Green modification of outer selective P84 nanofiltration (NF) hollow fiber membranes for cadmium removal

Nanofiltration (NF) is a membrane filtration process with a membrane pore size in the subnano scale. Due to its special rejection mechanisms: size exclusion and charge repulsion, NF is able to offer good rejections to multi-valent heavy metal ions without much compromising water fluxes as compared to those of reverse osmosis (RO) membranes. There is a growing interest in thin-film composite (TFC) NF hollow fiber membranes. The TFC layer can be formed on both inner and outer surfaces of hollow fiber membranes. The outer selective TFC hollow fiber membranes may have less mass transfer resistance and lower chances being blocked by foulants compared to the inner ones. Thus, the outer selective TFC fibers can be fabricated into smaller fiber dimensions with a higher package density in modules. Despite these advantages, the molecular design of an ideal TFC layer on the outer surface of hollow fiber membranes remains a challenge, making the mass production difficult. Since most TFC layers are made from interfacial polymerization of mphenylenediamine (MPD) and trimesoyl chloride (TMC), removing excess solution after the MPD coating is the major obstacle in the process. Vacuum assist interfacial polymerization or wipe drying are required to make a defect free polyamide layer. Another obstacle is that a large amount of alkane solvents has to be used during interfacial polymerization, which brings extra costs and environmental issues. Eliminating alkane solvents is critical to lower manufacture costs and yield a green production process. In this study, green methods to prepare the outer selective TFC hollow fiber membranes were explored by firstly modifying the membrane substrate with polyethyleneimine (PEI) and then by water soluble small molecules such as glutaraldehyde (GA) and epichlorohydrin (ECH). Using P84 polyimide as the substrate, not only do these modifications decrease substrate's pore size, but also vary surface charge by making the membranes less positively charged. As a result, the resultant membranes have higher rejections against salts such as Na₂SO₄, NaCl and MgSO₄. The PEI and then GA modified membrane has the best separation performance with a NaCl rejection over 90 % and a pure water permeability of 1.74 ± 0.01 Lm^-2bar^-1h^-1. It also shows an impressive rejection of 94 % for CdCl₂ removal during long-term stability tests. The CdCl₂ rejection remains higher than 90% when testing it from 5 to 60°C. This study may provide useful insights for green manufacturing of next-generation NF hollow fiber membranes.