Catalytic Ozonation Using MnO2-Enabled Membranes: Toward Direct Delivery of Hydroxyl Radicals

This study demonstrates the use of a MnO2-coated gas-permeable membrane for efficient radical delivery to water. MnO2 of various morphologies─including nanowires (NW), nanosheets (NS) and nanoflowers (NF)─were synthesized, characterized, and evaluated according to their catalytic ozonation. In the presence of dissolved ozone, all forms of suspended MnO2 resulted in elevated hydroxyl radical exposure but still differed across morphologies. MnO2 NS resulted in a more efficient catalytic ozonation per mass and was thus synthesized on gas-permeable membrane tubes as a proof-of-concept. Polydimethylsiloxane (PDMS) membrane tubing was used as a platform, as it has been shown to enable effective passive diffusion of ozone driven by concentration gradients. The coated membrane allowed direct hydroxyl radical generation in a two-step process. First, the gaseous ozone passes through the inner side of the membrane and is delivered as dissolved ozone at the outer layer (i.e., to the solution). Second, once the dissolved ozone comes into contact with the deposited MnO2 NS layer, it immediately converts to radicals, allowing for an increase of up to 43% removal of ozone-resistant compounds without additional chemicals. Overall, direct hydroxyl radical delivery using MnO2-enabled membranes may offer a new opportunity for effective catalytic ozonation water treatment applications.