Immobilization of chromium (VI) evaluated by binding isotherms for ground granulated blast furnace slag and ordinary Portland cement

The interaction mechanisms of the Cr(VI) ion in presence of GGBFS and OPC were evaluated by chromium binding isotherms and by pore solution analysis. The chromium in the final leaching solution was measured and the solid samples were investigated by SEM and by XRD. GGBFS was more efficient than OPC in fixing Cr ions at lower initial concentrations. However, from an initial Cr(VI) concentration of 2000 and 5000 mg/L, OPC was more efficient.

For an initial Cr(VI) concentration of 50 000 mg/L, around 145 mg Cr/g was fixed by OPC and only 8 and 55 mg Cr/g were fixed by GGBFS in alkaline and water-based immersion solutions, respectively. The Cr-bearing phases identified by XRD and SEM are: CaCrO₄ and CaCrO₄·2H₂O, C–S–H and calcium aluminate phases. The pore solution chemistry indicates that a value around 92% of chromium was retained by GGBFS and 87% by OPC.