Sorption of uranium(VI) on clay in absence and presence of humic acid

In Germany, the investigated host rocks for nuclear waste disposal in deep geological formations are salt, crystalline rock and argillaceous rock. Natural clay contains organic matter, such as humic acid (HA), which has an influence on the sorption of metal ions. In this work, we studied the uranium(VI) sorption onto the reference clay mineral kaolinite and the natural opalinus clay in absence and presence of HA. The results are compared with our previous studies focused on kaolinite [1].
The influence of the background electrolyte on uranium(VI) sorption onto kaolinite was investigated. The NaClO4 electrolyte previously used [1] was compared with synthetic opalinus clay pore water (I = 0.42 M, pH 7.6) [2]. The experiments show that a lower amount of uranium(VI) is adsorbed onto the clay when the opalinus clay pore water is used. This behavior can be explained with the uranium(VI) speciation. At pH 7.6, the speciation in opalinus clay pore water is dominated by the neutral aquatic Ca2UO2(CO3)3 complex, which has a lower sorption affinity towards kaolinite than the charged UO2(CO3)34-, (UO2)2CO3(OH)3- and UO2(CO3)22- species formed in the NaClO4 system (I = 0.42 M) at pH 7.6.
In the presence of HA (10 mg/L, 50 mg/L), in opalinus clay pore water a lower percental amount of uranium(VI) is adsorbed onto kaolinite. But the concentration of HA has no influence on the amount of sorbed uranium(VI). The reason is that HA has no influence on the uranium(VI) speciation in opalinus clay pore water at pH 7.6, thus, no uranium(VI) humate complexes are formed. The decrease of sorbed uranium(VI) in presence of HA is only due to the competition for surface binding sites between uranium(VI) and HA. These results are in contrast to the results obtained in the NaClO4 system (I = 0.1 M) [1]. There, the speciation of uranium(VI) in NaClO4 changes because aqueous uranium(VI) humate complexes are formed. Due to this and due to a competition for surface binding sites, the percental amount of sorbed uranium(VI) decreases. This effect is increased with increasing HA concentration.
Furhermore, the uranium(VI) sorption onto kaolinite and opalinus clay in opalinus clay pore water was compared. Results show a stronger sorption of uranium(VI) onto kaolinite (2.8 ± 0.2 µg/m²) than onto opalinus clay (0.049 ± 0.002 µg/m²). Since opalinus clay has a higher specific surface area than kaolinite (opalinus clay: Asp = 42 m²/g; kaolinite: Asp = 11.7 m²/g [1]), and contains a higher amount of iron-containing minerals (opalinus clay: 5 wt.% Fe; kaolinite: 0.27 wt.% Fe [1]), which are able to sorb uranium(VI) very well [3], this result is unexpected. To clarify this observation, leaching experiments with opalinus clay applying the two background electrolytes are performed in order to determine the release of ions into the solution in dependence on time and pH and to investigate their influence on the uranium(VI) sorption. We studied the pH dependence of the uranium(VI) sorption onto opalinus clay in absence and presence of HA, and compared this with previous results obtained for kaolinite [1].

[1] A. Křepelová et al., Radiochim. Acta 2007, 94, 825.
[2] F.J. Pearson, PSI Internal Report TM-44-98-07 1998, Paul Scherrer Institute, Villigen, Switzerland.
[3] K. Schmeide et al., Radiochim. Acta 2000, 88, 723.