Journal of Electroanalytical Chemistry 683 (2012) 14–20

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Journal of Electroanalytical Chemistry

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Structural effects on electrosorptive behavior of aromatic organic acidsfrom aqueous solutions onto activated carbon cloth electrodeof a flow-through electrolytic cell

Edip Bayram, Erol Ayranci ⇑Akdeniz University, Department of Chemistry, 07058 Antalya, Turkey

a r t i c l e i n f o

Article history:Received 29 May 2012Received in revised form 24 July 2012Accepted 25 July 2012Available online 2 August 2012

Keywords:Aromatic organic acidActivated carbon clothElectrosorptionWater treatmentRegeneration

1572-6657/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jelechem.2012.07.028

⇑ Corresponding author. Tel.: +90 242 3102315; faxE-mail address: eayranci@akdeniz.edu.tr (E. Ayran

a b s t r a c t

A flow-through electrolytic cell, constructed specially for electrosorption processes, was connected to aUV–Vis spectrophotometer to examine the electrosorptive behaviors of benzoic acid (BA), phthalic acid(PA) and nicotinic acid (NA) from aqueous 0.01 M Na2SO4 solutions onto activated carbon cloth (ACC)electrode. Effect of pH on electrosorptive behavior of BA was determined. Competitive electrosorptionfrom binary mixtures of BA and NA was carried out. In situ regeneration of ACC was examined by revers-ing the polarization direction upon which electrodesorption of 95% of electrosorbed NA was achieved.Kinetic data of both open circuit adsorption and electrosorption were found to obey successfully topseudo-first-order law and rate constants were determined. Appreciable increase in the removal rateof aromatic organic acids was achieved by electrosorption compared to open circuit adsorption. Rate con-stants for electrosorption at +600 mV polarization were found to decrease in the order of NA > PA > BA.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Throughout the past several decades, an increasing trend in theextensive emission of various organic and inorganic pollutants intothe ecosystems has been witnessed [1], underlying the exponentialgrowth rate of population and social civilization, and developmentof industry and technology as its key drivers [2]. Charged and/orionizable aromatic compounds including dyes, acids and pesticidesconstitute an important group of substances responsible fromwater pollution. Some of the techniques used in purification treat-ment of waste waters containing these pollutants are flocculation,coagulation, precipitation, adsorption, membrane filtration, elec-trochemical techniques, ozonation and fungal decolorization [3].Chemical degradation by oxidative agents such as chlorine consti-tutes one of the most relevant and effective methods, but it mayproduce some toxic secondary products, such as organochlorinecompounds [4]. Although, biodegradation process is cheaper thanother methods, the recalcitrant nature of aromatic organic mole-cules, together with their toxicity to microorganisms, makesaerobic treatment difficult [5]. Adsorption onto activated carbonsis among the most utilized techniques. Its major drawbacks areshort lifetime of activated carbons due to low regeneration capac-ities and slow speed of adsorption process originating from thediffusion limitations [6,7].

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: +90 242 2278911.ci).

Potential induced adsorption, named as electro adsorption orshortly electrosorption, has been developed as an environmentallyfriendly technology for removing toxic pollutants from aqueoussolutions, especially very dilute ones [7–11]. The essence of elec-trosorption is charging and discharging the electrical double layerat the surface of electrodes by applying potential or current. Due toits reversibility, electrosorption does not only increase the adsorp-tion ability of the adsorbent, but also changes the adsorption ten-dency by controlling the direction of polarization to reach anadsorption–desorption cycle providing an attractive approach forin situ regeneration of the adsorbent [6,11]. The reversibility ofelectrosorption, therefore, plays a promising role in its practicalindustrial applications such as water purification, solvent recycle,and the enrichment of valuable substances.

Appreciable enhancements were obtained in the removal rate ofpolycyclic aromatic dyes [6,11], pyridine derivates [7] and a pesti-cide [12] from dilute aqueous solutions upon polarization of high-area porous activated carbon cloth electrodes since they behave ashaving a quasi-three-dimensional structure [13]. It is stated that[6,7,11,12] electrosorption of aromatic organic molecules ontoactivated carbon cloth electrode is not diffusion controlled sinceits rate is mainly determined by the applied current or potential.

