Reduction and Removal of Aqueous Cr(VI) by Glow Discharge Plasma

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Reduction and Removal of Aqueous Cr(VI) by Glow Discharge Plasma at the GasSolution Interface Zhigang Ke, Qing Huang,* Hong Zhang, and Zengliang Yu Key Laboratory of Ion Beam Bio-engineering, Institute of Biotechnology and Agriculture Engineering, Hefei Institutes of Physical Science, Chinese Academy of Sciences, China

bS Supporting Information ABSTRACT: Aqueous chromium(VI) reduction and removal induced by glow discharge taking place at the gassolution interface in an argon atmosphere was studied. The effect of initial pH and hydroxyl radical scavenger (ethanol) on the reduction efficiency was examined. High reduction efficiency was obtained when initial pH e 2.0 or g 8.0. In particular, addition of ethanol into the solution substantially increased the reduction efficiency and facilitated chromium removal from the solution in the form of sediment after discharge. The optimum pH values for Cr(VI) removal were within 6.07.0. Fourier transform-infrared (FTIR) spectroscopy and X-ray diffraction (XRD) analysis confirmed that the main constituent of the sediment is chromium hydroxide.

’ INTRODUCTION Large volumes of Cr-containing effluents are produced from industries such as metal finishing, leather tanning, dyeing, electroplating, and textile factories.1 Cr exists mainly in two stable oxidation states in natural systems, i.e., hexavalent(VI) and trivalent(III) and they have distinct properties. Cr(III) is relatively harmless and has lower solubility in water. In contrast, Cr(VI) imposes severe environmental threat and causes healthy problems due to its toxicity, mobility, mutagenicity, and carcinogenicity.1 So the reduction of Cr(VI) to Cr(III) and removal of the heavy metal ions are deemed as a key process for treatment of Cr(VI)-containing wastewater.2,3 To date, many methods for purification of Cr(VI)-containing effluents have been proposed, which include adsorption, ion exchange, membrane separation, and chemical reduction/precipitation approaches.4,5 Among the conventional techniques for Cr(VI) treatment, chemical reduction/precipitation is the most widely applied method, in which Cr(VI) is at first reduced to Cr(III) by a reducer such as ferrous sulfate, sodium sulfite, or metabisulfite, and then Cr(III) can be precipitated out of solution in the form of chromium hydroxide (Cr(OH)3).68 Although this method is effective, excess chemicals are required for the sufficient Cr(VI) reduction, and this would lead to a lot of sludges or harmful gases which can cause secondary pollution. Another r 2011 American Chemical Society

widely used method is adsorption. The major challenge of this method is the recovery of adsorption columns. Recently, some researchers have applied the electrochemical method for Cr(VI) removal,9 but the produced Fe3+ arising from dissolution of iron electrodes would coprecipitate with Cr(OH)3 in the form of ferric hydroxide (Fe(OH)3) and so large volumes of sludges are produced as well. Studies concerning Cr(VI) reduction induced by γ-irradiation10 or discharge plasma taking place in solution11 were reported several years ago. In both cases Cr(VI) can be effectively reduced in proper circumstances and hydroxyl radical scavengers can promote the reduction efficiency. But no precipitation or removal of Cr was reported in these cases. Over the past few years, electrical discharge has been applied successfully on many aspects, such as analysis of heavy metals in solution,12 synthesis of nanoparticles,1315 and removal of organic pollutants.16 It is especially useful in disposing pollutants, especially organic pollutants from wastewater, because it is an environment-friendly method and can treat wastewater effectively.17,18 However, to the best our knowledge, few studies Received: December 31, 2010 Accepted: August 2, 2011 Revised: June 23, 2011 Published: August 02, 2011 7841

dx.doi.org/10.1021/es201680m | Environ. Sci. Technol. 2011, 45, 7841–7847

Environmental Science & Technology

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have been reported on the simultaneous reduction and removal of aqueous Cr(VI) by means of electrical discharge from aqueous solutions. The present study seeks to reduce and remove aqueous Cr(VI) by utilizing glow discharge plasma which takes place at the gassolution interface in an argon atmosphere. The Cr(VI) reduction and/or removal efficiency dependent on initial pH and hydroxyl radical scavenger were investigated. In particular, the different effect of pH on Cr(VI) reduction and the precipitation of Cr in the presence of ethanol were scrutinized. Characterization of the sediment was achieved via FTIR and XRD analysis and the underlying mechanisms are discussed.

