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

Yunyi Li; Wenshuai Zhou; Xinmiao Huang; Yutong Wang; Jialiang Liang; Yangsheng Liu; Meiping Tong

doi:10.1016/j.jhazmat.2023.133112

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

J Hazard Mater. 2024 Mar 5:465:133112. doi: 10.1016/j.jhazmat.2023.133112. Epub 2023 Nov 28.

Authors

Yunyi Li¹, Wenshuai Zhou², Xinmiao Huang², Yutong Wang², Jialiang Liang², Yangsheng Liu³, Meiping Tong³

Affiliations

¹ Key Laboratory of Three Gorges Reservoir Region's Eco-Environment, Ministry of Education, Chongqing University, Chongqing 400045, PR China. Electronic address: liyunyi@cqu.edu.cn.
² Key Laboratory of Three Gorges Reservoir Region's Eco-Environment, Ministry of Education, Chongqing University, Chongqing 400045, PR China.
³ College of Environmental Sciences and Engineering, Peking University, Beijing 100871, PR China.

PMID: 38043420
DOI: 10.1016/j.jhazmat.2023.133112

Abstract

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 Å). 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 δ-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.

Keywords: Chromium phosphate; Groundwater; Risk; Soil; Stability.