Batch laboratory electrosorption studies provide useful infor-mation toward the application of the removal of specific wasteconstituents. Although such batch processes are easy to apply inthe laboratory, they are less convenient for industrial applications.Fixed-bed columns are widely used in various chemical industries

Pt wire

Stainless Steel Mesh

ACC (WE)

Reference Electrode

O-Ring

Counter Electrode

Outlet

Inlet

(II)

(I)

(III)

Fig. 1. Schematic representation of flow-through electrosorption cell. (I): Teflonbody; (II): threaded Teflon plug; (III): outlet part.

E. Bayram, E. Ayranci / Journal of Electroanalytical Chemistry 683 (2012) 14–20 15

since their operation is simple, continuous and more practical inwastewater treatment. Therefore, studies on laboratory scale con-tinuous–type electrosorption are required to determine its applica-bility and to improve the efficiency for removing various pollutantsfrom waste-waters. Electrosorption of low molecular weight aro-matic organic molecules, amino acids, a peptide and a proteinwas studied by Woodard et al. [14] with a flow-through electro-lytic system having carbon fiber electrodes. They investigated theinfluence of pore size distribution and surface functional groupsof adsorbent, as well as the solubility and ionization of adsorbateon electrosorption. Ban et al. [15] investigated the potential useof electrosorption/electrodesorption with granular activated car-bon pack-bed electrodes for treatment of industrial effluents. Theyderived electrosorption isotherms of aromatic organic molecules atdifferent conditions and compared with those obtained from flowthrough column studies. They proposed two flow-throughelectrochemical cell configurations for electrosorptive removal ofaromatic organic molecules using activated carbon fibers whichwere found to be more effective than granular activated carbonsused as pack-bed electrodes. More recently, Kitous et al. [16] car-ried out studies on flow-through electrosorption of metribuzinpesticide onto granular activated carbon bed electrode. They foundthat, the capacity of electrode was increased more than 100% whendesired electrical potential was applied in comparison with theconventional granular activated carbon column in similar experi-mental conditions without electrical potential. In our recent papers[6,11], we have quantitatively examined the rates and extents ofadsorptive and electrosorptive removal of polycyclic aromatic ba-sic dyes in relation to the properties of polarization treated ACCand basic dyes using a batch type reactor. The purpose of the pres-ent study is to investigate the electrosorptive behaviors of threearomatic organic acids; benzoic acid (BA), phthalic acid (PA) andnicotinic acid (NA), from aqueous solutions onto high areaactivated carbon cloth (ACC) pack-bed electrode using a speciallydesigned and constructed flow-through electrochemical cellconnected, online, to an UV–Vis spectrophotometer for in situ mon-itoring of adsorbate concentration. Competitive electrosorption ofbinary mixtures of aromatic organic acids and regeneration ofACC electrode was also aimed.

2. Experimental section

2.1. Materials

The high specific surface area ACC was obtained from SpectraCorp. (MA, USA) coded as Spectracarb 2225. Before it was used inthe adsorption/electrosorption experiments, the ACC was firstwashed with warm deionized water by a procedure described pre-viously [8,10]. The washed and dried ACC was cut in circular shapeto fit into its compartment in the electrolytic cell (Fig. 1), weighedaccurately and kept in a desiccator for use in electrosorption stud-ies. BA, NA, PA and Na2SO4 were purchased from Aldrich. Theirmass fraction purity was at least 0.99 and they were used afterdrying under vacuum without further purification. High puritywater, used in all experiments, was from 18.2 MX Milli-Q UV(Millipore) water system.

2.2. Design and construction of flow-through electrolytic cell

Schematic representation of the flow-through electrolytic cellused to carry out the electrosorption experiments is shown inFig. 1. Electrolytic cell was produced from Teflon material (I). Theflow channel (1.5 cm i.d.) was drilled through the Teflon body. Astainless steel mesh was placed on half-way through the flowchannel as a base for ACC. It also helps to establish the electrical

contact with ACC and to ensure a homogenous potential distribu-tion. A threaded Teflon plug (II) with a hole of 0.8 cm was screwedthrough the flow channel to press the ACC pieces onto stainlesssteel mesh. Two pieces of circular shaped ACC (total mass of40 ± 0.2 mg) pressed between threaded plug and stainless steelmesh, in contact with a Pt wire, was used as the pack-bed workingelectrode (WE). The height of bed was 0.2 cm. An Ag/AgCl referenceelectrode (RE) (BAS, MF-2030, Bioanalytical systems Inc. USA) wasintroduced into the flow channel through a hole in the Teflon bodypositioned 90� to the channel. Any leakage was prevented by com-pression fitting using an O-ring. A piece of coiled platinum wire fit-ted to the outlet part (III) of the cell was used as the counterelectrode (CE).

