Layered Mn oxides such as triclinic and hexagonal birnessites are often found as fine-grained aggregates and coatings on soils. Triclinic Na-birnessite has octahedral sheets containing Mn3+ and Mn4+, with hydrated interlayer Na+ (Post et al. 2003). The octahedral sheets of hexagonal birnessite contain only Mn4+ and vacancies, with H+ or other cations in the interlayer above vacancies (Lanson et al. 2000). These structural discrepancies result in varied total uptake and rates of metal sorption. Triclinic birnessite converts to hexagonal birnessite in low pH solutions, and hexagonal birnessite is often called “acid birnessite”. On the other hand, experiments in which fungi and bacteria precipitate birnessite at circumneutral pH have generated only the hexagonal variety. These experiments often use growth media with buffers such as HEPES. The buffers’ effect on Mn oxide precipitation has not yet been evaluated. To test the buffers’ influence on the Mn oxide generated, synthetic triclinic birnessite was reacted with two common biological buffers alongside a buffer-free control experiment. Specifically, triclinic birnessite reacted with a 20 mM pH 7.00 HEPES solution and a 20 mM pH 7.00 MES solution. For the control, triclinic birnessite reacted with an unbuffered pH 7.00 solution. All reactions ran for 14 days in the dark. Solutions were measured for pH changes, and birnessites were characterized with X-ray diffraction. The HEPES buffer induced a full transformation from triclinic to highly crystalline hexagonal birnessite, as the pH remained constant with a final pH of 6.98. The MES buffer induced a partial transformation from triclinic to poorly crystalline hexagonal birnessite, and pH increased from 7.00 to 7.14. The unbuffered pH 7 solution induced a partial transformation to poorly crystalline hexagonal birnessite, with a large pH increase from 7.00 to 8.03. We suggest that triclinic birnessite hydrolyzes H2O at pH 7, and interlayer Na+ exchanges with H+, transforming to hexagonal birnessite. In buffered solutions, triclinic birnessite uptakes H+ from the buffer’s weak acid, and the buffer further dissociates to adjust for pH changes, producing more H+, and perpetuating the change from triclinic to hexagonal birnessite. The rate and extent of transformation depend on the weak acid’s buffering capacity. Our work suggests that biomineralization experiments involving Mn oxide phases should be interpreted cautiously, especially when a buffer is implicated.

Transformation of triclinic to hexagonal birnessite by common biological buffers

249th National Meeting (2015)

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