Journal of Industrial and Engineering Chemistry 18 (2012) 780–784

Treatment of pollutants in wastewater: Adsorption of methylene blue ontoolive-based activated carbon

Monica Berrios, Marıa Angeles Martın *, Antonio Martın

University of Cordoba (Spain), Department of Inorganic Chemistry and Chemical Engineering, Campus Universitario de Rabanales, Edificio Marie Curie (C3),

Planta Baja, 14071 Cordoba, Spain

A R T I C L E I N F O

Article history:

Received 16 May 2011

Accepted 5 August 2011

Available online 12 November 2011

Keywords:

Activated carbon

Adsorption isotherms

Kinetics studies

Methylene blue

Olive stones

A B S T R A C T

This study used olive stone-based activated carbon for the removal of methylene blue from wastewater in

order to evaluate the adsorption capacity of the carbon. The equilibrium and kinetics of adsorption were

examined at 258, 308, 358 and 40 8C and several agitation speeds. Type III adsorption isotherms

corresponding to physical adsorption in a multilayer system were used for the methylene blue system.

The equilibrium data for methylene blue adsorption showed a good fit to the Freundlich equation. The

kinetic data was analysed to determine kinetic constants and order of reaction. Kinetics was evaluated by

means of an n-order model, showing that the reaction was a first-order reaction. The results indicated

that olive stone-based activated carbon could be used as a low-cost alternative to commercial activated

carbon for the removal of organic compounds from wastewater. However, due to its microporosity, the

application of this type of activated carbon was found to be suitable for molecules smaller than

methylene blue.

� 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

Contents lists available at SciVerse ScienceDirect

Journal of Industrial and Engineering Chemistry

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

1. Introduction

The textile industry requires large amounts of water andproduces highly polluted wastewater containing different types ofdyes [1]. The main problem involved in decontaminating textilewastewaters is the removal of colour, since no single process iscurrently capable of generating adequate effluents [2]. Most dyeshave an adverse impact on the environment as they are consideredtoxic and have carcinogenic properties, which make the waterinhibitory to aquatic life [3].

Biological treatment processes are reported to be efficient inchemical oxygen demand reduction, but are largely ineffective inremoving colour from wastewater. Hence, research has beenconducted on physico-chemical methods for colour removal intextile effluent. These studies include the use of coagulants,oxidising agents, photocatalysis, ultrafiltration, electrochemicaland adsorption techniques [4,5].

Among the various treatment technologies available, adsorp-tion onto activated carbon has proven to be one of the mosteffective and reliable physico-chemical treatments. However,commercially available activated carbons are very expensive [6].The carbon derived from agricultural wastes is gaining importance

* Corresponding author. Tel.: +34 957 21 86 24; fax: +34 957 21 86 25.

E-mail address: iq2masam@uco.es (M.n).

1226-086X/$ – see front matter � 2011 The Korean Society of Industrial and Engineer

doi:10.1016/j.jiec.2011.11.125

due to its low price and suitability for the removal of organic andinorganic pollutants from wastewater [7]. Although, theseagricultural wastes can be also used as biosorbents directly [8–10]. In this sense, Nieto et al. [11] studied the ability of crude olivestones, a residue of the olive-oil industry, for the adsorption of ironpresent in the industrial wastewaters. Researchers have studiedthe production of activated carbon from palm-tree cobs, plumkernels, cassava peel, bagasse, jute fiber, rice husks, date pits,nutshells, wood, maize cob, cotton seed shell, rubber seed coat,apricot stone, almond shell, pongam seed coat, coconut shell,orange peel, walnut stone, bamboo dust, sunflower seed hull andpeach stone as has been detailed in the literature [6,7,12–18]. Littleinformation has been reported about the particular case ofactivated carbon from olive stones [19–21]. In Mediterraneancountries, olive stones and residues are a cheap and quite abundantagricultural waste [21].

Activated carbon has a porous structure with a large internalsurface area. Four consecutive mass transport steps are associatedwith the adsorption of solute from solution by porous adsorbent asfollows: the adsorbate migrates through the solution to theexterior surface of the adsorbent particles, molecular diffusiontakes place in the boundary layer, solute is moved from the particlesurface into the interior site by pore diffusion and finally theadsorbate is adsorbed into the active sites at the interior of theadsorbent particle. This phenomenon takes relatively long contacttime [6,21].

ing Chemistry. Published by Elsevier B.V. All rights reserved.

