Experimental Research On Dissimilatory Iron Reduction Of Characterized Trivalent Iron Minerals

There exist great amount of iron-containing minerals in the environment, and oxidation/reduction is easy to occur to the iron element, thus a complex series of iron-minerals will form. Transformation in mineral phases may bring about changes in contents of economical ores in a mining area, cause heavy metal pollution, and even affect regional magnetic susceptibility. Up to now, bio-oxidation of iron minerals has been widely investigated, while research in dissimilatory iron reduction of iron minerals has only rised in recent years. Characteristic iron ores and trivalence iron minerals were found in sulfide mining areas, in which coexists a lot of toxic elements. To look into transformation path of iron minerals, is of great importance in choosing the right time or place to conduct mining development, and prevent and cure heavy metal pollution that accompanying such minerals.This thesis choose the most commonly found trivalence iron minerals-jarosite, goethite, and sediments in mining area as the target, have them react with deeply researched dissimilatory iron reducing bacteria Shewanella oneidensis MR-1. The goal is to study the reduction process of iron in trivalence iron minerals, find out the secondary minerals and the sequence of their appearance, so as to investigate the mechanism of iron mineral bioreduction. In the meantime, to establish a preliminary understanding on the behavior of heavy metal that accompanying iron minerals during dissimilatory iron reduction. To observe the influence of bacteria and anaerobic environment on iron bio-reduction, here we roughly set up three reaction system: anaerobic-bio-system, anaerobic-system, and aerobic-bio-system. The study focuses on the iron bio-reduction process in anaerobic-bio-system, as well as features of As release.Through ICP-OES, IC, titration method, and o-phenanthroline method, we studied and contrasted ion concentration changes in experiment systems, and found that under anaerobic conditions bacteria notably implemented the dissolution and reduction of iron minerals. By using SEM, TEM, XRD, Raman, DRS, XPS, XAFS and STXM etc. methods, an array of iron-containing intermediate products and bivalent end minerals were tested. Concentrations of Fe2+and Fe3+raised larged in the early days of reduction, trivalent iron minerals were dissolved and reduced; in the late stage, concentrations of iron in solution declined, goethite, green rust, and amorphous ferric/ferrous oxides were produced as intermediate minerals, and finally transformed to end products such as magnetite, siderite and vivianite, etc. According to bio-reduction experiments on lab synthesized jarosite and goethite, the thesis proposed conceptual models of dissimilatory iron reduction of the two minerals. In initial stage, iron mineral is reduced by bacteria, Fe2+and elements that compose the mineral (such as K+, Fe3+, SO42-, etc) are released; With time goes by, released Fe2+increase a lot, remaining mineral residue start to generate middle minerals like goethite/green rust through crystal growth ore Oswald ripening, Fe2+in solution begin to decrease; In the late phase, unsteady intermediate products transform to more stable end product as magnetite, siderite and vivianite; Finally, since Fe2+adsorb to mineral surface in large number, end product precipitate on surface of trivalent iron minerals, and secreted extracellular polymeric substance or iron sediment wrap up cells, causing the depression of bacteria activity, further reduction of iron minerals are prevented, and dissimilatory iron reduction reached cessation period.The thesis takes reference of the reduction model of single minerals, analyses the bioreduction mechanism of trivalent iron sediments from mining areas, and its influences on the activity of accompanying heavy metals. The bioreduction effect is even more thorough on natural sediments than synthesized minerals. In static system, microbial reduction changed the initial orange color, which refers to oxidation state, to dark green, which represents reduction environment. And flower-like goethite/hematite and columnar vivianite was first detected. In flow system, different reduction stages appeared in different depth of sediments, the lower the sediment, the later to be reduced. By monitoring the release of heavy metal, Arsenic demonstrates a most characteristic release rate:Microbial effects multiplied the release amount of As; While As was largely released over reduction process, the unstable intermediate products were easy to adsorb As, finally transform to stable minerals and immobilize As. Investigating the dissimilatory iron reduction of trivalent iron minerals, are of significance to geochemical behavior of concomitant heavy metals.