Cellulose Nanofibers for Sustainable Separations

Today, there is an urgent need for products made of renewable and sustainable materials that are biodegradable, carbon-neutral, as well as possess low environmental and human health risks. Cellulose, the integral component of any plant biomass, is the most abundant biopolymer on earth. It has been broadly utilized by the society as a renewable and sustainable material for many applications from engineering, textiles to papermaking for thousands of years. While the native hierarchical structure formation of cellulose in plants is well suited for these applications, the properties, functionality, durability, and uniformity that are required for more advanced applications, such as separation membranes, energy storage media, and high-performance composites, cannot be achieved with the native hierarchical structure of lignocellulose plants. To overcome these challenges, one pathway is to break down the hierarchical structure of cellulosic biomass into a nanoscale form (i.e. nanocellulose) and then reconstruct them. Nanocellulose is known to have high mechanical property, good chemical stability, abundant surface functionality, and high surface area. This chapter deals with the process and property (especially surface functionality and charge group) of nanocellulose and its applications in sustainable water purification. Specifically, three different topics are covered: (i) the production of negatively charged nitro-oxidized cellulose nanofibers (NOCNFs) for metal ion removal; (ii) the manufacturing of thin-film nanofibrous composite (TFNC) membranes prepared from TEMPO-oxidized CNFs for low-fouling ultrafiltration (UF); and (iii) the preparation of positively charged cationic dialdehyde nanocellulose for chromium removal.