Mechanisms Of Lead Adsorption By Rice Waste Biochar Produced At Different Temperatures And Suggestions For Environmental Application

Pyrolysis temperature significantly affects biochar's properties and the way of heavy metals removal.In this study,rice straw biochars(RSB300,RSB500 and RSB700)and rice husk biochars(RHB300,RHB500 and RHB700)were prepared under the temperature of 300?,500? and 700? respectively.The effects and mechanisms of pyrolysis temperature on lead adsorption by rice straw biochar and rice husk biochar were researched by batch adsorption test,micro characterization analysis and sequential leaching,and the environmental applications were suggested in this study.The results are as follows:Rice straw biochar produced at higher temperature has higher p H value(7.89,10.40,10.68,respectively)and higher surface area(6.766,22.383,115.465m2/g).Therefore,the rice straw biochar produced at higher temperature has higher adsorption capacity for lead(maximum adsorption capacities Qmax were 100.27,164.62,198.18mg/g,respectively).As the pyrolysis temperature increased,the functional groups(like O-H,C=O,and C-H)decreased,and the aromatic structure increased.The fitting results showed that,the adsorption of lead by RSB300 and RSB500 was good described with Langmuir model,the adsorption was a monolayer adsorption;while Freundlich model was suitable with the data of RSB700.The adsorption was nonuniform and multi-layer adsorption.The adsorption of lead by three biochars in solution was suitable with the pseudo-second-order kinetics equation,showed that the adsorption reaction is mainly chemical adsorption.The XRD and FT-IR results suggest that rice straw biochar can adsorb lead by surface precipitation.(Pb C2O4)was formed on RSB300,(Pb3(CO3)2(OH)2)was formed on RSB500 and RSB700.The results of sequential leaching showed that,11.34% of the lead on RSB300 belonged to the exchangeable fraction,while the exchangeable lead of RSB500 and RSB700 was less than 1%.More than 99% of the lead was on the RSB500 and RSB700,which belongs to the acid soluble fraction and the non-bioavailable.Based on the results,RSB500 and RSB700 are more appropriate for lead contaminated soil remediation compared with RSB300.As the pyrolysis temperature rose from 300? to 700?,the surface areas of rice husk biochar(0.632,45.274,193.149m2/g,respectively)and p H value(7.13,9.46,9.80)increased significantly,while the number of functional groups(such as carboxyl)decreased.The relationship of maximum adsorption capacity(Qmax)for lead was RHB300(14.1mg/g)< RHB500(21.7mg/g)< RHB700(26.7mg/g).The adsorption isotherms of lead by three kinds of rice husk biochar were suitable with Freundlich model,the adsorption was non-uniform and multi-layer adsorption;the kinetics fitting was more suitable with the pseudo-second-order kinetics equation,and the adsorption reaction was mainly chemical adsorption.The XRD and FT-IR results suggest that rice husk biochar can adsorb lead by surface precipitation.After adsorption,Pb Se was formed on RHB300,(Pb3(CO3)2(OH)2)was formed on RHB500 and RHB700,(Pb5(PO4)3X(X =Cl,F,OH))was formed on all rice husk biochars.Although,the Qmax value of RHB300 was the lowest,the exchangeable lead fraction was the largest(RHB300,RHB500 and RHB700 was 2.61,0.223,0.377mg/g,respectively;21.8%,1.80%,0.94% of the total adsorption capacity).In summary,lead adsorbed by RHB300 was easy to release,The adsorption efficiency was about 1/4 of the original RHB300 after six adsorption-desorption of RHB300,RHB300 may have better recycling usage and may be suitable for water treatment.The lead adsorbed on RHB500 and RHB700 was mainly acid soluble fraction and non-bioavailable fraction,RHB500 and RHB700 may be suitable for lead contaminated soil remediation with less environmental risks.