Nanofiltration membranes prepared from pristine and functionalised multiwall carbon nanotubes/biopolymer composites for water treatment applications

A filtration method was used to prepare buckypaper (BP) membranes for water desalination. The samples are made from a biopolymer (chitosan) containing functionalized multiwall carbon nanotubes (MWCNTs) with –NH2 and −COOH moieties. Comprehensive characteristics of all BP membranes were systematically investigated. This included optimisation of sonication time, electrical and mechanical properties (ranging from 7 ± 1 to 69 ± 1 S/cm and tensile strengths between 17 ± 2 and 60 ± 2 MPa, respectively), contact angle (36 ± 3° to 105°±2°), morphology, surface area (ranging from 12 ± 2 m2/g to 112 ± 4 m2/g), surface charge, water permeability (from 0.5 ± 0.1 to 6.5 ± 0.2 L/m2 h bar), and salt rejection. The results indicate that the properties of the BP membrane have been significantly affected as the top layer by the type of multi-walled carbon nanotubes. With MWCNTs−COOH / chitosan BP membranes, the highest permeability towards water was achieved while the MWCNTs-NH2/chitosan membranes provided the best performance in salt rejection by properly balancing amine groups on the top layer, which could be separate monovalent and multivalent cations from salt solutions. Moreover, for monovalent cations (e.g. Na+) BP membranes (MWCNTs/chitosan and MWCNTs-NH2/chitosan) with sufficiently balanced amine groups on the top layer are shown to be much bigger rejection than for higher-valent cations (e.g. Mg2+).