The Impact of Humic Acid Coating on the Adsorption of Radionuclides (U-232) by Fe3O4 Particles

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Abstract
This study investigates the interaction of humic acid (HA) with magnetite nanoparticles and its impact on the adsorption behavior of the HA-coated magnetite (Fe3O4) nanoparticles towards uranium (U-232) in aqueous solutions. The particle surface modification was performed using HA solutions of varying concentrations (0.01, 0.1, and 1.0 g/L). Zeta potential measurements revealed a significant shift in surface charge—from positive values (+13 mV) for unmodified particles to negative values (down to −30 mV) due to the presence of carboxylic moieties on the particle surface. Batch adsorption experiments at pH 5.6 demonstrated that increasing HA coating markedly improves the U-232 adsorption, with Kd values rising by up to an order of magnitude compared to unmodified Fe3O4 nanoparticles. The enhanced performance is linked to both the greater number of surface-active sites and the increased negative surface charge introduced by the HA layer. Despite the HA coating, the hydrodynamic diameter of the particles remains largely unaffected, preserving colloidal stability. The latter is also corroborated by SEM-EDX analysis. Overall, this work highlights the role of HA in the adsorption behavior of magnetite particles towards (radio)toxic metal ions, which is of particular interest regarding their mobility in the geosphere and their removal from contaminated waters.

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