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Journal of Hazardous Materials
j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / j h a z m a tTreatment of Reactive Black 5 by combined electrocoagulation–granular
activated carbon adsorption–microwave regeneration process
Shih-Hsien Chang
a,∗, Kai-Sung Wang
a, Hsiu-Hao Liang
a, Hsueh-Yu Chen
a, Heng-Ching Li
a,
Tzu-Huan Peng
a, Yu-Chun Su
b, Chih-Yuan Chang
baDepartment of Public Health, Chung-Shan Medical University, 110 Chen-Kuo N. Road, Taichung 402, Taiwan, ROC bInstitute of Environmental Engineering, National Chiao-Tung University, Hsinchu, 300, Taiwan, ROC
a r t i c l e i n f o
Article history: Received 14 June 2009
Received in revised form 20 October 2009 Accepted 21 October 2009
Available online 30 October 2009 Keywords: Azo dye GAC Microwave irradiation Toxicity
a b s t r a c t
Treatment of an azo dye, Reactive Black 5 (RB5) by combined electrocoagulation-activated carbon adsorption–microwave regeneration process was evaluated. The toxicity was also monitored by the Vibrio fischeri light inhibition test. GAC of 100 g L−1 sorbed 82% of RB5 (100 mg L−1) within 4 h.
RB5-loaded GAC was not effectively regenerated by microwave irradiation (800 W, 30 s). Electrocoagulation showed high decolorization of RB5 within 8 min at pH0of 7, current density of 277 A m−2, and NaCl of
1 g L−1. However, 61% COD residue remained after treatment and toxicity was high (100% light
inhibi-tion). GAC of 20 g L−1effectively removed COD and toxicity of electrocoagulation-treated solution within 4 h. Microwave irradiation effectively regenerated intermediate-loaded GAC within 30 s at power of 800 W, GAC/water ratio of 20 g L−1, and pH of 7.8. The adsorption capacity of GAC for COD removal
from the electrocoagulation-treated solution did not significantly decrease at the first 7 cycles of adsorp-tion/regeneration. The adsorption capacity of GAC for removal of both A265(benzene-related groups) and
toxicity slightly decreased after the 6th cycle.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Textile wastewater is a major water pollution source in devel-oping countries and often contains high concentrations of unfixed dyes (about 20%). Azo dyes are of great concern because of their widespread use, toxic aromatic amine intermediates, and recalci-trance for aerobic wastewater treatment[1]. Several techniques have been applied to remove dyes from wastewater, including adsorption, chemical oxidation, electrochemical degradation, and advanced oxidation processes. However, their low removal abilities or high costs often limit their application[2].
Electrocoagulation (EC) is a simple and cost-effective approach for wastewater treatment [3,4]. According to recent reports, electrocoagulation can treat dye and actual textile wastewater effectively and rapidly[4–7]. EC usually utilizes aluminum or iron as sacrificial anodes to produce metallic hydroxide coagulants, which can remove dyes from wastewater through precipita-tion and adsorpprecipita-tion [5]. Chloride is often used as supporting electrolyte. The important factors influencing the effectiveness of dye and dyehouse effluent treatment with EC are nature
∗ Corresponding author. Tel.: +886 4 24730022x11799; fax: +886 4 22862587. E-mail address:shchang@csmu.edu.tw(S.-H. Chang).
of dye, electrode material, current density, solution pH, and electrolyte concentration and type [4,7–10]. Emamjomeh and Sivakumar[4]reviewed the fundamental theories of electrocoagu-lation and its application on dye/textile and organic and inorganic pollutants removal. Chatzisymeon et al.[11]utilized electrocoag-ulation to treat textile dyes and actual dyehouse effluents. They reported that the performance of electrocoagulation increased with increasing current intensity and salinity and decreasing solution pH.
During EC, chloride is anodically converted to active chlorine, which can oxidize the dye, leading to decolorization[4,10,12]. Even though EC can effectively decolorize dye wastewater, effluents may contain high concentrations of COD, residual chlorine/hypochlorite, and toxic intermediates. These may pose risks to environmental health. Both the chlorine and intermediates should be removed before discharge. However, the studies on removal of toxic EC-treated dye solution are limited.
Granular activated carbon (GAC) with extended porous sur-face area and sursur-face activity can effectively adsorb various classes of dyes [13]. However, because of its high cost, the exhausted GAC needs to be regenerated for reuse. The common techniques for GAC regeneration include thermal, chemical, and biological regeneration[14]. Their high-energy consumption, long regeneration time, and adsorption capacity loss often limit their application. Microwave (MW)-assisted granular activated carbon
0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.10.088
Fig. 1. Molecular structure of C.I. Reactive Black 5 (RB5, azo dye),max= 597 nm.
