Sorption and abiotic transformation of monensin by iron and manganese oxides

Monensin, an ionophore antibiotic, is commonly administered as a feed additive to cattle and poultry. A large percentage of the administered dose is excreted in animal waste, which is often applied to agricultural fields as fertilizer. The objective of this work is to gain insight into the fate of monensin in soil by investigating the interactions between monensin and common soil minerals, including sorption and transformation to unmonitored partial oxidation products. Batch sorption experiments across varying conditions (i.e., pH, ionic strength) and desorption experiments (i.e., methanol, PO₄^3-, methyl tert-butyl ether) were used to determine the extent to which a selection of common redox-active soil minerals [birnessite (δ-MnO₂), goethite (α-FeOOH), hematite (α-Fe₂O₃)] can bind and transform monensin. Monensin was bound by hematite (pH < 7.5, up to 7.5 mmol kg^-1), goethite (pH < 7.5, up to 3.4 mmol kg^-1), and birnessite (pH < 7, up to 0.1 mmol kg^-1). Combined sorption and transformation were the greatest for hematite and the lowest for birnessite. Sorption to hematite was more reversible than to goethite. Each desorption from goethite recovered <10% of sorbed monensin, whereas desorption from hematite recovered up to 69% of sorbed monensin, dependent on the solution. The potential for iron and manganese (hydr)oxides to abiotically transform monensin through reductive dissolution to partial oxidation products was evaluated by mass spectral analysis following sorption experiments. Additionally, the dominant sorption mechanism was inferred through ATR-FTIR spectroscopy, via examination of the carboxylate peak separation differences, on goethite and hematite to be bridging bidentate.