The Effect Of Vanadium On Microbial Fe (Ⅲ) Reduction In Paddy Soils

The microbiological reduction process of Fe (III), which is a common process in thenatural world, is one of the most important inhale styles of microorganism, especially in theanaerobic environment. The reduction of Fe (III) in paddy soil is very complicated, and iseasy to be affected by the electron donors and acceptors in the surroundings. Moreover, manyother heavy metal ions, such as Cr (VI), Au (III), Hg (II), V (V), etc., can also serve as thecompetition on the electrons; and such competition can change the electron flows during themetabolism of microorganism. V (V) has been attracting people interests in the contaminationof soils, mainly because that V (V) can threaten both people's health condition and crops'growing. The microbiological reduction process of Fe (III) can help to reduce toxic V (V) intonon-toxic ions by changing the electron flow. Hence, it is quite valuable to study therelationship between Fe (III) reduction and V (V) reduction in paddy soil under anaerobiccondition. For further study, it will be worthwhile to study the competition between the twoprocesses, as well as the detailed mechanisms for them. However, the reduction of V (V) isonly on the start stage, and few reports have been made both in China and abroad, especiallyon microbiological remediation. This thesis will be focus on the microbiological reduction ofV (V) in soil, taking advantage of the operation process and mechanisms for Fe (III) reduction.We are expecting to study the relationships and electron flows between the two reducingprocesses under thermostatic and anaerobic conditions. Hopefully, we can get the theoreticalframework for the prevention and remediation of V (V) contaminations.There are several different soil resources for this projects, and three incubation systems,namely anaerobic mud culture (AMC), mixed culture of the microbial community of soilextract (MCSE) and typical iron reduction of dominant bacteria culture (IRBC), were used,with extra variables of Fe (III), glucose, and V (V) solution, to test the interaction betweenreduction of Fe (III) and V (V). Meanwhile, the addition of carbon source was tested. We havegot the following conclusions:1) In AMC, the addition of carbon source will accelerate the microbial reduction of Fe(III) and V (V). The addition of glucose can help to improve the activity of microorganism,with a better usage of environmental metal ions (high oxidation state) as electron acceptorsfor oxidizing, so as to increase the reaction rate of metal ion reductions. The addition of glucose and iron oxide will both help the microbial reduction of Fe (III) and contribute to thereduction of V (V).2) In AMC, V (V) has a positive effect on the microbial reduction of Fe (III), with ahigher reacting rate, while, in the meantime, without any increase on the total reducingamount, which is related with the actual Fe (III) reducing systems in soil.3) In AMC, we have proposed a possible mechanism for the reduction of Fe (III) and V(V): V (V) is first taken as the electron acceptor by microorganism, and gets reduced to lowoxidation state; the product can serve as a reducing agent to improve the reduction of Fe (III),and then is re-oxidized to V (V). The amount of Fe (III) in soil can probably control thechemical state of V (V).4) Microorganism can directly take V (V) as electron acceptor, and gets it reduced.Because of the differences of both microbial species and structures, paddy soil from differentareas has different reducing potential of V (V). In my choices of paddy soil, AK and HZ aremore promising in the reducing process, on both maximum reducing rates and minimumreducing time. The reducing ability of four different soils is as follows: AK>HZ>JL>SC. Theoriginal concentration of V (V) will not affect the microbial reducing ability.5) In MCSE, with the existence of both iron oxides and metavanadates, the reduction ofFe (III) trends to be prior to that of V (V). The reason lies most probably in the species of ironoxides. The synthesized Fe (OH)3has a relatively large surface area, and is easier to bereduced by the ion reducing microorganism. Such structure can absorb the metavanadatesstrongly, thus will affect the reducing process of V (V).6) During the study of three different culture systems, there are several observations foreach: in AMC, V (V) cannot be fully reduced; in MCSE, V (V) can be reduced bymicroorganism completely to100%, which is mainly because of the expelling of soil mattersand soil particles; in IRBC, the V (V) reduction ability is between the former two, and V (V)cannot be removed completely, which means that the complex microbial community has abetter tolerance and reducing ability.