Iron-rich red mud and iron oxide-modified biochars: A comparative study on the removal of Cd(II) and influence of natural aging processes.

Red mud (RM) is a byproduct of various processes in the aluminum industry and has recently been utilized for synthesizing RM-modified biochar (RM/BC), which has attracted significant attention in terms of waste reutilization and cleaner production. However, there is a lack of comprehensive and comparative studies on RM/BC and the conventional iron-salt-modified biochar (Fe/BC). In this study, RM/BC and Fe/BC were synthesized and characterized, and the influence on environmental behaviors of these functional materials with natural soil aging treatment was analyzed. After aging, the adsorption capacity of Fe/BC and RM/BC for Cd(II) decreased by 20.76% and 18.03%, respectively. The batch adsorption experiments revealed that the main removal mechanisms of Fe/BC and RM/BC are co-precipitation, chemical reduction, surface complexation, ion exchange, and electrostatic attraction, etc. Furthermore, practical viability of RM/BC and Fe/BC was evaluated through leaching and regenerative experiments. These results can not only be used to evaluate the practicality of the BC fabricated from industrial byproducts but can also reveal the environmental behavior of these functional materials in practical applications.