Electrochemical Regeneration of Cr(VI) Saturated Granular and

Article pubs.acs.org/IECR

Electrochemical Regeneration of Cr(VI) Saturated Granular and Powder Activated Carbon: Comparison of Regeneration Efficiency Belgin Karabacakoğlu* and Ö znur Savlak Faculty of Engineering and Architecture, Chemical Engineering Department, Eskişehir Osmangazi University, Eskişehir, Turkey ABSTRACT: In this study, the regeneration of commercially available granular and powder activated carbons contaminated with Cr(VI) were investigated by electrochemical method. The regeneration of spent carbons was conducted in a batch stirred electrochemical reactor under different operating conditions in potentiostatic mode. The effects of operating parameters such as cell voltage, electrolyte concentration, pH and regeneration time on the regeneration efficiency and energy consumption were determined. The data of isotherm experiments were fitted to Langmuir, Freundlich, and Redlich−Peterson models. Surface characterization of fresh and regenerated activated carbons was examined using BET, SEM, and Boehm titration methods. Approximately 70% regeneration efficiency was achieved for granular and powder activated carbons. method uses a strong electrical field (in kV levels) and expensive equipment. Therefore, the use of an economically efficient regeneration method without creating secondary pollution is quite important for large-scale applications of AC in wastewater treatment. Electrochemical regeneration is an effective and environmentally friendly regeneration method, and it was suggested first by Narbaitz and Cen.15 The electrical current is used to regenerate the exhausted AC in an electrolyte solution. The electrochemical regeneration mechanism of AC involves electrodesorption and destruction of the desorbed pollutants from AC surface through the electrode reactions.16 The electrochemical regeneration method has some advantages, compared to the traditional methods. The most important advantage of the electrochemical method is to convert the pollutants desorbed from the adsorbent surface to less-harmful components via electrode reactions and reactions occurred in solution.17 It is also a simple and safe method, and this method can be applied at ambient temperature and pressure.18 Also, it is convenient for use in small- and medium-sized treatment facilities.17,19 Weng and Hsu19 reported that the regeneration efficiency of granular AC using an electrochemical method was higher than those of the alternative regeneration methods, such as ultrasound, steam, and base washing. Berenguer et al.14 compared the chemical, thermal, and electrochemical regeneration methods for phenol adsorbed on granular AC. Their results indicated that the electrochemical method is more efficient, in terms of regeneration efficiency and porosity recovery. Weng and Hsu20 compared the chemical and electrochemical regeneration of Zn adsorbed activated carbon in a recent study. They found that, when electrical current was applied to the regeneration column, the regeneration efficiency reached 88.3%, while the regeneration efficiency was only 25.3% in chemical regeneration. They also demonstrated that the cost of electrochemical regeneration of GAC was ∼40% of the thermal regeneration.

1. INTRODUCTION Activated carbons (ACs) have been used in widespread applications in the abatement of pollution, because of their unique surface properties, such as high surface area and high pore volume. Although activated carbon is an excellent adsorbent, the spent ACs led to serious environmental problems. If an AC has been used in a treatment process for a specific period of time, its surface becomes saturated and treatment process is nullified.1 Once the activated carbon has been exhausted, it should be disposed of in accordance with legal regulations or regenerated using an appropriate method. The reuse of spent AC is very important in view of sustainability. For this reason, the regeneration of AC is necessary for environmental and economical requirements. There are several regeneration methods for activated carbons such as thermal,2 chemical,3 catalytic oxidation,4 supercritical water,5 microwave regeneration,6 dielectric barrier discharge plasma,7 and electrochemical8−11 methods. Thermal regeneration is the most preferred process, because of its simplicity and high efficiency.12 Adsorbed material can be completely removed, especially at high temperature, during thermal regeneration, but the porosity of the AC is not completely recovered, because of the blockage of AC pores by some carbonization products. Moreover, attrition and burnoff is inevitable in thermal regeneration processes, and they lead to a loss of AC of ∼10%−20% (by weight). Another widely used method is chemical regeneration of AC by means of a solvent. This method is simple and inexpensive, but the desorbed pollutants from the AC surface are not destroyed and they are transported to the solvent. The solvent has the potential for secondary pollution.13 Furthermore, expensive purification steps may be necessary to recover the solvent.14 The catalytic regeneration of AC is an efficient method, but it needs the use of an additional reagent such as ozone or modification of AC with catalyst. The supercritical water process is very efficient, especially for the organics such as phenol, but the process is expensive, because of the necessity of very high temperatures and pressure. The drawbacks of microwave heating are a lack of uniformity in heating and also high investment costs. The dielectric barrier plasma technique is a promising regeneration method, but this © 2014 American Chemical Society

