Arsenite Binding to Natural Organic Matter: Spectroscopic Evidence for Ligand Exchange and Ternary Complex Formation

The speciation of As in wetlands is often controlled by natural organic matter (NOM), which can form strong complexes with Fe­(III). Here, we elucidated the molecular-scale interaction of arsenite (As­(III)) with Fe­(III)–NOM complexes under reducing conditions. We reacted peat (40–250 μm size fraction, 1.0 g Fe/kg) with 0–15 g Fe/kg at pH <2, removed nonreacted Fe, and subsequently equilibrated the Fe­(III) complexes formed with 900 mg As/kg peat at pH 7.0, 8.4, and 8.8. The solid-phase speciation of Fe and As was studied by electron paramagnetic resonance (Fe) and X-ray absorption spectroscopy (As, Fe). Our results show that the majority of Fe in the peat was present as mononuclear Fe­(III) species (<i>R</i>_Fe–C = 2.82–2.88 Å), probably accompanied by small Fe­(III) clusters of low nuclearity (<i>R</i>_Fe–Fe = 3.25–3.46 Å) at high pH and elevated Fe contents. The amount of As­(III) retained by the original peat was 161 mg As/kg, which increased by up to 250% at pH 8.8 and an Fe loading of 7.3 g/kg. With increasing Fe content of peat, As­(III) increasingly formed bidentate mononuclear (<i>R</i>_As–Fe = 2.88–2.94 Å) and monodentate binuclear (<i>R</i>_As–Fe = 3.35–3.41 Å) complexes with Fe, thus yielding direct evidence of ternary complex formation. The ternary complex formation went along with a ligand exchange reaction between As­(III) and hydroxylic/phenolic groups of the peat (<i>R</i>_As–C = 2.70–2.77 Å). Our findings thus provide spectroscopic evidence for two yet unconfirmed As­(III)–NOM interaction mechanisms, which may play a vital role in the cycling of As in sub- and anoxic NOM-rich environments such as peatlands, peaty sediments, swamps, or rice paddies