2.3. Description of the system and the procedure for the on-linespectrophotometric analysis

Experimental set-up of continuous flow electrosorption processand method for on-line concentration measurements was givenpreviously [17]. The system consists of a UV–Vis spectrophotome-ter (Varian Carry 100, UV–Vis spectrophotometer) equipped with aquartz flow cell (Hellma) for on-line concentration measurements,a peristaltic pump for the circulation of the adsorbate solution inthe system through silicon tubing (0.2 cm i.d.), a beaker containingadsorbate solution on a magnetic stirrer, a potentiostat/galvanostat(Gamry Instruments Inc.) and the flow-through electrolytic cell asdescribed in the previous subsection. A glass electrode connectedto a pH meter was immersed into the adsorbate solution forin situ pH measurements. Flow-through electrolytic cell was posi-tioned vertically and operated in up flow plug mode. Polarizationof ACC pack-bed electrode was achieved by the potentiostat/galva-nostat connected to the electrolytic cell.

3. Results and discussion

3.1. Properties of ACC

Electrosorption is non-Faradaic process and it is important toavoid the Faradaic reactions originating from both ACC and adsor-bate solution during electrosorption. Polarization of activated car-bons provokes Faradaic decomposition of water below �0.60 V vs.

E vs (Ag/AgCl)/V

-0,6 -0,3 0,0 0,3 0,6

I/A

-0,030

-0,015

0,000

0,015

0,030

0,045

Na2SO4

Na2

Na2

Na2

SO4+BA

SO4+FA

SO4+NA

Fig. 2. Cyclic voltammograms of 0.01 M Na2SO4 and 1 � 10�3 M aromatic organicacid solutions in 0.01 M Na2SO4 taken in electrolytic cell containing 40 mg ACC asWE, Pt plate as CE, and Ag/AgCl as RE, at a sweep rate of 5 mV s�1.

0

0.8

1.6

2.4

200 230 260 290

Abs

orba

nce

Wavelenght / nm

BA

NA

PA

Fig. 3. UV spectra of 2 � 10�4 M aromatic organic acids in 0.01 M Na2SO4 solutionsat their natural pH values of 4.28, 3.98 and 3.62 for BA, PA and NA, respectively.

16 E. Bayram, E. Ayranci / Journal of Electroanalytical Chemistry 683 (2012) 14–20

Ag/AgCl according to Eq. (1) and above +0.62 V vs. Ag/AgCl at pH6.5 according to Eq. (2) causing remarkable changes on propertiesof activated carbons [18];

2H2Oþ 2 e� ! H2 þ 2OH� ð1Þ

2H2O! O2 þ 4Hþ þ 4e� ð2Þ

Changes in properties of ACC upon polarization were alreadyinvestigated in our previous works [6,19]. It was found that neitherporosity nor surface chemistry of ACC is altered by negative polar-ization. However, positive polarization at potentials higherthan � (+800) mV vs. Ag/AgCl was found to provoke surface oxida-tion and minor textural changes due to the evolution of O2 on ACCaccording to the Eq. (2). The data on surface textural properties ofACC, cathodically polarized ACC at �1.5 V (ACC�1.5) and anodi-cally polarized ACC at 0.8 V (ACC + 0.8) in 0.01 M Na2SO4 solutionextracted from our previous work [19] are listed in Table 1.

It is recalled that, SO2�4 ions of Na2SO4 (used as supporting elec-

trolyte) do not specifically adsorb onto ACC [8]. Taking all theseprevious findings into account, electrosorption studies carriedout in this work were restricted to a potential range from�600 mV to +600 mV in order to observe, purely, the electrosorp-tion behaviors of BA, NA and PA and to understand the effects ofstructural differences between these aromatic organic acids ontheir electrosorption. In order to make sure that BA, NA and PAare electroinactive in this potential range, cyclic voltammetricstudies were carried out in 0.01 M Na2SO4 solution with and with-out BA, NA and PA at a sweep rate of 5 mV s�1 using Ag/AgCl as RE,ACC as WE and Pt plate as RE. Voltammograms given in Fig. 2 showthat there is no Faradaic reactions taking place with BA, NA or PAwithin the specified potential range.

