Enhanced Redox Conversion of Chromate and Arsenite in Ice

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Enhanced Redox Conversion of Chromate and Arsenite in Ice Kitae Kim and Wonyong Choi* School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea ABSTRACT: The redox transformation of trace elements is critically affected by properties of the environmental media. While the environmentally important redox chemical reactions in aquatic environments have been extensively studied, those in the ice phase have been studied in only a few cases. In this work, chromium and arsenic species were selected as the model inorganic oxyanion contaminants for the study of redox chemical transformation in ice. We investigated (1) the reduction of hexavalent Cr(VI) (as chromate) by model organic acids (e.g., citric and oxalic acid) and (2) the simultaneous removal of Cr(VI) and As(III) (as arsenite) in ice phase in comparison with their counterparts in aqueous solution. The reduction of Cr(VI) by various organic acids (electron donors) was negligible in ambient aqueous solution but was significantly accelerated in ice. The simultaneous reduction of Cr(VI) and oxidation of As(III) in ice phase proceeded stoichiometrically, whereas their mutual conversion was insignificant in aqueous solution. The enhanced redox conversion of Cr(VI)/ As(III) in ice is ascribed to the freeze concentration of both electron donors (e.g., organic acids, arsenites) and protons in the ice crystal grain boundaries. When the concentrations of both electron donors and protons were highly raised to an extreme, the removal rates of Cr(VI) in aqueous solution approached to those in ice. This specific combination of Cr(VI)/As(III) redox couple may provide an example that represents innumerable redox conversion reactions that could be greatly accelerated in ice/snow-covered or frozen environments.

’ INTRODUCTION Most chemical reactions in aqueous solution are generally decelerated as temperature decreases. However, some bimolecular chemical reactions are reported to be accelerated in frozen solution, in which the solutes are concentrated as they are excluded from the crystalline ice lattice into liquid-like grain boundary region (freeze concentration effect).1-7 For example, Takenaka et al.6,7 found that the oxidation of nitrite (NO2-) with dissolved oxygen to nitrate (NO3-) is accelerated by a factor of 105 upon freezing the aqueous nitrite solution. In general, the surface and interface (water/ice or air/ice) of ice provides a unique chemical environment where the chemical transformations proceed very differently from their counterparts in water and air.8,9 It has been also reported that the photochemical processes taking place in the ice phase can be very different from those in the aqueous phase and might play an important role in the chemical transformation of persistent organic pollutants in cold environments.10,11 In addition, our recent study found that the photoreductive dissolution of iron oxide particles (a common dust component) is highly enhanced in the ice phase.12 Therefore, the chemical processes involving various organic and inorganic species in the icy environment are believed to be quite different from their aqueous counterparts. Understanding redox chemical processes in the environment is a critical subject in environmental chemistry and provides basic information for preserving and remediating the affected environment. The mobility, toxicity, bioavailability, and environmental fate of trace inorganic elements are influenced by their redox speciation.13 For example, chromium and arsenic species are r 2011 American Chemical Society

ubiquitous in the environment and can be involved with various natural and industrial processes. Chromium is a common element in Earth’s crust and is used in many industries such as metallurgy, dye and pigment, electroplating, leather tanning, and corrosion inhibition. The most stable oxidation state of chromium in the environment are Cr(III) and Cr(VI). Cr(VI) is the thermodynamically stable oxidation state of chromium in an oxidizing environment and is present predominantly in the anionic form of HCrO4- and CrO42- in the pH range 310.14,15 The conversion of Cr(VI) to Cr(III) can occur in the presence of various reductants such as Fe(0), Fe(II), S(II), and organic compounds.16-18 Arsenic is emitted from either anthropogenic sources such as fertilizers and wood preservatives or natural weathering and dissolution of As-bearing minerals with its common oxidation state of As(III) and As(V).19,20 Among their various oxidation states, the hexavalent Cr(VI) as chromate or dichromate and the trivalent As(III) as arsenite are known to be the most toxic. Therefore, the reduction of Cr(VI) (chromate) to Cr(III) (chromium hydroxide) and the oxidation of As(III) (arsenite) to As(V) (arsenate) are highly desirable for the remediation of the environment contaminated with this species. The redox transformation of chromium and arsenic in the environment can be influenced by the presence of various organic and inorganic compounds,16-21 which has been frequently Received: October 18, 2010 Accepted: February 8, 2011 Revised: January 20, 2011 Published: February 23, 2011 2202

