Environ. Sci. Technol. 2010, 44, 6331–6336
Electrochemically Enhanced Removal of Polycyclic Aromatic Basic Dyes from Dilute Aqueous Solutions by Activated Carbon Cloth Electrodes EDIP BAYRAM AND EROL AYRANCI* Department of Chemistry, Akdeniz University, 07058 Antalya, Turkey
Received December 10, 2009. Revised manuscript received July 2, 2010. Accepted July 6, 2010.
Open-circuit (OC) adsorption and electrosorption behaviors of three polycyclic aromatic dyes from dilute aqueous solutions onto activated carbon cloth (ACC) were investigated. The selected dyes were crystal violet (BB-3), basic blue7 (BB-7), and basic blue11 (BB-11). OC adsorption and electrosorption processes were monitored by in situ UV-visible spectrophotometry. Electrosorption was carried out by polarization of an ACC electrode, galvanostatically. Considerable enhancements in removal capacity and duration of the dyes were achieved upon polarization of ACC. Kinetic data for OC adsorption and electrosorption were successfully treated according to pseudofirst-order law, and rate constants were determined. Adsorption isotherms were derived, and the data were treated according to Langmuir and Freundlich equations. Both the rate and extent of adsorption and electrosorption of dyes were found to increase in the order of BB-7 < BB-11 < BB-3. This order was discussed in terms of correlation between sizes of dye species and of ACC pores. Electrodesorption experiments were carried out to explore possibilities of regeneration of ACC.
Introduction Textile industries are among the most polluting industries in terms of the volume and the complexity of treatment of its effluents discharge. Wastewaters generated by textile industries are known to contain large amounts of toxic aromatic compounds including synthetic dyes. Thus, removal of synthetic dyes from wastewater before discharging to environment and from raw water before offering it to public use is essential for the protection of health and environment. Some of the techniques used in treatment of wastewaters containing dyes are flocculation, coagulation, precipitation, adsorption, membrane filtration, electrochemical techniques, ozonation, and fungal decolorization (1). Among these techniques adsorption onto activated carbons has been shown to be an effective technique with its efficiency, capacity, and applicability on a large scale to remove dyes. However, time required for adsorption, sometimes, is long (2). Electrochemical treatment processes for pollutants are quite unique in the sense that they do not require any added chemical to carry out oxidation or reduction reactions, nor do they involve any hazardous materials, only inert electrodes. On the other hand, electrosorptive removal techniques * Corresponding author phone: +90 242 3102315; fax: +90 242 2278911; e-mail:
[email protected]. 10.1021/es101177k
2010 American Chemical Society
Published on Web 07/19/2010
employing high surface area electrodes of nanotextured materials, such as activated carbons, have been developed as an environmentally friendly technology for removing toxic pollutants from aqueous solutions; especially very dilute ones (3-7). Electrosorption is generally defined as adsorption on the surfaces of charged electrodes by applying potential or current. Electrochemical polarization of adsorbents could increase the adsorption rate and capacity of the adsorbent. It can also accomplish desorption of various ions and organic substances at selected potentials and polarization directions to restore the adsorption capacity of adsorbents (4, 5). Use of ACC material with high specific surface area provides a convenient means of controlling the adsorption and desorption processes due to its developed porosity, high electroconductivity, and high adsorption capacity. It exhibits promising applications in purification of wastewater, especially in dilute solutions, with the advantages of low energy cost, high removal efficiency, and enhanced adsorption rates of pollutants (3-6). Due to such advantages electrosorptive removal of organic impurities is effective, simple, and economic as compared with more commonly used direct electrolysis methods. This method is being employed successfully, during the past decade for the electrosorptive removal of toxic inorganic ions (6-10) and a few organic molecules such as phenol, pyridine, and aniline (3-5, 11). However, only a limited number of works report the application of this technique to the removal of more complicated aromatic compounds (4, 12, 13). Adsorption and electrosorption of basic dyes onto carbon materials, to the best of our knowledge, has not been reported so far. In this regard, information to be obtained from the adsorption and electrosorption studies of basic dyes would be useful in environmental applications for pollutants of high molecular weight and complex structure. It is to be noted that we have initiated a work (14) on detailed electrochemical characterization of ACC and its electrosorptive behavior toward a cationic dye of basic blue7 (BB-7) in different supporting electrolytes. In the present study, we report further detailed systematic studies on the removal of organic dyes containing polyaromatic rings namely, crystal violet (BB-3), basic blue7 (BB-7), and basic blue11 (BB-11), from dilute aqueous solutions by electrosorption onto high specific surface area ACC electrodes monitored by in situ UV-visible spectroscopy. The adsorption process was also investigated under open-circuit conditions for comparison. Dyes were selected systematically differing in weight, size, and number of aromatic rings in order to determine the effects of molecular size and structure on electrosorption.
