(239b) Polymeric Nano-Metal Composite Membranes for Water Remediation

Responsive polymers react to external stimuli like temperature, pH, radiation, ionic strength or chemical composition. These polymers in combination with nanomaterials provide novel hybrid properties such as bactericidal, electrical, hydrophilic, catalytic, selectivity, etc., and they can be self-supported or in a convenient matrix such as a membrane.

These membranes, functionalized with polymers having pH responsiveness (hydrogels), is a promising research field for numerous applications. The functional groups of the hydrogels gain or lose protons based on the pH, collapsing or extending their network, making them swell or un-swell solutions. These nanostructured materials can be exploited to synthesize metal nanoparticles and as ion exchangers. They also prevent the loss of nanomaterials to the environment and improve reactivity due to larger surface areas, expanding its range of applications to selective catalysis, remediation and mining.

The present work describes â??greenâ? techniques used to create nanocomposites based on polyvinilidene fluoride (PVDF) hollow fiber (HF) and flat sheet (FS) membranes, both sponge-like and thin. Hydrophobic HF membranes were synthesized with 57.2% porosity and 0.72 µm mean pore size. These membranes were then hydrophilized by a water-based process of cross-linking polyvinylpyrrolidone (PVP) by persulfate (PS). Though, hydrophilization of PVDF membranes using PVP is known in industry, as a post-process has only been implemented at lab-scale.

The PVP cross-linking is confirmed but also a ring-opening is present due to etherification. Hydrophilization was measured by contact angle changes (CA) (76.3° to 60.3°) and surface free energy calculations (40.3 to 49.8 mJ·m^-2). HF and FS were subsequently functionalized with a poly(acrylic acid) (PAA) hydrogel through free radical polymerizations (thermal or redox) using PS as initiator. This work advanced FS membrane functionalization from lab-scale to full-scale by modifications of the polymerization procedures.

Swelling tests were performed to hydrogels/functionalized membranes finding responsiveness up to 1676% swelling at pH 9.0. Also, mesh size values (ξ) were calculated showing an inverse relationship with membrane permeability, tuning the flux through the membrane. An increase of ξ from 5.89 to 8.33 nm (pH 3.0 to 9.0) represented a 93% permeability decrease (53.9 to 3.4 L·m^-2·h^-1) at pressures between 1.0 and 4.0 bar.

Throughout a double ion exchange of sodium/iron and a subsequent reduction, bimetallic Fe/Pd nanoparticles (60 to 300 nm) were synthesized. The Fe exchanged was almost 500 mg Fe/g PAA. Similarly, it was possible to use the exhausted accelerants of Fe ions of the redox polymerization to synthesize nanoparticles, but having only 9.2 mg Fe/g PAA of loading.

Finally, depending on the Pd concentration in the membrane, the catalytic activity in the reduction of trichloroethylene (TCE) was studied, having enhanced reactivities (up to ten times) in sponge-like membranes compared to previous studies. Using hydrogels alone, similar results to nanoparticles in solution phase were found without nanoparticle loss. 80% of reduction was achieved within 2 hours with chloride production near to stoichiometric values (3/1), demonstrating absence of intermediates.

This research is supported by the National Science Foundation EPSCoR, and by the National Institute of Environmental Health Sciences (P42ES007380). HF membranes were synthesized at the Singapore Membrane Technology Center, NTU (Singapore). Nanostone Water (Oceanside, CA) provided support and collaboration in the development of full-scale flat sheet membranes.