dx.doi.org/10.1021/es103513u | Environ. Sci. Technol. 2011, 45, 2202–2208

Environmental Science & Technology

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investigated. However, the redox chemical reactions of the inorganic oxyanions in ice have never to our knowledge been studied despite their importance in the environment. In this work, chromium and arsenic species were selected as the model inorganic oxyanion contaminants for the study of redox chemical transformation in ice. We investigated both the reductive conversion of Cr(VI) and the simultaneous conversion of Cr(VI) and As(III) in ice in comparison with their counterparts in aqueous solution. A striking difference in the redox behavior between the ice and aqueous phases was observed, and the experimental parameters affecting the process, the mechanism, and the environmental implications are discussed.

’ EXPERIMENTAL SECTION Materials. Chemicals and reagents used in this work include the following: Na2Cr2O7 3 2H2O (Cr(VI), Aldrich), NaAsO2 (As(III), Aldrich), Na2HAsO4 3 7H2O (As(V), Kanto), molybdate reagent solution [containing hexaammonium heptamolybdate ((NH4)6Mo7O24.4H2O), sulfuric acid (H2SO4), potassium antimony(III) oxide tartrate (K(SbO)C4H4O6.0.5H2O), Fluka], oxalic acid (Aldrich), citric acid (Aldrich), formic acid (Aldrich), butyric acid (Aldrich), ethylenediaminetetraacetic acid disodium salt dehydrate (Na2EDTA, Aldrich), acetone (JUNSEI), H2SO4 (Aldrich), and diphenyl-carbazide (Aldrich). All chemicals were of analytical grade. Suwannee River fulvic acid (FA) and humic acid (HA) were purchased from the International Humic Substances Society (http://www.ihss.gatech.edu). Deionized water used was ultrapure (18 MΩ•cm) and prepared by a Barnstead purification system. Freezing and Thawing of Reaction Solution. A conical centrifuge tube of 15 mL was used as a reactor. For reaction in ice, the aqueous solution containing 20 μM or 200 μM of Cr(VI) with organic acid or As(III) was put in the tube reactor, which was subsequently placed in a stainless tube rack in a cryogenic bath containing ethanol cooled at a desired temperature (usually -20 °C) for solidification. The solution samples were gradually frozen within 30 min. Therefore, defining “t = 0” in the reaction kinetic measurements was not easy. In this study, the time zero refers to the point when the aqueous sample was put into the cryogenic bath preset at the reaction temperature (-20 °C). Aqueous reactions of Cr(VI) were also carried out as a control at 25 °C using the same experimental setup. After reaction, the frozen reactor was thawed usually within 10 min in a beaker containing lukewarm water (30-40 °C). All reactions were repeated three times to confirm the reproducibility. Analysis. The concentrations of Cr(VI) and As(V) were measured spectrophotometrically using the DPC (diphenyl carbazide) method22 and molybdene blue method,23 respectively. Aliquots of 0.5 mL sample solution were withdrawn from the centrifuge tube after thawing the ice sample for Cr(VI) measurement. The 100 μL of prepared DPC reagent (containing 25 mL of acetone, 250 μL of H2SO4, and 0.05 g of DPC) was added to a vial containing 2.5 mL of deionized water and 0.5 mL of the sample solution. The vial was mixed vigorously and kept for 30 min before the analysis. The absorbance measurements at 540 nm (ε = 6850 M-1cm-1) were done using a UV/visible spectrophotometer (Libra S22, Biochrom). The variation of the measured absorbance within 1 h was less than 5%. For the quantification of As(V), the 100 μL of ascorbic acid solution (10 ( 0.5 g/100 mL) and 200 μL of molybdate reagent solution were added to a vial containing 3 mL of deionized water and 1 mL of

