2015 Volume 48 Issue 7 Pages 592-599
Polychlorinated biphenyls (PCBs) have been widely used as flame retardants, dielectric and heat transfer fluids, and plasticizers in industrial and household products. Due to their toxicity, carcinogenicity, and teratogenesis, these compounds are considered as persistent organic pollutants (POPs). In this study, 3,3′,4,4′-tetrachlorobiphenyl (PCB77) was used as a model compound to study the sorption kinetics and thermodynamics between PCBs and humic acid (HA). Dynamic test results showed that the sorption process was divided into a fast and a slow stage and that sorption equilibrium occurred after 12 h. Fitting results from the five adsorption kinetic models showed that a pseudo-second order kinetic model, Elovich model, and double-constants model accurately described the sorption process, and indicated that PCB-HA sorption is likely a complex mass transfer process comprised of liquid film diffusion, surface adsorption, and intraparticle diffusion. In addition, sorption–desorption isotherm fitting results showed that a Freundlich model accurately predicted the sorption-desorption process. Using the hysteresis index (HI), distinct desorption hysteresis was obtained (HI>1). Thermodynamic analysis results showed that the variation in the free energy of sorption was −6.01 kJ/mol at a temperature of 15°C. As this value was less than 40 kJ/mol, the interaction between PCB77 and HA was mainly comprised of physical sorption. PCB77-HA sorption was a spontaneous and exothermic process related to the combined action of Van der Waals attractions, hydrophobic bonding, dipole forces, and hydrogen bonding. Experimental results illustrated that HA exhibited a high sorption affinity for PCB77. The equilibrated sorption efficiency was determined to be 67.71% (solution to soil ratio of 400 : 1). Sorption was found to be irreversible, indicating that PCB77 can potentially accumulate in the soil organic matter of the topsoil layer. Findings from this study provide an improved understanding of the sorption kinetics and thermodynamics of PCBs associated with HA, as well as a theoretical foundation for the use of exogenous humic acid to control soil PCBs mobility and reduce the diffusion of PCBs in low SOM soils.
