Sustainable solidification/stabilization of chromium (VI)-contaminated soil using enzyme-carbonated reactive magnesia...

The sustainable solidification/stabilization (S/S) of Cr(VI)-contaminated soils holds significant environmental value. In this study, an innovative enzyme-carbonated reactive magnesia cement (ECRMC) method was proposed for the Cr(VI)-contaminated soil solidification/stabilization. A systematic investigation was conducted to validate the feasibility and performance of the ECRMC-treated soils, focusing on the key parameter effects of urea pre-hydrolysis time and RMC content. Experimental results demonstrated that the ECRMC technology can significantly enhance soil strength while reducing Cr(VI) leaching, achieving a maximum UCS of 6.96 MPa at 6 h and a 97% reduction in Cr(VI) leaching at 24 h (25% RMC). The 6-24 h pre-hydrolysis range serves as a goal-driven decision interval based on strength-priority or safety-priority requirements. The S/S performance of the ECRMC technology is governed by urea pre-hydrolysis time and RMC content. Urea pre-hydrolysis treatment significantly enhanced the formation of hydrated magnesium carbonates (HMCs) by avoiding the enzyme inactivation after mixing with RMC, leading to a 680% increase in UCS, while increasing RMC content further improved UCS but concurrently reduced the degree of carbonation (DC). The formed HMCs and brucite were identified as the dominant products through the microstructural analysis. Their synergistic effects of adsorption, co-precipitation, and physical encapsulation contributed to both improved mechanical strength and efficient Cr(VI) immobilization. Furthermore, a quantitative relationship between Cr(VI) fixation and DC was revealed by incorporating the reaction pathways of RMC under hydration and carbonation conditions, enabling quantitative evaluation of Cr(VI) fixation. This study provides a comprehensive theoretical foundation and practical guidance for the low-carbon, effective, and sustainable solidification/stabilization of Cr(VI)-contaminated soils through the ECRMC technology.