Ultra-fine cellulose nanofibers: new nano-scale materials for water purification

Abstract

The major challenges in membrane technology for water filtration are the development of new materials (e.g. high durability, low cost and low environmental concerns) and new structures (e.g. directed water channels) that can produce high permeation flux (thus low energy input) while maintaining a high selectivity or rejection rate. This highlight discusses the utilization of ultra-fine cellulose nanofibers (UCN, diameter 5–10 nm), made by TEMPO/NaBr/NaClO oxidation of natural cellulose (e.g. wood pulp), in different nanofibrous composite formats that can meet this challenge for microfiltration (MF) and ultrafiltration (UF) applications. The unique features of ultra-fine cellulose nanofibers include small diameter, high surface-to-volume ratio, easy surface functionality, good mechanical properties and good chemical resistance. The electrospun nanofibrous scaffolds with fine pore size defined by the fiber diameter could be used to remove waterborne bacteria at two to three times higher flux when compared to that of commercial MF membranes (e.g., Millipore GS9035). When UCN were used as a functionalized adsorbent infused in the asymmetric two-layered non-woven fibrous format, the membranes exhibited a high ability to remove bacteria (by size exclusion) and viruses (by adsorption) simultaneously. When UCN were used as the barrier layer in an asymmetric three-layered non-woven fibrous format containing fibers of different diameters (from 5 nm to 20 µm), the membranes exhibited a two- to ten-fold increase in permeation flux over commercial membranes for ultrafiltration of oil and water emulsions (e.g., for purification of bilge water in ships or industrially produced water).