Functionalized halloysite nanotubes incorporated thin film nanocomposite nanofiltration membrane for treatment of wastewaters containing metal ions

The high concentrations of heavy metal ions in industrial effluents are one of the most challenging wastewaters to deal with. In this study and in order to change the inner diameter of the halloysite nanotubes (HNT), their internal surface was coated by different polymers (polydopamine, polyaniline and polystyrene) and then, the modified nanotubes were incorporated in the polyamide thin film NF membrane for the treatment of metal ions solutions. Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD) and Brunauer-Emmett-Teller (BET) studies indicated that the internal coating was successful. Furthermore, the neat and modified nanofiltration (NF) membranes were characterized in terms of morphology, contact angle, water flux and metal ion rejection. The modified membranes displayed lower contact angle values (29.4% reduction) that can be linked to the hydrophilic groups in the structure of the coated polymers and the smoother surface of the thin film nanocomposite (TFN) membrane. In addition, the permeation fluxes of the modified membranes were improved significantly (33.4% improvement) without any major reduction in their ion rejection; that shows HNTs act as the channels to transfer water through the membrane structure while the coating of the inner surface of HNT reduces the size of the channel and makes more repulsion and steric hindrance for the ion to pass through the nanotubes. Among the fabricated membranes, the NF membrane with 0.05 wt% polystyrene-coated HNT showed the lowest contact angle (55.30o) and the highest water permeation flux (27.51 L m-2 hr-1), compared to the neat TFC membrane (15.26 L m-2 hr-1) and the NF membrane with 0.05 wt% unmodified HNT (20.62 L m-2 hr-1).