Researchers at Nanyang Technological University in Singapore have developed composite nanofiltration (NF) hollow fibre membranes desirable for water softening under low operating pressure (<2 bar).
The thin-film selective layer of the composite hollow fibre was formed through interfacial polymerisation on the inner surface of a polyethersulfone (PES) ultrafiltration (UF) membrane substrate. Branched polyethyleneimine (PEI) and trymesoyl chloride (TMC) were employed as the monomers in aqueous and organic phases, respectively.
It was found that a proper molecular weight of PEI and the presence of sodium dodecyl sulfate (SDS) in the aqueous phase are important for a successful interfacial polymerisation reaction.
The resulting membrane prepared with optimised preparation parameters possesses a positively charged thin-film selective layer with pure water permeability (PWP) of about 17 l/m2h bar and a molecular weight cutoff (MWCO) of around 500 Da, which translates into an effective pore diameter of approximately 1.29 nm.
With combined separation mechanisms of Donnan exclusion and steric hindrance, it was possible to achieve MgCl2 and MgSO4 rejections of 96.7% and 80.6%, respectively, when tested for 1000 ppm feed solutions at 2 bar operating pressure.
In addition, for a 3000 ppm total dissolved salt (TDS) feed stream containing salt mixtures, the membrane rejections for Mg2+ and Ca2+ ions were found to be around 90%, while the water flux was about 20 l/m2h at 2 bar pressure. This suggests the potential of the newly developed composite hollow fibres for effective water softening.