Research On Mechamism And Long-term Performance Of A New Phosphate-based Binder For Stabilization Of Soils Contaminated With High Levels Of Zinc And Lead

The remediation of high concentration heavy metals contaminated industrial sites is currently an emergent concern in geoenvironmental engineering practice.As a widely used remediation technology,Stabilization/solidification(S/S)involves mixing binders and contaminated soils to reduce the mobility of heavy metals and enhance the strength of the soils by both physical and chemical means.The commonly used binders in S/S method are highalkali cementitious materials,such as Portland cement(PC),quicklime and pulverized fly ash.However,under long-term external conditions such as freeze-thaw cycling,carbonation,acid rain and sulfate attack,heavy metals in the cement-stabilized soils would leach easily.In addition,the presence of high concentration heavy metals in the soils,such as Zn and Pb contaminants,has significant negative effect on the hydration of the cement-based binders.Moreover,both Zn and Pb contaminants are commonly encountered in contaminated industrial sites in China.Therefore,it is necessary to develop alternative binders to stabilize soils that have relatively high-level of Zn and Pb contaminants.This dessertation is financially supported by the Key Program of National Natural Science Foundation of China(No.41330641),the National High Technology Research and Development Program of China(No.2013AA06A206),the General Program of National Natural Science Foundation of China(No.41472258),the Key Program of Basic Research Plan of Jiangsu(No.BK2010060),the Graduate Research and Innovation Plan for Regular Institution of Higher Learning in Jiangsu(No.CXZZ 0159),and the Scientific Research Founation of Graduate School of Southeast University(No.YBJJ1241).This study develops KMP as a new phosphate-based binder.KMP consists of oxalic acid-activated phosphate rock(OPR),monopotassium phosphate(KH2PO4)and reactive magnesia(MgO).Under extreme weather conditions such as freeze-thaw cycling and carbonation,the mechanisms of Zn and Pb immobilized by KMP are interpreted based on the results of X-ray diffraction(XRD),scanning electron microscope(SEM),energy spectrum analyses(EDS),and mercury intrusion porosimetry(MIP)analysis.In addition,the leachability,strength and ecotoxicity characteristics of KMP-stabilized soils with Zn or Pb contaminant individually and Zn and Pb contaminants together are investigated using laboratory tests such as acid buffering capacity test,toxic characteristics leaching procedures(TCLP),pH-dependent leaching test,modified European Community Bureau of Reference(BCR),unconfined compression tests(UCTs),and luminescent bacteria test(LBT)analyses.Moreover,the effects of widely varying pH of simulated acid rain on the leaching characteristics of KMP stabilize Zn and Pb contaminated soils are examined by semi-dynamic leaching tests.Microsturctural characteristics of stabilized contaminated soils after soaking are evaluated.For all the tests,Portland cement is also tested as a conventional binder.The main contents and results are simply introduced as below.(1)The study and practices of S/S method are reviewed.The efficiency and mechanism of phosphate as a binder are summarized.The long-term performance of stabilized heavy metal contaminated soils is introduced.The effect of rainfall on diffusion properties is demonstrated.The problems in previous studies are discussed.(2)The basic physico-chemical properties of the test materials are summarized.The preparation methods of acidified phosphate rock,KMP binder,contaminated soils,stabilized contaminated soils,simulated acid rain solution are demonstrated.The methods and tests of micromechanism,leaching,strength,and ecotoxicity characteristics are listed.