Effect of experimental variables onto Co2+ and Sr2+ sorption behavior in red mud-water suspensions

Abstract

ABSTRACTThe prospects of rinsed red mud (alumina production residue) utilization for liquid radioactive waste treatment have been investigated, with Co2+ and Sr2+ as model cations of radioactive elements. To evaluate the sorption effectiveness and corresponding binding mechanisms, the process was analyzed in batch conditions, by varying experimental conditions (pH, Co2+ and Sr2+ concentrations in single solutions and binary mixtures, contact time, and the concentration of competing cations and ligands common in liquid radioactive waste). Comparison of the Co2+ and Sr2+ sorption pH edges with the red mud isoelectric point has revealed that Co2+ removal took place at both positive and negative red mud surface, while Sr2+ sorption abruptly increased when the surface became negatively charged. The increase of initial cation content and pH resulted in increased equilibrium times and sorption capacity and decreased rate constants. From single metal solutions and various binary mixtures, Co2+ was sorbed more efficiently and selectively than Sr2+. While Sr2+ sorption was reduced by coexisting cations in the order Al3+ ≥ Ca2+ >Na+ ≥Cs+, removal of Co2+ was affected by Al3+ species and complexing agents (EDTA and citrate). Desorption of Co2+ was negligible in Ca2+ and Sr2+ containing media and in solutions with initial pH 4–7. Sr2+ desorption was generally more pronounced, especially at low pH and in the presence of Co2+. Collected macroscopic data signify that Co2+ sorption by red mud minerals occurred via strong chemical bonds, while Sr2+ was retained mainly by weaker ion-exchange or electrostatic interactions. Results indicate that the rinsed red mud represent an efficient, low-cost sorbent for Co2+ and Sr2+ immobilization.