Independent Fractionation of Mercury Isotope ... - ACS Publications

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Mass-Dependent and -Independent Fractionation of Mercury Isotope during Gas-Phase Oxidation of Elemental Mercury Vapor by Atomic Cl and Br Guangyi Sun,†,‡ Jonas Sommar,*,† Xinbin Feng,*,† Che-Jen Lin,†,# Maofa Ge,∥ Weigang Wang,∥ Runsheng Yin,§,⊥ Xuewu Fu,† and Lihai Shang† †

State Key Laboratory of Environmental Geochemistry (SKLEG), Institute of Geochemistry and §State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China ‡ University of Chinese Academy of Sciences, Beijing 100190, China # Center for Advances in Water and Air Quality, Lamar University, Beaumont, Texas 77710, United States ∥ Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ⊥ Environmental Chemistry and Technology Program, University of WisconsinMadison, Madison, Wisconsin 53706, United States S Supporting Information *

ABSTRACT: This study presents the first measurement of Hg stable isotope fractionation during gas-phase oxidation of Hg0 vapor by halogen atoms (Cl•, Br•) in the laboratory at 750 ± 1 Torr and 298 ± 3 K. Using a relative rate technique, the rate coefficients for Hg0+Cl• and Hg0+Br• reactions are determined to be (1.8 ± 0.5) × 10−11 and (1.6 ± 0.8) × 10−12 cm3 molecule−1 s−1, respectively. Results show that heavier isotopes are preferentially enriched in the remaining Hg0 during Cl• initiated oxidation, whereas being enriched in the product during oxidation by Br•. The fractionation factors for 202Hg/198Hg during the Cl• and Br• initiated oxidations are α202/198 = 0.99941 ± 0.00006 (2σ) and 1.00074 ± 0.00014 (2σ), respectively. A Δ199Hg/Δ201Hg ratio of 1.64 ± 0.30 (2σ) during oxidation of Hg0 by Br atoms suggests that Hg-MIF is introduced by the nuclear volume effect (NVE). In contrast, the Hg0 + Cl• reaction produces a Δ199Hg/Δ201Hg-slope of 1.89 ± 0.18 (2σ), which in addition to a high degree of odd-mass-number isotope MIF suggests impacts from MIF effects other than NVE. This reaction also exhibits significant MIF of 200Hg (Δ200Hg, up to −0.17‰ in the reactant) and is the first physicochemical process identified to trigger 200Hg anomalies that are frequently detected in atmospheric samples.

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INTRODUCTION Mercury (Hg) is a ubiquitous semivolatile persistent toxicant regulated by the legally binding Minamata Convention signed in October 2013.1Hg naturally exists as a mixture of seven stable isotopes (196Hg, 198‑202Hg, and 204Hg) in the environment primarily in elemental (Hg0) and mercuric (HgII) oxidation states,2 whereas a small fraction of HgII can be transformed to neurotoxic methylmercuric (CH3Hg) species and dimethylmercury (CH3)2Hg during microbial processes.3 The atmosphere is the most important transient reservoir of Hg released from natural and anthropogenic emission sources.4 With a total burden exceeding 5000 Mg, atmospheric Hg is dominated by Hg0 (>95%) with a mean lifetime of 0.8−1.7 years, allowing for hemispherical to global scale transport.5 The ultimate fate of atmospheric Hg0 is physical removal (deposition) usually preceded by oxidation to HgII species. Atmospheric oxidation of Hg0 occurs largely in the gas-phase, whereas the rates of © XXXX American Chemical Society

aqueous phase reactions in deliquescent aerosols are relatively slower on a unit-volume-of-air basis inherently limited by the low water solubility of Hg.6 Gaseous HgII species readily partitioning on aerosols are highly soluble, resulting in deposition within a few weeks. The importance of gas phase chemistry in the biogeochemical cycling of Hg has been reassessed over recent decades, mainly a consequence of the disclosure of atmospheric Hg0 depletion events (AMDEs) occurring in the boundary layer (BL) of polar7,8and subpolar9 regions. AMDEs, to a lesser degree of completion, have also been observed at the midlatitudes in marine airmasses10and in the free11 and upper12 troposphere. Specifically, AMDEs in the Received: April 4, 2016 Revised: July 6, 2016 Accepted: August 8, 2016

