Fate and Transport of Pharmaceuticals in Iron and Manganese Binary Oxide Coated Sand Columns

Predicting the fate and transport of pharmaceuticals in terrestrial environments requires knowledge of their interactions with complex mineral assemblages. To advance knowledge along this front, we examined the reactivity of pipemidic acid (PIP), a typical quinolone antibiotic, with quartz particles coated with a mixture of manganese oxide (MnO₂) and goethite (α-FeOOH) under static and dynamic flow conditions. Batch and dynamic column experiments showed that PIP binding to MnO₂ proceeded through a heterogeneous redox reaction, while binding to goethite was not redox-reactive. Mixed columns of aggregated goethite-manganese particles however enhanced redox reactivity because (i) goethite facilitated the transport of dissolved Mn(II) ion and increased the retention of PIP oxidation products, and (ii) MnO₂ was protected from passivation. This mobility behavior was predicted using transport models accounting for adsorption and transformation kinetics of PIP on both goethite and MnO₂. This work sheds new light on reactivity changes of mixtures of Fe and Mn oxides under flow-through conditions and will have important implications in predicting the fate and transport of redox-active organic compounds as well as development of new geomedia filters for environmental remediation.