草炭土及其有机质组分对PCB138吸附动力学和热力学研究

选取六氯联苯PCB138为目标污染物,有机质含量较高的草炭土为供试土壤,提取草炭土中的胡敏酸和胡敏素,研究PCB138在草炭土原土壤、胡敏酸和胡敏素中的吸附动力学和热力学。实验结果表明,PCB138在草炭土原土壤、胡敏酸和胡敏素中的吸附是一个快速吸附过程,240 min后吸附达到动态平衡,吸附速率常数分别为0.014、0.011、0.005,吸附速率由快到慢依次为原土壤>胡敏酸>胡敏素。准一级、准二级、Elovich和Weber-Morris动力学模型分析结果显示,准二级动力学模型能更好地描述其吸附过程,可决系数r2在0.988-0.994间。在实验条件下,随着PCB138在土壤中浓度增大,土壤各组分对其吸附量也随之增大。温度由25℃升高到45℃时,土壤各组分对PCB138的吸附量由1.40-17.7μg·g-1增长到1.56-17.9μg·g-1,即随着温度的升高吸附量略有增大。热力学吸附过程采用Freundlich、Langmuir和Temkin等温吸附模型进行拟合,拟合结果显示,Freundlich等温吸附模型拟合效果更好。在吸附时间为240 min的实验条件下,计算得到的有机碳均一化分配系数Koc值表明PCB138在胡敏酸中吸附能力最强。由吉布斯方程得到的3个热力学参数:吸附自由能(ΔG0)、焓变(ΔH0)和熵变(ΔS0)值均大于0,这表明草炭土对PCB138的吸附是一个非自发,吸热且吸附体系的混乱度增大的过程。该研究可为探讨PCBs在不同类型土壤中的环境行为提供一些基础数据,为PCBs的污染控制提供理论依据。

Adsorption Thermodynamics and Kinetics of PCB138 on Peat Soil and Its Different Soil Organic Matter Fractions

PCB138 was selected as the target pollutant, and the peat soil with high organic matter content as the test soil in this experiment. The adsorption kinetics and thermodynamics of PCB138 on the natural soil, humic acid and humin extracted from the peat soil were investigated. The results showed that the adsorption of PCB138 on natural soil, humic acid and humin was a rapid adsorption process. The dynamic equilibrium time was 240 min. The adsorption rate constants of natural peat soil, humic acid and humin were 0.014, 0.011 and 0.005, respectively. The order of adsorption rate from fast to slow was natural peat soil>humic acid>humin. The analytical results of pseudo first-order kinetic model, pseudo second-order kinetic model, Elovich kinetic model and Weber-Morris kinetic model showed that the adsorption of PCB138 onto natural peat soil and its components could be better described by a pseudo second-order kinetic model. The coefficient of determination is between 0.988 and 0.994. Under the experimental conditions, the adsorption capacity to PCB138 increased with the increase of the concentration of PCB138 in peat soil and its organic components. The adsorption capacity of soil components to PCB138 increased from 1.40-17.7 μg·g-1 to 1.56-17.9 μg·g-1, with the temperature increasing from 25 ℃ to 45 ℃. This displayed that the adsorption capacity of PCB138 on peat soil and its organic matter components increased with the increase of temperature. The thermodynamic adsorption process was fitted by Freundlich, Langmuir and Temkin isotherms. The fitting results demonstrated that the adsorption isotherms were well described by the Freundlich isothermal model. When the adsorption time is 240 min, the calculated Koc value of organic carbon homogenization distribution coefficient showed that the strongest adsorption capacity on PCB138 was humic acid. Three thermodynamic parameters calculated from the isothermal adsorption experiment indicated that the change values of Gibbs free energy, enthalpy and entropy were all greater than zero. Thermodynamics parameters demonstrated that the adsorption process of PCB138 on peat soil was a non spontaneous, exothermic and increasing disorder of adsorption system. This study can provide data for the study of environmental behavior of PCBs in different components of soil, and provide theoretical basis for the pollution control of PCBs.