Transformation of arsenic in offshore sediment under the impact of anaerobic microbial activities.

Sediment bound arsenic usually undergoes phase transformation processes when it is transported and buried in deeper settings. This work investigated anaerobic microbial mediated speciation change of the arsenic in offshore sediment and monitored the transformation process of oxyhydroxide associated arsenate to sulfide associated forms. The fate of arsenic and possible pathways of transformation were discussed based on quantitative analysis of aqueous and solid arsenic and iron, and qualitative characterization using X-ray absorption near edge spectroscopy (XANES). Arsenic was released and reduced upon development of anoxic conditions but was resequestered by authigenic minerals later. Most of the arsenic in the sediment was converted to orpiment-like material. Sulfide may have played double roles in arsenic redistribution process, i.e. promoting arsenic release from host oxyhydroxides in early stage and removal of arsenite from solution in the form of arsenic sulfide in later stage. The findings have implications about the pathways of arsenic transformation when arsenate is transported and buried below redox boundaries in offshore sediment.