Figure 1. Cr(VI) reduction after 24 h in the presence of various organic acids as an electron donor (ED) in aqueous solution at 25 °C and ice phase at -20 °C. Experimental conditions were as follows: [Cr(VI)]i = 20 μM, [ED] = 600 μM, [humic or fulvic acid] = 1 ppm, pHi = 3.0 except for citric and oxalic acid (pHi = 3.3-3.4).

the sample solution. The vial was mixed vigorously and kept for 1 h before the analysis. The absorbance measurements at 870 nm (ε = 19550 M-1 cm-1) were done using the UV/visible spectrophotometer. The absorbance calibrations for the determination of [Cr(VI)] and [As(V)] were carried out using freshly prepared standard solutions of Na2Cr2O7 3 2H2O and Na2HAsO4 3 7H2O, respectively. The optical image of ice was obtained with a Zeiss JENALAB-pol polarizing microscope equipped with a Linkam LTS 350 thermal stage (temperature range of -196 to 350 °C) and a Linkam LNP94 liquid nitrogen pump. One or two droplets of sample solution were dropped onto a cover glass and put on the stage. The desired temperature (-20 °C) of the airtight stage containing the sample solution was controlled by the liquid nitrogen pump. It took 2 min to cool the stage to -20 °C.

’ RESULTS AND DISCUSSION Cr(VI) Reduction by Organic Acids. The reduction of Cr(VI) was investigated in the presence of several organic acids (e.g., oxalic and citric acids that may be commonly present in the environment) as an electron donor (ED). Figure 1 shows that the addition of various acids as an ED markedly enhanced the removal of Cr(VI) in ice phase, while their presence little influenced the removal efficiency in aqueous solution. The rate of Cr(VI) reduction in ice was particularly fast with oxalic and citric acid, whereas formic and acetic acids were no better than the case without ED. Cr(VI) forms a chromate ester intermediate with organic ED as a precursor to the reduction of Cr(VI), which is followed by an inner-sphere electron transfer.24,25 Therefore, the properties of EDs related with the ability to form the chromate ester and the subsequent electron transfer sensitively affect the reduction of Cr(VI). The oxidation potentials of the organic acids (vs NHE: ED f aCO2 þ bHþ þ ce-) tested in Figure 1 are as follows: 0.31 V for formic acid, -0.075 V for acetic acid, 0.63 V for oxalic acid, 1.3 V for citric acid, and 1.4 V for EDTA.26-28 Humic and fulvic acids with many functional groups should serve as an ED. The reduction of Cr(VI) by fulvic acid was previously investigated.25 The observed trend is that the stronger reductants (having more positive oxidation potential) like EDTA, oxalic and citric acids are more efficient in Cr(VI) reduction than the weaker ones. However, other structural properties should play 2203

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Figure 3. Temperature effect on the reduction of Cr(VI) (after 3 h) in the presence of oxalic acid in aqueous solution and ice. For freezing, the aqueous sample was rapidly put into the cryogenic bath preset at each temperature. Therefore, the freezing speed, though uncontrolled, should be faster at lower temperature. Experimental conditions were as follows: [Cr(VI)]i = 20 μM, (b) [oxalic acid]i = 600 μM, pHi = 3.0.

the unit pH. The net effect of the proton concentration is to favor reaction 5 to the forward direction

Figure 2. (a) Cr(VI) reduction in the presence of citric or oxalic acid as an ED in ice phase at -20 °C. Experimental conditions were as follows: [Cr(VI)]i = 20 μM, no pH adjustment (pHi = 5.1-5.2 with 6 μM acid; pHi = 4.2-4.3 with 60 μM acid; pHi = 3.3-3.4 with 600 μM acid). The reaction time in ice phase means the time after the aqueous sample was put into the cryogenic bath preset at -20 °C. The lines between data points were drawn only as a visual guide. (b) Effect of pH and [oxalic acid] on Cr(VI) reduction in aqueous solution at 25 °C. The experimental error bars are usually smaller than the symbol size and not shown.

