Isomorphous Substitution Of Cd/Zn In Goethite And The Effect On Structure And Properties

Iron oxide often accumulates various heavy metal ions.Adsorption and isomorphous substitution are two of the main association mechanisms of heavy metals with iron oxides.Although there have been some reports on the effect of isomorphous substitution of Cd and Zn on the structure of goethite,it was generally tested at a higher temperature?70°C?.The soils in tropical and subtropical areas are mostly at atmosphere temperature,then it is not clear whether Cd and Zn can isomorphous substitute the iron in goethite at environment-related temperatures,what amount of lattice can be substituted,and how the effect of the metal substitution on the tunnel size and morphology in the goethite structure;Further,the synergetic or antagonistic effects of coexisting Cd and Zn on substitution for lattice Fe in gothite and on the mineral structure and properties has not been reported yet.Relevant research on the above issues will help to have a better understanding the forms of Cd and Zn in iron?hydr?oxides and the transformation and fate of Cd and Zn in contaminated soil and also provide a useful strategy to investigate the speciation of heavy metals in complex matrixes.In this paper,a series of different gradients of single Cd-or Zn-or co-substituted goethite were prepared at RT by co-precipitation method,taking Cd and Zn single-substituted goethite synthesized at70?as a control,and powder X-ray diffraction?XRD?,elemental analysis,transmission electron microscopy?TEM?,acid dissolution experiments,synchrotron X-ray absorption fine structure spectroscopy?XAFS?,and high-energy X-ray total scattering?PDF?,etc.were used to study the effect of single-or co-doping on the structure and properties of goethite,the crystal chemistry of Cd in goethite as well as the effect of crystallization temperature on the structure and properties of single Cd and Zn substituted goethite.The main results obtained are as follows:1.Treatment of the Cd-Fe coprecipitates obtained by co-precipitation method at room temperature with acidic acid oxalate/ammonium oxalate solution could not completely remove non-crystalline or weak crystals.There were still some ferrihydrite residues and underdeveloped small goethite particles in the precipitate,the proportion of which were gradually increased with the increase of Cd content.The dissolution reactions of these samples could be divided into two processes,1)the dissolution of ferrihydrite and small particle goethite,and 2)the dissolution of crystalline goethite minerals.The relative proportions of goethite and ferrihydrite in each sample and the relative contents of Cd in these two components were quantitatively analyzed by combining powder XRD quantitative phase analysis,dissolution experiment,Fe K-edge and Cd K-edge extended X-ray absorption spectroscopy linear combination analysis.The increase in the peak intensity at 4.04?of the atom pairing distribution function pattern of these treated samples was probably caused by the presence of Cd?OH?₂in the precipitate with high Cd content,rather than the presence of Cd²⁺in the 2×1 tunnel in goethite was ruled out.2.After aging under normal temperature or 70°C for 72 h,both Cd²⁺and Zn²⁺can replace lattice Fe³⁺in the goethite structure.After further treatment of the oxalate treated Cd-Fe coprecipitates by 2 mol/L nitric acid solution,pure Cd or Zn isomorphous substituted goethite can be obtained.The maximum doping amounts of Cd were 4.26mol%and 9.08 mol%at 25°C and 70°C,respectively While those for Zn were 7.98mol%and 7.31 mol%respectively.The increase in temperature contributed to the enhanced isomorphous substitution of Cd in goethite,but had little effect on the substitution of Zn.The cell parameters of Cd substituted goethite obtained at both temperatures were positively correlated with the increase of Cd doping.Increasing aging temperature changed the influence trend of Zn doping on the lattice parameter a and cell density?.Since bond dissociation energy of Me-O is decreased in the order of Fe-O?407k J/mol?>Zn-O?<250 k J/mol?>Cd?236 k J/mol?,the mineral dissolution rate was increased with the increase of Cd and Zn doping level.,Zn substituted goethite with high content obtained at 70?was easier to be dissolved than that of Cd substituted analogue.3.At 25?,Cd inhibited the overall growth of goethite crystals,but the aspect ratio remained basically unchanged;while at 70°C,the goethite crystal particles changed from rod-shaped to longer-length spindle particles with the increase of Cd doping.At room temperature or 70?,Zn doping had almost no effect on the morphology of goethite.Because Zn had different effects on the growth of different crystal planes,the aspect ratio of goethite crystals was reduced During the change from the rod to needle.When Cd²⁺and Zn²⁺replaced Fe³⁺in the goethite structure,it had a certain impact on the local environment of Fe³⁺and the goethite tunnel structure.The incorporation of Cd reduced the average coordination distance of Fe-O,shrank the[Fe O₆]octahedron,and reduced E'?Fe-M?/CS?Fe-M?,and changed the 2×1 octahedral tunnel void in the goethite structure accordingly.However,Zn doping had almost no obvious effect on the coordination distance of Fe-O and E?Fe-M?for samples obtained at room temperature or70?,but the ratio of E'?Fe-M?/CS?Fe-M?decreased slightly in the samples obtained at25?,while the ratio first decreased and then increased for samples synthesized at 70?.However,there were only minor changes.4.Cd and Zn could co-substitute for lattice Fe in goethite during crystallization at25?.In the co-substituted system,the maximum total doping amount could reach 7.97±0.02 mol%.As compared with Cd²⁺,the ionic radius of Zn²⁺is closer to that of Fe³⁺,Zn²⁺and Fe³⁺have a smaller lattice energy difference,and Zn²⁺has a larger thermodynamic distribution coefficient in goethite,so Zn²⁺preferred to replace lattice Fe than Cd when both cations existed.While Zn had an inhibitory effect on Cd incorporation,low concentration of Cd had an inhibitory effect on Zn but high content of Cd promoted the Zn incorporation into the goethite structure.When the total doping level was very low,the goethite morphology was mainly affected by Zn,and the crystal particles became longer.When the total doping amount was high,the goethite crystal grew unevenly with more fine particles.When the Zn doping amount was low,the change of the unit cell parameters of goethite was mainly affected by Cd.When the Zn content was high,Zn dominated the change trend of the unit cell parameter a.The unit cell parameters b and c both increased with the increase of the total substitution amount,which was the result of mutual effect of Cd and Zn.The co-substitution of Cd and Zn weaken the mineral chemical stability.The dissolution rate was mainly affected by Cd content.In co-substituted goethite,Cd and Zn were evenly distributed inside the mineral structure,and Cd was easier to be dissolved than Zn.With the increase of the total substitution amount,the average bond length of Fe-O tended to increase,E'?Fe-M?/CS?Fe-M?tended to decrease,and the 2×1 octahedral tunnel void in goethite structure changed accordingly.Because Cd²⁺has a large ion radius,the shrinkage and distortion of the goethite structure caused by the substitution of Cd made E?Zn-M?,E'?Zn-M?and CS?Zn-M?negligible change,while Cd-O and E'?Cd-M?and CS?Cd-M?in the Cd coordination were obviously lengthened.