| Recently, the continuous development of industry has brought significantly negative effects to the environment. For example, soil and water pollution by heavy metals (such as Pb2+or Cd2+etc.) occured frequently and it has attracted the attention of the whole society. In addition, the diffusion and migration of contamination in soil are also essential. The convection-dispersion parameter of clay is fundamental data for the evaluation and remediation of heavy metal contaminated soil. The existing methods for measuring this parameter have many uncertainy factors, such as pretreatment of samples, sample state, experiment conditions and so on. In this thesis, the interaction between heavy metal and kaolin clay are researched, such as:permeability, adsorption, diffusion and dispersion. The method of measuring convection-dispersion parameter of heavy metal in the low-permeable clay is proposed. The main conclusions include:(1) Well linear relation between void ratio of kaolin and logarithmic of permeability is acquired. The permeability decreases with the decrease of void ratio. By consolidation, the saturated permeability of soft and hard kaolin can be as low as1×10-7cm/s, but the sandy kaolin fails to reach this magnitude. Conbined with the physical properties, permeability and the conveniences of sample preparation, the soft kaolin is recommended as the materal of dispersion and column experiment.(2) In adsorption experiments, the adsorption of Pb2+and Cd2+in kaolin clay mainly happens in the first0.5hours, and adsorption equilibrium time of Pb2+and Cd2+is6.4hours, respectively. With the increased initial concentration of Pb2+or Cd2+, the total adsorption amount of Pb2+or Cd2+increases gradually until the adsorption equilibration, while on the contrary, the adsorption ratio decreases at the same time. The capability of kaolin adsorption for Pb2+is better than that for Cd2+, and saturated adsorption amount of Pb2+and Cd2+is5.190.1.243mg/g, respectively.(3) For Pb2+and Cd2+polluted soil, digestion method and HCl extract method are better than deionized water extract method, and digestion method is the best among the three methods. The concentration of contaminant in deionized water extract method is similar with that in void water, and in digestion method the concentration of contaminant can be regarded as the total contaminant amount. Both the deionized water extract and digestion method are suggested to be used when dealing with the soil samples of kaolin either polluted by heavy metals or in the diffusion and migration experiment.(4) The effective diffusion coefficient of Cl" in kaolin clay is related to pore velocity. The bigger void ratio is with the bigger effective diffusion coefficient and bigger curve factor. The dispersion coefficient of Cl-in kaolin is related to pore velocity, the bigger pore velocity leads to the bigger dispersion coefficient, while the degree of dispersion which is0.003m has no relationship with pore velocity.(5) In diffusion experiments of kaolin clay, the concentration of Pb2+or Cd2+with deionized water extract method is smaller than that with digestion method, and this shows that most of Pb2+or Cd2+adsorbed on kaolin clay doesn’t desorb in the deionized water extract method. So as a result, the retardant factor calculated in digestion method is bigger than that in deionized water extract method, and the retardant factor of Pb2+in kaolin clay is bigger than that of Cd2+in both two methods which is corresponded with the adsorption capacity of kaolin clay.(6) In soil column experiments, the retardant factor of Pb2+or Cd2+in kaolin clay varies with the hydraulic head, as a result of the difference of flow velocity. With high hydraulic head, the flow velocity is quick, and the retardant factor of Pb2+or Cd2+is small. With low hydraulic head, the flow velocity is slow, and the retardant factor of Pb2+or Cd2+is big. |
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