Dual-layer polybenzimidazole/polyethersulfone (PBI/PES) nanofiltration (NF) hollow fiber membranes for heavy metals removal from wastewater

We have designed and characterized a high performance dual-layer nanofiltration (NF) hollow fiber membrane for effective removal of heavy metal ions (Cd²⁺, Cr2O72- and Pb²⁺) from model wastewater. The membrane was fabricated by the simultaneous co-extrusion of polybenzimidazole (PBI) and polyethersulfone (PES)/polyvinylpyrrolidone (PVP) dopes through a triple-orifice spinneret using a dry-jet wet phase inversion process. PBI was chosen as the outer selective layer because of its superior chemical resistance and unique charge characteristics, while a PES/PVP blend was employed as the support layer because its reasonable cost, superior spinnerablity, hydrophilic nature, good mechanical properties and easy formation of porous membranes. In addition, PVP is miscible with both PBI and PES. The newly developed dual-layer NF membrane has superior rejections to various salts. The rejections of the membrane to Mg²⁺ and Cd²⁺ achieve 98% and 95%, respectively. By changing the pH of the solution, the rejections to Cr2O72- and Pb²⁺ can reach more than 98% and 93%, correspondingly. Experimental results indicate that the high rejections are owing to the following factors: (1) a narrow pore size distribution membrane with a mean effective pore radius of 0.32nm and a molecular weight cut off (MWCO) of 249Da; (2) an enhanced Donnan exclusion effect due to the amphoteric PBI charge property; (3) low adsorptions of heavy metals on the PBI surface due to its hydrophilic nature.