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DOI: 10.1021/acs.est.6b01668 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

laboratory experimental conditions34,36,41 be a substantial sink of •HgX in competition with reactions 3 and 4. Efforts to determine the Hg stable isotopic ratios of various sources, sinks, and pools, as well as Hg isotope fractionation via various geochemical processes in the past 15 years have largely improved our understanding of Hg biogeochemistry.42 Massdependent fractionation (MDF, expressed as δ 202Hg43) of Hg isotopes have been observed during various physical (e.g., volatilization, evaporation, adsorption, and dissolution), chemical (e.g., thermal and photoreduction), and biological (e.g., reduction, methylation, and demethylation) processes. Beyond oxygen and sulfur, Hg is the single metal that so far has been distinguished to display large mass-independent fractionation (MIF, expressed as Δ values).45 MIF of odd isotopes (199Hg and 201Hg), mainly understood by nuclear volume effect (NVE) and magnetic isotope effect (MIE), has been shown during several processes such as aqueous HgIIphotoreduction,46−48 equilibrium Hg0 evaporation,49,50 dark HgII (aq) reduction,51 and aqueous HgII-thiol complexation.52 To date, >10‰ overall spread of Δ199, 201Hg values has been reported mainly in Earth’s natural surface reservoirs such as soil/sediment, water/snow, atmosphere, and biological samples, which have demonstrated to be mainly a result of HgII photoreduction in condensed phases.43,53−55 More recently, significant MIF of 200Hg has been observed in samples predominantly from atmospheric studies42 that so far are quite limited both spatially and temporally. In general, the vapor phase (Hg0) samples are often characterized by slightly negative Δ200Hg,56−60 whereas precipitation more consistently displays positive Δ200Hg.58,61,62 It has been hypothesized that atmospheric Hg0 oxidation processes would introduce 200Hg MIF.61 However, up to now, no data are yet available for Hg isotope fractionation during atmospheric Hg oxidation and other processes involving Hg species in aerosols, cloud droplets, and heterogeneous reactions at interfaces/surfaces and therefore warrants investigation. This study presents the first measurement of Hg stable isotope fractionation during gas-phase oxidation of Hg0 vapor by Cl and Br atoms. The overall rate coefficient of the Hg0 + X• reaction is evaluated using a relative rate (RR) technique deploying simultaneous monitoring of the loss of Hg0 and an organic reference species (org) with a known rate constant (korg+X•) as inputs. The diagnostic changes in Hg isotopic composition observed during the oxidation experiments are evaluated as a Rayleigh distillation.63 The theoretical rationale for Hg0 oxidation by atomic halogens, the links to previous reported isotope data of atmospheric Hg, and the implications on the global Hg geochemical cycling are presented.

Arctic and Antarctica after polar sunrise are in concert with the ozone depletion events and have been linked to BrO (=Br• + BrO•) gas-phase chemistry amplified through the so-called bromine explosion” mechanism13supportedby “hydrocarbon clock” observations14,15and/or direct or indirect16 spectroscopic measurements. In addition, in the boundary layer of the Dead Sea (Israel), all-seasonal partial to near-complete AMDEs have been observed, with an intensity generally corresponding to an increased levels of BrO.17 In addition to BrO18−21 gasphase oxidation with O322,23 and •OH24,25 has been identified as potential oxidants of Hg0 in the atmosphere. The significance of O3, •OH, and BrO to initiate direct Hg0 oxidation is however disputed based on the account of theoretical studies.26,27 Homogeneous Hg0 oxidation in the gas phase by halogen atoms (X•) follows several steps and commences through a barrierless atom−atom recombination28 to form the HgX radical: k1

Hg + X• + M → •HgX + M

(1)

The exothermic bond formation between Hg and X (increasing in the order Cl > Br > I)29 releases excess energy that must be removed from •HgX by collisional deactivation with gas bulk molecules (M = N2, etc.) to curb dissociation into reactants (the reverse of reaction 1): •

k2

HgX + M → Hg + X• + M

(2)

