Environ. Sci. Technol. 2011, 45, 307–312
Oxidation of Sulfoxides and Arsenic(III) in Corrosion of Nanoscale Zero Valent Iron by Oxygen: Evidence against Ferryl Ions (Fe(IV)) as Active Intermediates in Fenton Reaction S U - Y A N P A N G , †,‡ J I N J I A N G , * ,†,‡ A N D J U N M A * ,†,‡ State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, China, and Environmental Science and Engineering Research Center, Harbin Institute of Technology Shenzhen Graduate School, Shenzhen, China
Received October 31, 2009. Revised manuscript received November 16, 2010. Accepted November 22, 2010.
Previous studies have shown that the corrosion of zerovalent iron (ZVI) by oxygen (O2) via the Fenton reaction can lead to the oxidation of various organic and inorganic compounds. However, the nature of the oxidants involved (i.e., ferryl ion (Fe(IV)) versus hydroxyl radical (HO•)) is still a controversial issue. In this work, we reevaluated the relative importance of these oxidants and their role in As(III) oxidation during the corrosion of nanoscale ZVI (nZVI) in air-saturated water. It was shown that Fe(IV) species could react with sulfoxides (e.g., dimethyl sulfoxide, methyl phenyl sulfoxide, and methyl p-tolyl sulfoxide) through a 2-electron transfer step producing corresponding sulfones, which markedly differed from their HO•-involved products. When using these sulfoxides as probe compounds, the formation of oxidation products indicative of HO• but no generation of sulfone products supporting Fe(IV) participation were observed in the nZVI/O2 system over a wide pH range. As(III) could be completely or partially oxidized by nZVI in airsaturated water. Addition of scavengers for solution-phase HO• and/or Fe(IV) quenched As(III) oxidation at acidic pH but had little effect as solution pH increased, highlighting the importance of the heterogeneous iron surface reactions for As(III) oxidation at circumneutral pH.
Introduction Over the past decade, it has been shown that the corrosion of zerovalent iron (ZVI or Fe0) by oxygen (O2) can produce reactive oxidants capable of oxidizing various organic and inorganic compounds (e.g., refs 1-3). A better understanding of the iron-catalyzed oxygenation at ambient conditions has important implications for both natural and technical systems. For instance, the iron-catalyzed oxidation of As(III) by O2 might be one of the most important abiotic transformation pathways for As(III) and might help to explain the appearance of As(V) in groundwater containing As(III) and * Corresponding author phone: 86-451-86283010; fax: 86-45182368074; e-mail:
[email protected] (J.M.);
[email protected] (J.J.). † State Key Laboratory of Urban Water Resource and Environment. ‡ Environmental Science and Engineering Research Center. 10.1021/es102401d
2011 American Chemical Society
Published on Web 12/06/2010
Fe(II) after it has been pumped to the surface, as well as the occurrence of As(V) in engineered ZVI treatment system (in conventional and nanoscale forms) for As(III) removal (e.g., refs 3-5). Recently, Sedlak and co-workers (6, 7) have systematically investigated the kinetics and mechanism of oxidant production during the corrosion of nanoscale ZVI (nZVI) in airsaturated water over a wide pH range, and concluded that oxidation was attributable to the generation of Fenton reagent. At acidic pH, O2 accepted two electrons from Fe0 surfaces to produce hydrogen peroxide (H2O2) in the initial step (reaction 1), which was either reduced to water by another 2-electron transfer from Fe0 surfaces (reaction 2), or reacted with ferrous ion (Fe(II); the product of reaction 1) to produce hydroxyl radical (HO•) (reaction 3) (e.g., ref 8). 0 Fe(S) + O2 + 2H+ f Fe(II) + H2O2
(1)
0 Fe(S) + H2O2 + 2H+ f Fe(II) + 2H2O
(2)
Fe(II) + H2O2 f Fe(III) + HO• + HO-
(3)
