Transformation And Crystal Growth Mechanisms Of Hausmannite And Manganite

Manganese oxides act as important active components in soil minerals,and are widely distributed in soils,sediments,and marine nodules.Manganese oxides have the advantages of small particle size,large specific surface area and high reactivity,etc.,and thus can control the geochemical cycling of heavy metals and nutrients through adsorption,ion exchange,coprecipitation and redox reaction.Metal elements(such as Na,K,Mn and Fe)usually coexist with manganese oxides,and will incorporate into the structure or react with manganese oxides.Those interaction will result in the changes of mineral crystal structure,physicochemical properties,and reactivity,and affect the fates of related elements.The transformation of manganese oxides and their interaction with Mn(II)and Fe(II)gained much attention,however,most current reported works are focused on the high valent state manganese oxides like birnessite,pyrolusite and cryptomelane.The study of the transformation of low valence manganese oxides including hausmannite and manganite remains largely unclear.This work investigated the transformation of hausmannite and the interaction mechanisms of hausmannite and manganite with Mn(II)/Fe(II).The concentration of Mn(II)and Fe(II)was determined,and the crystal structure,micromorphology and components of intermediates were characterized.The effect of p H,cations and dissolved oxygen on the transformation and crystal growth process of minerals were also studied.The main results are as follows.1.Hausmannite was disproportionated to high valence manganese oxides?-Mn O₂ and was then transformed to tunnel-structured manganese oxides and p H and cations influenced the tunnel size of the final products.Hausmannite was transformed into manganite at p H5.0–9.0 under anoxic conditions.The dissolution of hausmannite was initiated and promoted by protons(?7.0),and the decrease of p H accelerated its conversion to Mn(IV)oxides.The tunnel-structured Mn(IV)oxide was generated via two steps during the dissolution process of hausmannite at p H?3.0.Hausmannite was disproportionated to?-Mn O₂ at first,which was then transformed to?-Mn O₂ in the presence of Na⁺and H⁺through the transfer of electrons from adsorbed Mn(II)to structural Mn(IV).The disproportionation of hausmannite to?-Mn O₂ was not affected by other cations,while the presence of K⁺promoted the further transformation of?-Mn O₂ to cryptomelane.The structural rearrangement process of?-Mn O₂was the rate-determining step for the formation of final products.2.Adsorbed Mn(II)on hausmannite surface was catalytically oxidized to manganite under oxic conditions,and p H and oxygen partial pressure influenced the formation rate of manganite.Single hausmannite were gradually oxidized to manganite at p H 7.0 in air atmosphere.After the addition of Mn(II)in the reaction system,the transformation rate of hausmannite to manganite decreased.With the increase of initial Mn(II)concentration,the transformation rate and the crystallinity of formed manganite both decreased.Mn(II)oxidation rate and the transformation rate of hausmannite to manganite both increased with increasing oxygen partial pressure.Although Mn(II)can be catalytically oxidized by hausmannite,its oxidation rate was very low.The formation rate of manganite in the reaction system of hausmannite and Mn(II)reached the lowest in neutral p H condition.In acidic p H condition,the disproportionation of hausmannite to manganite was accelerated.A higher p H(?9.0)also resulted in a higher oxidation rate of Mn(II)to manganite.3.The catalytic oxidation of Mn(II)by manganite led to its epitaxial growth,and the crystal growth rate was affected by p H value.Adsorbed Mn(II)was directly oxidized to manganite on the surface,and the oxidation rate increased with increasing p H and oxygen partial pressure.Electrons are likely transferred in bulk manganite during the oxidation process of Mn(II).Although manganite induced Mn(II)oxidation and the epitaxial growth of bulk particles,there was no obvious change in mineral phase.Dissolved oxygen accelerated the growth of manganite in Mn(II)solution.Compared with the absence of manganite,the presence of manganite inhibited the formation of hausmannite from Mn(II)oxidation by oxygen in air at higher p H(?9.0).4.Adsorbed Fe(II)was catalytically oxidized by hausmannite,and Fe(II)concentration,p H and dissolved oxygen influenced their reaction rate.The oxidation rate Fe(II)by air was very low and was accelerated in the presence of hausmannite at p H 5.0.Fe(II)was first oxidized to ferrihydrite,and was then progressively transformed to goethite and lepidocrocite in the reaction system of Fe(II)and hausmannite.Hausmannite was simultaneously reduced to Mn(II).Hausmannite acted as the major oxidant in the initial stage,and the coverage of formed iron oxides on hausmannite surface inhibited the further reaction of Fe(II)and hausmannite.In the later stage,Fe(II)can be catalytically oxidized by the newly formed iron oxides and thus increased its consumption,however,released Mn(II)in the reaction system had no obvious effect on Fe(II)oxidation.The oxidation rate of Fe(II)by different oxidants followed the order of hausmannite>(hausmannite+O₂)>O₂.That is,the presence of oxygen decreased the oxidation rate of Fe(II)by hausmannite.With the decrease of p H to 3.0,the reaction rate of Fe(II)and hausmannite decreased,and goethite was formed as the major product.Oxygen in air had no effect on the reaction rate of Fe(II)and hausmannite at lower pH.