Nanofiltration applications of tough MWNT buckypaper membranes containing biopolymers

The ability of biopolymers (bovine serum albumin, lysozyme, chitosan, gellan gum and DNA) to facilitate formation of aqueous dispersions of MWNTs was investigated using a combination of absorption spectrophotometry and optical microscopy. Subsequently, self-supporting carbon nanotube membranes, known as buckypapers (BPs), were prepared by vacuum filtration of the dispersions. Microanalytical data obtained from the BPs confirmed the retention of biopolymers within their structures. Tensile test measurements performed on the BPs showed that incorporation of the biopolymers resulted in significant improvements in mechanical properties, compared to analogous BPs containing MWNTs and the low molecular mass dispersant Triton X-100. For example, MWNT/CHT BPs (CHT=chitosan) exhibited values for tensile strength, ductility, Young's modulus and toughness of 28±2 MPa, 5.3±2.7%, 0.9±0.3 GPa and 1.7±0.3 J g-1, respectively. Each of these values are significantly greater than those obtained for MWNT/Trix BPs, prepared using a low molecular weight dispersant (6±3 MPa, 1.3±0.2%, 0.6±0.3 GPa and 0.10±0.06 J g-1, respectively). This significant improvement in mechanical properties is attributed to the ability of the long biopolymer molecules to act as flexible bridges between the short CNTs. All BPs possessed hydrophilic surfaces, with contact angles ranging from 29±2° to 57±5°. Nitrogen gas porosimetry showed that the BPs have highly porous internal structures, while scanning electron microscopy (SEM) showed their surface morphologies have numerous pore openings. The permeability of the BPs towards water, inorganic salts, and dissolved trace organic contaminants (TrOCs), such as pharmaceuticals, personal care products, and pesticides, was investigated through filtration experiments. Of the twelve TrOCs investigated in this study, nine were rejected by more than 95% by BPs composed of MWNTs and chitosan. The latter BPs also demonstrated good rejection of both NaCl (30-55%) and MgSO4 (40-70%).