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 SO₄²⁻ 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 SO₄²⁻ were investigated in-depth. It is found that the operational mode, applied voltage and initial SO₄²⁻ concentration significantly affect the removal of U and SO₄²⁻ by FCDI. The removal efficiency of U and SO₄²⁻ were above 98 % at 75 min under optimal condition, although U in groundwater mainly existed in the form of uncharged UO₂(SO₄), followed by UO₂²⁺ and UO₂(SO₄)₂²⁻. UO₂²⁺ and UO₂(SO₄)₂²⁻ in groundwater migrated into the two poles and were quickly absorbed by flow electrode, which promoted the dissociation of UO₂(SO₄) or complexation of UO₂(SO₄) with SO₄²⁻. In addition, the anion exchange membrane can absorb UO₂(SO₄) through complexation. These resulted in the efficient removal of U(VI). FCDI can reduce the U and SO₄²⁻ concentration of the contaminated water (C_U = 10 mg L⁻¹, C_SO4²⁻ = 5 g L⁻¹) to a value lower than the Chinese emission limit (U: 300 μg L⁻¹; SO₄²⁻: 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.