Arsenite
Binding to Natural Organic Matter: Spectroscopic
Evidence for Ligand Exchange and Ternary Complex Formation
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Abstract
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><sub>Fe–C</sub> = 2.82–2.88 Å), probably accompanied by small Fe(III)
clusters of low nuclearity (<i>R</i><sub>Fe–Fe</sub> = 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><sub>As–Fe</sub> = 2.88–2.94
Å) and monodentate binuclear (<i>R</i><sub>As–Fe</sub> = 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><sub>As–C</sub> = 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