Abstract
This study investigates ionic transport and selectivity in a thin-film composite nanofiltration membrane (NF270) operated in pressure-driven and electrodialysis processes. The perm-selectivity and permeabilities of anions (SO42− and NO3−, SO42− and Cl−, NO3− and Cl−) and cations (Mg2+ and Na+) were studied in an electrodialysis cell operated with NF membrane (ED-NF) at varied trans-membrane potentials (0.5–2 V) and in conventional nanofiltration process. The permeabilities of all ions in the electrochemical process were one to four orders of magnitude lower than permeabilities reported for the pressure-driven NF operated with the same membrane. In the pressure-driven process, a high selectivity of mono-to di-valent ions was obtained, with anions selectivity higher than cations selectivity (highest Na+ to Mg2+ selectivity value of 4.0 vs. lowest Cl− to SO42− selectivity value of 11.6). In contrast, in ED a low perm-selectivity of mono-to di-valent ions was obtained (1.1–1.2 for Na+/Mg2+, and 2.4–3.4 for Cl−/SO42−).Our findings show that: (1) ionic transport in thin film composite membranes operated in electrochemical processes cannot be described by conventional ion exclusion models applied to pressure-driven processes, such as Born or Donnan exclusion mechanisms; and (2) a different, currently unknown, mechanism dominates the ion separation.
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