Advanced Oxidation Processes with Carbon Nanotubes: Surface-Promoted Formation of Hydroxyl Radical During Ozonation

Advanced Oxidation Processes (AOPs), which utilize hydroxyl radical (^•OH) to chemically degrade contaminants, are becoming increasingly utilized in water and wastewater treatment for disinfection and removal of organic micropollutants via chemical oxidation reactions. Catalytic ozonation represents an alternative AOP in which a solid substrate such as activated carbon or common metal oxides are used to promote ^•OH formation from ozone. This project explores the efficacy of using multi-walled carbon nanotubes (MWCNTs) as new materials for promoting ^•OH production during ozonation. Using para-chlorobenzoic acid (p-CBA) as a hydroxyl radical probe, aqueous suspensions of functionalized (i.e., surface oxidized) and nonfunctionalized MWCNTs increased ^•OH concentrations in bulk solution by an order of magnitude or more over levels observed with ozone alone and resulted in ^•OH levels comparable to more traditional ozone-based AOPs (e.g., hydrogen peroxide with ozone). Production of ^•OH was greatest on oxidized MWCNTs, and evidence suggests this enhanced ^•OH production cannot be explained solely by the increased dispersivity of oxidized MWCNTs in aqueous suspensions. Chemical routes for MWCNT functionalization were used to systematically vary the surface oxygen content of MWCNTs, and the ability of these functionalized materials to generate •OH was subsequently determined in batch ozonation experiments, again using p-CBA as a radical probe.  Complementary surface analysis with X-ray photoelectron spectroscopy (XPS) combined with chemical derivatization was used to correlate ^•OH production not only to total MWCNT surface oxygen content, but also to the concentration of specific functional groups (hydroxyl, carbonyl and carboxylic acid groups) resulting from oxidation. Results to date show that ^•OH production increases with MWCNT surface oxidation, and that carboxyl surface groups appear largely responsible for this increase in •OH production.