Crystallinity And Stability Of Iron Oxides Formed From Fe(Ⅱ)oxidation: The Influence Of Coexisting Minerals

Iron oxide is an active mineral component widely existing in soils.The oxidation of Fe（Ⅱ）in water often forms iron oxide（magnetite and maghemite）,iron hydroxide oxide（lepidocrocite and goethite）,and ferrihydrite with poor crystallization,which is one of the main ways to form iron oxides in nature.Many factors in the soil-forming environment,such as anions,organic matter and heavy metals,affect rates of Fe（Ⅱ）oxidation and the types,adsorbability of resulting iron minerals.However,previous studies on the effect of other co-existing minerals in soil on the oxidation and hydrolysis of Fe（Ⅱ）to iron oxides were lacking.Herein,the effects of five common minerals（Montmorillonite K10,Kaolin,Si O₂,Al₂O₃ and Ca CO₃）coprecipitated with two kinds of iron salts（FeCl₂ and FeSO₄）on the primary and aged products of Fe（Ⅱ）oxidation were investigated.The structure and properties of Fe（Ⅱ）oxidation products were analyzed by acid dissolution experiments,X-ray diffraction（XRD）,Brunauer-Emmett-Teller（BET）,Fourier transform infrared spectroscopy（FTIR）,X-ray photoelectron spectroscopy（XPS）,thermomagnetic curve（M-T）,hysteresis loops（HY）,thermogravimetry-scanning calorimetry curves（TG-DSC）,electron spin resonance（ESR）.The main conclusions of this thesis are as follows:（1）The initial synthetic iron oxides of FeSO₄-NaOH system are goethite and lepidocrocite,while magnetite and maghemite are formed in the primary products of FeCl₂-NaOH system.The aged products of these two systems are maghemite and hematite.Therefore,in the Fe（Ⅱ）oxidation process,Cl^-retards the oxidation of Fe（Ⅱ）.Both types of anions do not change the aging pathway of the primary products,but affect the proportion of mineral components,stability and magnetism of the aged products.（2）In FeSO₄ system,montmorillonite with high specific surface area accelerated the oxidation of Fe（Ⅱ）and promotes the formation of ferrihydrite.During the oxidation and crystallization of Fe（Ⅱ）,both Si and Al released by the dissolution of montmorillonite and kaolin inhibit the formation of FeOOH,but Si has a more significant inhibitory effect on lepidocrocite.Compared with FeSO₄ system,Si O₂,Al₂O₃ and Ca CO₃ all promote the formation of lepidocrocite during Fe（Ⅱ）oxidation.The types of aged products of the five coexisting mineral systems are consistent with those of FeSO₄ system,but the proportion of mineral components and properties of the aged products are different from those in FeSO₄ system.（3）In the Cl^--rich systems,kaolin and montmorillonite inhibit the formation of lepidocrocite compared with FeCl₂ system.Compared with FeSO₄ system,montmorillonite has less effect on the Fe（Ⅱ）oxidation products in FeCl₂ system,while kaolin has no significant effect on the Fe（Ⅱ）oxidation process in the two iron salt systems.The presence of Si O₂and Ca CO₃in FeCl₂ system promotes the formation of ferrihydrite and inhibits the conversion of ferrihydrite to hematite and goethite.Increasing the addition amount of Si O₂can inhibit the crystallization of lepidocrocite.The presence of Al₂O₃and Ca CO₃in FeCl₂ system,the formation of goethite is inhibited.Ca CO₃ in FeCl₂ system promotes the crystallization of lepidocrocite and magnetite.Compared with FeCl₂ system,Si O₂,Al₂O₃ and Ca CO₃ inhibit the oxidation of magnetite,hinder the formation of akaganeite and promote the formation of lepidocrocite.The proportion of mineral components and properties of minerals in the aged products are changed by the five coexisting minerals.