to 500 W (Fig. 6).

With an increase in ultrasonic power from 100 to 500 W, the TC degradation rate

increased from 57.7% to 88.51%. It is clear that the increase of ultrasound intensity to

a specific high level promoted the degradation of S2O82-

and generation of reactive

species and consequently increased the rate of TC degradation [51]. In addition, the

increase of the power intensity lead to significant increase of the number of cavitation

bubbles [65]. These results are similar to the results obtained by other researches, who

reported an increase in the degradation rate corresponding to the increase in ultrasonic

power [65-68].

Fig. 6. Effect of Ultrasound Power on TC degradation ([TC] = 0.104 mM; [S2O82-

] = 4

mM; pH = 10; US: 35 KHz; T = 25 0C)

3.7. The effect of temperature on degradation of tetracycline

The effect of temperature on TC degradation rate with different levels (25 to 65 0

C) is

shown in Fig. 7. Increasing temperature from 25 to 65 oC enhanced the degradation

rate constant from 0.0175 to 0.1573 min -1

. Complete degradation of TC occurs after

60, 90 and 120 min of reaction at 65, 55 and 45 oC respectively. PS activation can be

perform by heat to produce SO4-•

radical as shown in Eq. (15). Therefore, the

complete degradation of TC by high temperature could be a result of thermally

activated S2O82-

. In addition, increasing temperature dramatically improved the

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cavitational activity and chemical effects, resulting in further degradation rate of TC

by US/S2O82-

process [26, 28].

S2O8−2 +

Termal−→ 2SO4

−• (30 0C < T < 99

0C) (15)

activation

To evaluate the effect of ultrasound on the process kinetics, significant parameters

such as activation energy (Ea) which plays a remarkable role, must be take into

consideration. The effect of temperature on the rate of the reaction and rate constant

(k) is obtained by Arrhenius equation as shown in Eq (16) [41].

LnK = A exp (−Ea

RT ) (16)

Arrhenius plot can be applied to determine the activation energy at different

temperatures by graphing ln k (rate constant) versus 1/T (kelvin). The graph between

ln k and 1/T is a straight line with an intercept of ln A and the slop of the graph is

equal to –Ea/R, where R is a constant equal to 8.314 J/mol-K. Based on Arrhenius plot

(Fig.8), the activation energy values of 42.66 (kJ/mol) was obtained for degradation of

TC by S2O82-

/US process. It means that for a successful reaction, the colliding

molecules must have a total kinetic energy of 42.66 kJ/mol.

Fig. 7. Effect of temperature on degradation TC

Fig. 8. Arrhenius equation graph representing the temperature dependence on

chemical reaction rate ([TC] = 0.104 mM; [S2O82-

] = 4 mM; pH = 10; US: Power:

500W, 35 KHz)

3.8. The effect of humic acid on the degradation of tetracycline

Natural organic materials (NOM, e.g., HA) usually exist in water supplies; thus, the

effect of humic acid on TC degradation rate was also investigated (Fig. 9). The

experimental data fitted by first order kinetics, with a k value of 2.37 ×10−2

min−1

,

1.27 ×10−2

min−1

and 0.87 ×10−2

min−1

at HA concentrations of 5, 10 and 20 mg/L,

respectively (Table 2). The results suggest that a small concentration of HA (5 mg/L)

accelerated TC degradation rate, whereas a negative effect in the oxidation process

was obtained at HA concentrations of 10–20 mg/L. Addition of 10 and 20 mg HA

caused a decrease in TC degradation rate from 88.51% (without HA) to 78.92% and

64.65%, respectively. The results indicated that hydroxyl and sulfate radicals were

initially consumed by HA more than TC, and HA acted as a hydroxyl radical source at

low concentrations and as a strong hydroxyl radical scavenger at high concentrations

[55]. However, the negative effect of HA was eliminated by adding an excess of

S2O82-

. More than of 90% TC degradation was obtained by 6 mM S2O82-

while 4mM

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S2O82-

and 20 mg/L HA resulted in about 64% TC degradation by 4mM S2O82-

and 20

mg/L HA. The results correspond with results obtained by Li et al., [45] who

investigated sono-activated persulfate oxidation of 1,1,1-trichloroethane (TCA) from

aqueous solution and reported higher TCA degradation rate in the presence of HA at

values less than 4.46 mg/L and lower degradation rate at values more than 8.92 mg/L.