the role as well. For example, all efficient EDs are multidentate ligands, which might be related with the formation of chromate ester intermediates. Figure 2a compares the time profiles of Cr(VI) removal in ice in the presence of oxalic or citric acid. Most Cr(VI) was removed within the first hour, and the removal rate was faster with higher concentration of the organic acid. The reduction of Cr(VI) in ice was pH dependent and became insignificant above pH 5 (data not shown). The enhanced removal of Cr(VI) in ice is most likely due to the freeze concentration effect. The concentrations of solutes can be highly elevated in the grain boundary region of ice, where the enhanced local concentrations of Cr(VI), ED, and protons can influence the kinetics of the chemical transformation. It was previously estimated that the local concentration of acids in the ice grain boundary region is increased by 2-3 orders of magnitude in contrast to the aqueous solution.29,30 The coupled redox reactions of Cr(VI) and oxalic acid (reactions 1, 3, and 5) indicate that the reduction of Cr(VI) should be favored in the presence of more protons and more organic EDs, which was experimentally observed for the reduction of Cr(VI) in aqueous solution.25,31 The Nernst equation (eqs 2 and 4) predicts that the reduction potential of Cr(VI) should change by þ0.138 V and the oxidation potential of oxalic acid by -0.059 V upon lowering

HCrVI O-4 þ 7Hþ þ 3e-aq f Cr3þ þ 4H2 O E01 ¼ 1:38V NHE

ð1Þ

E1 ¼ E01 - 0:02logð½Cr3þ =½HCrO4 Þ - 0:138pH

ð2Þ

2CO2 þ 2Hþ þ 2e-aq f C2 O4 H2 ðoxalicacidÞ E03 ¼ - 0:49V NHE ð3Þ E3 ¼ E03 - 0:03log½oxalic þ 0:059log½CO2 - 0:059pH ð4Þ Net reaction: þ 3þ þ 6CO2 þ 8H2 O 2HCrVI O4 þ 3C2 O4 H2 þ 8H f 2Cr

ðE5 ¼ E1 - E3 Þ

ð5Þ

It should be noted that the removal of Cr(VI) was enhanced in the ice phase compared to the aqueous phase even in the absence of organic acids (see Figure 1). The reduction of Cr(VI) without organic acid is thermodynamically possible as shown in reaction 616 and should be pH-dependent. The pH effect is also shown in Figure 1. The ice-phase reduction of Cr(VI) without organic ED was negligible at pH 5.6 but clearly notable at pH 3. It seems that the reduction of Cr(VI) in ice in the absence of organic ED is driven only when protons are highly concentrated in the grain boundary region. 4HCrVI O-4 þ 16Hþ f 4Cr3þ þ 3O2 þ 10H2 O ðE06 ¼ 0:15V NHE Þ ð6Þ To investigate the concentration effect of EDs and protons on the Cr(VI) reduction kinetics, the reduction of Cr(VI) was carried out in aqueous solution with either excessive concentration of oxalic acid (60 mM) or very low pH (1.5). Let us take the reference condition of [oxalic] = 0.6 mM and pH = 3.3 under which the reduction of Cr(VI) in ice is almost completed within an hour whereas that in water is negligible. Either lowering pH to 1.5 ([Hþ] increase by 60 times) or raising [oxalic] to 60 mM (100-fold increase) from the reference condition is not sufficient enough to accelerate the reduction rate (Figure 2b). According to the Nernstian equation of reaction 5, elevating [oxalic] and [Hþ] by 100 and 60 times increases E5 by 0.06 and 0.14 V, respectively. 2204



Figure 4. (a) Time profiles of Cr(VI) removal and As(V) production in the presence of As(III) in aqueous solution at 25 °C and in ice at -20 °C. Experimental conditions were as follows: [Cr(VI)]i = 200 μM, [As(III)]i = 300 μM, pHi = 3.0. (b) Effect of pH and [As(III)] on Cr(VI) reduction in aqueous solution at 25 °C.

