Mechanism Of Enhancing Leaching Remediation Of Heavy Metal Contaminated Soil

In order to disclose the leaching kinetics and the Pb and Cd removing mechanism from the contaminated soil were sampled form 0²0 cm layer of a cultivated land near a zinc refinery plant in Liaoning province.Soils were separated into five size fractions?>500?m,100⁵00?m,50¹00?m,2⁵0?m and<2?m?according to the Chinese system of soil particle grade classification standard.The magnetic properties and metal?Pb and Cd?concentrations were measured in all fractions and the bulk soils.The three-step BCR sequential extraction procedure were applied to all fractions and surface samples.The pollution load index?PLI?,frequency magnetic susceptibility(?_fd)and other parameters were also calculated.The batch leaching experiments were used in washing the contaminated soil by tartaric acid,citric acid,oxalic acid and FeCl₃-citric acid.Results indicated that the contents of Pb and Cd,PLI,?_fdd tended to increase with the particle size decreased when the particle diameter<100?m?50¹00?m?2⁵0?m?<2?m?.There was significant positive correlation between the content of Pb and Cd,PLI,?_lff and?_fd.There was very significant positive correlation between the content of Pb and Cd,PLI,?_lff and?_fdd at the level of P=0.01when the particle diameter is<2?m and>500?m.Under the experimental conditions the leaching amount and efficiency of Cd and Pb were increased with the washing time;The washing process of Pb and Cd can be divided into rapid reaction stage,slow reaction stage and desorption equilibrium stage;Washing kinetics of the heavy metal from the soil sample is a heterogeneous diffusion process and it could be better described with Elovich equation.The optimum leaching conditions of three organic acids on leaching Cd and Pb from soil were as follows:the ratio of solution to solid was 20,eluent concentration was 0.2 mol/L,temperature was 30?,pH value was 3.0.When FeCl₃-citric acid as leaching agent,with the decrease of pH?6³,except 5?,the increased of T?25?⁴5??and IS?CaCl₂??0.01 mol/L⁰.5 mol/L?,the washing removal amount of Cd and Pb was increased.The results showed that with the increase of eluent concentration?0.01 mol/L⁰.5 mol/L?,soil pH value decreased,Eh value increased,and the change of soil pH and Eh were more obvious of FeCl₃-citric acid than those of FeCl₃ or citric acid;the 4 kind washing solutions on the acid extractable and the reducible can be better base on the forms change of Pb and Cd in the soil after washing.Orthogonal experiment show that the removal rate of Pb and Cd was the highest when the temperature was 55?,pH=3,FeCl₃:citric acid=20 M:200 M,C?CaCl₂?=200 M.the primary and secondary factor that effect the leaching efficiency of Pb and Cd is temperature,pH,FeCl3:ci-tric acid and IS.