Competitive Incorporation Of Al And Cd In Goethite And Local Environment Around Cd

Goethite is one of the most common and major Fe oxides in the natural soil environment.It has a high specific surface area and large surface reaction site density,always serves as high efficient absorbent of many nutrients and inorganic and organic pollutants.Also goethite is of great significance in terms of migration and transformation of heavy metals and bioavailability.With regards to the goethite in heavy metal contaminated soil in the south,it can be associated with sveral foreign elements except Al such as Cd incorporated into the structure of goethite by isomorphous substitution.In laboratory,the single metal incorporation in goethite,eg.Al,Cd,have been investigated and there was no study about co-incorporation of these metals.A comprehensive understanding of multi-metal sunstitution in goethite is required.This paper used the homogeneous precipitation method to synthesize different series of Al,Cd-containing goethite samples which were conducted under the room temperature and high PH value.The main object of this work is to investigate the substitution of Fe(III)by Al(III)and Cd(III)in goethite and the effects of its consequence changes of both ions on the competitive relationship of Al and Cd co-substitution and the Cd configuration in goethite.The main related results are as follows:1.When the total initial metal content of Al and Cd was kept constant(Al+Cd=x mol%),with increasing Cd and decreasing Al initial content,the amount of Cd has a significant negative correlation with ? Al.When the Al content was kept constant(Al=4,10 mol%),Al substitution decreased(? Al increased)with the increment of Cd substitution and Cd substitution was unaffected by the nominal content of Al.When Al and Cd content was the same,Cd substitution has a significant positive correlation with ?Al.2.When Al and Cd existed,the Rietveld refinement results of all samples indicates that unit cell parameters b,c and volume all have a significant positive correlation with Cd substitution.Unit-cell parameter a for some series samples(Al+ Cd= 20%,Al=x mol%)was closely related with Cd substitution.Overall,cell parameter a has a tendency to increase.3.The SEM and TEM images show that when the initial content of Al and Cd was kept constant,length of particles shortened and width remained a little change with the increase in Cd content and decrease in Al content.Al and Cd inhibit crystal growth.Besides,the inhibiting effect of Cd on crystal growth dominants over the facilitation of Al on crystal growth.When Al and Cd concentration was maintained the same,with the increase in Al and Cd,the particles of goethite transformed from thin and long needles to short and broad rod-like.4.When the total content of Al and Cd was kept constant at low level(5%,10%),with the increase in Cd concentration and decrease in Al concentration,HOH of water bending frequencies decreased and ? O-H band gradually increased.The ? O-H and ? O-H shows no obvious change.The IR band of Fe-O shifted to lower wavenumbers;When Al content was kept constant,with the increasing Cd concentration,IR band at 3417cm-1and3117cm-1shifted to higher wavenumbers,and the higher the Al content,the greater the increment of the wavenumbers.The H-O-H bending and the Fe-O strenching frequencies decreased with the increase in Cd substitution.When Al and Cd content was kept the same,with increasing Al and Cd content,H-O-H strenching band shifted to higher wavenumbers and water bending IR band kept unchanged.But when Al concentration increased to 10%,the water bending frequencies decreased by 30cm-1,in-plane O-H band increased,difference between ? O-H and ? O-H decreased.The Fe-O band at 617cm-1shifted to lower wavenumbers.5.When Al content was kept constant,Cd starting from a nominal content of 0 to 8mol%,Fe-O distances gradually decreased,E(edge-sharing Fe-Me1 distances)and E'(edge-sharing Fe-Me2 distances)had a marked decrease and DC(corner-sharing Fe-Me3distances)had a slight increase.There was a gradual increase in E/E'.When the concentration of Cd increased to 10 mol%(Al10Cd10),Fe-O distances enlarged,but when compared with pure-goethite,the distances were still a little shorter,and E,E' and DC all had a distinct increase.When compared Al2Cd8 and Al10 Cd8,it indicates that the increase in Al concentration will lead to a decrease in E,E' and DC,from 2.984 to 2.978?,3.159 to 3.135 ?,3.397 to 3.389 ?,respectively.E/E' increased from 0.945 to 0.950 ?.According to the Cd K-edge EXAFS,for samples with high Cd content,the configuration of Cd shows an obvious change compared with those with low Cd content.Some Cd were deduced in the vacant double rows but not the middle of geometric center site.