Quantum chemical calculations to determine partitioning coefficients for HgCl₂ on iron-oxide aerosols

Journal Article · 30 April 2018 · Science of the Total Environment

DOI: https://doi.org/10.1016/j.scitotenv.2018.04.289 · OSTI ID:1543593

Tacey, Sean A. ^[1]; Xu, Lang  ^[1]; Szilvási, Tibor ^[1]; Schauer, James J. ^[1]; Mavrikakis, Manos  ^[1]

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1. University of Wisconsin, Madison, WI (United States)

Gas-to-particle phase partitioning controls the pathways for oxidized mercury deposition from the atmosphere to the Earth's surface. The propensity of oxidized mercury species to transition between these two phases is described by the partitioning coefficient (K_p). Experimental measurements of K_p values for HgCl₂ in the presence of atmospheric aerosols are difficult and time-consuming. Quantum chemical calculations, therefore, offer a promising opportunity to efficiently estimate partitioning coefficients for HgCl₂ on relevant aerosols. In this study, density functional theory (DFT) calculations are used to predict K_p values for HgCl₂ on relevant iron-oxide surfaces. The model is first verified using a NaCl(100) surface, showing good agreement between the calculated (2.8) and experimental (29–43) dimensionless partitioning coefficients at room temperature. Then, the methodology is applied to six atmospherically relevant terminations of α-Fe₂O₃(0001): OH-Fe-R, (OH)₃-Fe-R, (OH)₃-R, O-Fe-R, Fe-O₃-R, and O₃-R (where R denotes bulk ordering). The OH-Fe-R termination is predicted to be the most stable under typical atmospheric conditions, and on this surface termination, a dimensionless HgCl₂ K_p value of 5.2 x 10³ at 295 K indicates a strong preference for the particle phase. Here this work demonstrates DFT as a promising approach to obtain partitioning coefficients, which can lead to improved models for the transport of mercury, as well as for other atmospheric pollutant species, through and between the anthroposphere and troposphere.

• Research Organization: Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States). National Energy Research Scientific Computing Center (NERSC); Argonne National Laboraatory (ANL), Argonne, IL (United States); Pacific Northwest National Laboratory (PNNL), Richland, WA (United States); University of Wisconsin, Madison, WI (United States)
• Sponsoring Organization: USDOE Office of Science (SC); Air Force Office of Scientific Research (AFOSR)
• Grant/Contract Number: AC02-05CH11231; AC02-06CH11357
• OSTI ID: 1543593
• Journal Information: Science of the Total Environment, Journal Name: Science of the Total Environment Journal Issue: C Vol. 636; ISSN 0048-9697
• Publisher: ElsevierCopyright Statement
• Country of Publication: United States
• Language: English