| With the rapid development of urbanization and industrialization, a large number of chemical fertilizer and pesticide application and many heavy metal pollutants were carried into the farmland soil directly or indirectly. The availability of lead (Pb) or cadmium (Cd) would decreased with the time after the polllutants of Pb or Cd was added into the soil, this process was affected by soil mechanical composition, cation exchange capacity(CEC), organic matter, pH, and so on. Some research about the single factor have been done, but the soil as a complicated heterogeneous system with various factors existing and interacting, it would have more practical significance to research the effect of soil properties on the stabilization processes of Cd and Pb in the natural conditions.The stabilization processes of Cd and Pb in single and combined metal-contaminated soil of China were studied. MgCl2 extraction was employed to examine the available speciation of Cd, Pb in soils. Cd, Pb or their mixture were added into twenty-seven soil samples respectively. Each treatment were sampled at 2rd,7th,15th,30th,60th,90th,180th, and 360th day after pollution and then extracted using 0.5 mol·L-1 MgCl2. Four kinds of dynamical equations were used to simulate the stabilization process of Cd or Pb, then, the optimum fitting equation was selected, the parameters modeled by dynamical equations about the stabilization processes of Cd and Pb were gained. By means of regression analysis and path analysis, the effect of different soil properties (CEC, organic matter, soil mechanical composition and pH) on the stabilization processes of Cd and Pb was researched, the most important factor was confirmed. The results had been drawn as follows:1) After 500 mg·kg-1 Pb,1.0 mg·kg-1 Cd or 500 mg·kg-1 Pb+1.0 mg·kg-1 Cd were added in soil, soil availability of Pb and Cd were decreased rapidly during the first some days, then, decreased slowly, and stabilized after 30 day's incubation. The differences of content of available Pb or Cd in the 27 soil samples were very significant, generally, in the red soil with low pH, the content of available Pb or Cd was higher, while, in the soil samples with high pH, such as fluvo-aquic soils, cinnamon soil, black limestone soil and so on, the content of available Pb or Cd was lower. In the red soil, black soil and fluvo-aquic soil, which taken from the long-term positioning sites, the content of available Pb or Cd in the NPKM and 1.5NPKM treatment with the high pH and high organic matter content were lower than that in the N, NP, and NPK treatment.2) The stabilization process of Cd and Pb in the soil were simulated by first-order exponential decay function, second-order equation, Elovich equation, and two-constant equation. The second-order equation and first-order exponential decay function perfectly described the changes of available Pb and Cd with incubation time in the 27 soil samples, the correlation coefficients of the equations came up to the most significant standard (r>0.90), and the second-order equation was the optimum model, because its correlation coefficients was the highest. Then, the correlation coefficients of two-constant were between 0.53 to 0.93, all reached the significant standard. Elovich equation was the worse model than other models, especially, in red soil NPK treatment, Red sandstone, lateritic red earths, black soil PK treatment and NPKM treatment, the equation describing stabilization process of Pb in these soils were worst.3) The equilibrium concentration (y0) can be gained by the second-order equation. In the group of single Pb contaminated soils, the value of yo was highest in granitization red soil (239 mg·kg-1),while it was lowest in black limestone soil(0.360 mg·kg-1). In the group of single Cd contaminated soils, the value of yo was highest in red soil NP treatment (0.554 mg·kg-1), while it was lowest in black limestone soil (0.093 mg·kg-1). In the Pb-Cd combined metal-contaminated soils, the existing of Pb increased the content of availability of Cd, except in black soil NPK treatment, black soil 1.5NPKM treatment and red sandstone, this would increase the harmfulness of Cd in the environmental. Whereas these had no significant effect of Cd on the availability of Pb, perhaps, it was because that the competitiveness of Cd was weaker than Pb and it had some related to the low concentration of added Cd.4) The stabilization velocity (B), which obtained from the two-constant equation, reflected the rate of content of available heavy metal decreasing with the time. The value of B was larger, the velocity of stabilization process was faster; if not, the velocity of stabilization process would lower. In the group of Pb, the value of B was largest in black limestone soil(0.482), while it is least in the red soil N treatment (0.051), In the group of Cd, the value of B also was largest in black limestone soil(0.216), while it was least in the red soil N treatment (0.055). In the single Pb or Cd contaminated soils, generally, the stabilization velocity was slower in the red soils than that was in the black soils and fluvo-aquic soils. In the Pb-Cd combined contaminated soils, the stabilization velocity of Pb or Cd was affected by the interaction of ions, so it had large diversity between different soils.5) The stabilization velocity and equilibrium concentration of Cd or Pb were influenced by soil properties. Path analysis indicated that pH contributed greatly to affect the equilibrium concentration of Cd or Pb in the stabilization process, showed that the increase of pH could reduce effectiveness of Cd or Pb, reduced the risk of environmental pollution in agricultural production. CEC and clay content also had some effect on the equilibrium concentration of Cd and Pb, but it were realized by the pH. The organic matter and pH were the major factors of affecting the stabilization velocity of Cd and Pb, with the increasing content of organic matter and pH, the stabilization velocity of Cd and Pb accelerated. To some extent, the increasing of CEC also could quickened the stabilization process of Cd and Pb, but the clay content had little effect on the stabilization velocity. |
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