Apart from a pressure dependence, the equilibrium constant k1/ k2 decreases sharply with rising temperature.30 Furthermore, the fate of the thermal labile •HgX also depends on the concentration of atmospherically more abundant reactive species (Y•) such as NO2, NO3, HO2, ClO, BrO, IO, or I31 that to a greater extent than X• (Cl, Br) and in contrast to e.g. O2 and NO30 accomplish oxidation to stable HgII molecules: •

k3

HgX + Y • + M → XHgY + M

(3)

During AMDEs, BrO is demonstrated as a major candidate to participate in reaction 3. However, its viability to initiate oxidation is debated.26,32,33 In addition to Br,18,19,21,34 atomic chlorine is a potent oxidant of Hg018,21,35−37 while the rate of the reaction between Hg0 and I• is orders of magnitude lower38 and HgI is the weakest bound of the HgX molecules.29 In turn, the direct oxidation of Hg0 by the free halogens (X2 = Cl2 and Br2) viable through the insertion reaction Hg + X2 → XHgX is strongly exothermic but nevertheless slow under atmospheric conditions due to large reaction barriers,28 whereas the abstraction Hg + X2 → •HgX + X• proceeds only with significant rate at high-temperature conditions.39 However, X2 is an eminent scavenger of HgX37,40 via •

k4

HgX + X 2 → XHgX + X•

2. MATERIALS AND METHODS Chemicals and Reagents. Chemicals and reagents used in this study are tabulated and described in the Supporting Information (SI Materials). Photolysis Experiments. Figure S1 shows the experimental apparatus and analytical system for investigating the reaction kinetics and corresponding isotope fractionation. Photolysis experiments were performed using 160 L collapsible FEP Teflon-film chambers equipped with a valve for filling reaction mixtures and collecting samples (Figure S1a). Mixtures of Hg0 vapor, halogen atom precursors, and reference organics (RR-experiments only) were introduced into the reaction chamber, diluted by zero air to atmospheric pressure (750 ± 1 Torr), and allowed to mix in the dark for at least 4 h. The

(4)

Albeit the fast kinetics observed in the Hg0 + Cl• reaction, it is not considered as an important sink for Hg0 in the atmosphere due to the low estimated mean Cl• concentrations.18 The HgX radical−radical recombination •

k5

HgX + •HgX + M → Hg 2X 2 + M

(5)

yielding semivolatile Hg2X2 is a fast reaction34,36,41 but has no importance whatsoever in ambient air due to low atmospheric • HgX concentrations. However, reaction 5 may at certain B


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Environmental Science & Technology chamber, housed in a temperature controlled (298 ± 3 K) incubator (2 m3), was surrounded by light banks of four 40 W fluorescent lamps illuminating mainly UV−B (290 to 315 nm, peak at 302 nm). The Cl and Br atoms were produced in situ via intermittent photolysis of precursors CCl3C(O)Cl and CHBr3, respectively. Hg0 oxidation was allowed to proceed along with the photochemical production of Cl• and Br• for up to 30 and 150 min of total illumination respectively with or without a reference compound present. Light hydrocarbons (ethane and 2-chloropropane for Cl•; ethane and propene for Br• reactions) with well-established korg+X• (deriving from independent determinations, see Table S4) were selected as references. The selected references have korg+X• in the same order of magnitude as the expected kHg+X• and induce less complex secondary chemistry than for larger hydrocarbons. The relative consumption of the reference compounds and Hg0 was measured using a gas chromatograph with flame ionization detection (Model 6820, Agilent Technologies) and a cold vapor atomic fluorescence spectrophotometer (Model 2500, Tekran Instruments), respectively. Further details are supplied in Supporting Information, SI RR-experiments. The ratio kHg+X•/korg+X• was determined using

xxx

Hg /198Hg ratios for the residual and initial Hg0, respectively; and α is the ratio of the heavy-to-light isotope oxidation rates. Introducing the δ-notation, this equation can be approximated by69 ln

