Chelating, Reducing, and Adsorbing Agents in Geopolymers for Heavy Metals Stabilization from Galvanic Sludge

Hazardous galvanic sludge waste (GSW) from the electroplating industry, produced at 100,000–150,000 tonnes/year in the EU and containing high concentrations of Cr and Ni was successfully treated using metakaolin-based geopolymers via Stabilization/Solidification (S/S). The experimental design incorporated chelating (sodium diethyl dithio carbamate, C₅H₁₀NS₂Na, DTC), reducing (sodium sulfide, Na₂S), and adsorbing (hydroxyapatite, Ca₅(PO₄)₃(OH), Hap) agents separately to improve heavy metal immobilization. The results demonstrated that Na₂S drastically decreased Cr release by −98.7% by reducing mobile Cr(VI) to insoluble Cr(III). DTC reduced Ni leaching by −93.4%, forming sparingly soluble Ni(II)(DTC)₂ complexes that precipitated within the matrix. Hap enhanced Ni retention by 55.5% via cation exchange but was ineffective for Cr due to electrostatic repulsion with the anion Cr(VI)O₄²⁻ at the geopolymer’s high pH. This work is the first to apply geopolymerization coupled with these chemical agents for S/S of as-received galvanic waste, offering a highly efficient, low-carbon strategy to manage this hazardous industrial residue.