Theoretical study on the mechanism of removing harmful anions from environment via Mg2Al layered double hydroxide

Abstract

Environmental pollution caused by harmful anions is becoming an increasingly prominent problem and should be addressed urgently. Layered double hydroxides (LDHs) have been used as excellent adsorbents and stabilizers. Here, three removal mechanisms (surface adsorption, anion exchange and ligand exchange) of Mg2Al–NO3 LDH for 13 harmful anions (F−, Cl−, Br−, I−, ClO3−, BrO3−, IO3−, CrO42−, SeO42−, MoO42−, AsO43−, VO43−, PO43−) are investigated by density functional theory (DFT). The adsorption energy of anions on 003 and 110 surface, the Gibbs free energy change of anion exchange and ligand exchange reactions are calculated thermodynamically. It is found that both divalent and trivalent harmful anions can be removed by the above three mechanisms. All anions except I− can undergo surface adsorption, with divalent and trivalent anions preferring to interact with 110 surface and monovalent more easily adsorbed on 003 surface. Kinetically, the energy barriers of the rate–determining steps for three mechanisms are further calculated. The results indicate that all harmful anions removed by LDHs favor the surface adsorption mechanism. Anion exchange mechanism cannot occur for anions with Mulliken electronegativity and ionic potential below 1.7 eV and 5 nm−1, respectively. Generally, the higher the charge of the oxyanion, the easier the ligand exchange mechanism.