Mechanism Of Sorption, Desorption, Diffusion And Remediation Of Heavy Metals In Soils

Widely distributed soil pollution by heavy metals requires studies of fate and transport of heavy metals in typical soils in China. This dissertation is designated to investigate the adsorption/desorption behavior and mechanism of four heavy metals (Cd, Cu, Pb, Zn) on two typical soils (Kaolin and Q₄ Loess) on the basis of laboratory experiments. Several factors (i.e. ionic strength, pH, temperature, solid solution ratio etc.) have been studied with regarding their effects on adsorption/desorption characteristics and the relevant mechanisms. A novel parameterization method was put forward to calculate the transport parameters (partitioning coefficient and effective diffusion coefficient). A novel desorption partitioning coefficient was defined along with a novel parameterization method. Novel analysis scheme and the theoretical model were both developed to simulate the remediation of heavy metals polluted soil using a comlexing agent. A series of significant conclusions were drawn accordingly.1) The adsorption of heavy metals on soils is controlled by ion type, soil constituent, equilibrating duration, pH, solid solution ratio etc. The calcite in loess was found greatly responsible for adsorption of Pb(II) and Cu(II) instead of Zn(II) and Cd(II). The silty quartz in loess was found for the first time responsible for adsorption of Pb(II), Cd(II), Zn(II). Kaolinite can react with heavy metals in the form of surface complexes to give rise to the adsorption capacity of soils.2) The hysterisis behavior of desorption from soils and the re-adsorption of heavy metal complexes on soil components were experimentally verified and theoretically characterised. The desorption behavior of four metal cations from Kaolin and loess were systematically investigated and found correlated with ion type, initial ion concentration, soil component and desorption method etc. The proposed desorption-process-oriented desorption partition coefficient could take into account of solute concentration and desorption method and thus appeared superior to the convetional definition.3) A new method was established and incoporated into a compiled code to parameterize the transport of pollutant in soil on the basis of total mass concentrations. The proposed method was testified by analyzing both the reported data and test results in this study. This method was characterized with clear concepts, convenient handling and stable results.4) Diffusion behavior of heavy metals in one dimensional soil column was studied by experiments. The parameterized method was verified and some improvement was made. The inhibiting Pb(II) and Cd(II) diffusion ability of loess was found better than kaolin. The effective diffusion coefficient of metal ions in soil columns became smaller along with the increasing radius of metal ions: Pb(II)d obtained by anti-analysis with soil columns was much less than that obtained by Batch method and some suggestions were proposed.5) A theoretical model (decaying transport theory and complexing simplification) was put forward and was experimentally verified reasonable to simulate the remediation of polluted soil. Column leaching tests showed that the complexing agent NTA can greatly improve the remedying performance of heavy metals polluted soil. NTA could remove more Cd than Pb from polluted soil, and could remove more pollutant from kaolin than from loess, and thus appears applicable in pump-and-treatment remediation of contaminated sites.