Synthesis and characterization of thin film nanocomposite reverse osmosis membrane for salt and boron removal

Abstract

In this study, the effects of organic solvents, heat treatment methods, postinterfacial polymerization (IP) rinsing (prior to membrane heat treatment) and additives on the properties of thin film composite (TFC) membranes were investigated prior to the fabrication of thin film nanocomposite (TFN) membranes incorporated with inorganic nanomaterials. It was found that the preservation of substrate pore structures and the removal of excess monomers and organic solvent from the membrane surface are imperative to fabricate reproducible TFC membranes with consistently high water flux and salt rejection. The main findings from investigating the IP parameters are i) keeping the substrate at minimal heat exposure could prevent substrate pore collapse that potentially reduces the membrane water permeability, ii) rinsing membranes with pure n-hexane after IP resulted in membranes having higher pure water flux (PWF) without significantly decreasing solute rejection, iii) the membrane performances became practically the same after post-IP rinsing, regardless of the solvent used in the IP reaction and iv) membranes fabricated using triethylamine-camphorsulfonic acid-sodium dodecyl sulfate (TEA-CSA-SDS) additives exhibited higher PWF and salt rejection than the membranes fabricated in the absence of the additive. For the TFN membranes, it was found that nanomaterial structures (i.e., sizes and shapes) affect the separation performance of the resultant TFN membranes. Noticeably, titanium-based nanomaterial in spindle-like nanoporous structure (f-nTiO2) yielded membrane of better filtration performances than its tubular structure – functionalized titanate nanotube (f-TNT). Compared to TFN-f-TNT membrane, TFN-f-nTiO2 membrane possessed greater water flux (4.26 vs. 3.36 L/m2·h·bar), NaCl (98.04 vs. 97.28%) and boron rejection (54.82 vs. 48.86%). Ultimately, the incorporation of nanomaterial into membrane selective layer was found to improve membrane water flux at the expense of NaCl and boron rejection in comparison to the TFC membranes. Surface coating of TFN membranes with polyvinyl alcohol (PVA) was found to be effective to recover membrane solute rejection, with slight reduction in water flux. The synergic effect of nanomaterial incorporation and PVA coating resulted in improved membrane water flux without trading off its solute rejection.