Fabrication of polybenzimidazole (PBI) nanofiltration hollow fiber membranes for removal of chromate

The feasibility of employing nanofiltration for the removal of chromate from wastewater was investigated. Polybenzimidazole (PBI) was used to fabricate nanofiltration hollow fiber membrane with outstanding mechanical strength and chemical stability through the phase-inversion technique. The PBI NF membranes have a mean effective pore size of 0.348 nm in radius, the molecular weight cut off of 525 Da, and pure water permeability of 1.86 × 10^-5 l m^-2 h^-1 Pa^-1 (1.86 l m^-2 h^-1 bar^-1). The PBI membranes showed different rejection to electrolytes in the order of R_{MgC l}2 > R_{MgS O}4 > R_{N a}2 _{S O}4 > R_NaCl. The surface charge properties were clarified by filtering NaCl solution and the PBI membranes showed a V-shape trend of NaCl rejection with an increase in pH. In addition, higher separation ratios of (SO₄^2-/Cl^-) and (Mg²⁺/Na⁺) were obtained from investigating the ion fractionation performance of the PBI membranes when using binary model solutions of NaCl/Na₂SO₄ and NaCl/MgCl₂. Effect of pH on the chromate rejection was rationalized based on an equilibrium diagram of the Cr(VI)-H₂O system. It is concluded that chromates can be effectively separated from their basic solutions by the PBI NF hollow fibers while maintaining their chemical and mechanical stability under high pressures and alkalinity.