| Heavy metal such as lead (Pb)is toxic to humans,especially young children,and toanimals.Metal-contaminated soil is a primary source of metals exposure to youngchildren.One of the primary sources of metals contamination is industrial activityincluding mining tailing,leading to metals accumulation in the soils and risk to human'shealth through soil ingestion and the food chain such as vegetables.The previous reportby our research group showed that there were thousands hectares of soil and plant heavilycontaminated by Pb and Zn in Shangyu,Zhejiang Province,China.Therefore,increasingawareness of the hazard that heavy metals can cause to humans make it necessary toremediate metal contaminated soils.This dissertations aimed to understand the mechanism of phosphorus (P)inducedlead immobilization in the soil contaminated by Pb/Zn mining tailings in Shangyu,Zhejiang Province,China,to assess the effects on the toxicity and bioavailability of Pband Zn after P application,and to figure out the most cost-effective P source and itsapplication rate,using X-ray diffraction (XRD),model MINTEQ,toxicity characteristicleaching procedure (TCLP),sequential extraction procedure,and plant indicator.Themain experiments and conclusion are as follows:(1)XRD pattems were analyzed to observe the mineralogy of P with orwithout chlorine (Cl)induced Pb immobilization in water and soil environment.It wasfound that occurrence of pyromorphite [Pb5(PO4)3Cl,ClP] and lead phosphate hydroxide[Pb10(PO4)6(OH)2,PH] was evident in the treated bentonite each for P addition with orwithout Cl 24 hours later.Furthermore,pyromorphite like compounds were alsoidentified in the P treated soils one month later.It was determined that the primarymineralogy of P induced Pb immobilization was through pyromorphite like compoundsformation after P addition.Therefore,it was theoretically proved to apply P materials for remediation of soil which was contaminated by Pb/Zn mining tailings in Shangyu,Zhejiang Province,China.(2)Effect of chlorine (Cl)on in situ lead immobilization using phosphorus ina soil contaminated by lead and zinc mining tailings was evaluated.The results showedthat water soluble and exchangeable Pb was reduced by 92.0 %～95.1% in the soil after Papplication.It was also suggested that application dosage of phosphorus at the P / Pbmolar ratio of 0.6,was enough to remediate Pb toxicity in the soil.Compared to withoutCl treatment at the level of molar ratio of 0.6 P / Pb,water soluble and exchangeable Pbin the soil treated with Cl was reduced markedly.It was concluded that the effectschlorine addition on in situ lead (Pb)immobilization using phosphate were improved.Visual MINTEQ model was employed to figure out Pb and P species distribution andsaturation indices for minerals in the soils treated by P and Cl.The results showed that Pbactivity was controlled by lead phosphate in the soil,especially pyromorphite[Pb5(PO4)3Cl],which would be the reason for Pb bioactivity reduction in the soil afterapplication of Cl and P.The effective Pb bioactivity reduction indicated that addition ofCl was necessary to improve in situ lead immobilization using phosphorous in thelead-contaminated soil.(3)The objective of this work was to evaluate the efficiency of calcium (Ca),pH,and phosphorus on the leachability of Pb and Zn in the contaminated soil.It wasfound out that the effects of phosphorus on the leachability of Pb and Zn wassignificantly affected by pH,and pH of 6.0～7.4 would be the best condition for metalimmobilization by phosphorus.It was worthwhile to noted that there would be a biggestreduction of Pb and Zn leachability (up to 68 % and 94 %)when the condition pH wasadjust to 6.5～7.5 after SSP application,and that would be useful for in situ technique.Itwas evidently that Zn was mobilized by Ca treatments with various pH conditions,but noPb.In neutral condition,the bioavailability of Pb and Zn in the contaminated soil couldbe synchronously farthest reduced after combined used of Ca and P with a 0.4～1.0 mole ratio of Ca/P application rate.(4)The effects of three phosphorus fertilizers (single superphosphate-SSP,calcium magnesium phosphate-CMP,and phosphate rock-PR)on chemical fractions oflead (Pb)in a soil contaminated by Pb/Zn mining tailings using a sequential extractionprocedure were evaluated.All three phosphorus fertilizers reduced Pb solubility andbioavailability effectively.It was suggested that application rate of phosphorus fertilizersat the level of 7.0 molar ratio of P / Pb,was enough to remediate Pb toxicity in the soil.At the level of 7.0 molar ratio of P / Pb,the effect of Pb remediation by phosphorusfertilizers followed the order in terms of the sum of decreased nonresidual fractions:SSP(85 %)≈CMP (83 %)＞＞PR (16 %).If linking up chemical fractions of Pb in soil withits bioavailability,the effect of Pb remediation by three phosphorus fertilizers followedthe order:CMP (94 %)≈SSP (92 %)＞PR (61%).Meanwhile,all three phosphorusfertilizers treatments effectively transformed soil Zn from water soluble (excluding SSP),exchangeable,and organic fractions to the residual fraction relative to the control.PR wasthe most economic amendment in terms of the most bioavailable (exchangeable)fractionof Pb (up to 79 %),and Zn (up to 66 %)in soil.It was suggested that the effectiveconversion of Pb and Zn from potentially available fractions to the residual fractionindicated a potential of phosphorus fertilizers for in-situ Pb immobilization in soilscontaminated by Pb/Zn mining tailings.