C0 (MB) = 0.508 mg/L

Ce (mg/L )

q e (m

g/g)

25ºC30ºC35ºC40ºC

1.0

0.06 0.08 0.10 0.12

0.8

0.0

0.6

0.2

0.4

Fig. 1. Adsorption isotherms for the MB solution and olive stone-based AC system.

M. Berrios et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 780–784 781

Determinations of surface area can be made by fitting the BETequation to the isothermal equilibrium data obtained. However,these values are not a true indication of the adsorption capacity ofan activated carbon applied in liquid-phase adsorption studies. It istherefore more logical to determine the porous structure bycombining both the gas-phase and the liquid-phase adsorptionequilibrium data. The literature indicates that the adsorption ofphenol, methylene blue, caffeine and iodine from the aqueousphase is a useful tool for product control in the manufacture ofactivated carbon [22–24].

In addition to determining the porous structure of activatedcarbon, methylene blue can be employed as a thiazine (cationicor basic) dye; the most commonly used dye for colouring amongall other dyes of its category. It is generally used for dyeingcotton, wool, and silk [25] and has a number of biological uses.However, given that methylene blue has various harmful effectson human beings, it is of utmost importance to remove it fromwastewater. Methylene blue dissociates in aqueous solution aselectrolytes into methylene blue cation and the chloride ion.Because the coloured cation is retained at great length by severaladsorbents [3,26], methylene blue was selected as the adsorbatein this study.

This research study aimed to evaluate the adsorption potentialof olive stone-based activated carbon for methylene blue from asynthetic wastewater as olive stones are a very abundant andinexpensive material in Mediterranean countries. The kinetic andequilibrium data of adsorption studies were processed tounderstand the adsorption behaviour of the dye molecules ontothe activated carbon. Although this activated carbon is commer-cial, it was selected because no data about this behaviour has yetbeen reported.

2. Materials and methods

2.1. Materials

Methylene blue (MB) supplied by Panreac (Spain) was used asan adsorbate and was not purified prior to use. The molecularweight (g/mol), molecular volume (cm3/mol) and moleculardiameter (nm) of MB are 319.85, 241.9 and 0.8, respectively [27].

Olive stone-based activated carbon (AC) was supplied by Ibericade Carbones Activos S.A. Textural characterization of the AC wascarried out by N2 adsorption at 77 K using Micromeritic ASAP 2020in our laboratory. The BET surface area, total pore volume andaverage pore diameter of the AC were found to be 587 m2/g,0.333 cm3/g and 2.27 nm, respectively.

2.2. Analysis of methylene blue concentration

The MB concentration in the supernatant solution before andafter adsorption was determined using a UV–vis spectrophotome-ter S-20 (BOECO, Germany) at 664 nm. The calibration curve wasvery reproducible and linear over the concentration range used inthis work.

2.3. Equilibrium experiments

The equilibrium tests were performed in 4 Erlenmeyer flasks(1 L) in which 500 mL of MB solution (initial concentration at0.5 mg/L) were placed. The agitation speed was fixed at 50 rpm andthe AC doses (0.25, 0.50, 1.00 and 2.00 g) were added to the MBsolutions and kept in an isothermal shaker SI 50 GIRALT (STUARTSCIENTIFIC, UK) at different temperatures (258, 308, 358 and 40 8C).The contact time to reach equilibrium between the solid phase andthe liquid phase was approximately 11.5 h (that was checked inprevious assays). The aqueous samples (10 mL) were centrifuged at

8000 rpm for 5 min and filtered prior to analysis in order to removesuspended particles of AC.

2.4. Kinetic experiments

The kinetic tests were carried out in a similar way to theprevious equilibrium tests. Several amounts of AC (1, 2, 4 and 8 g)were added to the MB solutions (initial concentration at 5 mg/L)and kept in an isothermal shaker SI 50 GIRALT (STUART SCIENTIFIC,UK) at different temperatures (258, 308, 358 and 40 8C) andagitation speeds (50, 100, 150, 200 and 250 rpm). The aqueoussamples (10 mL) were taken at time intervals of up to 95 min. Thesamples were centrifuged at 8000 rpm for 5 min and filtered priorto analysis in order to remove suspended particles of AC.

The amount of adsorption or adsorption capacity q (mg/g) wascalculated by

q ¼ C0V0 � CiVi

W(1)

where C0 and Ci (mg/L) are the liquid-phase concentrations of MBat initial and any time (Ct) or equilibrium (Ce), respectively,obtaining the amount of adsorption at time (qt) or the amount ofadsorption at equilibrium (qe). V0 is the initial volume of thesolution (0.5 L), Vi is the real volume when sampling and W is themass of the AC used (g).

