Arsenate incorporation in gypsum probed by neutron, X-ray scattering and density functional theory modeling

The ability of gypsum, a common sulfate mineral, to host arsenic atoms in its crystalline structure, is demonstrated through experimental structural studies of the solid solutions formed upon synthetic coprecipitation of gypsum (CaSO4 x 2 H2O) and arsenic. Neutron and X-ray diffraction methods show an enlargement of the gypsum unit cell proportional to the concentration of arsenic in the solids and to the pH solution value. The substitution of sulfate ions (SO4(2-)) by arsenate ions is shown to be more likely under alkaline conditions, where the HAsO4(2-) species predominates. A theoretical Density Functional Theory model of the arsenic-doped gypsum structure reproduces the experimental volume expansion. Extended X-ray Absorption Fine Structure (EXAFS) measurements of the local structure around the arsenic atom in the coprecipitated solids confirm solid state substitution and allow some refinement of the local structure, corroborating the theoretical structure found in the simulations. The charge redistribution within the structure upon substitutions of either the protonated or the unprotonated arsenate species studied by means of Mulliken Population Analyses demonstrates an increase in the covalency in the interaction between Ca(2+) and AsO4(3-), whereas the interaction between Ca(2+) and HAsO4(2-) remains predominantly ionic.