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

ABSTRACT

The 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.

KEYWORDS:

• Red mud
• liquid radioactive waste
• IEP; ⁶⁰Co
• ⁹⁰Sr
• sorption
• desorption
• competition

Funding

This work was supported by the Ministry of Education Science and Technological Development of the Republic of Serbia (Project III 43009).