Efficient separation of sulfate (SO 4 2- ) and chloride (Cl - ) in impaired waters is critical for mitigating mineral scaling in applications such as water desalination, resource recovery from brines, and the chlor-alkali process. In this study, we demonstrate that concentration gradient-driven transport across anion-exchange membranes (AEMs) enables exceptionally high Cl - /SO 4 2- selectivity. This selectivity arises from two coupled mechanisms: preferential partitioning of Cl - within the membrane and uphill transport of SO 4 2- against its external concentration gradient. Concentration gradient-driven transport in AEMs is governed by co-ions (e.g., Na + ), and these co-ions are predominantly balanced by Cl - when the membrane is contacted with mixed NaCl/Na 2 SO 4 solutions, resulting in selective Cl - transport. The Cl - / SO 4 2- selectivity is further enhanced by uphill SO 4 2- transport, driven by pronounced differences in competitive counter-ion partitioning at the two membrane-solution interfaces. Notably, high selectivity is observed with different commercially available AEMs and maintained across a wide range of solution compositions. These findings establish concentration gradient-driven transport as a promising and energy-efficient strategy for Cl - / SO 4 2- separation. • Anion-exchange membranes can separate Cl - /SO 4 2- via concentration gradients. • Transport governing co-ions (Na + ) are primarily balanced by Cl - ions. • SO 4 2- undergoes uphill transport against external concentration gradient. • Commercial anion-exchange membranes exhibit similarly high Cl - /SO 4 2- selectivity. • High Cl - /SO 4 2- selectivity is maintained across wide solution compositions.
Patel et al. (Fri,) studied this question.