Kinetics Of Cation And Oxyanion Adsorption And Desorption On Ferrihydrite:Developing A Unified Model

With the rapid development of economy,the problem of environmental pollution is becoming more and more serious,which threatens people's life and health especially heavy metal pollution.industrial and agricultural production will produce many heavy metal which can cause serious pollution when they are discharged into soil and water.Heavy metal pollution in the environment mainly includes lead,copper,nickel,chromium,arsenic and cadmium.Ferrihydrite is a kind of iron hydroxide,which is widely distributed in soil,and it has a strong adsorption capacity to heavy metals in the environment due to its large surface area and strong surface activity,which can greatly restrict the transfer and transformation of heavy metals in the environment.Therefore,understanding the reaction of toxic ions and ferrihydrite in soil is very important for predicting the dynamic behavior of pollutants in soil environment.This study explored typical heavy metal ions Pb(?),Cu(?),Cr(?),As(?),Cd(?)and Ni(?)adsorption/desorption kinetics behaviors on ferrihydrite under different reaction conditions,and established a unified mechanic kinetic model based on CD-MUSIC.At the same time,the complexation characteristics of metal ions on two kinds of ferrihydrite aggeragate(gellike ferrihydrite and dense ferrihydrite)were respectively investigated.In this study,the adsorption/desorption kinetics experiments of Pb(?),Cu(?),Cr(?),As(?),Cd(?)and Ni(?)complexing on ferrihydrite were conducted by a stirred-flow method.The characteristics of morphology and distribution of metal ions on ferrihydrite were studied by using a high resolution scanning electron microscopy(STEM).A series of balanced adsorption experiments were carried out to improve the calibration of CD-MUSIC model and provided a basis for the development of kinetic model.The stirred-flow experimental results showed that metal ions adsorption and desorption kinetic reaction on ferrihydrite highly depended on reaction pH,initial metal concentration,ferrihydrite concentration and varied significantly among different metal.The STEM/EDS results showed that the ferrihydrite aggregate was a kind of porous and amorphous substance similar to a cloud.After a period of adsorption,the metal ions were rapidly complexed and distributed evenly on the microporous edge of ferrihydrite aggregates.The agglomeration structure of gellike ferrihydrite was relatively loose,but the dense ferrihydrite was thick and the combination of small ferrihydrite particles was tighter compared with gellike ferrihydrite.Due to the decrease of interstitial water and void content among dense ferrihydrite particles after freeze-drying,its open structure collapses irreversibly,therefore,the specific surface area and effective binding sites amount of dense ferrihydrite for complexation with heavy metals were relatively fewer than gellike ferrihydrite.The results of equilibrium adsorption experiment showed that CD-MUSIC can fit the equilibrium adsorption curves of metal ions on gellike ferrihydrite under different reaction conditions,and can also fit the equilibrium adsorption curves of metal ions on dense ferrihydrite through adjusting the ferrihydrite concentration,which provided a solid foundation for the development of dynamic model.The study about heavy metal ions Pb,Cu,Ni complexing on dense ferrihydrite found that the adsorption capacity and adsorption rate of dense ferrihydrite was far less than gellike ferrihydrite under the same experimental conditions,due to the tight aggregation structure of dense ferrihydrite caused by freeze drying and the decrease of surface area and effective complexing sites,as a result,metal ions could only be adsorbed on a small mount of binding sites,and could not reach and contact with most binding sites of dense ferrihydrite in short adsorption time.In this study,ferrihydrite concentrations in CD-MUSIC and kinetic model were adjusted by equilibrium adsorption experiment or initial debugging of kinetic model,which make kinetic model could explain the adsorption and desorption kinetics of heavy metal ions Pb,Cu and Ni on dense ferrihydrite.Based on the experimental results,this study developed a unified kinetic model for oxyanion/cationic ions adsorption/desorption on ferrihydrite based on equilibrium model CD-MUSIC.The kinetic model could describe the kinetic experimental results of gellike ferrihydrite and dense ferrrihydrite,and separately considered the role of vrious ferrihydrite site in the complexation of metal ions on ferrihydrite.The equilibrium property of metal ions complexing on ferrihydrite was combined to the kinetic model through established the following two relationships: the relationship between the adsorption /desorption rate coefficient and the equilibrium distribution coefficient predicted by CD-MUSIC;and the relationship between the surface complexation constant in CD-MUSIC model and the desorption rate coefficient.The adsorption and desorption rate coefficients of the kinetic model were obtained through model optimization.The establishment of kinetic model would be conducive to predicting the influence of ferrihydrite on dynamic behavior of heavy metals in environment,and it was of great significance to the environmental restoration and risk assessment of heavy metal pollution.