Efficient adsorption of phosphate on magnetic Fe₃O₄@MOF@LDH superstructures: Kinetics, thermodynamics, and mechanisms

Phosphorus contamination in water systems poses a significant environmental threat, necessitating the need for effective phosphate removal methods. A novel magnetic composite, magnetic Fe₃O₄@MIL-100(Fe)@Mg-Al layered double hydroxide (LDH), synthesized through a solid-state transformation of MIL-100(Fe) from Fe₃O₄ followed by in-situ growth of Mg-Al LDH. This innovative hierarchical core/shell/shell structure leverages the magnetic properties of Fe₃O₄ for easy separation, utilizes MIL-100(Fe) to grow and orient the LDH, and exploits the large ion exchange capacity of Mg-Al LDH nanosheets for efficient phosphate capture. Our experiments demonstrated rapid phosphate removal exceeding 95 % within 10 min, achieving a final concentration of 25.5 μg/L from an initial concentration of 1 mg/L. The adsorption kinetics conformed to a pseudo-second order model, and isothermal data fit the Langmuir model. Thermodynamic analysis indicated spontaneous and exothermic adsorption, with an activation energy of 15.76 kJ mol^-1. Enthalpy and entropy findings suggest a decrease in randomness during the adsorption process. Density Functional Theory (DFT) calculations revealed that phosphate ions interact strongly with Al sites in the LDH and Fe-O nodes in MIL-100(Fe). Phosphate recovery and sorbent regeneration are accomplished through a simple alkaline wash, which concentrates the recovered phosphate by 4.8 times. This study highlights the potential of Fe₃O₄@MIL-100(Fe)@Mg-Al LDH as a sustainable and efficient adsorbent for phosphate pollution mitigation, offering significant contributions to environmental protection and resource conservation.