Single-Step Topochemical Synthesis of NiFe Layered Double Hydroxides for Superior Anion Removal from Aquatic Systems

Layered double hydroxides (LDHs) have attracted significant attention as adsorbents for the removal of anions from wastewater. However, it is challenging to develop a simple, economical, and environmentally friendly method for fabricating efficient LDH adsorbents. In this paper, we present an alternative approach for preparing a superb NiFe LDH adsorbent via a single-step topochemical synthesis method based on density functional theory (DFT) calculation. The NiFe LDH adsorbent [Ni_0.75Fe_0.25(OH)₂]·(CO₃)_0.125·0.25H₂O was obtained via the topotactic transformation of an oxide precursor (NaNi_0.75Fe_0.25O₂), which was prepared by utilizing the high-temperature flux method, in ultrapure water. When the oxide precursor was soaked in ultrapure water, the host layer valence state changed from Ni³⁺ and Fe³⁺ to Ni²⁺ and Fe³⁺, and carbonate (CO₃^2-) ions were simultaneously intercalated in the interlayer. Thereafter, the CO₃^2- ions were deintercalated by Cl^- ions to increase the adsorption capacity. The adsorbent exhibited high crystallinity, cation state, and porosity, and unique particle shape. In addition, it showed superior adsorption capacities of approximately 194.92, 176.15, and 146.28 mg g^-1 toward phosphate, fluoride, and nitrate ions, respectively. The adsorption capacity toward all the anions reached over 70% within 10 min. The adsorption behavior was investigated by performing from adsorption kinetics, isotherm, and thermodynamics studies. The results showed that the anions were endothermically and spontaneously chemisorbed through an ion exchange process onto the adsorbent in a monolayer. In addition, the as-prepared NiFe LDH adsorbent showed high stability after multicycle testing.