Linear Free Energy Relationship for Actinide Sorption to Graphene Oxide

Th(IV) and Np(V) sorption to graphene oxide (GO) was studied as a function of pH from 0-7.5 and analyte concentrations (0.01-1 mg/L for Th(IV) at pH 3 and 0.005-10 mg/L for Np(V) at pH 7). Starting at pH 1, greater than 90% Th(IV) sorption to GO occurred while significant Np(V) sorption to GO started at pH 5. Surface complexation modeling (SCM) using an electrostatic double layer model simultaneously modeled Th(IV) and Np(V) sorption to GO over the pH and the analyte concentration ranges. The SCM indicated that Th(IV) complexation to sulfonate sites dominated at a low pH 0-3 and its complexation to carboxylate sites dominated at a higher pH 3-7.5. In contrast, Np(V) showed a stronger affinity for sulfonate sites than carboxylate sites over the pH and concentration ranges examined in this work. Combining the results from a previous study on Eu(III) and U(VI) sorption to GO, the affinities of actinide/lanthanide sorption to GO was found to follow the trend in actinide/lanthanide ion effective charges (e.g., Th⁴⁺ (+4) > UO₂²⁺ (+3.2) > Eu³⁺ (+3) > NpO₂⁺ (+2.2)), which is similar to actinide sorption to iron oxides and clay minerals. Moreover, a linear free energy relationship was observed between the stability constants for actinides and Eu(III) (with the oxidation state from III to VI) complexation to carboxylate sites on GO and the stability constants for their complexation to carbonate.