The relative contributions of Mn(III) and Mn(IV) in manganese dioxide polymorphs to bisphenol A degradation.

Polymorphs of MnO comprise Mn(III) and Mn(IV), which are both strong oxidants capable of BPA degradation, but their relative contributions are unclear. To advance process understanding, the reactivities of biogenic MnO prepared using Roseobacter sp. AzwK-3b and synthetic MnO (i.e., hexagonal and triclinic birnessite) toward BPA were compared. Both colloidal and particulate biogenic MnO, as well as triclinic birnessite, showed insignificant reactivity towards BPA, but degradation did occur when pyrophosphate (PP), a ligand for Mn(III), was present. Despite higher Mn(III) content of triclinic birnessite (38.6 %), only hexagonal birnessite with an Mn(III) content of 30.4 % degraded BPA without PP, and no rate increases were observed following the addition of PP. Similarly, colloidal MnO degraded BPA with nearly double the rate measured with particulate MnO (i.e., 1.24 ± 0.10 versus 0.73 ± 0.08 h), even though the Mn(III) contents were only 10 % different. The Mn(III) release rates from each MnO polymorph in the presence of PP correlated more strongly with the observed BPA degradation rates than with Mn(III) content, suggesting that both Mn(III) release rate and Mn(III) content govern MnO-mediated BPA degradation. In natural settings, Mn(III) generally occurs in complexed form suggesting that laboratory testing should include ligands to derive environmentally relevant information about MnO-mediated degradation of BPA and other compounds of concern.