Sustainable chemical approach for room temperature immobilization by geopolymerization of heavy metals cations and amphoteric moieties from galvanic waste

This study investigates the ability of an aluminosilicate network, characteristic of geopolymers , to immobilize heavy metals. In order to optimize the incapsulation of caions and anions the geopolymeric formulation was combined with reducing and chelating agents, sodium sulfide (Na₂S), ferrous sulfate heptahydrate (FeSO₄·7H₂O), dithiocarbamates (DTC), ethylenediaminetetraacetic acid (EDTA) and citric acid (C₆H₈O₇) in order to prevent metals release. Galvanic sludge, from the electroplating industry, containing high levels of Cr and Ni (27 wt% and 14 wt% of dry waste, respectively) was used as received. The effectiveness of metal immobilization was assessed by determining the incorporated metal content in each sample through digestion analyses, followed by leaching tests to evaluate metal release. Considering that Cr is also present in its hexavalent form as an anion, a speciation analysis for this element was conducted on the leachates. Among the stabilizing agents tested, FeSO₄ and Na₂S as reducing agents and DTC, as chelating/reducing, were the most effective, achieving Cr immobilization up to 99.73% and significantly lowering the fraction of Cr(VI) relative to total Cr release. This study proposes geopolymers, known for their sustainability in terms of low CO2 emisisons, as matrix with targeted redox and chelation chemistry for real high Cr-Ni waste, effectively controlling chromium speciation and limiting Cr(VI) mobility even under highly alkaline conditions.