(GAC) regeneration recently attracted attention because it has the advantages of short reaction time, energy savings, and high degradation[15]. The factors influencing MW regeneration per-formance are activated carbon property, nature of dye, oxygen supply, MW power, and irradiation time[15–17]. MW regeneration has been used on GAC exhausted by dye [14,17,15], pharma-ceuticals[18], and phenolic compounds[16,19,20]. However, the studies on MW generation of dye intermediate-loaded GAC are limited.
Bioassays provide valuable information to reflect the toxicity of mixed solution on living organisms. Vibrio fischeri, a marine lumi-nescent bacterium, can rapidly and sensitively detect a variety of toxic substrates. The toxic chemicals can interfere with the respi-ratory electron transfer system in V. fischeri and inhibit its light production. V. fischeri bioassay can be used as a warning for biotox-icity, because structural determinants associated with V. fischeri are similar to those associated with mammals and fish[21]. V. fischeri light inhibition test is often used to evaluate the biotoxi-city of effluents. The V. fishcheri light inhibition test has been used to evaluate the toxicity of dye and textile wastewater during dif-ferent treatment processes[11,22–25]. For example, Girotti et al. [24]reported that bioluminescent bacteria can be used to moni-tor industrial wastewater during treatment. Wang et al.[22]found that RB5 intermediates generated during ozonation are toxic to V. fischeri. Chatzisymeon et al.[11]found that the toxicity of textile effluents increased sharply after electrochemical oxidation. In our previous study, we also reported that the toxicity of C.I. Acid Black 1 solution treated by Fenton oxidation is higher than that by Fe0/air treatment[25].
In this study, an azo dye, I.C. Reactive Black 5 (RB5), was selected as the model dye. RB5 is a common di-azo reactive dye used for dying cotton and other cellulose fibers [10]. Several studies have reported that RB5 and its degradative intermedi-ates are biotoxic[22,23,26]. RB5 above 100 mg L−1 is 100% toxic to Daphnia magna [26]. The RB5 intermediates generated dur-ing ozonation and electrochlorination are toxic to V. fischeri [22,23]. The aims of the study are to investigate (1) the fea-sibility of combined electrocoagulation–GAC adsorption to treat RB5, (2) the effectiveness of microwave irradiation on GAC regen-eration, and (3) the toxicity evolution of dye solution during treatments.
2. Materials and methods
2.1. Chemicals
The RB5 (C.I. No. 20505; C26H21Na4N5O19S6, M.W. = 991.8 g mol−1, 55% purity, Fig. 1) was purchased from Sigma–Aldrich and was used as received. Na2S2O3·5H2O was obtained from Osaka (analytic grade, Japan). Sodium chloride was obtained from Panreac (analytic grade, purity 99%, Spain).
2.2. Electrocoagulation experiment
EC experiment was conducted in a 600 mL beaker containing 500 mL of RB5. If not mentioned otherwise, the RB5 concentra-tion used in this study was 100 mg L−1. The anode and cathode material was cast iron. The immerged area of the electrode was 4 cm× 7 cm, and the gap between cathode and anode was 1 cm. The electrodes were rinsed in 30% HNO3and washed with distilled water before the experiment. The EC experiment was conducted under galvanostatic conditions using a regulated DC power supply (GPC-3030D, Taiwan). The solution was continuously mixed by a magnetic stirrer. The initial solution pH was adjusted with diluted sodium hydroxide or diluted hydrochloric acid and measured by pH meter (Cyberscan 510, Taiwan).
The UV–visible spectrum during the RB5 degradation was measured at 200–800 nm using a UV–visible spectrophotome-ter (Shimadzu, UV-mini 1240, Japan). The maximum absorption wavelength for RB5 is 597 nm at pH 7. The concentration of the EC-treated RB5 solution was measured based on the constructed calibration curves at absorption wavelength of 597 nm. A 5-point calibration was carried out. The apparent molar extinction coeffi-cient (ε) of the RB5 observed at 597 nm was 45,500 M−1cm−1. The sample was diluted with distilled water if the absorbance exceeded the range of calibration curve. COD was determined according to standard method for examination of water and wastewater [27].
2.3. GAC adsorption experiments
A commercial coconut-based granular activated carbon (GAC, 4.76 mm diameter, 10 mm length, Hong-Yu, Taiwan) was used as received. The surface area of GAC was measured by BET (Brunauer–Emmett–Teller-method). BET surface area was mea-sured at 77 K on a BET Specific Surface Area Analyzer (Micromeritics Gemini 2370C). N2adsorption–desorption on the surface was used to determine textural properties. The specific surface area of GAC was 763 m2g−1. The pHzpcof GAC was measured by Zeta-Meter 3.0+ (Zeta-Meter Inc., USA). The pHzpcof the GAC was 5.5.
For GAC adsorption experiment, known amounts of GAC ash were added to a 300 mL flask containing 100 mL of either untreated RB5 or EC-treated RB5 solution. The initial solution pH was adjusted by addition of 0.1 M of either NaOH or HCl. The flask was sealed with parafilm and shaken in a water bath (20◦C, 50 rpm). At pre-set time intervals, 3 mL samples were taken, filtered by Whatman GF/A, and analyzed by UV–vis spectrophotometer (Shimadzu, UV-mini 1240, Japan). COD was used to determine the organic content of RB5 and intermediates in the solution. The adsorption capacity of GAC was calculated according to the difference of COD amounts before and after GAC adsorption.
