(571d) Chitosan-Graphene Oxide Composite Membranes for Water Filtration

Chitosan (CS) is a natural, biologically-derived polymer in which deacetylated and acetylated units are randomly distributed. CS can be produced by treating crustacean shells such as shrimp crab, and squid shell with alkali sodium hydroxide. The use of CS as a polymer film, membrane, or scaffold has been explored for different applications, including drug delivery, wound healing, tissue engineering, biosensors, and water treatment. Due to its biocompatibility, biodegradability, low toxicity, and antibacterial and hemostatic properties, CS continues to be explored as a key biopolymer and potential low-cost, naturally-occurring alternative to petroleum-based synthetic polymers. Moreover, CS contains amino and hydroxyl functional groups. The functional groups make CS hydrophilic and amenable to adsorption-based dye and heavy metal removal from water. Two critical challenges in CS utilization as a membrane material in water treatment applications are the weak mechanical properties and the solubility of CS in acidic aqueous environments. To overcome these drawbacks, modification methods, including cross-linking strategies and the use of mechanical reinforcement agents (e.g., graphene oxide, hydroxyapatite, cellulose whiskers, clay), can address these two challenges and result in a more robust membrane material. The use of nanoscale additives has shown particular promise. The greater available surface area per unit mass or volume and the unique properties of materials at the nanoscale allow greater mechanical stability and benefit membrane performance, as well as add additional functionality (e.g., reactivity).

Due to its hydrophobic nature, pristine graphene has a tendency to aggregate, which can reduce the available surface area and its usefulness as a nanofiller. To address this challenge, hydrophilic graphene oxide (GO) is produced by chemical modification of graphene, where oxidation causes the addition of hydroxyl, carboxyl, and epoxide functional groups to the basal planes and edges of the graphene sheets. Low cost, high surface area, water solubility, and high negative charge density make GO a notable adsorbent for removal of heavy metal ions and cationic dyes from water. The oxidative surface modification of GO also enables GO to be a useful, dispersible nanofiller for water filtration membranes, due to the strong interactions between hydrophilic polymer functional groups and GO.

In this talk, we will discuss characterization and performance results for a set of chitosan-graphene oxide composite membranes, where two types of graphene oxide, a nanoscale GO and a granular GO, were evaluated and compared to results for chitosan-only and GO-only membranes. Electron microscopy of membrane surfaces and cross-sections demonstrate significant differences in structural organization and GO layering within the composite membrane materials. Elemental analysis by energy dispersive x-ray spectroscopy identified the presence of aluminum in GO-only membranes and no aluminum in chitosan-GO composites, as is expected from differences in membrane synthesis. X-ray diffraction measurements show that a GO-only membrane has a high level of order with a calculated interlayer spacing of 8.38 Å, which increases slightly to 8.52 Å when wetted with pure water. In comparison, chitosan-GO composite membranes result in broad diffraction peaks and the loss of apparent ordered interlayer spacing upon wetting. X-ray photoelectron spectroscopy results confirmed the presence of aluminum in GO membrane samples, as well as a decrease in amine and an increase in protonated amine and amide contributions in the nitrogen N1s spectrum when GO was added to chitosan. This increase in both amide and protonated amine species suggests oxidation and protonation of the amine groups on the chitosan polymer backbone through interactions with the oxygen-based functional groups on the surface of GO. Membranes were tested for pure water flux and rejection with model water contaminant methylene blue; performance results will be discussed for methylene blue concentrations of 1 â?? 100 mg/L.