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Oxidation of arsenite (As(Ul)) is a critical yet often weak link in many current technologies for remediating contaminated groundwater. We report a novel, efficient oxidation reaction for As(lll) conversion to As(V) using commercial available peroxymonosulfate (PMS). As(lll) is rapidly oxidized by PMS with a utilization efficiency larger than 90%. Increasing PMS concentrations and pH accelerate oxidation of As(lll), independent to the availability of dissolved oxygen. The addition of PMS enables As(lII) to oxidize completely to As(V) within 24 h, even in the presence of high concentrations of radical scavengers. On the basis of these observations and theoretical calculations, a two-electron transfer (i.e., oxygen atom transfer) reaction pathway is proposed. Direct oxidation of As (III) by PMS avoids the formation of nonselective reactive radicals, thus maximizing the adverse impact of coexisting organic matter and maximizing the utilization efficiency of PMS. Therefore, this simple approach is considered a cost-effective water treatment method for the oxidation of As(lll) to As(V). [ABSTRACT FROM AUTHOR]

Arsenic and chromium are often abundant constituents of acid mine drainage (AMD) and are most harmful as arsenite (As(III)) and hexavalent (Cr(VI)). To simultaneously change their oxidation state from As(III) to As(V), and Cr(VI) to Cr(III), is a potentially effective and attractive strategy for environmental remediation. The coabundance of As(III) and Cr(VI) in natural environments indicates their negligible direct interaction. The addition of H2O2 enables and greatly accelerates the simultaneous oxidation of As(III) and reduction of Cr(VI). These reactions are further enhanced at acidic pH and higher concentrations of Cr(VI). However, the presence of ligands (i.e., oxalate, citrate, pyrophosphate) greatly retards the oxidation of As(III), even though it enhances the reduction of Cr(VI). To explain these results we propose a reaction mechanism where Cr(VI) is primarily reduced to Cr(III) by H2O2, via the intermediate tetraperoxochromate Cr(V). Cr(V) is then involved in the formation of •OH radicals. In the presence of ligands, the capacity of Cr(V) to form •OH radicals, which are primarily responsible for As(III) oxidation, is practically inhibited. Our findings demonstrate the feasibility for the coconversion of As(III) and Cr(VI) in AMD and real-world constraints to this strategy for environmental remediation. [ABSTRACT FROM AUTHOR]

This study describes the sorption of As(V) and As(III) to schwertmannite as a function of pH and arsenic loading. In general, sorption of As(V) was greatest at low pH, whereas high pH favored the sorption of As(III). The actual pH of equivalent As(V) and As(III) sorption was strongly loading dependent, decreasing from pH ∼ 8.0 at loadings <120 mmolAs molFe-1 to pH ∼ 4.6 at a loading of 380 mmolAs molFe-1. Sorption isotherms for As(V) were characterized by strong partitioning to the schwertmannite solid-phase at low loadings and sorption capacities of 225-330 mmolAs(V) molFe-1 at high loadings. In contrast, the As(III) isotherms revealed a weak affinity for sorption of As(III) versus As(V) at low loadings yet a greater affinity for As(III) sorption compared with As(V) at high loadings (when pH > 4.6). Sorption of As(V) and As(III) caused significant release of SO42- from within the schwertmannite solid-phase, without major degradation of the schwertmannite structure (as evident by X-ray diffraction and Raman spectroscopy). This can be interpreted as arsenic sorption via incorporation into the schwertmannite structure, rather than merely surface complexation at the mineral—water interface. The results of this study have important implications for arsenic mobility in the presence of schwertmannite, such as in areas affected by acid-mine drainage and acid-sulfate soils. In particular, arsenic speciation, arsenic loading, and pH should be considered when predicting and managing arsenic mobility in schwertmannite-rich systems. [ABSTRACT FROM AUTHOR]

The effect of acid-volatile sulfide (AVS) oxidation on Fe-S cycling and water quality in coastal flood plain drains from acid-sulfate soil landscapes was examined using natural sediments and synthetic iron monosulfide. Oxidation of AVS occurred rapidly (half-time ≤ 1 h) and produced elemental sulfur (S80(s)) and iron oxyhydroxide (FeOOH(s)). The initial rapid AVS oxidation process occurred without significant acidification or changes to the aqueous-phase composition. Severe acidification (pH < 4) occurred only once S80(s) began to oxidize to SO4 (within 2–3 days of the initial AVS oxidation). Our results demonstrate, for the first time with natural sediments, a significant pH-buffered (near-neutral) AVS oxidation step with the trigger to acidification being the oxidation of S80(s). Acidification resulted in the pH-dependent release of large amounts of Al, Mn, Ni, and Zn even though the sediment metal content was low. [ABSTRACT FROM AUTHOR]

We examined processes regulating reduced inorganic sulfur (RIS) speciation in drain sediments from coastal acid sulfate soil (ASS) landscapes. Pore water sulfide was undetectable or present at low levels (0.6-18.8 μM), consistent with FeS(s) precipitation in the presence of high concentrations of Fe2+ (generally >2 mM). Acid-volatile sulfide (AVS), with concentrations up to 1019 μmol g-1, comprised a major proportion of RIS. The AVS to pyrite-S ratios were up to 2.6 in sediment profiles containing abundant reactive Fe (up to ∼4000 μmol g-1). Such high AVS:pyrite-S ratios are indicative of inefficient conversion of FeS(S) to pyrite. This may be due to low pore water sulfide levels causing slow rates of pyrite formation via the polysulfide and H2S oxidation pathways. Overall, RIS speciation in ASS-associated drain sediments is unique and is largely regulated by abundant reactive Fe. [ABSTRACT FROM AUTHOR]

Oxidation of arsenite (As(III)) is a critical yet often weak link in many current technologies for remediating contaminated groundwater. We report a novel, efficient oxidation reaction for As(III) conversion to As(V) using commercial available peroxymonosulfate (PMS). As(III) is rapidly oxidized by PMS with a utilization efficiency larger than 90%. Increasing PMS concentrations and pH accelerate oxidation of As(III), independent to the availability of dissolved oxygen. The addition of PMS enables As(III) to oxidize completely to As(V) within 24 h, even in the presence of high concentrations of radical scavengers. On the basis of these observations and theoretical calculations, a two-electron transfer (i.e., oxygen atom transfer) reaction pathway is proposed. Direct oxidation of As(III) by PMS avoids the formation of nonselective reactive radicals, thus minimizing the adverse impact of coexisting organic matter and maximizing the utilization efficiency of PMS. Therefore, this simple approach is considered a cost-effective water treatment method for the oxidation of As(III) to As(V).