Study On Passivation Remediation Of Thallium-contaminated Farmland Soil By Iron-manganese-based Biochar

Thallium（Tl）is a typical highly toxic heavy metal.Due to its lithophile and chalcophile properties,the mining activities of Tl-bearing mineral led to serious Tl enrichment in farmland soil near the mining area.In Lanmuchang area,Guizhou Province,extremely Tl pollution of the local soil is originated from special geological processes and mining activities.The contents of Tl in farmland soil near the mining area range from 40 to 124mg/kg.Rice,the main food crop,is widely planted locally.Its health risk caused by Tl pollution has adverse impact on local residents.Biochar loaded Fe/Mnoxide（MFBC）is selected to stabilize heavy metal in the farmland soil from this area.Biochar（BC）and blank sample（KB）are served as the control groups to analyze the stabilization effect of MFBC on Tl polluted soil.The passivation effect is measured by the content of Tl in the bioavailable state in soil（the extracted state of 0.5 M HCl）.Finally,A pot experiment was established to determine the accumulation of Tl in different tissues of rice before and after passivation,combined with the response of microbial community in rhizosphere soil.Thereby,it develops targeted interventions for the remediation of Tl polluted farmland soil.The main findings of this study are as follows:（1）After BC and MFBC were added to the polluted soil,the value of pH could effectively improve.In the whole passivation period,the Tl passivation effect of BC displays a trendless fluctuation.A better passivation effect is observed in MFBC with basically increased following dosage.Compared with KB（0.76 mg/kg）,MFBC-20 has the highest passivation rate（83.9%）and the lowest Tl content（0.12 mg/kg）.Results of XRD show the findings of Cancrinite,Gibbsite and Hydrate calcium silicate.The ability to adsorb metals is related to their porous characteristics.SEM-EDS results show that at the micron level,the concentration of Tl on solid particles is usually consistent with that of Feand Mn.Besides,the bioavailable Tl showed significant negative correlation with Feand Mn.The passivation of soil Tl is obviously related to the Feand Mnadded to the composite.As shown in the results of TEM-EDS,some Fe/Mnhigh valence oxide minerals such as pyrolusite（MnO₂）and goethite（FeO（OH））were found at the micron level,and the existence of lignite（Tl₂O₃）was also found.XPS results also confirm that the peak area of Tl（Ⅲ）increased with the application of MFBC composites.The mechanisms of Tl passivation in soil by MFBC are mainly divided into:a)the increase of soil pH value leads to the increase of exchangeable sites of metal cation adsorption and metal immobilization,so as to achieve good passivation effect;b)Minerals occurring at different passivation times will also affect the activity of Tl;c)The main function of Feoxide is to provide the adsorption site of metal and show a certain oxidation ability.The main function of Mnis performed to oxidize the low valence metal such as Tl（Ⅰ）to Tl（Ⅲ）.（2）During the rice pot experiment,the content of Tl in rice plants decreased significantly after application of MFBC-20.Tl content of rice plant in KB is 114.01 mg/kg,which shows that rice plants have a strong enrichment effect on Tl.The Tl content of rice plants with MFBC-20 is 25.55 mg/kg,which is 51.27%lower than that of KB,and TF values of rice plants planted on MFBC-20 are lowest（0.21）.This may be mainly the avoidance mechanism of plants,which transports Tl to root vacuoles for detoxification,so as to limit the entry of Tl into cells and upward migration.Therefore,MFBC can reduce the uptake of Tl by rice by reducing the bioavailable Tl in soil and promoting root detoxification.（3）Further research on microbial diversity of rice rhizosphere soil shows that,micropores and macropores in biochar contain more oxygen and air content,which will provoke the decomposition of soil organic matter（SOM）,and the resulting available C leads to the promotion of copiotrophic taxa（i.e.Actinobacteria and Proteobacteria）.The microorganisms in the soil passivated by MFBC are more likely to form modular synergy or antagonism.