Immobilization potential of Cr(VI) in sodium hydroxide activated slag pastes

This study investigated the immobilization potential of alkali-activated slag (AAS) pastes for Cr(VI) by examining compressive strength, toxicity characteristic leaching procedure (TCLP) and generalized acid neutralization capacity tests. Alkaline digestion, total acid digestion, XRD, FTIR and SEM-EDS were carried out to clarify the immobilization mechanism. The AAS pastes gave high compressive strengths, which contributed to the physical encapsulation of Cr. The addition of Cr(VI) induced an increase in compressive strength compared to Cr(VI)-free pastes. The leaching results showed that AAS pastes exhibited effective immobilization for Cr(VI) and that the leachability was strongly dependent on the NaOH dosage, water to slag ratio, initial Cr(VI) content, leachate pH and curing duration. The initial Cr(VI) content up to 1.5wt% by weight of slag was well immobilized with the total-Cr leachability below the TCLP regulatory limit of 5mg/L. The digestion results demonstrated that the reduction of chromium from hexavalent to trivalent played a significant role in the immobilization of Cr(VI) in AAS without any additional reductants. The XRD and SEM-EDS results suggested the formation of a clear CrO₄-U phase as the primary retention phase for unreduced Cr(VI). Therefore, alkali-activated slag binder is effective in the immobilization of Cr(VI)-bearing wastes.