| In natural environment, the reduction of ferric iron is caused mainly by microorganisms, and the dissimilatory Fe(Ⅲ) reduction is the most important way for microorganisms to reduce ferric iron. In dissimilatory Fe(Ⅲ) reduction, ferric iron reduction connects with the microbial metabolism directly, so the transformation of Fe(Ⅲ) to Fe(Ⅱ) can promote the degradation of organic pollutant. And the result of reduction of Fe(Ⅲ) to Fe(Ⅱ) can also cause some significant changes of soil properties. Therefore, people are paying more and more attention to the microbial reduction of ferric iron now. In this experiment, paddy soil was chosen as research materials, in which the reduction and oxidation of iron occurs periodically, and we studied the microbial reduction characteristic of ferric iron in seven paddy soils by the method of incubating the paddy soil samples under flooded and airtight condition in constant temperature incubators at 15℃, 25℃ and 35℃. The paddy soil samples were collected from the paddy fields in seven province and regions: Shannxi, Jilin, Sichuan, Hunan, Jiangxi, Zhejiang and Guangxi. Our aim is to find out the characteristics, the laws and the limits of microbial reduction of ferric iron in different paddy soils in China, and to provide more scientific base for further study on the microbial reduction of ferric iron and its environmental significance. The primary results were as follows:(1) The reaction of ferric iron reduction has obvious staged feature because the curve shapes of concentrations of extractable Fe(Ⅱ) and total Fe increasing with incubation time is 'S'. The reaction process can be divided into four stages: preliminary-stage, fast-stage, deceleration-stage and stable-stage. The amount of Fe(Ⅱ) produced in fast-stage is about 80% of total Fe(Ⅱ) produced in the whole reaction, and that produced in deceleration-stage is about 20% of total Fe(Ⅱ). Before the reaction reached the stable-stage, the extractable Fe(Ⅲ) had disappeared, and the time when extractable Fe(Ⅲ) disappeared is similar with the time that fast-stage transform to deceleration-stage. This shows that the ferric iron reduced in fast-stage is easy to dissolve and that reduced in deceleration-stage is harder to dissolve relatively.(2) The final amount of Fe(Ⅱ) in soil is different between different paddy soil. The rang of Fe(Ⅱ) concentration is 1700-8300mgFe/kg, and the sequence is Jilin paddy soil > Puji paddy soil > Hunan paddy soil > Sichuan paddy soil > Zhejiang paddy soil > Jiangxi paddy soil > Guangxi paddy soil. The final concentration of Fe(Ⅱ) in every paddy soil is smaller than free iron oxide content and larger than amorphous iron oxide content, and has great significant correlation with amorphous iron oxide. This shows the ferric irons reduced by microorganisms in incubation are amorphous iron oxide and some other activity iron oxides. The concentration of Fe(Ⅱ) in Jilin paddy soil and Puji paddy soil is higher than those collected in South of China. This shows the paddy soils in North of China have more activity iron oxides than that in South of China.(3) Every reaction stage of ferric iron reduction was shortened when temperature was raised. At higher temperature, the rate of reaction of ferric iron reduction is larger than that at lower temperature. And at the fast-stage, the rate of reaction increases 2-4 times when temperature was raised per 10℃. At higher temperature, the final amount of Fe(Ⅱ) in every paddy soil is larger than that of Fe(Ⅱ) at lower temperature.(4) The kinetic characteristic of the microbial reduction of ferric iron in paddy soils was studied. At different temperature, the best model to describe the kinetic characteristic of ferric iron reduction is not entirely similar among seven paddy soils, that shows soil properties and temperature have important effects on mechanism of ferric iron reduction. But for most soils, the model ln (1-C/Cmax)=A+Bt is better than the other models for describing the kinetic characteristic of ferric iron reduction.(5)... |
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