| Iron oxides are common compounds which are widespread in nature. The diversity and the different thermodynamic stability of iron oxides give rise to the interconversions between different forms. The study on the formation and transformation of iron oxides, especially the transformation of ferrihydrite has been paid more attention. In recent years, our group has explored the transformation ferrihydrite in the presence of trace of Fe(II). Our earlier investigation focused mainly on the transformation of ferrihydrite prepared by FeCl3 and NaOH solutionsin the presence of trace of Fe(II), which is termed NaOH medium. During the research, we found that there are obviously differences in the transformation of ferrihydrite prepared by the different precipitation agents such as NaOH and NH3·H2O. One of our aims is to investigate the effect of the different precipitation agents on the transformation of ferrihydrite and understand the nature of the transformation under different conditions so that we can control the transformation from ferrihydrite to the goal product. The present work aims at understanding the nature of the transformation of ferrihydrite prepared in the NH3·H2O medium in the presence of trace Fe(II). We try to reach the goal to controll the transformation from ferrihydrite to goethite by investigating the transformation mechanism of ferrihydrite in ammonium and amine media, etc. Not only is this study of academic importance but also of practical value. The reason is that iron oxides are important chemical materials and they exist widespreadly in nature, especialy surface environment of the earth and this system is often taken as a model system in the study on the property of colloid and interface for other metal oxides. At the same time, we really hope that the investigation in the current system becomes a nice example for the control transformation process based on the theoretical research. The main research aspects are summarized as follows.(1) The phase transformation of ferrihydrite, prepared by using FeCl3 and NH3·H2O as raw materials, was studied in the presence of trace Fe(II) in detail. The effect of various factors such as the initial pH, the transformation temperature and the ferric hydroxide formation conditions as well as the concentrations of Fe(II) and Fe(III) ions was discussed. The results show that the main products obtained in the NH3·H2O medium areα-FeOOH and/orα-Fe2O3. The species and amount of product have a close relationship with the temperature. Pure goethite was obtained at T≤40℃and the mixture of goethite and hematite at 40℃9, both a low pH value and slow heating rate favor the formation of goethite and a high pH value and fast heating rate favor the formation of hematite. The above results were given a reasonable explaination by the catalytic dissolution-reprecipitation mechanism and the catalytic solid-state transformation mechanism.(2) Compared the transformation of ferrihydrite prepared in the NH3·H2O medium with that in the NaOH medium, it was found that the differences between the two medium in the transformation result from the complexation between Fe(III) dissolved from ferrhydrite and N in NH4+ ions. On the one hand, the presence of these complexation has affected the electron transfer between Fe(II) and Fe(III) ions, which leads to the decrease in the dissolution rate of ferrihysrite as well as the transformation of ferrihydrite. On the other hand, the formation of soluble complexes of ferrihydrite with ammonia favors the formation of cation polymer [Fe(OH)2 (H2O)4/2+]n, thus, the formation of goethite.(3) In order to avoid the influence of NH4+, NaOH was used to prepare ferrihysrite. In the presence of hexamethylene tetramine, ethanolamine, diethanolamine and triethanolamine, the phase transformation of ferrihydrite was investigated at low teperatures and near neutral pH. The products were characterized by such technologies as IR,XRD and TEM. The results show that the presence of above additives favors that formation of goethite. The results were attributed to the complexation between Fe(III) dissolved from ferrhydrite and N in the amine.(4) In order to avoid the influence of NH4+, NaOH was used to prepare ferrihysrite. In the presence of C2O42- and F- ions, the phase transformation of ferrihydrite was investigated. The results show that the presence of above additives favors that formation of goethite.(5) Based on the above results, the conditions to control the transformation of ferrihydrite to goethite were found. That is, CFe(Ⅲ)=0.5mol/L, nFe(Ⅱ)/nFe(Ⅲ)=0.02, pH=7,25°C≤T≤40°C,n6H12N4/nFe(Ⅲ)=1.8, or nC2H7NO/nFe(Ⅲ)=2, nC4H11NO2/nFe(Ⅲ)=0.4, nC2O42- /nFe(Ⅲ)=0.05, nF- /nFe(Ⅲ) =0.3.
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