2. Experimental Section Materials. The ACC was obtained from Spectra Corp. It is coded as Spectracarb 2225 and has a very high specific surface area. BB-3 (407.9 g · mol-1), BB-11 (458.1 g · mol-1), and BB-7 (514.2 g · mol-1) with the highest available purity specifications (>95%) were purchased from Aldrich. The molecular structures of the three basic dyes are shown in the SI as Figure S1. All other reagents were Merck grade. Bidistilled deionized water was used in all experiments. Characterization of ACC. Before it was used in the characterization and adsorption/electrosorption experiments, the ACC was first washed with warm deionized water by a procedure as described in our previous works (2, 8). The washed and dried ACC was then cut in desired dimensions, weighed accurately, and then kept in a desiccator for further use. The surface properties of ACC such as the BET specific surface area (SBET), total pore volume (Vtot), micropore volume VOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
6331
TABLE 1. Properties of the Adsorbent ACC specific surface area total pore volume micropore volume mesopore volume total acidic group content carboxylic group content lactonic group content phenolic group content total basic group content pHPZC EPZC
1870 m2 g-1 0.827 cm3 g-1 0.709 cm3 g-1 0.082 cm3 g-1 0.345 mmol · g-1 0.139 mmol · g-1 0.079 mmol · g-1 0.123 mmol · g-1 0.396 mmol · g-1 7.2 154 mV
(Vmicro), mesopore volume (Vmeso), and pore size distribution (PSD) were determined by nitrogen adsorption. Prior to nitrogen adsorption experiments to determine surface properties, ACC samples were degassed at 130 °C under vacuum (up to 10-6 Torr) for 12 h. The N2 adsorption data were obtained with a Quantachrome Autosorb-1-C/MS apparatus over a relative pressure ranging from 10-6 to 1. The SBET was calculated from the linear part of the adsorption isotherm using the BET method. Vtot was estimated from the amount adsorbed at a relative pressure of 0.99. The Dubinin-Radushkevich theory was employed for estimating Vmicro. Vmeso was obtained from (Vtot - Vmicro). PSD was calculated using the density functional theory (DFT) method. All the calculations were done by using the software of the instrument which assumes slit shapes for pores. The pHpzc which is the pH of the solution when the net charge on the ACC piece dipped into it is zero was determined using the batch equilibrium method reported by Babicˇ et al (15). The contents of acidic and basic surface groups on the ACC samples were determined according to the Boehm method (16). Potential of zero charge (EPZC) is the potential at the surface when the net charge on it is zero. The immersion method was adopted for measuring the EPZC of ACC. The immersion potential of a material in the absence of any Faradaic process should be equal to EPZC (17). It is known that SO42- ions do not specifically adsorb onto ACC (8). Thus the immersion potential of ACC to be measured in the Na2SO4 solution will be its EPZC. Before the measurement of EPZC, an ACC piece of about 12 mg and 0.01 M Na2SO4 solution were degassed separately, by applying vacuum for a short period. Then the ACC sample was inserted into the Na2SO4 solution and N2 was bubbled through the solution. Finally, potential was measured using a standard three electrode system connected to a potentiostat/galvanostat (Gamry Instruments Inc.) interfaced to another computer with its own software. The working electrode (WE) was 12 mg of ACC attached to a platinum wire, the counter electrode (CE) was a Pt plate, and the reference electrode (RE) was a Ag/AgCl electrode (BAS, MF-2030, Bioanalytical Systems Inc. USA). Various properties of ACC are collectively given in Table 1. Important features to note from Table 1 are possession of very high BET specific surface area, microporous character of the pores, and existence of surface oxygen groups. The PSD curve, given as SI, Figure