3.2. Chemical nature and optical absorption characteristics of aromaticorganic acids

UV spectra of the aromatic organic acids studied are shown inFig. 3.

Separate calibration experiments were run to determine themolar absorptivities (e) at the specified wavelengths (k) requiredfor calibration. Chemical, spectral and calibration data for aromaticorganic acids studied are given in Table 2.

Since, 0.01 M Na2SO4 solution was used as inert supportingelectrolyte in order to provide the required conductivity in electro-sorption studies, the same solution was also used in calibrationexperiments as solvent to make the matrix similar. pH of the solu-tions was adjusted by H2SO4 or NaOH when necessary. Absorbancevs. concentration data for each single compound was treatedaccording to the Lambert–Beer law by linear regression analysisto determine e at each k and the correlation coefficient, r.

3.3. Electrosorptive behaviors of aromatic organic acids

Electrosorption of BA, PA and NA from their solutions in 0.01 MNa2SO4 at different polarization conditions was monitored spectro-photometrically by the system and procedure described above.Absorbance data, extracted from kinetic scans, in certain timeintervals until equilibrium were converted into concentration data

Table 1Surface textural properties of ACC and polarization treated ACC samples [19].

Sample Specific surface area (m2 g�1) Total pore volume (cm3 g

ACC 1596 0.697ACC + 0.8 1541 0.655ACC�1.5 1518 0.662

using the corresponding calibration relation and then plotted as afunction of time for each process. The same initial concentration(2 � 10�4 M), volume (200 mL) of adsorbate solution and mass ofACC electrode (40.0 ± 0.2 mg) were used in each electrosorptionrun for a reasonable comparison of results. All the adsorption/elec-trosorption runs were repeated at least three times and average re-sults are reported here.

The results of electrosorption experiment sets carried out foreach aromatic organic acid are shown in Fig. 4a for NA, 4(b) forPA and 4(c) for BA. Open circuit (OC) adsorption data of aromaticorganic acids were also included in these figures for comparison.

Electrosorption data were recorded upon �600 mV polarizationuntil equilibrium (point A of curve I) and then polarization direc-tion was reversed (from point A) and applied until equilibration.The same procedure was repeated by polarizing first at +600 mVuntil equilibrium (point A of curve II) and then reversing the polar-ization direction to �600 mV (from point A) until equilibration.Comparing the equilibrium concentrations after initial electrosorp-tion at point A with that of OC adsorption at the same time showsthat the extent of removal of organic acids increases with positivepolarization and decreases with negative polarization of ACC. The

�1) Micropore volume (cm3 g�1) Average pore diameter (Å)

0.695 23.70.654 24.80.659 24.6

Table 2Chemical, spectral and calibration data for aromatic organic acids.

Adsorbate Molecular structure pKa1 pKa2 pH k (nm) e (L mol�1 cm�1) r

Benzoic Acid O

OH

4.20a – 4.28 226 8155 0.9969

4.28 261 638 0.9966

2.93 230 10206 0.999710.54 224 8170 0.9999

Phthalic acid O

OH

O

OH

2.92a 5.41a 3.98 229 4621 0.9999

3.98 280 943 0.9996

Nicotinic acid

N+

H

O

OH

2.05a 4.81a 3.62 226 3024 0.99683.62 261 4030 0.9945

a From Ref. [20].

0

0,6

1,2

1,8

0 40 80 120 160 200

time / min

c /

10-4

M

c /

10-4

M

c /

10-4

M

0

0,6

1,2

1,8

0 40 80 120 160 200 240

time / min

0

0,6

1,2

1,8

0 40 80 120 160

time / min

x2

I

II A

A

x1x1, x2

I

IIA

A

x1

x2

I

II

A

A

(b)(a) (c)

Fig. 4. Electrosorptive behaviors of (a) NA, (b) PA and (c) BA upon polarization of ACC first at �600 mV (curve I up to A), then at +600 mV (curve I after A) and first at +600 mV(curve II up to A), then at �600 mV (curve II after A). Initial adsorbate concentration is 2 � 10�4 M in 0.01 M Na2SO4 solution, mass of ACC is 40 mg and volumetric flow rate is10 mL min�1 in all experiments. x1 and x2 are concentration differences between the subsequent electrosorption steps at equilibrium. (o) open circuit adsorption.