Received: Revised: Accepted: Published: 13171

January 13, 2014 June 20, 2014 July 30, 2014 July 30, 2014 dx.doi.org/10.1021/ie500161d | Ind. Eng. Chem. Res. 2014, 53, 13171−13179

Industrial & Engineering Chemistry Research

Article

The procedure to determine the regeneration efficiency was applied as follows:27 (i) Initial Adsorption: Before electrochemical regeneration, the activated carbons were saturated using 350 mg/L Cr(VI) solution. The concentration of Cr(VI) was determined according to the previous experimental studies.28 The 0.5 g of PAC or 1 g of GAC was mixed with 200 mL of Cr(VI) solution in a 250-mL flask for 24 h at 25 °C in a thermostatic bath shaker. The initial pH of the Cr(VI) solution was 2.5. After adsorption, the solution was filtered and the Cr(VI) concentration of the filtrate was measured. The separated AC was washed three times with 250 mL of distilled water to remove unadsorbed Cr(VI). (ii) Electrochemical Regeneration: The effects of applied voltage, regeneration time, electrolyte concentration, and initial pH on the regeneration efficiency and energy consumption were investigated (Table 1). In each

The electrochemical regeneration of commercial AC loaded with organics such as toluene, phenol, and dyes, and heavy metals such as arsenic and zinc was investigated by different researchers, and they achieved regeneration efficiencies of 88%−100%,20−25 but this method was not investigated for Cr(VI) adsorbed granular and powder ACs. The objective of this study is to regenerate commercial powder and granular ACs saturated by Cr(VI) via an electrochemical method. The effects of applied potential, electrolyte concentration, pH, and time were investigated, in terms of regeneration efficiency and energy consumption. In addition, the surface properties of fresh and regenerated ACs were characterized by Brunauer−Emmett−Teller (BET), scanning electron microscopy (SEM), and Boehm titration methods.

2. EXPERIMENTAL SECTION 2.1. Materials. A commercial powder activated carbon (PAC) (Merck) and a granular activated carbon (GAC) (Picacarb 830, Pica) were used in the experiments. The Cr(VI) solutions were prepared from K2Cr2O7. The pH of the solutions was adjusted with 0.1 M H2SO4 and 0.1 M NaOH solutions. The NaCl was used for the preparation of electrolyte solutions. All chemicals are analytical grade and supplied from Merck. 2.2. Analysis of Cr(VI). The concentration of Cr(VI) was determined spectrophotometrically by 1,5-diphenylcarbazide method.26 For the analysis, 1 mL of 20% (w/w) H2SO4 solution and 1 mL of 1% 1,5-diphenyl carbazide were added to the 1 mL of sample and then it was diluted to 100 mL with distilled water. The absorbance of the strongly red-violet color complex was measured by an ultraviolet−visible (UV-Vis) spectrophotometer (Aquamate, Thermo Electron Co.) at a wavelength of 540 nm. 2.3. Equilibrium Time. The equilibrium time was determined for both PAC and GAC by mixing a known mass of activated carbon with 200 mL of the 350 mg/L of Cr(VI) solutions (initial pH 2.5) in a 250-mL flask in thermostatic shaker bath at 25 °C. 2.4. Electrochemical Regeneration. The regeneration apparatus consist of a DC power supply, a magnetic stirrer, and an electrochemical reactor, as shown in Figure 1. A batch

Table 1. Experimental Results for GAC and PAC E (V)

NaCl (%)

initial pH

t (min)

REPAC (%)

EC-PAC (kWh/kg)

REGAC (%)

EC-GAC (kWh/kg)

