Study On The Phase Transformation Mechanism Of Ferrihydrite In The Presence Of Al Or Ga

Iron oxides are widespread compounds in nature, which almost exist in all of the global system. The diversity of iron oxides gives rise to the complicated interconversions between different species. Thus, much attention has been paid to the formation and phase transformation, especially the transformation of precursor"ferrihydrite". The transformation products of ferrihydirte areα-FeOOH,γ-FeOOH orα-Fe2O3, which depend on the component of solution, temperature, reaction time and additional ions and molecules etc. Earlier research showed trace of Fe(II) has obvious catalytic role in the phase transformation of ferrihydirte. The present work is aimed at having an insight into the influence of trace of additional ions which coexist with ferrihydirte. The work helps to investigate the transformation of iron oxides in presence of other substances, and then supplies further experimental basis of catalytic phase transformation mechanism of ferihydrite with Fe(II). The main research aspects are summarized as follows:In the process of ferrihydirte phase transformation and catalytic phase transformation, the effects of the concentration of Al(III), initial pHs, the methods of doping on the reaction were investigated. The result showed that trace of Al(III) restrained the transformation of ferrihydrite , the reaction velocity slowed down as the concentration of Al(III) increased. The methods of doping displayed different effects on the transformation process of ferrihydrite with or without Fe(II). The transformation time of ferrihydrite prepared by the way of subsequent hydrolysis was shorter than that by combined hydrolysis in the absence of Fe(II). Solid state transformation was the major mechanism. The transformation time of ferrihydrite prepared by the way of combined hydrolysis was shorter than by subsequent hydrolysis in the presence of Fe(II) and the transformation of ferrihydrite involved dissolution-reprecipitation. In suitable conditions(initial pH=9,nAl/nFe≤0.18), plate-likeα-Fe2O3 particles were attained from Al -doped ferrihydrite. The formation mechanism of the plate-likeα-Fe2O3 particles involved dissolution-reprecipitation.As cognation of Al(III), Ga(III) played some role on ferrihydrite transformation. The effects of the concentration of Ga(III), initial pHs, the methods of doping on the reaction were also investigated. The results showed that trace of Ga(III) inhibited the transformation of ferrihydrite, which was similar to Al(III). The methods of doping displayed different effects on the transformation process of ferrhydrite with or without Fe(II). The research was focused on pH ranged from 5 to 9. At pH5, needlelikeα-FeOOH particles formed from ferrihydrite with Ga-doped, while at pH9 the phase transformation product was sphericalα-Fe2O3 particles. As the concentration of Ga(III) increased(≥0.07M),α-FeOOH particles of different morphologies formed. The different ionic radii and ionic property of Al(III) and Ga(III) is responsible for the difference of transformation between Al-ferrihydrite and Ga- ferrihydrite.