Comparative Research On Sorption Mechanisms For Polychlorinated Biphenyls In Three Representative Soils

According to Polychlorinated Biphenyls (PCBs) pollution and distribution characteristics in Chinese soils, this research selected three different soils—loess, blue purple clay and paddy soil—as the geosorbents to carry out sorption research, compared the sorption characteristics and sorption mechanisms of PCBs in the three soils from the view of sorption kinetics and thermodynamics, and explored the relationships between the structural characteristics of soils and their sorption capacities. The results of research are listed as follows.(1) Blue purple clay and paddy soil exhibiting powerful PCBs sorption capabilities are good geosorbents for PCBs, while loess has low PCBs sorption capacities.The batch experiments showed that the apparent equilibriums of 10 PCBs in the three soils were reached at 24 h, and the sequence of equilibrium sorption capacities of three soils for every PCB congener was:blue purple clay>paddy soil>loess, which was consistent with the contents of organic matter of the soils selected. The sorption amounts of blue purple clay, paddy soil and loess for PCBs were 1.5×10-1-9.1μg/g,1.4×10-1-7.9μg/g and 6.3×10-3-4.2×10-1μg/g, respectively, and the sorption capabilities of blue purple clay and paddy soil were found to be 17.64-24.00 times and 15.24-21.76 times that of loess. Blue purple clay and paddy soil are good geosorbents for PCBs; while loess with low sorption capabilities for PCBs can be recommended as covering material for soils (blue purple clay and paddy soil) polluted by PCBs.(2) PCBs sorption in three soils fit the pseudo-second-order equation well, and the intraparticle diffusion and film diffusion were the sorption controlling steps.Three sorption kinetic equations (pseudo-first-order equation, pseudo-second-order equation and Elovich equation) were applied to simulate the sorption kinetic data, and the pseudo-second-order model could describe PCBs sorption in the three soils well with R2>0.953 and a good agreement between the experimental qe,exp values and the calculated qe,cal values. Two stages were found in all sorption process, the initial stage of rapid sorption and the following stage of slow sorption, which were determined by the numbers and capabilities of sorption sites. The film diffusion and intraparticle diffusion were the sorption controlling steps, together determining the sorption rapid of PCBs in all three soils.(3) PCBs sorption in loess fit a linear model, while PCBs sorption in blue purple clay and paddy soil indicated nonlinear sorption characteristics under laboratory condition.Three sorption isotherm equations were applied to simulate the experimental data, and the result showed that, when the concentrations of Aroclor1260 were 0.1-0.8 mg/L, the sorption behaviors of 10 PCBs in loess fit a linear model with R2>0.96; however, the nonlinear sorption isotherms of PCBs in blue purple clay and paddy soil were observed (n>1) and the Freundlich model could describe the sorption behaviors of PCBs in three soils well (R2>0.92). That could be explained by the relative content of two-phase (amorsphous phase and condensed phase) organic matter.According to the thermodynamic parameters calculated, the results showed that PCBs sorption in the three soils was a spontaneous sorption process (ΔG0>0), and the sequence of driving force for PCBs sorption in the three soils at room temperature (22℃) was:blue purple clay>paddy soil>loess. The sorption amounts of PCBs in blue purple clay and paddy soil decreased slightly with temperature raised from 22 to 46℃, while the sorption amounts of PCBs in loess increased in the same range of temperature. Therefore, loess as the covering material could be reached the best effect in room temperature (22℃). The calculated values ofΔS0 andΔH0 were positive in loess suggesting that the process was endothermic and the sorption of PCBs increased the disorder in the system; whereas for blue purple clay and paddy soil, both were negative indicating that the process was exothermic and the sorption of PCBs decreased the randomness at the solid-solution interface. The interaction force related with entropy change affected the sorption of PCBs in blue purple clay and paddy soil and hydrophobic effect was the main sorption driving force for PCBs sorption in these two soils.