Geochemical modelling of arsenic adsorpt

In natural environments, arsenic chemistry is dominated by the reactions of its two predominant soluble forms arsenate and arsenite. To predict the fate of arsenic in the environment, it is necessary to consider processes that act to restrict its mobility. The mobility of arsenic is strongly influenced by adsorption reactions to particle surfaces. Arsenate and arsenate may form surface complexes with a number of different oxides, including Fe, Al, Mn and Ti oxides. The focus of this chapter is on the adsorption of As(III) and As(V) to the surfaces of oxide surfaces, in particular Fe oxides. We have analysed the existing data for arsenite and arsenate adsorption to ferrihydrite and goethite. Spectroscopic results show that arsenate form bidentate binuclear complexes under all conditions; for arsenite, evidence has been found both for a bidentate binuclear complex and for a weaker outer-sphere complex, which may be of some importance at low ionic strength. We optimized As adsorption parameters for two surface complexation models, Diffuse Layer Model (DLM) and Three-Plane CD-2 MUSIC Model (TPCD) taking into account these spectroscopic evidence. For arsenate adsorption to ferrihydrite, the new DLM constants imply stronger binding than the previous compilation by Dzombak & Morel (1990), whereas for arsenite the revised DLM constants are in reasonable agreement. The surface complexation models could not be optimized satisfactorily for data sets in which the dissolved arsenite concentration at equilibrium was large.