(367h) Kinetics of Elementary Arsenic and Selenium Reactions

As environmental regulations pertaining to mercury are becoming much stricter, other trace metals, particularly arsenic and selenium, are beginning to be examined as well. A noted source of compounds containing these elements is coal combustion flue gases and, as such, the mechanism(s) of their removal is a topic of much attention. Given that any removal strategy will be dependent upon the speciation, and the speciation in turn will be dependent upon the reaction kinetics of the flue gas, determination of the kinetic parameters of reactions involving these metals is key to developing effective removal techniques.

Previous experimental work has been performed to determine the speciation for a variety of conditions, however the nature of many of the compounds created as a result of the combustion process makes them undetectable to current experimental techniques. To develop a more complete understanding of the overall speciation it is thus necessary to determine the importance of these compounds. To accomplish this, computational chemistry techniques are employed to determine the kinetic parameters of the elementary reactions taking place within the combustion flue gas environment.

Selenium and arsenic reactions believed to take place in the flue gases of coal combustion facilities were investigated. Prior theoretical work involving various As and Se species was completed using DFT and a broad range of ab initio methods. Building upon that work, the present study is a determination of the kinetic and thermodynamic parameters of the reactions, Se + O2 → SeO + O and As + HCl → AsCl + H at the CCSD/RCEP28VDZ and QCISD(T)/6-311++G(3df,3pd) levels of theory, respectively. Transition state theory was used in determining the kinetic rate constants along with collision theory as a means of comparison. The calculated Keq values are compared to experimental data, where available.