**3. Results and Discussion**

#### *3.1. Removal of Potassium*

Because potassium would be co-extracted with rubidium and cesium in the extraction process, the perchloric acid (HClO4) was added into simulated brine to selectively precipitate potassium perchlorate (KClO4) at −5 ◦C and 10 min. The precipitation percentages in different pH values are shown in Figure 3 and the final compositions of simulated brine are shown in Table 2. In this procedure, the precipitation percentage of KClO4 was 98.5% at −5 ◦C and pH 2.

**Figure 3.** Precipitation percentage of KClO4 in different pH value at 10 min.

**Table 2.** Final composition of simulated brine


#### *3.2. First Stage of Solvent Extraction for Cs*

In the first stage of solvent extraction, t-BAMBP was used to separate Cs and other metal ions. Due to the property of t-BAMBP, only K, Rb, and Cs were able to be extracted efficiently. In this study, the values of them in the first stage of solvent extraction were analyzed by ICP-OES and turned into the extraction percentage.
