Adsorption And Immobilization Of Heavy Metals By Modified Biomass Ash

Biomass ash can be applied to immobilize heavy metals(HMs)in soils,due to its alkaline,high specific surface area,and SiO2/Al2O3 components.To enhance the immobilization efficiency,this study aims to destroy stable silicon/aluminum oxides structure through promoting the formation of amorphous silica-alumina components by chemical modification and to increase its adsorption capacity for heavy metals.Therefore,two modified biomass ashes(MBA and PBA)were prepared by hydrothermal modification with KOH and KH2PO4.The specific surface area,morphology,mineral composition and functional groups in original biomass ash(BA)and modified biomass ash(MBA)were investigated by BET,SEM,XRD,FTIR and XPS.The adsorption and desorption characteristics of heavy metals by BA and MBA and their immobilization effects in contaminated soil were investigated by solution system experiments and soil incubation.The passivation effects of BA alone and combined addition of phosphate(BAP)and iron oxide amendment(BAPFe)on Cu,Cd and Pb in contaminated soil were also investigated.The main results are as follows:(1)The adsorption capacity of MBA and PBA on Cu2+,Cd2+ and Pb2+ increased by 1.79,3.22,0.22 and 0.22,1.45,1.86 folds compared with BA at same initial concentration,respectively.The adsorption capacity of MBA on Cu2+ and Cd2+ was better than PBA.In addition,the adsorption capacity of PBA for Pb2+ was much higher than MBA.(2)The isothermal adsorption of Cu2+,Cd2+ and Pb2+ by BA and MBA were well fitted by the Langmuir model,indicating the adsorption process of BA and MBA was monomolecular layer adsorption.The maximum adsorption amounts of Cu2+,Cd2+ and Pb2+ by MBA were 270 mg·g-1,51.6 mg·g-1 and 227 mg·g-1,respectively,which were 1.79,3.07 and 0.22 folds higher than BA,respectively.Moreover,the total desorption rate of MBA for Cu2+ and Pb2+ was reduced from 4.57%-6.07%and 0.82%-4.25%to less than 0.02%,and from 3.63%-7.69%to 0.62%-5.84%for Cd2+.It indicated that the alkaline modification not only improved the adsorption capacity of MBA for heavy metals,but also increased the retention capacity and reduced the re-release risk of adsorbed heavy metals.The kinetic adsorption of Cu2+,Cd2+ and Pb2+ by BA and MBA were better fitted by the pseudo-second-order kinetic model,indicating the adsorption process of BAs on these heavy metals is a multi-step reaction.The intraparticle diffusion model fitting results showed it included film diffusion and pore diffusion phases.Furtherly,film diffusion was the dominant step and pore diffusion was the limiting factor in the adsorption rate.Furthermore,KOH modification increased the specific surface area and pore volumes of biomass ash.FTIR results showed that the Si-O-Si vibration peak of MBA was enhanced and the carbonate binding peak appeared in the MBA after saturation adsorption on Cu2+,Cd2+ and Pb2+.XRD displayed the presence of silica-aluminate components in MBA,the reduction of silica-aluminate characteristic peaks in MBA after saturated adsorption of Cu2+,Cd2+ and Pb2+,and detected crystals were CdCO3,PbCO3 and CuO,respectively,which is consistent of XPS findings.The above results indicated that the adsorption mechanisms of modified BAs on Cu2+,Cd2+ and Pb2+ included co-precipitation,chelation of silica-aluminate active components,and cation exchange.(3)The BA treatments with the application at ratio of 0.1%-0.6%additions(w/w)increased soil pH by 0.14-0.64 units and the activated silicon content by 3.4-65 mg·kg-1,and reduced available Cu,Cd and Pb by 11.1%-50.9%,4.20%-64.1%,9.81%-58.8%compared to CK,respectively.Meanwhile,MBA treatments increased soil pH by 0.21-1.2 units and the activated silicon content by 6.00-84.4 mg·kg-1 and reduced the available Cu,Cd and Pb by 8.36%-84.9%,13.6%-61.0%,13.4%-87.6%,respectively.It can be concluded that MBA can be more effective in reducing the availability of Cu,Cd and Pb in soil than BA.In addition,BA and MBA could promote the transformation of Cu and Pb from exchangeable fraction to carbonate-bond,iron/manganese-bond and organic matter fractions,and promote the transformation of Cd from exchangeable fraction to organic matter and residual fractions.(4)Soil pH in BA,BAP and BAPFe treatments were increased by 0.26-0.62,0.40-1.28 and 0.40-1.30 units compared to CK,respectively.BAP and BAPFe significantly increased available phosphorus by 1.31-3.66 and 1.36-3.72 folds compared to BA,respectively,and reduced soil activated silica content by 25.2%-40.2%and 27.6%-40.0%.It can be concluded that BAP and BAPFe can be more effective to reduce the availability of Cu,Cd and Pb in soil than BA.Simultaneously,BAP and BAPFe could promote the transformation of Cu,Cd and Pb from exchangeable fraction to potentially active and stable fractions.This study showed that modified biomass ash had high adsorption capacity for heavy metals and could significantly decrease the availability of soil heavy metals.This study will provide some theoretical support for the remediation of contaminated soil by biomass ash.Figures[41]Tables[26]References[136]...