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3556315
Sorption of methylated arsenic onto mackinawite (FeS)
Date
April 5, 2021
Session: Molecular Processes at Mineral-Water Interfaces:
In anaerobic subsurface environments, arsenic (As) is mobilized by the reduction of arsenate (As(V)) to more mobile arsenite (As(III)) and by the reductive dissolution of iron (Fe) oxide phases. In conditions with elevated sulfur concentrations, the production of sulfide (S(-II)) by sulfate-reducing bacteria leads to the precipitation of iron sulfide (FeS) minerals. Disordered mackinawite is the first FeS mineral to form in these sulfidic environments and has been observed as an effective adsorbent for inorganic As(V) and As(III). Growing attention to the global problem of As uptake into rice has directed a renewed interest to the fate and transport of methylated As species in anaerobic paddy soil environments. However, interactions of methylated arsenic species with FeS minerals have not been studied and the potential role of mackinawite in immobilizing monomethylarsonic acid (MMAs(V)) and dimethylarsinic acid (DMAs(V)) in sulfidic conditions remains largely unexplored. This contribution investigates the sorption of MMAs(V) and DMAs(V) onto synthetic mackinawite to assess the effects of FeS precipitation on the mobility of methylated As compounds, and the effect of methyl group substitution on As-FeS interactions. Through a combination of sorption isotherm experiments, kinetic studies, and X-ray absorption spectroscopy (XAS), we examine effects of the As:FeS ratio on FeS solubility, determine mechanisms for MMAs(V) and DMAs(V) sorption on FeS surfaces, and evaluate the reversibility of sorption reactions. FeS solubility increased at higher As:FeS ratios. Freundlich model fits indicated that affinity constants of FeS for MMAs(V) and DMAs(V) were similar. Furthermore, XAS analysis of mackinawite reacted with MMAs(V) and DMAs(V) revealed that As was reduced and there was a difference in As-S coordination with increasing methyl group substitution. There was no evidence for abiotic demethylation via reactions between methylated As species and FeS. This research demonstrates that MMAs(V) and DMAs(V) are effectively immobilized via sorption onto mackinawite surfaces and is critical in further studying how the extent of sorption of As onto FeS minerals may be influenced by factors affecting environmental conditions such as pH, ionic strength, and the presence of natural organic matter (NOM), which has shown to be highly reactive towards metals and mineral surfaces.