The solid-solution HxPO4@Fe3O4 synthesized by irradiation enhances the adsorption capacity of tungsten(VI) and anti-interference of coexisting oxoanion in waste water: Surface oxolation interaction

Abstract

Tungsten (W), mainly existing as hexavalent oxidation state in nature, has been considered as an increasingly prominent waterborne pollutant, but it is also a critical metal. Therefore, it is important to develop a material to adsorb W(VI) from the waste water containing W(VI) to realize the high-efficiency enrichment and recovery of W(VI). Inspired by self-assembly of phosphate and W(VI) ions in the acidic solutions to form phosphotungstic polyoxoanions (e.g., Keggin-type PW₁₂O₄₀³⁻), in this work, we have loaded the phosphate ion to surface of the magnetic Fe₃O₄ nanoparticles (Fe₃O₄ NPs) by β particle irradiation to synthesize a composite adsorbent, H_xPO₄@Fe₃O₄ solid-solution (SS). This novel adsorbent is very efficient for W(VI) adsorption and recovery. The results of a batch of adsorption experiments indicate that H_xPO₄@Fe₃O₄ SS has a high adsorption efficiency for W(VI) in aqueous environments and strong anti-interference of coexist ions especially PO₄³⁻. Under room temperature conditions (∼25 °C), the maximum adsorption capacity reaches 88.44 mg g⁻¹. This method not only ensures a substantial increase in W(VI) adsorption but also provides a facile means for adsorbent recovery, while minimizing additional environmental burdens. Based on the characterization results from Fourier transform infrared, Zeta potential, and X-ray photoelectron spectroscopies, we speculated that the possible mechanism of the adsorption process is that formed W(VI) polyoxoanions in acidic solutions are initially attracted by electrostatic attraction to the positively charged H_xPO₄@Fe₃O₄ surface, and then oxolation reaction occurs between W-OH of polyoxoanions and phosphate hydroxyl (P-OH) on the H_xPO₄@Fe₃O₄ surface to form P-O-W bridging oxygen group. This adsorption mechanism is also reflected in the adsorption kinetics, following a pseudo-second-order kinetic model, and adsorption thermodynamics, exhibiting a large enthalpy change (−76.3 kJ/mol) and a significant entropy reduction (−174.5 J/(mol K)).