2.4. Microwave regeneration of GAC experiment
Distilled-deionized water was used for all microwave regen-eration cycles. The operating conditions for MW regenregen-eration of exhausted GAC were 20 g of GAC in 1 L of distilled-deionized water. The solution pH was adjusted to 7 by diluted NaOH and HNO3. A domestic MW oven (2450 MHz, 800 W, Panasonic-NEV27, Taiwan) was used. After MW irradiation treatment, the solution was with-drawn, centrifuged, and analyzed for COD, UV–vis spectrum, and biotoxicity. The regenerated GAC was added into EC-treated RB5 solutions (pH 7.8) to evaluate its adsorption capacity.
2.5. V. fischeri light inhibition test
The marine luminescent bacteria V. fischeri (NRRL B-11117), obtained from DSMZ (Germany), was employed to evaluate the
biotoxicity of dye solution during treatments. The cultivation of luminescent bacteria and toxicity evaluation procedure were according to ISO 11348-1 standard protocol. The solution sample was adjusted to pH 7.0± 0.2. V. fischeri was exposed to the solution samples for 5 min as determined by a luminometer at 15◦C. Phe-nol was used as the positive control with EC50ranging from 13 to 26 mg L−1. Toxicity was expressed as the light inhibition ratio and was calculated as follows[28]:
inhibition ratio (%)=I0× fk− If
I0× fk × 100%,
where fkwas the correction factor at t = 5 min, and fk= Ikc/I0c. I0cand Ikcwere the luminescence intensity of the control sample at t = 0 and 5 min, respectively. I0and Ifwere the luminescence intensity of the sample at t = 0 and 5 min, respectively. The bioluminescence intensity of V. fischeri may decrease with exposure time. The cor-recting factor fkis used to correct the luminescence intensity of the test sample at t = 0 min. Therefore, in this study, the inhibition ratio (percentage) was used to represent the biotoxicity of pollutants instead of inhibition percentage of the bioluminescence.
3. Results and discussion
3.1. GAC adsorption of RB5 and MW regeneration of exhausted GAC
Adsorption of RB5 onto GAC was evaluated at solution pH of 5.6. As expected, RB5 removal increased with an increase in the GAC dose (Fig. 2a). GAC of 100 g L−1 removed 82% of COD after 4 h adsorption. Since GAC is costly, the regeneration of exhausted GAC by MW was evaluated and the operating con-ditions were pH of 5.6, GAC/distilled-deionized water ratio of 20 g L−1, and irradiation time of 30 s. After microwave irradia-tion, the regenerated GAC was again added into the 100 mg L−1of untreated RB5 solution to evaluate its adsorption capacity.Fig. 2b indicates that the MW-regenerated GAC only removed 36% COD from the untreated RB5 solution. It is possible that the short irradiation time could not effectively regenerate the exhausted GAC.
3.2. RB5 treatment by electrocoagulation
Treatment of RB5 solution by EC was investigated.Fig. 3a depicts the influences of current density (CD) on RB5 decolorization at the operating conditions: initial solution pH of 7 and NaCl of 1 g L−1. An increase in current density enhanced decolorization. This is because the high CD enhanced the generation of iron hydroxide coagulant (Eqs.(1) and (2))[8]and active chlorine (Eqs.(3) and (4)) [10,29]. The active chlorine also can enhance the production of iron hydroxide coagulants (Eq.(5)) and directly decolorize dye (Eq.(6))
[4,10,12], resulting in enhancement of decolorization
anode : Fe(s)→ Fe2++ 2e− (1)
solution : Fe2++ O2+ H2O→ Fe(OH)3+ H+ (2)
anode : 2Cl−→ Cl2+ 2e− (3)
solution : Cl2+ H2O→ HOCl + HCl (4)
solution : HOCl+ 2Fe2+→ 2Fe3++ Cl−+ OH− (5) solution : dye+ Cl2→ oxidizeddye + 2Cl− (6) Energy consumption is an important concern for dye treatment. In this study, the energy consumptions for RB5 decolorization at CD of 277 and 544 A m−2were 0.017 and 0.034 kWh g−1, respectively. Therefore, CD of 277 A m−2was selected for sequential study.
