(478b) Functionalized Carbon Nanotube Nanocomposite Membranes for Water Desalination

Cost-effective purification and desalination of water is a global challenge. Reverse osmosis (RO), the current method of choice, is a membrane-based semipermeable process that separates salts from water, and it is limited by the intrinsic properties of the membranes used in the process. Biofilm fouling on the surface of the membranes is another critical problem in RO applications. In this study, we utilized zwitterion functionalized carbon nanotubes (CNTs) and incorporated them in polyamide thin film composite membranes to improve membrane permselectivity and surface biofouling resistance for water desalination. The CNTs within the polyamide layer were partially aligned via a vacuum filtration step during membrane synthesis, and the fabrication of the zwitterionic CNT/polyamide nanocomposite membranes was optimized by varying the loadings of CNTs and the crosslink density of the polyamide. The permeation flux was found to increase by approximately three-fold as the fraction of CNTs was increased from 0 to 20 wt% for all the salt solutions tested in our study. The steric hindrance to transport due to the zwitterionic functional groups prevents the permeation of the majority of ions through the CNT pores resulting in high salt rejection. The charges of the zwitterionic groups do not appear to produce a significant Donnan potential to reject ions, and thus surface charge had no effect on membrane selectivity. The semi-aligned zwitterionic CNTs appeared to have their tips exposed at the membrane surface and interacted with the feed water to form a strong hydration layer, resulting in improved surface biofouling resistance. The adsorption rate of protein foulants on the nanocomposite membrane surface was significantly reduced compared to the control membrane without CNTs, and the adsorbed fouling layer could be easily removed by flushing with water. After washing, the nanocomposite membrane recovered 100% of the decreased water flux whereas the control membrane only recovered 10% of the decreased flux resulting in a permanent loss of 30% in water permeation.