1 1.5 2 1 1 1 1 1 1 1 1 1

1 1 1 1 2 3 2 2 2 2 2 2

2.48 2.46 2.51 2.50 2.47 2.44 2.50 3.60 4.50 2.47 2.48 2.47

60 60 60 60 60 60 60 60 60 30 60 120

68.3 69.6 70.3 68.3 69.5 69.7 69.5 71.3 59.6 66.7 69.5 58.9

0.233 0.450 0.667 0.233 0.333 0.467 0.333 0.300 0.267 0.150 0.300 0.600

68.2 69.1 69.8 68.2 68.8 68.9 68.8 69.9 58.5 64.4 68.8 41.1

0.057 0.150 0.314 0.057 0.129 0.343 0.129 0.100 0.086 0.064 0.129 0.257

experiment, the saturated AC was placed in the electrochemical reactor consisting of 350 cm3 of the NaCl electrolyte. At the end of the regeneration, the cell content was filtered; the activated carbon washed with distilled water and dried at 105 °C. (iii) Readsorption: Dried and regenerated activated carbons were weighed, and readsorption was carried out at the identical conditions (Cr(VI) concentration = 350 mg/L, pH 2.5) with the initial adsorption step. In the experiments conducted with PAC, a loss in mass of PAC occurred because of filtration. Thus, the readsorption experiments for PAC were performed using 0.3 g of regenerated PAC and 120 mL of Cr(VI) (350 mg/L) solution to keep the mass/volume ratio constant. No loss of mass occurred in GAC. The mass of the adsorbent was the same in each initial adsorption step since fresh AC was used in each regeneration experiment. The Cr(VI) uptake value per gram of AC was calculated using the difference of initial and equilibrium concentrations:

Figure 1. Regeneration setup. [Legend: a, electrodes; b, electrochemical cell; c, activated carbon; d, magnetic stirrer; and e, DC power supply.]

cylindrical electrochemical cell was used in the regeneration experiments. Two stainless steel electrodes (316L), with dimensions of 5 cm × 8 cm × 0.2 cm, placed 10 cm away from each other, were used as the anode and cathode. The immersed areas of anode and cathode were 25 cm2. The reactor was stirred by a magnetic stirrer in order to enhance the mass transfer. The electrochemical regeneration experiments were conducted at constant voltage and ambient temperature.

q (mg/g) =

(C0 − Ce)V m

(1)

where C0 and Ce are the initial and equilibrium Cr(VI) concentrations, respectively; V is the solution volume (in liters), and m is the mass of activated carbon (in grams). 13172

dx.doi.org/10.1021/ie500161d | Ind. Eng. Chem. Res. 2014, 53, 13171−13179


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Therefore, in the adsorption experiments, HCrO4− and Cr2O72− are the predominantly anionic forms of Cr(VI).30 3.1. Equilibrium Time. The effect of time for the adsorption process was investigated. The adsorbed chromium onto powder and granular activated carbons versus time is shown in Figure 2.

The regeneration efficiency of activated carbons (RE) was calculated according to the following equation, which was suggested by Narbaitz and Cen:16 ⎛q ⎞ RE (%) = ⎜⎜ r ⎟⎟ × 100 ⎝ qf ⎠

(2)

where qr (mg/g) and qf (mg/g) are the Cr(VI) adsorption capacities of the regenerated and fresh activated carbons, respectively. The energy consumption per unit mass of the regenerated activated carbon (EC) is also calculated according to the following equation: EC (kWh/kg) =

(EIt ) × 10−3 m

(3)

where E is the applied potential (in volts), I the current intensity (in amperes), t the treatment time (in hours), and m the mass of the AC (given in kilograms). 2.5. Adsorption Isotherms. The adsorption isotherms for fresh and regenerated activated carbons were prepared by mixing 0.1 g of activated carbon with 50 mL of Cr(VI) solution at the initial concentration range of 70−400 mg/L for 24 h at 25 °C (initial pH = 2.5). The ACs used in isotherm studies were regenerated under the conditions of 1.5 V, 2% NaCl, and initial pH of 3.6, which are the optimum conditions of the electrochemical regeneration, in view of regeneration efficiency and energy consumption. 2.6. Activated Carbon Characterization. The porous textures of the fresh and regenerated carbons were investigated by physical adsorption of N2 gas at −196 °C using an automatic adsorption instrument (Autosorb 1 C, Quantachrome). The samples were degassed at 110 °C under vacuum. The T-plot method was used with P/P0 < 0.005 and the surface area was calculated by application of the BET equation. The surface morphologies of ACs were examined by scanning electron microscopy (SEM) (JEOL, Model JSM-5600 LV). 2.7. Boehm Titration. The surface chemistry of ACs was examined using the Boehm titration method.29 A 0.2 g sample of the activated carbon was added to 20 mL of the 0.05 M NaOH or HCl solutions. The Erlenmeyer flasks were sealed and shaken for 24 h, the solution was filtered, and then 5 mL of the solution was titrated by the acid or base solution. Total surface acidic or basic functional groups (SFG) were determined according to the following equation: SFG (mmol/g) =

0.05f (Tb − T )5/20 w

Figure 2. Effect of time on the adsorption of Cr(VI). [Conditions: C0 = 350 mg/L; V = 250 mL; mPAC = 0.5 g, mGAC = 0.7 g; initial pH = 2.5; T = 25 °C.]