S2, showed that the majority of pores have diameters narrower than 20 Å. Description of the Procedure for Adsorption/Electrosorption Studies by in Situ Optical Absorbance Measurements. A specially designed V-shaped cell was used to carry out the adsorption/electrosorption studies and to perform in situ concentration measurements by means of UV-visible absorption spectrophotometry. The diagram of the cell and its brief description are given in the SI as Figure S3. The detailed description of the procedure for adsorption/ electrosorption experiments was given in our recent work (14). In the present work the program for monitoring 6332
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 16, 2010
spectrum in certain time intervals during adsorption/ electrosorption was run on the computer, interfaced to the spectrophotometer (Varian Carry 100, UV-visible spectrophotometer), in “scanning kinetics” mode, rather than kinetics mode applied in the previous work (18). Working in this mode allows monitoring the changes that might occur during the course of adsorption/electrosorption in the spectrum of adsorbate solution. It was also possible to determine absorbance vs time data at any selected wavelength over the scanned range from the scans. On the other hand, pH of the solutions appreciably changes upon polarization of ACC (14). pH of the dye solutions reached values as low as 3.65 by cathodic polarization and as high as 10.15 by anodic polarization during the electrosorption studies. However, such variations in pH did not affect our analyses because it was found from studies on the dependence of absorption spectra of dyes on pH that the bands at 315, 316, and 303 nm for BB-7, BB-11, and BB-3, respectively, were not affected from pH changes. Absorption characteristics of the dyes and a typical diagram showing the pH dependence of the spectrum of BB-7 are given in the SI and in Figure S4. Treatment of Data. Absorbance data, obtained in certain time intervals until equilibrium at 25 °C, were converted into concentration data using the corresponding calibration relations and then plotted as a function of time for each system at specified conditions. The initial concentrations (6.0 × 10-5 M), volumes of dye solutions (20 mL), and the mass of ACC (12.0 ( 0.1 mg) were taken to be the same for each system to make the comparison of adsorption/electrosorption behaviors of basic dyes easy. The reason for keeping initial dye concentration at such a low level is to get reliable results from analysis by in situ UV spectroscopy. At higher concentrations deviations from Beer’s law may occur. Kinetic treatment of data for adsorption/electrosorption of basic dyes onto ACC was made on the basis of the pseudofirst-order model, since this model was found to apply satisfactorily for adsorption/electrosorption of many organic species (11, 12). The linear form of the pseudo-first-order model can be formulated as (19) ln(qe - qt) ) ln(qe) - k1t
(1)
where qe and qt are the amounts of dye adsorbed at equilibrium and at time t, respectively, and k1 is the rate constant. The extent of adsorption/electrosorption can be quantified by calculating the percentage of dye removed at equilibrium using the following equation %Req)[(c0 - ce)/c0]·100
(2)
where c0 and ce are initial and equilibrium concentrations of basic dyes, respectively. Percentage removal of basic dyes at a reference time of 100 min (%R100) was also calculated by replacing ce in eq 2 with the concentration at 100 min to make the comparison of the rate and the extent of adsorption/ electrosorption under similar conditions. The amount of BB-7 adsorbed per unit area of ACC at equilibrium, Γeq, was calculated using the following equation (5) Γeq)[(c0 - ce)V]/(Sm)
(3)
where c0 and ce have the same meaning as in eq 2, V is the volume of the solution, S is the BET surface area per unit mass of ACC, and m is the mass of ACC used.
