Adsorption Characteristics And Mechanisms Of Cadmium From Aqueous Solutions By Plant-derived Biochars

Studies on the adsorption of heavy metal by biochars,which provides not only a theoretical basis for the treatment of heavy metal pollution,but suggest a new way for the recycling of agricultural and forestry waste.In this study,vetiver-derived biochar（VBC300,VBC500,and VBC700）,eucalyptus-derived biochar（EBC300,EBC500,and EBC700）and rice straw-derived biochar（RBC300,RBC500,and RBC700）were prepared under the pyrolysis temperature of 300,500,and 700 ^oC,respectively.The physicochemical properties of biochars were determined through BET-N₂,Zeta potential and elemental analysis.Based on the comparison of adsorption characteristics of Cd²⁺by biochars,rice-straw biochar showed better performance;Moreover,the adsorption mechanisms of rice-straw biochar was discussed by qualitatively and quantitatively using SEM-EDS,XRD,FTIR and Boehm titration.（1）The effect of feedstock and pyrolysis temperatures on the physicochemical properties of biochars.With the increase of pyrolysis temperature,the yield of three biochars gradually decreased,the ash content increased,the pH was all alkaline and increased,the specific surface area gradually increased,and the Zeta negative charge amount increased.Among them,the relationship between temperature and yield,ash and pH showed extremely significant.It is noteworthy that the ash content of rice-straw biochar reached up to 42.43%and the maximum specific surface area reached up to 242.53 m²/g,which were much higher than those of the other biochars.The water-soluble K⁺,Na⁺,CO₃^2-,and PO₄^3-were rich in content,and the Si element content was also high.（2）Adsorption characteristics of Cd²⁺by biochars.The adsorption kinetics of the biochars followed pseudo-second-order kinetics model,indicating the adsorption was mainly chemical.The equilibrium time followed in the descending order of BC700,BC500 and BC300.Vetiver and eucalyptus biochars were more fitted by the Freundlich model,while rice straw biochar was better fitted by the Langmuir model.The maximum adsorption followed in the order of BC700>BC500>BC300.It is worth noting that the time for rice straw biochar to reach equilibrium is relatively short,especially for RBC700,with its equilibrium time was only 5 minutes when the initial concentration of Cd²⁺was 50 mg/L.The maximum adsorption capacity of rice-straw biochar was higher than that of the other biochars,especially RBC700（127.46 mg/g）.（3）Adsorption mechanisms of Cd²⁺by rice-straw biochar.The main mechanisms included precipitation mechanism,ion exchange mechanism,functional group chelating mechanism,and Cd²⁺-pbonding mechanism.Among them,precipitation and ion exchange mechanism were predominant adsorption mechanisms.The contribution of these two mechanisms to overall adsorption was in the range of 76.10%to 80.74%.Functional groups chelation mechanism and Cd²⁺-pbonding mechanism were also important,both of which the contribution to the overall adsorption ranged from 19.26% to 23.90%.