Effect Of Biochar Loaded With Nano-zero Valent Iron On The Activity Of Heavy Metals In Uranium-contaminated Soil And Its Mechanism

With the increasing demand of energy development,nuclear energy plays an important role in the world energy system.A large number of uranium（U）mines are exploited leaving behind excessive U tailings and U-bearing wastewater.These activities exert significant impact on the soil nearby uranium mine and pose a threat to the health of local residents.Nanometer zero-valent iron（NZVI）has the characteristics of small particle size,large specific surface area and strong reducibility.It can react effectively with heavy metal such as uranium（U）in environmental media.However,its reactivity is affected by easy agglomeration.Some studies have found that biochar（BC）can be used as an effective carrier of NZVI,which enhances the stability while maintaining its reduction performance.In addition,it can greatly improve the phenomenon of agglomeration and deactivation of NZVI.However,most researches focused on the remediation of U pollution by biochar loaded nanometer zero-valent iron（NZVI@BC）in water treatment.The research on the remediation of heavy metal contaminated soil is relatively less.The feasibility of this composite in the process of remediating U contaminated soil caused by mining and the mechanism of heavy metal dynamic evolution have not been systematically explored.Based on this,this study uses NZVI@BC to treat U contaminated soil in U mining area and laboratory simulated U contaminated soil.The effects of NZVI@BC on p H and the activity of heavy metals such as U in aforementioned two types of soil are investigated in conjunction with the response of soil mineral composition changes and soil microbial community structure.It can provide basic data and theoretical reference for related U contaminated soil remediation practice in the future.The main results and findings of this study are as follows:（1）Both NZVI@BC and BC can effectively improve the value of p H in soil.Under the same dosage,the soil with NZVI@BC can maintain a higher p H value more persistently,and its average p H of soil is 0.65 units higher than that of soil with BC.Whether it is U contaminated soil in U mining area or simulated uranium contaminated soil,the addition of NZVI@BC increases the active content of uranium.Compared with the blank sample,the application of NZVI@BC will increase the water-soluble U and weak acid extractable U in the soil by 13.0%-1371.5%and7.0%-196.0%,respectively.In addition,the application of NZVI@BC will also affect the weak acid extractable content of lead（Pb）,cadmium（Cd）,chromium（Cr）and manganese（Mn）in soil.The results of characterization showed that with the increase of NZVI@BC dosage,the proportion of Phengite in the two soils increased by6.3%-92.2%,while the proportion of Muscovite decreased by 3.3%-34.5%.The increase in the proportion of Phengite may be due to the isomorphism of Fe²⁺produced after oxidation of NZVI,which replaces some heavy metal ions(such as Al³⁺)in minerals（such as Muscovite）.The occurrence of this phenomenon led to the decline of the absorption capacity of raw minerals to heavy metals.The heavy metal ions which originally existed in the mineral lattice enter the environment,and finally increase the active content of heavy metals in the soil.After adding NZVI@BC,Calcite appeared in the uranium contaminated soil of the mine,while Berlinite appeared in the simulated uranium contaminated soil.The emergence of Calcite may be caused by soil carbonation,while Berlinite may be caused by the combination of PO4^2-and Al³⁺in soil.Furthermore,the change of metabolic activity of microorganisms may also be a factor affecting the activity of the studied heavy metals.（2）The abundance and diversity of soil microbial community changed significantly with the addition of NZVI@BC.The abundance and diversity of soil microbial community decreased significantly（P<0.05）on the 15th day and then increased gradually on the 60th day.Compared with the blank sample,microbial community structure changes obviously in the direction of utilizing iron efficiently,with the addition of NZVI@BC.After the addition of NZVI@BC,the abundance of Firmicute increased at the phylum level,resulting in the dominate position of plant probiotics and related microorganisms that could produce siderophore（such as Paenisporosarcina,Citricoccus,Gaiella and Nocardioides）in the soil.The metabolism of some related microorganisms（such as Paenisporosarcina and Mesorhizobium）increased the active contents of U,Pb,Cd,Cr and Mn in the soil.Paenisporosarcina can accelerate the soil carbonation and the dissolution of silicate minerals in the soil,and release the heavy metals fixed by silicate minerals into environment.Mesorhizobium can produce siderophore.Siderophore can chelate Fe³⁺and other heavy metal ions,improving the migration of heavy metals in soil.In addition,the addition of NZVI@BC strengthen the metabolic ability of iron oxidizing bacteria in the soil.Thus the combination of iron oxidizing bacteria and sulfur oxidizing bacteria made U（IV）continuously oxidized to U（VI）,which increase the content of U active phase.In this research,the NZVI@BC composite based on NZVI and BC is not suitable for the direct remediation of uranium-contaminated soil.The microbial community and composition in the soil may restrict the efficiency of remediation materials.It points out the future research direction of remediation materials and technologies for heavy metal pollution in specific soils.At the same time,the active content of the heavy metal increase greatly due to the addition of NZVI@BC,which create the necessary basic conditions for the phytoremediation of uranium contaminated soil.