The Effects Of Cd,Co And Zn Substitution On Lepidocrocite?Feroxyhyte And Akaganeite

Several kinds of iron?hydro?oxides,including goethite??-FeOOH?,hematite??-Fe₂O₃?,lepidocrocite??-FeOOH?,akaganeite??-FeOOH?and feroxyhyte??-FeOOH?exist in soils and sediments.Though many studies have conducted on heavy metal substitution in goethite and hematite,few studies have been conducted on cation substitution for Fe in lepidocrocite,akaganeite and feroxyhyte yet.In this work,substitution of Cd,Co or Zn for Fe in lepidocrocite,akaganeite and feroxyhyte were systematically studied.The main conclusions contained are as follow:1.Cd can be incorporated into feroxyhyte?1.59 mol%?,but failed to enter the lepidocrocite??-FeOOH?,akaganeite??-FeOOH?.When the initial Co/Fe ratio was consistent,the largest substitution level of Co in lepidocrocite?2.03mol%?was far lower than that in feroxyhyte?14.55mol%?,indicating that Co showed a significant different substitution among the three minerals.There were approximately⁸%of Zn entering the lepidocrocite and feroxyhyte:In the lepidocrocite,⁶0%was attached to the surface,and the other⁴0%was incorporated into the structure of lepidocrocite,and the crystal particles became significantly smaller as the Zn content increased.In the feroxyhyte,the ratio of these two forms was⁹0%and¹0%,and the crystal particles also showed a tendency to decrease as the Zn content increased.Almost none of the three heavy metal ions can enter into the akaganeite.The substitution of the same metal in different types of FeOOH are quite different.Among them,feroxyhyte is the easiest one for metal ions incorporation,followed by lepidocrocite,and then the akaganeite.This is greatly related with the mineral structures.According the order of feroxyhyte,lepidocrocite and akaganeite,the proportion of edge-sharing FeO₆ octahedra gradually decreases,while the proportion of corner-sharing FeO₆ octahedra coordination increases.The freedom of the space become smaller and the numbers of removable Fe-O bonds also decrease.Therefore,metal substitution is easier to occur relatively in minerals with greater spatial freedom such as layered structures,2.Co substitution in lepidocrocite decreased the unit cell parameter c.As the Co?II?gradually changed to Co?III?when the doping amount is 1.09 mol%,the radius became smaller and formed a smaller cobalt-oxygen octahedron,so the unit cell parameters a,b increased first and then decreased.Zn substitution inhibited the growth of the lepidocrocite along the a-axis and b-axis,causing structural defects.The addition of Cd,Co and Zn made the area of the lepidocrocite particle larger and the density of interlacing between the layers smaller.3.In feroxyhyte,the Cd substitution increased the unit cell parameters a and c and had a greater effect on c.Since the radius of Cd²⁺?0.95??is much larger than that of Fe³⁺?0.645??,the Cd substitution made the original ferrite octahedron become a larger Cd oxygen octahedron.It can also be seen from the SEM results that the area of the feroxyhyte particles are obviously increased,and the irregular shape is changed into an irregular sheet.The Co substitution decreased the unit cell parameters a and c,the area of particles decreased first and then increased.When the doping amount reaches 12.19 mol%,the layered stack changed to a staggered stack.The effect of Zn on feroxyhyte was different from the other two cations.The addition of Zn reduced the area and the size of particle,but the minerals had still layered structures.