Inhibition of Oxyanions on Redox-driven Transformation of Layered Manganese Oxides

• DOI: 10.1021/acs.est.0c06310
• Publication Date: 2021-02-19T10:55:52Z
• Authors: Peng Yang; Ke Wen; Kevin A. Beyer; Wenqian Xu; Qian Wang; Dong Ma; Juan Wu; Mengqiang Zhu
• Keywords: Layered Manganese Oxides Layered ma.; Manganese; Ecology; Chemical Sciences not elsewhere classified; Physiology; transformation; solution chemistry control; TMO; Oxides; Biochemistry; δ- MnO 2; 01 natural sciences; Mn oxides; oxyanion; Mn 3 O 4; Manganese Compounds; Adsorption; Molecular Biology; Oxidation-Reduction; Neuroscience; Biotechnology; 0105 earth and related environmental sciences

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 δ-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, δ-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 δ-MnOOH and Mn3O4 with minor poorly crystallized α- and γ-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 δ-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.