Batch experiments with U, Np and Nd
In context of the WEIMAR-Project batch sorption experiments onto orthoclase and muscovite were conducted to create new data. The selected parameters for experiments at the HZDR are:
- Elements: U(VI), Np(V), Nd(III)
- Element oncentrations: 10-5 mol L-1 and 10-6 mol L-1
- Background electrolyte: 0.01 mol L-1 NaClO4, 1 mol L-1 NaClO4
- Solid-to-liquid ratio: 1/20 and 1/80 g ml-1
- pH: 5 - 8
At first all experiments will be performed with U(VI). The experiments with Np(V) and Nd(III) will follow later.
Sorption experiments for U(VI) with orthoclase and muscovite were done. As orthoclase a Norflot Kali 600 from sibelco Germany GmbH was used and the muscovite is from normag GmbH (China). Both minerals were used with a grain size of 63-200 µm. All experiments are carried out under ambient atmosphere to keep the oxidation state of U(VI).
All batch-experiments were performed in the same way. The minerals and the background solution were equilibrated and the pH-values were checked and readjusted periodically until the final pH was stable. During equilibration the samples were shaken. If the pH was stable the U(VI) solution was added. Afterwards the pH-values were directly readjusted and the samples were shaken (24 h for the selection of the solid-to-liquid ratio, 1 week for all other batch experiments). This time is used as equilibration time for U(VI) with the mineral and the background solution. Then the samples were centrifuged and the supernatant was analyzed via ICP-MS.
For further characterization TRLFS measurements and flow-through experiments were done.
Selection of the solid-to-liquid ratio (SLR)
To determine the optimal SLR at which the sorption of uranium is neither too low nor too high, a pretest is done with different SLR. For this pretest the pH-value was set to 6 and an uranium concentration of 10-6 mol L-1 was used. As possible SLR were investigated: 1/10, 1/20, 1/40, 1/80, 1/160 and 1/320.
In Fig. 1 the sorbed amount of U(VI) onto orthoclase at different solid-to-liquid ratios is shown.
Fig. 1: Percentage of 10-6 mol L-1 U(VI) adsorbed onto
orthoclase at different solid-to-liquid ratios at pH 6.
At a SLR of 1/20 the amount of adsorbed U(VI) shows a high affinity to adsorb at the mineral. This high amount of adsorbed U(VI) is good to see differences when sorption conditions were changed thus this ratio is used in further experiments. The SLR of 1/80 is in the range where nearly half of the added uranium adsorbs. To see a difference to the SLR of 1/20 this ratio is also used for following investigations.
An experiment was done to determine the ideal time for uranium sorption onto the minerals. For this experiment suspensions of a solid-to-liquid ratio of 1/20 and 1/80 were prepared at an pH of 6.5 and the uranium concentration was set to 10-6 mol L-1. After different times (6; 24; 72; 168 h) the samples were centrifuged and afterwards analysed with ICP-MS. The results for both solid-to-liquid ratios for muscovite are shown in Fig. 2.
Fig. 2: Sorption kinetics of U(VI) onto muscovite,
M/V [g ml-1] 1/20 blue, 1/80 red.
The uranium sorption onto muscovite shows still an increase for both investigated solid-to-liquid ratios after three days. After one week no significant increase is detectable. The kinetic experiments for orthoclase show similar results. Because of this all further sorption experiments of U(VI) will be done with a sorption time of one week (7 days).
Sorption according to pH
Batch sorption experiments onto orthoclase and muscovite were performed at two solid-to-liquid ratios and two concentrations of U(VI) (10-5 mol L-1 and 10-6 mol L-1) according to different pH values. The results indicate that the sorption maximum of uranium onto these two minerals is between pH 6 and 7. A higher amount of mineral increases the sorbed amount of U(VI) due to more sorption places. As well as a lower concentration of U(VI) increase the sorbed amount because less U(VI) is in solution that has to bind on the mineral. Exemplarily the results for muscovite and an U(VI) concentration of 10-6 mol L-1 are shown in figure 3.
Fig. 3: Sorption of 10-6 mol L-1 U(VI) on muscovite.
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