The concentration of Cr(VI) decreased sharply with time for the first 5 h and then reached an equilibrium value within ∼24 h both for GAC and PAC. 3.2. Electrochemical Regeneration. In the regeneration experiments, the adsorbed anionic Cr(VI) moieties are desorbed to the solution with the aid of electrical current and NaCl electrolyte. The ionic strength values of the regeneration solutions varied as 0.17 M (1% NaCl), 0.256 M (1.5% NaCl), and 0.34 M (2% NaCl). The pH of the regeneration solution changed over the range of 2.5−6.5, according to initial pH, voltage, and time. Visual Minteq 3.1 was used for the detailed analysis of Cr(VI) distribution, as a function of pH, and it was presented in Figure 3.

(4)

where Tb (given in milliliters) is the titration value of acid or base for blank experiment, T is (in milliliters) the value of acid or base consumed, f is the factor of titration solution, (5/20) is the diluting factor, 0.05 is the molarity of the acid or base solution, and w (in grams) is the mass of AC.

Figure 3. Cr(VI) speciation diagram as a function of pH. [Conditions: ionic strength = 0.34 M; Cr(VI) concentration = 350 mg/L, T = 25 °C.]

3. RESULTS AND DISCUSSION Chromate speciation is strongly dependent on pH and Cr(VI) concentration. Temperature and ionic strength of the solution also affect the distribution of the species. The initial pH of adsorption experiments was 2.5, and the Cr(VI) concentration was 350 mg/L (6.7 × 10−3 M) in the adsorption experiments. The HCrO4−, Cr2O72−, and CrO42− ions are the major anionic forms of Cr(VI) in the pH interval between 1 and 12.30−32

As seen in Figure 3, the distribution of the Cr(VI) species is ∼80% HCrO4− and ∼20% Cr2O72− in the pH interval of 1−5. The CrO42− species exists at pH >5. The possible reactions occurred in electrodes during regeneration are as follows:33,34 Anode: Fe → Fe2 + + 2e− 13173

(5)



Article

2H 2O → O2 (g ) + 4H+ + 4e−

(6)

2Cl− → Cl 2(g ) + 2e−

(7)

istic color of chromium(III). Furthermore, the regeneration solution was analyzed at the end of the regeneration time and there were no Cr6+ ions in the solution. The regeneration efficiencies of the ACs were measured at different cell voltage values while keeping other experimental conditions constant. As seen in Table 1, the regeneration efficiencies of both PAC and GAC increase slightly as the voltage increases. This tendency was also observed in the other electrochemical regeneration studies.20,36 On the other hand, the increase in cell potential results in increased energy consumption. For this reason, the regeneration must be conducted at optimum voltage. The applied regeneration voltage of 1.5 V was considered to be more suitable by assessing the regeneration efficiency and energy consumption. 3.2.2. Effect of NaCl Concentration. There are different electrolytes, such as Na2CO3, NaHCO3, Na2SO4, and NaCl, for the electrochemical regeneration of AC in earlier studies.17,36,37 Their results demonstrated that the best electrolyte is NaCl, in terms of high regeneration efficiency. For this reason, NaCl was selected as the electrolyte. The supporting electrolyte increases the regeneration efficiency of AC.19 As the concentration increases from 1% to 3%, the effect of the NaCl concentration on the regeneration efficiency is similar for PAC and GAC. The regeneration efficiency increased slightly as the electrolyte concentrations increased. The number of Na+ and Cl− ions increases while the NaCl concentration increases. The Cl− ions make anionic chromium(VI) species from AC surface easy to desorb. Similar results have been seen in the literature.21,27 The current density increases as the salt concentration increases. The rising of current density enhances mass transfer and electrode reactions, but the energy consumption increases with the increasing electrolyte concentration. NaCl is one of the best electrolytes for chromium(VI) electrocoagulation.38 In addition, Cl− ions accelerate the dissolution of iron by pitting corrosion.39 All chromium(VI) species in the solution can be completely reduced at acidic pH, and NaCl promotes the reduction rate of chromium(VI). 3.2.3. Effect of Regeneration Time. Regeneration time is a very important parameter, in terms of process economy, and a shorter treatment time is preferred in industrial-scale applications. The regeneration efficiency increased from 30 to 60 min for PAC and GAC, but a decrease was observed for a treatment time of 120 min for all carbons. The energy consumption also increased with the increase in time, as shown in Table 1. The electrochemical oxidation or reduction of desorbed moieties during the regeneration of AC is mainly advantageous in wastewater treatment, but this phenomenon may hinder the porosity recovery, because of pore blocking by these moieties. Moreover, the blockage of porosity increases with increased current and regeneration time. 18 For this reason, the regeneration efficiency may decrease. Berenguer et al.18 achieved a regeneration efficiency of 85% for phenol adsorbed by GAC, and they declared that further improvement seemed to be impossible, even under the optimal conditions, because of the adsorption of desorbed or oxidized products of phenol during electrochemical regeneration, especially for long regeneration times. Another problem with long regeneration times is a possible accumulation of the reaction products on the electrode surface, which leads to passivation of the electrode.17 Therefore, a regeneration time of 60 min is sufficient for PAC and GAC. 3.2.4. Effect of Initial pH. The initial pH of the electrolyte solution influences both electrode and solution reactions. Low

