Trace metal uptake by biogenic manganese oxides with structural variability: Connecting lessons from coal mine drainage remediation and the laboratory

Manganese (Mn) oxide formation in modern day environmental systems is likely often mediated via microbial Mn(II) oxidation. Such biogenic Mn oxides are typically poorly crystalline Mn(III/IV) (hydr)oxide phyllomanganate minerals with large sorption capacities for trace metals (e.g., Ni, Zn, Co), often serving as the dominant control on trace metal concentration and speciation in environmental and metal-polluted sites. Such processes are particularly important in coal mine drainage (CMD) sites throughout the Northeastern United States, in which Mn remediation poses an ongoing challenge. Passive Mn removal beds rely on microbial Mn oxidizers to form phyllomanganates that in turn sequester other trace metal contaminants. This talk examines one CMD remediation system with a unique opportunity for study: the Eastern side contains hexagonally-symmetric phyllomanganates while the Western side contains triclinic-type birnessites and higher aqueous Mn(II) concentrations. Aqueous Ni, Zn, and Co concentrations are positively correlated with aqueous Mn(II), with solid-associated Ni, Zn, and Co concentrations consistently higher in the Western side (with triclinic birnessite) than in the Eastern side (with hexagonal birnessite).

To further investigate the role of phyllomanganate structures on trace metal sequestration, Ni uptake by mycogenic phyllomanganates (formed by the fungus Stagonospora sp. SRC1lsM3a) under two timing-of-addition scenarios was investigated: Ni sorption to pre-existing Mn oxides and Ni coprecipitation with Mn oxides. Mycogenic coprecipitation of Mn with Ni results in the formation of a hexagonally symmetric, potentially vacancy-rich birnessite, while the Ni-free phyllomanganates have more disordered, pseudo-orthogonal structures that become hexagonally-symmetric upon reaction with Ni. Overall macroscopic Ni uptake from solution is unaffected by aging, while incorporation into the phyllomanganate sheet increases over time in both Ni addition scenarios. We suspect that biomass-bound Ni serves as a source for progressive Ni incorporation into mycogenic birnessites. These results highlight the importance of aqueous geochemistry in determining phyllomanganate structures and trace metal adsorption.