Semi-volatile organic compounds (SVOC), such as di-2-ethylhexyl phthalate (DEHP), have not only caused chemical sensitivity, but also trigger asthma. SVOCs are adsorbed onto airborne particles as well as exist as a gas phase in indoor air, because these compounds have low vapor pressure. Therefore, the mechanism of partitioning between air and particles for SVOCs in indoor environments is crucial to evaluate their exposure on humans. However, there are currently few studies into SVOC adsorption that considered indoor environmental factors. In this study, the effect of several factors on the mechanism of DEHP adsorption onto airborne particles was investigated experimentally. In particular, the residence time and surface characteristics (such as specific surface area and pore size distribution) of airborne particles were focused on.
The surface concentration of airborne particles was evaluated using two methods. A spherical approximation was used for one evaluation, and the specific surface area of the test particles was used for the other. Carbon black and silica were selected as the compositions for the test particles in the DEHP adsorption experiments, and four types with different surface characteristics were prepared for each composition.
As a result of the experiments, a strong correlation was expressed between particle surface concentrations with a specific surface area and the DEHP particulate concentrations. The result suggested that DEHP was adsorbed on the surface of the pores of the airborne particles. In addition, there was no significant difference between DEHP adsorption amounts for carbon black and silica. This fact indicated that the composition of the airborne particles did not affect the mechanism of DEHP adsorption on airborne particles.
The residence times of airborne particles in the test chamber were controlled at 7.5, 15, 30, 60, and 120 minutes (at air exchange rates of 8.0, 4.0, 2.0, 1.0, and 0.50 per hour respectively). The difference in the residence times of the particles had no effect on the amount of DEHP adsorption for any type of test particles. Furthermore, from the viewpoint of the theoretical time calculated to reach adsorption equilibrium, the experimental and theoretical results suggested that adsorption reaches equilibrium under general indoor conditions.
Moreover, the Junge model revised in this study was proposed to evaluate the DEHP particulate ratio φ under general indoor conditions. The revised model demonstrated good agreement with the experimental results. Additionally, the validities of predicted values using the original Junge model and the revised model for variations in DEHP gas concentrations were compared. The result suggested that the original Junge model might overestimate φ under indoor DEHP gas concentration conditions (DEHP gas concentration > 0.1 μg/m3).
