Heterogeneous Reduction Pathways for Hg(II) Species on Dry Aerosols: A First-Principles Computational Study

Legacy Mercury Re-emission and Subsurface Migration at Contaminated Sites Constrained by Hg Isotopes and Chemical Speciation

The re-emission and subsurface migration of legacy mercury (Hg) are not well understood due to limited knowledge of the driving processes. To investigate these processes at a decommissioned chlor-alkali plant, we used mercury stable isotopes and chemical speciation analysis. The isotopic composition of volatilized Hg(0) was lighter compared to the bulk total Hg (THg) pool in salt-sludge and adjacent surface soil with mean ε202HgHg(0)-THg values of -3.29 and -2.35‰, respectively. Hg(0) exhibited dichotomous directions (E199HgHg(0)-THg = 0.17 and -0.16‰) of mass-independent fractionation (MIF) depending on the substrate from which it was emitted. We suggest that the positive MIF enrichment during Hg(0) re-emission from salt-sludge was overall controlled by the photoreduction of Hg(II) primarily ligated by Cl- and/or the evaporation of liquid Hg(0). In contrast, O-bonded Hg(II) species were more important in the adjacent surface soils. The migration of Hg from salt-sludge to subsurface soil associated with selective Hg(II) partitioning and speciation transformation resulted in deep soils depleted in heavy isotopes (δ202Hg = -2.5‰) and slightly enriched in odd isotopes (Δ199Hg = 0.1‰). When tracing sources using Hg isotopes, it is important to exercise caution, particularly when dealing with mobilized Hg, as this fraction represents only a small portion of the sources.

Theoretical Study of the Monohydration of Mercury Compounds of Atmospheric Interest

The structures, vibrational frequencies, and model IR spectra of the monohydrates of oxygenated mercury compounds (BrHgO, BrHgOH, BrHgOOH, BrHgNO₂, BrHgONO, and HgOH) have been theoretically studied using the ωB97X-D/aug-cc-pVTZ level of theory. The ground state potential energy surface exhibits several stable structures of these monohydrates. The thermodynamic properties of the hydration reactions have been calculated at different levels of theory including DFT and coupled-cluster calculations DK-CCSD(T) with the ANO-RCC-Large basis sets. Standard enthalpies and Gibbs free energies of hydration were computed. The temperature dependence of Δ_rG°(T) was evaluated for the most stable complexes over the temperature range 200-400 K. Thermodynamic data revealed that the highest fraction hydrated at 298 K and 100% relative humidity will be BrHgNO₂-H₂O at ∼5%.

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.

An updated review of atmospheric mercury

The atmosphere is a key component of the biogeochemical cycle of mercury, acting as a reservoir, transport mechanism, and facilitator of chemical reactions. The chemical and physical behavior of atmospheric mercury determines how, when, and where emitted mercury pollution impacts ecosystems. In this review, we provide current information about what is known and what remains uncertain regarding mercury in the atmosphere. We discuss new ambient, laboratory, and theoretical information about the chemistry of mercury in various atmospheric media. We review what is known about mercury in and on solid- and liquid-phase aerosols. We present recent findings related to wet and dry deposition and spatial and temporal trends in atmospheric mercury concentrations. We also review atmospheric measurement methods that are in wide use and those that are currently under development.

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

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 HgCl₂K_p value of 5.2 × 10³ at 295 K indicates a strong preference for the particle phase. 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.

Automated Calibration of Atmospheric Oxidized Mercury Measurements

The atmosphere is an important reservoir for mercury pollution, and understanding of oxidation processes is essential to elucidating the fate of atmospheric mercury. Several recent studies have shown that a low bias exists in a widely applied method for atmospheric oxidized mercury measurements. We developed an automated, permeation tube-based calibrator for elemental and oxidized mercury, and we integrated this calibrator with atmospheric mercury instrumentation (Tekran 2537/1130/1135 speciation systems) in Reno, Nevada and at Mauna Loa Observatory, Hawaii, U.S.A. While the calibrator has limitations, it was able to routinely inject stable amounts of HgCl₂ and HgBr₂ into atmospheric mercury measurement systems over periods of several months. In Reno, recovery of injected mercury compounds as gaseous oxidized mercury (as opposed to elemental mercury) decreased with increasing specific humidity, as has been shown in other studies, although this trend was not observed at Mauna Loa, likely due to differences in atmospheric chemistry at the two locations. Recovery of injected mercury compounds as oxidized mercury was greater in Mauna Loa than in Reno, and greater still for a cation-exchange membrane-based measurement system. These results show that routine calibration of atmospheric oxidized mercury measurements is both feasible and necessary.