Cathode: HCrO4 − + 7H+ + 3e− → Cr 3 + + 4H 2O

(8)

Cr2O7 2 − + 14H+ + 6e− → 2Cr 3 + + 7H 2O

(9)

CrO4 2 − + 4H 2O + 3e− → Cr 3 + + 8OH−

(10)

−

2H 2O + 2e → H 2(g) + 2OH

−

(11)

2H+ + 2e− → H 2(g)

(12)

The favorable reaction for the anode is ferrous oxidation, with respect to the standard potentials. Among the cathode reactions, the most favorable reactions, in terms of standard potential, are the reductions of HCrO4− and Cr2O72−. In this study, stainless steel was used as the anode and the cathode in regeneration experiments. When stainless steel is the anode, Fe2+ ions are generated by anodic dissolution (eq 5) Moreover, the released Fe2+ ions can react with Cr(VI) moieties from desorbing of the AC surface and it provides the electrocoagulation of Cr(VI). As a result, as seen in the following reactions, the Cr(VI) ions that are not reduced at the cathode can be reduced chemically by Fe2+ ions. HCrO4 − + 3Fe 2 + + 7H+ → Cr 3 + + 3Fe3 + + 4H 2O (13)

Cr2O7

2−

+ 6Fe

2+

+

+ 14H → 2Cr

3+

+ 6Fe

3+

+ 7H 2O (14)

Canizares et al.35 had used stainless steel as the anode and the cathode for regenerating GAC-adsorbed phenol, but the produced Fe3+ ions reacted with phenol in solution and produced insoluble iron-phenol complexes. On the other hand, Cr(VI) anions cannot generate a complex with Fe2+ or Fe3+. The regeneration experiments were conducted at constant voltage (potentiostatic mode). The regeneration efficiencies and energy consumptions were determined for various cell voltages, NaCl concentrations, and pH and regeneration time values. The results of the regeneration experiments for GAC and PAC are given in Table 1. As seen in Table 1, the regeneration efficiency of PAC is slightly greater than that of GAC. It is explained by the effect of the AC particle size on the regeneration efficiency. The regeneration efficiency decreased with increasing particle size. The nature of carbon affects the regeneration efficiency.17 Similar results were demonstrated by Narbaitz and Cen.15 3.2.1. Effect of Voltage. The increase in the applied potential promotes the anode and cathode reactions according to Faraday’s law. Generally, the applied potential affects not only regeneration efficiency but also transformation of desorbed moieties from the AC surface to less-harmful ones. This is also valid for other electrochemical applications such as regeneration of GAC saturated by phenol.18 In this study, desorbed species were reduced to the less-harmful chromium(III) species, according to eqs 8−10, 13, and 14. The most important advantage of the electrochemical regeneration technique is converting the desorbed pollutant to harmless compounds, in addition to recovery of surface characteristics of AC. In the regeneration experiments, the color of the electrolyte solution changed from transparent to blue-green, which is the character13174



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Table 2. Isotherm Constants Langmuir Model Fresh-GAC Reg-GAC Fresh-PAC Reg-PAC

Freundlich Model

Redlich−Peterson Model

qm

b

R2

kF

n

R2

A

B

g

R2

94.51 65.02 116.28 86.20

0.0623 0.0161 1.5926 0.1186

0.983 0.997 0.995 0.998

22.624 5.714 70.880 25.508

3.817 2.475 9.615 4.310

0.953 0.979 0.866 0.951

11.56 1.21 88.08 15.73

0.2200 0.0308 0.8367 0.2761

0.89767 0.91495 0.97749 0.92288

0.984 0.981 0.972 0.961

pH values benefit the electrochemical reduction of Cr6+ to Cr3+, since the H+ ions are necessary for this reaction, as seen in eqs 8−10. Also, Cr6+ may reduce to Cr3+ via Fe2+ in acidic solution (eqs 13 and 14), which is revealed at the anode. The electrochemical and chemical reduction of Cr6+ consumes considerable amounts of H+ ions. Hexavalent chromium anions (Cr6+) were reduced to Cr3+ and these ions can be exchanged by the positively charged ions existing on the AC surface.40 The consumption of H+ ions due to the release of H2 gas results in a significant increase in the solution pH, and most of the Cr3+ and Fe2+ may precipitate as follows: Cr 3 + + 3OH− ↔ Cr(OH)3 ↓