Fig.9. Effect of HA on the TC degradation ([TC = 0.104 mM; [S2O82-

] = 4 mM; US

Power: 500 W, 35 KHz; pH = 10; T = 25 0C).

3.9. Effect of radical scavenger on the degradation of tetracycline

Sulfate and hydroxyl radicals were typically intended as primary oxidizing species for

degradation of organic pollutants in sono-activated persulfate processes at ambient

temperatures. To investigate the contribution of the oxidizing species, tert-butyl

alcohol (TBA) as OH• scavenger and methanol (MA) as SO4

-• and OH

• scavenger

were used. According to Eqs. (19) and (20), the reaction constant of TBA for OH•

radical is 418–900 times greater than the rate constant for SO4-• radical [45, 53].

According to Eqs. (17) and (20), the reaction constant of MA for OH•

radical is

approximately 300 time greater than the rate constant for SO4-•

radical [45, 53].

t-BuOH + OH• ~ 3.8-7.6 × 10

8 M

-1 S

-1 (17)

t-BuOH + SO4-• ~ 4-9.1 × 10

5 M

-1 S

-1 (18)

MA + OH• ~ 9.7 × 10

8 M

-1 S

-1 (19)

MA + SO4-• ~ 3.2 × 10

6 M

-1 S

-1 (20)

It is apparent that the TC degradation rate dramatically decreased after adding

alcohols (Fig.10). After addition of TBA and MA (molar ratio [TBA]: [TC] = [MA]:

[TC] = 1000:1), the TC degradation rate decreased from 88.51% (without scavengers)

to 35.48% and 28.24%, respectively. In the case of TBA addition, OH• radical was

scavenged and TC was degraded by SO4-• radical, while in the case of MA addition,

both SO4-• and OH

• were scavenged, which lead to a very low degradation rate. The

results showed that by the addition of the same amount of MA, a relatively smaller

reduction was found in the degradation process in compared to TBA. OH• radical was

found to be is predominant radical for the degradation of TC by US/S2O82-

process at

pH=10. Our results were similar to those acquired by Liang and Su [63] who applied a

chemical probe technology on thermally activated persulfate oxidation process to

recognize the oxidizing species, and concluded that OH• was the predominant radical

at pH=10.

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Fig. 10. Effect of scavengers on the TC degradation ([TC] = 0.104 mM; [S2O82-

] = 4

mM; US power: 500 W, 35 KHz; pH = 10; T = 25 0C).

3.10. Effect of water matrix on the degradation of tetracycline

To evaluate the effect of the water matrix on the degradation of TC, experiments were

done using deionized water (DW), bottled water (BW), tap water (TW) and

wastewater effluent (WE). The chemical characteristics of the waters used in this

study are illustrated in Table 3.

Table 3

Chemical characteristics of the waters used in this study

Parameters deionized water

(DW)

bottled water

(BW)

tap water

(TW)

wastewater effluent

(WE)

pH 6.6 7.6 7.4 7.9

EC (μS /cm) 0.067 409 458 863

COD (mg/L) ND ND ND 25

HCO3− (mg/L) 0 200 176 164

Cl − (mg/L) 0 12 23 92

SO42-

(mg/L) 0 34 46 74

NO3- (mg/L) 0 5.5 6.7 24

Na+

(mg/L)

K+

(mg/L)

0

0

21

1.2

24

12

50

24

Ca2+

(mg/L) 0 60 67 98

Mg2+

(mg/L)

TH (mg/L)

0

0

12

200

13

225

58

475

The effect of water matrix on the degradation of TC is shown in F.g.11. The

experimental data followed by first order kinetics, with a k value of 1.75×10−2

min−1

,

2.22×10−3

min−1

, 1.90×10−2

min−1

and 1.25×10−2

min−1

in DW, BW, TW and WE,

respectively (Table 2). This result was similar to the results reported by Lopez-

