Synthesis and thermodynamic properties of arsenate and sulfate-arsenate ettringite structure phases

Arsenic is a toxic and carcinogenic contaminant of potential concern. Ettringite [Ca(6)Al(2)(SO(4))(3)(OH)(12)·26H(2)O] has the ability to incorporate oxyanions as a solid solution with SO(4)(2−), which could lower the soluble oxyanion concentrations. Therefore, ettringite containing SO(4)(2−) and AsO(4)(3−) has been synthesized. Results indicated that AsO(4)(3−) could substitute for SO(4)(2−) inside the channels of ettringite in the form of HAsO(4)(2−), and a linear correlation existed between X(initial solution) and X(solid). The thermodynamic characterization of the solid samples was investigated by means of Visual MINTEQ, a freeware chemical equilibrium model, and the solubility product logK of -48.4 ± 0.4 was calculated for HAsO(4)–ettringite at 25°C. The Lippmann phase diagram and X(HAsO4)–X(HAsO4,aq) plot showed that the solid solution series containing arsenate has HAsO(4)-poor aqueous solutions in equilibrium. These findings can be helpful to arsenate solidification and arsenate leaching modeling projects.