A unique high performing nanofiltration membrane using complementary phase additives in tuning polyamide layer to treat salt rich stream of cellulosic fibre industry.

Abstract

A thin-film composite (TFC) nanofiltration (NF) membrane was developed using additives polysodium 4-styrenesulfonate (PSS) and tri-n-butyl phosphate (TBP) in the aqueous and organic phases to attain the synergistic effects. The effects of concentration of PSS and TBP in the respective phases were investigated on the interfacial polymerization (IP) reaction and their impact on the membrane performance. Use of sole additives led to the enhanced cross-linking and denser polyamide (PA) layer but incorporation of complementary phase additives resulted thinner and looser morphology with higher negative surface charge enhancing the nano-filtration performance. The TFC-3 membrane with optimum PSS (0.05 wt%) and TBP concentration (0.005 wt%) showed the best performance with water flux (31.4 L.m⁻².h⁻¹), rejection of sodium sulfate (90 %), and flux recovery ratio (95 %) at 8.3 bar pressure and 100 L.h⁻¹ crossflow rate in the synthetic solution. The efficacy of the membranes was tested at the same operating conditions with the sulfate-rich industrial stream using the TFC-3 membrane and achieved 17.3 ⁻².h⁻¹ average permeate flux, 75 % rejection of total dissolved salts, and 80 % rejection of organics during long-run filtration study. Further, a multi-component coupled concentration polarization and pore flow model was formulated to describe the transport of the ions as well as organic (uncharged hemicellulose) in order to predict the concentration of salt, COD in the permeate as well as the permeate flux. The membrane pore charge density was found to −21.6 mEq.L⁻¹. The model predictions agreed closely with the experimental data. This work not only demonstrated the influence of the synergistic effects of complementary phase additives during interfacial polymerization but also provided a sustainable TFC nanofiltration membrane suitable for the treatment of the industrial process stream.