Enhancing Cd(II) sorption by red mud with heat treatment: Performance and mechanisms of sorption

Red mud is a waste generated from the aluminum industry in large quantities. The potential of red mud as a sorbent for beneficial reuse has been the focus of research efforts. However, the limited sorption capacity of red mud has hindered its applications in the removal of environmental pollutants. In this study, the feasibility of heat treatment in improving the sorption of toxic Cd(II) by red mud was investigated in the temperature range of 200-900 °C. Heat treatment at 500 °C resulted in the highest sorption capacity (42.64 mg g^-1) and the fastest sorption rate. Further analyses revealed that heat treatment at 500 °C led to significant increases in specific surface area (32.77 m² g^-1), which likely contributed to the enhanced Cd(II) sorption performance. Notably, heat treatment at 500 °C nearly doubled Cd(II) sorption stability as compared with that of raw red mud, as demonstrated by leaching experiments with simulated rainwater. Sequential extraction and XPS analyses indicated that specific sorption was the predominant mechanism involved in Cd(II) removal by red mud heat-treated at 500 °C (RM500). The strength of specific sorption following heat treatment likely contributed to the increase in sorption stability due to the formation of inner-sphere complex (-OCdOH). Metal-metal ion exchange was identified as another sorption mechanism, which, however, likely had only a limited effect on Cd(II) sorption performance. As the final pH (6.57) of the sorption system was typically lower than the pH_PZC (about 10.6) of RM500, positive charges would develop on the red mud surface and impede the retention of Cd(II) cations, resulting in weak electrostatic attraction between Cd(II) cations and red mud. In summary, heat treatment at 500 °C considerably enhanced the capacity, rate and stability of Cd(II) sorption by red mud, suggesting red mud could be optimized by heat treatment as a more effective sorbent for Cd(II) removal. These findings represent the first mechanistic characterization of Cd(II) sorption by heat-treated red mud, providing much needed insights into the potential strategies to enhance the effectiveness of red mud in the sorptive removal of toxic heavy metals.