Study and Comparison of Microbial Reduction of Structural Fe (III) in Nontronite, Goethite and Hematite

Microbial reduction is an important process that affects properties of ferric clay mineralas and iron biogeochemical cycling in natural environments. Oxyhydroxides and phyllosilicates are two major sources of iron in soils، rocks and sediments that potentially have ability to bioreduced over the time. In this study، comparison of bioreduction in two main sources of iron (oxides and phyllosilicates) in the presence of Shewanella sp was carried out. For this purpose an experiment was conducted under laboratory conditions in the form of a completely randomized design with 16 treatments and 3 replications. Treatments included bioreduction of goethite، hematite and two type of nontronite with and without electron transfer (AQS) and controls (all mentioned treatments in the absence of Shewanella sp). The results revealed that bioreduction in goethite، hematite and two type of nontronite increased in the presence of AQS. The extent of reduction in the presence of AQS was 21%، 9%، 3% and 8. 7% in NAu-2، NAu-1، hematite and goethite respectively. These results have shown when iron oxides (goethite and hematite) and iron silicates are the dominant form of ferric iron in soils and subsurface sediments، Shewanella sp can survive and produce significant amounts of Fe (II). In the identical conditions of bioreduction (concentration of electron donor and acceptor and equal number of bacterial cell) size and surface area of mineral play an important role in efficiency of bioreduction.