Abstract
BackgroundIn soils and sediments, manganese oxides and oxygen usually participate in the oxidation of ferrous ions. There is limited information concerning the interaction process and mechanisms of ferrous ions and manganese oxides. The influence of air (oxygen) on reaction process and kinetics has been seldom studied. Because redox reactions usually occur in open systems, the participation of air needs to be further investigated.ResultsTo simulate this process, hexagonal birnessite was prepared and used to oxidize ferrous ions in anoxic and aerobic aqueous systems. The influence of pH, concentration, temperature, and presence of air (oxygen) on the redox rate was studied. The redox reaction of birnessite and ferrous ions was accompanied by the release of Mn2+ and K+ ions, a significant decrease in Fe2+ concentration, and the formation of mixed lepidocrocite and goethite during the initial stage. Lepidocrocite did not completely transform into goethite under anoxic condition with pH about 5.5 within 30 days. Fe2+ exhibited much higher catalytic activity than Mn2+ during the transformation from amorphous Fe(III)-hydroxide to lepidocrocite and goethite under anoxic conditions. The release rates of Mn2+ were compared to estimate the redox rates of birnessite and Fe2+ under different conditions.ConclusionsRedox rate was found to be controlled by chemical reaction, and increased with increasing Fe2+ concentration, pH, and temperature. The formation of ferric (hydr)oxides precipitate inhibited the further reduction of birnessite. The presence of air accelerated the oxidation of Fe2+ to ferric oxides and facilitated the chemical stability of birnessite, which was not completely reduced and dissolved after 18 days. As for the oxidation of aqueous ferrous ions by oxygen in air, low and high pHs facilitated the formation of goethite and lepidocrocite, respectively. The experimental results illustrated the single and combined effects of manganese oxide and air on the transformation of Fe2+ to ferric oxides.Graphical abstract:Lepidocrocite and goethite were formed during the interaction of ferrous ion and birnessite at pH 4-7. Redox rate was controlled by the adsorption of Fe2+ on the surface of birnessite. The presence of air (oxygen) accelerated the oxidation of Fe2+ to ferric oxides and facilitated the chemical stability of birnessite
Highlights
IntroductionManganese oxides and oxygen usually participate in the oxidation of ferrous ions
In soils and sediments, manganese oxides and oxygen usually participate in the oxidation of ferrous ions
The stimulated redox behaviors of ferrous ions and birnessite were studied, and the influence of Fe3+, Mn2+, temperature, and the presence of air on the transformation process of ferric oxides was investigated with pH 4-7
Summary
Manganese oxides and oxygen usually participate in the oxidation of ferrous ions. The reduction of U(VI) and the oxidation of ferrous sulfides including pyrrhotite and pyrite are usually catalyzed by Fe2+ as FeII–FeIII− cycling works as electron transporter [7,8,9]. Under oxidizing conditions, green rust may influence radionuclide mobility by catalyzing their transformation to a higher oxidation state [10]. These redox cycles of FeII–FeIII− were sometimes derived by biogeochemical pathway [1, 11]. A comprehensive attention has been given for the oxidation of Fe(II) in environment system
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