不同类型耕地紫色土中3,5,6-三氯-2-吡啶醇迁移试验与模拟

3,5,6-三氯-2-吡啶醇(TCP)的化学结构稳定、水溶性高、运移能力强,对水体具有潜在的污染风险。在有机质含量低、大孔隙度高、导水性好的紫色土地区,风险更加显著。为研究TCP在紫色中的迁移规律,该研究依据紫色土的典型耕作类型,在中国科学院盐亭国家紫色土农业生态试验站采集3组土样(小麦-玉米轮作的坡地、水稻-油菜轮作的水旱农田和萝卜-白菜套种的菜地),通过批量平衡法研究紫色土对TCP的吸附特征,并采用稳定流场饱和均质土柱的易混合置换试验研究TCP的动态迁移过程,最后对其迁移动态进行模拟。结果表明:紫色土对TCP的吸附特征呈线性,在坡地、水旱农田和菜地中的吸附系数分别为1.94、1.22和1.02 L/kg,且黏土含量和矿物组成是主要影响因子;TCP的出流平衡浓度分别为初始浓度的77%(坡地)、84%(水旱农田)和92%(菜地),相应的平衡时刻分别为2.88PV、4PV和6.5PV,表明TCP对环境的污染风险较高;用非平衡两点对流弥散模型模拟TCP在3种耕作条件下迁移,表明TCP以瞬时吸附为主,其水动力弥散系数和分形系数在坡地、水旱农田和菜地中依次减小,但一阶动力学常数依次增大。研究结果为探索TCP在紫色土壤中的迁移机制和预测防止TCP对环境的污染提供了参考依据。

Experiment and model simulation of 3,5,6-TCP transport in different farmland purple soils

Abstract: 3,5,6-trichloro-2-pyridinol (TCP), the main degradation product of pesticide chlorpyrifos and herbicide triclopyr, exhibits strong anti-degradation ability, high water solubility and high migration capability, which would easily lead to soil and water pollution. This situation would be worse in the purple soil distribution areas, most in the upstream regions of Yangtze River, due to soils with low organic matter content, large pores with high water conductivity and erosion. In this study, three typical farmland soil samples were collected based on major land use of purple soil from Chinese Academy of Agricultural Agro Ecological Experimental Station (Yanting), including a sloping field sample located at higher relative elevation, a paddy field sample at lower relative elevation and a vegetable field sample close to the residential areas, corresponding to the crop rotation of wheat-corn, rice-rape and radish-cabbage, respectively. Physical properties of these soil samples were measured, including particle-size fraction, organic matter content, bulk density, pH value and content of clay minerals (montmorillonite and kaolinite). Then, the batch equilibrium method was used to determine the liquid - solid partition coefficient of the TCP to reveal the absorbing characteristics of these purple soil samples to TCP; meanwhile, miscible displacement experiment was carried out under steady-state flow condition (12 mL/h) with Br- as a conservative tracer (50 mg/L), from which dispersion coefficient for TCP transport could be estimated by inverse simulation. Finally, the appropriate models were selected to simulate the physical and chemical processes of TCP in purple soil columns. The results were as follows: 1) The absorbing characteristics of purple soil to TCP were well described by the linear form of Freundlich Model, and the adsorption coefficients of sloping field, paddy field and vegetable field were 1.94, 1.22 and 1.02 L/kg, respectively. The content of clay and its components (montmorillonite and kaolinite) were the main factors affecting the adsorption coefficients; 2) The equilibrium outflow concentration of TCP in the outflow was 77%, 84% and 92% (relative concentration to the TCP outflow) respectively and the equilibrium time was at 2.88 PV, 4 PV and 6.5 PV, suggesting high pollution risk to deep soil layer, surface water and groundwater. Especially in the slope field, TCP displayed more easily and faster to enter the groundwater through infiltration and rivers through the surface runoff and lateral flow at the slope. 3) The two sites non-equilibrium convection-diffusion model could well simulate the transportation of TCP through purple soil columns, with a determination coefficient larger than 0.90 and the mean square errors (MSE) ranging from 0.009 to 0.021. These hydrodynamic and absorbing parameters from the experiment or reverse simulation revealed the reasons why TCP transportation was different among the three farmland purple soils: in the transient-absorption-dominated uniform soil environment, the main factor affecting dispersion coefficient was the flow velocity, which was mainly controlled by soil porosity or bulk density under the condition of stead flow without pressure; and dispersion coefficient and first-order kinetic rate constant showed an increase trend from vegetable field to paddy field, and then sloping field, while fractal coefficient displayed a decrease trend. These results are important to disclose TCP transfer mechanism in purple soil, and can also provide valuble information for preventing pollution caused by the pesticide and herbicide, especially their degradation productions, to the soil and water environment in the upstream regions of Yangtze River.