3. Results and discussion

3.1. Adsorption equilibrium

The adsorption of dyes from the liquid to solid phase can beconsidered a reversible reaction with equilibrium establishedbetween the two phases [28]. The adsorption isotherm (qe vs. Ce)indicates how the adsorption molecules distribute between theliquid phase and the solid phase when the adsorption processreaches an equilibrium state. The analysis of the equilibrium data ofthe isotherm models is very important for the use of adsorbents [12].

Fig. 1 shows the adsorption isotherms at four temperatures forthe MB solution and olive stone-based AC system. According toBrunauer et al. [29] and Hinz [30], the adsorption isotherms havethe same shape that the type III isotherms or S1 isotherms (Gilesclassification), which correspond to physical adsorption in amultilayer system where no difference is noticed between thefilling of the first layer and the other layers.

The Freundlich isotherm is the earliest known relationshipdescribing the adsorption equation. This fairly satisfactory

Table 1Freundlich constants for the MB solution and olive stone-based AC system at

different temperatures.

Temperature (8C)

25 30 35 40

KF� 103 (mg/g)(L/g)1/n 936.9 734.1 1744.1 132.7

n 0.165 0.163 0.150 0.172

r2 0.999 0.999 0.998 0.997

M. Berrios et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 780–784782

empirical isotherm can be used for non-ideal adsorption thatinvolves heterogeneous surface energy systems and is expressedby the following equation:

qe ¼ KF C1=ne (2)

where KF is a rough indicator of the adsorption capacity and (1/n) isthe adsorption intensity. In general, the KF value increases as theadsorption capacity of adsorbent for a given adsorbate increases.The magnitude of the exponent (1/n) indicates easy uptake ofadsorbate from aqueous solution. A value for (1/n) below oneindicates a normal Langmuir isotherm, while (1/n) above one isindicative of cooperative adsorption [12].

The Freundlich constants (KF and n) were calculated bynonlinear regression using Sigmaplot1 11.0 software. The resultsand the regression coefficients are shown in Table 1.

As can be observed in the regression coefficients, the adsorptionof MB from wastewater on olive stone-based AC follow theFreundlich isotherm at all tested temperatures. According toHameed et al. [12], our (1/n) values above 1 were indicative ofcooperative adsorption. This means that the binding of a MBmolecule to one site on AC influences the affinity of other sites.Similar values of n were found by Avom et al. [24] for palm treecobs-based AC, indicating the heterogeneity of the AC surface.

When the temperature increased, the MB amount in solid phasedecreased in the equilibrium (Fig. 1). Hence, the effectiveness ofthe adsorption process decreased at a high temperature becausethe desorption process took place at higher temperatures.Therefore, the MB concentration in liquid phase increased.

Based on the experimental data, an empirical equation wasobtained to relate qe to temperature (T) and Ce for the olive stone-based AC. This relationship is shown in the three-dimensionalgraph in Fig. 2.

Fig. 2. Three-dimensional graph of the relationship between qe, temperature (T) and

Ce.

It is believed that adsorption of organics onto AC depends onboth the pore structure and surface chemical properties of carbonas well as the adsorbate. Dye adsorption tests help to determinethe capacity of carbon to adsorb molecules of a particular size. TheMB molecule has a minimum molecular diameter of 0.8 nm andcannot enter pores with a diameter of less than 1.3 nm [28,31].Therefore, it can only enter the larger micropores, but most of it islikely to be adsorbed in mesopores. Despite the high surface area ofolive stone-based AC (587 m2/g) as other authors have reported forolive stone-based activated carbon [32], the adsorption capacity ofMB in aqueous solution (for example qe = 0.858 mg/g at 25 8C withan AC dose of 0.5 g/L) was poor due to the molecular diameter ofMB and the AC pore size distribution as described in Section 2.Although this AC has a high surface area, its application could bemore suitable for molecules smaller than MB due to the highmicroporosity of AC.

3.2. Adsorption kinetics

The kinetic adsorption data was evaluated to understand thedynamics of the adsorption process. The MB molecules mustovercome three stages before coming into contact with the activesites of AC. These stages are the migration of the solute through thesolution to the exterior surface of the adsorbent particles,molecular diffusion in the boundary layer and solute movementfrom particle surface into the interior site by pore diffusion.Adsorption itself could be considered almost instantaneous if thephenomenon was purely a physical process, while mass transfercould be minimized by means of suitable agitation speeds.

