Mechanism Of Cadmium And Atrazine Adsorption By Biochar

The application of biochar has attracted extensive attention of researchers.Longer pollution by biochar control function is based on certain assumptions such as the precondition of high stability of biochar.The biochar into the soil,will inevitably occur in the process of biological and non-biological varying degrees of aging,including water-soluble components loses in the process（water soluble mineral ions and soluble organic matter）.The loss of water-soluble components will change the specific surface area of biochar and reduce the content of water-soluble mineral ions,which will reduce or inhibit the fixation effect of biochar on pollutants.In addition,complex organic pollutants and heavy metal ions often coexist in the soil environment,and the existence of co-existing pollutants will affect the fixation effect of biochar on pollutants.Therefore,it is of great significance to evaluate the environmental effects of biochar to understand the influence of water-soluble components on the adsorption of pollutants by biochar and the co-adsorption behavior of organic pollutants and heavy metal ions.In this study,biochars and washed biochars were prepared at different temperatures（200,400 and 600℃）,using pine chip（PC）and corn straw（CS）.Through the characterization of biochar before and after washing,the effect of washing on the physicochemical properties of biochar was analyzed.On this basis,batch adsorption experiments were conducted to explore the effect and mechanism of water washing on the adsorption of cadmium and atrazine on biochar,and the effect of coexisting atrazine on the adsorption of cadmium on biochar.The main findings are as follows:（1）We prepared biochar and washed biochar at different pyrolysis temperatures（200,400 and 600℃）.With the pyrolysis temperature increase,the p H and ash content of the biochar increased;the C content increased,the H and O contents decreased,and the H/C and O/C of the biochar gradually decreased,indicating reduced polarity of biochar and enhanced aromaticity.The specific surface area gradually increases,the number of oxygen-containing functional groups decreases,the mineral components continue to enrich,and the content of soluble ions K⁺,Ca²⁺,Mg²⁺and PO₄^3-decreases.After washing to remove soluble components,the p H and ash content of biochar,and the content of soluble mineral components decreased.（2）The results of adsorption experiments on cadmium show that:The adsorption isotherms of pine chip（PC）biochar and corn straw（CS）biochar at 200℃ and 400℃ for Cd²⁺can be well fitted by the LM model,and the adsorption isotherms of Cd²⁺by the CS biochars at 600℃ can be well fitted by the FM model.With the pyrolysis temperature increases,the adsorption of Cd²⁺on CS and PC biochars basically showed a trend of first decrease and then increase,CS biochars has the best adsorption performance at 400℃,and PC biochars has the best adsorption performance at 200℃.The mechanism of Cd²⁺adsorption by biochar pyrolysis temperature≤400℃ is coprecipitation and ion exchange;while the main mechanism for Cd²⁺adsorption on biochars with pyrolysis temperature>400℃ was cation-πinteraction.After removing the soluble components,the adsorption of Cd²⁺by biochars decreased by 25%-42%and 15%-40%,respectively.The decrease in the content of dissolved mineral ions weakens the co-precipitation and ion exchange between Cd²⁺and biochar.Adsorption of cadmium by biochar decreased.Coprecipitation,cation-πinteraction,complexation and cation exchange are the main mechanisms of Cd²⁺adsorption by biochar.（3）The results of the adsorption experiments of ATZ showed that the FM model was more suitable for the data fitting of the biochars adsorption of ATZ.The adsorption capacity of original CS biochars and PC biochars to ATZ was CS6>CS4>CS2 and PC6>PC4>PC2.After washing,the adsorption capacity of CS biochars at 400 and 600℃ to ATZ was enhanced.This was because the removal of water-soluble components in biochar exposed more pore structure,and the increase of pore size and specific surface area was conducive to the adsorption of ATZ by biochar.The adsorption capacity of 600℃ PC biochars to ATZ decreased during the washing process,which was related to the reduction of specific surface area caused by washing.This indicates that the adsorption of ATZ on high-temperature biochar is greatly affected by specific surface area and pore size.Distribution,hydrogen bonding,π-πEDA,hydrophobicity and pore filling are the main mechanisms of ATZ adsorption by biochar.（4）Results of co-adsorption experiments:The FM model is more suitable for describing the adsorption of Cd²⁺on CS biochars and PC biochars in the co-adsorption system.ATZ promoted the adsorption of Cd²⁺on CS2 and CS6,but had little effect on the adsorption of Cd²⁺on CS4.ATZ inhibited the adsorption of Cd²⁺on PC biochars,and only promoted the adsorption of high concentration of Cd²⁺on PC4.The FM model was more suitable for describing the adsorption of Cd²⁺on the washed CS biochars in the co-adsorption system,and the LM model was more suitable for describing the adsorption of Cd²⁺on the washed PC biochars.The presence of ATZ promoted the adsorption of Cd²⁺on washed PC biochars and CS2W,while it had little effect on the adsorption of Cd²⁺on CS4W and CS6W.（5）The main reason why coexistence of ATZ can promote the adsorption of Cd²⁺on biochar is that（1）ATZ can form a complex with Cd²⁺,and biochar can provide part of the adsorption sites for Cd²⁺after adsorbing ATZ;（2）ATZ molecules contain lone pair electrons,which can provide lone pair electrons for Cd²⁺adsorption to produce cationi-πeffect.The reasons for the inhibition may be:（1）Cd²⁺,ATZ and organic matter combine to form larger complexes,which block the pores of biochar,which is not conducive to the adsorption of Cd²⁺through coprecipitation;（2）Cd²⁺and ATZ compete for adsorption sites when adsorbed on biochar.point（oxygen-containing functional groups and pi electrons）.