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Vega, Alejandra ; Planer-Friedrich, Britta ; Pastén, Pablo A.:
Arsenite and arsenate immobilization by preformed and concurrently formed disordered mackinawite (FeS).
In: Chemical Geology.
Bd. 475
(2017)
.
- S. 62-75.
ISSN 0009-2541
DOI: https://doi.org/10.1016/j.chemgeo.2017.10.032
Abstract
Arsenic (As) immobilization in aqueous environments is controlled by sorption and/or coprecipitation with iron (Fe) oxyhydroxides under oxic conditions and with ferrous iron (Fe(II)) and sulfide (S(-II)) minerals under anoxic conditions. Shifts to reducing conditions trigger the release of dissolved As by reductive dissolution of Fe oxyhydroxides. More stringent reducing conditions in sulfur (S) rich environments form solid S(-II) phases that immobilize As. This work studied the association of arsenite and arsenate with freshly-formed disordered mackinawite (FeS), a model Fe and reduced S mineral of environmental relevance. The association of As to preformed FeS (Set P experiments) was contrasted to the case when As is present during FeS precipitation (Set C experiments). Most abiotic studies of As reactivity under reducing conditions have been done under Set P setting, despite that redox shifts are likely to induce environments analogous to Set C experiments. Dissolved As removal was initially higher for Set C compared to Set P experiments at least by a factor of two, except for arsenate with low initial redox potential. Set C experiments had a rapid initial As sorption, followed by some As desorption. When stringent reducing conditions ensue, the extent and rate of As removal by FeS is misrepresented by sorption experiments to preformed FeS. A continuous release of As concurrent with the formation of thioarsenates –a dissolved form of As with weaker binding to Fe minerals- was observed in Set C experiments with arsenite. The formation of thioarsenates occurred preferably in Set C experiments at low redox potential, while it was negligible for arsenate experiments. Remarkably, the occurrence of thioarsenates disputes a widespread notion that their formation is inhibited by Fe(II)-S(-II) affinity and precipitation as FeS. This study shows the potential of mackinawite to scavenge arsenite and to decrease its mobility in reduced sediments and groundwater and highlights the role of thioarsenates in As mobility in systems with changing redox conditions where the reactive solid forms in presence of the contaminant.