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

Heterogeneous Chemistry of Mercuric Chloride on Inorganic Salt Surfaces

Gaseous oxidized mercury (GOM) is a major chemical form responsible for deposition of atmospheric mercury, but its interaction with environmental surfaces is not well understood. To address this knowledge gap, we investigated the uptake of gaseous HgCl₂, used as a GOM surrogate, by several inorganic salts representative of marine and urban aerosols. The process was studied in a fast flow reactor coupled to an ion drift-chemical ionization mass spectrometer, where gaseous HgCl₂ was quantitatively detected as HgCl₂·NO₃^-. Uptake curves showed a common behavior, where upon exposure of the salt surface to HgCl₂, the gas-phase concentration of the latter dropped rapidly and then recovered gradually. None of the salts produced a full recovery of HgCl₂, indicating the presence of an irreversible chemical reaction in addition to reversible adsorption, and all salts showed reactive behavior consistent with the presence of surface sites of a high and a low reactivity. On the basis of the decrease in the uptake coefficient with increasing concentration of gaseous HgCl₂, we conclude that the interaction follows the Langmuir-Hinshelwood mechanism. The reactivity of a deactivated salt surface after uptake could be partially restored by cycling through an elevated relative humidity at atmospheric pressure. The overall surface reactivity decreased in the series Na₂SO₄ > NaCl > (NH₄)₂SO₄ > NH₄NO₃. The uptake on NH₄NO₃ was nearly fully reversible, with low values of the initial (0.4 × 10^-2) and steady-state (3.3 × 10^-4) uptake coefficients, whereas Na₂SO₄ was significantly more reactive (3.1 × 10^-2 and 1.7 × 10^-3). Depending on the aerosol loading, the lifetimes of gaseous HgCl₂ on dry urban and marine particles (as pure (NH₄)₂SO₄ and NaCl, respectively) were estimated to range from half an hour to about a day.

Binding of Organophosphorus Nerve Agents and Their Simulants to Metal Salts

Nerve agents (NAs) pose a great threat to society because they are easy to produce and are deadly in nature, which makes developing methods to detect, adsorb, and destroy them crucial. To enable the development of these methods, we report the use of first principles electronic structure calculations to understand the binding properties of NAs and NA simulants on metal salt surfaces. We report calculated Gibbs free binding energies (G_BE) for four NAs (tabun (GA), sarin (GB), soman (GD), and venomous X (VX)) and five NA simulants (dimethyl methylphosphonate (DMMP), dimethyl chlorophosphate (DMCP), trimethyl phosphate (TMP), methyl dichlorophosphate (MDCP), and di-isopropyl methylphosphonate (DIMP)) on metal perchlorate and metal nitrate salts using density functional theory. Our results indicate a general trend in the binding strength of NAs and NA simulants to metal salt surfaces: MDCP < DMCP < GA < GD ≈ GB < TMP < VX ≈ DMMP < DIMP. Based on their binding properties on salt surfaces, we identify the most effective simulant for each of the studied NAs as follows: DMCP for GA, TMP for GB and GD, and DMMP for VX. To illustrate the utility of the binding energies calculated in our study, we address the design of NA sensors based on the competitive binding of NAs and liquid crystalline compounds on metal salts. We compare our results with previous experimental findings and provide a list of promising combinations of liquid crystal and metal salt systems to selectively and sensitively detect NAs. Our study highlights the great value of computational chemistry for designing selective and sensitive NA sensors while minimizing the number of very dangerous experiments involving NAs.