| Abstract: |
In natural environments, arsenic chemistry is dominated by the reactions of its two predominant soluble forms, arsenate and arsenite. To predict the fate of As in the environment, it is necessary to consider processes that act to restrict its mobility. The mobility of As is strongly influenced by adsorption reactions to particle surfaces. Arsenate and arsenite 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 oxides, in particular Fe oxides. We have analysed the existing data for arsenite and arsenate adsorption to ferrihydrite and goethite. Spectroscopic results show that arsenate forms 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, the diffuse double-layer model (DLM) and the three-plane CD-MUSIC model (TPCD), taking into account the spectroscopic evidence. For arsenate adsorption to ferrihydrite, the new DLM constants imply stronger binding than the previous compilation by Dzombak and 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 larger than 10 mM; the reasons for this are discussed. Simulations of competition effects show that o-phosphate competes strongly with arsenate over the whole pH range. Silicic acid and carbonate are important competitors in the circumneutral pH range, while sulphate may have a small competitive effect at low pH. Humic substances are important competitors when a large part of the Fe oxides is covered with humic substances. By contrast, calcium promotes arsenate adsorption at alkaline pH because of surface charge effects. |