E. Bayram, E. Ayranci / Journal of Electroanalytical Chemistry 683 (2012) 14–20 17

sharp increases and/or decreases observed in solution concentra-tions after reversing the polarization direction at point A are themanifestations of electrosorption/electrodesorption behaviors ofaromatic organic acids in Na2SO4 solutions. The differences be-tween equilibrium concentrations after the initial negative polari-zation and after the subsequent positive polarization, x1, and alsobetween equilibrium concentrations after initial positive polariza-tion and after the subsequent negative polarization, x2, (Fig. 4)were calculated. It was found that, absolute values of x1 and x2

are almost equal to each other for the three aromatic organic acids.By dividing the arithmetic mean of x1 and x2 to initial concentra-tions we obtained the unitless active fraction, previously definedby Woodard et al. [14] for the explanation of the electrosorptivebehaviors of organic molecules. In our case, active fraction corre-sponds to the fraction of molecules reversibly adsorbed or des-orbed as the potential of ACC is switched from �600 mV to+600 mV or vice versa. It was calculated as 0.252, 0.166 and0.091 in a decreasing order for NA, PA and BA, respectively. This or-der is proportional with the order of percentage of ionic speciesthat exist in solutions of NA, PA and BA at their natural pH valuesof 3.62, 3.98 and 4.28, respectively. These percentages can easily becalculated from the initial adsorbate concentrations and pKa val-ues given in Table 2. These calculations yielded BA to be 54% in an-ionic, 46% in neutral form, PA to be 89% in monohydrogen

phthalate, 8% in neutral, 3% in phthalate form and NA to be 91%in zwitterionic, 7% in anionic and 2% in neutral form at theirrespective natural pH values. The correlation between active frac-tions and the degree of ionization suggests that electrostatic inter-actions between charged ACC and ionic aromatic organic acidspecies play an important role in adsorption/desorption underpolarization.

Several other factors should also be considered concerning themechanism of electrosorption. It is expected that solute moleculesare adsorbed with their aromatic rings facing the graphene layersat the surface of ACC for maximizing the dispersive p-system inter-actions. Such orientation seems to be favored in most organic aro-matics at small coverage conditions [21]. On the other hand, sincethe charge density on the electrode surface increases upon polari-zation, charge-dipole interactions between ACC surface and thearomatic acids with permanent dipoles continue to be operativein an enhanced manner. The dipole moments of NA, PA and BAare 3.05 D [22], 2.59 D [23] and 1.21 D [24], respectively. They fol-low the same order as active fractions. All these interactions are ex-pected to govern the observed rates and extents of adsorption/electrosorption. A similar mechanism has been proposed foradsorption/electrosorption of aniline [25,26] and bentazon [12]on activated carbon fibers with electrosorption taking placethrough the aromatic ring approaching the charged carbon surface.

18 E. Bayram, E. Ayranci / Journal of Electroanalytical Chemistry 683 (2012) 14–20

3.4. Effect of pH on electrosorption

Aromatic organic acids exist in water as a mixture of neutraland ionic forms. The ratio of these forms changes depending onthe pH of solution. The two forms of BA absorb UV light at slightlydifferent wavelengths (Table 2): benzoate, predominant form at pH10.54, at 224 nm and BA, predominant form at pH 2.54, at 230 nm.This allows monitoring the two species simultaneously by analyz-ing the adsorbate solution as a mixture of two components accord-ing to Lambert–Beer law. So, it would be interesting to see how theconcentrations of benzoate and BA change during the course ofelectrosorption at different pH values leading to crucial informa-tion about the nature of interactions between adsorbate and ACCsurface. Similar situation also exists for PA and NA in water but de-tailed binary analysis is not possible for these species due to close-ness of kmax values of the neutral and ionic species for them. Thesimultaneous analysis for neutral BA and benzoate species in BAsolution was achieved spectrophotometrically by evaluating thetotal absorbances at two wavelengths, 224 nm (k1) and 230 nm(k2), extracted from the scans recorded during the course of elec-trosorption in certain time intervals [17]. The total absorbance atk1, Ak1

total , can be given by

Ak1total ¼ ek1

1 c1 þ ek12 c2 ð3Þ

0

0,6

1,2

1,8

time / min

c / 1

0-4 M

c / 1

0-4 M

c / 1

0-4 M

0

0,6

1,2

1,8

time / min

0

0,6

1,2

1,8

0 40 80 120

0 40 80 120

0 40 80 120

time / min

(a)