(15)

Fe2 + + 2OH− ↔ 3Fe(OH)2 ↓

(16)

regenerated PACs were 0.995 and 0.998, respectively. Similarly, the correlation coefficients of fresh and regenerated GACs were 0.983 and 0.997, respectively. The Langmuir model fitted well to the experimental data for fresh and regenerated activated carbons. This is probably due to the homogeneous single layer adsorption characteristic of ACs for chromium(VI). In addition to the Langmuir isotherm model, the Freundlich and Redlich−Peterson models43 were applied to experimental data. The Freundlich equation is expressed by the following equation: qe = k f Ce1/ n

where kf and n are the Freundlich constants. The constants kf and n were calculated by plotting the graph between ln qe vs ln Ce. The magnitude of n gives an idea of favorability of the adsorption. A value of n between 2 and 10 indicates good adsorption capacity.43 As seen in Table 2, the n values of ACs are >2. Also, if n > 1, adsorption is favored by physisorption.44 The Redlich−Peterson isotherm is known as a threeparameter model and its equation is as follows:

On the other hand, higher acidic pH facilitates the precipitation of Cr3+ ions, but the formation of hydroxide makes the filtration of activated carbon difficult at the end of the regeneration. The increase in pH reduces the adsorbability of chromium(VI) while enhancing the desorption from the AC surface. The reduction potential of chromium(VI) to chromium(III) also decreases with increasing pH, according to the Nernst equation.41 Thus, the regeneration should be performed at the highest possible acidic pH value. In this case, choosing an optimum pH value was needed to be determined to obtain both satisfactory adsorption and an efficient desorption. Another advantage of increasing the chosen pH value is that the energy consumption reduces slightly with the increase in the pH value during regeneration. The electrolyte pH of 3.6 was chosen as being optimum, in terms of higher regeneration efficiency. The pH variation of the electrolyte solution was measured at the end of the regeneration process. Significant variation in the electrolyte pH was not observed, because of undivided cells and stirring, especially after regenerating times of 30 and 60 min. The initial pH values of 2.5 and 3.6 increased to ∼2.62 and 4.7 in 60 min, respectively. The green precipitate of Cr(III) was seen in an initial pH of 4.5 or after a regeneration time of 120 min, in which the pH value increased to ∼6−6.7. 3.3. Adsorption Isotherms. The Langmuir equation is one of the most widely implemented methods in adsorption studies. The linearized form of the Langmuir equation is as follows:42 Ce C 1 = + e qe qmb qm

(18)

qe =

ACe 1 + BCe g

(19)

where A, B, and g are the Redlich−Peterson parameters. The isotherm constants of Redlich−Peterson model were obtained using professional graphic software Origin (trial version 9.1) and listed in Table 2. As seen in the table, Langmuir and Redlich−Peterson models give a high R2 values than the Freundlich model and data. Similar results were obtained for Cr(VI) adsorption in literature.43,44 Figures 4 and 5 show the experimental and predicted isotherms for the adsorption onto PAC and GAC, respectively.

(17)

where qe is the quantity of chromium(VI) adsorbed per unit mass of AC (mg/g) at equilibrium, Ce is the equilibrium concentration (mg/L) of chromium(VI) in solution, qm (mg/g) is the maximum adsorption capacity, and b (L/mg) is a constant related to the activation energy. The values of Ce/qe vs Ce were graphed; qm and b constants were calculated from the intercept and slope. Langmuir constants and correlation coefficients of ACs are given in Table 2. The regression coefficients for fresh and

Figure 4. Isotherms for the adsorption of chromium(VI) onto fresh and regenerated PAC. [Adsorption conditions: m = 0.1 g; C0 = 70−400 mg/ L; V = 50 mL; initial pH = 2.5; T = 25 °C. Regeneration conditions: initial pH = 3.6; 2% NaCl; E = 1.5 V; t = 60 min.] 13175



