Study On Passivation Efficiency And Mechanism Of Fe-based Biochar For Cadmium And Arsenic

The economic rapidly development cost a large number of heavy metals discharged into the environment.Heavy metal pollution of farmland soil will cause harm to the health of livestock and human beings through the food chain.Soil management is extremely urgent.Biochar,especially with heavy metal oxides,which has better adsorption performance for heavy metals,has been widely used as environmental remediation material in recent years.In the treatment of Cd and As pollution in soil,biochar has a superior passivation and remediation effect on heavy metals.In this paper,rice straw was used as raw material to prepare iron-based modified biochar（Fe@BC）by stirring and impregnation method.The adsorption effect of Fe@BC on Cd and As in solution was investigated,and the adsorption mechanism was preliminarily explored.Fe@BC was applied to the passivation and stabilization remediation of Cd-As co-contaminated farmland soil.And the passivation and stabilization efficiency was studied.To provide technical reference for remediation of heavy metal contaminated soil.The main research contents of this paper were as follows:（1）A new type of iron-based biochar（Fe@BC）was prepared by stirring impregnation method.Scanning electron microscopy（SEM）,X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FTIR）and X-ray photoelectron spectroscopy（XPS）were used to characterize the morphology,structure and elemental composition.The results showed that compared with BC,Fe@BC had larger specific surface area and more functional groups such as:C-O,C-H and Fe-O.（2）Batch adsorption experiments were conducted to investigate the effects of Fe@BC on the adsorption of Cd²⁺and As³⁺by Fe@BC.The adsorption kinetics,isotherm and thermodynamics were analyzed to clarify the process and mechanism.The results showed that comparing with BC,the Fe@BC on Cd（II）and As（Ⅲ）had better adsorption performance,and the maximum adsorption capacities is 131.93 mg/g and 98.79 mg/g,respectively.The isothermal adsorption curve accorded with the Freundlich model.The adsorption kinetics of both Cd²⁺and As³⁺by Fe@BC conformed to the pseudo-second-order kinetics model.The adsorption rate was mainly controlled by chemisorption.Thermodynamic results show that the adsorption process was spontaneous endothermic.The removal mechanisms of Cd²⁺and As³⁺by Fe@BC mainly include precipitation,ion exchange,complexation of surface functional groups of biochar with Cd and As,and physical adsorption.Fe@BC is a potential adsorption material for Cd²⁺and As³⁺.（3）The remediation effect of Fe@BC on Cd-As compound contaminated farmland soil was investigated by soil culture in laboratory.The results showed that the available state contents of Cd and As in soil decreased significantly after adding 2%Fe@BC.After 120 days of culture,the best passivation efficiency for Cd and As,were anaerobic Fe@BC（62.41%）,aerobic-Fe@BC（45.61%）,respectivetly.Both BC and Fe@BC increased soil p H value,SOM and CEC,while the soil Eh value decreased.The application of Fe@BC significantly increased the available phosphorus and rapidly available potassium in soil,and the soil fertility was improved.（4）for further proven Fe@BC into after the passivation mechanism of heavy metal contaminated soil,the morphology distribution of Cd and As in soil,the distribution of Fe and both correlation analysis were invesgated.The result showed:the Fe@BC promoted the conversion of Cd and As in soil from active state with high bioavailability to stable state such as Fe oxide bound state,crystalline Fe bound state and residue state,which significantly reduced the migration and biotoxicity of Cd and As in soil.Fe@BC increased soil Fe（Ⅱ）content,reduced the amorphous iron oxide,free iron oxide and complexing iron oxide content,to reduce the leaching and toxicity of heavy metals in soil.Fe（Ⅱ）and complexing iron oxide showed significant negative correlation relationship with each morphology of Cd and As,the amorphous iron oxide were positively correlated with exchange-Cd,carbonate-Cd,as well as exchange-As and the combination of crystalline-iron-As,indicating that the soil Fe forms are the important factors that affect heavy metal passivation and effectiveness.