|Other Abstract||Phosphorus is an essential mineral element for life activities in aquatic ecosystems. Due to extreme sensitiveness of hydrobiology, excess loading of phosphorus would render water bodies eutrophicated. There are various phosphorus species in sediments that can contribute to water column, but iron phosphate fraction is regarded predominant because of high instability in sediment environment. Iron oxides, one of main sediment components, play an important role in buffering the change of sediments in physico-chemistry and biology properties through oxidation-reduction reaction. Therefore, with water environment deteriorated, the attention is increasingly focused on the oxidation-reduction of iron oxides associated to the availability of iron phosphate fraction in sediments.
In the thesis, a high pure strain of Fe (III) reducing bacteria was prepared with the sediment sampled from Yuehu Lake of Wuhan, and amorphous Fe (III) oxide was synthesized to gain a model of ferric oxides for bio-reduction of ferric oxides. Simultaneously, some criteria of reaction system such as soluble phosphate, sorbed phosphate, Fe (II) and Fe (III) were monitored with the purpose to characterize the relationship between availability of iron phosphate fraction and bio-reduction of ferric oxides. The results were as following:
(1) A optimal habitat for ferric reducing bacteria was the iron oxide-water interface. With biomass increasing, the augment of ferric reducing bacteria was toward the directions of both iron oxide and water column. The ferric reducing bacteria obtained in the thesis seemed anaerobic, and oxygenation might impede their activities.
(2) Ferric reducing bacteria can effectively promote reduction of ferric oxides. The production of Fe (II) was increased over incubation time with a maximum reached. The extent of ferric oxide bio-reduction depended on incubation medium. Compared with the residue of aquatic plants, acetate induced ferric reducing bacteria to reduce less of Fe (III) due to a weak activity.
(3) Iron phosphate availability decreased was coupled with bio-reduction of ferric iron oxides. Phosphate existed in the suspension of Fe (III) oxides in three types, e. g. both specific and electrostatic adsorption phosphate as well soluble phosphate. As Fe (III) reduced, phosphate species would be changed with specific adsorption phosphate increasing but both electrostatic sorption and soluble phosphate decreasing. Provided that a certain amount of Fe (II) was produced, the latter were decreased to a level not be detected.
(4) Sorption of amorphous ferric oxide to phosphate could be characterized by mathematical model. On the basis of several hypotheses, a model of iron sorption to phosphate was given, e. g. TPad = (KFe(II) - KFe(III)) * MFe(II) + KFe(III) M, which TPad denoted the total of phosphate sorbed, KFe(II) and KFe(III) denoted the amount of phosphate sorbed by unit Fe(III) and Fe (II) respectively, MFe(II) denoted the amount of Fe (II), and M represented the total of iron in the system of interest. KFe(II) obtained was two times as much as KFe(III), showing a stronger ability of Fe(II) to sorb phosphate than Fe (III).
(5) The mechanism by which iron oxides sorbs phosphate was inferred based on the parameter values. Ratio of phosphate sorbed to Fe (III) (P/Fe (III)) was responsible for the amount of phosphate sorption with more phosphate sorbed as P/ Fe (III) was higher. Interpretation to the results might be related to the increase of iron oxide surface induced by addition of phosphate. With the production of Fe(II), Fe (II) preferred to sorb the available phosphate, which was operationally defined as both electrostatic sorption and soluble phosphate, and transformation of the phosphate adsorbed specifically by Fe (III) to adsorbed specifically by Fe (II) was enhanced until distribution equilibrium of phosphate in both Fe (II) and Fe (III) states reached.|