(b)

(c)

Fig. 5. Electrosorption of BA species from aqueous solutions at (a) pH: 4.28, (b) pH:2.93, (c) pH: 10.54, flowing at a rate of 5 mL min�1 onto 40 mg ACC electrode uponpolarization at +600 mV: BA ( ), benzoate ( ) and sum of BA and benzoate ( ).

and that at k2, Ak2total ,can be given by

Ak2total ¼ ek2

1 c1 þ ek22 c2 ð4Þ

where e is the molar absorptivity of the species at the wavelengthindicated as a superscript, c1 and c2 are concentrations of benzoateand BA species, respectively. The light path does not appear in theabove equations, since 1 cm cuvette was used in all measurements.e values were determined in separate calibration experiments at pH10.54 for benzoate and at pH 2.93 for BA, and are given in Table 2.Simultaneous solutions of Eqs. (3) and (4) give concentrations ofbenzoate and BA in the adsorbate solution at any time during elec-trosorption. Concentration variations of benzoate anion, BA inmolecular form and the sum of the two are plotted separately asa function of time in Fig. 5 during electrosorption at +600 mV andat a volumetric flow rate of 5 mL min�1.

It is seen that the concentration of neutral form of BA in adsor-bate solution is rapidly decreased almost to zero level over120 min electrosorption period at pH 4.28 (Fig. 5a). However, theconcentration of benzoate remained almost constant during thefirst 40 min of electrosorption. Then a slight gradual decrease is ob-served in benzoate concentration which is expected to be due to itshydrolysis to neutral BA molecules as the already existing neutralBA molecules are depleted by electrosorption. Of course, a smallamount of benzoate may also have been electrosorbed. However,it is clear from Fig. 5a that at pH 4.28 the unelectrosorbed BAremaining in the solution is in benzoate form at the end of120 min. The enhanced rate and capacity of electrosorption of BAat more acidic medium with a pH 2.93 can visually be seen inFig. 5b. The neutral form of BA is the predominant form and it isthe form being electrosorbed. Benzoate anions are predominantspecies of BA in solution at pH 10.54 and a slight gradual decreasein their concentration is observed during electrosorption (Fig. 5c).The lower electrosorption rate and capacity in this basic mediumthan in acidic media is probably due to the competition betweenthe benzoate and OH� anions for the positively charged ACC sur-face [27]; obviously OH� ions win this competition.

3.5. Kinetics of electrosorption

It is known that the kinetic data for adsorption/electrosorptionof many species onto ACC electrodes [6,7,10–12] fit quite well topseudo-first-order kinetic model [28]. This is probably becausethe overall process is not diffusion controlled and the rate of theprocess depends on the applied potential or current [7,11]. There-fore, this model was also applied to the current data presented inFig. 4 to investigate its applicability to the present systems. Linearform of pseudo-first-order model can be formulated as;

lnðqe � qtÞ ¼ lnðqeÞ � k1t ð5Þ

where qe and qt are the amounts of adsorbate adsorbed at equilib-rium and at time t, respectively, and calculated as described

Table 3Kinetic and capacity parameters for adsorption/electrosorption of aromatic organicacids.

Molecule Polarization (mV) 103k1 (min�1) r %Req teq (min)

BA OC 35.1 0.9998 52 124�600 – – 44 15+600 43.4 0.9996 75 110

PA OC 34.0 0.9994 45 110�600 – – 42 80+600 46.9 0.9993 63 144

NA OC 28.8 0.9983 45 114�600 – – 45 60+600 51.5 0.9992 65 120

0

0,6

1,2

1,8

0 40 80 120 160 200

time / min

c / 1

0-4 M

Fig. 6. Electrosorption of BA ( ) and NA ( ) from their equimolar (1 � 10�4 Meach) mixture in 0.01 M Na2SO4 solution onto 40 ± 0.5 mg ACC electrode at avolumetric flow rate of 10 mL min�1. Polarization was +600 mV up to 110 min. andthen at �600 mV. ( ) shows the sum of BA and NA concentrations.