In order to evaluate the influence of agitation speed (rpm) onthe external mass transfer, several experiments were carried out inthe range of 50–250 rpm for all the temperature conditions and ACdoses selected. Fig. 3 shows the influence of agitation speed on theadsorption capacity at 25 8C and an AC dose of 8 g/L. As can beobserved, no differences were detected between 100 and 250 rpm.However, when the agitation speed was increased from 50 to100 rpm, the adsorption capacity (q) was enhanced. This increasehighlighted that the mass transfer through the solution and in theboundary layer did not limit the adsorption process when anagitation speed of 100 or higher was selected.

Once the external mass transfer did not limit the adsorptionprocess, the kinetics was evaluated at 100 rpm. As can be observedin Fig. 4, the q vs. t plots for all temperatures and AC doses werefound to rise exponentially to the maximum for an AC dose of 4 g/L.Temperature did not significantly influence the adsorption

25ºC, 8 g AC/L

time (min)

200 40 60 80 100

q (m

g/g)

50 rpm100 rpm150 rpm200 rpm250 rpm

0.6

0.4

0.5

0.0

Fig. 3. Effect of agitation speed on the adsorption capacity at 25 8C, 8 g AC/L and an

initial MB concentration of 5 mg/L.

4 g AC/L, 100 rp m

time (min)200 40 60 80 100

q (m

g/g)

25ºC30ºC35ºC40ºC

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Fig. 4. Fitting of experimental data to exponential function at 100 rpm and AC dose

of 4 g/L.

Table 2Kinetic constants of the n-order model.

AC dose (g/L)

2 4 8 16

K (1/min) 0.020 � 0.002 0.030 � 0.009 0.057 � 0.004 0.050 � 0.003

M. Berrios et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 780–784 783

capacity in the range studied for temperature and time (25–40 8Cand 100 min).

Several mathematical expressions explain the increase in MBconcentration in a dose of AC with greater adsorption time. Moststudies (i.e. Langergen and Svenska) use first- and second-orderkinetic equations to model adsorption capacity [12]. Lee [33]applied pseudo second-order kinetic equations to study theadsorption of erythrosine dye from aqueous solution usingactivated carbon. In order to determine the exact order of reaction,the following generalised kinetic equation was employed:

dq

dt¼ Kðqmax � qÞn (3)

where K is the kinetic constant and qmax is the maximumadsorption capacity for each temperature and AC dose. The kineticconstant and the order of reaction can be calculated by linearisingEq. (3):

logdq

dt

� �¼ log K þ n log ðqmax � qÞ (4)

as shown in Fig. 5 for an AC dose of 2 g/L. The order of reaction (n)was found to be 1 or close to 1 in all the experiments. This resultcoincides with other authors [4,6,7,34]. The mean kinetic constants(K, 1/min) for each AC dose (2, 4, 8 and 16 g/L) can be observed in

2 g AC/L, 100 rpm

(qmax - q) (mg/g)

(dq/

dt) (

mg/

g·m

in)

25ºC30ºC35ºC40ºC

0.01 0.10.001

0.1

0.01

1 10

Fig. 5. Kinetic plots for the removal of MB by adsorption on olive stone-based AC.

Table 2. These kinetic constants were similar to the resultsobtained by Santhy and Selvapathy [7] for the adsorption ofreactive dyes. The kinetic constant increased from 0.020 to 0.030and 0.057 (1/min) for AC doses of 2, 4 and 8 g/L, respectively. Athigher AC doses, the kinetic constant remained approximatelystable, indicating that the adsorption rate did not increase whenthe AC dose was higher than 8 g/L.

4. Conclusions

This study showed that olive stone-based activated carbon canbe used for the removal of organic compounds from aqueoussolution under a wide range of conditions. Type III adsorptionisotherms were used for the MB system. These isothermscorrespond to physical adsorption in a multilayer system whereno difference is noticed between the filling of the first layer andthe other layers. Adsorption behaviour was described by theFreundlich isotherm with n values that demonstrate theheterogeneity of the AC surface. Kinetics was evaluated by meansof an n-order model, showing that the reaction was a first-orderreaction. The olive stone-based AC characterization showed highmicroporosity, thus indicating that the poor adsorption capacityresults for MB are due to the molecular properties of thiscompound.

Acknowledgements

The authors are very grateful to Iberica de Carbones and Juntade Andalucia (PAIDI group RNM-271) for funding this research. Wealso wish to express our gratitude to laboratory technicianInmaculada Bellido Padillo for her help.

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