Mesoporous violarite FeNi₂S₄ nanosheets as a high-performance adsorbent for selective recovery of Au(III) ions

The recovery of gold (Au(III)) from aqueous solutions has become increasingly important due to the growing production of gold-based devices and applications. This study investigates the synthesis, characterization, and adsorption properties of cubic spinel-phase FeNi₂S₄ nanosheets, synthesized via a hydrothermal method. The as-made mesoporous FeNi₂S₄ adsorbent demonstrates excellent performance in selectively trapping Au(III) ions, achieving an adsorption efficiency of over 98 % and a high adsorption capacity ranging from 224 to 307 mg/g at pH 2.5. A comparative evaluation of adsorption isotherms identifies the Sips model as the best fit (R² = 0.995), reflecting the complexity of the adsorption mechanism. The kinetic analysis of Au(III) adsorption shows a complex interplay of surface phenomena and diffusion processes, with excellent fits to the PFO, Avrami, and Bangham models (R² ≈ 0.99). Additionally, the two-step Weber-Morris model provides insights into the nature of the adsorption process. Thermodynamic analysis, including Gibbs free energy (ΔG), confirms the spontaneous nature of the adsorption. The FeNi₂S₄'s consistently high performance across multiple cycles underscores its robustness and scalability as a material for selective Au(III) adsorption from aqueous solutions. The adsorption mechanism was clarified through zeta potential and X-ray photoelectron spectroscopy measurements, identifying a strong chemisorption interaction between Au(III) and FeNi₂S₄ sorbent. The FeNi₂S₄ nanosheets exhibit significant potential for Au(III) trapping, offering a promising and sustainable solution for precious metal extraction.