GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 57-8
Presentation Time: 3:50 PM


YANG, Peng1, POST, Jeffrey E.2, WANG, Qian1, XU, Wenqian3, GEISS, Roy4, MCCURDY, Patrick4 and ZHU, Mengqiang1, (1)Department of Ecosystem Science and Management, University of Wyoming, Laramie, WY 82071, (2)Department of Mineral Sciences, Smithsonian Institution, Washington, DC 20013, (3)X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, IL 60439, (4)Department of Chemistry, Colorado State University, Fort Collins, CO 80523

Hexagonal birnessite, a typical layered Mn oxide (LMO), can adsorb and oxidize Mn(II) and thereby transform to Mn(III)-rich hexagonal birnessite, triclinic birnessite, or tunneled Mn oxides (TMOs), remarkably changing the environmental behavior of Mn oxides. We have determined the effects of coexisting metal cations and oxyanions on the transformation by incubating Mn(II)-bearing δ-MnO2 at pH 8 under anoxic conditions for 25 d. In the Li+, Na+, and K+ chloride solutions, the Mn(II)-bearing δ-MnO2 first transforms to Mn(III)-rich δ-MnO2 or triclinic birnessite (T-bir) due to the Mn(II)-Mn(IV) comproportionation, most of which eventually transform to a 4 × 4 TMO. In contrast, Mn(III)-rich δ-MnO2 and T-bir form and persist in the Mg2+ and Ca2+ chloride solutions. However, the presence of surface-adsorbed Cu(II) and phosphate strongly inhibit or slow down the transformation of Mn(II)-bearing δ-MnO2 to T-bir and TMOs. The stabilizing power of cations and oxyanions on the δ-MnO2 structure positively correlates with their binding strength to δ-MnO2 (Li+, Na+, and K+ < Mg2+ and Ca2+ < Cu(II) and phosphate). Since the adsorption of metals and oxyanions decrease the surface energy of minerals, our finding suggests that the surface energy largely controls the thermodynamic stability of LMOs. Our study indicates that the adsorption of divalent metal cations, particularly transition metals, and strongly adsorptive oxyanions can be an important cause of the high abundance of LMOs, rather than the more stable TMO phases, in the environment.