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respectively; and Dp is the particle diameter of the ACs. The SBET recovery is the ratio of SBET values of the regenerated and fresh ACs. Activated carbons have micropores, mesopores, and macropores. Narrow distribution of micropores is useful for effective adsorption, but desorption and regeneration of this ACs is more difficult.18 Therefore, ACs with high mesoporosity regenerate easier.30 According to the BET results, the surface and pore recovery of PAC is higher than that of GAC; 63% of the pores of GAC are micropores, while 42% of the pores of PAC are micropores. As seen in Table 3, PAC has a higher mesoporosity than GAC. These results are also in accordance with the regeneration efficiencies in Table 1. The surface area recovery in PAC is slightly higher than GAC. The recovery of micropores and total pores of PAC is also significantly higher than GAC. Narbaitz and McEwen46 studied electrochemical regeneration of field-spent GAC-loaded natural organic matter. They demonstrated that the regeneration efficiency depends on the loading time of the pollutants onto GAC and the nature of adsorbed material. The regeneration efficiency of the laboratoryloaded GAC with natural organic matter in a 2-week period was 87%. On the other hand, the regeneration efficiency of the fieldloaded GAC over a long time was a maxiumum of 20%. If the adsorbate molecules did not penetrate deeply into the internal micropores, it would be easier to desorb these molecules during regeneration.47 3.5. SEM Photographs. Figures 6 and 7 show SEM photographs of the PAC and GAC, respectively. As seen in

Figure 5. Isotherms for the adsorption of chromium(VI) onto fresh and regenerated GAC. [Adsorption conditions: m = 0.1 g; C0 = 70−400 mg/ L; V = 50 mL; initial pH = 2.5; T = 25 °C. Regeneration conditions: initial pH = 3.6; 2% NaCl; E = 1.5 V; t = 60 min.]

As seen in these figures, the calculated data from models for fresh and regenerated ACs were fitted well to the experimental data. The qm values were also used for the calculation of the regeneration efficiency of ACs, according to the following equation:19 qm,reg RE (%) = × 100 qm,fresh (20) where qm,reg and qm,fresh are the maximum adsorption capacities of the regenerated and fresh ACs, respectively. The maximum adsorption capacities of the fresh and regenerated PACs are 116.28 and 86.20 mg/g, respectively. The regeneration efficiency of PAC is 74.1% under these conditions. The maximum adsorption capacities of the fresh and regenerated GACs are 94.51 and 65.02 mg/g, respectively. The regeneration efficiency of GAC is 68.8%. The advantage of choosing this method is to cover a wider concentration range. The experimental errors may be less significant in this case, since a curve fitting is applied to the obtained experimental results. 3.4. BET Analysis. The surface area and pore volume are among the important textural characteristics that determine the adsorption capacities of the ACs. Table 3 shows the BET results of the fresh and regenerated activated carbons. As shown in the table, the regenerated samples have smaller pore volumes and surface area, compared to the fresh ACs. This may be due to the pore blockage or pore destruction as a result of the chemical and physical effects of the electrochemical regeneration. The occupation of the adsorption sites of the ACs by residual chromium(VI) and destruction products such as chromium(III) may also be another reason.45 In Table 3, SBET is the surface area of the ACs; Vmicro, Vmeso, and Vtotal are the micropore, mesopore, and total pore volume of ACs,

Figure 6. SEM photographs of (a) fresh and (b) regenerated PAC.

these figures, electrochemical regeneration led to textural changing. It was appeared that the PAC was irregular in shape; after the regeneration, the clustering was seen. As shown in Figure 7a, GAC had irregular pores; after regeneration the pores on GAC surface was decreased. 3.6. Boehm Titration. ACs have surface acidic and basic groups. These groups are very important in determining the adsorption capacity of the activated carbons. The regeneration

Table 3. Physical Properties of Activated Carbons

GAC R-GACa PAC R-PACa a

SBET (m2/g)

VMicro (cm3/g)

VMeso (cm3/g)

VTotal (cm3/g)

Dp (Å)

928 580 1010 646

0.3180 0.2289 0.2680 0.20380

0.1843 0.0656 0.3726 0.2857

0.5023 0.2945 0.6406 0.4895

21.65 20.30 27.64 30.31

SBETrecovery (%)

Vmicro recovery (%)

Vmeso recovery (%)

62.5

71.9

35.6

63.9

76.1

76.7

Vtotal recovery (%) 58.6 76.4

Regeneration conditions: cell voltage = 1.5 V; initial pH = 3.6; 2% NaCl; time = 60 min. 13176



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GAC, because of the mass loss of PAC during filtration. The GAC was loaded with chromium(VI) and then regenerated in NaCl electrolyte solution. This procedure was repeated seven times consecutively. The results of the regeneration cycle are shown in Figure 8. There was no notable decrease in the

Figure 7. SEM photographs of (a) fresh and (b) regenerated GAC.

