Arsenic binding onto phyllosilicates and glutathione: Soil immobilisation and human excretion mechanisms

The binding of As (both III and V) on phyllosilicates, such as micas (biotite), clay (montmorillonite), or Callovo Oxfordian clayrock (rich in smectite/illite) suspensions, and with glutathione in presence of selenium has been investigated by chemical and spectroscopic means. Sorption is fast for As(V) and slow for As(III). The kinetics of As(III) adsorption on micas in presence of dissolved oxygen suggests a surface catalyzed oxidation of As(III). The As(V) sorbed on the edge of the phyllosilicate crystal is present as a binuclear bidentate inner-sphere surface complex, in which As binds two adjacent Al and Mg octahedra. This As(V) complex is rather stable against reduction by the bio-mimetic electron shuttle AH(2)DS(2-), as observed in experiments in which alternating oxic and anoxic conditions are imposed. Therefore, although usually assumed to be of neglictable importance, e.g. compared to Fe oxyhydroxides, phyllosilicates could represent an important sink of As in soils and sediments. The arsenic which is not retained by phyllosilicates (and Fe oxyhydroxides) reaches the aquifer drinking water reservoirs where it threatens roughly 80 million people in SE Asia. Although chronic arsenic exposure gives rise to arsenicosis, a syndrome that is characterized by skin lesions, increased risks to cancers of the lung, skin, bladder, and liver, and death, it was found to be inhibited by selenium, thanks to the formation and excretion of a seleno-bis(S-glutathione) arsinium [(GS)(2) AsSe](-) ion. Interaction of arsenic with clays and glutathione was investigated by X-ray absorption spectroscopy and molecular dynamics modeling.