Insight into migration of Cr(VI) in self-hardening slurry materials for trench cutoff wall

The migration and immobilization of heavy metals in soil and groundwater pose significant environmental challenges, particularly in the context of Cr(VI), a highly toxic and mobile contaminant. Self-hardening slurry materials, commonly used for trench cutoff walls, have gained great attention due to their potential for pollutant containment. However, the relationship between their adsorption properties and pollutant diffusion behaviors remains poorly understood. This study investigates the adsorption properties and free-diffusion behaviors of Cr(VI) in two self-hardening slurry materials, SCB (slag-cement-bentonite) and MASB (MgO activated slag-bentonite), to better understand the pollutant migration and immobilization mechanisms. Adsorption experiments reveal that MASB exhibits a higher Cr(VI) adsorption capacity than SCB, resulting in greater retardation factors and longer breakthrough times, improved by 57.6% and 94.9% at Cr(VI) concentrations of 2600 mg/L and 26000 mg/L, respectively. However, free-diffusion tests show that MASB has a larger migration depth and shorter breakthrough times, reduced by 68.2% and 57.9%. Microscopic analysis reveals that the presence of stable and ion-exchangeable hydration products (hydrotalcite-like phases) in MASB enhances its Cr(VI) adsorption capacity but provides favorable conditions for Cr(VI) diffusion. SCB demonstrates superior retardation to Cr(VI) in free-diffusion due to the transformation of AFm phases absorbed with Cr(VI) into stable AFt phases.