The Sequestration Mechanism Of Typical Iron Oxides To Heavy Metals And Its Application In Tailing Passivation

A large number of metal sulfide tailings are accumulated in the tailings pond.These tailings often migrate to the environment through dust,surface runoff and underground runoff,and caused a serious heavy metal pollution.Adsorption and surface passivation are very effective methods to reduce the migration and bioavailability of heavy metals in tailings.Iron minerals and natural organic matter are widely present in the epigenetic environment,which have a great significance to the form,migration and transformation of heavy metals.Therefore,it is necessary to explore the dynamic interaction among iron minerals,organic matter and the heavy metals for understanding the environmental geochemical process of heavy metals in mine tailings.This can also provide theoretical and technical support for the passivation of heavy metals in metal sulfide tailings.Although the coupling of organic matter and iron minerals on the environmental behavior of heavy metals has been studied,the mechanism and quantitative of the coupling reaction of heavy metals on interface between iron minerals and organic carbon is still poor.In this study,the characterization of 3D-EEM,Q-Exactive LC-MS/MS,DFT,Zeta potentials,AAS and UV-vis,XRD and HRTEM were used to elucidate the influence of microscopic mechanism among organic matter,iron mineral and heavy metals（Cr,Cu）.Besides,the application of iron mineral materials in the passivation of mine tailings was investigated.The major findings of this paper are as follows:（1）Small molecular organic matter was preferentially fractionated on the surface of ferrihydrite.Chromate mainly exists in the form of HCr O₄^-under acidic conditions,and this made it easier been adsorbed on the surface of ferrihydrite and organic matter through electrostatic attraction.DOM fractionation on the surface of ferrihydrite can reduce Fe（III）to Fe（II）,and then promote the reduction of Cr（VI）to Cr（III）on the surface of ferrihydrite.The transformation of ferrihydrite showed that ferrihydrite was mainly transformed to goethite and hematite after the reaction of DOM and chromate.Besides,bracewellite（Cr O（OH））generated as an accompanied mineral during the transformation of ferrihydrite in the present of Cr（VI）.Based on the DFT theoretical calculation,we concluded that Cr（VI）mainly in the form of HCr O₄^-was more feasible to be adsorbed on iron-oxide tetrahedron by inner-sphere monodentate mononuclear configurations.（2）The mineralization of iron minerals mediated by FA and the fixation mechanism of hydrated copper ion on the microscopic interface of iron minerals and FA were revealed.The results of characterization suggested that with the increase of FA content,the coprecipitates minerals of FA-Fe（III）from goethite（α-Fe OOH）to hematite（α-Fe₂O₃）and then to amorphous minerals.The results of m?ssbauer spectroscopy indicated that with the increase of FA content,the octahedral structure will collapse and tetrahedral structure will increase in the iron mineral.Chemical extraction experiment and interaction region indicator explained that van der Waals force and the weak hydrogen bond（C-H...O）)were the major interaction between FA and iron minerals.Copper ions adsorbed and precipitated on the surface of iron mineral were mainly in the form of Cu-O functional group or have coordination reaction with FA on the surface of iron mineral.The signals of Cu have a good correlation with C,Fe,O on the surface of iron mineral.This indicated the adsorption of Cu（II）on the surface of iron minerals were homogeneous.Finally,DFT theoretical calculation revealed the microscopic molecular mechanism between hydrated copper ion and iron minerals.The results showed that the bidentate binuclear coordination of hydrated copper ion with iron minerals and carboxyl functional groups of FA were more stable than other coordination models.（3）The adsorption mechanism of copper ions on{001}and{100}facet hematite crystal was elucidated.The results of adsorption kinetics showed that the adsorption process of Cu（II）on HNRs and HNPs were mainly controlled by chemisorption,and the adsorption sites were evenly distributed on the hematite surface.The sequestration of FA on mineral surface caused HNRs have the lower Zeta potential and smaller particle size than HNPs at the same p H,and this suggested Cu（II）was more feasible adsorbed on the surface of HNRs.The signals of C,Fe,O and Cu are highly correlated.The signal of Cu at the edge of HNPs was obviously stronger than that in other regions,indicating that copper ions may be adsorbed on the edge of HNPs through surface precipitation.On the other hand,the signal of Cu on HNRs surface was highly correlated with the signal of Fe,O and C,indicating that copper ions can also be fixed on the surface of iron minerals though the coordination with FA.（4）The main mineral compositions of the tailings of CDK and TL were kaolinite,goethite,calcite,quartz,magnetite and hematite et al.The total element analysis of minerals by ICP-MS showed that Fe,Si,S,K and Ca were the main elements in tailings,while Mn,Cu,Zn,Cr and Pb were the main heavy metal elements.The leaching experiments of Mn,Zn and Cu in CDK and TL tailings samples was studied by iron mineral and organic matter complex（Fe-FA（8m M））.The results showed that the accumulative contents of Mn,Zn and Cu during the leaching experiment were basically consistent with the increase of time,and the leaching of heavy metals were obeyed Mn>Cu>Zn;The passivation efficiency of Fe-FA（8 m M）to leaching Mn,Zn and Cu of CDK and TL tailings are 85%,86%,95%and 88%,85%and 95%,respectively.