E. Bayram, E. Ayranci / Journal of Electroanalytical Chemistry 683 (2012) 14–20 19

previously [10]. k1 is the rate constant. k1 values were evaluatedfrom the linear regression analysis of ln(qe � qt) vs. t data foradsorption/electrosorption studies and are tabulated in Table 3 to-gether with correlation coefficients. It should be recognized thatthe OC adsorption data from batch systems may not be treatedaccording to the pseudo-first order model when the intraparticlediffusion is the rate determining step. However, in flow-throughsystems diffusion limitation originating from the external masstransfer is minimized [29] as a result of forcing the adsorbate ontothe ACC pores by flow regime. The applicability of pseudo-first-or-der model to both flow-through OC adsorption and electrosorptionprocesses is confirmed by correlation coefficients which are allgreater than 0.99 (Table 3).

Another quantitative measure of the extent of adsorption/elec-trosorption is the removal percentage of adsorbate at equilibrium,%Req, calculated through the following equation;

%Req ¼ ½ðc0 � ceÞ=c0� � 100 ð6Þ

where c0 and ce are initial and equilibrium concentrations of BA insolution, respectively. %Req values and equilibrium times (teq) are gi-ven in the last two columns of Table 3.

Rate constants (k1) calculated for OC adsorption of aromatic or-ganic acids showed a decreasing trend in the order of BA > PA > NAwhich can be explained by the electrostatic interactions betweenionic forms of adsorbates and ACC surface. The natural pH valuesof aromatic organic acid solutions in water are smaller than thepHPZC of ACC which was determined previously as 7.4 [6,10]. Thisindicates that the surface of ACC in solutions of organic acids ispositively charged. So, the rate and extent of adsorption are deter-mined mainly by the electrostatic attractions between the posi-tively charged ACC surface and the anionic adsorbate species andalso by the dispersion interactions. NA exists 87% in zwitterionicform (negative charge is on carboxylate and positive charge is onN center) and 9% in anionic form at natural pH of 3.62. Electrostaticrepulsion is expected between NA and ACC surface and is probablyresponsible from the relatively slow OC adsorption rate with thesmallest k1 value (Table 3).

Positive polarization causes an increase in the rate of adsorptionand %Req compared to OC adsorption; 24%, 38% and 79% increasesin the rate constant (k1) and 44%, 40% and 44% increases in thecapacity parameter (%Req) were observed for BA, PA and NA,respectively, upon +600 mV polarization. The observed order ink1 values under +600 mV polarization is the same as that in activefractions (see Section 3.3) and reverse of that in k1 values of OCadsorption, which can be related to the dipole moments of aro-matic organic acids to be operative under polarization conditions[10,21]. In the case of negative polarization a decrease is observedin solution concentration at initial stages of processes, probablydue to the grater rate of adsorption than electrodesorption of BA(curve I in Fig. 4). However soon, electrodesorption rate reachesthe adsorption rate and then equilibrium is reached. This can beconfirmed by the much lower teq values at �600 mV polarizationthan at OC adsorption and at +600 mV polarization (Table 3). Sinceboth adsorption and electrodesorption processes are taking placesimultaneously during negative polarization of ACC electrode, thedata could not be treated according to pseudo-first-order rateequation.

In order to see the improvement in the removal rate of aromaticorganic acids by electrosorption in flow-through mode, k1 valuesobtained in this study can be compared with those of our earlierwork obtained in batch mode [30–32]. In batch operation the high-est k1 value for the OC adsorption of BA from 1.96 � 10�4 M aque-ous solution at pH 3.7 onto ACC was 0.0143 min�1 [30]. Thepresent k1 value for OC adsorption is 2.5 times and k1 value underpolarization at +600 mV is 3.0 times greater than that in batchoperation. The reason for this enhancement in rate is the improved

mass transfer rate in flow-through mode and also the polarization.Similarly, comparing the k1 values previously obtained in batchmode for NA [31] and PA [32] with the present k1 values, it is foundthat, 1.8-fold and 2.3-fold enhancements for OC adsorption of NAand PA, respectively, and 3.2-fold enhancements for electrosorp-tion of both NA and PA under +600 mV polarization were obtainedin flow-through mode. It should be noted that neither shift in kmax

nor new peaks was observed on scanning-kinetic spectra duringthe course of electrosorption/electrodesorption studies, confirmingthe structural integrity of aromatic organic acids and absence ofany Faradaic reactions between adsorbate and ACC electrode.

