Crosslinked layer-by-layer polyelectrolyte nanofiltration hollow fiber membrane for low-pressure water softening with the presence of SO42− in feed water

• Crosslinking and polyelectrolyte deposition were used to make low-pressure NF membrane.
• The membrane has water softening capability with the presence of SO42− in the feed.
• The possible structure of the deposited layers was proposed based on XPS analysis.
• The membrane showed >10 LMH/bar and >95% rejection to 1000 ppm MgCl2, MgSO4, Na2SO4 at 2 bar.

In this study, the glutaraldehyde (GA) crosslinking was adopted in combination of layer-by-layer (LBL) polyelectrolyte deposition to develop a novel hollow fiber NF membrane with low-pressure water softening capability for synthesized hard water containing divalent counterion SO42−. The challenges for the new membrane include (a) the presence of SO42− in the feed water, which tends to deteriorate positively charged NF membrane retention to divalent ions; (b) high salinity feed water up to 10,000 ppm total dissolved salt (TDS); (c) 2 bar operating pressure that requires the membrane to have a good Mg2+/Na+ selectivity and less mass transfer resistance in the substrate; and (d) better separation performance as compared with the state-of-the-art and commercial NF membranes.Based on the membrane characterization results, the possible structure of the deposited layers and the effect of GA crosslink was proposed. It was found that the crosslinking could tighten the membrane surface pores with increased hydrophilicity while the membrane surface charge was reduced. Therefore, mainly based on the size exclusion, the crosslinked LBL*1.5C membrane showed more than 10 LMH/bar permeability with above 95% rejection to each of 1000 ppm MgCl2, MgSO4 and Na2SO4 solution under 2 bar pressure. It was also proved to be more suitable for the low-pressure water softening application at 2 bar in all tested feed water with the presence of SO42− and TDS up to 10,000 ppm in terms of both permeability and hardness removal efficiency when compared with commercial NF 270 and NF 90 membranes.