The adsorption and oxidation properties of birnessite minerals are largely dictated by structural Mn(III) and vacancy concentrations. Previous studies propose that vacancy–adsorbed Mn(II) is oxidized by structural Mn(IV) to form Mn(III) that enters the vacancy, increasing Mn(III) and decreasing vacancy concentration. However, the mechanisms are doubted because the previous experiments were conducted in air where atmospheric oxygen can also oxidize Mn(II) to Mn(III). In addition, pH ≥ 8 conditions were used and whether the reactions occur at more environmentally–relevant pH (pH &lt; 8) remains unknown. Here, we incubated Mn(II)–preloaded birnessite (4 g/L δ-MnO<sub>2</sub>, Mn(II)/solid Mn ratio r = 0, 0.081 or 0.137) in 100 mM NaCl solution at pH 6, 7 or 8 in an anaerobic chamber for 25 days. The solids sampled at various time intervals were characterized for structural and compositional changes using synchrotron X-ray techniques. Results show that at r = 0, the vacancy–adsorbed Mn(III) in δ-MnO<sub>2</sub> enters the vacancies at all pHs, which is more favored at higher pH. At r = 0.081 and pH 8, additional Mn(III) is formed by the Mn(II)–Mn(IV) redox reaction, which enters vacancies faster than those at r = 0 and pH 8. Triclinic birnessite (TricBir, containing 1/3 Mn as Mn(III)) starts to form at ~15 days and continues to grow within the studied time period. At r = 0.137 and pH 8, more TricBir forms with a faster rate than at r = 0.081 and pH 8. Surprisingly, at this ratio, TricBir even forms at pH 7. But at both pH 7 and 8, the TricBir transforms into an unknown new phase after ~15 days. This work indicates that Mn(II)–birnessite reactions affect structural Mn(III) and vacancy concentrations at pH ≤ 8 under anoxic conditions, improving the current understanding of the interplay between birnessite and Mn(II) in redox fluctuated environment.

Reactions of birnessite with Mn(II) under anoxic conditions

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.