The adsorption characteristics of nitrate on Mg-Fe and Mg-Al layered double hydroxides in a simulated soil solution

Four types of layered double hydroxides (LDHs) synthesized by coprecipitation method including Mg-Al and Mg-Fe in 3:1 and 4:1 ratios of M²⁺/M³⁺ were compared in terms of chemical structure, nitrate adsorption isotherm and kinetics, thermodynamic parameters, anion exchange capacity and nitrate selectivity in the presence of common anions found in soil solutions. Moreover, the influence of calcination on nitrate adsorption was studied. Results showed that the Mg-Al-LDH (4:1) was most effective for the removal of nitrate and its selectivity for nitrate adsorption was remarkable in the presence of phosphate, sulfate and bicarbonate. The nitrate adsorption on Mg-Al-LDH (4:1) followed a pseudo-second order model with rate constant of 0.623gmmol^-1min^-1. Furthermore, the kinetic data were described very well with the intraparticle diffusion model with rate constant of 0.046mmolg^-1min^-0.5. The nitrate adsorption isotherm of Mg-Al-LDH (4:1) described well with the Freundlich model as indicated by the high values of R². The negative values of δH_r^o indicated that the nitrate adsorption on the LDHs was exothermic in nature. Calcination of the LDHs had no significant effect on increasing nitrate adsorption.