Effects Of Differerent Electron Donors On Microbial Reduction Of Iron Oxide

Iron presents in abundance as variety of Fe(III) oxides in crust. Fe(III) in environment undergoes reduction under anaerobic condition. And this process is a microbial dissimilatory reduction, namely an anaerobic respiration with Fe(III) as the terminal electron acceptor. Dissimilatory Fe(III) reduction showed important geological and environmental significances. Fe(III) minerals reduction increases the release of Si, P, which results in a high availability of these nutrient elements for plant growth. The group of dissimilatory Fe(III) reducers showed a high capacity of metabolism, and could broadly use diverse electron acceptors including many toxic heavy metals and metalloids such as Mn(Ⅳ),Cr(Ⅵ),Co(III),As(Ⅴ),Se(Ⅴ) as well as some radioactive elements U(Ⅵ),Tc(VII),V(Ⅴ). Thus these organisms played a key role in reductive detoxification and immobilization of these toxic metals in environments. Under Fe(III) or other metal oxides reduction, the dissimilatory Fe(III) reducers could stimulate the decomposition of some organic contaminants including aromatic compounds, halogenated aromatics, some dyes and long-chain aliphatic acids. In anoxic environment, Fe(III) reduction also effectively outcompeted the electron flow to methanogenesis and inhibited methane production. Furthermore some dissimilatory Fe(III) reducing bacterial could transfer electron to electrode to generate electricity and showed promising potentials in clean energy generation. In this paper, three paddy soil samples collected from Sichuan, Jiangxi, Jilin were used and the soil solutions were incubated under anoxic atmosphere. The effects on Fe(III) reduction of different components in culture were investigated. And Fe(III) reductions utilizing glucose and simple organic acids were also preliminarily studied. Some conclusive results were obtained as following. (1) Adding suitable concentrated NH4Cl, phosphate and KCl in culture stimulated the Fe(III) reduction to some extent and accelerated the Fe(III) reduction, but affected the stable Fe(II) accumulation slightly. Higher phosphate concentration inhibited Fe(III) reduction. When the concentration of phosphate in the culture reached 44 mmol/L, the final Fe(II) accumulation was just 80% of the no addition control. Adding higher concentration of NH4Cl and KCl in the culture, the stimulation of Fe(III) by N and K nutrients decreased suggesting a possible inhibition of microbes and its activities in Fe(III) reduction by chloridion.. (2) NaCl inhibited the Fe(III) reduction utilizing glucose as substrate. The acidogenic fermentation of glucose maybe was more sensitive to NaCl than Fe(III) reduction in use of simple fatty acids produced in fermentation of glucose. And the fermentation of glucose was almost inhibited sufficiently when the salinity less than 10.0g /L of NaCl, in which the chloridion was the putative inhibitor. (3) Fe(III) reductions were distinct in the incubations of different microbes from the three paddy soil samples with glucose, citrate, succinate, pyruvate, propionate, acetate and formate as substrate respectively. Under Fe(III) reduction, the incubation of microbes from Sichuan paddy soil responded to these substrates quickly and showed a higher activity with highest values of k (Fe(III) reduction reaction rate constant), lowest values of Tvmax (incubation time of maximal reaction rate) and highest Fe(II) accumulation in unit carbon (pyruvate and propionate excluded ). Propionate was hardly used to reduce Fe(III) in the incubation of microbes from Jiangxi paddy soil, and succinate for Fe(III) reduction in incubation of microbes from Jilin paddy soil. Pyruvate could be readily used coincidently in the incubations of three paddies microbes under Fe(III) reduction showing some similarities in substrate utilities of microbes under Fe(III) reduction. (4) Logistc and exponential models could both well represent Fe(III) reduction process. Logistic formula was more suitable for modeling Fe(III) reduction in incubations at 30℃and exponential formula got satisfied fitting results for the Fe(III) reduction at 20℃and 10℃. (5) Activation energy was not suitable to represent the energy barriers in Fe(III) reduction, but could well indicate the sensitivity of Fe(III) reduction to temperature variation. The calculated activation energy in three paddies microbe incubation uniformly showed highest values for the utilities of pyruvate, medial for formate and lowest for acetate. Fe(III) reduction in incubations of microbes from Jilin paddy soil utilizing pyruvate and microbes from Sichuan paddy soil with acetate were impacted greatly by temperature variation from 10℃to 20℃. And other incubations were more sensitive to variation from 20℃to 30℃under Fe(III) reduction. Summarily incubations of microbes from Jilin paddy was not as sensitive as the other two paddy soil microbes incubation in Fe(III) reduction.