Effects Of Several Metal Ions On Formation/Transformation,Structure And Properties Of Iron Oxides

As an important component of soil,iron oxides have significant impact on the physicochemical properties of soil,due to their large specific surface area and strong reactivity,which determines the geochemical behavior and fate of many nutrients,heavy metals and organic pollutants in the soil.The formation and transformation of iron oxides in soil is often accompanied by metal cations,and the structure and properties of iron oxides are affected by metal substitution.Thus,the relationship between iron oxides and metals cations is important in environmental geochemistry.There are still many unresolved problems about the effect of metal ions on iron oxides,such as the effects of metal cations on the structure of iron oxides during the formation and transformation process,the same/different effect of different metal cations substitution on the structure of iron oxides,and how the structure modification caused by metal cations substitution lead to changes in the properties of iron oxides.In this paper,the formation of ferrihydrite from Fe3+and transformation of ferrihydrite to hematite was studied by XRD,TEM,SEM,UV-vis,FTIR,XPS,EXAFS and other technologies to reveal the effect of metal cations on the formation and transformation of iron oxides and the effect of different metal cations on the structure and properties of iron oxides.The results are as follows:1)The influence of Al³⁺and Cr³⁺on the formation of ferrihydrite was studied.Al³⁺and Cr³⁺are easier to combine with Fe³⁺due to lower electronegativity,inhibiting the formation ofμ-oxo dimer and ferrihydrite nuclei Increase of structure defects in ferrihydrite nuclei caused by Al³⁺and Cr³⁺will provide more sites for the formation and growth of ferrihydrite,which leads to faster growth rate of ferrihydrite.2)The effect of Al³⁺substitution on the transformation of ferrihydrite to hematite was studied.The experimental results show that not only the attachment of particles but also the Ostwald Ripening process exist during the transformation of ferrihydrite to hematite.During the transformation of ferrihydrite to hematite,structural topological transformation occurred,and face-sharing Fe-Fe bonds were formed,resulting in shorter Fe-Fe₁ bond length.Al³⁺will hinder the transformation rate of ferrihydrite but will enhance the formation of face-sharing Fe O₆ octahedron.3)How structure modification caused by Al³⁺substitution leads to the change of hematite morphology is studied.Al³⁺substitution will cause the shift of XRD peak position to high angle and decrease of cell parameters.Al³⁺substitution will cause larger increase of O-O and Fe-Fe bonds length in a-b plane of hematite than the increase of O-O and Fe-Fe bonds length along c-plane,resulting in the density of Fe and O planes in a-b plane decreases less than that along c-axis.The change of Fe and O density results in the change of hematite morphology from rhombohedral to disk-shape plate,and the change of main crystallographic plane from（104）to（110）.4)The effect of different metal cations substitution on the structure and properties of hematite was studied.Substitution of Cr³⁺leads to decrease of cell parameter a and slight increase cell parameter c,while substitution of Ni²⁺,Cu²⁺,Zn²⁺,Pb²⁺cause increase cell parameter a and c.Significant hydroxylation happens in Ni²⁺,Cu²⁺substituted hematite due to the formation of structural defects and imbalance charge caused by low valance;while weaker hydroxylation happens in Cr³⁺substituted hematite only due to the formation of structural defects.Substitution of Cr³⁺,Ni²⁺,Cu²⁺,Zn²⁺,Pb²⁺will cause the increase ratio of face-sharing Fe-Me bond length to edge-sharing Fe-Me bond length,leading to the morphology of hematite change from rhombohedral to disk-shape plate.5)The effect of substitution of manganese ions with variable valences on the structure and properties of hematite was studied.The substitution of Mn²⁺in hematite is mainly in the form of Mn³⁺.Substituted hematite with low Mn²⁺addition also contains a small amount of Mn²⁺and Mn⁴⁺;while substituted hematite with high Mn²⁺addition only contains a small amount of Mn⁴⁺.Increase addition of Mn²⁺results in different contents of Mn with different valences,leading to cell parameter and the ratio of face-sharing Fe-Me bond length to edge-sharing Fe-Me bond length increase first,then decrease.