Effect of temperature and air movement on the flux of elemental mercury from substrate to the atmosphere

Abstract

The flux of elemental mercury vapor from intact mill tailings (36 to 1270 μg Hg/g), soil (7 μg Hg/g), and cinnabar ore (934 μg Hg/g) was measured as a function of temperature (20°- 6O°C) and wind velocity (0.2-0.8 m/s) using a controlled environment, open gas-exchange system. Continuous air movement over core surfaces in the gas-exchange chamber resulted in a logarithmic decline in mercury flux with time. Measurement of the effect of environmental parameters on mercury flux was done after attainment of a quasi steady state of flux. Prior to attainment of this state the activation energy for mercury flux was less than the molar heat of vaporization of element mercury (14 kcal/mol). At steady state the substrate-to-air flux of mercury vapor increased logarithmically with temperature, mimicking the element's vapor pressure curve; and activation energies (16.4 to 25.7 kcal/mol) for mercury flux were higher than the molar heat of vaporization of elemental mercury due to physicochemical properties of the soil (e.g., porosity, organic matter, clay content) that affect gas-phase mercury transport and fate. A change in wind velocity from 0.2 to 0.8 m/s resulted in an increase in mercury flux by a factor of 2 for a core with > 150 μg Hg/g and no significant response from two cores with <150 μg Hg/g. Using data from gas-exchange experiments, equations were derived for predicting the response of mercury flux to a range of temperatures and wind velocities for a variety of substrate mercury concentrations. The equations and the results of this study are used to predict the flux of mercury to the atmosphere from substrate enriched in mercury.