Layered double hydroxides as versatile materials for detoxification of hexavalent chromium: Mechanism, kinetics, and environmental factors

Hexavalent Cr (Cr^VI) compounds are a common environmental pollutant that causes carcinogenesis and interferes with the normal process of the cells. Layered double hydroxides (LDHs) as a class of ionic solids with hydroxylated metals in two layers and an intermediate layer containing water molecules and anions, e.g., CO₃²⁻, SO₄²⁻, Cl⁻, can be used for wastewater treatment, controlled drug delivery, pharmaceuticals, cosmetics, and electrochemical sensors. The use of LDHs to remove Cr^VI from aqueous environments has been considered because they have advantages such as biocompatibility and high efficiency. Cr^VI is converted to the less dangerous trivalent Cr (Cr^III) by some of the LDHs, e.g., MgFe LDH, MgAl LDH, and NiFe LDH. In this review, after a summary of the LDHs principle, the main mechanisms of Cr^VI removal, including adsorption, interlayer ion exchange, and redox process, and the kinetics of Cr^VI removal have been described. The influential factors in Cr^VI removal efficiency by LDHs, including modification of LDHs, pH, and the effect of LDH interlayer anions, were reviewed in depth. The appropriate selection of interlayer anion, using intercalated LDHs, LDHs composite developing, as well as divalent and trivalent metals of LDHs, is effective in its performance; Also, modifying LDHs and preparing LDH-composites can improve the efficiency of removing Cr^VI.