Low-pressure reverse osmosis membrane made of cellulose nanofiber and carbon nanotube polyamide nano-nanocomposite for high purity water production

Abstract

Cellulose nanofiber (CNF) and multiwall carbon nanotube (CNT) nanocomposite polyamide membranes were synthesized and used in a low-pressure reverse osmosis (RO) system. Incorporating CNT and CNF into the polyamide structure (PA-CNT/CNF) allows the homogenous dispersion within the polymeric matrix, increasing its hydrophobic character and surface oxygen functionalities while reducing its surface roughness when compared to plain polyamide (PA), PA-CNT, and PA-CNF membranes. The resultant nanocomposite was tested at 0.75 MPa and sodium chloride salt concentration of 500 ppm resulting in a rejection of 99.47 % and water permeability of 1.65 m3/m2day. The rejection and permeation performance is highly competitive compared with commercial RO membranes. A 2in spiral module built with the PA-CNT/CNF membrane and tested for SiO2, NaCl, and CaCl2 rejection showed a reduction in its permeability of 24.9 % after 100 h of operations, while the commercials showed a decrease of 49.42 %. The present RO membrane module achieved a calcium rejection of 97.5 % and a boron rejection of 50.4 %. The 2in PA-CNT/CNF spiral module was tested in a two-step RO filtration system for high pure water (HPW) production and compared with a TW30 module with NSF certification. The PA-CNT/CNF module revealed a permeability reduction of 9.07% after 30 h operation, while TW30 had a reduction of 20.3%, which is about twice the durability. Molecular dynamics simulation showed that the CNF increased water hydration and boron diffusion on the membrane while CNT increased charge transfer to the PA structure. This polymeric nanocomposite has potential use in producing high purity water for brackish water or wastewater reuse for applications in the emerging semiconductor or pharmacology industry.