3.6. Competitive electrosorption and regeneration of ACC

Aromatic organic acids are usually present in waste waters as amixture of various organic pollutants. Thus, information to be ob-tained from the competitive electrosorption would be useful inenvironmental applications for organic pollutants. For this pur-pose, an electrosorption study on an equimolar mixture of BAand NA was carried out. The process was followed by in situ UVspectroscopy at scanning kinetic mode. Adsorption data were ex-tracted at 226 nm for BA and 261 nm for NA from the scans. Theywere converted into concentrations from simultaneous solution ofEqs. (3) and (4) using 226 nm for k1 and 261 nm for k2 and therespective calibration relations (Section 3.4). Results are presentedin Fig. 6.

The general electrosorptive behaviors of both components inthe mixture resemble to those the behaviors they show when theywere alone in the solution as presented in Fig. 4. The active fractionvalues calculated for comparing the electrosorptive behaviors ofBA and NA in mixture are 0.147 and 0.491, respectively. Althoughthe active fractions are increased due to the decreased initial con-centrations of each adsorbate [14], the order is the same as thatwhen they were alone. The extent of electrodesorption was greaterfor NA than for BA as can be seen from the comparison of curvesafter reversing the polarization direction in Fig. 6. About 71% ofelectrosorbed NA was electrodesorbed.

The reversibility of the process is important in applications ofelectrosorption especially in waste water treatments for in situregeneration of electrode. Since active fraction is increased bydecreasing the initial concentration of adsorbate, electrosorptionof NA from its more dilute solution (4 � 10�5 M) in 0.01 M Na2SO4

was carried out onto 40 ± 0.5 mg ACC. A continuous cyclic electro-sorption test was performed to decide whether electrosorption

0

0,1

0,2

0,3

0,4

0 60 120 180 240 300 360 420

time / min

c / 1

0-4 M

1st cycle 2nd cycle 3rd cycle

Fig. 7. Concentration vs. time plot for NA recorded during the electrosorption upon+600 mV polarization ( ) and subsequent electrodesorption upon �600 mVpolarization ( ) of ACC; mass of ACC: 40 ± 0.5 mg, initial NA concentration:4 � 10�5 M in 0.01 M Na2SO4.

20 E. Bayram, E. Ayranci / Journal of Electroanalytical Chemistry 683 (2012) 14–20

process would be reversible and ACC can be regenerated in situ,and reused many times. The result of three successive electrosorp-tion/electrodesorption cycles was shown in Fig. 7.

Complete removal was not achieved upon +600 mV polarizationof ACC for the first 80 min. However, after reversing the polariza-tion direction, rapid increases were observed in solution concen-tration of NA and reached to a maximum value of 3.8 � 10�5 M,which corresponds to 95% of its initial concentration, in 50 minperiod. It should also be visualized from the steepness of curvesthat the electrodesorption rate is much higher than the electro-sorption rate. Almost reversible electrosorption/electrodesorptionprocess was observed as indicated by similar behaviors displayedin 2nd and 3rd cycles under specified conditions. The results arepromising in terms of regeneration of ACC.

4. Conclusions

The rates and extents of adsorption/electrosorption aregoverned by dispersion interactions as well as charge-dipole inter-actions between ACC surface and the aromatic organic acids. Themagnitude of the rate constants for electrosorption upon+600 mV polarization was found to decrease in the order of NA >PA > BA, consistent with the order of active fractions and dipolemoments of aromatic organic acids. Studies on competitiveelectrosorption/electrodesorption between BA and NA showed thatelectrosorptive behaviors of both components resemble to thosethey show when they were alone in the solution. The rate and ex-tent of electrodesorption was much greater for NA than BA, in thepresence of one another. Successive reversible electrosorption/

electrodesorption processes in dilute solutions of NA have provenefficient in situ regenerability of ACC. Future studies, focused ondifferent configurations of the electrolytic cell and operation mode,are expected to help improvements in waste water treatment.

Acknowledgements

The support of this work by the Scientific Research ProjectsCoordination Unit of Akdeniz University is acknowledged.

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