3. Results and Discussion Adsorption Behaviors of BB-7, BB-11, and BB-3 on Open Circuit (OC). The OC adsorptive removal of BB-7, BB-11, and BB-3 was investigated in aqueous solutions at natural
TABLE 2. Adsorption/Electrosorption Parameters of Basic Dyes molecule
polarization/mA
ultimate potential/mV
103 k1 /min-1
r
teq/min
%R100
%Req
1012Γeq /mol.cm-2
BB-7
OC -0.5 -1.0 -2.0 +0.5 +1.0 +2.0 OC -0.5 -2.0 OC -0.5 -2.0
196 -820 -1250 -1685 640 1320 1570 185 -790 -1445 185 -770 -1490
2.41 7.90 12.5 26.9 5.60a 3.90 12.5 29.5 8.51 13.9 32.1
0.9905 0.9994 0,9986 0.9979 0.9956 0.9974 0.9997 0.9953 0.9950 0.9976 0.9929
1470 308 292 148 92a 780 268 86 596 255 84
16 38 57 93 21a 30 56 100 56 60 100
53 63 81 100 21a 71 76 100 87 84 100
3.35 3.86 5.40 6.65 1.40a 4.47 4.06 5.58 5.16 4.26 5.54
BB-11 BB-3
a
Calculated for the first 100 min. See text for details.
FIGURE 1. Open circuit adsorption behaviors of basic dyes from aqueous solutions onto ACC. pH (around 6.4 for all three) with in situ UV monitoring at wavelengths of 315, 316, and 303 nm, respectively, until equilibrium is reached. Adsorption equilibrium times (teq) are given in Table 2. Figure 1 shows the changes in concentrations of basic dyes in solution upon OC adsorption onto ACC over the first 650 min period. Kinetic and capacity parameters k1, %R100, %Req, and Γeq, calculated according to the equations described in the treatment of data subsection, are presented in Table 2. All these parameters show that both rate and extent of adsorption of the dyes increase in the order of BB-7 < BB-11 < BB-3. The regression coefficients for the linear treatment of kinetic data were all greater than 0.99, confirming the acceptability of the applied pseudo-first-order model. Favorable electrostatic interactions between dyes and the surface are not expected at the natural pH of around 6.4, because the ACC surface acquires a slight positive charge at this pH which is below the pHPZC of 7.2. Thus, even a slight electrostatic repulsion of positively charged basic dyes may occur (20). The main type of attractive force to be effective on OC adsorption of the three basic dyes from their aqueous solution at natural pH is expected to be dispersion forces between π-electrons of polycyclic aromatic rings and of the ACC surface. These attractive forces together with possible H-bonding interactions between secondary amine groups of BB-7 or BB-11 and surface functional groups overcome the slight unfavorable electrostatic interactions resulting in the observed OC adsorption parameters given in Table 2 and in Figure 1. On the other hand, the order of rate and extent of adsorption is further determined by the size of adsorbate species as will be discussed in the next subsection.
Adsorption Isotherms of Basic Dyes. Adsorption isotherms were derived to help in understanding the OC adsorption mechanism of basic dyes onto ACC. A detailed description of determination of adsorption isotherms and treatment of isotherm data according to the well-known Langmuir and Freundlich models is given in the SI, and the isotherms are presented in Figure S5. It was concluded from the isotherm studies that the extent of adsorption of basic dyes is mainly determined by the molecular sizes of dyes. BB-3 and BB-11 were found to adsorb onto ACC in horizontal orientation and in monolayer coverage. BB-7 was found to adsorb in multilayer coverage (see the SI for details). Electrosorptive Behavior of Basic Dyes. Cyclic voltammetric studies were carried out in order to determine the electrochemical stability of the three basic dyes. Voltammograms of the aqueous 0.01 M Na2SO4 solution, with and without dye, were recorded at a scan rate of 5 mVs-1 in the potential range between -1.50 V and +1.50 V vs Ag/AgCl reference electrode using ACC as a working electrode. Voltammograms are shown in the SI, Figure S6. No differences were observed in the cyclic voltammograms upon inclusion of dyes in the solution, indicating the electrochemical stabilities of dyes in this potential range. The electrochemical stability of dyes was also checked by blank electrosorption experiments with bare Pt electrodes. No change in dye concentration was detected. Electrosorption studies of BB-7, BB-11, and BB-3 were carried out in their water solutions. Preliminary