Is crystalline chromium phosphate environmentally stable? A study on the formation, dissolution and oxidation risk of CrPO4.6H2O

Hexavalent chromium (Cr(VI)) contamination in soil and groundwater is usually remediated via reduction techniques. The formation of crystalline chromium phosphate (CrPO4.6 H2O) occurs as a byproduct during Cr (VI) remediation processes in the presence of phosphate, yet its stability in the environment has received limited attention. In this study, the formation conditions, structure, properties, and risks associated with the dissolution and oxidation of CrPO4.6 H2O were comprehensively assessed. Results showed that crystalline CrPO4.6 H2O was formed under pH 5 - 7 at room temperature. CrPO4.6 H2O exhibits higher dissolution risk compared to Cr (OH)3.3 H2O due to a long Cr-P bond (4.2 angstrom). H+ and OH- increased the risk of dissolution at pH 5 and 11, respectively, owing to the formation of CrH2PO42+ and Cr(OH)4-. In addition, under faintly acidic conditions, the high solubility of CrPO4.6 H2O increases the risk of oxidation; under neutral and weakly alkaline conditions, the presence of positively charged Cr(H2O)63+ structures on the surface elevates its susceptibility to contact and oxidation by 8-MnO2 compared to Cr(OH)3.3 H2O. Specifically, at pH 11, the conversion of CrPO4.6 H2O to Cr (OH)3.3 H2O results in similar oxidation risks for both Cr(III) precipitates.