Structural Transformation of Birnessite by Fulvic Acid under Anoxic Conditions

The structure and Mn(III) concentration of birnessite dictate its reactivity and can be changed by birnessite partial reduction, but effects of pH and reductant/birnessite ratios on the changes by reduction remain unclear. We found that the two factors strongly affect the structure of birnessite (δ-MnO₂) and its Mn(III) content during its reduction by fulvic acid (FA) at pH 4-8 and FA/solid mass ratios of 0.01-10 under anoxic conditions over 600 h. During the reduction, the structure of δ-MnO₂ is increasingly accumulated with both Mn(III) and Mn(II) but much more with Mn(III) at pH 8, whereas the accumulated Mn is mainly Mn(II) with little Mn(III) at pH 4 and 6. Mn(III) accumulation, either in layers or over vacancies, is stronger at higher FA/solid ratios. At FA/solid ratios ≥1 and pH 6 and 8, additional hausmannite and MnOOH phases form. The altered birnessite favorably adsorbs FA because of the structural accumulation of Mn(II, III). Like during microbially mediated oxidative precipitation of birnessite, the dynamic changes during its reduction are ascribed to the birnessite-Mn(II) redox reactions. Our work suggests low reactivity of birnessite coexisting with organic matter and severe decline of its reactivity by partial reduction in alkaline environment.