tests showed that the initial dye concentration of 6 × 10-5 M is sufficient for providing the required conductivity due to the charged nature of the dye species used. For investigating the electrosorptive behavior of basic dyes, adsorption data were recorded on OC condition for the first 60 min, followed by electrosorption under galvanostatic negative polarization for the next 50 min. Then polarization direction was reversed in order to observe the electrodesorption behavior for the possible regeneration of ACC. It was mentioned earlier that polarization can be accomplished under either galvanostatic or potentiostatic conditions. Niu and Conway (21) stated that galvanostatic polarization is much to be preferred as it is more convenient and gives rise to larger removal rates than those attainable using the potentiostatic technique under otherwise corresponding conditions. Thus galvanostatic polarization was preferred in the present study. Although the applied potential is attained in a short time in potentiostatic polarization, a decrease is observed in electrosorption rate due to a limitation in electrical double layer charging at high area carbon electrodes. Since a small current (a few mA) is applied in galvanostatic polarization, potential rises slowly. VOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
6333
FIGURE 2. Open circuit adsorption (s), electrosorption (from A to B) at -0.5 mA (0), - 1.0 mA (O), and -2.0 mA (∆), and electrodesorption (after B) at +0.5 mA (0), +1.0 mA (O), and +2.0 mA (∆) for BB-7. The OC adsorption behavior followed by electrosorption behavior under polarization at -0.5 mA, -1.0 mA, or -2.0 mA and finally electrodesorption behavior under reversed polarization at +0.5 mA, +1.0 mA, or +2.0 mA for BB-7 are shown in Figure 2. It is seen that negative polarization of ACC by passing a current of -0.5 mA galvanostatically through ACC causes an increase in adsorption corresponding to a further decrease in concentration of BB-7 in solution after OC adsorption. It has been suggested that the polarization effects arise (especially for ionized adsorbates) from changes of surface charge-density on the C surface upon positive or negative polarization and by associated changes of orientation of water dipoles and strength of their electrostatic adsorption (22). Hence, electrostatic attraction between positively charged BB-7 and negatively charged ACC results in a decrease in BB-7 concentration in solution. It is clear from the Figure 2 that electrosorptive removal rate of BB-7 depends on the applied current and dramatically increases with increasing cathodic current from -0.5 to -2.0 mA. The fact that the electrosorptive removal rate depends on applied current shows that the overall electrosorption process is not controlled by the diffusion of BB-7 from bulk solution onto the ACC surface (3). It is to be noted that further increase in negative polarization beyond -2.0 mA may cause undesired electrochemical reactions such as electrochemical oxidation/reduction at working electrode or counter electrode, especially at N centers of dye species. Thus polarization studies were carried out only at -2.0 mA for the other two dyes, and the results are presented in Figure 3a for BB-11 and in Figure 3b for BB-3. The dramatic enhancement of adsorption upon polarization is clearly observed for BB-11 and BB-3. The absence of undesired electrochemical reactions and thus the structural integrity of the dyes have been confirmed by the observation that the scanning-kinetic data, recorded during the course of electrosorption of dyes, showed no shift in the absorption bands. These scanning-kinetic outputs are given in the SI as Figures S7, S8, and S9 for BB-7, BB-11, and BB-3, respectively. Electrodesorption behaviors of BB-7, BB-11, and BB-3 are seen after point B in Figures 2, 3a, and 3b, respectively. It is clear that complete electrodesorption was not achieved by reversal of the direction of polarization. Only 40% of electrosorbed BB-7 was desorbed, at the most, by anodic currents of +2.0 mA. Some of the electrosorbed dye molecules may not leave the pores due to blockage by direct oxidation 6334
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 16, 2010
