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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.

No MeSH data available.


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The relationship of consumed Fe2+ and releasedMn2+ concentration in the initial reaction stage of 20 mMFe2+ and 1.0 g L−1 birnessite under differentconditions: 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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Fig12: The relationship of consumed Fe2+ and releasedMn2+ concentration in the initial reaction stage of 20 mMFe2+ and 1.0 g L−1 birnessite under differentconditions: 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 redox rate was further analyzed and confirmed by comparing the relationship ofconsumed Fe2+ and released Mn2+ concentrationin the initial stage. As discussed above, ΔFe/ΔMn slope demonstrates the chemical stability andoxidation capacity of birnessite. From Eq. (1), ΔFe/ΔMnslope of about 1.86 (mass ratio) suggested the balance of adsorption/oxidation ofFe2+ and the reduction/release of Mn2+, andhigher slope of ΔFe/ΔMn indicates higher adsorption/oxidation rate ofFe2+ and higher chemical stability of birnessite. As shown inFig. 12, this slope increased with an increase in pH and adecrease of temperature. These results further revealed that properly increasing alkalinity favoredthe adsorption and oxidation of Fe2+ on the surface of birnessite,resulting in the increase of the release Mn2+ concentration. Therefore,ΔFe/ΔMn slope decreased with an increase in pH of reaction system. The lowest slope of 1.70 and thegreatest slope of 5.69 were obtained at 10 and 40 °C, respectively. High temperature and proper highpH accelerated the redox reaction and corresponding Mn2+ dissolving, andadsorption might be the major reaction at lower temperature.Fig. 12


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 relationship of consumed Fe2+ and releasedMn2+ concentration in the initial reaction stage of 20 mMFe2+ and 1.0 g L−1 birnessite under differentconditions: 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

Fig12: The relationship of consumed Fe2+ and releasedMn2+ concentration in the initial reaction stage of 20 mMFe2+ and 1.0 g L−1 birnessite under differentconditions: 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 redox rate was further analyzed and confirmed by comparing the relationship ofconsumed Fe2+ and released Mn2+ concentrationin the initial stage. As discussed above, ΔFe/ΔMn slope demonstrates the chemical stability andoxidation capacity of birnessite. From Eq. (1), ΔFe/ΔMnslope of about 1.86 (mass ratio) suggested the balance of adsorption/oxidation ofFe2+ and the reduction/release of Mn2+, andhigher slope of ΔFe/ΔMn indicates higher adsorption/oxidation rate ofFe2+ and higher chemical stability of birnessite. As shown inFig. 12, this slope increased with an increase in pH and adecrease of temperature. These results further revealed that properly increasing alkalinity favoredthe adsorption and oxidation of Fe2+ on the surface of birnessite,resulting in the increase of the release Mn2+ concentration. Therefore,ΔFe/ΔMn slope decreased with an increase in pH of reaction system. The lowest slope of 1.70 and thegreatest slope of 5.69 were obtained at 10 and 40 °C, respectively. High temperature and proper highpH accelerated the redox reaction and corresponding Mn2+ dissolving, andadsorption might be the major reaction at lower temperature.Fig. 12

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