Incorporation of Functionalized Halloysite Nanotubes (HNTs) into Thin-Film Nanocomposite (TFN) Nanofiltration Membranes for Water Softening

Abstract

Incorporating nanoparticles (NPs) into the selective layer of thin-film composite (TFC) membranes is a common approach to improve the performance of the resulting thin-film nanocomposite (TFN) membranes. The main challenge in this approach is the leaching out of NPs during membrane operation. Halloysite nanotubes (HNTs) modified with the first generation of poly(amidoamine) (PAMAM) dendrimers (G1) have shown excellent stability in the PA layer of TFN reverse-osmosis (RO) membranes. This study explores, for the first time, using these NPs to improve the properties of TFN nanofiltration (NF) membranes. Membrane performance was evaluated in a cross-flow nanofiltration (NF) system using 3000 ppm aqueous solutions of MgCl2, Na2SO4 and NaCl, respectively, as feed at 10 bar and ambient temperature. All membranes showed high rejection of Na2SO4 (around 97–98%) and low NaCl rejection, with the corresponding water fluxes greater than 100 L m−2 h−1. The rejection of MgCl2 (ranging from 82 to 90%) was less than that for Na2SO4. However, our values are much greater than those reported in the literature for other TFN membranes. The remarkable rejection of MgCl2 is attributed to positively charged HNT-G1 nanoparticles incorporated in the selective polyamide (PA) layer of the TFN membranes.