Volatile organic compounds (VOC) are among the most common wastewater pollutants and are often removed by the process of air-stripping. Since VOC molecules in solution must adsorb to the air-liquid interface before evaporating, it is possible to use surface tension measurement to monitor the change in interfacial properties. This paper demonstrates that axisymmetric drop shape analysis (ADSA) can be used for surface tension measurement and offers several advantages: ADSA works in dynamic situations, such as those encountered in wastewater treatment where surface properties are anticipated to be changing constantly. ADSA only needs small amounts of wastewater for analysis, making it safe for those who must handle these compounds. To explain the experimental results, it was found that diffusion controlled adsorption alone could not explain the processes occurring at the surface. A new kinetic model based on statistical rate theory (SRT) was developed and applied to explain the process of evaporation, which was then combined with a diffusion controlled adsorption model. The diffusion-SRT model was tested against experiment by comparing the equilibrium exchange rates, J(0), obtained from experiments, and also the value calculated based on a Maxwell-Boltzmann distribution. For the tetrachloroethylene (PCE) system, it was found that SRT yielded an equilibrium exchange rate of the order of magnitude of 10(-5) mol/m(2) per s for the first 1000 s of the experiment, in agreement with the value calculated from a Maxwell-Boltzmann distribution. Though it was expected that the equilibrium rate remains the same throughout the process, it actually changed by an order of magnitude after the first 1000 s and further as the experiment continued. This is due to the fact that the diffusion portion of the new model (used to explain the unidirectional adsorption of molecules from liquid to surface) is only a short time approximation and would not be applicable for the full 6000 s of the experiment. (C) 2003 Elsevier B.V. All rights reserved.
