Adsorption And Immobilization Of Heavy Metals By Modified Fly Ash

Fly ash can be applied to immobilize heavy metals(HMs)in soils,due to its high specific surface area,alkaline and oxygen-contaning groups.To enhance the immobilization efficiency,this study aims to destroy stable vitreous structure by chemical modification and increase its adsorption capacity for heavy metals.Since the solubility product constant of HMs phosphate precipitation is lower than those of carbonate and hydroxide precipitation,calcium hydroxide(Ca(OH)2)and potassium dihydrogen phosphate(KH2PO4)are used to modify fly ash in this study.Therefore,the specific surface area surface morphology,mineral composition and surface functional groups of original fly ash(FA)and modified fly ash(FAKCa)were investigated by BET,SEM,XRD,FTIR and XPS.The adsorption and desorption characteristics of heavy metals by FA and FAKCa and their immobilization effects of heavy metals in contaminated soil were investigated by solution system experiments and soil incubation.The main results are as follows:(1)The adsorption amount of Pb and Cd by fly ash before and after modification was well fitted with Langmiur model(R2=0.993-0.998),indicating it is kind of monomolecular layer adsorption.The maximum adsorption capacity(Qmax)of FAKCa on Pb and Cd is 1.47 and 0.37 folds higher than that of FA,respectively,and the adsorption capacity for Pb by FA and FAKCa is significantly higher than that of Cd.The adsorption kinetics of Pb and Cd by fly ash before and after modification was the best fitted with the pseudo-second-order kinetics model(R2=0.997-0.999),indicating it is mainly chemical adsorption.In addition,SEM results showed that the spherical particles of fly ash are destroyed after modification,and the specific surface area increases which was consistent with BET results.XRD results showed that FAKCa saturated adsorption of Pb and Cd resulted in the corresponding carbonates and Pb oxide precipitation.The results of FTIR analysis showed that the O-H vibration peak of fly ash was enhanced and the O-C-O group was added,which was consistent with the XRD analysis results.XPS analysis results not only confirmed XRD results,but also showed that FAKCa adsorbed Pb and Cd to produce alkali metal oxides.The above results indicated that the adsorption mechanism of modified fly ash on Pb and Cd may exist precipitation besides surface chemisorption.(2)A 45-day soil incubation experiment was conducted to investigate the metal immobilization effects of FA and FAKCa under 70%maximum field water holding capacity.Results showed that 0.1%-0.6%FA treatments increased soil pH by 0.04-0.29 units and the available Pb,Cu and Cd were decreased by 3.30%-29.9%,0.61%-8.03%and 2.40%-22.1%,respectively.0.1%-0.6%FAKCa treatments increased soil pH by 0.25-1.00 units,available silicon content was increased by 2.67-31.94 mg·kg-1,and the available Pb,Cu and Cd were decreased by 45.2%-98.0%,7.04%-70.3%and 25.4%-47.3%,respectively.It can be concluded that FAKCa is more effective to decrease the availability of Pb，Cu and Cd in soil compared with FA.In addition,FA and FAKCa could promote the transformation of Pb,Cu and Cd from exchangeable to residual fraction.(3)A 45-day soil flooding incubation experiment was conducted to investigate the metal immobilization effects of FA and FAKCa Results showed that 0.1%-0.6%FA treatments increased soil pH by 0.05-0.24 units,and the available Pb,Cu and Cd werc decreased by 2.54%-16.4%,0.85%-0.97%and 7.41%-13.4%in the first 30 days.Meanwhile,0.1%-0.6%FAKCa treatments increased soil pH by 0.24-0.91 units,available silicon content was increased by 7.69-46.11 mg·kg-1,and the available Pb,Cu and Cd were decreased by 17.1%-94.7%,0.71%-74.2%and 18.1%-23.1%,respectively.The available and chemical morphologies of HMs are the similar to those under 70%maximum field water holding capacity.This study shows that chemical modification can significantly enhance the adsorption capacities for heavy metals by fly ash,and decrease the availability of heavy metal in contaminated soils.It would provide a new idea for soil remediation and resource utilization of fly ash.Figure[43]Table[29]Reference[173]...