Modelling the reaction of uranium with carboxylic groups on surfaces through mono- and multi- dentate surface complexes on the basis of pH and redox potential

Abstract

An analytical expression is proposed to simulate effects of pH and redox potential (E) on the sorption of uranium onto bioorganic model particles in saline or other aquatic environments. The elaborated expression is intended to avoid use of the classical approach of sorption which relies on experimental data and empirical models. The goal is to produce an expression that provides a distribution coefficient (Kd e.g. mL g⁻¹) as function of pH, E and ligand concentration (through complex formation in solution) by applying a surface complexation model on one type of mono-dentate surface sites> (SuOH) as well as utilizing multi-dentate surface sites> (SuOH)_c. The formulation of the worked out expression makes use of correlations between the surface complexation and hydrolysis constants for all species and sorption sites. The model was applied to the sorption of uranium onto bioorganic sites with and without carbonates in solution e.g. Log Kd: + 2.75 at pH 8 for 2 sites per nm². The calculated distribution coefficients were found very sensitive to the presence of carbonates, e.g. Log Kd: − 7.0 at pH 8 for 2 × 10⁻³ M total carbonate. The potential reduction of uranium U(VI) and its complexes (carbonates) which are the primary stable species in surface waters, to U(IV) during sorption was simulated in association with a decrease in the redox potential and was found generally below the redox stability limits of water. The calculated distribution coefficient values were validated by the values reported in literature for the sorption of uranium onto specific adsorbents. The investigated simulations are also applicable to the sorption of other redox sensitive elements.