’ MATERIALS AND METHODS Reagents. A stock solution of Cr(VI) (400 mg 3 L1) was

prepared by dissolving an appropriate quantity of potassium dichromate (K2Cr2O7) in distilled water. The stock solution was further diluted with distilled water to prepare desired concentrations. Ten percent (V:V) H2SO4 and 1 M NaOH were used for adjustment of the initial pH value. All chemicals were commercially available in analytical grade and used without further purification. Discharge Apparatus. The main constitution of the experimental setup is similar to the one reported previously19 but with some technical improvement for facilitating discharge, as shown in Figure S1 in the Supporting Information (SI). Briefly, a stainless steel needle and a stainless steel plate were used as the anode and cathode, respectively. The anode was placed ∼35 mm above the solution surface and the cathode was submerged in the solution. Both electrodes were connected to a DC power supply. When the voltage exceeded the threshold value, discharge plasma formed at the gassolution interface and then kept steady by lowering the voltage. Before discharge argon gas was introduced into the reactor to remove air. When the discharge was steady, the current was about 40 mA and the voltage was about 1300 V ((10%). During the discharge treatment, the solution was magnetically stirred and a small aliquot was drawn periodically for the Cr(VI) and total-Cr-ion analysis. For the experiments with additives, certain percentages of additives, such as dimethyl sulphoxide (DMSO), methanol, ethanol, isopropanol, and 2-butyl alcohol were added into the solutions initially. Measurement of the Concentration of Cr(VI) and Total Cr. The concentration of Cr(VI) remaining in the solution was determined colorimetrically using the diphenylcarbazide (DPC) method introduced by Clesceri et al. with a detection limit of 4 μg 3 L1.20 In a 25-mL volumetric flask, 0.25 mL sample was added and then the mixture was titrated to the tick with distilled water. Then 1.25 mL of 10% (V:V) H2SO4 and 1.25 mL of freshly prepared 2.5 g 3 L1 DPC in acetone were added to the volumetric flask. The mixture was vortexed sufficiently and let to stand for 15 min for full color development. The absorbance at wavelength of 540 nm was examined by a UVvis spectrometer (SHIMADZH UV-2550). The measurements of the total Cr and Fe concentration were made using an atomic absorption spectroscopy (AAS, AA800, Perkin-Elmer) with a detection limit of 0.0010.1 mg 3 L1. Measurement of the Concentration of Hydrogen Peroxide. The concentration of H2O2 in the discharge treated water samples was determined spectrophotometrically at 410 nm, after mixing with titanium sulfate in acidic condition.21

Figure 1. Effect of initial pH on Cr(VI) reduction induced by glow discharge plasma taking place at the gassolution interface in an argon atmosphere. (The data are the means of two separate runs. Inset is the change of solution pH before and after discharge treatment, [Cr(VI)] = 40 mg 3 L1, volume = 20 mL, temperature = room temperature, discharge time =10 min.).

Measurement of Nitrate. Measurement of nitrate was performed by ion chromatography (ICS-3000, Dionex). Characterization of the Sediment by FTIR and XRD. The sediment formed during discharge was washed with distilled water three times. After heating at 55 °C and/or 400 °C for 1 h, the FTIR spectra of the samples prepared as KBr pellets were recorded by a FTIR spectrometer (Bruker, ALPHA-T). The XRD analysis of the samples was carried out with a Philips X’Pert diffractometer. Elemental Analysis of the Sediment. Elemental analysis of the sediment was performed by an elementar analyzer (Vario EL III, Elementar).

