Bisulfide Reaction with Natural Organic Matter Enhances Arsenite Sorption: Insights from X‑ray Absorption Spectroscopy

Terrestrial ecosystems rich in natural organic matter (NOM) can act as a sink for As. Recently, the complexation of trivalent As by sulfhydryl groups of NOM was proposed as the main mechanism for As–NOM interactions in anoxic S- and NOM-rich environments. Here we tested the molecular-scale interaction of bisulfide (S­(-II)) with NOM and its consequences for arsenite (As­(III)) binding. We reacted 0.2 mol C/L peat and humic acid (HA) with up to 5.8 mM S­(-II) at pH 7 and 5, respectively, and subsequently equilibrated the reaction products with 55 μM As­(III) under anoxic conditions. The speciation of S and the local coordination environment of As in the solid phase were studied by X-ray absorption spectroscopy. Our results document a rapid reaction of S­(-II) with peat and HA and the concomitant formation of reduced organic S species. These species were highly reactive toward As­(III). Shell fits of As <i>K</i>-edge extended X-ray absorption fine structure spectra revealed that the coordination environment of trivalent As was progressively occupied by S atoms. Fitted As–S distances of 2.24–2.34 Å were consistent with sulfhydryl-bound As­(III). Besides As­(III) complexation by organic monosulfides, our data suggests the formation of nanocrystalline As sulfide phases in HA samples and an As sorption process for both organic sorbents in which As­(III) retained its first-shell oxygens. In conclusion, this study documents that S­(-II) reaction with NOM can greatly enhance the ability of NOM to bind As in anoxic environments