ARSENITE SORPTION AND CO-PRECIPITATION WITH CALCITE

Abstract

Sorption of As(III) by calcite was investigated as a function of As(III) concentration, time and pH. The sorption isotherm, i.e. the log Γ As(III) vs. log [As(OH)3° / Assat] plot is S-shaped and has been modelled on an extended version of the surface precipitation model [Farley, K.J., Dzombak, D.A., Morel, F.F.M., 1985. A surface precipitation model for the sorption of cations on metal oxides. Journal of Colloid and Interface Science 106 (227-242); Wersin, P., Charlet, L., Kathein, R., Stumm, W., 1989. From adsorption to precipitation: sorption of Mn2+ to FeCO3(s). Geochimica et Cosmochimica Acta 53, 2787-2796]. At low concentrations, As(OH)3° is adsorbed by complexation to surface Ca surface sites, as previously described by the X-ray standing wave technique [Cheng, L.W., Fenter, P., Sturchio, N.C., Zhong, Z., Bedzyk, M.J., 1999. X-ray standing wave study of arsenite incorporation at the calcite surface. Geochimica et Cosmochimica Acta 63 (19-20), 3153-3157]. The inflexion point of the isotherm, where As(OH)3° is limited by the amount of surface sites (ST), yields 6 sites nm- 2 in good agreement with crystallographic data. Beyond this value, the amount of sorbed arsenic increases linearly with solution concentration, up to the saturation of arsenic with respect to the precipitation of CaHAsO3(s). The solid solutions formed in this concentration range were examined by X-ray and neutron diffraction. The doped calcite lattice parameters increase with arsenic content while c/a ratio remains constant. Our results made on bulk calcite on the atomic displacement of As atoms along [0001] direction extend those published by Cheng et al. [Cheng, L.W., Fenter, P., Sturchio, N.C., Zhong, Z., Bedzyk, M.J., 1999. X-ray standing wave study of arsenite incorporation at the calcite surface. Geochimica et Cosmochimica Acta 63 (19-20), 3153-3157] on calcite surface. This study provides a molecular-level explanation for why As(III) is trapped by calcite in industrial treatments. © 2006 Elsevier B.V. All rights reserved.