Biogeochemical transformations of arsenic in circumneutral freshwater sediments.

Arsenic is a wide-spread contaminant of soils and sediments, and many watersheds worldwide regularly experience severe arsenic loading. While the toxicity of arsenic to plants and animals is well recognized, the geochemical and biological transformations that alter its bioavailability in the environment are multifaceted and remain poorly understood. This communication provides a brief overview of our current understanding of the biogeochemistry of arsenic in circumneutral freshwater sediments, placing special emphasis on microbial transformations. Arsenic can reside in a number of oxidation states and complex ions. The common inorganic aqueous species at circumneutral pH are the negatively charged arsenates (H2As(V)O4(-) and Has(V)O4(2-)) and zero-charged arsenite (H3As(III)O3(0)). Arsenic undergoes diagenesis in response to both physical and biogeochemical processes. It accumulates in oxic sediments by adsorption on and/or co-precipitation with hydrous iron and manganese oxides. Burial of such sediments in anoxic/suboxic environments favors their reduction, releasing Fe(II), Mn(II) and associated adsorbed/coprecipitated As. Upward advection can translocate these cations and As into the overlying oxic zone where they may reprecipitate. Alternatively, As may be repartitioned to the sulfidic phase, forming precipitates such as arsenopyrite and orpiment. Soluble and adsorbed As species undergo biotic transformations. As(V) can serve as the terminal electron acceptor in the biological oxidation of organic matter, and the limited number of microbes capable of this transformations are diverse in their phylogeny and physiology. Fe(III)-respiring bacteria can mobilize both As(V) and As(III) bound to ferric oxides by the reductive dissolution of iron-arsenate minerals. SO4(2-)-reducing bacteria can promote deposition of As(III) as sulfide minerals via their production of sulfide. A limited number of As(III)-oxidizing bacteria have been identified, some of which couple this reaction to growth. Lastly, prokaryotic and eukaryotic microbes can alter arsenic toxicity either by coupling cellular export to its reduction or by converting inorganic As to organo-arsenical compounds. The degree to which each of these metabolic transformations influences As mobilization or sequestration in different sedimentary matrices remains to be established.