Re-dissolution of Cadmium Sulfide formed by Calcium Polysulfide in the Presence of Dissolved Oxygen

 Precipitating cadmium in groundwater as sulfide using calcium polysulfide (CPS) has advantages over hydroxide precipitation, as it is less affected by pH variations and exhibits a lower solubility product constant. However, dissolved oxygen (DO) in groundwater can oxidize the precipitated cadmium sulfide (CdS), leading to its remobilization. From this perspective, this study aimed to evaluate the long-term stability of cadmium precipitated by CPS and to elucidate the stabilization mechanisms.

 First, under anoxic conditions, Cd²⁺ (100 mg/L) was completely precipitated as CdS by adding CPS. The precipitates were then oxidized in ambient air in two different conditions: (1) with the supernatant (SN) and (2) without the supernatant (in deionized water, DI). Both conditions were subjected to stirring at 250 rpm to maximize DO contact. After 27 days of exposure, the Cd release was only 4.2% in SN, while in DI, 60.5% of the Cd²⁺ was re-dissolved, indicating more than 10 times greater dissolution. In addition, X-ray photoelectron spectroscopy (XPS) analysis revealed that the Cd in SN remained consistent with the CdS reference peaks. Based on the measurements of pH, DO, ion chromatography (IC), and zeta potential results, it was concluded that polysulfide (S_x^2-) and bisulfide (HS^-) in the SN supernatant reacted with DO, preventing CdS oxidation. Moreover, the formation of elemental sulfur (S⁰) from S_x^2- and HS^- oxidation contributed to physical sequestration, which inhibited the dissolution of CdS. The experiments were conducted under higher DO levels than typical groundwater, creating a more oxidative environment. Thus, our research suggests that CPS could effectively stabilize cadmium in groundwater, with minimal risk of remobilization under the presence of DO.