Effects Of Some Common Metal Ions In Soils On The Formation And Transformation Of Green Rust And Their Accumulation Speciation

Iron oxides are ubiquitous in soil environment,containing a large amount of metal ions.The iron oxides influence the environmental geochemical behaviors of metal ions by adsorption,co-precipitation and isomorphous substitution.However,during the formation processes of iron oxides by Fe²⁺oxidation,the accumulation speciation and mechanisms of different metal ions in iron oxides,especially its dynamic change during mineral conversion and the effects on the formation and transformation of green rust,have been less addressed.Green rust plays an important role in controlling the formation of crystalline iron oxides in the environment and the migration and fate of pollutants and nutrients with weak crystallinity,large surface area,high adsorption affinity and reduction activity,and unstable structure.Therefore,in this study,the effects of many different metal ions on the crystal structure changes,formation and transformation,and mineralogical mechanisms of metastable green rust were investigated by X-ray diffraction（XRD）and Rietveld refinement,X-ray absorption spectroscopy（XAS）,Fourier transform infrared spectroscopy（FTIR）,and transmission electron microscopy（TEM）and solution chemistry methods.From the viewpoint of environmental mineralogy and soil chemistry,it reveals the behaviors,processes,and relationships between green rust and various metal ions and improves our understanding of heavy metals fate in soils.The main results are listed as follows:1.Effects of Na⁺,Mg²⁺and Al³⁺with different ionic valence on the formation and transformation of GR2(SO₄^2-)The formation and transformation of GR2 in the presence of three typical different valence metal ions were studied systematically.In the systems where metal ions（Me）were added before GR2 formation（system A）,the formation rate of GR2 was Al³⁺>Na⁺>Mg²⁺at Fe/Me=24,and when Fe/Me=6,it was Al³⁺>Mg²⁺>Na⁺.From the XRD patterns when GR2 was completely formed,the characteristic peaks of GR2 didn’t shift,that means,metal ion substitution did not affect the lattice spacing of GR2.However,in the Al system,a small amount of weak crystallinity goethite appeared simultaneously,indicating that Al promoted oxidation transformation of GR2.The oxidation rate of GR2is consistent with the formation rate.In the systems where metal ions（Me）were added after GR2 formation（system B）,the oxidation rate was faster than that of system A.But at the same concentration,the order of the formation rate is similar to system A.The final products of GR2 were mixed with goethite and lepidocrocite under all conditions.System B had more goethite in the final products than system A under the same conditions.And bubbling N₂ into system B for a period of time could increase the content of goethite in the final products.When the Ion valence increased from one to three,the content of goethite and SO₄ adsorbed in the products increased,whereas the particle size gradually decreased.Consequently,in the co-hydrolysis-oxidation-precipitation process of Fe²⁺and Me,the GR2 doped with metal ions which have higher valence,smaller Ksp and bigger hydrolysis rate was inclined to be transformed into goethite,but the crystallinity and particle size decreased.2.Effects of Ni²⁺and Cu²⁺with different redox ability on the formation and transformation of GR2(SO₄^2-)The formation and transformation of GR2 in the presence of Ni²⁺and Cu²⁺with the same ion valence but different redox characteristics were studied.In the system A,both the formation and transformation rate were Cu²⁺>Ni²⁺,indicating that Cu²⁺whose oxidability was stronger than that of Ni²⁺,was more favorable to formation and oxidation transformation of GR2 and contributed to a faster rate.The higher metal ions concentrations were,the greater rates it had.In the system B,similar ion effects and concentration effects were emerged during the transformation of GR2.Compared with the control（Na）system,the GR2（001）plane peaks in the presence of Ni²⁺and Cu²⁺shifted to the left,indicating that Ni²⁺and Cu²⁺had entered the structure of GR2.The final transformation products of GR2 were mixed of goethite and lepidocrocite under all conditions.With increasing the metal ions concentrations,the crystallinity and particle size decreased.Compared to the system where Cu²⁺was added before GR2 formation at Fe/Cu=24,it was more favorable to the formation of geothite for the added-after.In the process of formation and transformation,Ni²⁺and Cu²⁺in the mineral were gradually released to solution until the end of the transformation,as the pH decreased.The acid dissolution kinetics showed that Ni uniformly distributed in the crystal structure of the products,while Cu was mainly enriched on the surface of iron oxides;With