Effect Of Biochar On Adsorption-desorption Of Thiamethoxam In Soil

In recent years,biochar has been widely studied and applied in soil organic pollution.Adsorption is the main process of removing organic pollutants by biochar.But adsorption is not permanent,desorption of some organic pollutants from biochar could not be ignored.In addition,promotion of biochar on the degradation of organic pollutants could be removed more thoroughly.Therefore,it is necessary to conduct systematic research on the adsorption-desorption process and chemical degradation of organic pollutants.This paper first explored the influence of pyrolysis temperature and water washing treatment of corn straw biochar on its physicochemical properties,selected thiamethoxam as the target pollutant,and explored the adsorption-desorption kinetics,isothermal adsorption-desorption process and hydrolysis of thiamethoxam in different biochar and soil mixtures.Secondly,effects of the interaction mode between biochar and soil on the adsorption-desorption kinetics,isothermal adsorption-desorption process and hydrolysis of thiamethoxam were explored to clarify the migration and transformation process of thiamethoxam in soil mixture system.The main research contents and results include following parts:(1)Due to surface of organic carbon of biochar usually contains functional groups such as hydroxyl,carboxyl and carbonyl,biochar has high polarity.With increase of pyrolysis temperature,functional groups of biochar disappear and form pore structure composed of aromatic carbon,aromaticity and specific surface area of biochar increase significantly.Meanwhile,Residual inorganic ash caused partial pore clogging and covering.Most of biochar ash could be removed by washing,and exposure of aromatic carbon further increases its aromaticity.(2)Adsorption of thiamethoxam on the three kinds of washing biochar has obvious fast adsorption stage and slow adsorption stage,and formation and increase of biochar pore structure increased contribution of fast adsorption stage of adsorption process.Secondly,the significant difference(P<0.05,n=3)between the Kf value(29124.66)and nads value(0.17)of SX700 and the other two types of biochar,which is attributed to its rich micropore structure with more adsorption sites.Adsorption kinetics experiments of soil mixtures show that biochar could significantly affect adsorption capacity of soil mixtures.However,since adsorption capacity of soil mixture is much smaller than that of biochar,the interaction between soil and biochar(organic matter encapsulation or biochar pore plugging)leads to a decrease in adsorption sites and prevents pore structure from adsorbing thiamethoxam.Therefore,as the addition ratio increases,adsorption capacity of soil mixture increases nonlinearly.The further study of isothermal adsorption experiment showed that K_fvalue of soil mixture increased significantly with increase of addition ratio,and relative content of biochar increased to provide more adsorption sites.(3)Desorption kinetics of biochar indicate that its pore structure properties significantly affect desorption process of thiamethoxam.Secondly,according to isothermal desorption parameters and desorption hysteresis coefficient HI value,the HI value of SX700 is larger,indicating that adsorption-desorption process of thiamethoxam may be controlled by the micropores and mesopores of biochar,which was attributed to fact that the adsorption-desorption process was not a simple reversible process.In the pure soil and soil mixture with SX300 and SX500,adsorption quantity and desorption quantity showed a very significant positive correlation(P<0.01,n=10),and increase in desorption quantity is attributed to the easier desorption of thiamethoxam from low affinity adsorption sites.In addition,interaction between soil and biochar not only decreased some adsorption sites,but also hindered desorption of thiamethoxam adsorbed to a certain extent.The difference of HI value in soil mixture indicated that different interaction modes had different effects on adsorption and desorption processes of thiamethoxam.(4)In the adsorption-desorption kinetics experiments of three biochars,the degradation rate of thiamethoxam were 11.58-34.59%,and degradation quantity and adsorption quantity of SX300 and SX700 showed significant positive correlation(P<0.01,n=3),while the degradation quantity and adsorption quantity of SX500 showed significant negative correlation(P<0.01,n=3),and showed largest degradation rate.The three biochars have more alkaline substances and higher p H value,SX300 and SX700 mainly undergo hydrolysis by adsorbing thiamethoxam and contacting it with alkaline functional groups.The weakly acidic environment of pure soil results in a very low degradation rate of thiamethoxam(5.57%)The degradation quantity and adsorption quantity of nine soil mixtures showed a highly significant positive correlation(P<0.01,n=9).The hydrolysis of thiamethoxam may mainly occur in the solid phase part,due to the interaction between biochar and soil,which makes it difficult for the alkaline ash of biochar to enter the liquid phase.