Complexation of Arsenite, Arsenate, and Monothioarsenate with Oxygen-Containing Functional Groups of Natural Organic Matter: An XAS Study

• Arsenic (As) is reported to be effectively sorbed onto natural organic matter (NOM) via thiol coordination and polyvalent metal cation bridged ternary complexation. However, the extent of sorption via complex-ation with oxygen-containing functional groups of NOM is poorly understood. By equilibrating arsenite, arsenate, and monothioarsenate with purified model-peat, followed by As K-edge X-ray absorption spectroscopic analysis, this study shows that complexation with oxygen-containing functional groups can be an additional or alternative mode of As sorption to NOM. The extent of complexation was highest for arsenite, followed by monothioarsenate and arsenate. Complexation was higher at pH 7.0 compared to 4.5 for arsenite and arsenate and vice versa for monothioarsenate because of partial transformation to arsenite at pH 4.5. Modeling of the As K-edge extended X-ray absorption fine structure data revealed that As center dot center dot center dot C interatomic distances were relatively longer in arsenate- (2.83 +/- 0.01 angstrom) and monothioarsenate-treated peat (2.80 +/- 0.02 angstrom) compared to arsenite treatments (2.73 +/- 0.01 angstrom). This study suggests that arsenite was predominantly complexed with carboxylic groups, whereas arsenate and monothioarsenate were complexed with alcoholic groups of the peat. This study further implies that in systems, where NOM is the major sorbent, arsenate and monothioarsenate can have higher mobility than arsenite.

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NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geokemi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geochemistry (hsv//eng)

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