Solubility product of chromate analogue of hydrogarnet (Ca3Al2[CrO4]0.12[O4H4]2.88(s)) at 25 °C

Chromium (Cr)-hydrogarnet (Ca3Al2 [CrO4](0.12)[O4H4](2.88(s))) is a critical host of hexavalent chromium [Cr(VI)] in cementitious and alkaline solids such as chromium ore processing residue (COPR). Leaching from COPR often results in Cr(VI) contamination of soil and groundwater. Although Cr-hydrogarnet has been synthesized and characterized previously, its solubility product (K-sp) is poorly constrained. Accurate determination of this thermodynamic parameter is critical to quantify the extent of Cr(VI) dissolution from COPR in water. In this contribution, the K-sp of Cr-hydrogarnet was determined by (i) hydrothermal synthesis of the solid at 150 degrees C in alkaline conditions in two batches; and (ii) batch equilibration experiments over 745 d from undersaturation and over 60 d from supersaturation. For each synthesized batch of solids, Cr-hydrogarnet solubility in ultrapure water at 25 degrees C in triplicate reactors was measured. Time-dependent aqueous samples were collected, and concentrations of Cr(VI) and co-solutes and pH were measured until a steady state was attained from undersaturation, after which the state was perturbed by the addition of 7x Cr(VI) to trigger the supersaturation experiment. X-ray diffraction and SEM-EDS analyses on the reacted solids confirmed the presence of Cr-hydrogarnet and its constituent elements, respectively. In addition, calcite and aluminum oxyhydroxides were formed as minor secondary phases. The log ion activity products at steady states attained from undersaturation and supersaturation were 72.36 +/- 0.86 and 72.75 +/- 0.14, respectively. A mean log K-sp of 72.55 +/- 0.87 at 25 degrees C for the reaction Ca3Al2 [CrO4](0.12)[O4H4](2.88(s)) + 11.52H(+) <-> 3Ca(2+) + 2Al(3+) + 0.12CrO(4)(2-) + 11.52H(2)O was proposed. Furthermore, the new thermodynamic constant was used to analyze data from two previous COPR equilibration studies in water and identify the governing mechanisms of Cr(VI) release.