Therefore, the combination of both should increase E5 by 0.2 V (if neglecting the temperature difference between water and ice). The reduction of Cr(VI) in water is significant and comparable to the ice case only if both ED and Hþ are highly concentrated (by about 100-fold) ([oxalic] = 60 mM, pH = 1.5). Although the actual concentration enhancement in the grain boundary liquid layer might be far higher than 100-fold increase, preparing even higher concentrations of oxalic acid and protons in water was practically difficult. Nevertheless, the results in Figure 2b show the qualitative trend that concentrating both ED and protons in water enhances the redox conversion rate, approaching the level observed in ice. Therefore, the accelerated reduction of Cr(VI) in ice should be most likely ascribed to the freeze concentration of both Hþ and EDs in the ice grain boundary region. The temperature effect on the reduction of Cr(VI) by oxalic acid in ice was also investigated between -10 to -196 °C (Figure 3). The efficiency of Cr(VI) reduction was relatively invariable between -10 to -25 °C but rapidly decreased when the ice temperature dropped below -25 °C. There are some factors that can be related with the temperature. The liquid content in the grain boundary region might be smaller at lower temperature and the freezing speed should be faster at lower temperature. It was previously observed that the freeze concentration effect was much higher under slow freezing at higher temperature compared to fast freezing at lower temperature.4 This is consistent with the temperature-dependent behavior shown in Figure 3. At the liquid nitrogen temperature (-196 °C) where the sample froze immediately, the

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Figure 5. Cr(VI) reduction as a function of [As(III)] in aqueous solution at 25 °C and in ice phase at -20 °C. Experimental conditions were as follows: (a) [Cr(VI)]i = 20 μM, (b) [Cr(VI)]i = 200 μM, pHi = 3.0, reaction time 1 h.

reduction of Cr(VI) was markedly retarded compared to the reduction at higher temperature. This is most likely ascribed to the rapid freezing that might minimize the freeze concentration effect. Simultaneous Reduction of Cr(VI) and Oxidation of As(III). The reductive conversion of Cr(VI) was also carried out in the presence of As(III) instead of organic acids because As(III) may serve as an alternative ED. In Figure 4a, the reduction of Cr(VI) and the oxidation of As(III) were simultaneously monitored in both aqueous and ice phases as a function of reaction time. The redox conversion was negligibly slow in water but markedly accelerated in ice. The removal of Cr(VI) and the accompanied production of As(V) were completed within 1 h (including about 20 min solidification time at -20 °C). The oxidation of As(III) alone (without Cr(VI)) was negligible in both aqueous and ice phases. The reduction of Cr(VI) was investigated as a function of As(III) concentration both in aqueous solution and ice, and the results are compared in Figure 5. Regardless of the initial level of [Cr(VI)] (20 and 200 μM), the removal of Cr(VI) in ice phase was enhanced with increasing [As(III)], whereas there was little effect of [As(III)] on the removal of Cr(VI) in aqueous phase. To investigate how the freeze concentration affects the redox conversion of Cr(VI)/As(III) in ice, the redox reaction was carried out in aqueous solution with higher concentrations of As(III) and Hþ as the case of Cr(VI)/oxalic acid (Figure 2b). From the reference condition of [As(III)] = 0.3 mM and pH 3 under which the redox conversion of Cr(VI)/As(III) in ice was completed within an hour, but its counterpart in aqueous 2205



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Table 1. Stoichiometric Reduction of Cr(VI) and the Accompanying Production of As(V) at Various pH in Ice Phasea -Δ[Cr(VI)] (μM)

Δ[As(V)] (μM)

-Δ[As(V)]/[Cr(VI)]

pH 2

191.2

285.1

1.49

pH 3 pH 5

190.7 161.1

272.2 203.9

1.43 1.27

pH 7

169.1

217.3

1.29

pH 9

81.2

110.7

1.36

Scheme 1. Illustration of Redox Conversion of Cr(VI) and As(III) in the Confined Grain Boundary Region of Icea

Experimental condition: [Cr(VI)]i = 200 μM, [As(III)]i = 300 μM, reaction time 1 h at -20 °C. a

solution was negligible, increasing the concentration of either [As(III)] or [Hþ] by 10-fold did enhance the redox conversion rate. However, it was not as fast as that in the ice reference condition. When both [As(III)] and [Hþ] were raised by 10fold, then the removal rate of Cr(VI) in aqueous solution was comparable to that in ice (Figure 4b). The quantitative stoichiometry of the reaction between Cr(VI) and As(III) can be expressed in terms of the combined half reactions (reaction 1 þ reaction 7) to yield reaction 8.32,33 H2 AsV O-4 þ 3Hþ þ 2e-aq f HAsIII O2 þ 2H2 O E07 ¼ 0:4V NHE ð7Þ

a

III þ 3þ 2HCrVI Oþ 3H2 AsO4 þ 3HAs O2 þ 5H f 2Cr 4 þ 2H2 O

The optical image of the polycrystalline ice clearly shows the grain boundary region in which the redox couples can be concentrated. The artistic drawings of substrates concentrated in the grain boundaries are superimposed on the optical image.

