Immobilization Of Heavy Metals Lead(Pb) And Cadmium(Cd) In Biomass By Pyrolysis

Adsorptive removal of lead（Pb）,cadmium（Cd）and other heavy metal pollutants using biomass like agricultural residues is one of the research hotspots for remediation of heavy metal pollution,and has the advantages of low cost,wide biomass availability and simple operation.However,how to improve the adsorptive removal efficiency and safely dispose of the biomass material after adsorbing heavy metals are two major problems that restrict the promotionand application of this technology.To address these issues,in this paper,the biomass including rice straw,peanut shells and celery were selected as the raw materials.First,the biochars were prepared by pyrolysis of biomass so as to increase the adsorption to Pb and Cd,and the adsorption capacity and mechanisms by biochars were investigated.Second,the biomass after adsorption of Pb and Cd were treated with direct pyrolysis or phosphate-pretreated pyrolysis so as to dispose safely the contaminated biomass,by which the biomass could be transformed into stable chars while heavy metals be immobilized in the chars.The main research findings include:Biochars derived from rice straw and peanut shell showed relatively high adsorption of Cd²⁺,and the maximum adsorption capacity(Q_max)obtained by fitting with Langmuir model was 13.3 mg/g and 15.8 mg/g,respectively.According the analysis results of XRD,FT-IR and SEM-EDX,adsorption of Cd²⁺by the biochars was mainly attributed to precipitation,and the acid-soluble components（AS）made the major contribution to the adsorption.The adsorption of Cd²⁺by peanut shell and rice straw biomass was studied,and the biomass after adsorption of Cd²⁺was treated with pyrolysis methods so as to immobilize Cd.The research results showed that the direct pyrolysis treatment significantly reduced the leaching rate（TCLP method）and extraction rate（DTPA method）of Cd and improve the stability of Cd.Pretreatment of Cd-contaminated biomass with phosphate at a molar ratio of n[PO₄^3-]/n[Cd²⁺]=5:1 and 10:1 increased the recovery of Cd in chars to more than90%,and the DTPA extraction rate was further reduced to less than 1%.The Cd in chars obtained by phosphate-pretreated pyrolysis was transformed into more stable species including both organic-matter-bonding species and residuals,according to the analysis results using Tessier sequential extraction method.XPS analysis indicated the formation of stable C-O-P and C-P bonds in the Cd-containing chars.There are a large amount of metal elements like K,Ca,Na and Mg in the celery-derived biochars,and the maximum adsorption capacity(Q_max)for Pb²⁺reached to304 mg/g that is higher than other biochars reported previously.It was found that the contribution by water-soluble fraction of celery biochars accounted for 60%to the adsorption of Pb²⁺,after separation of various biochar factions through water-washing and acid soaking.The high adsorption was mainly due to precipitation of Pb²⁺by the celery biochars according to analysis results of XRD、SEM-EDX and FT-IR.The maximum adsorption capacity(Q_max)of celery biomass to Pb²⁺reached 132mg/g.The DTPA extraction ratio of Pb was reduced from 68.1%in the Pb-adsorbed biomass to 29.3%in the chars obtained by direct pyrolysis of the Pb-adsorbed biomass,and was further dropped to less than 2%in the chars obtained by phosphate-pretreated pyrolysis.The Pb in chars was transformed into more stable species including both organic-matter-bonding species and residuals,according to the analysis results using Tessier sequential extraction method,and stable lead phosphates were formed in the chars according to the XRD analysis results.