Uranium(VI) sorption complexes on silica in the presence of calcium and carbonate

Uranium sorption on minerals and related solids depends to a large degree on its aqueous speciation. The present work attempts to understand the U(VI) sorption behavior on silica under environmentally relevant conditions, i.e. at neutral to weakly alkaline pH and in the presence of dissolved calcium and carbonate. Under these conditions, Ca(UO₂)(CO₃)₃^2- and Ca₂(UO₂)(CO₃)₃(aq) complexes emerge as the dominant aqueous U(VI) species. The U(VI) sorption affinity was measured as a function of contact time, solution pH, and humic acid. The U(VI) sorption decreased with increase of pH and was not affected by the addition of 50 mg/L humic acid. On the other hand, nitric acid was more effective than EDTA and carbonate at desorbing U(VI). Generally, the U(VI) sorbed on silica at neutral pH was less readily desorbed than that sorbed at higher pH values. Therefore, the U(VI) complex favorably sorbed on silica at the neutral pH is more strongly bound to the silica surface than that sorbed at higher pH values. Time-resolved laser fluorescence spectroscopy confirmed the results of the batch sorption experiments and revealed the presence of two surface U(VI) complexes with fluorescence lifetimes 251 ± 8 μs and 807 ± 24 μs.