Immobilization of Cadmium (Cd) and Zinc (Zn) Using Steel Slag Fines

The immobilization potential of dissolved Cd and Zn (each ~750 mg/L) in independently packed columns of freshly crushed steel slag fines (SSF) media was evaluated. The SSF media was characterized as USCS SP type soil with <5% passing the No. 200 (0.075 mm) sieve and the flow rates were on the order of a meter per day with a residence time on the order of 3.5 hours for a 25-cm long column containing approximately 5 kg of SSF media. The influent pH of the Cd and Zn solutions were on the order of 5.8 and 7.2, respectively, and the pH of the effluent for the test duration of 100 pore volume (PVs) remained above 11.5. The dissolved concentrations of Cd in the effluent remained below the detection limit (0.05 mg/L) for the entire test duration; whereas the Zn concentrations exceeded 1 mg/L at approximately 48 PV. Independent acid neutralization capacity (ANC) testing of Cd-And Zn-spiked SSF media (~500 mg/L each) coupled with geochemical modeling indicated that Cd was insoluble within the SSF. Similar trends were initially observed with the Zn testing suite, except that the aqueous solubility minimum of Zn occurs near pH=9; and its amphoteric behavior above that pH value meant it prematurely broke through the packed SSF column.