Study On Remediation Of Lead And Cadmium Pollution In Farmland Soil By Biochar Curing Agent

In recent years,environmental problems due to soil pollution have occurred frequently.The problem of heavy metal pollution in soil has become one of the environmental pollution problems that need to be solved urgently.Among many technologies for remediation of soil pollution,solidification/stabilization remediation technology is recognized as an efficient and economic remediation technology due to its low cost and remarkable effect.Biochar is considered to be a solidified material with good environmental benefits because of its advantages such as large specific surface area and large porosity.In this study,a variety of biochars were prepared for the remediation of Cd and Pb in the soil.The modified tobacco straw biochars were selected as the materials for solidifying heavy metal lead and cadmium through adsorption experiments in aqueous solutions.Firstly,the adsorption kinetics of modified tobacco straw biochar to Cd²⁺and Pb²⁺was studied in aqueous solution,and the rate-limiting factors of the reaction were discussed.Then the modified tobacco straw biochar,humic acid,and superphosphate were mixed in the proportion of 5%,3%,and 2%per 100g of soil and applied to the soil to investigate its lead and cadmium in contaminated soil.At the same time,the effect on the physical and chemical properties of the soil was also studied.The following conclusions are obtained:?1?In this paper,corn stalks,corn cobs,peanut husks,tobacco straws,and industrial wastes are selected as raw materials to prepare biochar.First,each material was cracked at 300?,400?,500?,600?,and then modified with sodium hydroxide solution.A total of 20 kinds of biochar were prepared.The yield of all biochar is between 27.13%and 71.79%.The yield of biochar obtained from the cracking of peanut shells at 600?was 71.79%,and the yield of other biochars was between 27%and 45%,indicating that the yield of biochar is generally low.?2?For each biochar,preliminary experiments of Cd and Pb were carried out,and it was found that they had good adsorption effects on Cd and Pb.In the aqueous solution,the removal efficiency of cadmium of each biochar material is above 98%,and the removal efficiency of lead is between 76%and 99.99%.The biochar obtained during the cracking at 400?has the best adsorption effect on Cd²⁺and Pb²⁺,the removal efficiency of Cd²⁺is above 98%,and the removal efficiency of Pb²⁺is above 99%.?3?The quasi-first-order kinetic fitting R² of the modified tobacco straw biochar to heavy metal ions Cd²⁺in aqueous solution is 0.0059,and the quasi-second-order kinetic fitting effect R² is 1,which conforms to the quasi-second-order kinetic model.This indicates that the chemical adsorption controls the reaction rate.The quasi-first-order kinetic fitting R² of the modified tobacco straw biochar to heavy metal ions Pb²⁺in aqueous solution is 0.0094,and the quasi-second-order kinetic fitting effect R² is 1,which conforms to the quasi-second-order kinetic model.This indicates that the chemical adsorption controls the reaction rate.?4?The modified tobacco straw biochar,humic acid,and superphosphate were mixed at a rate of 5%,3%,and 2%per 100g of soil and applied to the soil.After 60 days,the pH of the soil can be adjusted between 7 and 8,and the cation exchange capacity of the soil sample is increased from 12.8?19cmol/kg to 17?22.2cmol/kg.?5?Modified tobacco straw biochar,humic acid,and superphosphate were mixed at a rate of 5%,3%,and 2%per 100g of soil and applied to the soil.After 30 days of reaction,the proportion of residual cadmium increased from 15.9%?91%of the original soil to21.4%?96.5%.After 30 days of reaction,the water-soluble content of cadmium in the soil sample decreased from 3?15.88?g/kg to 1.2?9.0?g/kg.After 60 days,the content of weak acid extractable state of cadmium decreased from the original highest proportion of 50.77%to6.66%.The water soluble content of cadmium is reduced to 1.2?5.0?g/kg.?6?Modified tobacco straw biochar,humic acid,and superphosphate at a rate of 5%,3%,and 2%per 100g of soil,and then applied to the soil,on the 30th day,the original untreated Compared with soil samples,the proportion of residual Pb in the soil increased from0.21%?48.65%of the original soil to 34.45%?74.53%.The content of weak acid extractable state of lead decreased from 40.89%?82.98%of the original soil to 1.18%?10.83%.The water-soluble content of lead is reduced from 28.10mg/kg?41.10mg/kg to 7.20mg/kg?23.7mg/kg.After 60 days,the proportion of the residue state of lead increased to 45.75%?80.87%,the content of the weak acid extractable state of lead was reduced to 0.45%?1.67%,and the content of water-soluble Pb was reduced to 0.70mg/kg?14.90mg/kg.With the increase of time,the form of lead in soil samples is mostly stable.