The Effect And Mechanism Of Controlling Heavy Metals In Soil Of Rare Earth Mining Area By Navel Orange Peel Biochar Supported Graphene Oxide

During the mining process of ion-absorbed rare earth mines,the associated typical heavy metals were migrated and released to the surrounding water and soil under hydraulic and chemical disturbances,which poses a threat to the regional ecological environment.Therefore,it has become a research hotspot and difficulty to explore the mechanism and technology of heavy metal pollution control in rare earth mining area.Biochar can provide an effective new way for the treatment of heavy metal pollution because of its good adsorption performance.However,there are some problems in the single biochar materials,such as limited specific surface area,low porosity and few adsorption sites,which limit its wide application.At the same time,the influence of biochar on the accumulation and release of heavy metals and the analysis of the process of heavy metal migration and transformation in rare earth mining soil by mathematical model should be further studied.In this paper,the biochar supported graphene oxide（BGO）material was prepared by using navel orange peel and natural graphite as raw materials.The effects of single factor conditions on the adsorption of Pb²⁺were investigated.The optimal adsorption parameters were analyzed by response surface methodology under the conditions of pH value,rotation number and dosage.the adsorption isotherm and kinetic model fitting experiments were carried out according to the optimal adsorption parameters,and the mechanism of BGO adsorption of Pb²⁺was explained using SEM-EDS,XPS,XRD,FTIR and other characterization techniques.The original ores and tailing soils of ion-absorbed rare earth mining areas were used.The change characteristics of occurrence form were compared by adding BGO materials.On this basis,the soil column leaching experiment was carried out to explore the change characteristics of heavy metal content in leaching filtrate and the vertical migration law of heavy metals in soil,and the accumulation and release model of heavy metals under leaching condition was determined.The main conclusions are as follows:（1）The BGO composites material were successfully prepared by improved Hummer impregnation co-pyrolysis method.Compared with virgin biochar,the adsorption capacity of Pb²⁺on biochar supported graphene oxide was significantly increased by 97.3%,78.4%and38.31%,respectively.The maximum adsorption capacity of BGO for Pb²⁺was 105.75 mg/L at pH 6,but the adsorption capacity of BGO for Pb²⁺was inhibited at pH≤4.With the increase of adsorbent dosage and ionic strength,the adsorption capacity of BGO for Pb²⁺first increased and then decreased.（2）The results of response surface methodology optimization design showed that the order of influence on adsorption of Pb²⁺was:pH>dosage>revolution.The optimal conditions were as follows:pH value was 4.97,revolution was 172.97 r/min,and BGO dosage was 0.086g.In the adsorption-desorption experiment,the desorption efficiency is between 54.3%and63.3%.The process of Pb²⁺adsorption by BGO is spontaneous and endothermic,mainly through electrostatic interaction and surface complexation.It is a heterogeneous adsorption process with heterogeneous surface,including surface adsorption,external liquid film diffusion and intra particle diffusion.（3）After 30 days of cultivation with biochar composite materials,the pH value and organic matter content increased,and the acid extractable Pb content in raw ore and tailings soil decreased by 17%and 8.6%,respectively.The concentration of heavy metals in leaching solution increased first and then decreased and tended to be stable with the increase of leaching time.Biochar composite material treatment can significantly increase the resistance and control effect of heavy metal migration behavior in soil.The accumulation and release characteristics of heavy metals in soil can be divided into two stages:rapid and slow release stage.（4）The release rate of heavy metals in the soil of the biochar composite material is lower than without addition.The modified Elovich can better describe the process of heavy metal release,and the optimal kinetic fitting model is different in the two stages,which shows that the release process of heavy metals in soil is not a single reaction process,but also a complex reaction process controlled by multiple diffusion mechanisms.The acid-extractable heavy metals can be migrated and are enriched in the lower layer.The results showed that the contents of Pb and Mn removed from the tailings soil were reduced by 2.5%and 28.4%,respectively,and the experimental group of the original ore soil decreased by 5.7%and 1.1%compared with the control group.Biochar composites materials can effectively inhibit the migration of heavy metals by increasing soil pH,surface complexation,and precipitation.