FIGURE 3. Open circuit adsorption (9), electrosorption (from A to B) at -2 mA (0), and electrodesorption (after B) at +2.0 mA (0) for (a) BB-11 and (b) BB-3. of ACC under anodic potential leading to formation of oxygen complexes which are known to be located mainly at the edges of the basal planes and at the entrance of the pores in activated carbons (23). The oxygen groups are ideal sites for water adsorption, favoring the creation of water clusters at the entrance of the pores. Those hydration clusters might effectively hinder desorption of basic dyes from pores, especially micropores, into the bulk solution. Hence, only dyes, which were adsorbed into the mesopores or macropores, can be electrodesorbed to a certain extent. In order to support the prediction of formation of oxygen groups at the surface upon polarization, total acidic groups and pHPZC of ACC (polarized potentiostatically at 1.5 V which is the ultimate potential reached on electrosorption upon galvanostatic positive polarization for 90 min) were determined. Total acidic group content was found to be 0.983 mmol · g-1, almost three times that of pristine ACC (Table 1), and pHPZC was found to be 3.95, almost half of the pHPZC of pristine ACC. The effect of this oxidation of ACC on electrosorption of dyes will be discussed in the following subsection. Kinetics of Electrosorption of Basic Dyes. A series of experiments were conducted for detailed investigation of electrosoption kinetics of basic dyes. The changes in concentration with time during electrosorption of BB-7 under polarization at -0.5 mA, -1.0 mA, and -2.0 mA are shown in Figure 4a where OC adsorption behavior was also included for comparison. The data were treated according to pseudofirst-order model by regression analysis based on eq 1. Rate constants, k1, and corresponding regression coefficients, r,
FIGURE 4. Open circuit adsorption (x) and electrosorption of BB-7 from aqueous solutions onto ACC by polarization at (a) -0.5 mA (0), -1.0 mA (O), and -2.0 mA (∆) and (b) +0.5 mA (9), +1.0 mA (b), +2.0 mA (2). The solid lines represent the simulation data based on the pseudo-first-order equation. The dashed line shows electrosorptive behavior of BB-7 onto electrochemically oxidized ACC (at 1.5 V for 90 min in 0.01 M Na2SO4) at -2.0 mA polarization. are given in Table 2. Well fits arise between experimental and simulated data as seen visually from Figure 4a and numerically from the regression coefficients in Table 2 (r > 0.99), confirming the validity of the presumed pseudo-firstorder model. Rate constants for the electrosorption of BB-7 increase about 3-fold, 5-fold, and 11-fold at -0.5, -1.0, and -2.0 mA polarization, respectively, compared to that for OC adsorption. Capacity parameters, %R100, %Req, and Γeq, were also determined for these processes and given in Table 2. %Req values increase and teq decrease with increasing negative current. Almost complete removal of BB-7 from solution, under the specified conditions, is achieved by polarization at -2.0 mA within 148 min. Afterward, recording of absorbance was continued over three days to check the possibility of OC desorption. However, no increase was observed in absorbance. Electrosorption of BB-7 was also examined on preelectrochemically oxidized ACC in order to see the effect of oxidation. ACC was first oxidized at 1.5 V for 90 min in 0.01 M Na2SO4. Then this ACC was used in electrosorption of BB-7. Resulting electrosorptive behavior is shown with a dashed line in Figure 4a. The apparent decrease in rate and extent of electrosorption onto oxidized ACC upon -2.0 mA polarization compared to that onto pristine ACC was taken as a support for the idea of blockage of pores. Figure 4b shows the electrosorptive behavior of BB-7 under polarization at +0.5 mA, +1.0 mA, and +2.0 mA. OC adsorption behavior was also reproduced in this figure. In spite of the expectation of repulsion of cationic dye species by ACC upon positive polarization, about a 2-fold increase is observed in the rate constant for adsorption during polarization at +0.5 mA. This is probably due to the interactions between positively charged ACC surface and π-electrons of aromatic rings in BB-7. Similar behaviors were reported for other aromatic compounds (11, 12, 18). However after about 90 min, electrostatic repulsion overcomes the dispersion forces as a result of further charging of ACC and slight electrodesorption takes place. For the case of polarization at +1.0 mA and +2.0 mA, electrostatic repulsions become effective at much earlier stages so that no net adsorption take place. Thus kinetic and capacity parameters could not be calculated for these cases. Kinetic data for the electrosorption under polarization at -0.5 mA and -2.0 mA are given in Figure 5a for BB-11 and in Figure 5b for BB-3 together with OC data. Increasing enhancements with increasing negative current in rate and
