Adsorption Performances and Mechanisms of MgFe₂O₄ Spinel Toward Gallium (III) from Aqueous Solution

The European Union regards gallium as a crucial element. Because of that, the retrieval of gallium ions from secondary sources through diverse methodologies is of the utmost significance in an actual economical context. The primary goal of this study was to explore the viability of MgFe₂O₄ spinel as an adsorbent material for Ga(III) ions recovery from aqueous solutions. A spinel adsorbent material was synthesised by using the sol-gel synthesis method. After preparation, the obtained spinel was subjected to a thermal treatment, which resulted in modifications of its crystalline structure and morphology, in concordance with the calcination temperatures. Specifically, two distinct temperatures of 260 and 650 °C were utilised in the process, which was conducted in air. The second objective was represented by the physicochemical characterisation of the newly prepared adsorbent material by using various analytical techniques, e.g., Fourier transform infrared spectroscopy (FT-IR), atomic force microscopy (AFM) and magnetic measurements. The optimal conditions for Ga(III) adsorption were established (S:L ratio, solution pH, contact time, temperature, initial Ga(III) concentration). Simultaneously, the obtained experimental data were modelled to prove the fact that the pseudo-second-order model explained the studied kinetics process and established its mechanism. Intraparticle diffusion was also studied to highlight the rate-determined step during the Ga(III) adsorption process. The equilibrium of the process was also studied, establishing that the Sips isotherm fitted the experimental data best, with a correlation coefficient R²~1, indicating that the studied adsorption process was homogeneous, the maximum adsorption capacity of spinel being 24.7 mg Ga (III)/g MgFe₂O₄. Thermodynamic parameters, involving ΔG°, ΔH° and ΔS°, were also calculated; negative values of ΔG° indicated that the adsorption was spontaneous. ΔH° proved to be endothermic, and the calculated ΔS° values being positive confirmed the fact that the process was spontaneous.