Stabilization of Cr3+ using iron rich slag-derived phosphate cement: An integrated solution for heavy metals and hazardous solid wastes

Abstract

A novel iron-based phosphate cement (IPC), derived from iron-rich smelting slag (ISS), was developed as a sustainable and efficient binder for the stabilization/solidification of trivalent chromium (Cr³⁺). The mechanical properties, hydration behavior, microstructure, leaching toxicity, chromium chemical forms, and environmental safety of chromium-stabilized iron phosphate cement (CIPC) were thoroughly evaluated. The results showed that, with a mass ratio of ISS to ammonium dihydrogen phosphate (ADP) of 2.0, and even with the addition of 20 % chromium nitrate nonahydrate (CN), the compressive strength of CIPC reached 4.2 MPa after curing for 28 d. Furthermore, chromium leaching was well below 1 mg/L, significantly lower than the GB 5085.3–2007 standard limit of 15 mg/L, demonstrating the effective encapsulation of Cr³⁺ due to IPC's high early strength. In the IPC system, Cr³⁺ was primarily stabilized by forming CrPO₄ and Cr_xFe_1-x(OH)₃ co-precipitates, which were further solidified through the physical encapsulation of IPC hydration products, such as (NH₄)₂Fe(PO₃OH)₂·4H₂O, (NH₄)(Mg,Ca)PO₄·H₂O, and FePO₄. This process resulted in a solidification efficiency of up to 99 %. BCR analysis confirmed that more than 98 % of the chromium in the CIPC remained in a stable residual form. Finally, the ecological risk index (PERI) was found to be 23.52, far below the safety threshold of 150, indicating the solidified material's long-term environmental safety. This study provides an innovative approach for the reutilization of ISS while effectively stabilizing/solidifying chromium.