Interaction of europium with humic acid covalently bound to silica beads

The uptake of Eu³⁺ (a trivalent actinide homolog) by Aldrich humic acid covalently bonded to an inorganic support is examined. Two types of covalent linkages are used and the synthetic routes to produce the resins are discussed. The use of these resins excludes having to account for humic acid desorption from the surface, yields a well characterized system, and allows the experiment to focus on and account for the role of humic acid in the sorption of Eu. The proton exchange capacity of the resins is examined by titration and the differences observed are traced to the resin synthesis. Europium sorption experiments are performed at pH 4 and pH 6 in 0.1 M NaClO₄. Kinetic experiments show equilibrium is reached in 24 hours. The kinetic data are used to evaluate the loading capacity, with results similar to equilibrium experiments. The complexation results are evaluated based on the metal ion charge neutralization model. For the resins an effect of pH and resin synthesis route on the Eu uptake is observed. The uptake increases with pH for both resins. The resin HA-Epo (Epoxy linkage) has a higher metal binding at pH 4, while the resin HA-HAB (2-hydroxylazobenzene linkage) had more proton exchange sites occupied by metal ions at pH 6. Overall, more Eu is bound to HA-Epo at pH 6 since its proton exchange capacity is higher. The evaluated stability constants vary slightly and show a dependence on the linkage group but are similar to literature values that examined complexation by aquatic humic acid analyzed with the same model. This result supports the utility of the metal ion charge neutralization model and the applicability of the resulting stability constants.