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Interaction mechanisms and kinetics of ferrous ion and hexagonal birnessite in aqueous systems.

Gao T, Shen Y, Jia Z, Qiu G, Liu F, Zhang Y, Feng X, Cai C - Geochem. Trans. (2015)

Bottom Line: The formation of ferric (hydr)oxides precipitate inhibited the further reduction of birnessite.The presence of air accelerated the oxidation of Fe(2+) to ferric oxides and facilitated the chemical stability of birnessite, which was not completely reduced and dissolved after 18 days.The presence of air (oxygen) accelerated the oxidation of Fe2+ to ferric oxides and facilitated the chemical stability of birnessite.

View Article: PubMed Central - PubMed

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 People's Republic of China.

ABSTRACT

Background: In 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.

Results: To 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 Mn(2+) and K(+) ions, a significant decrease in Fe(2+) 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. Fe(2+) exhibited much higher catalytic activity than Mn(2+) during the transformation from amorphous Fe(III)-hydroxide to lepidocrocite and goethite under anoxic conditions. The release rates of Mn(2+) were compared to estimate the redox rates of birnessite and Fe(2+) under different conditions.

Conclusions: Redox rate was found to be controlled by chemical reaction, and increased with increasing Fe(2+) 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 Fe(2+) 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 Fe(2+) 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.

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XRD patterns of solid products of 20 mM Fe2+ oxidized by1.0 g L−1 birnessite in air at different times
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Fig7: XRD patterns of solid products of 20 mM Fe2+ oxidized by1.0 g L−1 birnessite in air at different times

Mentions: In soils and sediments, redox reactions are usually driven by oxygen, ferric irons,manganese oxides and microorganisms [8, 39, 40]. To simulatethe abiotic oxidation behavior of Fe2+ by birnessite in an open system,air was admitted into the reaction system, and intermediate products were characterized. As shown inFig. 7, a mixture of birnessite and lepidocrocite wasproduced within 6 days, and then goethite was formed after 10 days. The fact that the participationof oxygen improved the chemical stability of birnessite was further verified by the concentration ofreleased Mn2+ as shown in Fig. 5b. The remove rate of Fe2+ concentration significantlyincreased, compared with the reaction under anoxic condition, and Fe2+concentration decreased to about zero at 1 h (Fig. 5).However, the concentration of generated Mn2+ andK+ just reached about 170 and 70 mg L−1 after6 h, respectively. These results indicated the oxidation of Fe2+ byoxygen and the improvement of the chemical stability of birnessite. The chemical stability ofbirnessite was improved in the presence of oxygen, likely due to the fact that the newly formedMn(III) from Mn(IV) in birnessite would be re-oxidized by oxygen in air [28, 30, 34, 39, 41]. In our previous work, during the oxidation process of solublesulfide by todorokite and oxygen, the reaction rate was controlled by the rate of diffusion ofsoluble sulfide and todorokite, and the admission of oxygen reduced the initial oxidation rate ofsoluble sulfide by todorokite due to the decrease of active Mn(III) content in manganese oxide owingto the oxidation of Mn(III) to Mn(IV) by oxygen [28].On the other hand, oxygen would directly oxidize ferrous ions, and the consumption of oxidantbirnessite decreased in the same reaction system. The improved redox stability of birnessite wasfurther confirmed by TEM image as shown in Fig. 4d. Althoughthe morphologies of birnessite became pulverized and indistinct, the aggregates could be observed.These particles were completely dissolved and unobservable as the redox reaction occurred innitrogen atmosphere for the same time (Fig. 4c). There wasno obvious change for the concentration of released K+ likely due toion-exchange by Fe3+/2+ and H+ during theinitial stage and the complete reduction and dissolve during the final stages.Fig. 7


Interaction mechanisms and kinetics of ferrous ion and hexagonal birnessite in aqueous systems.

Gao T, Shen Y, Jia Z, Qiu G, Liu F, Zhang Y, Feng X, Cai C - Geochem. Trans. (2015)

XRD patterns of solid products of 20 mM Fe2+ oxidized by1.0 g L−1 birnessite in air at different times
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4585411&req=5

Fig7: XRD patterns of solid products of 20 mM Fe2+ oxidized by1.0 g L−1 birnessite in air at different times
Mentions: In soils and sediments, redox reactions are usually driven by oxygen, ferric irons,manganese oxides and microorganisms [8, 39, 40]. To simulatethe abiotic oxidation behavior of Fe2+ by birnessite in an open system,air was admitted into the reaction system, and intermediate products were characterized. As shown inFig. 7, a mixture of birnessite and lepidocrocite wasproduced within 6 days, and then goethite was formed after 10 days. The fact that the participationof oxygen improved the chemical stability of birnessite was further verified by the concentration ofreleased Mn2+ as shown in Fig. 5b. The remove rate of Fe2+ concentration significantlyincreased, compared with the reaction under anoxic condition, and Fe2+concentration decreased to about zero at 1 h (Fig. 5).However, the concentration of generated Mn2+ andK+ just reached about 170 and 70 mg L−1 after6 h, respectively. These results indicated the oxidation of Fe2+ byoxygen and the improvement of the chemical stability of birnessite. The chemical stability ofbirnessite was improved in the presence of oxygen, likely due to the fact that the newly formedMn(III) from Mn(IV) in birnessite would be re-oxidized by oxygen in air [28, 30, 34, 39, 41]. In our previous work, during the oxidation process of solublesulfide by todorokite and oxygen, the reaction rate was controlled by the rate of diffusion ofsoluble sulfide and todorokite, and the admission of oxygen reduced the initial oxidation rate ofsoluble sulfide by todorokite due to the decrease of active Mn(III) content in manganese oxide owingto the oxidation of Mn(III) to Mn(IV) by oxygen [28].On the other hand, oxygen would directly oxidize ferrous ions, and the consumption of oxidantbirnessite decreased in the same reaction system. The improved redox stability of birnessite wasfurther confirmed by TEM image as shown in Fig. 4d. Althoughthe morphologies of birnessite became pulverized and indistinct, the aggregates could be observed.These particles were completely dissolved and unobservable as the redox reaction occurred innitrogen atmosphere for the same time (Fig. 4c). There wasno obvious change for the concentration of released K+ likely due toion-exchange by Fe3+/2+ and H+ during theinitial stage and the complete reduction and dissolve during the final stages.Fig. 7

Bottom Line: The formation of ferric (hydr)oxides precipitate inhibited the further reduction of birnessite.The presence of air accelerated the oxidation of Fe(2+) to ferric oxides and facilitated the chemical stability of birnessite, which was not completely reduced and dissolved after 18 days.The presence of air (oxygen) accelerated the oxidation of Fe2+ to ferric oxides and facilitated the chemical stability of birnessite.

View Article: PubMed Central - PubMed

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 People's Republic of China.

ABSTRACT

Background: In 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.

Results: To 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 Mn(2+) and K(+) ions, a significant decrease in Fe(2+) 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. Fe(2+) exhibited much higher catalytic activity than Mn(2+) during the transformation from amorphous Fe(III)-hydroxide to lepidocrocite and goethite under anoxic conditions. The release rates of Mn(2+) were compared to estimate the redox rates of birnessite and Fe(2+) under different conditions.

Conclusions: Redox rate was found to be controlled by chemical reaction, and increased with increasing Fe(2+) 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 Fe(2+) 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 Fe(2+) 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.

No MeSH data available.


Related in: MedlinePlus