E08 ¼ E01 - E07 ¼ 0:98V NHE ð8Þ Reaction 8 is thermodynamically spontaneous and indicates that the theoretical molar ratio of As(III) to Cr(VI) is 1.5. To confirm the quantitative redox reaction between Cr(VI) and As(III), the stoichiometric relationship was investigated at various pH (2 - 9) since both Cr(VI) reduction and As(III) oxidation should be pH dependent (see Table 1). The redox conversion of Cr(VI)/As(III) was markedly accelerated in all pH range in ice phase, whereas that in aqueous solution was negligible. Although the rate of Cr(VI) reduction by organic acids in ice was significant only at pH < 5 (data not shown), Cr(VI) reduction by As(III) in ice was efficient over the wide pH range. The molar ratios of oxidized [As(III)] to reduced [Cr(VI)] in ice phase range in 1.2-1.5, which is close to the stoichiometric ratio of 1.5 according to reaction 8. Environmental Implications. We investigated (1) Cr(VI) reduction by model organic acids (e.g., citric and oxalic acid) and (2) the simultaneous removal of Cr(VI) and As(III) in ice phase in comparison with their counterparts in aqueous solution. The reduction of Cr(VI) in the presence of either organic acid or As(III) as an ED, which proceeds very slowly in aqueous solution, is greatly accelerated in ice. The accelerated reduction of Cr(VI) by freezing is ascribed to not only the increased concentration of EDs but also the elevated concentration of protons in the ice grain boundary region. The freeze concentration effect on the Cr(VI)/As(III) redox conversion is illustrated in Scheme 1. The background optical image clearly shows the grain boundary region of several μm thick. Cr(VI), As(III), and protons all can be concentrated in this confined boundary region where the unfrozen liquid state exists.4-6 Although we do not have a direct evidence that the redox conversion reactions are actually taking place in the liquid-like boundary region, the experimental data and known facts about ice chemistry strongly support this scenario. The enhanced activities of the relevant species in this confined region should accelerate the overall redox conversion reaction.

Chromium and arsenic species can be found in atmospheric water, soil, groundwater, and industrial wastewater. Therefore, the frozen environment may affect their redox transformation. While the single component (chromium or arsenic only) redox system has been widely investigated, the coexistence of chromium and arsenic species in the environment is not uncommon. The reaction of Cr(VI) with As(III) in atmospheric water markedly shortens the half-life of Cr(VI) (by reduction to Cr(III)).18 The freezing of atmospheric droplets in the upper troposphere can further shorten the lifetime of Cr(VI) in the presence of As(III). To take another example, copper, chromium, and arsenic (CCA) mixture have been widely used as a wood preservative and the CCA-treated woods are widespread for outdoor uses. They may leach out CCA chemicals into the surrounding soil as a result of weathering and eventually into groundwater.34-36 The simultaneous removal of Cr(VI) and As(III) in industrial wastewater, acid mine drainage (AMD), and soil was the subject of previous studies.37,38 Chromium and arsenic are often associated with pyrite ores and found together in AMD.38 The results of the present study imply that both chromium and arsenic in soils contaminated with CCA or AMD may undergo accelerated redox conversion when soil-water is frozen in winter. Although most aquatic chemical reactions are retarded in the frozen phase, some redox conversion can be accelerated on the contrary. This enhanced redox conversion in ice should not be limited to the specific case of Cr(VI)/As(III). There are numerous candidates of the redox couples between various inorganic/organic species of which conversion is negligibly slow in aquatic environments but markedly accelerated in icy environments. The thermochemically feasible redox reactions can be constituted in innumerable ways by combining two different half reactions of inorganic and organic species. Although the environmentally important redox chemical reactions in aquatic environments have been extensively studied and reported in the literature, those in the ice phase have been rarely studied. Their chemical 2206


Environmental Science & Technology kinetics and mechanisms in ice might be often different from those in aqueous solution. Understanding the redox transformation of various inorganic/organic species in ice phase may provide newer views and insights on the environmental chemical processes in the cold regions (e.g., upper troposphere, permafrost, polar/high latitude environment and midlatitudes during winter season) where the freeze-thaw cycles continue. The field of studying chemical reactions in ice is just emerging with many opportunities waiting to be investigated.

