Mercury is a pervasive and highly toxic environmental pollutant. Major anthropogenic sources of mercury emissions include artisanal gold mining, cement production, and combustion of coal. These sources release mostly gaseous elemental mercury (GEM), which upon entering the atmosphere can travel long distances before depositing to environmental waters and landforms. The deposition of GEM is relatively slow, but becomes greatly accelerated when GEM is converted to gaseous oxidized mercury (GOM) because the latter has significantly higher water solubility and lower volatility. Modeling GOM deposition requires the knowledge of its molecular identities, which are poorly known because ultra-trace (tens to hundreds part per quadrillion) level of GOM in the atmosphere makes its experimental detection and analysis a formidable task. It is here where computational methods can help address the GOM molecular identity problem. Accordingly, the two major goals of this work are to (a) develop a computationally inexpensive approach for assessing accurate thermochemistry of GOM species and (b) investigate ion-molecule reactions of GOM species in order to assist experimentalists in the development of a novel detection method.
The first goal addresses the question of what are some of the molecular identities of GOM species that could be present in combustion and atmospheric environments. Ab initio and density functional theory calculations are used in combination with the methods of isodesmic and isogyric work reactions in order to calculate accurate heats of formation for GOM species that can form in reactions of GEM with atomic halogens, OH, OCl, and OBr. The accuracy of the calculations is assessed by comparing the calculated values against experimental data and also data from rigorous and computationally expensive state-of-the-art ab initio calculations. Bond dissociation energies (BDE) are determined from the heats of formation and used as a measure of the stability of the GOM species studied.
The second goal of this work addresses the question of how can GOM species be measured in the atmosphere in real-time while retaining speciation information, using chemical ionization mass spectrometry. Ab initio and density functional theory calculations are used to determine structures of products of ion-molecule reactions and calculate associated reaction enthalpies and Gibbs free energies. The obtained data are used to identify reagent ions that can be used for atmospheric detection of GOM. The calculations provide an understanding of the complex ion-molecule chemistry that occurs during the chemical ionization process.
The implications of this body of work are as follows. A low computational cost methodology is established that can be used to study a wide range of GOM species outside the scope of this work. The thermochemistry of the GOM species calculated in this work can serve as the foundation for future kinetic studies with the goal of improving the reaction mechanism in global transport models to provide a better understanding of the global mercury budget. Reagent ions identified in this work can be used for real-time speciation of GOM in the atmosphere, using chemical ionization mass spectrometry.
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