Effect of iron redox transformations on arsenic solid-phase associations in an arsenic-rich, ferruginous hydrothermal sediment

Well-constrained laboratory incubations of a ferruginous marine hydrothermal sediment from Santorini, Greece, were used to elucidate the effect of microbially induced redox transformations on arsenic speciation and mobility. Despite naturally high arsenic concentrations (∼400mg/kg), the sediment has a low As:Fe ratio (1:1000wt/wt). Acetate-amendment of sediment, extracted from the naturally-occurring suboxic–anoxic (Eh −60 to −138mV) transition zone, promoted Fe(III) reduction, and increased the concentration of Fe(II) from ∼40% to ∼60% in the bulk sediment. Sulfate, which was present at lower concentrations, was also reduced. Phylogenetic 16S rRNA and dsr gene analysis suggested that Fe(III) and sulfate were reduced by bacteria related to Malonomonas rubra and Desulfosarcina variabilis, respectively. Arsenic remained predominantly as arsenic trioxide (As₂O₃) throughout the amendment experiment. However, the percentage of total arsenic present within poorly-crystalline iron oxides decreased from ∼69% to ∼32%, while the percentage incorporated within crystalline iron-containing minerals or sorbed to surfaces via inner-sphere complexes increased significantly (to 22% and 30%, respectively). Re-oxidation of the system with nitrate resulted in incomplete reduction of the nitrate pool, and partial re-association of arsenic with the poorly-crystalline iron fraction. Exposure to air led to virtually complete reversal of the arsenic partitioning, and oxidation of 71% As(III) to As(V). During aeration, oxidation of sediment-bound sulfur/sulfide occurred, alongside an observed ∼63% decrease in arsenic bound to this minor component. Analogous trends in arsenic-sediment associations were observed in the natural, unamended sediment depth-profile, whereby a greater proportion of arsenic (34% As(III), 66% As(V)) was bound within poorly-crystalline iron oxides at the sediment–water interface. Arsenic (96% As(III)) was increasingly incorporated within well-crystallized forms of iron with depth and decreasing Eh values. At the greatest depth sampled (35cm) arsenic increased substantially within the sulfide/organic fraction. Results here contribute to existing evidence that arsenic is not necessarily released from iron-rich sediment systems under conditions of anoxia, but that Fe(II)-bearing minerals forming concomitantly can immobilize arsenic in the solid-phase. Such results may have implications for other systems with high Fe:As ratios.