’ RESULTS Effect of Initial pH on Cr(VI) Reduction. In this experiment, 20 mL of K2Cr2O7 solutions at concentration of 40 mg 3 L1 Cr(VI) with different initial pH values from 2.0 to 11.0 were placed into the reactor for discharge treatment. A 0.5-mL sample was drawn periodically from the solution and then analyzed for Cr(VI) concentration. The results of pH effect on Cr(VI) reduction are shown in Figure 1 (the data are the means of two separate runs). It was observed that the reduction efficiency was lower within the initial pH range 3.06.0 compared to pH e 2.0 or g 8.0. Nearly all Cr(VI) in solution with initial pH = 2.0 was reduced within 2 min of discharge treatment but this value is only 21% when initial pH = 3.0. Surprisingly, a high Cr(VI) reduction efficiency was obtained in alkaline solution, which was distinct from most previously reported cases.11,22,23 Almost 93.5% Cr(VI) with initial pH = 8.0 were reduced at the end of discharge treatment for 10 min, which was as high as the condition with initial pH = 2.0. The inset of Figure 1 shows the change of solution pH before and after discharge treatment. Control experiments with variable initial pH values from 2.0 to 11.0 were also conducted in the absence of plasma discharge. The results, as shown in Figure S2 in SI, indicate that only pH adjustments cannot influence the concentration of Cr(VI) in solution. Effect of Ethanol on Cr(VI) Reduction. To investigate the mechanism of Cr(VI) reduction, hydroxyl radical scavenger was added into the solution initially followed by discharge treatment. 7842

dx.doi.org/10.1021/es201680m |Environ. Sci. Technol. 2011, 45, 7841–7847


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Figure 2. Effect of ethanol on Cr(VI) reduction induced by glow discharge plasma taking place at the gassolution interface in an argon atmosphere. ([Cr(VI)] = 40 mg 3 L1, volume = 20 mL, both initial pH = 5.9, temperature = room temperature, discharge time = 10 min).

Figure 4. FTIR spectra of the sediment after heating at 55 (a) and 400 °C (b) for 1 h with the comparison of reference spectra of Cr(OH)3 and Cr2O3.

Figure 3. Effect of initial pH on Cr(VI) removal in the presence of 1% (V:V) ethanol. ([Cr(VI)] = 80 mg 3 L1, volume = 25 mL, temperature = room temperature, discharge time = 15 min.).

In this case, the additive ethanol was selected as the hydroxyl radical scavenger. The stimulative effect of ethanol on Cr(VI) reduction is obvious, as shown in Figure 2. Higher percentage (>98%) of Cr(VI) reduction was obtained in the presence of 1% (V:V) ethanol compared to the case without additive (about 80%) after discharge treatment for 10 min. Another interesting phenomenon with addition of ethanol was that some sediment was precipitated out of the residual solution after discharge treatment. After full precipitation of the sediment, the color of the solution changed from yellow to colorless, as seen in Figure S3 in SI. A fraction of the supernatant was drawn and filtrated with 0.22-μm cellulose acetate membrane for determination of the total Cr by AAS. It was found that the total Cr content in the supernatant decreased dramatically. This indicates that the presence of ethanol promoted not only the Cr(VI) reduction but also the removal of total Cr from the solution. Furthermore, the effect of ethanol concentration on Cr removal was investigated through batch experiments carried out by varying ethanol concentration from 0.1% to 10% (V:V) with fixed volume (25 mL) and initial pH (5.9) of K2Cr2O7 solution at concentration of 80 mg 3 L1 Cr(VI). The results are plotted in Figure S4 in SI. With increase in ethanol concentration, the

Figure 5. XRD pattern of the sediment after heating at 55 and/or 400 °C for 1 h, respectively.

fraction of Cr(VI) removed from solution increased and the maximum removal efficiency (93%) was obtained when the ethanol percentage achieved 2% (V:V). Effect of Initial pH on the Removal of Cr(VI) in the Presence of Ethanol. Besides the effect on reduction, pH is also an important factor on the removal of Cr(VI).24,25 The pH effect on Cr(VI) removal induced by glow discharge plasma in argon in the presence of ethanol was investigated for initial pH values ranging from 2.0 to 9.0. After full precipitation of the sediment out of the discharge treated solution, the concentration 7843



