Abstract
Layered manganese (Mn) oxides, such as birnessite, can reductively transform into other phases and thereby affect the environmental behavior of Mn oxides. Solution chemistry strongly influences the transformation, but the effects of oxyanions remain unknown. We determined the products and rates of Mn(II)-driven reductive transformation of-MnO2, a nanoparticulate hexagonal birnessite, in the presence of phosphate or silicate at pH 6-8 and a wide range of Mn(II)/MnO2 molar ratios. Without the oxyanions,-MnO2 transforms into triclinic birnessite (T-bir) and 4 × 4 tunneled Mn oxide (TMO) at low Mn(II)/MnO2 ratios (0.09 and 0.13) and into-MnOOH and Mn3O4 with minor poorly crystallized α-and-MnOOH at high Mn(II)/MnO2 ratios (0.5 and 1). The presence of phosphate or silicate substantially decreases the rate and extent of the above transformation, probably due to adsorption of the oxyanions on layer edges or the formation of Mn(II,III)-oxyanion ternary complexes on vacancies of-MnO2, adversely interfering with electron transfer, Mn(III) distribution, and structural rearrangements. The oxyanions also reduce the crystallinity and particle sizes of the transformation products, ascribed to adsorption of the oxyanions on the products, preventing their further particle growth. This study enriches our understanding of the solution chemistry control on redox-driven transformation of Mn oxides.
| Original language | English |
|---|---|
| Pages (from-to) | 3419-3429 |
| Number of pages | 11 |
| Journal | Environmental Science and Technology |
| Volume | 55 |
| Issue number | 5 |
| DOIs | |
| State | Published - Mar 2 2021 |
| Externally published | Yes |
Funding
This work was supported by the U.S. Department of Energy Experimental Program to Stimulate Competitive Research Office for financial support (DOE-EPSCoR DE-SC0016272). We acknowledge beamline scientists Yongfeng Hu and Qunfeng Xiao at the Soft X-ray Microcharacterization Beamline at the Canadian Light Source for their technical assistance in data collection. This research utilized resources of the APS, a U.S. DOE Office of Science User Facility, operated for the DOE Office of Science by the Argonne National Laboratory under Contract No. DE-AC02-06CH11357. A portion of the study was performed at the Canadian Light Source, which is supported by the Natural Sciences and Engineering Research Council of Canada, the National Research Council Canada, the Canadian Institutes of Health Research, the Province of Saskatchewan, Western Economic Diversification Canada, and the University of Saskatchewan.