Phyllomanganates from the birnessite mineral family are ubiquitous in natural aqueous geochemical systems, where they are important regulators of the mobility and speciation of trace metals and metalloids. Biogeochemical manganese redox cycling in suboxic and anoxic environments places these Mn(III/IV)-oxide minerals in contact with aqueous Mn(II). Interactions between dissolved Mn(II) and birnessite may affect the mineral structure in various ways, ranging from subtle modifications of the layer and interlayer composition to full reductive transformation of birnessite into secondary Mn(II/III)-phases. Our recent application of Mn-54 radiotracers has revealed that interaction with dissolved Mn(II) additionally activates dynamic recrystallization of the birnessite sorbent, whereby the mineral undergoes continuous regeneration through dissolution-reprecipitation processes that lead to the exchange of Mn atoms between the aqueous and solid phase. We propose that the main mechanism responsible for these transformation and recrystallization processes is interfacial Mn(II)-Mn(IV) comproportionation, which involves electron transfer from adsorbed Mn(II) to lattice Mn(IV) producing Mn(III). The transient nature of the Mn(III) intermediates enables tracking of these processes with Mn-54 tracers. This talk will demonstrate application of these radiotracers to monitor Mn(II)-driven recrystallization and reductive structural transformation of vernadite in anoxic systems, complemented with structural characterization using XRD, XAS, and IR analyses.

Tracking the recrystallization and transformation of birnessite in reducing environments with <sup>54</sup>Mn radiotracers

255th National Meeting (2018)

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Tracking the recrystallization and transformation of birnessite in reducing environments with 54Mn radiotracers

Tracking the recrystallization and transformation of birnessite in reducing environments with ⁵⁴Mn radiotracers