| Biochar has unique pore structure and a large number of oxygen-containing functional groups,which can improve soil p H,cation exchange capacity（CEC）and water holding capacity,so that it has great environmental benefits in increasing crop yield,reducing soil greenhouse gas emissions,improving soil fertility and repairing polluted soil.In this paper,biochar was prepared at 400℃with sulphate lignin,rice straw,rice husk and cow dung as raw material,with the element analyzer,specific surface and pore size distribution tester,scanning electron microscope（SEM）,Fourier transform infrared spectrometer（FT-IR）to explore the difference in physicochemical properties of the biochar from different raw materials.On this basis,the adsorption capacity of four biochars for Cr（Ⅵ）was studied.Batch adsorption experiments and the effect of Cr（Ⅵ）solution p H and pyrolysis temperature were investigated.The effects of biochar and biochar composite microorganisms on the speciation of heavy metals in contaminated soil were studied by soil culture experiments in laboratory,and the characteristics of chemical speciation of Pb（Ⅱ）and Cr（Ⅵ）with time were analyzed.Pot experiment was conducted to study the biomass of pakchoi on contaminated soil by biochar and biochar composite microorganisms.（1）The yield,p H,ash content and CEC were determined and analyzed.LBC400 had the highest yield（59.4%）,the highest C content（weakly acidic）,the lowest ash content and CEC,and the highest CEC of cow dung carbon was 141.41cmol/kg.Except for the weak acidity of the sulfate lignin biochar,the other biochar were alkaline.CEC sizes were as follows:cow dung biochar（NBC400）>rice straw biochar（SBC400）>sulfate lignin biochar（LBC400）>rice husk biochar（RBC400）.There are a lot of oxygen-containing functional groups on the surface of biochar,and there is no obvious difference in the types and number of them.LBC400 has higher specific surface area（111.10 m~2/g）and pore volume（0.19 cm~3/g）,which are 27.4 times and 9.5 times of RBC400,respectively.The adsorption performance showed that the adsorption capacity of Cr（Ⅵ）on LBC400 was 5.23 mg/g.（2）Batch adsorption experiment results show that the LBC400 has the best adsorption performance for Cr（Ⅵ）,and the optimal p H is 2.When the initial concentration was 30-70mg/L,the adsorption dose was 2 g/L and the adsorption temperature was 25℃,the removal rate of Cr（Ⅵ）by biochar was almost 100%.The adsorption process of Cr（Ⅵ）on LBC400was more consistent with the quasi-second-order kinetic model and Langmuir model,which indicated that the adsorption process was mainly monolayer adsorption,and the maximum adsorption capacity was 37.2 mg/g.Thermodynamic parametersΔG,ΔH andΔS indicate that the adsorption process is spontaneous endothermic and biochar has a good affinity for Cr（Ⅵ）adsorption.The results of repeated experiments showed that biochar could be reused,and the removal rate of Cr（Ⅵ）was 63.16%after the third reuse.（3）After adding NBC400 and Bacillus subtilis,soil p H increased with the increase of addition level and culture time,while LBC400 was the opposite.In the treatment of Pb（Ⅱ）contaminated soil,Pb（Ⅱ）residue increased by 11.99-13.34%（NBC400）and11.20-13.50%（LBC400）with the application rate of NBC400 and LBC400 from 1%to 5%,respectively.The change of residue in the treatment group with Bacillus subtilis is as follows:10.65%（Bacillus subtilis）,12.96-13.87%（1-5%NBC400 combined treatment with Bacillus subtilis）,11.35-16.91%（1-5%LBC400 combined treatment with Bacillus subtilis）.The above results showed that biochar and Bacillus subtilis had good passivation effect on heavy metals.The effect of NBC400 on Pb（Ⅱ）was better than that of LBC400.At the same time,the combined treatment of biochar and Bacillus subtilis had better effect,among which5%NBC400-Bacillus subtilis had the best repair effect.（4）In the treatment of Cr（Ⅵ）contaminated soil,the residue of Cr（Ⅵ）increased10.46-11.12%（NBC400）and 11.11-15.44%（LBC400）with the application rate of NBC400and LBC400 from 1%to 5%,respectively.The change of residue in the treatment group with Bacillus subtilis is as follows:2.97%（Bacillus subtilis）,11.77-14.78%（1-5%combined treatment of NBC400 and Bacillus subtilis）,8.88-9.35%（1-5%combined treatment of LBC400 and Bacillus subtilis）,indicating that The passivation effect of LBC400 on Cr（Ⅵ）was better than that of NBC400.Meanwhile,the combined treatment of biochar and Bacillus subtilis weakened the passivation effect,and 5%LBC400 showed the best repair effect.（5）After LBC400 and NBC400 were added in Pb（Ⅱ）and Cr（Ⅵ）contaminated soil with different supplemental levels（1-5%）,the biomass and plant height of Pakchoi increased with the increase of the dosage of biochar samples.The fresh weight of 5%LBC400 and NBC400in Pb（Ⅱ）contaminated soil was 2.00 and 2.01 times that of CK,respectively.In Cr（Ⅵ）contaminated soil,the fresh weight was 1.47 times and 2.08 times of CK,respectively.However,the biomass variation trend of biochar and Bacillus subtilis was similar to that of LBC400 and NBC400 alone,in which 5%NBC400-Bacillus subtilis had the best effect,and the fresh weight in Pb（Ⅱ）and Cr（Ⅵ）contaminated soil was 2.60 times and 2.33 times that of CK,respectively.The addition of biochar and Bacillus subtilis to Pb（Ⅱ）and Cr（Ⅵ）contaminated soil can reduce the concentration of heavy metals in Pakchoi,and 5%NBC400-Bacillus subtilis treatment can reduce the concentration of heavy metals in Pakchoi most obviously,reaching 28.12 mg/kg.5%LBC400 treatment reduced Cr（Ⅵ）content most obviously,reaching 28.14 mg/kg. |
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