1000 + δ xxxHg = (α xxx/198 − 1) ·ln fR 1000 + (δ xxxHg)0

(7)

where (δxxxHg)0 is the initial δxxxHg-value of the reactant Hg0 and the slope represents αxxx/198 − 1 (units per mille). Hence, α < 1 indicates that the lighter isotopes react faster than the heavier (kinetic isotope effect, KIE), and α > 1 suggests that heavier ones react faster (inverse KIE). In this study, both normal and inverse KIEs were observed, and the magnitude of fractionation is indicated by the absolute deviation of α from 1. Quality Assurance. In the RR-experiments, the precision in Hg0 and reference compound measurements was determined to 4% and 3% (Relative σ), respectively, from analysis of replicate gas samples withdrawn from the reaction chamber filled with [Hg0]0 and [org]0 in zero air. The recovery efficiencies of Hg0 and mercury products by the WCHgS method were evaluated for each sample by comparing the loaded Hg0 mass and stipulated f R in the gas reaction mixture with that of Hg present in the bubblers II&III and I, respectively. For Hg0, recoveries were in the range of 98 to 103%, and the relative differences between duplicate samples were below 4%. In contrast, this analysis recovered only 5 to 24% of products. In several of the experiments, the inside of the evacuated reaction chamber was rinsed rigorously with a HNO3/HCl mixture, then the acid aliquots were combined and analyzed for total Hg. Adding up the yield from WCHgS analysis, this resulted in nearquantitative product recoveries (83−90% and 92−98% in Cl and Br atom experiments, respectively, SI Table S1). The long-term reproducibility of isotope measurements was assessed by repeated measurements (for every 10−20 samples) of UM-Almadén standard solution over different analytical sessions relative to NIST-3133. The overall average and uncertainty of UM-Almadén (δ202Hg: −0.59 ± 0.09‰; Δ199Hg: −0.03 ± 0.04‰; Δ200Hg: −0.01 ± 0.03‰ Δ201Hg: −0.02 ± 0.04‰; 2σ, n = 9) agreed well with the previous studies.52,68,70 The external reproducibility of our method was also evaluated using secondary standard solutions, NIST 3177 (δ202Hg: −0.52 ± 0.09‰; Δ199Hg: 0.00 ± 0.03‰; Δ200Hg: 0.00 ± 0.03‰; Δ201Hg: −0.02 ± 0.04‰; 2σ, n = 51) and IGCAS Fluka (SI, Table S2). Data uncertainties (±2σ) reported here reflect the larger values of either the external precision of the replication of the standard solutions (UMAlmadén, NIST 3177, and IGCAS Fluka) or sample replicates.

ln{[Hg 0]0 /[Hg 0]t } = kHg + X•/korg + X•·ln{[org]0 /[org]t } (6)

where the subscripts 0 and t refer to initial concentration and concentration at time t after photolysis started, respectively. Plots of ln{[Hg0]0/[Hg0]t} vs ln{[org]0/[org]t} should align to a straight line with a slope of kHg+X•/korg+X•. During kinetic experiments involving measurement of Hg isotopic ratios without organic references (detailed in SI Isotope fractionation experiments), the fraction of remaining Hg0 f R = [Hg0]t/[Hg0]0 measured by CVAFS was systematically varied in the range 1 > f R> ∼ 0.1 (∼0.25 for the Br• reaction). When an experiment reached the stipulated f R, the reaction mixture was evacuated from the chamber by a vacuum pump into a wet chemical Hg speciation (WCHgS) sampling train (details in SI Isotope fractionation experiments). Using the WCHgS method without particulate measurements, the final reaction mixture was also sampled in selected experiments for the presence of aerosols and their size distribution using a scanning mobility particle sizer (SMPS).64 In our study, consecutive runs always included a replacement of used chambers with new conditioned65 ones to avoid the initial presence of wall deposits that in any experiment may cause catalytic removal of Hg0.66 Mercury Concentration and Isotope Composition Measurements. Total Hg concentration was measured using dual stage Au amalgamation and CVAFS detection (Model 2500, Tekran Instruments),67 and Hg isotopic ratios were determined by MC-ICP-MS (Nu-Plasma II, Nu instruments).68Further details are presented in SI Hg concentration and isotope measurements. Following the nomenclature suggested in Blum and Bergquist,43 we report our results as δxxxHg[‰] = [((xxxHg/198Hg)sample)/xxxHg/198Hg)NIST3133 − 1] × 103 to describe MDF and ΔxxxHg = δxxxHg − δ202Hg · βxxx, where βxxx is 0.252 for 199Hg, 0.502 for 200Hg, and 0.752 for 201 Hg, respectively, to describe MIF of these three isotopes. The kinetic fractionation factors (α) of Hg0 during the gasphase oxidation by Cl and Br atoms were estimated by Rayleigh distillation63 describing an irreversible sink of pseudo-first xxx/198 · f R(α −1), where R and R0 are kinetic order: Rxxx/198 = Rxxx/198 0