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Table 1. Assignment of the FTIR Bands of the Sediment after Heating at 55 °C for 1 h wavenumber (cm1)

assignment

535 850

CrO lattice vibration out-of-plane bending vibrations of carbonate

26 26

1378

coordinated or adsorbed carbonate

26

1495

coordinated or adsorbed carbonate

26

1625

coordinated or adsorbed H2O

26

3400

the stretching of surface hydroxyls originated

2729

references

from and dissociative chemisorption of

bombard the water molecules under the acceleration of cathode drop near the solution surface. Water molecules in solution can be ionized and activated by collision with the bombarding ions, and primary free radicals, including hydroxyl radical, hydrated electron, and hydrogen radical are produced. Furthermore, dimerization of the free radicals or interaction of their precursors induce the formation of molecules such as hydrogen peroxide and hydrogen gas. These products then diffuse into the body of the solution and interact with the solute molecules. Cr(VI) existing in the solution in our case can be reduced by the reaction with produced reductive species, such as hydrated electron, hydrogen radical, and hydrogen peroxide.33,34 þ 3þ Cr2 O2 þ 7H2 O K ¼ 2:1 1010 L=ðmol 3 sÞ, 7 þ 14H þ 6e f 3Cr

water and to the OH stretching of the

of residual total Cr in the supernate was determined using AAS and the results are presented in Figure 3. As seen from the figure, the optimal initial pH values for Cr(VI) removal were approximately from 6.0 to 7.0. With initial pH from 2.0 to 6.0, the removal efficiency increased with increasing of pH and reached a maximum around pH = 6.0 and 7.0, and decreased when pH g 8.0. To be noted, the optimal initial pH value for Cr(VI) reduction was different from the optimal pH value for Cr removal. The former was pH e 2.0 or g8.0 and the latter was around pH = 6.0 and 7.0. Characterization of the Sediment. Characterization of the sediment after drying at different temperatures (at 55 °C and/or 400 °C for 1 h) was achieved via FTIR (Figure 4) and XRD analysis (Figure 5). Assignment of the FTIR bands of the sediment after heating at 55 °C are summarized in Table 1. Reference Cr(OH)3 was obtained according to the method mentioned by Ratnasamy et al.28 The CrO lattice vibration band at 535 cm1 26 appeared in the sample after drying at 55 °C for 1 h, as seen in Figure 5a. According to the assignment, it is concluded that the main constituent of the sediment is Cr(OH)3. The difference of the FTIR spectra between the sample and reference Cr(OH)3 is due to decomposition of water from the Cr(OH)3 sample during drying.28 Appearance of H2O and carbonate bands in the spectrum is due to the strong affinity of Cr(OH)3 for both substances.26 In another experiment, the sediment was placed into a muffle for heating at 400 °C for 1 h in air. Both the FTIR spectrum and XRD pattern of the heated (400 °C) sample show the main characteristic peaks of chromic oxide (Cr2O3). This further confirms that the main constituent of the sediment is Cr(OH)3 because Cr(OH)3 can be transformed into Cr2O3 when heating at high temperature.28 Additional weak peaks at 950 cm1, 1050 cm1, and 1623 cm1 not due to Cr2O3 appear in the FTIR spectrum of the sample, implying that some other components except Cr(OH)3 also exist in the sediment formed during discharge.

’ DISCUSSION Various active species including hydroxyl radical, hydrated electron, hydrogen peroxide, and hydrogen radical are formed in solution when high voltage is applied to generate discharge plasma over the solution surface.16,30,31 The mechanism for the formation of these reactive species in solution is explained according to previous reports3032 in the following. Energetic ions, mainly being H2O+ produced by the electron impact ionization of H2O molecules in the cathode dark space,32 can

ð1Þ

pH ¼ 7

nondissociated water molecules

þ 3þ Cr2 O2 þ 7H2 O K ¼ 2:0 1010 L=ðmol 3 sÞ, 7 þ 8H þ 6H• f 2Cr

ð2Þ

pH ¼ 7 þ 3þ 2HCrO þ 3O2 þ 8H2 O 4 þ 3H2 O2 þ 8H h 2Cr

ð3Þ

The reduction of Cr(VI) by hydrogen peroxide is thermodynamically favorable because the standard electrode potential is +1.08 V vs standard hydrogen electrode (SHE) for Cr(VI) reduction at pH = 2.0, higher than the standard electrode potential for hydrogen peroxide oxidation which is +0.56 V vs SHE.35 The reduction rate strongly depends on the pH of the reaction mixture and decreases with increasing pH (as shown in Figure S5 in SI).34 Cr(VI) exists mainly in the form of H2CrO4, HCrO4 (less than 0.01 g 3 L1, while in high concentration Cr(VI) exists in the form of Cr2O72-), and CrO42- in solution and they can be transformed into each other with change of pH.36 HCrO4 is the predominant form in acidic solution and CrO42- is the predominant form in neutral or basic solution, respectively. The equilibrium of Cr(VI) in solution is37 H2 CrO4 f HCrO4 þ Hþ pKa ¼ 1:20 2 HCrO þ Hþ pKa ¼ 6:52 4 f CrO4

