Geopolymer-Based Stabilization of Heavy Metals, the Role of Chemical Agents in Encapsulation and Adsorption: Review

This review provides a comprehensive analysis of the role of chemical agents in enhancing the performance of geopolymers for the stabilization and adsorption of heavy metals. Geopolymers, synthesized from aluminosilicate sources activated under alkaline conditions, are recognized for their versatile structural and environmental benefits, including low carbon emissions and high chemical resistance. Their unique Si-O-Al framework supports both stabilization/solidification (S/S) and adsorption processes, making them an ideal polymeric matrix for the immobilization of hazardous heavy metals in contaminated environments. The review categorizes the heavy metal immobilization mechanisms into physical encapsulation, ion exchange, hydroxide precipitation, and chemical complexation, depending on the specific metal species and geopolymer formulation. The introduction of chemical stabilizing agents, such as dithiocarbamate, sodium sulfide, and trimercaptotriazine, significantly improves the encapsulation efficacy of geopolymers by promoting targeted reactions and stable metal complexes. These agents enable the effective S/S of metals, such as lead, cadmium, and chromium, reducing their leachability and environmental impact. In addition to solid waste management applications, geopolymers have shown promising adsorption capabilities for aqueous contaminants, with chemical modifications further increasing their affinity for specific heavy metals. This review evaluates the impact of different agents and synthesis conditions on the overall performance of geopolymers in heavy metal immobilization, highlighting advances in environmental applications and future research directions for sustainable hazardous waste treatment.