Methods for removing anions from an aqueous solution include contacting the aqueous solution with an initial organic phase composition in a primary stage to form a first mixture, the initial organic phase composition including a quaternary amine and a weak organic acid; separating a nitrate-depleted raffinate from the first mixture; mixing the remaining organic phase (now containing nitrate) with a first basic solution to obtain a second mixture; separating an aqueous phase sulfate-containing scrub solution from the second mixture; mixing the remaining organic phase with a second basic solution to form a third mixture; and separating the third mixture into an aqueous phase nitrate-rich solution and a secondary organic phase composition. The secondary organic phase composition can be recycled. The raffinate, the sulfate-containing scrub solution, and the nitrate-rich solution can then be further processed.