The effect of NaCl on RB5 decolorization was evaluated. An increase in the NaCl dose enhanced decolorization. A high chloride concentration can enhance decolorization because (1) it increases solution conductivity, thus reducing the energy consumption[10], and (2) it increases the active chlorine production[10,12,29]. The effect of initial solution pH on decolorization by EC was also eval-uated. The operating conditions were CD of 277 A m−2and NaCl of 1 g L−1.Fig. 3c indicates that the initial solution pH did not obviously affect RB5 decolorization. On the other hand, solution pH can influ-ence (1) the oxidation kinetics of Fe2+to Fe3+[10], (2) the surface charge of coagulating particles[8,10], (3) the ratio of hypochlor-ous acid (E0= 1.49 V) and hypochlorite ion (E0= 0.94 V) in solution. The phenomenon that there was no difference in decolorization between pH 4 and 9 was also was observed by S¸engil and Özacar [10].
The changes of solution pH during electrocoagulation at differ-ent initial solution pH (5, 7, and 9) were monitored.Fig. 3d indicates the solution pH increased with increased time. The pH of these three solutions reached 9.5–9.6 at 6 min, and after that the solution pH changed slightly. The solution pH most probably increased for all studied initial pH’s as a consequence of cathodic OH−formation (Eq.(7))[7].
cathode : 2H2O(aq)+ 2e−→ H2(g)+ 2OH−(aq) (7) Fig. 3e shows the influence of RB5 concentrations on decoloriza-tion. The EC conditions were pH of 7, CD of 277 A m−2, and NaCl of 1 g L−1. The increase in RB5 concentration reduced the decol-orization. For example, EC completely decolorized RB5 of 50 mg L−1 within 4 min. In contrast, when RB 5 concentration increased to 200 mg L−1, 91% color removal was obtained after 10 min. It is probable that the adsorption capacity of metallic hydroxide flocs was not enough for dye adsorption at RB5 of 200 mg L−1 [30]. COD removal and toxicity are also important parameters for efflu-ent quality. Fig. 3f indicates that even though 99% of RB5 was decolorized at 8 min, only 39% of COD removal was achieved.
Fig. 2. GAC adsorption of Reactive Black 5: initial and after regenerated by microwave irradiation; (a) GAC adsorption of RB5 (100 mg L−1), pH0of 5.6, adsorption time of 4 h,
Fig. 3. Electrocoagulation (EC) treatment of Reactive Black 5. If not mentioned, RB5 of 100 mg L−1, pH0of 7, current density of 277 A m−2, and NaCl of 1 g L−1; I: color removal
influenced by (a) applied current density, (b) NaCl, and (c) initial solution pH. (d) Changes of solution pH during EC. (e) Color removal influenced by RB5 concentration; II: (f) color and COD removal by EC; III: (g) UV–vis spectrum change during EC; IV: (h) Vibrio fischeri light inhibition of EC-treated NaCl-only and RB5 sets before and after 20 g L−1
Na2S2O3addition, EC treatment time of 8 min.
COD removal did not increase obviously with a prolongation of the EC time. Also, EC and active chlorine did not further remove RB5 intermediates. When operating conditions were current den-sity of 277 A m−2, NaCl of 1 g L−1, initial solution pH of 7, and electrocoagulation time of 10 min, the weight loss of anode was 0.091 g and 0.316 kg of solids produced per cubic meter of treated effluent resulting from the metallic hydroxides formation and dye removal.
The UV–vis spectrum change of RB5 solution during EC was also investigated. For RB5, the adsorption peaks at wavelength 597 and 310 nm are attributed to azo bond and naphthalene components, respectively[8].Fig. 3g illustrates that the UV–vis band absorp-tion at 320–700 nm completely diminished after 8 min. However, there still was a UV-band absorption peak at about 265 nm. Libra et al.[31] reported that the RB5 anchor group metabolite p-aminobenzene-2-hydroxyethylsulfonic acid (p-ABHES) has an
absorption peak at 265 nm. The A265 of the electrocoagulation-treated RB5 solution should be attributed to benzene-related intermediates. Chemical identification was not conducted in this study to identify the intermediates. Prolongation of EC time to 10 min did not further diminish the UV-bands at 200–320 nm. This implies that EC and active chlorine did not effectively remove the A266-related intermediates.
EC with chloride as supporting electrolyte can generate toxic products such as chlorine/hypochlorite and dye-byproducts. The V. fischeri light inhibition test was used to monitor the toxic-ity changes in solution. The NaCl-only set (NaCl of 1 g L−1, CD of 277 A m−2, pH0of 7 and 8 min treatment) was evaluated before and after Na2S2O3addition. First, the background toxicity of Na2S2O3 to V. fischeri was evaluated.Fig. 3h indicates that 20 g L−1only had a slight influence on V. fishcheri (3% light inhibition). The NaCl-only set exhibited high toxicity (100% light inhibition) before Na2S2O3 addition. Na2S2O3 of 20 g L−1 effectively reduced the toxicity of chlorine/hypochlorite generated during EC. Therefore, 20 g L−1of Na2S2O3was used to remove the toxicity of chlorine/hypochlorite in the subsequent study. The toxicity of EC-treated RB5 solution (ECTS, RB5 of 100 mg L−1, NaCl of 1 g L−1, CA of 277 A m−2, pH0of 7 and 8 min treatment) was also investigated.Fig. 3h shows that tox-icity of the ECTS was high (100% light inhibition) before Na2S2O3 addition. After addition of Na2S2O3, the toxicity of ECTS was still high (97% light inhibition). This suggests that RB5 intermediates were also a major toxicity source of the ECTS. Even though EC rapidly decolorized RB5, only 39% COD removal was achieved and the ECTS was highly toxic. The COD residue and toxicity should be mitigated before discharge.
