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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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XRD patterns of solid products of 20 mM Fe2+ oxidized by1.0 g L−1 birnessite with pH 5.5 in nitrogen atmosphere at differenttimes
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Fig2: XRD patterns of solid products of 20 mM Fe2+ oxidized by1.0 g L−1 birnessite with pH 5.5 in nitrogen atmosphere at differenttimes

Mentions: The redox reaction was conducted in nitrogen atmosphere to create an anoxiccondition at pH 5.5, and solid products were characterized by XRD at different times as shown inFig. 2. After 5 min of reaction, a mixture of birnessite andamorphous ferric oxides was formed, and then lepidocrocite (γ-FeOOH, JCPDS No. 76-2301) was formedafter 2 h. Birnessite was completely reduced due to the disappearance of characteristic diffractionpeaks after 2 days as shown in Fig. 2. The increase ofdiffraction peaks of goethite (α-FeOOH, JCPDS No. 81-0464) and a relative weakening of lepidocrociteis possibly due to the transformation from lepidocrocite to goethite after 8 days. Thetransformation from lepidocrocite to goethite in the nucleation process was influenced by multiplefactors [31, 32]. This transformation would be inhibited by traces of silicate, aluminate andstannate [32], and markedly interfered by Ti(IV),Cu(II), Cr(III) and Ni(II), and the coexistence of Fe(II) and SO42- is necessary for this transformation [31]. Inthe current system, both SO42− and Fe2+ participated in the reaction, andthe transformation from lepidocrocite to goethite occurred. However, the transformation fromrelatively metastable lepidocrocite to goethite is extremely slow at ambient temperature[33]. The presence of Mn2+and other transition metal ions likely assisted this progress, and the newly releasedMn2+ could incorporate in goethite [2, 31–33]. Conversely, aqueous Fe2+ can induce the release ofstructural manganese from manganese-doped goethite [2].Fig. 2


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 with pH 5.5 in nitrogen atmosphere at differenttimes
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

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

Fig2: XRD patterns of solid products of 20 mM Fe2+ oxidized by1.0 g L−1 birnessite with pH 5.5 in nitrogen atmosphere at differenttimes
Mentions: The redox reaction was conducted in nitrogen atmosphere to create an anoxiccondition at pH 5.5, and solid products were characterized by XRD at different times as shown inFig. 2. After 5 min of reaction, a mixture of birnessite andamorphous ferric oxides was formed, and then lepidocrocite (γ-FeOOH, JCPDS No. 76-2301) was formedafter 2 h. Birnessite was completely reduced due to the disappearance of characteristic diffractionpeaks after 2 days as shown in Fig. 2. The increase ofdiffraction peaks of goethite (α-FeOOH, JCPDS No. 81-0464) and a relative weakening of lepidocrociteis possibly due to the transformation from lepidocrocite to goethite after 8 days. Thetransformation from lepidocrocite to goethite in the nucleation process was influenced by multiplefactors [31, 32]. This transformation would be inhibited by traces of silicate, aluminate andstannate [32], and markedly interfered by Ti(IV),Cu(II), Cr(III) and Ni(II), and the coexistence of Fe(II) and SO42- is necessary for this transformation [31]. Inthe current system, both SO42− and Fe2+ participated in the reaction, andthe transformation from lepidocrocite to goethite occurred. However, the transformation fromrelatively metastable lepidocrocite to goethite is extremely slow at ambient temperature[33]. The presence of Mn2+and other transition metal ions likely assisted this progress, and the newly releasedMn2+ could incorporate in goethite [2, 31–33]. Conversely, aqueous Fe2+ can induce the release ofstructural manganese from manganese-doped goethite [2].Fig. 2

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