This study introduces a new and previously unconsidered effect of soluble Mn(III)-pyrophosphate (Mn(III)-PP) on the formation of layered birnessite nanosheets. Recent studies revealed the abundance of soluble Mn(III) species as complexes with organics (e.g., siderophore) and inorganics (e.g., pyrophosphate). However, their impact on the formation of Mn(III/IV) oxide is elusive. Here, using photochemically-assisted fast abiotic oxidation of Mn<sup>2+</sup>(aq) in nitrate solution with varied pyrophosphate conditions (0.3 mM, 0.5  mM, 1 mM, and 2 mM PP), we studied changes in the stacking behaviors and structure of disordered birnessite nanosheets. With an increase in PP concentration, a higher concentration of soluble Mn(III)-PP formed from the oxidation of Mn<sup>2+</sup>(aq) to Mn(III). On the other hand, faster oxidation of Mn(III)-PP to Mn(IV), and consequent formation of disordered birnessite nanosheets, occurred under lower PP concentrations. Furthermore, the formation rate of Mn(III)-PP and disordered birnessite nanosheets affected the thickness of the nanosheets and the structure of the produced birnessite. The higher the PP concentration, the thicker the nanosheet layers. In 1 mM and 2 mM PP, orthogonal birnessite formed, which has up to 33 % Mn(III) in a layer and fewer vacant sites than hexagonal birnessite. In 0.3 mM and 0.5 mM PP, hexagonal birnessite formed. Hexagonal birnessite usually has more vacant sites than orthogonal birnessite and has more interlayer Mn(III). These findings can shed light on the unknown and crucial roles of soluble Mn(III) and PP in birnessite formation in environmental systems, and suggest how natural birnessite can occur with variable oxidation states.

How soluble Mn(III)-pyrophosphate affects the formation of layered birnessite nanosheets by photochemically-assisted Mn oxidation?

253rd National Meeting (2017)

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