Effects Of Hydrochar On The Speciation Of Lead In Contaminated Soil

In recent years,the remediation technology of lead passivation in soil through the application of biochar（including hydrochar and pyrolytic charcoal）has received much attention due to its cost-effectiveness advantage,but little has been reported on the effect of changes in the chemical properties of the biochar surface on the effectiveness of lead passivation.In this study,walnut shell hydrochar（WHC）and wood chip hydrochar（SHC）were prepared by hydrothermal carbonization,and two modified types of hydrochars,WHC-H and WHC-A,were obtained after surface modification of WHC with H₂O₂ and Na OH.Four types of hydrochars（WHC,WHC-H,WHC-A and SHC）and commercial fruitwood pyrolytic charcoal（BC）prepared were taken,and the chemical properties and material structures of the five biochar surfaces were characterized and comparatively analysed by XPS,FTIR and SEM.A comparison of the adsorption mechanism and effectiveness of BC and four hydrochars on Pb（II）in water through adsorption experiments;a study of the effect of five biochar species on Pb fugacity patterns and effectiveness after entering contaminated soil.Three types of walnut shells,WHC,WHC-H and WHC-A,were added to lead-contaminated soil and their effectiveness in fixing lead was examined using plant root secretions,and the phytotoxicity of the three hydrochars additions to the soil was evaluated by the germination rate of wheat seeds.The results of the study show that:（1）Hydrochars with a yield in the range of 55%-68%and a surface area derived mainly from the inter-particle voids.The p H of the hydrochar aqueous dispersion is acidic,whereas the pyrolytic carbon aqueous dispersion is alkaline and has a larger specific surface area.The main reason for this difference is the degree of pore development and the decarboxylation of surface carboxyl groups during the carbon formation process.（2）Oxygen-containing functional groups such as hydroxyl and carboxyl groups were significantly increased in the hydrochar modified by H₂O₂,but there was no significant change in the specific surface area of the hydrochar.After the Na OH modification treatment the oxygen-containing functional groups were reduced,but the specific surface area of the hydrochar increased by 53.98%;both H₂O₂ and Na OH changed the morphological structure of the hydrochar.（3）The adsorption experiments showed that the adsorption of Pb（II）in the aqueous phase by the five biochar species was in accordance with the quasi-secondary kinetic model and was chemisorption.The adsorption isotherm analysis shows that the adsorption of Pb（II）by biochars is consistent with the Langmuir model and is monolayer;both parameters 1/n and K_L in the isotherm model indicate that WHC-H has the strongest affinity for Pb（II）.（4）The addition of WHC,SHC and BC to the contaminated soil reduced the plant effectiveness of lead（DTPA extraction）in the experimental group at 30 days,with WHC having 4.3%and 2.8%less effective lead than BC and SHC.The results of the BCR graded extraction method analysis showed that the percentage of weak acid extractable（F1）and reducible states（F2）in the contaminated soil treated for 60 days was reduced by 5.87%-10.51%,respectively,compared to the control group,indicating that the transportability of lead in the contaminated soil was reduced,with better fixation by WHC.（5）Under the condition that WHC,WHC-H and WHC-A were all added at 7%,the effective Pb content in the soil was reduced by 13.60%,19.32%and 14.53%,respectively,after 30 days of application compared with the control group.The F1 form of Pb in the soil was reduced by 1.42%,1.71%and 2.47%respectively compared to the control,and the percentage of Oxidizable state（F3）and Residual state（F4）in the soil treated with WHC-H and WHC-A increased significantly,by 6%-10%compared to the control.（6）After drenching experiments with the root secretions of three plants,ophiopogonis,radish and spinach,the amount of lead in the drench solution decreased as the amount of WHC,WHC-H and WHC-A applied increased.The BCR results showed that the F1+F2 ratio of lead in soil decreased after the three charcoal additions,all three charcoals were effective in fixing lead in soil,and WHC-H was the most effective;all three charcoals were phytotoxic and inhibited the germination of wheat seeds,and the inhibitory effect became stronger as the amount added increased.