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