Adsorption of Phthalates on Impervious Indoor Surfaces

Abstract

Sorption of semivolatile organic compounds (SVOCs) onto interior surfaces, often referred to as the "sink effect", and their subsequent re-emission significantly affect the fate and transport of indoor SVOCs and the resulting human exposure. Unfortunately, experimental challenges and the large number of SVOC/surface combinations have impeded progress in understanding sorption of SVOCs on indoor surfaces. An experimental approach based on a diffusion model was thus developed to determine the surface/air partition coefficient K of di-2-ethylhexyl phthalate (DEHP) on typical impervious surfaces including aluminum, steel, glass, and acrylic. The results indicate that surface roughness plays an important role in the adsorption process. Although larger data sets are needed, the ability to predict K could be greatly improved by establishing the nature of the relationship between surface roughness and K for clean indoor surfaces. Furthermore, different surfaces exhibit nearly identical K values after being exposed to kitchen grime with values that are close to those reported for the octanol/air partition coefficient. This strongly supports the idea that interactions between gas-phase DEHP and soiled surfaces have been reduced to interactions with an organic film. Collectively, the results provide an improved understanding of equilibrium partitioning of SVOCs on impervious surfaces.

Modeling the formation and growth of organic films on indoor surfaces

“…15,39,54 This means that an instantaneous equilibrium is assumed between the adsorbedphase SVOC and the air immediately adjacent to the surface, that is, In practical applications, it is often assumed that the rate of kinetic adsorption is much faster than the rate of transport through the concentration boundary layer.…”

“…2,9 These compound classes are potentially associated with a range of adverse health effects including endocrine disruption, asthma, and allergies. 2,15 The formation of a thin organic film or "grime layer" on these surfaces therefore represents a sink for SVOCs, forming a reservoir that may subsequently re-emit SVOCs over long periods of time. 13,14 Understanding SVOC behavior in the indoor environment and associated human exposure pathways requires identification of their sources, characterization of their physicochemical properties, and knowledge of their transport mechanisms ( Figure 1).…”

“…Indoor surfaces are critical mediators of these processes, as SVOC partitioning to surfaces depletes the gas phase and drives additional emission from their source materials. 2,15 The formation of a thin organic film or "grime layer" on these surfaces therefore represents a sink for SVOCs, forming a reservoir that may subsequently re-emit SVOCs over long periods of time. 2,16,17 Thin films of deposited material are ubiquitous on impervious indoor surfaces such as window glass and walls.…”

“…A recent study by Wu et al 15 provides direct experimental evidence that the formation of organic films on a range of impervious surfaces creates a "new" surface with more uniform physical/ chemical properties, despite the highly varied underlying surface materials. This experimental result, which suggests that many indoor surfaces may have similar surface properties, may accelerate our understanding of indoor chemistry, but observational constraints and comprehensive models remain limited.…”

“…In fact, previous work suggests that adsorption of SVOCs onto impervious surfaces reaches equilibrium, with an equilibrium thickness that might be thinner than the "critical thickness" needed to transition from adsorption to absorption and therefore to continued growth. 15 Indeed, it may be that the formation of a surface film that is sufficiently thick to enable subsequent growth by absorption is facilitated by condensation of supersaturated vapors near surfaces due to temperature differentials. The importance of condensation for the formation of surface films on outdoor surfaces has been demonstrated before, 18,20,26 and it might therefore be reasonable and even necessary to include this mechanism in considerations of surface film formation indoors.…”