Red Mud-Sodium Alginate Hydrogels for Synergistic Adsorption of Pb(II), Cd(II), and Cu(II) from Water: Mechanistic Insights

To overcome the inherent limitations of pristine red mud, such as low adsorption capacity and the risk of secondary toxic leaching, a red mud-sodium alginate (RMSA) hydrogel was synthesized via a calcium chloride cross-linking method for heavy metal remediation. Batch experiments and characterization analyses revealed that the sequestration of heavy metals is driven primarily by ion exchange and complexation, accompanied by specific mineral precipitation for Pb(II). Compared to pristine red mud, the RMSA composite exhibited significantly enhanced maximum adsorption capacities of 454.5, 151.0, and 86.8 mg g –1 for Pb(II), Cd(II), and Cu(II), respectively, well-described by the Langmuir and PSO models. In multimetal systems, RMSA demonstrated preferential uptake of Pb(II) due to its smaller hydration radius. Importantly, practical application tests validated the robust performance of RMSA in complex real wastewater, achieving an 85.5% removal rate for Pb(II). Furthermore, TCLP tests confirmed the absolute environmental safety of the spent hydrogel with a negligible leaching rate (<0.03%), effectively reducing secondary pollution risks. Combined with its moderate reusability over five regeneration cycles and a production cost that is 67.5% lower than that of commercial alternatives, RMSA represents a sustainable and cost-effective strategy for using waste to treat waste in practical wastewater purification and agricultural water management.