(5)A pot experiment was conducted to evaluated the effects of P amendmentson the toxicity and bioavailability of Pb and Zn in a soil contaminated by mining tailingsusing toxicity characteristic leaching procedure (TCLP)and water soluble,exchangeableleaching procedure,and to find out the appropriate application rate to reduced the soilTCLP Pb to below the EPA regulatory limit level.The results showed that TCLP Pbconcentrations were significantly decreased by up to 93.3 % for MPP treatments and upto 68.5 % for SSP treatments after P application,and the dose required to reduceleachable Pb below the EPA regulatory limit level was found to be around the molar ratio P/Pb=0.6 for MPP and 1.8 for SSP.Both MPP and SSP were found to reduce theexchangeable Pb and Zn concentrations,but variable in water soluble Pb and Zn.It wassuggested that all bioavailable Zn concentrations including water soluble,exchangeable,and TCLP in soil were significantly negative correlated to soil pH values,which mayindicate that the activity of Zn in the soil was mostly controlled by soil pH value evenafter P application.These results suggest that P as MPP and SSP successfully decreasedthe toxicity and bioavailability of Pb and Zn in the contaminated soil.(6)A pot experiment was conducted to determine the influence of phosphorus(P)application on the translocation and accumulation of lead (Pb),zinc (Zn),andcadmium (Cd)in a soil solid-liquid-plant system using a plant Swamp Morningglory(Ipomoea aquatica,Forsk)as indicator and Visual MINTEQ model.A contaminated soilcontaining 22830 mg·kg-1Pb,746 mg·kg-1Zn,and 10 mg·kg-1Cd was amended with P asKH2PO4 at 0:1,0.18:1,0.36:1,0.72:1,1.08:1,and 1.44:1 P:Pb mole ratios.The resultsshowed that Pb concentration in the root was significantly decreased after P application,while the Zn concentration were increased,but no impact on Cd concentration after Papplication,compared to the control.Further more,concentrations of Pb,Zn,and Cd inplant tissue were sharply reduced after P application by 53 %～92 %,35 %～71%,and 59%～86 %,respectively;Concentrations decreased exponentially with increasing amount ofP;Relationship between Pb and Zn (r=0.993,p＜0.01),between Pb and Cd (r=0.986,p＜0.01)suggested that there was a positive relationship between the mechanismscontrolling the phytoavailability of Pb,Zn,and Cd.It was suggested that Cdconcentrations in the soil solution were reduced generally by P application as a result ofthe increased soil pH,and Zn concentrations were no changed,but Pb concentrationswere increased.Species distribution showed that PbHPO4,PbOH+ and PbH2PO4+ werethe main complexation species of Pb in the soil equilibrium solution after P application.Itwas also suggested that Pb accumulation by root were negatively correlated to Pb in soilsolution with coefficient of-0.872 (p＜0.05),and that water soluble Zn and Cd concentrations were reduced while the concentrations in root tissues were increased afterP addition.These results indicate the difference between Zn,Cd and Pb and suggest thatP as KH2PO4 successfully decreased the phytotoxicity of Pb,Zn,and Cd in thecontaminated soil.(7)A field demonstration was conducted at a mining tailings-contaminatedsite to assess the effectiveness of P-induced Pb immobilization using XRD,TCLP,sequential extraction procedure,inorganic P fractionation procedure,and cabbage(Brassica campestris L.chinensis)as a plant indicator.Three phosphorus fertilizers(single superphosphate-SSP,calcium magnesium phosphate-CMP,and phosphaterock-PR)were applied as P amendments in the field demonstration.Lead immobilizationwas attributed to the P-induced formation of pyromorphite like compounds[Pb10(PO4)6(OH/Cl)2] and Pb-P-Ca/Fe compounds,which were identified in the treatedsoils.It was found that the addition of all three P fertilizers significantly decreased thepercentage of non-residual fraction (by 34 %～64 %)and TCLP (7 %～59 %)soil Pb,andthen reduced the uptake of Pb (16 %～58 %)by the cabbage compared to the control (CK).In addition,at a 0.8 molar ratio of P / Pb application rate,SSP and CMP treatmentsreduced TCLP Pb to below EPA regulatory limits level in the soils.The results showedthat the concentration of Zn in non-residual fraction (by 27 %～79 %),TCLP (4 %～39 %),and plant uptake (39 %～73 %)were significantly reduced after CMP and PR application.Zn concentrations in plant uptake (1%～12 %)were decreased after SSP application,while the TCLP concentrations were slightly increased,which were far below the EPAregulatory limits level.Results of regression analysis indicated that Ca8-P was theprimary form of P fertilizers to decrease the bioavailability of Pb and Zn in the soil-plantsystem.Therefore our field trial demonstrated that it was effective and feasible to reducePb,Zn availability in soil and cabbage contaminated by mining tailings using P fertilizersin China.(8)The objective of this study was to assess the stability of P-induced lead immobilization in the field scale using XRD,TCLP,and inorganic P fractionationprocedure.Lead immobilization was attributed to the P-induced formation ofpyromorphite like compounds [Pb10(PO4)6(OH/Cl)2] and Pb-P-Ca/Fe compounds,whichwere identified in the treated soils.Intensity of those peaks was increased along with theincreasing of treated time from 1 m to 6 m～24 m.It was found that there was an increasein TCLP Pb reduction along with the increasing of treated time from 1 m to 6 m～24 m,and that all three P fertilizers reduced TCLP Pb concentration in the treated soils to belowthe EPA regulatory limits level 24 months after P fertilizers application,which stronglydemonstrated that it was stable for P-induced lead and zinc immobilization in thecontaminated soil in the field.It was concluded that PR was the most cost-effectivematerial for lead immobilization considering both its long-term effectiveness and price.
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