Efficient removal of uranium and sulfate in acid contaminated groundwater by flow electrode capacitive deionization

In-situ leaching (ISL) causes non-negligible groundwater pollution. It is urgent to remediate the groundwater after ISL activities. In this study, we evaluated the effectiveness of flow electrode capacitive deionization (FCDI) to treat a simulated groundwater, the uranium (U) and SO 4 2− concentration of which are comparable to groundwater in acid in-situ leaching (AISL) uranium mine for the first time. Moreover, the removal mechanism of U and SO 4 2− were investigated in-depth. It is found that the operational mode, applied voltage and initial SO 4 2− concentration significantly affect the removal of U and SO 4 2− by FCDI. The removal efficiency of U and SO 4 2− were above 98 % at 75 min under optimal condition, although U in groundwater mainly existed in the form of uncharged UO 2 (SO 4 ), followed by UO 2 2+ and UO 2 (SO 4 ) 2 2− . UO 2 2+ and UO 2 (SO 4 ) 2 2− in groundwater migrated into the two poles and were quickly absorbed by flow electrode, which promoted the dissociation of UO 2 (SO 4 ) or complexation of UO 2 (SO 4 ) with SO 4 2− . In addition, the anion exchange membrane can absorb UO 2 (SO 4 ) through complexation. These resulted in the efficient removal of U(VI). FCDI can reduce the U and SO 4 2− concentration of the contaminated water ( C U  = 10 mg L −1 , C SO4 2−  = 5 g L −1 ) to a value lower than the Chinese emission limit (U: 300 μg L −1 ; SO 4 2− : 250 mg L −1 ) even after 18 cycles with each cycle operated for 120 min, which informed that FCDI system using activated carbon is of great potential for acidic contaminated water treatment.