High-Flux Positively Charged Nanocomposite Nanofiltration Membranes Filled with Poly(dopamine) Modified Multiwall Carbon Nanotubes

TLDR

Pristine carbon nanotubes are poorly dispersible in water, limiting their use in thin‑film nanocomposite membranes for water purification. The study aimed to develop high‑flux, positively charged nanofiltration membranes by embedding poly(dopamine)‑modified multiwall carbon nanotubes into polyamide thin‑film composites. PDA‑modified MWCNTs were uniformly dispersed in polyethyleneimine aqueous solutions and co‑polymerized with trimesoyl chloride in n‑hexane to form the nanocomposite membranes. The resulting PEI/PDA‑MWCNT/TMC membrane (M‑4) exhibited a 1.6‑fold increase in pure‑water permeability (15.32 L m⁻² h⁻¹ bar⁻¹) and selective rejection of multivalent cations (ZnCl₂ 93.0 %, MgCl₂ 91.5 %, CuCl₂ 90.5 %, CaCl₂ ≈ 90 %), making it suitable for water softening and heavy‑metal removal.

Abstract

The poor dispensability of pristine carbon nanotubes in water impedes their implications in thin-film nanocomposite membranes for crucial utilities such as water purification. In this work, high-flux positively charged nanocomposite nanofiltration membranes were exploited by uniformly embedding poly(dopamine) modified multiwall carbon nanotubes (PDA-MWCNTs) in polyamide thin-film composite membranes. With poly(dopamine) modification, fine dispersion of MWCNTs in polyethyleneimine (PEI) aqueous solutions was achieved, which was interracially polymerized with trimesoyl chloride (TMC) n-hexane solutions to prepare nanocomposite membranes. The compatibility and interactions between modified MWCNTs and polyamide matrix were enhanced, attributed to the poly(dopamine) coatings on MWCNT surfaces, leading to significantly improved water permeability. At optimized conditions, pure water permeability of the PEI/PDA-MWCNTs/TMC nanofiltration membrane (M-4) was 15.32 L m–2 h–1 bar–1, which was ∼1.6 times increased compared with that of pristine PEI/TMC membranes. Salt rejection of M-4 to different multivalent cations decreased in the sequence ZnCl2 (93.0%) > MgCl2 (91.5%) > CuCl2 (90.5%) ≈ CaCl2, which is well-suited for water softening and heavy metal ion removal.