Volatile Chlorinated Hydrocarbons Adsorption - Desorption Characteristics Of Typical Soils Of China

In recent years, soil pollution accidents had happened frequently in China, and had aroused wide attention of government and community. Adsorption is one of the principal mechanisms of soil contamination by volatile chlorinated hydrocarbons (VCHs). The adsorption behaviors of VCHs onto soil not only directly affect the concentration of VCHs and distribution in soil, but also influence the VCHs migration and transformation in soil, and also have a significant impact on the ability of remediation of contaminated site. Therefore, understanding the adsorption characteristics of VCHs onto soil has important practical significance for the investigation and remediation of contaminated sites. On the basis of reviewing the adsorption-desorption behaviors onto soils, the adsorption equilibrium relationships between VCHs gases and typical soils in China were quantitatively analyzed and methods used to predict quantity of pollutants in soil according to soil gas concentrations was established in this study. Besides, the desorption behavior of tetrachloroethylene (PCE) from long-term contaminated soils was studied using continuous air stripping. Thus, provide a theoretical basis for promoting the investigation and control of soil pollution in urban areas. The study mainly includes the following three parts:(1)The equilibrium adsorption of 6 VCHs gases onto 3 dry paddy soils was studied using static equilibrium adsorption experiments. Results showed that the equilibrium adsorption isotherms of dry soils could be well fitted with the Dubinin-Astakhov equation (R²>0.95). Parameters of the Dubinin-Astakhov equation were influenced by the characteristics of soils and VCHs. No correlation was found between the parameters and soil organic carbon content. The affinity coefficients (β) did not show significant dependence on the molecular volumes of VCHs but tended to increase along with the increase in the molecular polarities. The adsorption energies (E₀) of the reference compound (tetrachloroethylene) positively correlated to the pore volume with radius below the average (V<10nm), while poorly correlated to the specific surface area of the soil. The Dubinin-Astakhov equation could be used to predict the equilibrium adsorbed amounts of VCHs onto dry soils. The predicted values fitted well with the measured values (R2=0.98).(2)The equilibrium adsorption of 4 VCHs gases onto 8 moist soils were studied using dynamic equilibrium experiments. It was found that adsorption isotherms of trichloroethylene, tetrachloroethylene, 1,1,1-trichloroethane, could be described by Henry equation, but the adsorption isotherms of 1,1,2-trichloroethane was non-linear. The soil-water adsorption equilibrium coefficients KSW showed a good linear relationship with soils'organic carbon content (α). The adsorbed amounts of wet soils agreed well with the measured values (R2=0.98). For soils with water content between dry and moist soils, the adsorption onto the hydrated soil surface exposed directly to the vapor phase which was similar to dry soil can't be neglected. The relative saturation of soil (RS) and the fraction of soil surface exposed to the vapor phase (γ) can be obtained by combining mercury intrusion data and BET adsorption data. Thus, the adsorbed amounts by those soils can be estimated using the predicted equations of dry and moist soils.(3)Continuous ventilation experiment was conducted to study the desorption behavior from unsaturated soils which were long-term contaminated by PCE. The PCE concentration in the effluent air decreased with time of ventilation, mostly during the early stages of air injection. The PCE removal rate was related to the soil particle size distribution and soil organic carbon content. Some fraction of adsorbed PCE couldn't be removed at the end of ventilation, because the process of diffusion from soil was difficult due to the strong adsorption in soil organic carbon, crystallization of minerals and porous particle layers.