Experimental Study Of Immobilization Of Lead Contaminated Soil Using Phosphate And FeS Particles

Lead contamination is receiving increasing attention of the state and society.Chemical stabilization have been widely applied to immobilized lead contaminated soils,for it's advantages of short repaircycle,high efficiency and less cost.A lot of theoretical research and engineering practice were carried out,many achievements have been achieved,but there are also some problems,including:1)the high efficiency of immobilization agent,2)the long-term immobilization after repaired,3)the potential secondary pollution during the process of immobilization,4)the final destination of repaired soil.These problems have become the bottleneck of lead contaminated soil immobilization.The key to solve above problems is the choice of immobilization agent.In this paper,phosphate,FeS particles and composite reagent?combination of phosphate and FeS particles?is used to immobilize different initial concentration of lead contaminated soil.By comparing the three immobilization agents,study the immobilization effect,long-term stability and the secondary pollution,explore the influence factors of immobilization effect and the stabilization mechanism.Besides,we use the repair soil as landfill cover to investigate the mobility of lead and phosphorus under simulated landfill and rainfall conditions,on this basis,analyzing its suitability as landfill cover soil.The results show that:?1?All of Phosphate and FeS particles have high efficiency to immobilized lead contaminated soil,the efficiency increased with increasing of added amount of chemical and initial concentration of lead.For high lead concentration contaminated soil?7000mg/kg?,the effect of phosphate treated is better than FeS particles under the same drug dosing mole ratio.O n the contrary,for low lead concentration contaminated soil?500mg/kg,1000 mg/kg?,the stabilizing effect of FeS particles is better than phosphate.?2?The addition of FeS particles could significantly enhance the effect of immobilization compared to using phosphate separately,the efficiency increased with increasing of added amount of FeS particles.Besides,when the drug dosing amount remains the same,the the stabilizing effect of complex reagent is better than phosphate treatment.when drug dosing ratio PO₄^3-:Pb=10:1 and PO₄^3-:FeS:Pb=6:4:1,the leaching concentration of lead is 0.24 mg/L?0.19 mg/L respectively,which meet the landfill admission standards?<0.25 mg/L?.?3?The study of Long-term stability shows that all of phosphate and complex reagent immobilizationhave a good long-term stability,besides,it can keepeffect after the balance,the addition of FeS particles did't affected the long-term stability.For high lead concentration contaminated soil,when drug dosing ratio PO₄^3-:Pb=10:1 which the leaching of lead can meet the landfill admission standards,the leaching concentration of phosphorus up to 1064 mg/kg,the excessive amounts of phospho rusto could due to eutrophication risk.While,when use complex reagent immobilization,PO₄^3-:FeS:Pb=6:4:1,the leaching concentration of phosphorus is 443 mg/kg,decreased by 58%,we can see that the composite FeS particles and phosphate could not only ensure the long-term stability but also can effectively alleviate the eutrophication risk.?4?The mechanism of lead immobilization as follows:1)phosphate stabilization:As the introduction of phosphate,the surface electronegativity of soil increase,and then the free Pb²⁺were absorbed in the surface of soil particles to precipitate with phosphate,which could reduce the mobility of lead.Moreover,the dissolve phosphate could react directly withPb²⁺in the form ofprecipitate and ion exchange,eventually realizing the immobilization of lead.2)Complex reagent stabilization:After phosphate and FeS particles were added into soil,the PO₄^3-and FeS absorbed on the surface of soil particles could stabilize some free Pb²⁺through the precipitation,ion exchange,adsorption,and so on.Besides,the phosphate and FeS particles in the soil solution can directly react with the various forms of lead in soil through the precipitation,co-precipitation,ion exchange,adsorption to realize the stabilization of lead in soil.3)the effect of FeS:on one hand,through the adsorption of phosphate and lead in the surface of soil particles,it could provide a micro area environment for FeS,PO₄^3-and Pb,and thus promote the precipitation adsorption and ion exchange process.On the other hand,the reduction of phosphate and the precipitation,adsorption of phosphorus,could effectivelyrestrain leaching of phosphorus,and relieve secondary pollution caused by phosphate.?5?The physical and chemical character of soil immobilized by phosphate and complex reagent could not only meet the requirements of landfill cover,but also can facilitate the long-term stability of the pollutants in soil.Under the simulation of leachate leaching conditions,both of the leaching concentration of lead and p hosphorus in the range of typical garbage leachate pollutants concentration,however,the soil immobilized by phosphate showing higher lead and phosphorus leachability than treated by complex reagent.