3.3. GAC adsorption of EC-treated RB5 solution
The effect of GAC addition on the COD removal of ECTS was investigated (Fig. 4a). GAC adsorbed most RB5 intermediates from ECTS, and 20 g L−1of GAC significantly decreased COD/COD0from 61% to 7% after 4 h adsorption. However, an increase in the GAC
dose to 40 g L−1 did not further reduce COD. This suggests that a slight amount of intermediates present in the ECST was not effec-tively removed by GAC.Fig. 4b shows the spectrum change of ECTS by GAC adsorption. The A266decreased from 2.61 to 0.04 by 20 g L−1 of GAC after 4 h. This suggests that GAC effectively sorbed aromatic RB5 intermediates.
The effects of GAC addition on toxicity mitigation of the NaCl-set and ECST were also assessed.Fig. 4c illustrates that 20 g L−1of GAC completely removed the toxicity of the NaCl-only set (<1%) after 4 h contact. This is because GAC can act as catalyst to chemically reduce strong oxidants to nontoxic products, e.g. chlorine/hypochlorite can be reduced to chloride[32]. The influence of GAC on toxicity mitigation of ECST was also evaluated.Fig. 4d illustrates toxic-ity evolution of ECST by GAC before and after Na2S2O3 addition. GAC effectively reduced the toxicity of ECST within 4 h, whether Na2S2O3was added or not. This implies that GAC not only catalyt-ically reduced chlorine/hypochlorite to chloride but also adsorbed toxic RB5 intermediates.Fig. 4b and d indicates that the trends of A266and the intermediates toxicity were similar. This implies that the toxicity of RB5 intermediates may relate to A266-related byproducts in the ECTS.
Because there are strong oxidants in the ECST, the effect of MW irradiation on COD removal of ECST was assessed.Fig. 5a and b indicates that the microwave irradiation (800 W, 30 s) had only a slight effect on COD removal and spectrum change of ECTS. This suggests that microwave irradiation could not enhance the removal of RB5 intermediates while strong oxidants are present in the ECTS. To conclude, GAC effectively removed COD and toxicity from ECTS. However, because GAC is costly, regeneration of exhausted GAC is needed for economical feasibility.
3.4. Microwave regeneration of spent GAC
The adsorption capacity of MW regenerated GAC was used to evaluate the effectiveness of MW regeneration. First, the RB5 intermediate-exhausted GAC was irradiated in distilled-deionized
Fig. 4. GAC adsorption of electrocoagulation (EC)-treated RB 5 solution. EC conditions were RB5 of 100 mg L−1, current density of 277 A m−2, NaCl of 1 g L−1, pH0of 7 and
8 min treatment. If not mentioned, experimental conditions for GAC adsorption were GAC of 20 g L−1and pH0of 7.8. (I): (a) effect of GAC dose, (b) UV–vis spectrum change;
Fig. 5. Effect of microwave irradiation on electrocoagulation-treated RB 5 solution. EC conditions were RB5 of 100 mg L−1, current density of 277 A m−2, NaCl of 1 g L−1, pH0
of 7 and 8 min treatment. Microwave power of 800 W and irradiation time of 30 s: (a) COD removal and (b) UV–vis spectrum change.
water by microwave. Then, the regenerated GAC was reused for adsorption of ECST. The effect of MW irradiation time on MW regeneration was evaluated. The operating conditions were MW power of 800 W, GAC/water ratio of 20 g L−1, and pH0of 7.8.Fig. 6a indicates that an increase in MW irradiation time enhanced GAC regeneration. When 30 s of MW regeneration was applied, the adsorption capacity of the regenerated GAC was similar to that of virgin GAC (Fig. 4a) and decreased the remaining COD percent-age (COD/COD0) of ECTS from 61% to 7% within 4 h adsorption. The effect of GAC/water ratio on GAC regeneration was also inves-tigated.Fig. 6b shows that when the GAC/water ratio increased from 1:50 to 1:25, the adsorption capacity of regenerated GAC slightly decreased. The effect of the pH of the water on GAC regen-eration was assessed.Fig. 6c indicates that the initial water pH, ranging from 5.2 to 9.2, did not significantly affect GAC regen-eration. After MW regeneration, COD and toxicity of the water were 14 mg L−1 and 11% light inhibition, respectively, and it can be discharged directly or channeled to a wastewater treatment plant.
