## 2 Materials and Methods
### Procedures and Analytics
#### 3.1.2 Scrub 2 Section  
It was recognized that the amount of loaded organic phase from the extraction section test would be insufficient to run all further planned single centrifugal contactor tests. Therefore, a batch contact was conducted under the same conditions as in the extraction section test, i.e., an aqueous phase with the same composition as in the extraction section test was batch contacted for 30 min with the correspondent volume of ALSEP solvent. During phase separation, a small amount of white precipitation was encountered at the phase boundary, which was expected (it had been observed in previous studies) [35]. Based on the mass balance, the precipitate was found to be mainly composed of Sn, Mo, Zr, and Y. The organic phase was centrifuged to separate the precipitate, and the loaded organic phase from the batch contact was mixed with the loaded organic phase from the single centrifugal contactor extraction section test. This combined organic phase was then batch contacted under Scrub 1 conditions, i.e., contacted with the correspondent volume of a 1:1 mixture of Scrub 1 and Scrub 2 solutions for 30 min (a/o phase ratio of 2.5:1). The phases were separated to yield the loaded organic phase for the Scrub 2 single centrifugal contactor test. In this second batch contact, no precipitation was observed. The organic phase composition is shown together with the results of the single centrifugal contactor test in Table S2. The loaded solvent mainly contained An(III) and Ln(III) and some impurities of Y, Zr, Mo, Ru, Rb, Sr, and Fe.  
Figure 3 shows the Am, Cm, and Eu distribution ratios as a function of the experimental run time and equilibrium values in the Scrub 2 section single centrifugal contactor test. Distribution ratios and mass balances calculated from the transient and equilibrium samples, and stage efficiencies for all metal ions, are shown in Table S2. The stage efficiencies were calculated by dividing the equilibrium distribution ratio by the mean value of the transient samples distribution ratios from 80, 90, and 100 min. As the steady state had been already reached, taking the mean value of those three samples was done to reduce uncertainties. For An(III) and Ln(III), the stage efficiency was high. These elements stayed in the organic phase with high distribution ratios. The molybdenum stage efficiency was relatively low at 8%. However, this was caused by a steady-state distribution ratios of ca. 0.17 and a very low equilibrium \(D\) (0.01). Therefore, eight Scrub 2 stages were expected to be sufficient, although a higher number of stages would clearly be beneficial for more efficient scrubbing of Mo from the loaded organic phase. In the Scrub 2 section, Rb, Sr, and Cs also showed distribution ratios below 1 and would therefore be scrubbed. Co-extracted Zr, Y, Ru, and Fe would not be scrubbed in that section, as the distribution ratios were above 1.  
Figure 3: Am, Cm, and Eu distribution ratios as a function of the experimental run time (data points) and equilibrium values (horizontal lines) in the Scrub 2 section single centrifugal contactor test. “g”, “a”, and “1” indicate values measured by gamma and alpha spectrometry as well as ICP-MS, respectively.