Increased adsorption of aqueous zinc species by Ar/O₂ plasma-treated carbon nanotubes immobilized in hollow-fiber ultrafiltration membrane

Herein, we employed Ar/O₂ plasma treatment technique to functionalize multi-walled carbon nanotubes (MWCNTs), thus to improve their dispersion capability and adsorption for cationic metal ions in water. The results indicate that plasma treatment significantly increased the surface oxygen content, and modified surface functionality of MWCNTs without damaging the integrity of nanotube patterns, thus, exhibiting superior dispersion and adsorption properties. Moreover, nanocomposite membranes were fabricated by incorporating functionalized MWCNTs inside a polymeric substrate membrane with hollow-fiber configuration. These membranes were studied for the adsorptive filtration of zinc (Zn²⁺) and other metal cations from synthetic and real water. The adsorptive removal efficiency of zinc by the composite membrane with functionalized MWCNTs remained at above 90%, regardless of water chemistry conditions. Unlike CNT adsorption of organics reported in the literature, the presence of natural organic matter (NOM) did not adversely affect zinc removal, indicating lack of competitive adsorption between zinc ions and NOM. Furthermore, the self-fabricated membranes effectively removed zinc and iron from a secondary wastewater effluent in the presence of common co-existing ions such as Na⁺, Ca²⁺, Mg²⁺, Mn²⁺ and Fe²⁺, illustrating the selective adsorption of Zn from wastewater. Importantly, the adsorption capability of CNT membranes for zinc was conveniently regenerated under acidic condition. This verified that the CNT membranes retained the original metal removal capacity even after five consecutive adsorption–desorption cycles.