Novel polybenzimidazole (PBI) nanofiltration membranes for the separation of sulfate and chromate from high alkalinity brine to facilitate the chlor-alkali process

The potential of employing nanofiltration for the removal of sulfate from concentrated chlor-alkali brine has been investigated. Polybenzimidazole (PBI) was chosen to fabricate nanofiltration hollow-fiber membranes through the dry-jet wet phase-inversion technique because of its robust mechanical strength and excellent chemical stability. The feed solution was a concentrated brine consisting of 253.3 g L^-1 NaCl, 9.7 g L^-1 Na ₂SO₄, and 9.5 g L^-1 Na₂CrO ₄ with a pH value greater than 12.65. The FBI membranes showed high sulfate rejection (up to 98.4% at pH 13.25 and 25 bar) and low chloride rejection (less than 4.0%), thus simultaneously obtaining an extremely high di-/monovalent anion selectivity. In addition, the sulfate and chromate rejections increased with increasing solution pH and/or operating pressure. The impressive separation performance can be attributed to the unique pore and charge characteristics and superior chemical stability of PBI NF membranes. It was found that the mean effective pore size of PBI membranes is around 0.30 nm in radius, which is close to the sizes of hydrated sulfate and chromate anions but much larger than chloride anion size, which contributes to the high separation of divalent anions through size exclusion under high ionic strengths.