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

Arsenic is a toxic and carcinogenic contaminant of potential concern. Ettringite [Ca6Al2(SO4)3(OH)12·26H2O] has the ability to incorporate oxyanions as a solid solution with SO42−, which could lower the soluble oxyanion concentrations. Therefore, ettringite containing SO42− and AsO43− has been synthesized. Results indicated that AsO43− could substitute for SO42− inside the channels of ettringite in the form of HAsO42−, and a linear correlation existed between Xinitial solution and Xsolid. 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 HAsO4–ettringite at 25°C. The Lippmann phase diagram and XHAsO4–XHAsO4,aq plot showed that the solid solution series containing arsenate has HAsO4-poor aqueous solutions in equilibrium. These findings can be helpful to arsenate solidification and arsenate leaching modeling projects.