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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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The concentration of released Mn2+ in reaction process of20 mM Fe2+ and 1.0 g L−1 birnessite underdifferent conditions: a pH 4.0–7.0, 25 °C,N2; b pH 5.5, 10–40 °C,N2; c pH 4.0–7.0, 25 °C, air
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Fig11: The concentration of released Mn2+ in reaction process of20 mM Fe2+ and 1.0 g L−1 birnessite underdifferent conditions: a pH 4.0–7.0, 25 °C,N2; b pH 5.5, 10–40 °C,N2; c pH 4.0–7.0, 25 °C, air

Mentions: The influence of pH, temperature, and air on the release rate ofMn2+ was further studied in the oxidation process ofFe2+ of 20 mmol L−1 by birnessite of1.0 g L−1. As shown in Fig. 11a,Mn2+ release rate increased with an increase of pH in reaction system,suggesting redox rate increasing with an increase in OH− concentration.High pH might also decrease the solubility of Mn2+. Therefore, thedissolving rate is not controlled by the transport of reduced species away from the surface, that isto say, Mn2+ release into solution is not the rate-determining step[42], which agreed with the results and analysis forthe reaction systems with Fe2+ of different concentrations. Usually, theoxidation activity of manganese oxide is enhanced with increasing H+concentration [41, 42]. However, increasing pH enriched surface negative charge amount, consequently theadsorption capacity increased [44, 45]), and high pH thermodynamically accelerated redox reaction ofFe2+ and manganese oxides [20, 23].Fig. 11


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)

The concentration of released Mn2+ in reaction process of20 mM Fe2+ and 1.0 g L−1 birnessite underdifferent conditions: a pH 4.0–7.0, 25 °C,N2; b pH 5.5, 10–40 °C,N2; c pH 4.0–7.0, 25 °C, air
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

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

Fig11: The concentration of released Mn2+ in reaction process of20 mM Fe2+ and 1.0 g L−1 birnessite underdifferent conditions: a pH 4.0–7.0, 25 °C,N2; b pH 5.5, 10–40 °C,N2; c pH 4.0–7.0, 25 °C, air
Mentions: The influence of pH, temperature, and air on the release rate ofMn2+ was further studied in the oxidation process ofFe2+ of 20 mmol L−1 by birnessite of1.0 g L−1. As shown in Fig. 11a,Mn2+ release rate increased with an increase of pH in reaction system,suggesting redox rate increasing with an increase in OH− concentration.High pH might also decrease the solubility of Mn2+. Therefore, thedissolving rate is not controlled by the transport of reduced species away from the surface, that isto say, Mn2+ release into solution is not the rate-determining step[42], which agreed with the results and analysis forthe reaction systems with Fe2+ of different concentrations. Usually, theoxidation activity of manganese oxide is enhanced with increasing H+concentration [41, 42]. However, increasing pH enriched surface negative charge amount, consequently theadsorption capacity increased [44, 45]), and high pH thermodynamically accelerated redox reaction ofFe2+ and manganese oxides [20, 23].Fig. 11

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