Under neutral pH conditions, the oxidant production by reactions 1 and 3 was negligible, whereas the oxidation of Fe(II) by O2 was mainly responsible and nZVI simply served as a source of Fe(II). This conclusion was supported by the nearly identical oxidant production when the same concentrations of nZVI and Fe(II) were exposed to O2 (6, 7). The reaction of Fe(II) with O2 through a series of 1-electron transfer (reactions 4 and 5) also produced H2O2, which was subsequently converted to an oxidant by reacting with Fe(II). Fe(II) + O2 f Fe(III) + O•2
(4)
+ Fe(II) + O•2 + 2H f Fe(III) + H2O2
(5)
Furthermore, the recent work conducted by Sedlak and coworkers (6, 7) provides new evidence against HO• as a crucial Fenton intermediate at circumneutral pH. These authors found that formaldehyde production from methanol oxidation in the nZVI/O2 system at pH 6-8 was much higher than that obtained at slightly acidic pH 3-5, while the yield of oxidation products (i.e., acetone and para-hydroxylbenzoic acid (p-HBA)) from 2-propanol and benzoic acid was highest under acidic conditions, with little production at neutral pH. To account for the different patterns of oxidation product formation as a function of pH, they proposed a pH-dependent Fenton reaction mechanism; that was, nonselective HO• (reaction 3) was the main oxidizing species at low pH, whereas a more selective oxidant ferryl ion (Fe(IV)) (reaction 6) oxidized methanol but exhibited little reactivity toward 2-propanol and benzoic acid at circumneutral pH. Fe(II) + H2O2 f Fe(IV)(e.g., FeO2+) + H2O
(6)
However, the interpretation of these results as evidence for the occurrence of Fe(IV) at circumneutral pH does not seem convincing. For instance, Vermilyea and Voelker recently stated “while many researchers have attributed decrease in observed product yields as formation of a different (weaker) oxidant instead of HO•, an alternative explanation could involve solution interferences with the mechanism of product formation between HO• and the probe” (9). Similarly, Hug and co-workers (3, 4) also suggested that the Fenton oxidant shifted from HO• to Fe(IV) as solution pH VOL. 45, NO. 1, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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increased, based on the inability of 2-propanol, a typical HO• scavenger, to quench As(III) oxidation by ZVI or Fe(II) in air-saturated water at circumneutral pH (unlike at low pH). However, the reasoning to support the pH-dependent Fenton reaction mechanism does not seem compelling either. For instance, even though methanol (or ethanol) was recognized as a scavenger for HO• but also for Fe(IV) (6, 7), its effect on As(III) oxidation under the same condition as that in the case of 2-propanol was not addressed, which would otherwise provide more direct evidence for the occurrence of Fe(IV). The aim of this study was to reevaluate the relative importance of the oxidants HO• and Fe(IV) and their role in As(III) oxidation during the corrosion of nZVI by O2 via the Fenton reaction over a wide pH range of 2-9. First, we verified and modified an existing method using sulfoxides (e.g., dimethyl sulfoxide (DMSO), methyl phenyl sulfoxide (PMSO), and methyl p-tolyl sulfoxide (TMSO)) as indicators to distinguish between HO• and Fe(IV). Then, reactive oxidants produced in the nZVI/O2 system were identified and quantified using sulfoxides as well as methanol, 2-propanol, and benzoic acid as probe compounds. The latter three were also selected as probes in order to make a direct comparison with previous studies (6, 7). Further, the oxidation of As(III) by nZVI in air-saturated solutions and the effects of oxidant scavengers (e.g., DMSO, methanol, and 2-propanol) were evaluated. Finally, these results were compared to previous studies with regard to the nature of the oxidants produced in the nZVI/O2 system, and the possible reasons for the discrepancy were discussed.
