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 complexation to oxygen containing functional groups of NOM is poorly understood. By equilibrating arsenite, arsenate, and monothioarsenate with peat, followed by As K-edge X-ray absorption spectroscopic analysis, this study shows that complexation to the alcoholic 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 due to partial transformation to arsenite at pH 4.5. EXAFS modelling of the As K-edge spectra revealed monodentate and bidentate complexation for arsenite and tridentate complexation for arsenate and monothioarsenate to the alcoholic group. Similarly, the As…C interatomic distance was relatively longer in arsenate- (2.83 ± 0.01 Å) and monothioarsenate-treated peat (2.80 ± 0.02 Å) compared to arsenite-treatment (2.73 ± 0.01 Å). This study implies that depending on acidity of the NOM, arsenate and monothioarsenate can have a higher mobility than arsenite in NOM-rich environments.