Because of its dielectric property, GAC can absorb microwave energy and convert it to heat due to space-charge polarization[33]. Several studies have reported that mechanisms responsible for MW regeneration of GAC are catalysis[15,17,20], free radicals[34], and high-energy plasma[17,35]. Zhang et al.[15]suggested that the uneven surface of GAC can sorb microwave energy and form a large number of hot spots that can oxidize organic pollutants. Quan et al. [34]used salicylic acid as molecular probe to detect hydroxyl radi-cals. They reported hydroxyl radicals can be generated by activated carbon powder under microwave irradiation. Dawson et al.[35] observed the plasma phenomenon, which is induced by microwave irradiation in a granular activated carbon fluidized bed.
In this study, only a short MW time was needed (30 s) to regenerate GAC. This is because the COD loaded on GAC was low (3.36 mg g−1). Because the water temperature only increased from 25◦C to 59◦C after GAC regeneration, the loss of RB5 intermediates due to vaporization to air seems to be insignificant. Besides, the short irradiation time benefited the preservation of the microp-orosity of GAC despite the heat during GAC regeneration[16,36]. In
Fig. 6. Adsorption capacity of microwave-regenerated GAC for electrocoagulation-treated RB5 solution. Adsorption conditions were 20 g L−1of GAC and pH0of 7.8. If not
mentioned, the experimental conditions for GAC regeneration were 800 W, pH0of 7.8, and GAC/water ratio of 20 g L−1; (a) effect of MW irradiation time, (b) effects of
Fig. 7. Influences of adsorption/regeneration cycles on GAC adsorption capacity of electrocoagulation-treated solution. Adsorption conditions were 20 g L−1of GAC, pH0of
7.8, and adsorption time of 4 h. MW regeneration conditions were 800 W, pH0of 7, GAC/water ratio of 20 g L−1, and irradiation time of 30 s; (a) COD/COD0and toxicity and
(b) spectrum change of EC-treated RB5 solution after adsorption.
this study, we only evaluated the effectiveness of microwave irra-diation on regeneration of the RB5 intermediate-exhausted GAC. The surface of the treated GAC was not analyzed. MW irradiation may change the physicochemical surface properties of GAC, includ-ing the amount of catalyst surface area, total pore volume, pore size distribution, and surface chemistry[15–18]. The changes of physic-ochemical surface properties of GAC will influence the adsorption efficiency and rate. Further study on the surface of the treated GAC (exhausted and regenerated) is suggested.
Oxygen acts as an electron acceptor for organic oxidation during MW regeneration[17,20,34]. In this study, no oxygen was provided except for that dissolved in the water (DO of 6.4 mg L−1). Compared to solution at GAC/water ratio of 1:50, the solution at GAC/water ratio of 1:25 contained less oxygen, which may have been insuffi-cient for organic oxidation during MW regeneration of GAC. After MW regeneration of GAC, the water contained 0.014 mg L−1COD; this is because the microwave dielectric heating enhanced desorp-tion of RB5 intermediates from GAC[37,38]. However, the toxicity of water was slight (light inhibition of 11%).
3.5. Effect of cycles of adsorption/regeneration on GAC adsorption capacity
The effect of adsorption/regeneration cycles on GAC regenera-tion was investigated, and the adsorpregenera-tion capacity of GAC for ECTS was used as the index. The operating conditions for MW regener-ation were GAC/water of 20 g L−1, pH0of 7.8, and irradiation time of 30 s. As can be seen inFig. 7a, the adsorption capacity of GAC for COD removal from ECTS did not significantly decrease at the first 7 cycles of adsorption/regeneration. However, the adsorption capac-ity of GAC for removal of both toxiccapac-ity (Fig. 7a) and A266(Fig. 7b) slightly decreased after the 6th cycle.
Two mechanisms may be responsible for the decrease in the adsorption capacity of GAC for toxicity and A266removal: (1) The GAC loss during adsorption/MW regeneration cycles[16]. In this study, the GAC mass loss after 7 cycles of adsorption/regeneration was 3.5% compared to the virgin GAC. (2) Reduction of adsorption ability of GAC by MW irradiation[36]. In this study, an experiment was also conducted to investigate effects of MW irradiation-only on GAC. GAC was subjected to 30-s MW irradiation treatment for 7 cycles. Then, the MW-irradiated GAC was used for adsorption of ECTS. Spectrum result indicated that A266= 0.108 remained in the ECST after MW-irradiated GAC adsorption (data not shown).
Even though the regenerated GAC maintained its COD removal capacities after 7 cycles of adsorption/MW regeneration, the removal ability of the GAC for toxicity and A266slightly decreased. A small amount of new GAC is suggested to replenish the GAC abil-ity for toxicabil-ity and A266-related intermediate removal after the 6th cycle of adsorption/MW regeneration.
