title: Redox Reactions between Mn(II) and Hexagonal Birnessite Change Its Layer Symmetry.
abstract: Birnessite, a phyllomanganate and the most common type of Mn oxide, affects the fate and transport of numerous contaminants and nutrients in nature. Birnessite exhibits hexagonal (HexLayBir) or orthogonal (OrthLayBir) layer symmetry. The two types of birnessite contain contrasting content of layer vacancies and Mn(III), and accordingly have different sorption and oxidation abilities. OrthLayBir can transform to HexLayBir, but it is still vaguely understood if and how the reverse transformation occurs. Here, we show that HexLayBir (e.g., δ-MnO2 and acid birnessite) transforms to OrthLayBir after reaction with aqueous Mn(II) at low Mn(II)/Mn (in HexLayBir) molar ratios (5-24%) and pH ≥ 8. The transformation is promoted by higher pH values, as well as smaller particle size, and/or greater stacking disorder of HexLayBir. The transformation is ascribed to Mn(III) formation via the comproportionation reaction between Mn(II) adsorbed on vacant sites and the surrounding layer Mn(IV), and the subsequent migration of the Mn(III) into the vacancies with an ordered distribution in the birnessite layers. This study indicates that aqueous Mn(II) and pH are critical environmental factors controlling birnessite layer structure and reactivity in the environment.
authors.affiliation: Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtse River), Ministry of Agriculture, College of Resources and Environment, Huazhong Agricultural University , Wuhan 430070, China.
authors.forename: Huaiyan
authors.initials: H
authors.lastname: Zhao
authors.affiliation: Department of Ecosystem Science and Management, University of Wyoming , Laramie, Wyoming 82071, United States.
authors.forename: Mengqiang
authors.initials: M
authors.lastname: Zhu
authors.affiliation: Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth Sciences and Engineering, Nanjing University , Nanjing 210093, China.|Environmental Soil Chemistry Group, Delaware Environmental Institute and Department of Plant and Soil Sciences, University of Delaware , Newark, Delaware 19716, United States.
authors.forename: Wei
authors.initials: W
authors.lastname: Li
authors.affiliation: Department of Earth & Environmental Sciences, Rutgers University , Newark, New Jersey 07102, United States.
authors.forename: Evert J
authors.initials: EJ
authors.lastname: Elzinga
authors.affiliation: Geochemistry Department, Geology Institute, Universidad Nacional Autónoma de México (UNAM) , Mexico, D.F. 04510, Mexico.
authors.forename: Mario
authors.initials: M
authors.lastname: Villalobos
authors.affiliation: Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtse River), Ministry of Agriculture, College of Resources and Environment, Huazhong Agricultural University , Wuhan 430070, China.
authors.forename: Fan
authors.initials: F
authors.lastname: Liu
authors.affiliation: Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences , Beijing 100039, China.
authors.forename: Jing
authors.initials: J
authors.lastname: Zhang
authors.affiliation: Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtse River), Ministry of Agriculture, College of Resources and Environment, Huazhong Agricultural University , Wuhan 430070, China.|Environmental Soil Chemistry Group, Delaware Environmental Institute and Department of Plant and Soil Sciences, University of Delaware , Newark, Delaware 19716, United States.
authors.forename: Xionghan
authors.initials: X
authors.lastname: Feng
authors.affiliation: Environmental Soil Chemistry Group, Delaware Environmental Institute and Department of Plant and Soil Sciences, University of Delaware , Newark, Delaware 19716, United States.
authors.forename: Donald L
authors.initials: DL
authors.lastname: Sparks
mesh_terms: D000327:Adsorption; D008345:Manganese / Q000737:chemistry*; D010084:Oxidation-Reduction; D010087:Oxides / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D010087:Oxides; C505018:birnessite; D014867:Water; D008345:Manganese