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.