Facile synthesis of MgAl layered double hydroxides by a co-precipitation method for efficient nitrate removal from water: kinetics and mechanisms

Abstract

Recently, layered double hydroxides (LDH) have been proven to be promising adsorbents in the field of wastewater treatment mainly due to their unique characteristics. In this study, a series of Mg_xAl-LDH (x is the molar ratio of Mg/Al) were synthesized via a supersaturated co-precipitation method and used for the removal of low concentration nitrate (NO₃⁻). Based on the results, Mg₅Al-LDH showed a significantly enhanced adsorption capacity of 9.76 mg g⁻¹, and the removal efficiency of NO₃⁻ (C₀ = 1 mg L⁻¹) reached about 80% after 6 h at 25 °C and neutral pH. As proved by various characterization studies, changes in the molar ratio of Mg/Al would influence the surface active groups and layer distance of the synthesized LDH, thereby affecting the adsorption of NO₃⁻. In addition, the synthesized LDH presented a mesoporous structure, which can reduce the obstruction for adsorbing NO₃⁻. Studies on the reaction kinetics indicated that the NO₃⁻ adsorption behavior of Mg₅Al-LDH can be well fitted by the pseudo-second-order kinetic and Langmuir models, demonstrating that the resultant adsorption is a spontaneous, exothermic and entropy decreasing process with physical and chemical effects simultaneously. By further clarifying the mechanisms of NO₃⁻ removal, the synthesized Mg₅Al-LDH could be thought of as an efficient, environmentally friendly, and low-cost adsorbent to remove NO₃⁻ from the aquatic environment.