Reversible transformations of sulfamethoxazole and its submoieties by manganese-oxidizing bacteria and biogenic manganese oxides in the presence of humic substances

Manganese-oxidizing bacteria (MnOB) and biogenic manganese oxides (BioMnOx) play key roles in the breakdown of organic matter (including pollutants) in water and soil environments. The degradation of some organic compounds (such as sulfonamides selected in this study) by BioMnOx in the presence of active MnOB is poorly understood. Thus far, it has been shown that the transformation of sulfonamides by either BioMnOx or MnOB (but thus far not studied in a binary system) can be modulated using naturally occurring redox mediators, such as humic substances, leading to the formation of coupling products of unknown stability. The co-occurrence of sulfonamides, MnOB, BioMnOx, and humic constituents is pertinent to many natural and engineered settings. This study used syringaldehyde, which is a model humic constituent, to investigate the nature of modulation in a binary system of BioMnOx and MnOB for the first time. The MnOB strain <i>Pseudomonas putida</i> MnB6 was cultivated and used in batch degradation tests. Initial tests with eight sulfonamides showed comparably poor degradation. In the next step of this study, sulfamethoxazole (SMX) and two SMX submoieties (sulfanilamide (SNM) and 3-amino-5-methylisoxazole (ISX)) were examined. After 48-60 hours in active cultures with BioMnOx, the degradation of all the three substances was negligible. However, syringaldehyde increased the degradation efficiency by 26% for SMX, 58% for SNM, and 27% for ISX. The active culture and BioMnOx synergistically improved degradation, highlighting the importance of BioMnOx regeneration. Removal was partially reversible (10-30%) owing to the retransformation of the reaction products into parent compounds, which was induced by syringaldehyde depletion. Unstable reaction products were conjugates of SMX, SNM, and ISX with syringaldehyde or its oxidation product DMBQ (2,6-dimethoxy-1,4-benzoquinone). This deconjugation likely contributes to process reversibility, potentially negatively impacting the environment and the safety of water and wastewater treatment systems.