The oxidation potential of Cr(VI) increases with decrease of the pH value of the solution. Generally, Cr(VI) reduction is easier in acidic condition than in neutral or basic conditions.11,22,23 However, different pH effect on Cr(VI) reduction in our case has been observed. The reduction efficiency increases with increasing pH from 3.0 to 11.0. The reason for high Cr(VI) reduction efficiency in alkaline solution is due to more production of reductive species in soultion.31,38 Sahni et al. have quantified the reductive species produced by high voltage electrical discharge in water by using two reductive species probes, tetranitromethane (TNM, for hydrogen radical and superoxide anion) and nitroblue tetrazolium chloride (NBT, for superoxide anion). It was found that more reductive species, mainly superoxide anion, were produced in alkaline solution than in acidic solution.38 In addition, Thagard et al. have reported that silver ions could be reduced to elementary silver by discharge at pH = 8.0, and if solution pH was lowered to pH = 3.0 no reduction of silver ions was observed.31 They pointed out that the reason for that phenomenon was due to scavenging of hydrated electron in acidic condition. The 7844



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difference of the pH effect on Cr(VI) reduction between our work and Wang’s work11 may be due to the difference in experimental conditions. In Wang’s experimental setup both electrodes were submerged in solution and glow discharge plasma took place in solution. The Cr(VI) reduction efficiency decreased with increasing pH from 2.0 to 8.0 and no precipitation was reported in their case. For the investigation of the vast difference in Cr(VI) reduction between pH = 2.0 and pH = 3.0 two water samples with respective initial pH values at 2.0 and 3.0 were treated by argon discharge. One mL of the treated water was then added to 1 mL of K2Cr2O7 solution at concentration of 80 mg 3 L1 Cr(VI). Residual Cr(VI) concentration of the mixture after 30 min incubation at room temperature were determined. We observed that the treated water with initial pH = 2.0 induced a dramatically higher reduction efficiency for Cr(VI) compared with that case for pH = 3.0 (Figure S6 in SI). This indicates that some long-lived species existing in the treated water can reduce Cr(VI) to lower oxidation states. We speculate that this long-lived species is H2O2. The content of H2O2 formed in argon discharge treated water with initial pH ranging from 2.0 to 11.0 was measured (Table S1 in SI). Although the amounts of H2O2 produced at different pH values were similar, the reduction rate of Cr(VI) by H2O2 was remarkably larger at pH = 2.0 than at pH = 3.0 (as shown in Figure S5 in SI).34 This difference contributed to the large difference of Cr(VI) reduction between pH = 2.0 and pH = 3.0, as shown in Figure 1. In addition, the influence of Fenton reaction on Cr(VI) reduction during discharge can also be excluded. Only 1.6 mg 3 L1 iron, maybe released from the cathode electrode submerged in solution, was detected in 20 mL of treated water with initial pH = 2.0 after discharge treatment for 10 min. No iron was detected when initial pH g 3.0. Therefore, Fenton reaction was negligible or very weak when pH g 3.0 because of the absence of Fe. If Fenton reaction was very strong in solution with pH = 2.0 lower Cr(VI) reduction efficiency would be obtained in this case compared with the case pH g 3.0 because hydroxyl radical from Fenton reaction could reoxidize lower oxidation states of Cr to Cr(VI), as will be explained below. The pH change of the solution before and after discharge, as shown in the inset of Figure 1, is due to the reaction in the solution but not due to the contamination of argon with air, because the change of nitrate concentration in argon discharge treated water is negligible in comparison with the control water sample. In air atmosphere, pH value of K2Cr2O7 solution decreased from 5.9 to 2.7 after discharge treatment for 10 min and large amounts of nitrate ions were produced in air discharge treated water, as shown in Table S2 in SI. As one of the dominant and strongly oxidative species formed during discharge,39 hydroxyl radical can reoxidize the lower oxidation states of Cr to Cr(VI),40 and in addition, it can also react with hydrogen radical to produce water,41 the latter can act as a reducing reagent for Cr(VI) according to the reaction 2. Ethanol as a nontoxic hydroxyl radical scavenger can effectively promote discharge-induced Cr(VI) reduction, as manifested in our case. It reacts with hydroxyl radical more rapidly than with hydrated electron and hydrogen radical, as shown in the following reactions:33,42 OH• þ C2 H5 OH f H2 O þ C•2 H4 OH 1:9 109 L=ðmol 3 sÞ