performance of AC could also be related to its surface chemistry and microporous structure.14 The electrochemical regeneration of the AC changes its surface acidic and basic group distribution. Mehta et al.48 investigated the effects of the electrochemical treatment on the surface groups of AC. Their results indicated that the anodic and cathodic treatment of AC altered the acidic properties of AC and, hence, reduced the adsorptive capacity for phenol. In contrast, an increase in the acidic groups increases the adsorption capacity of the adsorbent for chromium(VI) adsorption. The electrochemical treatment of ACs increased the acidic functional groups and decreased the basic groups, as seen in Table 4. The ratio of the acidic groups to the total surface

Figure 8. Regeneration cycle of GAC. [Regeneration conditions: E = 1.5 V; 2% NaCl; initial pH = 3.6; t = 30 min.]

regeneration efficiency in the first four cycles and a significant decrease was observed in the fourth and subsequent cycles. A similar tendency was observed by Wang and Balasubramanian.24 These results showed that the electrochemical regeneration of GAC can be applied in a reasonable efficiency in consecutive cycles. In this study, the maximum regeneration efficiency was obtained as 73% for PAC and 70% for GAC. These efficiencies are relatively low, with respect to earlier studies in the area of electrochemical regeneration of organic loaded AC and carbonbased adsorbents. Brown and co-workers studied the electrochemical regeneration of carbon-based adsorbent loaded with phenol11 and organic dyes,27,52 and they achieved 100% regeneration efficiency in short treatment times. It is concluded that the regeneration efficiency is affected significantly by the adsorbent type. A high regeneration efficiency of GAC up to 100% has been achieved in the literature for organic saturated ACs.22,24 In our study, relatively low regeneration efficiency may be related to the type of the adsorbate.

Table 4. Boehm Titration Results GAC R-GACa PAC R-PACa

total acidic groups (mmol/g)

total basic groups (mmol/g)

0.0300 0.5650 0.0212 0.8252

0.2611 0.1537 0.9402 0.5484

a

Regeneration conditions: cell voltage = 1.5 V; initial pH = 3.6; 2% NaCl; time = 60 min.

functionality increased significantly from 10.3% to 78.2% for GAC, while it increased from 2.2% to 60.1% for PAC. Similar results were obtained by Asghar et al.49 with electrochemical regeneration of carboneous adsorbent and by Park and Kim50 with electrochemical treatment of AC fiber. Park and Kim50 demonstrated that the anodic surface treatment of AC resulted in the rising of acidic surface groups such as carboxylic, phenolic, and lactonic groups, and, therefore, the diffusion coefficient and adsorption rates of chromium(VI) increased. They also implied that the acid−base interactions are predominant for chromium(VI) adsorption; hence, the surface physical characteristics such as surface area and pore volume are less important. The reaction between the bichromate anion and the oxygen-containing functional group is as follows:51 Cx O + HCrO4 − + H 2O ↔ Cx OHOCr + + 2OH−

4. CONCLUSIONS The experimental results showed that the activated carbon (AC) loaded with chromium(VI) could be regenerated electrochemically with the regeneration efficiency of ∼70% for GAC and PAC. The applied potential, electrolyte concentration, and pH affected the regeneration efficiency of ACs. The optimum conditions are 1.5 V, 2% NaCl and pH 3.6, with respect to the regeneration efficiency and energy consumption. The energy consumptions of PAC and GAC at the optimum regeneration conditions were 0.182 and 0.131 kWh/kg, respectively. The adsorption capacities of the regenerated GAC and PAC were decreased compared to those of the fresh ACs. The BET analysis of ACs revealed a decreasing adsorption capacity of ACs. The recovery of surface area and pore volume for PAC is higher than that of GAC. The GAC and PAC had similar adsorption mechanisms for chromium(VI) and both fitted well the Langmuir isotherm model. The surface chemistry of the ACs changed significantly. The electrochemical regeneration with acidic NaCl electrolyte caused an increase in the acidic functional groups. On the other

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The increase in the acidic surface functional groups on activated carbon accelerates the mass transfer of chromium(VI) from bulk to AC surface.48 Therefore, more acidic functional groups result in more chromium(VI) adsorption capacity. 3.7. Regeneration Cycle. Reusability of ACs over several times is important, from an economical point of view. Regeneration cycle experiments were conducted with only 13177



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hand, it decreased the basic functional groups. The electrochemical regeneration of GAC can be applied in a reasonable efficiency in consecutive cycles at least four times. The loss of GAC did not occur in the experiments. It is very important for large-scale electrochemical regeneration. It was observed that the desorbed chromium(VI) from the activated carbon surface was reduced to chromium(III). This study shows that the electrochemical regeneration is a good choice for the ACs used in wastewater treatment to remove Cr6+ ions at a low concentration.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +(90)-222-2393750. Fax: +(90)-222-2393613. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Electrochemical Regeneration of Cr(VI) Saturated Granular and

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