4. Conclusions
Even though EC rapidly decolorized RB5 solution at 1 g L−1 of NaCl, current density of 277 A m−2, and pH0 of 7, the EC-treated RB5 solution still contained 61% of COD and was highly toxic (100% light inhibition). Both the strong oxidants and inter-mediates generated during EC were responsible for solution toxicity. The addition of 100 g L−1 GAC removed 82% of RB5 after 4 h adsorption. However, the adsorption capacity of regen-erated GAC for RB5 removal obviously decreased after 30-s microwave irradiation (800 W). GAC of 20 g L−1effectively removed COD and toxicity of ECTS after 4 h adsorption. RB intermediate-loaded GAC was effectively regenerated by MW irradiation at MW power of 800 W, GAC/water of 20 g L−1, and irradiation time of 30 s. Both irradiation time and GAC/water ratio were important factors for MW regeneration of exhausted GAC. The adsorption capacity of GAC for COD removal from EC-treated solution did not significantly decrease at the first 7 cycles of adsorption/regeneration. The adsorption capacity of GAC for removal of both A266and toxicity slightly decreased after the 6th cycle.
Acknowledgement
The authors gratefully acknowledge the National Science Coun-cil of the ROC (Taiwan) for financial support under Project No. NSC 97-2622-E-040-001-CC3.
References
[1] S.W. Oh, M.N. Kang, C.W. Cho, M.W. Lee, Detection of carcinogenic amines from dyestuffs or dyed substrates, Dyes Pigments 33 (1997) 119–135.
[2] A. Anjaneyulu, N.S. Chary, D.S.S. Raj, Decolourization of industrial effluents-available methods and emerging technologies—a review, Rev. Environ. Sci. Biotechnol. 4 (2005) 245–273.
[3] M. Kobya, M. Bayramogh, M. Eyvaz, Techno-economical evaluation of electro-coagulation for the textile wastewater using different electrode connections, J. Hazard. Mater. 148 (2007) 311–318.
[4] M.M. Emamjomeh, M. Sivakumar, Review of pollutants removed by electro-coagulation and electroelectro-coagulation/flotation processes, J. Environ. Manage. 90 (2009) 1663–1679.
[5] M. Kobya, O.T. Can, M. Bayramoglu, Treatment of textile wastewaters by elec-trocoagulation using iron and aluminum electrodes, J. Hazard. Mater. 100 (2003) 163–178.
[6] A. Alinsafi, M. Khemis, M.N. Pons, J.P. Leclerc, A. Yaacoubi, A. Benhammou, A. Nejmeddine, Electro-coagulation of reactive textile dyes and textile wastewa-ter, Chem. Eng. Process. 44 (2005) 461–470.
[7] C.A. Martínez-Huitle, E. Brillas, Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods: a general review, Appl. Catal. B 87 (2009) 105–145.
[8] S. Song, Z. He, J. Qiu, L. Xu, J. Chen, Ozone assisted electrocoagulation for decolorization of C.I. Reactive Black 5 in aqueous solution: an investiga-tion of the effect of operainvestiga-tional parameters, Sep. Purif. Technol. 55 (2007) 238–245.
[9] H.A. Moreno-Casillas, D.L. Cocke, J.A.G. Gomes, P. Morkovsky, J.R. Parga, E. Peter-son, Electrocoagulation mechanism for COD removal, Sep. Purif. Technol. 56 (2007) 204–211.
[10] ˙I.A. S¸engil, M. Özacar, The decolorization of C.I. Reactive Black 5 in aqueous solution by electrocoagulation using sacrificial iron electrodes, J. Hazard. Mater. 161 (2009) 1369–1376.
[11] E. Chatzisymeon, N.P. Xekoukoulotakis, A. Coz, N. Kalogerakis, D. Mantzavinos, Electrochemical treatment of textile dyes and dyehouse effluents, J. Hazard. Mater. B137 (2006) 998–1007.
[12] A.K. Golder, N. Hridaya, A.N. Samanta, S. Ray, Electrocoagulation of methy-lene blue and eosin yellowish using mild steel electrodes, J. Hazard. Mater. 127 (2005) 134–140.
[13] V.K. Gupta, Suhas, Application of low-cost adsorbents for dye removal—a review, J. Environ. Manage. 90 (2009) 2313–2342.
[14] M.K. Purkait, A. Maiti, S. DasGupta, S. De, Removal of congo red using activated carbon and its regeneration, J. Hazard. Mater. 145 (2007) 287–295.
[15] Z. Zhang, Y. Shan, J. Wang, H. Ling, S. Zang, W. Gao, Z. Zhao, H. Zhang, Investigation on the rapid degradation of congo red catalyzed by activated carbon powder under microwave irradiation, J. Hazard. Mater. 147 (2007) 325–333.
[16] X. Liu, X. Quan, L. Bo, S. Chen, Y. Zhao, Simultaneous pentachlorophenol decom-position and granular activated carbon regeneration assisted by microwave irradiation, Carbon 42 (2004) 415–422.
[17] Y. Zhang, X. Quan, S. Chen, Y. Zhao, F. Yang, Microwave assisted catalytic wet air oxidation of H-acid in aqueous solution under the atmospheric pressure using activated carbon as catalyst, J. Hazard. Mater. 137 (2006) 534–540.
