Study On Immobilization And Particle-size Effect Of Different Amendments On Lead Contaminated Soil

Soil is material basis and indispensable natural resources for humans.With acceleration of industrialization and urbanization,the problem of heavy metals pollution in soil is increasingly serious.There have been a lot of researches on in-situ immobilization remediation of soil contaminated with lead?Pb?,but the influence of addition of amendments on migration and transformation of Pb in different soil particle-sizes has not been explored.In this study,loess soil in Guizhou province was selected as the study object to be polluted by exogenous,three kinds of materials,goat manure,lime and phosphate,were used as amendments.The effects of different amendments on physical and chemical properties of soil and concentrations of Pb in different forms were compared by single and combined application experiments.Based on the international standard of soil particle-sizes classification,different soil particle-size fractions were extracted by ultrasonic,wet sieving and centrifugation.The concentrations and forms of Pb in different particle-size fractions were determined,microscopic characterization of different particle-size fractions before and after amendments were characterized.Meanwhile,the relationship between forms of Pb in different particle-size fractions and forms of Pb in bulk soil was analyzed,and bioaccessibility of Pb in the passivated clay fraction was determined.The main findings are as follows:?1?Soil pH values was affected significantly?P<0.05?by single and combined application of three amendments.In single application treatments,soil pH values were increased significantly?P<0.05?by addition of goat manure and lime,and decreased significantly?P<0.05?by addition of phosphate.In combination treatments,effect of phosphate or lime combined with goat manure on soil pH values were significantly lower?P<0.05?than that of single application treatments,these results indicating that goat manure could buffer the change of soil pH values.The concentrations of organic matter?OM?in bulk soils were affected by application of amendments,in which application of goat manure increased OM concentrations,while lime and phosphate significantly reduced?P<0.05?OM concentrations.The difference of OM concentrations between combined application groups and control check?CK?did not reach a significant level?P<0.05?.The effect of amendments addition on OM concentrations of soil particle-size fractions were different from that of bulk soil.OM concentrations of fine sand fractions and silt fractions were increased significantly by amendments application without goat manure.Concentrations of Olsen-P and CaCl₂-P were increased by all of treatments,the model of soil phosphorus leaching threshold value showing that soil phosphorus would leach while the concentrations of CaCl₂-P was more than 2.90 mg·kg^-1.?2?Soil DTPA-Pb concentrations were decreased significantly?P<0.05?through application of amendments.In single application groups,DTPA-Pb concentrations were reduced significantly?P<0.05?by different amounts of three kinds of amendments.Immobilization effect on DTPA-Pb was:phosphate>lime>goat manure while same amounts of amendments.At the same time,Immobilization effect of combined application group was not better than that of the single application treatment.Three kinds of amendments had different effects on soil TCLP-Pb concentrations.TCLP-Pb concentrations was decreased by adding goat manure,and concentrations of decrease was inversely related to the amount added.It was significantly increased?P<0.05?by adding lime,and the concentrations increased was inversely related to the amount added.The change of TCLP-Pb concentrations had not obvious regularity by application of phosphate and amendments combined,LP5 had the best Immobilization effect in all treatments,it was 71.60%.?3?The results of Pearson correlation analysis showed that there was no significant relationship between DTPA-Pb concentrations and concentrations of four forms of Pb in soil,which indicated that DTPA-Pb concentrations was not determined by the concentrations of single one form of Pb.There was a significant positive correlation?P<0.01?between TCLP-Pb concentrations and concentrations of weak acid-extractable fractions,indicated that it might be the main source of TCLP-Pb concentrations.Weak acid-extractable Pb and residual Pb concentrations were not significantly correlated with the other forms of Pb,but reducible and oxidizable Pb concentrations showed a significant negative correlation?P<0.01?,which suggested that availability of soil heavy metals might be changed by mutual transformation between reducible and oxidizable Pb after amendments added.?4?Pb distribution in different particle-size fractions was bimodal,and concentrations and speciation distribution in different particle-size fractions were affected by addition of amendments.XRD analysis showed that there were diffraction peaks in coarse sand fractions and fine sand fractions after amendments application in comparison with CK,but not in silt fractions and clay fractions,which indicated that hard soluble Pb compounds were mainly formed on coarse sand and fine sand fractions through amendments added.SEM-EDS results showed that surface of particle-size fractions became smoother and Pb concentrations increased after amendments application,indicated that amendment might interact with soil components to form crystal mineral coating,which bound heavy metal ions to soil particles and prevented heavy metal leaching.?5?Pearson correlation analysis between bulk soil and particle-size fractions showed that DTPA-Pb concentrations in bulk soil was positively correlated?P<0.05?with the weak acid-extractable Pb in coarse sand,fine sand and clay fractions,and also positively correlated?P<0.05?with the easy-to-use Pb in fine sand and clay fractions,indicated that plant available Pb in bulk soil mainly came from fine sand and clay fractions.There was a significant positive correlation?P<0.01?between TCLP-Pb concentrations in bulk soil and in silt and clay fractions,indicating that TCLP-Pb concentrations in bulk soil was mainly controlled by in silt and clay fractions.However,there were no significant positive correlation between bulk soil and the corresponding forms in each particle-size fractions,and a significant negative correlation between bulk soil and other forms,which showed that change of form distribution of Pb in soil after amendment added were caused by the common influence of various forms of Pb in different particle-size fractions.?6?DTPA-Pb concentrations were reduced by addition of amendments,but in vitro experiment showed that concentrations of bioavailable Pb in clay fractions increased after amendments treatment,among them,LP5 increased least,which was 30.43 mg·kg^-1,not significantly different?P<0.05?compared to CK,L1 increased most,which was 403.13 mg·kg^-1.