Interlayer-anion-controlled immobilization of chromate and chloride by Ca/Al layered double hydroxides

Understanding the interaction mechanisms between chromate (CrO₄^2-)/chloride (Cl⁻) and calcium-aluminum (Ca/Al)-layered double hydroxides (LDHs) is essential for effective immobilization of hazardous wastes using cementitious binders. This study elucidated the structural evolution of Ca/Al-LDH phases in Cr(VI)/Cl⁻ co-contaminated environments, focusing on the roles of interlayer anion chemistry. The results indicated that the type of interlayer anion critically determined chromate uptake efficiency, interlayer stability, and structural integrity. The Cl⁻- and SO₄^2--intercalated LDHs exhibited partial chromate substitution, with interlayer expansion from ∼8.4 Å to ∼10.2 Å. In contrast, the CO₃^2--based LDHs showed limited chromate uptake and structural disruption, maintaining a narrow d-spacing (∼7.6 Å). The Cl^⁻-rich environments promoted partial chromate intercalation while preserving well-defined layered morphologies and an octahedral Al-dominated coordination. However, the CO₃^2--based LDHs exhibited a pronounced structural disruption, decreased hydration, and an increase in tetravalent Al species upon exposure to chromate. We illustrated chromate intercalation pathways and the associated structural responses, highlighting how interlayer anion chemistry could regulate these processes. The pathway of chromate uptake was influenced by the interlayer anion size, geometry, and electrostatic interactions. Our findings provided fundamental insights into anion-driven structural transformations and offered practical strategies for designing LDH-based binders suitable for the stabilization/solidification of Cr(VI)-contaminated hazardous waste, such as incineration fly ash and ore processing residue.