The Effects Of Ni(?) And Al(?)substitution On Goethite And Hematite

Goethite and hematite are common iron?hydro?oxides in soils.They have large specific surface area,strong adsorption capacity,and thus play a very important role in the transformation and migration of trace substances?various inorganic and organic pollutants?in the natural environment.In heavily contaminated red soils around mining areas in southern China,as well as in the polluted paddy soils,goethite and hematite are commonly associated with various metal contaminants by adsorption and/or isomorphous susbtitution,which would greatly modify the structure and physicochemical properties of the minerals.The Al substituted in goethite and hematite are commonly observed and have been widely studied.In addition to Al,other heavy metal elements such as Ni have also received attention.There have been detailed reports on the substitution of Ni or Al in goethite and hematite under the laboratory conditions.However,these studies were limited to the single substitution of goethite or hematite by Ni or Al and did not consider the co-substitution,which is commom in heavy metal polluted systems.In this work we synthesized goethite and hematite with single substitution and co-substitution of Ni and Al,and EXAFS,XRD structural refinement,TEM and SEM techniques were used to study the interactions between Ni and Al and the effect on the physicochemical properties of the minerals.The main conclusions obtained are as following:1.Both Ni²⁺and Al³⁺can be incorporated into goethite and hematite structures through isomorphous substitution.The largest single substitution of Ni in goethite?3.0mol%?is far lower than the largest single substitution in hematite?8.6mol%?.This is because goethite is formed by the dissolution and recrystallization of ferrihydrite,while hematite is formed by the topological conversion of ferrihydrite,and the loss of Ni²⁺in the latter process is much smaller.Although the difference between the radius of Ni²⁺and Fe³⁺?7%?is much smaller than Al³⁺and Fe³⁺?21%?,Al³⁺is preferred for the substitution of Fe in both goethite and hematite.2.Increase of Ni or Al substitution increased the chemical stability of goethite.In the single substitution system,Ni is uniformly distributed in the goethite structure and slightly enriched in the surface layer,and Al is more likely to accumulate in the inner part;Ni and Al co-substitutionwill cause Ni and Al to be more easily dissolved from goethite.Ni had a greater influence on the dissolution behavior of Al when co-substituted.In the single substitution system,Al substitution results in a decrease of unit cell parameters a,b,and c,and Ni substitution results in an increase of unit cell parameters a,b,and c,and the unit cell parameters b and c had significantly linear relationship with the substitution amount of Ni or Al.In addition,the linear correlation between the unit cell parameter a and the substitution amount is reduced due to the structural defects inducedby substitution.In the co-substitution system,the unit cell parameters a and c are greatly affected by Al³⁺,while the unit cell parameter b is mainly affected by Ni²⁺.In the single substitution system,Ni substitution will not change the crystal morphology of goethite significantly,but Al substitution will make the crystallinity of goethite decrease.This effect has led to increase the width and reduce the aspect ratioof goethite particles.Ni and Al co-substitutionat high content will inhibit the nucleation and growth of goethite crystals,leading to a significant deterioration of the crystallinity.In the single substitution and co-substitution system,the increase in Ni substitution results in an increase in the ratio of Fe-MeE₂/Fe-MeCS?Me=Fe,Ni,Al?distances in goethite octahedra,while Al substitution results in a decrease in this ratio.This might be ascribed to the decrease of the ionic radius in the order Al³⁺?0.545??<Fe³⁺?0.645??<Ni²⁺?0.69??,which resulted in inconsistent effects of Ni²⁺and Al³⁺on Fe-MeE₂ and Fe-MeCS,and the corresponding changes in the 2×1 tunnel in the goethite structure.Especially when Ni,Al co-substitution at high content?GNi5Al20?caused a large distortion of the goethite structure.3.When Ni and Al substituted in hematite,Ni is uniformly distributed in the hematite structure,and Al is accumulated in the innerof hematite.The substitution does not significantly change the dissolution properties and stability of hematite,and Ni²⁺,Al³⁺does not affect the dissolution behaviors of Al and Ni in the mineral.Whether in single substitution or co-substitution,the unit cell parameters a and c of the hematite increase with the increase of Ni substitution,and decrease with the increase of Al substitution,and the effect of Ni²⁺on the cell parameters is greater.In the single substitutionsystem,the substitution of Ni has no obvious effect on the morphology of the hematite;when the Al content increases to a certain extent?HAl10?,the morphologychanges from rhombic to disk-like shape.when Ni,Al co-substitutionin hematite,the increase of Ni and Al content will make the face-sharing?alongc axis?Fe-Me?Me=Fe,Ni,Al?distance more significantly increase than the edge-sharing?in a-b plane?Fe-Me distance?increasing the ratio of Fe-MeF/Fe-MeE distance?.This effect caused a decrease of reticular densities in the directions parallel to c-axis and then favors the growth of the crystal in the direction of the a-b plane.As a result,the size of the hematite crystal in the c-axis?Crystal thickness?becomes smaller and the dimension in the a-b plane?Crystal width?becomes larger.And when the ratio of Fe-MeF/Fe-MeE distance is increased to a certain extent,the morphology of hematite is changed from rhombic to circular.When Ni and Al are substituted together in a relatively low content?HNi5Al5?,the morphology of hematite can be changed from rhombic to circular,which cannot be caused by single substitution of Ni?HNi5,HNi10?or single substitution of low content of Al?HAl5?.