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RESULTS AND DISCUSSION Wall Losses of Reactants and Reactions in the Dark. New bags used in the experiments throughout this study give very minor wall losses of Hg0 from the gas-phase that are best described by first-order decays (SI, Table S3). Exploratory experiments were conducted to elect precursors that absorb UV−B light significantly but without displaying discernible reactions with Hg0 vapor in the dark. It has been shown that free halogens (X2 = Cl2, Br2) as “clean” sources of X atoms are improper in static reactor studies involving Hg0.18 Therefore, we examined SOCl2 and (C(O)Br)2 photolytes exhibiting substantial photoabsorption cross section here (SI, Table S3). In spite that the compounds were purified after being received as prescribed in the literature,71,72 a substantial loss of Hg0 C


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modeling purposes, there is no numeric data available on the Hg + ClOO• reaction. Similar to our results (SI, Table S1), Hg0 vapor oxidation in static chambers has unanimously revealed that a large portion of Hg products exist as wall deposits.18,21,22 At the end of bag illumination experiments, SMPS measurements revealed the presence of aerosols in the Aitken mode. Most plausibly, they originate from vapor nucleation of mercury-containing products and display growth by the condensation of vapor constituents. Examples of particle size distribution deriving from completion of the Hg0 + X• reactions are shown in Figure S5. The Cl• initiated oxidation resulted in a mode of comparatively larger particles (Figure S5B) and is associated with a larger mass, which is plausibly driven by higher yield of Hg2Cl2 relative to Hg2Br2. After a wall-loss correction,76 we estimated that ∼3% and ∼15% of oxidized Hg0 was originally present as suspended particulates during Br• and Cl• initiated oxidation, respectively. Presumably, Hg2X2 but not HgX2 has sufficiently low vapor pressure to form new particles under the present experimental conditions, even if inconsistencies in literature data obtained at >400 K77−79 and their extrapolation to room temperature adds up to considerable uncertainties. Our observations are in contrast to the reported insignificant yield of suspended particulates during Hg + X• (X = Cl, Br) oxidation by Ariya et al.18 However, their conclusion relies primarily on filtration of the reaction mixture through a 0.5 or 1 μm Teflon grid that appear inept to significantly capture Aitken mode particles. Owing to nonquantitative product recoveries, the evaluation of isotope fractionation during Hg0 + X• reactions here is based on measurements of the Hg0 pool only (Figures 1−3, Figures S6−S9).

