Keywords: carbon nanotubes, conductive membranes, hollow fibers, crossflow, microfiltration
Summary:Carbon nanostructures have gained significant interest in membrane science as an additive for conferring conductive properties to nonconductive membrane surfaces. Membrane separation is ubiquitous in municipal and industrial wastewater treatment; however, the polymeric membranes used in these separations are susceptible to fouling, resulting in poor throughput and incurring large operating costs for backwashing and chemical cleaning. Electrically conductive membranes (ECMs) are of interest for their ability to mitigate fouling and prevent biofilm development. ECM research has focused predominantly on flat sheet membranes suitable for bench-scale laboratory studies. For industrial applications, hollow fiber (HF) membrane geometries are preferred due to their high packing density. A simple pressure or vacuum dead-end filtration approach is commonly used in current research to deposit conductive material onto the surface of flat sheet membranes. This technique is often directly applied in emerging conductive HF research; however, we have observed issues with maintaining a uniformly conductive surface via dead-end deposition of carbon nanotubes (CNTs) onto polyether sulfone (PES) support HF membranes. Despite high measured surface conductivity, we observed significant variation in conductivity along the length of the fiber. This nonuniformity may cause inconsistent antifouling performance under an applied potential. We aim to optimize this technique to enable uniform HF ECM fabrication by studying crossflow induced shear forces during deposition. We have developed a crossflow pressure deposition approach for adhering a thin film of CNTs onto the active surface of HF membranes. Carboxyl functionalized single wall CNTs were purchased and suspended with sodium dodecyl sulfate, polyvinyl alcohol (PVA) and other chemical cross-linkers. The final suspension was filtered continuously in crossflow along the active surface of commercial PES HF membranes (microfiltration, 0.2 μm pore size). The coated membranes were then cured at 100°C to adhere the PVA to the membrane surface. A wide range of design parameters affect shear forces (feed pressure, crossflow velocity) and CNT chemistry (CNT concentration, cross-linker length, chemistry and density). We quantified the impact of these parameters on composite membrane permeance and conductivity. Scanning Electron Microscopy and surface conductivity measured via four-point probe were used to assess coating thickness and conductivity along the fiber as a uniformity metric. We have found that relatively high feed pressures, low crossflow velocities and low CNT concentrations tend to produce more uniform, conductive coatings with measurable conductivities as high as 2500 S/m. This level of conductivity is sufficient to induce anti-fouling effects observed in literature. There is however a trade-off in membrane performance, with pure water membrane permeance decreasing by 70-80% of its initial value. We further explored fouling mitigation potential by applying direct current across a custom-built crossflow module fitted with electrodes. ECMs have the potential for significant operating and maintenance cost savings for water purification due to reduced fouling potential. Successful fabrication of HF ECMs will further the range of applications for ECMs into full-scale municipal and industrial wastewater treatment. Our crossflow deposition technique addresses fabrication challenges with uniformity observed in dead-end deposition. Consistent antifouling performance is essential for ECM use in large-scale treatment applications.