’ AUTHOR INFORMATION Corresponding Author

*Fax: þ82-54-279-8299. E-mail: [email protected].

’ ACKNOWLEDGMENT Funding for this work was provided by KOSEF NRL program (No. R0A-2008-000-20068-0), KOSEF EPB center (No. R112008-052-02002), KCAP (Sogang Univ.) funded by NRF (2009C1AAA001-2009-0093879), and Korea Polar Research Institute (PP11010). ’ REFERENCES (1) Betterton, E. A.; Anderson, D. J. Autoxidation of N(III), S(IV), and other species in frozen solution: A possible pathway for enhanced chemical transformation in freezing systems. J. Atmos. Chem. 2001, 40, 171–189. (2) Grannas, A. M.; Bausch, A. R.; Mahanna, K. M. Enhanced aqueous photochemical reaction rates after freezing. J. Phys. Chem. A 2007, 111, 11043–11049. (3) Grannas, A. M.; Jones, A. E.; Dibb, J.; Ammann, M.; Anastasio, C.; Beine, H. J.; Bergin, M.; Bottenheim, J.; Boxe, C. S.; Carver, G.; Chen, G.; Crawford, J. H.; Domin’e, F.; Frey, M. M.; Guzm’an, M. I.; Heard, D. E.; Helmig, D.; Hoffmann, M. R.; Honrath, R. E.; Huey, L. G.; Hutterli, M.; Jacobi, H. W.; Kl’an, P.; Lefer, B.; McConnell, J.; Plane, J.; Sander, R.; Savarino, J.; Shepson, P. B.; Simpson, W. R.; Sodeau, J. R.; Glasow, R. v.; Weller, R.; Wolff, E. W.; Zhu, T. An overview of snow photochemistry: evidence, mechanisms and impacts. Atmos. Chem. Phys. 2007, 7, 4329–4373. (4) Heger, D.; Jirkovsky, J.; Klan, P. Aggregation of methylene blue in frozen aqueous solutions studied by absorption spectroscopy. J. Phys. Chem. A 2005, 109, 6702–6709. (5) Takenaka, N.; Bandow, H. Chemical kinetics of reactions in the unfrozen solution of ice. J. Phys. Chem. A 2007, 111, 8780–8786. (6) Takenaka, N.; Ueda, A.; Daimon, T.; Bandow, H.; Dohmaru, T.; Maeda, Y. Acceleration mechanism of chemical reaction by freezing: the reaction of nitrous acid with dissolved oxygen. J. Phys. Chem. 1996, 100, 13874–13884. (7) Takenaka, N.; Ueda, A.; Maeda, Y. Acceleration of the rate of nitrite oxidation by freezing in aqueous solution. Nature 1992, 358, 736–738. (8) Park, S.-C.; Moon, E.-S.; Kang, H. Some fundamental properties and reactions of ice surfaces at low temperatures. Phys. Chem. Chem. Phys. 2010, 12, 12000–12011. (9) Boxe, C. S.; Colussi, A. J.; Hoffmann, M. R.; Perez, I. M.; Murphy, J. G.; Cohen, R. C. Kinetics of NO and NO2 evolution from illuminated frozen nitrate solutions. J. Phys. Chem. A 2006, 110, (10) Kahan, T. F.; Donaldson, D. J. Photolysis of polycyclic aromatic hydrocarbons on water and ice surfaces. J. Phys. Chem. A 2007, 111, 1277–1285. (11) Klanova, J.; Klan, P.; Nosek, J.; Holoubek, I. Environmental ice photochemistry: monochlorophenols. Environ. Sci. Technol. 2003, 37, 1568–1574.

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Enhanced Redox Conversion of Chromate and Arsenite in Ice

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