Experimental Section Materials. The stock suspensions of nZVI were prepared daily by aqueous-phase reduction of nitrogen-saturated ferric chloride solution through the dropwise addition of sodium borohydride solution as described previously (e.g., refs 5, 6). A detailed description of nZVI preparation and characterization is provided in the Supporting Information (SI) (Text S1 and Figures S1 and S2). The stock solutions of methyl p-tolyl sulfone (TMSO2) and methyl phenyl sulfone (PMSO2) were prepared by bubbling O3 through TMSO and PMSO solutions (pH 2), where sulfoxides were completely oxidized to corresponding sulfones via the following reaction (reaction 7 10, 11): O3 + PMSO(TMSO) f O2 + PMSO2(TMSO2)
(7)
Other details concerning the materials are provided in SI Text S1. Experimental Procedures. Oxidation of Selected Probe Compounds and As(III) by nZVI in Air-Saturated Water. To initiate the reactions, aliquots of freshly prepared nZVI stock suspension were added to air-saturated solutions containing individual probe compounds (or As(III)) and buffer at room temperature (21 ( 2 °C). The following buffers were used: sodium acetate for pH 4-5, 2-(N-morpholino)ethanesulfonic acid (MES) for pH 6, piperazine-N,N′-bis(ethanesulfonic acid)(PIPES) for pH 7, and sodium borate for pH 8-9. All buffer concentrations were 1-2 mM. MES and PIPES were selected because they do not form complexes with iron (6). Solutions at pH 2 and 3 were adjusted with HCl and NaOH. For these experiments, the pH varied by less than 0.3 units during the reactions. The reactors open to the atmosphere were placed on a shaker table at 150 rpm to keep the particles in suspension. At a predetermined time, samples were withdrawn and filtered immediately through 0.22 µm nylon filters for further analysis. Ozonation of Sulfoxides Catalyzed by Fe(II). To examine whether the reactions between Fe(IV) (in situ-formed by the reaction of O3 with Fe(II); reaction 8) and sulfoxides at pH 308
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2-9 of interest could also proceed via an oxygen-atomtransfer step as observed by Pestovsky and Bakac at strongly acidic pH (10, 11), the following experiments were conducted. O3 + Fe(II) f O2 + FeIVO2+(i.e., Fe(IV))
(8)
Aliquots of O3 stock solution were quickly added to pHbuffered solutions containing individual sulfoxides and/or Fe(II) under vigorous magnetic-stirring. The same pH buffers were used as those in the experiments of nZVI oxidation. Residual O3 was monitored during the reactions, and the final oxidation products (i.e., Fe(III) and sulfones) were analyzed after the reactions (i.e., the complete decomposition of O3). Actually, in experiments involving Fe(II), O3 decayed so quickly that no residue could be measured at the first data point time (≈10 s) under the conditions investigated. In this regard, the analysis of iron speciation and sulfone products was often conducted as soon as possible by acidifying the reaction solutions to minimize the interference of O2 reaction with Fe(II). Oxidation of Sulfoxides by Fe(VI). Experiments using ferrate (Fe(VI)) as a possible source of Fe(IV) to explore its reaction with sulfoxides at high pH were conducted. For these experiments, aliquots of Fe(VI) stock solutions were quickly added to solutions containing individual sulfoxides in large excess (10 mM) and borate buffer (10 mM; pH 8 and 9) under vigorous magnetic-stirring. After the complete consumption of Fe(VI), sulfone products were quantified. The effect of dissolved O2 on Fe(VI) oxidation of sulfoxides was also examined. For this purpose, the reaction solutions and buffers for the preparation of Fe(VI) stock solutions were separately purged with nitrogen gas for 30 min. Analytical Methods. Formaldehyde and acetone were determined by high-performance liquid chromatography (HPLC) and UV-vis absorbance detection after their 2,4dinitrophenylhydrazine derivatization (6). p-HBA and sulfone products were directly analyzed using HPLC with UV and fluorescence detection, respectively. The detection limits were
