Green and Sustainable Water Purification Nanocomposites

A. Sahu, J.C. Poler
University of North Carolina at Charlotte,
United States

Keywords: cellulose nanofibers, water purification, anion exchange resins, nanocomposites

Summary:

Water has been the most life-sustaining drink to humans and essential for survival of all organisms. The demand for clean and safe drinking water is a global challenge because of water scarcity, growth of human population, urbanization and anthropogenic pollution. Clean drinking water substantially impacts human health, ecosystems and economic conditions around the world. To address this issue, robust new water purifying processes are required that use low cost and less energy, and at the same time minimize the use of chemicals and impact on the environment. Sustainable materials like cellulose nanofibers are not only environmentally benign and cost effective, but also exhibit exceptionally high surface area, high mechanical strength and versatile surface chemistry. Due to these properties, it is a highly suitable candidate for formulating next-generation water purification systems. Robust cellulosic nanocomposites were synthesized by functionalizing short strands of anion exchange resins of poly(vinyl benzyl trimethylammonium chloride) poly(vbTMAC) using successive all-aqueous surface modification and functionalization steps. Oxidized cellulosic nanofibers were obtained by exfoliating cellulosic materials using well-established 2,2,6,6-Tetramethylpiperidin-1-yl)oxyl/sodium bromide/sodium hypochlorite oxidation mechanism in presence of sonochemistry. C6 carboxylate groups on cellulose nanofibers were cross linked with allyl amine hydrochloride via stable amide bonds using 1-Ethyl-3-(3-dimethyl-aminopropyl) carbodiimide/N-Hydroxysuccinimide. The resulting vinyl moieties hanging on the surface of modified cellulose nanofibers were made to react with known lengths of uniformly grown poly(vbTMAC) strands using modified activator regenerated by electron transfer – atom transfer radical polymerization mechanism under reflux. Functionalization of polyelectrolyte strands on cellulose nanofibers having high surface area to mass ratio resulted in increase in the availability accessible anion-exchange binding sites allowing faster sorption kinetics and higher adsorption capacity compared to currently commercialized material like magnetic ion exchange resins, DOWEX, etc. Presence of carboxylate group (~1600 cm-1) on cellulose was detected via Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR).9,10 The hydrodynamic diameter of cellulosic materials changed from 246 ± 9.06 nm to 21.1 ± 1.24 nm in nanofibers after C6 carboxylate oxidation and simultaneous exfoliation under sonochemistry. Kinetics of amide linkage was analyzed using proton nuclear magnetic resonance. Purified poly(vbTMAC) functionalized Cellulose Nanofibers Composite (poly-CNC) exhibited positive zeta potential confirming the conformal coating of positively charged polyelectrolyte on the surface modified cellulose. Successful functionalization of polyelectrolyte on surface modified cellulose nanofibers was confirmed by observing shift in C-H stretch from 3015 cm-1 to ~2900 cm-1 using ATR-FTIR. Morphology of poly-CNC films deposited on polypropylene membranes exhibited a mesh of nanofibers revealing nanochannels for easy flow of contaminant media. Poly-CNC fabricated films exhibited good absorption capacities for various types of pervasive anionic contaminants (per- and polyfluoroalkyl substances, pharmaceuticals, disinfection byproducts and pesticides) along with comparable water flux to typical ultrafiltration membranes. Regeneration and reusability of poly-CNC was performed via breakthrough curves using sodium fluorescein as the analyte. Synthesis, purification, kinetics, characterization and water purification testing of poly-CNC will be presented. Poly-CNC was used to fabricate inner walls of prototype column to generate minimal viable product in the form of green and sustainable water purifier.