Reaction of uranium with poly-hydroxy-aromatic groups on particles through mono- and multi-dentate surface complexes on the basis of pH and redox potential: A modelling approach

The analytical approach that was proposed in our recent paper has been applied to simulate effects of pH and redox potential (E) on the sorption of uranium onto potentially redox active bioorganic model particles in saline or other aquatic environments. Specifically herein, it is applied to the mono- and poly-hydroxy-aromatic (polyphenolic) sites which account for approximately 30% of bioorganic site capacity. The derived expression is aimed to avoid use of the classical approach of sorption, which requires experimental data and empirical models. The expression provides a distribution coefficient (K_d e.g. mL g⁻¹) as function of pH, E and soluble ligand concentration by considering a surface complexation model on mono- or multi-dentate complexation surface sites > Su(OH)_c. The application of the model uses correlations between the surface complexation constants and hydrolysis constants, for all potential species and all form of sorption sites. The model was used to quantify the uranium sorption onto hydroxy-benzene, dihydroxy-benzene, and dihydroxy-naphthalene sites with or without carbonates in solution. The latter is the primary interfering reagent in waters that decreases Log K_d. The calculated distribution coefficients were found sensitive to both pH and E and very sensitive to the presence of carbonates. The reduction of uranium U(VI), and its carbonate complexes, to U(IV) during sorption was simulated by decreasing the redox potential. It was found that the transition phase between U(VI) and U(IV) was generally below the redox stability limits of water. However, the reduction of U(VI) to U(IV) was found to be potentially associated with their reaction with the polyphenols, decreasing the redox potential subsequently. The calculated sorption coefficient values were validated using the values reported in literature for the sorption of uranium onto specific adsorbents. The methodology of the simulation is also applicable to the sorption of other redox sensitive elements, and with the addition of a scaling factor, it would allow the predictions of co-complexation phenomena by employing relevant site formulations. The oxidation of mono-hydroxy- benzene in di-hydroxy-benzene enhances the sorption of uranium by a factor 10⁶ which may be applied to its extraction from seawater. XX.