Penalver et al. [69] and Gomez-Pacheco et al. [70] which reported higher TC

degradation rate in real water (surface and ground water) and lower TC degradation

rate in wastewater effluent compared to ultrapure water. The improvement of TC

degradation rate in bottled and tap water compared to deionized water may be due to

the presence of natural organic matter in small levels, which act as sonosensitizer and

may lead to either direct degradation of organic compounds through energy

transferring or development of active species such as OH• radical. Moreover, recent

researchers suggested that activation of PS may also be accomplished by minerals and

organic matter [71]. It has been proved that PS activation is promoted by organic

compounds at basic pH [72]. The lower TC degradation rate in WE compared to DW

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may be due to higher levels of organic matter, that prevents the formation of OH•

radicals and reduces the TC degradation rate (69-70). Moreover, the high

concentration of carbonates, bicarbonates and chlorides in wastewater effluent may

act as hydroxyl radical scavenger according to Eqs. (20) to (22) [45, 50, 69, 70].

Cl− + OH• ↔ HCO3•− k = (6.1±0.8) × 10

9 M

-1 S

-1 (20)

OH• + CO32− → CO3

•− + OH− k = 3.9 × 108 M

-1 S

-1 (21)

OH• + HCO3− → CO3

•− + H2O k = 8.5 × 106 M

-1 S

-1 (22)

Fig. 11. Effect of water matrix on the TC degradation ([TC] = 0.104 mM; [S2O82-

] = 4

mM; US power: 500 W, 35 KHz; pH = 10; T = 25 0C).

3.11. Mineralization of tetracycline in the US/S2O82-

process

Performance of US/S2O82-

process for mineralization of TC is shown in the Fig. 12.

The results showed that efficiency of US/S2O82-

process in removal of TOC was

59.88%. Based on the results, it was found that the US/S2O82-

process was not able to

complete mineralization of TC. The incomplete mineralization implied that TC was

mainly transformed to byproducts. Further studies are required for identification of the

main byproducts of TC formed during sonochemical degradation.

Fig. 12. Removal of TOC by US/S2O82-

process ([TC] = 0.052 mM; [S2O82-

] = 4 mM;

US power: 500 W, 35 KHz; pH = 10; T = 25 0C).

3.12. Identification of intermediates and the proposed degradation pathway

The TC molecule was eluted at retention time of 2.405 min (Fig.13). The

identification of the TC degradation by-products was analyzed using Liquid

Chromatography Mass Spectrometry (Agilent 6410 Triple Quadrupole LC/MS) with a

20 μl solution injected to column. Mobile phase was a mixture of acetonitrile and 0.01

M oxalic acid solution (31:69, v/v) with a flow rate of 1.0 mL. min-1

. The obtained

MS spectra are represented in Fig. 14. After 120 min of reaction, three new flow peaks

were observed at retention times of 2.049, 2.368 and 6.658 min. In the flow peak of

2.049 min, two ions with m/z 445.1, 428.2 were identified. In the flow peak of 2.368

min, three ions with m/z 445.1, 428.2, and 366.1 were identified. In the flow peak of

6.658 min, two ions with m/z 410.1 and 351.2 were identified. The retention time of

three types of by-products was lower than that of TC, which implied that more polar

by-products were formed during the US/S2O82-

process [73-75]. As shown in Fig. 14,

an ion was observed with the retention time of 2.049 and 2.368 min, m/z 445.1 and

lower intensity compared with the 445.1 peak in the retention time of 2.405 min.

Usually, MS peaks are wide and broad and they may have width more than 1 min. in

this study, the peak of parent compound (TC molecular) has been started at 1.8 min

and continued up to 2.6 min and the maximum flow peak of the TC molecular was

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observed at retention times of 2.405 min. In TC degradation chromatogram (Fig. 14),

the parent compound was present and has been detected in the mentioned time range

(1.8-2.6 min). Therefore, it is assumed that two ions observed with the retention time

of 2.049 and 2.368 min and m/z 445.1 are not a new product of TC and likely are

corresponding to the protonated TC molecular ion [M+H]+. Based on the data shown

in Fig. 14, four major by-products with m/z of 428.2, 410.1, 366.1 and 351.2 may be

formed during US/S2O82-

process. The possible degradation pathway for tetracycline

antibiotic is shown in Fig. 15. In this study, degradation by-products and possible

reaction pathways are in accord with the results reported by other researchers [76-79].