vapor is observed in corresponding dark gas mixtures over time (SI, Table S3). For the latter compound, the reactivity may stem from Br2 produced from thermal decomposition and subsequent surface catalyzed oxidation. Finally, conventional UVC halogen atom initiators CCl3C(O)Cl and CHBr3 with cross sections sharply declining toward longer wavelengths were deployed. They exhibited inconsiderable dark reactions (losses of Hg0 ≤ 2.5% h−1, Table S3) and from the rates of decay of reference compounds in regular photolysis experiments, average Cl• and Br• concentrations were estimated to be in the concentration ranges (0.3−1.5) × 108 and (0.4−2.3) × 1010 atoms cm−3, respectively. Furthermore, blank experiments showed that no discernible dark reactions occurred between Hg0 and the reference as well as between reference and the halogen precursor relative to the wall losses observed for the single reference reactants in the absence of photolytes. Rate and Products of Hg0 Oxidation by Cl and Br Atoms. In the UVB photolysis experiments, the loss of Hg0 follows a first-order rate law f R = A · e−B·t. Representative f R vs photolysis time are displayed in Figure S2. In the RRexperiments, plots derived based on eq 6 show good linearity in all cases with small intercepts that are mostly statistically insignificant from zero (at 95% confidence). Figure S3 shows an example of relative plots with the Cl and Br atom reference reactants. For each reference, photolysis runs were repeated at least three times under different initial ratios of [Hg0]/[org]. The results are summarized in Table S4 including korg+X information with literature references. The final room temperature rate coefficients kHg+Cl• and kHg+Br• taken as averages are (1.8 ± 0.5) × 10−11 and (1.6 ± 0.8) × 10−12 cm3 molecule−1 s−1, respectively. The derived kHg+X represents the effective rate constant for the complex oxidation chemistry and falls within the range of rate coefficients at atmospheric conditions reported from related previous studies using static techniques18,21,73 summarized in Table S5. Here, absolute rate determinations and corresponding theoretical predictions of the elementary steps of the numbered reactions are tabulated together with those of direct oxidation of Hg0 by the free halogens that may have importance in our system. The Hg + Br• system has received more attention than that of Hg + Cl•; and experimental data, yet not in all cases determined at atmospheric temperatures, are available for the reactions 2−5. However, there are considerable discrepancies between experimental and theoretically derived rate constants for reactions 2(Br) and 3(Br) (Table S5). The only determination for k5 in the literature indicates that the dimerization of •HgX is more than 1 order of magnitude faster for Cl than for Br, but the rates have been listed as having high uncertainties.34,36 Numerical simulations using a box model (representing a wellmixed reaction chamber, MATLAB SimBiology, Mathworks Inc.) of the chemical processes were performed to investigate the temporal profiles of Hg product concentrations that would evolve during the photolysis experiments. Using theoretical data for k230,74 and k328,39 but otherwise experimental data as input with k5 set within reported errors, simulations displayed in Figure S4 imply that principally HgX2 rather than Hg2X2 is produced during the experiments. Nonetheless, in the Cl atom experiments, the relative importance of Hg2X2 formation is expected to be significantly higher. Moreover, additional loss of Hg0 by reaction with oxygenated species would be plausible in this system since abundant molecular oxygen produces a weakly bound adduct with atomic chlorine (ClOO•), which unlike its congener ClO•21 may react fast with Hg0.75 However, for

Figure 1. Linearized Rayleigh diagram for δ202Hg in Hg0 during Cl and Br atom oxidation experiments. Each point represents a single experiment. The slope from the linear fits represent 1000 · (α202/198 − 1), and the dashed envelopes are 95% confidence bands of the mean.

Mercury Isotope Fractionation in Residual Hg0 during Gas-Phase Oxidation. Kinetic fractionation factors α202/198 are calculated to be 0.99941 ± 0.00006 (2σ) and 1.00074 ± 0.00014 (2σ) using the linear regression of eq 7 for the Cl• and Br• experiments, respectively. However, the scatter around the fitted lines of Figure 1 is visible, and taking the confidence band of these slopes into account renders the obtained fraction factor of the Cl and Br atom reactions not statistically different in magnitude. More strikingly, when Cl removes Hg0, a normal KIE results in the faster removal of lighter isotopes in the reactant, whereas when oxidized by Br, remaining Hg0 becomes depleted in the heavier isotopes corresponding to an apparent D


