(423h) Modeling the Transport of Mercury in Indoor Spaces Following a Release of Mercury Droplets

A mathematical model for predicting potential exposure concentrations of mercury resulting from the breakage of mercury-containing products has been developed. This modeling work estimates airborne mercury concentrations in indoor spaces following the breakage and subsequent release of mercury from mercury-containing products, such as switches, thermostats, and hygrometers. This model was intended to estimate exposure on a per-breakage-event basis. Additional activity factors, such as the frequency of product breakages or the exposure duration to a broken product within these occupational settings, are not included. Two occupational settings are modeled: office and industrial.

This model, named the Mercury Evaporation in Indoor Spaces Model (MEISM), applies engineering principles of liquid-gas phase mass transfer to an industrial hygiene scenario. MEISM models the evaporation of mercury using boundary layer theory and estimates airborne concentrations as a function of time using a near-field – far-field approach to account for imperfect air mixing. The near-field – far-field approach divides the indoor space into two, well-mixed zones and calculates the airborne concentrations in each zone using a mass balance similar to that of a CSTR.

Multiple literature contributions have described the oxidation of mercury in a humid air environment. These studies have noted the exponential decline of airborne mercury concentrations after the initial release. This decline is likely due to the formation of a mercury oxide layer on the surface of the evaporating mercury, which provides a resistance to mercury evaporation. To account for this important phenomenon, MEISM used multiple mercury exposure studies from the literature to regress a first-order rate constant of the oxidation of mercury in humid air. The first-order rate of mercury oxidation is modeled as an exponential decay of mercury surface area available for evaporation. An Arrhenius relationship was used to describe the first-order rate constant as a function of temperature. These regressions resulted in an Arrhenius activation energy of 41.5 kJ/mol and a pre-exponential factor of 578,388 hr^-1. This model predicts a first-order rate constant of 0.0312 hr^-1 at 25 ^oC.

For the purpose of modeling, MEISM assumes the spilled mercury forms several hemispherical droplets on the floor, and the entire surface area of each hemisphere is available for evaporation. To capture uncertainties of the various model inputs, MEISM uses probabilistic modeling to estimate airborne mercury concentrations following a mercury release. Probability distributions were assigned to the model inputs of temperature, mercury mass within the product, number of mercury droplets formed, indoor air speed, and the oxidation first-order rate constant. For the breakage of a thermostat, switch, and hygrometer in both model office and industrial settings, MEISM predicted the following 95^th percentile peak and eight-hour time weighted average (TWA) concentrations in the near field (the zone closest to the mercury source):