Preparation Of Iron-rich Biochar Material And Its Experimental Study On Remediation Of Chromium-contaminated Soil

With the development of electroplating,chemical industry and leather industry,a large number of chromium-containing wastewater and chromium-containing sludge infiltration have caused serious pollution to the environment and soil.In recent years,the remediation of chromium-contaminated soil using iron-based materials,mainly including zero-valent iron（ZVI）and FeS₂,has been paid attention and studied by scholars at home and abroad.Due to their advantages of small particle size,large surface area and strong reducing ability,these materials can be effectively applied in the remediation of chromium-contaminated soil.However,with the deepening of the research,it is found that iron materials in the field of soil remediation are still have problems,such as easy to agglomerate,difficult to disperse and poor migration in the soil.To solve the above problems,this paper took the soil of a tanning sludge landfill in Jinan,Shandong as the research object,designed experiments to study the remediation effects of ZVI,pyrite and biochar on the chromium contaminated soil.In addition,using biochar as the supporting skeleton of iron salt remediation agent,iron-rich biochar was prepared by ball-milling method to study the remediation ability of the biochar to chromium contaminated soil.The main content of this paper is divided into the following parts:（1）ZVI and pyrite were applied to chromium-contaminated soil to investigate the effects of material dosage,remediation time and soil moisture content on the efficiency of chromium remediation.Meanwhile,the changes of soil p H,available iron content and chromium form before and after remediation were investigated.The results showed that the total chromium concentration in the leaching solution decreased with the increase of material dosage and soil moisture content.When the dosage was 8g/kg,the restoration effect was the best,and the content of chromium in the soil leaching solution restored by ZVI was 29.01mg/L,and that of the soil leaching solution restored by pyrite was 27.31mg/L.The restoration process basically reached equilibrium after 15 days.The application of ZVI and pyrite will decrease the soil p H value,and the chromium in the soil will be transformed to the form of low mobility and low bioavailability.In the ZVI remediation soil,the available iron content will increase rapidly with the increase of the dosage,and there is the problem of excessive iron release.（2）The biochar with different structures were prepared by ordinary pyrolysis method（MF type）,rapid pyrolysis method（WB type）and hydrothermal carbonization method（SR type）,and the prepared biochar was characterized and analyzed and the soil remediation test was conducted.The results showed that SR biochar had more oxygen-containing functional groups than MF biochar and WB biochar,and had stronger adsorption and complexation capacity for chromium in soil.The higher the preparation temperature,the higher the carbonization level of biochar,the stronger the thermal stability.The remediation effect was enhanced with the increase of material dosage and ambient temperature,and the total chromium concentration in the leached solution decreased continuously.When the amount of biochar was 10g/kg and the ambient temperature was 35℃,the restoration effect was the best.The application of biochar increased the soil p H,and the soil p H of SR biochar increased significantly.The analysis of chromium form before and after remediation showed that the bioavailability of chromium was reduced when chromium was transferred to OM and RES forms.（3）Using SR biochar,ZVI and pyrite as raw materials,iron-rich biochar was prepared by ball-milling method,and the biochar was characterized and analyzed,soil remediation and soil column leaching tests were conducted.The results show that ZVI or FeS₂ particles are loaded onto the surface of biochar in the form of micron aggregates.Ball-milling can form new surface functional group Fe-O of iron-rich biochar,and the thermal stability of iron-rich biochar is significantly improved by doping of iron salts.Iron doping of 2:1 ratio of iron rich biological carbon can better fixed total chromium in soil.After applyingⅢ-BC andⅥ-BC,total chromium content in soil solution were decreased by 6.49mg/L and 8.40mg/L.When applying amount of 5g/kg,Ⅲ-BC andⅥ-BC had the best repair effect to chromium-contaminated soil.After the application of iron-rich biochar,the available iron content in the soil is stable,which can effectively avoid the excessive release of iron into the environment.（4）In Soil column leaching experiment,iron-rich biochar has a strong fixation in chromium contaminated soil,leaching concentration of total chromium in the soil column that exertⅢ-BC andⅥ-BC were lower than CK soil column,and it can significantly reduce the upper chromium downward vertical migration from contaminated soil.Both the double constant rate equation and the parabolic diffusion equation fit well with the cumulative chromium release curve,that is,the two equations can describe the release process of chromium in the soil under leaching,indicating that the main release process of chromium is the diffusion within the soil particles.