FIGURE 5. Open circuit adsorption (x) and electrosorption of (a) BB-11 and (b) BB-3 from aqueous solutions onto ACC by polarization at -0.5 mA (0) and -2.0 mA (∆). The solid lines represent the simulation data based on the pseudo-first-order model. extent of electrosorption relative to OC adsorption are clearly seen for both dyes. The kinetic parameters determined from VOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
6335
the treatment of kinetic data according to the pseudo-firstorder model together with regression coefficients and capacity parameters, %R100, %Req, Γeq, and teq, are given in Table 2. Simulated data based on pseudo-first-order model are shown as solid lines in Figure 5a,b. Validity of pseudofirst-order model is confirmed by regression coefficients being greater than 0.99. Equilibrium times for both dyes are decreasing while the extents and rates of adsorption are increasing with increasing negative currents applied for polarization. It was exciting to see the complete removal of both dyes under the studied conditions within 85 min at -2 mA polarization. The main purpose of selection of the ACC material used in this study was its huge specific surface area. However it is recognized that as the specific surface area of an adsorbent material increases, its pore size decreases limiting the available surface area for adsorption of adsorbates of greater size, especially in cases where they are adsorbed horizontally to the surfaces. However, it can also be recognized that basic dyes may take up vertical or tilted positions in the case of electrosorption, its positively charged center pointing to the ACC surface as mentioned by Niu and Conway (4) in relation to pyridine electrosorption. However, it should also be noted that the total specific surface area of the present ACC is so large that even its 7% (133 m2 · g-1) available for BB-7, 14% (256 m2 · g-1) available for BB-11, or 26% (446 m2 · g-1) available for BB-3 are reasonably large for their electrosorptive removal (see the SI for details of obtaining specific area values in parentheses). Ideally, large surface area as well as large pore size are among the main desired properties of the adsorbents for electrosorptive removal of given adsorbates such as polycylic aromatic dyes. On the other hand when it comes to electrosorption of the studied dyes; the limitation by double-layer overlapping effect, discussed in detail by Yang et al. (24), may be expected due to the microporous character of ACC. However, it is known that this phenomenon occurs mainly in micropores depending on the thickness of double layer which decreases with ionic strength and applied current. Although the ionic strength is low in the present work, the ultimate potentials are quite high (Table 2) to keep the thickness of electrical double layer reasonably small. Furthermore, the smallest dye species in the present work has a diameter of 13.6 Å. Thus pores less than this size cannot be occupied by the dye species, and the double-layer overlapping effect is not expected for the present system.
Acknowledgments The support of this work by the Scientific Research Projects Coordination Unit of Akdeniz University is acknowledged.
Supporting Information Available Molecular structures of basic dyes, pore size distribution of ACC, description of electrolytic cell used in electrosorption studies, absorption characteristics of basic dyes, description of determination of adsorption isotherms, treatment of isotherm data, cyclic voltammograms, and scanning-kinetic outputs of basic dyes. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) Dabrowski, A. Adsorptionsfrom theory to practice. Adv. Colloid Interface Sci. 2001, 93, 135–224.
