Simultaneous separation and immobilization of Cr(VI) from layered double hydroxide via reconstruction of the key phases

Highlights: • The treatment of Cr(VI) in LDH interlayer with MgCl₂·6H₂O were investigated. • Part of Cr was volatilized as CrCl₃ and the other was fixed as MgCr₂O₄. • HCl accelerated the deconstruction of Cr(VI)-LDH, completely exposing Cr(VI). • The preferred pathways were elucidated by thermodynamic calculation. Layered double hydroxide (LDH) is one of the key host phases of Cr(VI) in the natural environment and chromite ore processing residue (COPR), causing serious pollution by Cr(VI). Therefore, efficient extraction or immobilization of the incorporated Cr(VI) in LDH is urgently needed. In this work, simultaneous separation and immobilization of Cr(VI) in LDH by using MgCl₂·6H₂O under thermal treatment is innovatively proposed. Cr was volatilized as CrCl₃ and was immobilized as MgCr₂O₄ accounted for 62.2% and 37.8%, respectively, under the optimal condition (the mole ratio of Cl/Cr is 9, 700 °C and 120 min). The underlying reaction mechanisms are as follows: (i) HCl produced by MgCl₂·6H₂O accelerates the destruction of Cr(VI)-LDH layer structure, completely exposing the incorporated Cr(VI), (ii) Cr(VI) is reduced to Cr(III) by Cl^-, part of which is directly immobilized as MgCr₂O₄, and the other part generates CrCl₃, which is volatilized or further combined with Mg²⁺ to form MgCr₂O₄. The total Cr leaching concentration of the practical COPR sample treated by this method dramatically decreases from 421 to 0.7 mg/L, well below the landfill standard limit (4.5 mg/L). This work provides an attainable strategy for thorough remediation of COPR and inspires the treatment of heavy metal-containing LDH.