The Effects Of Co And Al Substitution On Goethite And Hematite

Goethite and hematite are both the most common and stable iron oxides in the soil of southern China.They have characteristics such as small particle sizes,large specific surface area,structural defects and many active groups,and thus can adsorb many inorganic and organic contaminants.These adsorption behaviors have been widely studied in literatures.Many metal cations(Al³⁺,Ni²⁺,Cu²⁺,Zn²⁺,Cd²⁺,Pb²⁺,Cr³⁺,Mn³⁺and Co³⁺)can enter into the crystal lattices of goethite and hematite by isomorphous substitution of Fe³⁺,due to their similar properties and radius.However this phenomenon is not much focused.When redox conditions in soils change（such as dry-wet alternation,agronomical cultivation,and fertilization）,the metal elements in iron oxide may be released to the soil.Especially redox-active metal ions are more sensitive to the redox environment.Aluminum is the most common exotic cations existing in goethite and hematite minerals.However few studies were conducted on the interactions mechanisms of iron oxides with co-substition of Al³⁺and Co.Therefore,in this paper,Co,Al,single and co-doped goethite and hematite samples with different dopant concentrations were synthesized,and then carefully characterized by XRD,TEM,SEM,XPS,XAS spectra（EXAFS and XANES）.The effects of single or co-doping on the structure and properties of the mineral,the interaction mechanisms of Co and Al,and the valence of Co in these samples were also studied.The main results are as follows:1.Substitution of cobalt in goethite significantly increased the incorporation of aluminum while substitution of aluminum had almost no effect on the substitution of cobalt for Fe.In hematite,Co or Al substitution had no significant effect on the incorporation of Al or Co.When the total amounts of Co and Al was high,substitution of cobalt predominated over Al（Co 6.07mol%,Al 6.10mol%）in goethite（GCo5Al20）,while in hematite（HCo10Al10）,the actual substitution of cobalt and aluminum accounts for more than the initial addition（Co 11.68mo%,Al 14.10mol%）.2.When the substitution amount of cobalt in goethite was approximately 1 mol%,it mainly existed in the divalent form,and with the increase of the substitution,it changed to the trivalent state.The substitution amount of cobalt in hematite was⁹ mol%and it is mainly in a valence of+2.With the increase of the amount of substitution,it changed to+3.Co doping in goethite and hematite first increased and then decreased the unit cell parameters a,b,and c.This might be related to the change of the Co valence.3.Co-substitution of cobalt and aluminum in goethite reduced the aspect ratio（L/W）of goethite,and the particles became shorter and thicker,in which Al might play a major role.In hematite,the co-substitution generally made the hematite small round crystals.4.In goethite,both the single substitution and co-substitution decreased the bond lengths（d）of edge-sharing Fe-Me_E1（Me=Fe,Co,Al）and Fe-Me_E2,and corner-sharing Fe-Me_C.The ratio of dFe-Me _E2/dFe-Me_C also decreased.When the total replacement amount of Co and Al was high（GCo5Al20）,all d and dFe-Me_E2/dFe-Me_C decreased clearly,and there is a mutation in Fe edge k space.In the hematite,when aluminum single substituted and Co and Al co-substituted,the d of face-sharing Fe-Me_F increased with increasing the substitution of aluminum.The ratio of dFe-Me_F/dFe-Me_E increased.In the case of Co single substitution,dFe-Me_F,and dFe-Me_F/dFe-Me_E decreased.When Co and Al were co-substituted,dFe-Me_F,and dFe-Me_F/dFe-Me_E first increased（0.96774 to0.97729）,then decreased（0.96017）,and finally changed little（0.960⁰.961）with the increase of substitution of cobalt.