Study On Effect And Mechanism Of Passivation Remediation Of Simulated Lead And Cadmium Contaminated Soil

The in-situ passivation/stabilization remediation technology has been widely used in the remediation of soil heavy metal pollution due to its advantages of sim ple operation,low cost,quick results and obvious effects.This technique mainly adds different passivation materials to the soil,through regulating and changing the soil environment and the physicochemical properties of heavy metals in the soil,enabling heavy metals to undergo a series of physical and chemical reactions such as adsorption,surface complexation,oxidation-reduction,dissolution/precipitation,ion exchange and so on,thereby reducing their mobility and bio-availability to achieve the purpose of remediation of contaminated soils.The key to the application of this technology is to screen and apply the most suitable passivator materials for different soil environments as well as different types and characteristics of heavy metals.This article is based on a large number of domestic and foreign research literature on the remediation of heavy metal pollution in soil,taking a test rice field soil in Wangcheng District of Changsha as the research object,simulatedly cultivating lead（Pb）and cadmium（Cd）heavy metal contaminated soil by artificially adding a certain amount of heavy metal pollutants and us ing it as experimental soil.First,three kinds of different soluble phosphate materials（ammonium dihydrogen phosphate,calcium superphosphate and hydroxyapatite）were used to passivate the simulated Pb-,Cd-containing solis.Ethylene Diamine Tetraacetic Acid（EDTA）and physiologically based extraction test（PBET）leaching experiments were applied to study the effect of different phosphate passivation treatments on phyto-availability and bio-availability of Pb,Cd in soils as well as to evaluate and compare the passivation effects of three phosphates on Pb,Cd heavy metals,determining the appropriate type of phosphate and the optimal amount of phosphate on this basis.Then,adopting three kinds of amendments including superphosphate,humic acid,fly ash and their complex combination to passivate the articially simulated Pb-and Cd-containing solis.The passivation effeciency was evaluated by CaCl ₂ and Triethylenetriaminepentaacetic acid（DTPA）extraction method as well as BCR morphological classification experiment.Analyzing the correlation of soil pH values,contents of available phosphorus and contents of extractable Pb,Cd with CaCl₂,DTPA through pearson correlation matrix method to evaluate the influencing factors of passivation treatment of Pb-and Cd-containing solis by applying superphosphate,humic acid,fly ash and their combination.Finally,the microstructures and structures of the soil was explored by by X-ray diffraction（XRD）and scanning electron microscopy with X-ray energy dispersive spectroscopy（SEM-EDS）further to elaborate the passivation mechanism.The main results were as follows:（1）The addition of three different soluble phosphate materials can reduce the phyto-availability and bio-availability of soil heavy metals Pb and Cd in different degrees.The passivation effect was related to the type of phosphate and the amount of application and the order of passivation effect was:ammonium dihydrogen phosphate>calcium superphosphate>hydroxyapatite,for the same kind of phosphate material,the more phosphate was applied,the better the effect;In addition,applying ammonium dihydrogen phosphate and Calcium phosphate can lower the pH value of the soil and acidic soil solution was good for the dissolution of phosphates and heavy metal compounds,which makes it easier to form heavy metal phosphate precipitation.Therefore,the effect of applying ammonium dihydrogen phosphate and superphosphate was more effective than that of applying hydroxyphosphorus;For ammonium dihydrogen phosphate and calcium superphosphate,ammonium dihydrogen phosphate had a higher efficiency of passivating heavy metals in the soil,however,ammonium dihydrogen phosphate is a very soluble phosphate and its excessive application of ammonium phosphate may cause eutrophication,it should be used with caution;For the lower passivation effect of superphosphate,it can be considered in combination with other deactivators in order to ensure its better passivation effect,while avoiding adverse effects on the soil environment.（2）All passivation processings except single humic acid addition could reduce CaCl₂ and DTPA extration contents of Pb,Cd in soils,where the optimal efficiency could be achieved by the sequential addition order as superphosphate and humic acid,following by fly ash.In the presence of phosphate,humic acid and fly ash,the Pb and Cd could effectively change the chemical form of heavy metals Pb,Cd,promoting the conversion of two heavy metals from the more mobile form into the low-migration residue form,thereby effectively reducing the toxicity of Pb,Cd heavy metals.（3）There was a weakly positive correlation between soil pH and CaCl ₂,DTPA extraction content of Pb,with a negative correlation bet ween soil pH and CaCl₂,DTPA extraction content of Cd,while there was a significantly negative correlation between available phosphorous content and CaCl ₂,DTPA extraction content of Pb,Cd,suggesting its crucial role of available phosphorous contents to control the activity of Pb,Cd.（4）Throughout the XRD and SEM-EDS analyses,it can be found that ion exchange was the predominant mechanism in heavy metal passivation by single superphosphoshate,where the heavy metals were transformed into insoluble Ca-containing phosphate mixture.The dissolving/precipitation or surface adsoprtion could be concluded as the main mechanism in the combination of three passivation agents that converted heavy metals to lead phosphate precipitate(Pb ₃（PO₄）₂]or mixed heavy metal mineral[PbFe₃（SO₄）（PO₄）（OH）₆],so as to obtain superior heavy metals passivation achievement.