Comparing Cr(III) oxidation intermediated by different manganese oxidizing bacteria: Efficiencies, kinetic and key structure characteristics of biogenic manganese oxides.

The oxidation of Cr(III) to Cr(VI) plays a critical role in the biogeochemical cycling of chromium, particularly in soils. This study investigated Cr(III) oxidation mediated by three strains of manganese-oxidizing bacteria: Pseudomonas putida MnB1, Brevundimonas diminuta MnO-2, and Pannonibacter phragmitetus MnO-3 through generating biogenic manganese oxides (BioMnOx). The MnB1-generated BioMnOx exhibited superior Cr(III) oxidation efficiency (82.00 %) compared with the BioMnOx generated by MnO-2 (2.50 %) and MnO-3 (3.26 %). Structural characterization (XRD, SEM, TEM, XPS) demonstrated that MnB1-derived BioMnOx possessed three distinctive structural features: a disordered layered structure, abundant oxygen vacancies, and elevated Mn(III/IV) ratios, which synergistically enhanced active site availability for Cr(III) oxidation. Notably, only the MnB1 system effectively oxidized slightly soluble Cr(III) forms (e.g., Cr(OH)3, CrxFe1-x(OH)3), with these structural advantages directly accounting for its superior efficiency (82.00 %). Kinetic analysis showed that the Cr(III) oxidation followed a zero-order kinetics during the rapid oxidation phase and MnB1 achieved the highest oxidation rate with different Cr(III) forms. In the leaching experiment, both of the robust manganese oxidizing bacteria MnB1 and the indigenous microorganisms significantly enhanced Cr(III) oxidation in soil. These findings highlight the critical role of BioMnOx structure in driving Cr(III) oxidation and offer deeper insights into the biogeochemical cycling of chromium in the natural and contaminated environments.