Unveiling the Boron Adsorption Mechanism on Fe₃O₄ Nanoparticles by Fine-Tuning Hydroxyl Density with Oleic Acid

This study proposed a precise synthetic strategy for tailoring the surface chemistry of magnetic nanomaterials and established a structure-performance relationship between surface hydroxyl density and boric acid adsorption. By modulation of the molar ratio of oleic acid to the iron source in the solvothermal reaction, a gradient control of hydroxyl density on the surface of Fe₃O₄ nanoparticles was systematically achieved. A novel fluorine substitution XPS method was employed to quantitatively characterize the hydroxyl density, yielding a normalized F/Fe ratio range of 0.195-0.326, which validated the feasibility and controllability of the "fine-tuning" approach. As the hydroxyl density increased, the adsorption capacity of the material increased from 0.7 to 1.5 mmol/g. Comprehensive analysis of boric acid adsorption, including adsorption kinetics, isotherms, thermodynamics, and material characterization before and after adsorption, demonstrated that the adsorption mechanism primarily involved bidentate hydroxyl complexation via a trigonal coordination configuration. These findings provide a reproducible framework for surface modification of magnetic nanomaterials and offer experimental guidance for designing high-performance boric acid adsorbents.