(3)The interaction between KMP and Zn and Pb contaminants are discussed.The phases of major hydration products and chemical compound with Zn and Pb are quantitatively determined by XRD tests.In addition,the micro structural characteristics and chemical elements mass rate in typical areas of thsese phases are evaluated by SEM/EDS tests.Particularly,the variation of intensity of major products with initial Zn and Pb concentration is used to illustrate the changes of leaching and strength of KMP stabilize Zn and Pb contaminated soils.The main contents include:? the formation of Zn3(PO4)2·4H2O,CaZn2(PO4)2·2H2O and Pb5(PO4)3F in the stabilized soils may be the main mechanism for immobilization of Zn and Pb with the KMP binder.? the presence of magnesium phosphate hydration products such as Mg3(PO4)2,Mg3(PO4)2·8H2O,and MgHPO4·7H2O are the main source of the strength of the stabilized soils.(4)Based on the acid buffering capacity,pH-dependent,and BCR leaching tests,the mechanisms controlling the Zn and Pb release of KMP stabilized Zn and Pb contaminated soils in the long-term external conditions such as freeze-thaw cycling and carbonation are determined.The variation between acid buffering capacity with initial Zn and Pb concentration is discussed.In addition,the correlation between Zn,Pb and Ca,Mg in leachate is establised respectively.Moreover,the chemical forms of Zn and Pb for soils before and after reaction with leachant are compared.Compared with PC,the KMP stabilized soils have:? higher acid buffering capacity.? 2 to 3 of magnitudes lower leached amounts of Zn and Pb,which are stable for the leachate pH of 8 to 10.5 and 4 to 10.5 separately.? 2 to 3 times higher residual fraction(F4)of Zn and Pb separately.(5)The micromechanisms of KMP stabilized Zn and Pb contaminated soils in the long-term external conditions such as freeze-thaw cycling and carbonation are determined.By XRD,SEM/EDS,and MIP tests,variation of the cumulative pore volume,fitting parameters such as ? and ?i in Gaussian function change with initial Zn and Pb are discussed.In addition,the effects of carbonation on the transformation of major hydration products and chemical compound with Zn and Pb are demonstrated.Moreover,microstructural characteristics and chemical elements mass rate in typical areas are determined.The main contents include:? freeze-thaw can lead to the increase of cumulative pore volume(especially for macropores and entrained air voids,entrapped air voids,preexistnig microcracks).? the carbonation leads to the generation of carbonate and the decrease in soil pH.? the KMP stabilized soils have higher microstructural stability than that of PC under the extreme condition of freeze-thawing cylces and carbonation.(6)The long-term performances of KMP and PC stabilized Zn and Pb contaminated soils are discussed.Through freeze-thawing and carbonation tests,the mathematical relation between leached Zn and Pb concentration and electric conductivity(EC)and soil pH is established.The unconfined compressive strength(qu)is analyzed by considering dry density and soil pH.The rate of quality and volume loss as well as carbonation depth is discussed.Relative luminous intensity(I0)and the EC50 values are calculated from the LBT test.The main contents include:? freeze-thaw can lead to the increase of the leached Zn and Pb,decrease of strength,and rise of biotoxicity.? carbonation can cause the opposite effect of the long-term performances.? KMP stabilized soils have higher ability to resist action of freeze-thaw cycles and carbonation than that of PC.? The service life of KMP and PC stabilized Znl.5Pb2 contaminated soils are 50 years and 0.4 years,and the corresponding carbonation depth are 0.17 m and 0.03 m respectively.(7)The leachability of KMP and PC stabilized Zn and Pb contaminated soils in semi-dynamic tests is discussed.The variation of effective diffusion coefficient(D*)calculated from the results of semi-dynamic leaching test is analyzed by considering different leachant pH under the freeze-thawing cycles and carbonation conditions.The transformations of microstructural characteristics and chemical elements mass rate in typical areas before and after soaking in simulated acid rain solution are compared.The main contents include:?the D*values of Zn and Pb increase by 1 to 3 orders of magnitude as the pH of the leachant decreases from 5.0 to 2.0,while the change is small when the pH ranges from 5.0 to 7.0.?contrary to the negative effect of freeze-thawing cycles,the effect of carbonation causes the dicrease of D*values.? compared with PC,the D*values of KMP stablized contaminated soils are 3 to 4 orders of magnitude lower than that of PC.