The influence of ammonia and carbon dioxide on the sorption of a basic organic pollutant to a mineral surface

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Abstract
Indoor surfaces have a sorptive capacity for organic pollutants which may be significantly influenced by other gases and the pH of the surface. In this research, we examine the influence of a common indoor gaseous acid, CO2, and base, NH3, on the adsorption of a volatile organic base, trimethylamine (TMA), to a mineral surface, zirconium silicate beads. Varying ammonia and CO2 within concentration ranges of indoor relevance substantially influences the sorptive capacity of this mineral surface. Increasing the CO2 mixing ratio to 1000 p.p.m. enhances surface capacity of TMA by 40-50%; increasing the NH3 mixing ratio to 10 p.p.m. decreases the TMA surface capacity by approximately 5-80% depending on relative humidity. The phenomena of dissolution of TMA into bulk surface water and acid-base chemistry in the surface water do not adequately describe equilibrium adsorption on this surface. Instead, adsorption to the dry solid or to adsorbed water layers appears to dominate. Reduction in the equilibrium partition coefficient, ke, in the presence of NH3 is due to a competition between TMA and ammonia molecules for adsorption sites. Site competition appears to follow the Langmuir competitive model and most ke values range from 0.003-0.045 m. Sorptive interactions with indoor surfaces strongly influence indoor exposure to pollutants. For basic or acidic compounds, these interactions are themselves influenced by surface pH and competition with other acidic or basic gases such as CO2 and NH3. We show that CO2 tends to cause mineral surfaces to store more amines but NH3 tends to decrease this surface capacity. Given the typical range of indoor CO2 and NH3 concentrations, the indoor sorbtive capacity of amines on mineral surfaces may vary by greater than an order of magnitude.

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