Adsorption Of Pb²⁺ By Modified Walnut Shell Biochar And Its Passivation On Lead-contaminated Soil

With the rapid development of industry and agriculture,the problem of heavy metal lead pollution is becoming more and more serious.Lead is a nonessential element for biological development,and it is easy to accumulate through the food chain and threaten human health.Therefore,it has become a research hotspot to seek low-cost and efficient methods for lead-polluted water and soil.In this study,magnetic amino walnut shell biochar（CMBC）was prepared by modification of walnut shell biochar（BC）.General physical and chemical properties of CMBC were analyzed by a number of characterization methods to explore the adsorption mechanism and the effects of two materials on Pb（II）in water.And the effects of CMBC on the physicochemical properties,lead morphology and lead accumulation in lead-contaminated soil were investigated.The main results are as follows:（1）Magnetic amino walnut shell biochar（CMBC）was synthesized with walnut shell biochar by loading Fe₃O₄ particles and crosslinking chitosan.The material has rich functional groups,strong aromaticity and good adsorbability,which can improve the adsorption efficiency.And it can be used in the water with a wider range of p H and pollution concentration.Therefore,the adsorption performance of CMBC is better than that of BC.Langmuir model and PF-Second model are well matched with the adsorption process.（2）The removal rate of Pb（II）and adsorption capacity of CMBC are much higher than that of BC.The effects of initial conditions on the adsorption properties of BC and CMBC were investigated.When the initial p H is 5.0,both of BC and CMBC have the best adsorption performance.When the temperature is 298K,the removal rate of CMBC on 100 mg/L lead solution can reach 99.58%,which is 27.21%higher than that of unmodified walnut shell biochar.And the reusability of CMBC is better than BC.Because the active group on CMBC is more likely to form a more stable complexation with Pb（II）,Pb（II）is preferentially adsorbed in the double system of Pb（II）and Cd（II）coexist.（3）CMBC applied to the soil can improve the nutritional conditions of the soil and promote enzymatic catalysis.Therefore,it promotes the metabolism of organisms and microorganisms,which shows a considerable effect on improving the soil nutrient environment.（4）The morphology distribution of Pb changed after the addition of CMBC.When the applied amount of CMBC was 5%,the effect was the best.The acid extractable Pb and reducible Pb decreased 15.61%and 15.43%,and the oxidizable Pb and residue Pb increased 18.09%and12.95%respectively.It showed that CMBC reduced the mobility and bioavailability of lead and reduced toxicity.The main mechanisms of passivation of lead-contaminated soil are as follows:metal precipitation with salt ions,complexation with ionized groups,or metal chelates formed by intramolecular binding or intermolecular interaction with active functional groups.（5）The result of pot experiment of pakchoi indicated that CMBC can reduce the cumulative of Pb in plants,increase the biomass of different parts of packchoi and reduce the enrichment in the underground parts,so as to alleviate the stress of lead in the growing process of packchoi.