ð4Þ

eaq þ C2 H5 OH f H• þ C2 H5 O 1:0 106 L=ðmol 3 sÞ

ð5Þ

H þ C2 H5 OH f H•2 þ C•2 H4 OH 1:7 107 L=ðmol 3 sÞ

ð6Þ

To clarify that the accelerating effect of ethanol on Cr(VI) reduction was induced by discharge plasma but not the redox reaction between Cr(VI) and ethanol, control experiments with different percentages of ethanol in the absence of discharge treatment were carried out. The result (as shown in Figure S7 in SI) that Cr(VI) concentration in solution kept steady after incubation with different percentages of ethanol further confirms the function of ethanol as hydroxyl radical scavenger. Solubility of the produced Cr(III) strongly depends on pH value of the solution. Several hydroxyl species of Cr(III), including Cr(OH)4, Cr(OH)3, Cr(OH)2+, CrOH2+, Cr2(OH)24+, and Cr3(OH)45+, can be formed in Cr(III) aqueous solution.43 CrOH2+, Cr(OH)3, and Cr(OH)4 are the predominant species in Cr(III) aqueous solution at pH 3.86.4, 6.410, and 1014, respectively.44 However, in our case the presence of ethanol plays a crucial role in the precipitation of Cr(III) because no sediment formed if no ethanol was added. To further confirm the role of ethanol as precipitation promoter, other hydroxyl radical scavengers, such as methanol, isopropanol, 2-butyl alcohol, and DMSO were also tested. We found that all the alcohols can promote precipitation but DMSO cannot. Although the latter can apparently promote Cr(VI) reduction (about 99% and 80% of Cr(VI) were reduced in 20 mL of K2Cr2O7 solution at concentration of 40 mg 3 L1 Cr(VI) after discharge treatment for 10 min with 1% (V:V) DMSO and without DMSO, respectively), the total Cr content in the solution kept balance as confirmed by AAS. This may be due to different reaction mechanisms in scavenging hydroxyl radical by DMSO.45,46 Therefore, we conclude that alcohol not only scavenges hydroxyl radicals but also promotes formation of insoluble Cr-complex which helps the precipitation of Cr(III) out of the solution. Wiberg et al. have summarized the reasonable reaction between alcohol and Cr(VI)47 as follows: þ CH3 CH2 OH þ HCrO 4 þ H h CH3 CHOCrO3 H þ H2 O

ð7Þ

Our elemental analysis shows the existence of carbon element in the sediment, verifying that ethanol takes part in the reaction. This is because ethanol is the only chemical which contains carbon element in this reaction system. However, for the detailed mechanism of alcohol in promoting precipitation it needs further investigation. Argon was used in this work because it is inert and cannot react with water and solutes in solution. So it is beneficial for us to clarify the underlying mechanism for the reactions. In addition, argon discharge is easier to be initiated and more primary free radicals can be produced in solution compared with air discharge with the same input energy.39 This is because under the air atmosphere, a part of the input energy can be dissipated to dissociate nitrogen, whereas in argon discharge the input energy is mainly consumed to produce the primary free radicals in water.39 Actually, we have also applied air as the discharge atmosphere and found that sediment can also be precipitated out of the solution after discharge in the presence of ethanol (about 89% of Cr(VI) removed from 25 mL of K2Cr2O7 solution at concentration of 80 mg 3 L1 Cr(VI) after air discharge treatment for 15 min in the presence of 2% (V:V) ethanol). Taken together, only a small amount of alcohol was consumed in the reactions while it made a big difference not only in reduction but also removal of toxic chromium ions from solution. As such, this work demonstrated a novel and promising way in treatment of Cr(VI) containing industrial wastewater. 7845



’ ASSOCIATED CONTENT

bS

Supporting Information. Results of the measurements of hydrogen peroxide, nitrate in discharge treated water, the color photo of Cr(VI) solution after argon discharge in the presence of ethanol, estimation of reactor power and energy cost, etc. This information is available free of charge via the Internet at http:// pubs.acs.org/.