[18] C.O. Ania, J.B. Parra, J.A. Menéndez, J.J. Pis, Microwave-assisted regenera-tion of activated carbons loaded with pharmaceuticals, Water Res. 41 (2007) 3299–3306.
[19] I. Polaert, L. Estel, A. Ledoux, Microwave-assisted remediation of phenol wastewater on activated charcoal, Chem. Eng. Sci. 60 (2005) 6354–6359. [20] L. Bo, X. Quan, S. Chen, H. Zhao, Y. Zhao, Zhao degradation of p-nitrophenol in
aqueous solution by microwave assisted oxidation process through a granular activated carbon fixed bed, Water Res. 40 (2006) 3061–3068.
[21] H.S. Rosenkranz, J. Pangrekar, G. Klopman, Similarities in the mechanisms of antibacterial activity (MicrotoxTM assay) and toxicity to vertebrates,
ALTA-Altern, Lab. Anim. 21 (1993) 489–500.
[22] C. Wang, A. Yediler, D. Lienert, Z. Wang, A. Kettrup, Ozonation of an azo dye C.I. Remazol Black 5 and toxicological assessment of its oxidation products, Chemosphere 52 (2003) 1225–1232.
[23] K.S. Wang, H.Y. Chen, L.C. Huang, Y.C. Su, S.H. Chang, Degradation of Reactive Black 5 using combined electrochemical degradation-solar-light/immobilized TiO2film process and toxicity evaluation, Chemosphere 72 (2008) 299–305.
[24] S. Girotti, E.N. Ferri, M.G. Fumo, E. Maiolini, Monitoring of environmental pol-lutants by bioluminescent bacteria, Anal. Chim. Acta 608 (2008) 2–29. [25] S.H. Chang, S.H. Chuang, H.C. Li, H.H. Liang, L.C. Huang, Comparative study on
the degradation of I.C. Remazol Brilliant Blue R and I.C. Acid Black 1 by Fenton oxidation and Fe0/air process and toxicity evaluation, J. Hazard. Mater. 166
(2009) 1279–1288.
[26] S. Meric¸, D. Kaptan, T. Ölmez, Color and COD removal from wastewater contain-ing Reactive Black 5 uscontain-ing Fenton’s oxidation process, Chemosphere 54 (2004) 435–441.
[27] American Public Health Association, Standard Method for the Examination of Water and Wastewater, 5520 Chemical Oxygen Demand, 1995, pp. 5-12– 5-16.
[28] ISO Water Quality-Determination of the Inhibitory Effect of Water Samples on the Light Emission of Vibrio fischeri (Luminescent Bacteria Test). ISO 11348-1, International Standard Organization, Geneva, Switzerland, 1998.
[29] C.L. Yang, J. McGarrahan, Electrochemical coagulation for textile effluent decol-orization, J. Hazard. Mater. 127 (2005) 40–47.
[30] N. Daneshvar, H. Ashassi Sorkhabi, M.B. Kasiri, Decolorization of dye solution containing Acid Red 14 by electrocoagulation with a comparative investigation of different electrode connections, J. Hazard. Mater. 112 (2004) 55–62. [31] J.A. Libra, M. Borchert, L. Vigelahn, T. Strom, Two stage biological treatment of
a diazo reactive textile dye and the fate of the dye metabolites, Chemosphere 56 (2004) 167–180.
[32] W.J. Huang, H.H. Yeh, Reaction of chlorine with NOM adsorbed on powdered activated carbon, Water Res. 33 (1999) 65–72.
[33] A. Zlotorzynski, The application of microwave-radiation to analytical and envi-ronmental chemistry, Crit. Rev. Anal. Chem. 25 (1995) 43–76.
[34] X. Quan, Y. Zhang, S. Chen, Y. Zhao, F. Yang, Generation of hydroxyl radical in aqueous solution by microwave energy using activated carbon as catalyst and its potential in removal of persistent organic substances, J. Mol. Catal. A: Chem. 263 (2007) 216–222.
[35] E.A. Dawson, G.M.B. Parkes, P.A. Barnes, G. Bond, R. Mao, The generation of microwave-induced plasma in granular active carbons under fluidised bed conditions, Carbon 46 (2008) 220–228.
[36] C.O. Ania, J.B. Parra, J.A. Menéndez, J.J. Pis, Effect of microwave and conven-tional regeneration on the microporous and mesoporous network and on the adsorptive capacity of activated carbons, Microporous Mesoporous Mater. 85 (2005) 7–15.
[37] D. Bathen, Physical waves in adsorption technology—an overview, Sep. Purif. Technol. 33 (2003) 163–177.
[38] F.K. Yuen, B.H. Hameed, Recent developments in the preparation and regenera-tion of activated carbons by microwaves, Adv. Colloid Interface Sci. 149 (2009) 19–27.
