Uranium Adsorption at Increased and Ultra-Trace Levels by Humic Acid-Coated Alumina: Thermodynamic and Kinetic Studies

This study investigates the interaction of commercially available alumina (Al2O3) particles with humic acid salt (HA–salt) and the subsequent impact of HA-coating on uranium adsorption. Alumina particles were coated by immersion in HA–salt solutions of varying concentrations (0.01–1.0 g/L). Following, batch adsorption experiments were conducted using U-232 at ultra-trace levels, to evaluate distribution coefficients (Kd) at pH 4 and 7, and depleted uranium (DU) at elevated concentrations to assess maximum adsorption capacities (qmax), temperature dependence, and kinetics. HA-coating significantly enhanced uranium uptake, particularly at neutral pH for ultra-tracer levels, where the deprotonation of HA carboxylic groups favors inner-sphere complex formation. At pH 3 (and at relatively high concentrations), the results showed that adsorption appears to be due to the formation of outer-sphere complexes, with qmax values for particles with the highest HA–salt loading reaching up to 0.16 ± 0.02 mol/kg. Temperature studies indicated endothermic adsorption, while kinetic data revealed a two-step mechanism for HA-coated particles, involving most probably initial outer-sphere complexation followed by gradual inner-sphere complex formation. These findings highlight the critical role of HA surface modification in enhancing the affinity of alumina towards uranium, with implications for radionuclide mobility in the geosphere and the development of efficient sorbents for uranium-contaminated water treatment.