the lower concentrations of metal ions in the system B,the better crystallinity of the final products.Pseudo-first-order kinetics could fit well with the changes of Fe content and metal ion content in dissolution processes by HCl.The fitted rate constant was closely related to the crystallinity of the product.That means,the better crystallinity,the smaller dissolution rate constant.The above results revealed that the influence of two common transition metals in the environment on the formation,transformation,and accumulation mechanisms on iron oxides,which can provide a theoretical basis for the remediation of heavy metals contamination in soil or water circumstances.3.Effects of Mn²⁺on the formation and transformation of GR2(SO₄^2-)and its accumulation speciation and mechanismsThe effects of Mn²⁺on the formation and transformation of GR2 and their accumulation speciation and mechanisms were examined.In the system A,the crystallization of GR2 could be promoted at low concentration of Mn²⁺but retarded at high concentration.The effect of Mn substitution in the GR2 structure was not obvious.In the process of transformation,the high concentration of Mn²⁺and constant pH in system could promote Mn²⁺oxidation,and accelerate the transformation rate of GR2.With increasing the concentration of Mn²⁺,the ratio of lepidocrocite to goethite decreased slightly first and then increased significantly,and the particle sizes of lepidocrocite and goethite decreased.In the system B,constant pH could markedly decrease the ratio of lepidocrocite to goethite.Mn entered into the final products structure as the form of Mn（Ⅲ）.More content of Mn entered into lepidocrocte than goethite.These results have shed light on the effect of Mn on the process of formation-transformation and also provided a more in-depth understanding on the formation pathways and mechanisms of iron-manganese complexes in the environment.4.Effects of As（Ⅲ）and Cd（Ⅱ）on the transformation of GR2(SO₄^2-)and mechanismsAs（Ⅲ）or Cd（Ⅱ）added after the formation of GR2,under condition of the constant pH（7.3）,could enriched in the final products and significantly affectedtypes of final products.As（Ⅲ）could improve the stability of GR2 in the primary transformation stage,which implied a longer stable time.With the increase of As（Ⅲ）concentration,the final products changed from a mixture of lepidocrocite and goethite to pure lepidocrocite to mixed phased of lepidocrocite and ferric GR2（EX-GR2）,and the crystallite size of the final products gradually decreased.Bubbling extra air into the system for a period of time could decrease the crystallite size of the final products.Under all conditions,As（Ⅲ）in the solution was enriched by minerals and existed as the form of As（Ⅲ）,indicating As（Ⅲ）could be abundantly fixed and not be oxidized during the transformation of GR2.Extended X-ray absorption fine structure spectroscopy（EXAFS）results indicated that As（Ⅲ）formed inner-sphere surface complexes by bidentate mononuclear edge-sharing（²E）and bidentate binuclear corner-sharing（²C）on lepidocrocite.With the increase of Cd（Ⅱ）concentration,the final products changed from lepidocrocite and goethite to lepidocrocite.The great majority of Cd（Ⅱ）also entered into the final products during the transformation process,but it was slightly lower than the As（Ⅲ）system.Therefore,a large amount of pollutants could be enriched in green rust,which provides a possible way for contamination remediation in soil or water environment.5.Effects of Mn²⁺,Ni²⁺and Cu²⁺on the formation and transformation of GR1(CO₃^2-)The effects of Mn²⁺,Ni²⁺and Cu²⁺on the formation and transformation of GR1 were examined.Due to the different types of green rust and their pH and Eh variation rules during the process,these three metal ions showed some different effects on the two green rusts.Compared with the control（Na）system,the formation and transformation rate of GR1 could be increased in the presence of Mn²⁺,Ni²⁺and Cu²⁺and Mn system was the fastest.The crystallinity of GR1 could be promoted in the presence of Ni²⁺and Cu²⁺,at the meantime,there was also a small amount of GR2 formed.In the control（Na）system,the final product was acicular goethite and well crystallinity.In the case of Me systems,the crystallinity and particle size decreased and the final products changed from acicular goethite（Fe/Cu=24）to tabular lepidocrocite and acicular goethite（Fe/Mn=24）to mixed phases of platelets of ferric GR1（EX-GR1）,lepidocrocite and goethite（Fe/Ni=24）.Especially,in the Ni system,the crystallinity was the weakest and particle size was the smallest.The acid dissolution kinetics showed that Mn was located inside the iron oxides,Ni uniformly distributed in the crystal structure of the products,while Cu mainly enriched on the surface of the iron oxides.These results were consistent with the GR2 systems.