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Environmental Science & Technology inverse KIE. The regression analysis (Figure 1) shows that the Cl • experiments are better approximated by Rayleigh fractionation (constructed by using eq 7) than the Br• experiments (R-squared value of 0.967 vs 0.859). This may imply that KIE during Br atom initiated oxidation is not only controlled during consecutive oxidation steps but the combined result of counteracting processes. The Hg + Br• system is characterized by a slower reaction 1, a slower •HgX dimerization (5), and a faster dissociation of •HgX via reaction 2, where the latter rate in comparison k2(Br)/k2(Cl) is at least >9 based on theoretical calculations under our experimental conditions (Table S5).30,39,74,80,81 The net inverse KIE observed is likely to be introduced via the cyclic replenishment of Hg0 through channel 2, where a more rapid bond breaking is expected for the lighter HgBr molecule isotopomers as they form relatively weaker bonds.45 This pseudo-equilibrium in the Hg+Br• system induces equilibrium isotope effect (EIE) counteracting on normal KIE and yielding preferential heavy isotope enrichment in products displaying tighter bonds (i.e., HgBr2).82 At temperatures present in the upper atmosphere and during AMDEs in the polar regions, the rate of thermal dissociation of •HgX (k2) becomes profoundly smaller. Compared to 298 K, the dissociation lifetime of HgBr at 245 K (typical high Arctic AMDE temperature) increases by a factor ∼200 or more as predicted from theoretical calculations.30,80,81 Associated with a faster k1(Br), the oxidation mechanism grows toward irreversibility, which potentially may result in normal KIE at lower temperatures. Maximum fractionation relative to Hg0 predicted for HgX2 by first-principle quantum mechanical methods expressed as 52,82,83 1000 · ln α202/198 HgX2‑Hg0 is in the order of ∼2.2−2.3 (X = Cl) 82,83 and ∼2.0−2.1 (X = Br) and is largely augmented by NVE that go in the same direction as conventional MDF for the redox pair Hg0 - HgII45. The measured α202/198 in this study is much smaller than the predicted maximum redox EIE factors and also moderate compared to most of those reported during various aqueous kinetic chemical processes (e.g., reduction,46−48,51,53,84 alkylation53,85 and photodemethylation46,86), which also unanimously display progressive heavier Hg isotope enrichment in the reactant (α202/198 < 1). Figures 2 and 3 display the evolution of δ-values as a function of the fraction of Hg0 remaining in the reactant ( f R) during oxidation by atomic Cl and Br, respectively. Dashed are the Rayleigh curves derived from α202/198 by the kinetic law:87

Figure 3. Hg isotope fractionation (δxxxHg) in Hg0 during the Br atom initiated oxidation process. MDF model fits were plotted according to eq 7 and using the theoretical MDF factors obtained from eq 8.

α xxx/198 = (α 202/198)βxxx

(8)

The evolution of δ-values shows opposite directions during Cl-and Br-initiated oxidation. However, both processes are associated with δ199Hg and δ201Hg that are systematically higher than the estimated mass dependent Rayleigh curve (predicted by eq 8), indicating positive odd-MIF (Figures 2 and 3). For the Cl• reaction, δ200Hg data also do not match massdependent Rayleigh fractionation fits but in contrast display systematically lower values than expected, signifying negative MIF of 200Hg. As shown in Figure S6, the magnitude of Δ200Hg (up to −0.17‰) is much smaller than Δ199Hg (up to ∼1.2‰) and Δ201Hg (∼0.6‰) during the Cl• reaction. Nevertheless, samples at a higher conversion of Hg0 display significant Δ200Hg signatures that are negatively correlated with δ202Hg (p < 0.001). To our knowledge, this is the first observation of significant 200Hg MIF triggered and traceable to a constrained process. A recent study reported odd Hg-MIF during aqueous oxidation of Hg0 initiated by •OH, but this process did not show discernible 200Hg MIF88. The Δ200Hg of Hg0 during Cl atom initiated oxidation follows a Rayleigh-type fractionation (Figure S7) and is proportionate to the slightly negative Δ200Hg observed in atmospheric Hg0 samples collected in the U.S (−0.04 ± 0.09 (2σ)‰,58 −0.10 ± 0.02 (1σ)‰,56 −0.19 to −0.06‰,59 −0.12 to 0.03‰57) and Europe (the Pyrenees, −0.10‰ to 0.05‰89). The magnitude of odd-MIF in the Br• reaction is relatively smaller (Δ199Hg up to ∼0.4‰, Figure S8), but the evolution of δ199Hg points in an opposite direction compared to the remaining δ-values produced in the reactant (Figure 3). A similar “reversed” evolution of δ199Hg vs δ202Hg has been observed during aqueous photoreduction of HgII in complexes with a range of small organic compounds including reduced S groups (e.g., Hg(Cys)2).48 Furthermore, the ΔxxxHg values align with δ202Hg linearly and pass through the origin within errors (Figure S8). This also applies for 200Hg, but since a majority of Δ200Hg-signatures evolving is within error of accuracy (Table S2), a definite conclusion on whether diminutive even-MIF prevails cannot be drawn. This matter is further discussed in the following section. Specific Diagnostic Ratios of MDF and MIF. Ratios Δ199Hg/δ202Hg used as a diagnostic for processes involving MDF and odd-MIF42,57for the Hg + Cl• and Hg + Br• experiments are shown in Figures S6 and S8, respectively. The magnitude of Δ199Hg/δ202Hg in the reactants is nearly

