Schmal, S. (2022). Green Synthesis of Cellulose Graft Copolymers for Anion Exchange Water Purification. Unc Charlotte Electronic Theses And Dissertations.
This thesis describes the synthesis and characterization of novel cellulose graft copolymers that are used for the purification of drinking water. These nanomaterials function as bottle-brush anion-exchange resins with repeating quaternary ammonium polyelectrolyte chains covalently coupled to a cellulose scaffolding. In the synthesis process of cellulose nanoresin (CNR), commercial microcrystalline cellulose is oxidized at the C-6 position to form cellouronic acid via the (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO) radical while under sonication, or high-pressure homogenization. Polyelectrolyte chains are then synthesized with controlled strand lengths using the ARGET-ATRP mechanism. CNR was prepared in all-aqueous medium by Fischer-Speier esterification using a catalytic amount of hydrochloric acid. Carboxyl contents of TEMPO-oxidized nanocellulose (TONC) and of CNR were measured using conductometric titration, with values as high as 1.61 mmol g-1 for TONC and as low as 0.627 mmol g-1 for CNR respectively. Percent functionalization of CNR was calculated to be 61%. The hydrodynamic diameter and zeta potential of TONC and CNR were measured by dynamic light scattering (DLS). DLS data showed a decrease in the size of the particles in dispersion after the oxidation of cellulose. Consistent with our graft copolymer model, the particle’s size increased after polyelectrolyte functionalization. The surface charge of dispersed particles of commercial cellulose, TONC, and CNR also changed from neutral to negative to positive at pH 7, indicating successful synthetic procedures. CNR was further characterized by testing its functionality as a thin-film membrane and as an adsorbent material for anion exchange applications. CNR membranes were found to exhibit high water flux of >700 L m-2 h-1 bar-1 and high loading capacities of >20 mg g-1 when tested using a sodium fluorescein (NaFL) surrogate adsorbate. These materials exhibit fast kinetics, with equilibrium loading being achieved in a matter of seconds. CNR thin films are regenerable and reusable as water purification membranes, with a decrease in loading capacity of only 0.01 ± 0.01 mg NaFL per gram of CNR, or 0.03% after 40 cycles.
