Advances in X-ray detector technology have combined with developments in environmental cells to open a new frontier in dynamical crystallography. The high sensitivity of image plates and CCD cameras allow for the collection of full-circle diffraction patterns from samples in seconds, particularly when used in concert with synchrotron radiation. Flow-through environmental cells enable time-resolved X-ray diffraction (TR-XRD) studies of mineral-fluid reactions, and when the cells are coupled with automated fraction collectors it is possible to correlate changes in crystal structure and the evolution of fluid chemistry with unparalleled precision. Here we report results from our long-term study of the interaction of dissolved metal cations with triclinic Na-birnessite [Na0.58 (Mn4+1.42 Mn3+0.58) O4 . 1.5 H2O], whose structure consists of negatively charged sheets of MnO6 octahedra balanced by interlayer cations and water molecules. Birnessite occurs in a variety of soils at minor concentrations, but it can control the cycling of metals between solid and fluid phases because of its high redox and cation exchange capacities. Using TR-XRD, we have explored the behavior of synthetic Na-birnessite in the presence of acids, dissolved alkali and alkaline earth cations (K+, Cs+, Ca2+, Ba2+) and heavy metals (Cr3+, Pb2+). Acids – even when part of a Goods buffering system – induced a transition from triclinic to hexagonal symmetry. Group I and II cations are exchanged into the interlayer according to a two-step kinetic law that is mechanistically tied to a delamination-reassembly pathway. Rate laws that we have calculated for these exchanges suggest that the reaction orders for exchange are driven by hydration energies of the dissolved cations. In contrast, Pb2+ apparently is sterically hindered from exchanging into the interlayer and preferentially sorbs to the birnessite surface. As revealed by simultaneous TR-XRD and TR-XANES, Cr3+ similarly exhibited little evidence for cation exchange. Instead, Cr3+ oxidized to Cr6+ through a concomitant reduction of Mn3+ to Mn2+, thereby inducing a phase transition in birnessite from triclinic to hexagonal symmetry. At pH 4 and 5, oxidation was markedly less effective for hexagonal than triclinic birnessite.

Cation exchange and redox reactions in birnessite as revealed by time-resolved X-ray diffraction

255th National Meeting (2018)

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