The fragmentation pathways for the NH3 loss and the water loss including the

hydroxyl group bound at the C6 atom were proposed based on the literature reported

by Dalmzio et al. and Wang et al.[74,76].

Fig. 13. Liquid chromatography mass spectrometry (LC-MS) spectra of aqueous

solution containing 0.104 mM TC

Fig. 14. Liquid chromatography mass spectrometry (LC-MS) spectra of TC

degradation compounds in aqueous solution with reaction time of 120 min.

Fig. 15. Proposed degradation pathway for tetracycline antibiotic

Conclusion

In this study, TC was markedly decomposed using peroxydisulfate activated by

ultrasound irradiation. The investigation of the effect of different operational

conditions on the TC degradation revealed the following conclusions:

1. US alone had no significant effect on the degradation of tetracycline, but the

combination of Na2S2O8 and ultrasound irradiation (US/S2O82-

) could markedly

enhance TC degradation rate. Therefore, the main mechanism of TC degradation was

indirect oxidation.

2. After 120 min reaction, nearly 96.5 % of TC, 74% of COD and 61.2% of TOC

could be removed under the conditions of initial TC concentration = 0.052 mM, initial

pH=10, S2O82-

concentration = 4 mM, ultrasound frequency = 35 KHz and ultrasound

power = 500 W.

3. Increasing temperature, ultrasound power, initial Na2S2O8 concentration and

solution pH from 7 to 10 and decreasing initial tetracycline concentration and solution

pH from 7 to 4 substantially exacerbated the TC degradation rate.

4. The COD and TOC removal efficiency indicated that tetracycline transformed into

intermediate products without complete mineralization. The low TOC removal

efficiency may be due to generation of recalcitrant products under US/S2O82-

process.

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5. The effect of radical scavenger (methanol and tert-butyl alcohol) indicated that

sulfate hydroxyl radicals played main roles in degradation of tetracycline by

US/S2O82-

process at pH=10.

6. The sonochemical degradation of TC well followed Pseudo-first order kinetics

model (R2=0.99).

7. The presence of humic acid in lower concentrations (5 mg/L) accelerated TC

degradation rate, but reduced the TC degradation in concentrations 10 and 20 mg/L.

8. The TC degradation rate was relatively higher in drinking water compared to

ultrapure water. The lower TC degradation rate in secondary effluent than in ultrapure

water may be attributed to high concentrations of organic substances, which could

prevent the creation of activated radicals, reducing the TC degradation rate.

9. The lower TC degradation rate in the presence of high HA concentrations and

chemical composition of secondary effluent was offset using additional doses of

Na2S2O8.

10. Five major by-products were observed with m/z of 428.2, 409.1, 382.1, 365.1 and

357.2 during US/S2O82 process and pathway of TC degradation was proposed on the

basis of the identified intermediates.

The major conclusion of the present study was that the combination of peroxydisulfate

with ultrasound provides a substantial enhancement in TC degradation. Overall,

US/S2O82-

process was found to be a promising technique for TC degradation in

aqueous solutions.

Acknowledgement

This study was part of a PhD dissertation of the first author, performed at Department

of Environmental Health Engineering, School of Public Health, Tehran University of

Medical Sciences in 2015. We appreciate the financial support from Institute for

Environmental Research, Tehran University of Medical Sciences, Tehran, Iran.

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Fig. 1

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Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

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Fig. 6

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Fig. 7

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Fig. 8

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Fig. 9

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Fig. 10

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Fig. 11

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Fig. 12

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Fig. 13

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Fig. 14

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Fig. 15

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Highlights

The degradation kinetics of tetracycline was studied using US/S2O82-

process.

The effects of the main operational parameters were investigated.

Degradation pathway of tetracycline was also proposed.

Tetracycline degradation by-products were identified using LC- MS.

Dominant radicals were evaluated using radical scavengers.

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