Kinetic, isotherm and thermodynamic investigations of phosphate adsorption onto core–shell Fe3O4@LDHs composites with easy magnetic separation assistance

Abstract

In this study, three different magnetic core–shell Fe3O4@LDHs composites, Fe3O4@Zn–Al–, Fe3O4@Mg–Al–, and Fe3O4@Ni–Al–LDH were prepared via a rapid coprecipitation method for phosphate adsorptive removal. The composites were characterized by XRD, FTIR, TEM, VSM and BET analyses. Characterization results proved the successful synthesis of core–shell Fe3O4@LDHs composites with good superparamagnetisms. Batch experiments were conducted to study the adsorption efficiency of phosphate. Optimal conditions for the phosphate adsorption were obtained: 0.05g of adsorbent, solution pH of 3, and contact time of 60min. Proposed mechanisms for the removal of phosphate species onto Fe3O4@LDHs composites at different initial solution pH were showed. The kinetic data were described better by the pseudo-second-order kinetic equation and KASRA model. The adsorption isotherm curves showed a three-region behavior in the ARIAN model. It had a good fit with Langmuir model and the maximum adsorption capacity followed the order of Fe3O4@Zn–Al–LDH>Fe3O4@Mg–Al–LDH>Fe3O4@Ni–Al–LDH. Thermodynamic analyses indicated that the phosphate adsorption process was endothermic and spontaneous in nature. The three Fe3O4@LDHs composites could be easily separated from aqueous solution by the external magnetic field in 10s. These novel magnetic core–shell Fe3O4@LDHs adsorbents may offer a simple single step adsorption treatment option to remove phosphate from water without the requirement of pre-/post-treatment for current industrial practice.