Sorption of U(VI) onto Opalinus Clay: Effects of pH and humic acid

The U(VI) sorption onto Opalinus Clay, a natural clay rock from Mont Terri, Switzerland, was investigated as a function of pH in the absence and presence of humic acid (HA) under aerobic conditions using 0.1 M NaClO4 as background electrolyte ([U(VI)] = 1×10-6 M, [HA] = 50 mg/L, S/L = 4 g/L, pH 3-10). Due to leaching of Opalinus Clay, several competing ions are present in solution, where especially calcium and carbonate ions affect the speciation of U(VI) and HA in solution and consequently, their sorption properties. At pH > 7.5, the Ca2UO2(CO3)3(aq) complex is formed, which dominates the U(VI) speciation and sorbs only weakly onto Opalinus Clay. In the case of HA, the CaHA(II) complex dominates the HA speciation over a wide pH range. However, in the pH range 4.5-7.5, the presence of HA leads to the formation of the aquatic complex UO2(OH)HA(I), which causes a decreased U(VI) sorption. In the pH range from pH 7.5-8, HA seems to have no influence on the U(VI) sorption. The sorption characteristics of U(VI) on Opalinus Clay were predicted using PHREEQC. Thereby, the investigation was focused on the influence of the binding sites of the iron mineral goethite on U(VI) sorption onto Opalinus Clay.