Comparative study of Mg/Al-LDH and Mg/Fe-LDH on adsorption and loss control of 2,4-dichlorophenoxyacetic acid

Low efficiency and high surface runoff of 2,4-dichlorophenoxyacetic acid (2,4-D) from agricultural field threaten crop yield severely. Layered double hydroxides (LDH) have shown promising adsorption properties for 2,4-D. However, the comparison of two environmentally friendly LDHs (i.e. Mg/Al-LDH vs Mg/Fe-LDH) on adsorption of 2,4-D and corresponding intrinsic mechanisms are still unclear, and the studies on the leaching control of 2,4-D by LDHs in soil environment are particularly limited. In this study, Mg/Al-LDH and Mg/Fe-LDH were selected to conduct their adsorption kinetics experiment for 2,4-D combined with the characterization technology. The results showed that the adsorption capacity of Mg/Al-LDH and Mg/Fe-LDH for 2,4-D was up to 242 mg kg^-1 and 64 mg kg^-1, respectively, which were negatively correlated with pH. Adsorption mechanisms of both Mg/Al-LDH and Mg/Fe-LDH for 2,4-D are dominated by chemical adsorption, including electrostatic attraction and inner sphere complexation, but no interlayer adsorption mechanism. Mg/Al-LDH contains smaller metal ion radius, which provides greater surface charge density, resulting in greater electrostatic attraction and inner sphere complexation to 2,4-D compared to Mg/Fe-LDH. The greater adsorption capacity of Mg/Al-LDH for 2,4-D was driven by the higher adsorption energy (E_ads) and lower electron density, as corroborated by density functional theory (DFT) calculation. The soil column experiment further verified that Mg/Al-LDH could control the loss of 2,4-D more effectively, and the leaching amount could be significantly reduced by 61.7%, while the effect of Mg/Fe-LDH was only 24.2%. This study provides theoretical guidance for screening more potential LDH types to solve the leaching loss of 2,4-D from soil and improve its effectiveness in agricultural production.