’ AUTHOR INFORMATION Corresponding Author

*Phone: 86-551-5595261; fax: 86-551-5595261; e-mail: huangq@ ipp.ac.cn; address: P.O. Box 1138, Hefei 230031, P. R. China.

’ ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (Grant 2007CB936602) and the Hundred Talents Program of the Chinese Academy of Sciences. ’ REFERENCES (1) Kimbrough, D. E.; Cohen, Y.; Winer, A. M.; Crelman, L.; Mabuni, C. A critical assessment of chromium in the environment. Crit. Rev. Environ. Sci. Technol. 1999, 29, 1–46. (2) Dogan, N. M.; Kantar, C.; Gulcan, S.; Dodge, C. J.; Yilmaz, B. C.; Mazmanci, M. A. Chromium(VI) bioremoval by pseudomonas bacteria: Role of microbial exudates for natural attenuation and biotreatment of Cr(VI) contamination. Environ. Sci. Technol. 2011, 45, 2278–2285. (3) Graham, A. M.; Bouwer, E. J. Rates of hexavalent chromium reduction in anoxic estuarine sediments: pH effects and the role of acid volatile sulfides. Environ. Sci. Technol. 2010, 44, 136–142. (4) Owlad, M.; Aroua, M. K.; Daud, W. A. W.; Baroutian, S. Removal of hexavalent chromium-contaminated water and wastewater: A review. Water, Air Soil Pollut. 2009, 200, 59–77. (5) Wu, J.; Zhang, H.; He, P. J.; Yao, Q.; Shao, L. M. Cr(VI) removal from aqueous solution by dried activated sludge biomass. J. Hazard. Mater. 2010, 176, 697–703. (6) Wichern, D. A quick look at waste treatment with ferrous sulfate. Met. Finish. 2004, 102, 39–39. (7) McGuire, M. J.; Blute, N.; Seidel, C.; Qin, G.; Fong, L. Pilot-scale studies of hexavalent chromium removal. J. Am. Water Works Assoc. 2006, 98, 134–143. (8) Patterson, J. W.; Gasca, E.; Wang, Y. Optimization for reduction/ precipitation treatment of hexavalent chromium. Water Sci. Technol. 1994, 29, 275–284. (9) Bazrafshan, E.; Mahvi, A. H.; Naseri, S.; Mesdaghinia, A. R. Performance evaluation of electrocoagulation process for removal of chromium (VI) from synthetic chromium solutions using iron and aluminum electrodes. Turkish J. Eng. Env. Sci. 2008, 32, 59 –66. (10) Yuan, S. J.; Zheng, Z.; Mu, Y. Y.; Yu, X.; Zhao, Y. F. Radiationinduced reduction of chromium(VI) in aqueous solution by γ-irradiation in a laboratory-scale. J. Environ. Sci. 2006, 18, 254–258. (11) Wang, L.; Jiang, X. Z. Plasma-induced reduction of chromium(VI) in an aqueous solution. Environ. Sci. Technol. 2008, 42, 8492–8497. (12) Cserfalvi, T.; Mezie, P. Direct solution analysis by glow discharge: Electrolyte-cathode discharge spectrometry. J. Anal. At. Spectrom. 1994, 9, 345–349. (13) Shimizu, Y.; Kawaguchi, K.; Sasaki, T.; Koshizaki, N. Generation of room-temperature atmospheric H2/Ar microplasma jet driven with pulse-modulated ultrahigh frequency and its application to gold nanoparticle preparation. Appl. Phys. Lett. 2009, 94, 191504–1 1915043.

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Reduction and Removal of Aqueous Cr(VI) by Glow Discharge Plasma

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