(521a) Functionalized Membranes for Iron Capture and Subsequent Free Radical Reactions

The functionalization of membranes has led to the use of membrane-based processes in areas with which they are not traditionally associated. Reactive membranes synthesized through such methods offer enhanced reactivity due to increased surface area and low diffusion limitations. Our group has utilized these properties to create a reactive membrane capable of generating free radicals for the oxidation of unwanted organic compounds. Oxidative techniques utilizing free radical reactions have proven effective for the destruction of toxic organics as well as for various other forms of water treatment. Many free radical generating processes rely on the reaction of Fenton-like processes using ferrous iron and hydrogen peroxide to form hydroxyl radicals.

A functionalized membrane consisting of a support membrane coated with a polyelectrolyte offers an excellent platform for iron capture by ion exchange and subsequent oxidation (such as with hydrogen peroxide). Our group has performed in-situ polymerization of poly(acrylic acid) (PAA) inside the pores of a polyvinylidene fluoride (PVDF) membrane for Fe(II) capture. By convectively passing hydrogen peroxide through the membrane, we are able to generate hydroxyl radicals which can be used for contaminant degradation. As Fe(II) is converted to Fe(III), it is recaptured in the PAA matrix, where it continues to react with hydrogen peroxide. Recent studies have shown that zero-valent iron nanoparticles (nZVI), traditionally used in processes for reduction of contaminants, are capable of producing significant quantities of oxidants in the presence of oxygen and a chelate. These particles can be synthesized simply via the reduction of the iron ions captured within the PAA matrix. Particle formation within the membrane domain makes this technology applicable to flow-through processes which would be difficult to implement without particle immobilization.

Steady-state hydrogen peroxide degradation in a PAA/PVDF membrane containing Fe(III) showed first order reaction kinetics (modeled as a continuous stirred-tank reactor). This rate is approximately one order of magnitude greater than what would be expected from the literature-reported Fe(III) and hydrogen peroxide reaction kinetics, indicating an increased rate of hydrogen peroxide degradation for Fe(III) in its carboxylate complex form. In order to better understand the mechanism by which hydroxyl radicals are generated by the reaction of the ion-exchanged iron and hydrogen peroxide, it is important to quantify their production. This was accomplished through the use of a radical probe, such as benzoic acid. By varying hydrogen peroxide concentration and residence time, the rate of hydroxyl radical production can be altered, therefore determining both the extent and rate of benzoic acid oxidation. In order to show the applicability of these systems for the treatment of toxic organics, we have demonstrated successful degradation of pentachlorophenol (PCP). To our knowledge, this is the first study demonstrating controlled free radical generation within a PAA/PVDF membrane. Support of this research has been provided by NSF-IGERT and NIEHS-SBRP.