Adsorption and heterogeneous reduction of arsenic at the phyllosilicate-water interface

The Bengal Delta Plain (BDP) shallow aquifer is extensively used for public water supply but is severely polluted with naturally occurring arsenic, threatening the health of millions of people. Arsenic (III) is the most toxic and mobile aqueous arsenic species, but homogeneous reduction of As(V) is usually sluggish. Since in our study site field data indicate near equal amounts of As(III) and As(V), we investigated the heterogeneous reduction of arsenic at the muscovite-water interface, as muscovite is a mica frequently found in the BDP sediment. As(V) adsorption on Muscovite was studied as a function of pH and compared to As(III) adsorption. The pH dependence for As(III) and As(V) adsorption are remarkably similar, and this similarity contrasts with the distinct reactivity of many minerals such as Fe and Mn (oxyhydr)ox ides and other phyllosilicates towards the two species. The reacted muscovite samples were investigated by PIXE (Particle Induced X-Ray Emission) and XPS (X-Ray Photoelectron Spectroscopy)spectroscopy, and it was revealed that As(V) is completely reduced upon adsorption. This was also observed for muscovite flakes sampled in the tipper aquifer sediment, and thus only As(III) sorption oil muscovite was described using surface complexation modelling. In the presence of aqueous Fe(II), As(V) is also partly reduced to As(III) at the montmorillonite/water interface. The two investigated mineral surfaces have therefore been shown to catalyse, in different way, the reduction of As(V). Thus arsenic immobilization may be intimately linked to adsorption and electron transfer phenomena occurring at the phyllosilicate/water interface, and this may affect the transport of arsenic in anoxic groundwaters.