Development of hydroxyl and carboxylic acid functionalized CNTs–polysulphone nanocomposite fouling-resistant ultrafiltration membranes for oil–water separation

Fouling-resistant composite ultrafiltration (UF) membranes of hydroxyl functionalized multi-walled carbon nanotubes (MCNTs) ( $$_{\mathrm {{OH}}}\text {CNT}$$ )/polysulphone (PSF)/polyvinylpyrrolidone (PVP) and carboxylic acid functionalized MCNTs ( $$_{\mathrm {{COOH}}}\hbox {CNT}$$ )/PSF/PVP in weight ratios of 0.7/86.1/13.2 and 1.3/85.5/13.2 were prepared by phase inversion process from dimethylformamide solution. These membranes were characterized by infrared, scanning electron microscopy, atomic force microscopy, differential scanning calorimetry and water contact angle measurements. The molecular weight cut off values of these membranes were measured by permeating aqueous solutions of different molecular weight PEGs and dextrans. Among the fabricated UF membranes, $$_{\mathrm {{COOH}}}\text {C}_{{{1.3}}}\text {PS}_{{{85.5}}}\text {PV}_{{{13.2}}}$$ composite membrane showed a lower average contact angle ( $$62^\circ $$ ). The $$_{\mathrm {{COOH}}}\text {C}_{{{0.7}}}\text {PS}_{{{86.1}}}\text {PV}_{{{13.2}}}$$ membrane exhibited higher flux (99 $$\hbox {l}~\hbox {m}^-2~\hbox {h}^-1$$ ) than all the fabricated membranes (62–78 $$\hbox {l}~\hbox {m}^-2~\hbox {h}^-1$$ ) at 2 bar pressure. The flux recovery ratio values for bovine serum albumin feed solution were in the range of 63–72% for $$_{\mathrm {{COOH}}}\hbox {CNTs}$$ containing membranes and 53–64% for $$_{\mathrm {{OH}}}\text {CNTs}$$ membranes, whereas PSF/PVP blend membrane exhibited 45% at 2 bar applied pressure. The membranes containing 0.7 and 1.3 wt% carboxylic acid functionalized MCNTs showed 100% oil rejection when tested with an oil–water emulsion containing 1000 ppm lube oil. Conversely, membranes containing hydroxyl functionalized MCNTs showed relatively lower oil rejection (86–90%), whereas the PSF/PVP membrane showed 86% oil rejection.