Simultaneous stabilization of cadmium and arsenic in soil by humic acid and mechanically activated phosphate rock

Competition between phosphorus and arsenic limits the application of phosphate materials in soil remediation. However, it is possible to simultaneously stabilize arsenic and cationic metals by sensible use of phosphate's solubility. In this study, ball-milling and humic acid (HA) activated phosphate rock (PR) were used to stabilize cadmium (Cd) and arsenic (As) in soil. After 30 days of treatment with ball-milling and 5 % humic acid-activated phosphate rock (BMP-HA), the leaching concentrations of Cd and As in the soil decreased from 0.12 mg/L and 0.11 mg/L to 0.0086 mg/L and 0.019 mg/L, respectively. The availability of Cd and As was significantly reduced after BMP-HA treatment. The acid-soluble fraction of Cd decreased from 16.30 % to 1.22 %, indicating its transformation into more stable forms. The water-soluble and surface-adsorbed fraction of As decreased to 0.11 %, while the Ca-associated fraction of As increased from 25.87 % to 31.41 %. Ball-milling increased the specific surface area of PR, enhancing the adsorption and surface complexation of Cd. The addition of humic acid facilitated the dissolution of PR. However, the phosphate release rate in BMP-HA was insufficient to activate As. Meanwhile, the released Ca bound with As, further reducing its availability. Overall, BMP-HA proves to be an effective strategy for remediating cadmium and arsenic-contaminated soils.