Water Flow and Dissolved Mn Alter Transformation of Pipemidic Acid by Manganese Oxide.

Manganese oxides have been proposed as promising geomedia to remove trace organic contaminants in both natural soils and artificial infiltration systems. Although MnO-based redox processes have been largely investigated, little is known on the effects of water flow and dissolved Mn on manganese-mediated redox reactions in saturated porous media. Here, we have demonstrated that the reactive transport of a widely used quinolone antibiotic, pipemidic acid (PIP), in MnO-coated sand (MCS) columns is altered by the presence of dissolved Mn, generated as reduced ions or present in inflow solution. Decreasing the flow rate or flow interruption facilitated oxidation reactions and generated redox byproducts (Mn and PIP). However, preloading of MCS columns with dissolved Mn led to suppressed reactivity with PIP. When PIP and Mn are simultaneously injected, competition between PIP and Mn for binding at the edge sites takes place during the initial kinetic phase of reaction, while at a later breakthrough time Mn will occupy both edge and vacancy sites due to the continuous supply of Mn. We also developed a reactive transport model that accounts for adsorption kinetics to predict changes in transport behavior of antibiotics in the presence of different doses of dissolved Mn. This work has strong implications for an accurate assessment of the reactivity of manganese oxides used as engineered geomedia for quinolone remediation and in developing transport models of antibiotics in natural systems.