6336
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 16, 2010
(2) Hoda, N.; Bayram, E.; Ayranci, E. Kinetic and equilibrium studies on the removal of acid dyes from aqueous solutions by adsorption onto activated carbon cloth. J. Hazard. Mater. B 2006, 137, 344–351. (3) Niu, J.; Conway, B. E. Development of techniques for purification of waste waters: removal of pyridine from aqueous solution by adsorption at high-area C-cloth electrodes using in situ optical spectrometry. J. Electroanal. Chem. 2002, 521, 16–28. (4) Niu, J.; Conway, B. E. Molecular structure factors in adsorptive removal of pyridinium cations, 1,4-pyrazine and 1-quinoline at high-area C-cloth electrodes for waste-water remediation. J. Electroanal. Chem. 2002, 529, 84–96. (5) Ayranci, E.; Conway, B. E. Removal of phenol, phenoxide and chlorophenols from waste-waters by adsorption and electrosorption at high-area carbon felt electrodes. J. Electroanal. Chem. 2001, 513, 100–110. (6) Conway, B. E.; Ayranci, G.; Ayranci, E. Molecular structure effects in the adsorption behavior of some heterocyclic compounds at high-area carbon-cloth in relation to waste- water purification. Z. Phys. Chem. 2003, 217, 315–331. (7) Oren, Y.; Soffer, A. Water desalting by means of electrochemical parametric pumping.1. The equilibrium properties of a batch unit-cell. J. Appl. Electrochem. 1983, 13, 473–487. (8) Ayranci, E.; Conway, B. E. Adsorption and electrosorption at high-area carbon-felt electrodes for waste-water purification: Systems evaluation with inorganic, S-containing anions. J. Appl. Electrochem. 2001, 31, 257–266. (9) Afkhami, A.; Conway, B. E. Investigation of removal of Cr(VI), Mo(VI), W(VI), V(IV) and V(V) oxy-ions from industrial wastewaters by adsorption and electrosorption at high-area carbon cloth. J. Colloid Interface Sci. 2002, 251, 248–255. (10) Ying, T. Y.; Yang, K. L.; Yiacoumi, S.; Tsouris, C. Electrosorption of ions from aqueous solutions by nanostructured carbon aerogel. J. Colloid Interface Sci. 2002, 250, 18–27. (11) Han, Y.; Quan, X.; Chen, S.; Zhao, H.; Cui, C.; Zhao, Y. Electrochemically enhanced adsorption of aniline on activated carbon fibers. Sep. Purif. Technol. 2006, 50, 365–372. (12) Ania, C. O.; Be´guin, F. Mechanism of adsorption and electrosorption of bentazone on activated carbon cloth in aqueous solutions. Water Res. 2007, 41, 3372–3380. (13) Ban, A.; Chafer, A.; Wendt, H. Fundamentals of electrosorption on activated carbon for waste water treatment of industrial effluents. J. Appl. Electrochem. 1998, 28, 227–236. (14) Bayram, E.; Ayranci, E. Investigation of changes in properties of activated carbon cloth upon polarization and of electrosorption of the dye basic blue-7. Carbon 2010, 48, 1718–1730. (15) Babic´, B. M.; Milonjic´, S. K.; Polovina, M. J.; Kaludierovic´, B. V. Point of zero charge and intrinsic equilibrium constants of activated carbon cloth. Carbon 1999, 37, 477–481. (16) Boehm, H. P. Advances in Catalysis 16; Academic Press: New York, 1966; pp 179-274. (17) Tobias, H.; Soffer, A. The immersion potential of high surface electrodes. J. Electroanal. Chem. 1983, 148, 221–232. (18) Bayram, E.; Hoda, N.; Ayranci., E. Adsorption/electrosorption of catechol and resorcinol onto high area activated carbon cloth. J. Hazard. Mater. 2009, 168, 1459–1466. (19) Lagergren, S. About the theory of so-called adsorption of soluble substances. K. Sven. Vetenskapsakad. Handl. 1898, 24, 1–39. (20) Moreno-Castilla, C. Adsorption of organic molecules from aqueous solutions on carbon materials. Carbon 2004, 42, 83– 94. (21) Niu, J.; Conway, B. E. Adsorption of organics onto an high-area C-cloth electrode from organic solvents and organic solvent/ water mixtures. J. Electroanal. Chem. 2003, 546, 59–72. (22) Conway, B. E. Electrochemical Supercapacitors; Kluwer Academic/ Plenum: New York, 1999. (23) Donnet, J. B. Structure and reactivity of carbons: from carbon black to carbon composites. Carbon 1982, 20, 267–282. (24) Yang, K. L.; Ying, T. Y.; Yiacoumi, S.; Tsouris, C.; Vittoratos, E. S. Electrosorption of Ions from Aqueous Solutions by Carbon Aerogel: An Electrical Double-Layer Model. Langmuir 2001, 17, 1961–1969.
ES101177K
