Study On The Passivation Effect Of Biochar Materials On Cadmium Contaminated Facility Soil

The high multiple cropping index and high yield of facility cultivation make farmers invest a lot of manure and chemical fertilizer for a long time,which increases the surplus risk of nutrient elements and heavy metals in facility soil,leads to soil acidification and salinization,increases the activation risk of heavy metals in facility soil,and forms Cd accumulation in facility soil.Among them,the accumulation of heavy metals in facility soil can cause soil pollution and can also enter the human body through the food chain,endangering human health.Therefore,the problem of heavy metal pollution in facility soil has attracted wide attention.Passivation remediation can directly stabilize heavy metal elements by reducing the effectiveness of target heavy metals and can also reduce soil heavy metal activity by changing soil physical and chemical properties.Therefore,this study aims to passivate heavy metals in facility soil,explore the physical and chemical structure characteristics of different types and modified biochar,and analyze the passivation effect of biochar on heavy metals in facility soil.The main contents and conclusions are as follows:（1）In this paper,corn straw,corn cob and rice husk were used as biochar raw materials to prepare biochar（denoted as BC,MC and RH,respectively）at 500°C.The elemental composition of biochar was analyzed.The structural characteristics of different types of biochar were analyzed by scanning electron microscopy,Fourier transform infrared spectroscopy and X-ray diffraction.The effects of different kinds of biochar（BC,MC and RH）with different addition amounts（1.0%,3.0%and 5.0%）on soil properties and Cd speciation distribution in greenhouses with exogenous Cd were investigated by indoor simulation experiments.The results of biochar characterization of three different raw materials showed that the morphological structure of corn straw biochar prepared at 500°C was the best.The results of passivation test showed that the three biochars could increase the p H and organic matter content of the facility soil and decrease the activity of heavy metal Cd in the facility soil to varying degrees.Among them,the corn straw biochar with 5.0%dosage increased the p H and organic matter content of the facility soil and decreased the active state content of heavy metal Cd.Based on the comprehensive analysis of the passivation effect of three biochars on Cd in soil and their effects on soil physical and chemical properties,corn straw biochar was selected and modified to apply to Cd passivation test.（2）The effects of acid（HNO₃）and alkali（KOH）modification on the physicochemical structure of corn straw biochar were analyzed.Modified biochar was prepared by using corn straw biochar as raw material.By analyzing the physical and chemical properties of biochar before and after modification,the effect of modification treatment on the physical and chemical properties of biochar was clarified.The results showed that the modification method could simultaneously optimize the specific surface area of biochar,the number of oxygen-containing functional groups on the surface and improve the aromaticity.It can be seen from SEM scanning electron microscopy that the modified corn straw biochar has a tightly arranged homogeneous honeycomb microporous structure.XRD and FTIR analysis showed that the aromatization degree of the modified carbon was improved,the surface oxygen-containing functional groups were abundant,and the adsorption sites were increased.Boehm titration analysis showed that the total amount of oxygen-containing functional groups of corn straw biochar after KOH modification was more,the cation exchange capacity was enhanced,and the metal ligand complexation was stronger.（3）The effects of acid-modified corn straw biochar and alkali-modified corn straw biochar on soil properties and heavy metal Cd speciation distribution in facilities were studied.The results showed that the addition of 5.0%KOH-modified biochar had a better improvement effect on the physical and chemical properties and Cd speciation of Cd-contaminated facility soil:the p H value of facility soil increased by 0.46 units;the organic matter content reached 5.13%.The cation exchange capacity increased by about 40%.The available Cd content in the greenhouse soil was reduced by 0.825 mg/kg.The residual cadmium content increased by more than 1times.Through biochar modification and soil cadmium passivation test,KOH modified corn straw biochar passivation repair effect is better,which can provide a passivation repair technology for cadmium-exceeding facility soil.