Removal of cadmium and zinc by calcium polysulfide in acidic groundwater: Injection ratio and precipitation mechanism.

In this study, we conducted lab-scale experiments to assess the applicability of calcium polysulfide (CPS) for the removal of cadmium (Cd) and zinc (Zn) from acidic groundwater, with an emphasis on the injection ratio and removal mechanism. For the experiments, CPS was used to treat Cd⁺ and Zn⁺ contaminated solution. Solutions were purged with nitrogen gas, and CPS was injected in an anaerobic chamber. After 18 h, solutions were filtered, and heavy metal concentrations were measured using ICP-MS and ICP-OES. Total polysulfide (S⁻) concentration in CPS was determined by converting it to bisulfide (HS⁻) at pH 8.20 and quantifying via UV-Vis spectrophotometry. Precipitates were analyzed using XRD and SEM-EDS after centrifugation. The findings revealed that more than 99.5% of the heavy metals were removed when CPS/Cd (w/w) = 1.45 and CPS/Zn (w/w) = 2.50, determined as the injection ratios for maximum efficiency. These ratios were applicable when Cd and Zn coexisted. From the XRD and SEM-EDS analyses, it was clarified that Cd and Zn precipitated in sulfide forms, consistently showing the preferential precipitation of Cd because of the lower solubility of cadmium sulfide compared to zinc sulfide. In addition, the concentration of S in the 29% CPS solution was determined to be approximately 2.442 M. Finally, by comparing the injected S concentration with the concentration of heavy metals removed accordingly, it was concluded that CPS and heavy metals react with a 1:1 M ratio. Based on the above results and precise quantification method, our study suggests that CPS can be effectively utilized to address heavy metal contamination issues and may serve as a valuable tool for the remediation and management of contaminated groundwater globally.