Figure 2. Hg isotope fractionation (δxxxHg) in Hg0 during the Cl atom initiated oxidation process. MDF model fits were plotted according to eq 7 and using the theoretical MDF factors obtained from eq 8. E


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Environmental Science & Technology twice higher for the Cl• compared to Br• reactions (0.63 ± 0.09 (1σ) vs −0.32 ± 0.06 (1σ)) and is diagnostically divergent from that reported for aqueous photoreduction of HgII, photodemethylation of MeHg bound to dissolved organic matter (DOM), and light-induced reduction of HgII via ligands containing reduced sulfur (1.2,46 2.4,46 and −0.848expressed for the reactant, respectively) as well as photoreduction of HgII in snow derived from AMDEs (−3.560), dark reduction of HgII by DOM(−0.12 51), and equilibrium Hg0 liquid−vapor evaporation (−0.149,50). MIF of odd Hg isotopes is primarily explained by two mechanisms: MIE90 and NVE,91 both of which have been shown to give specific Δ199Hg/Δ201Hg ratios. For instance, most recent theoretical calculations52,82,83 have estimated that NVE would result in Δ199Hg/Δ201Hg of 1.6552 to 1.6683 which is supported empirically from studies on equilibrium Hg0 liquid−vapor evaporation,50 equilibrium HgII-thiol complexation,52and dark HgII reduction51 yielding values of 1.54 to 1.61. In turn, MIE is a pure kinetic mechanism arising from hyperfine interactions between nuclear and electronic spins and separates magnetic from nonmagnetic isotopes by spin and magnetic moment.90 It has been used to explain MIF during aqueous HgII photoreduction and MeHg photodegradation with Δ199Hg/Δ201Hg in the span from ∼1.0 to ∼1.4.46−48,86,92 Partly reversible reactions involving intermediate radical pairs are candidates to exhibit MIE that usually yield (+) odd-MIF in the reactant,90 which is in conformity with the result of this study. However, MIE requires spin-conversion that operates on time-scales longer than the duration of a collision of reactants in the gas-phase93 and therefore appears of little consequence in the present study. In Figure 4, Δ199Hg is plotted versus

at 25 °C cause Δ199Hg-signals up to −0.29 and −0.28‰, respectively, whereas the corresponding Δ200Hg-values are up to −0.05 and −0.04‰. A NVE mechanism agrees with our observations in the Br• initiated reaction of faster removal of heavier isotopes from the Hg0 pool. In the product(s) including Hg bonds with electronegative Br, the 6s orbital electron density in the valence band of Hg is reduced, and the enrichment of larger isotopes is preferred by NVE.91 In contrast, Cl•-initiated oxidation produces a suite of signatures: (1) large odd-MIF (Δ199Hg up to ∼1.2‰ in the reactant and down to −2.4‰ in the product), (2) discernible evolution of (−) Δ200Hg in the residual Hg0 pool and (+) Δ200Hg in the product(s) (Figure S9), and (3) an empirical Δ199Hg/Δ201Hgratio ∼1.89, whereof especially argument (1) urges contribution from mass-independent effects other than NVE. Moreover, an adequately constant Δ200Hg/Δ199Hg ratio of −0.16 ± 0.03 (2σ) is obtained from linear regressions of Δ199Hg vs Δ200Hg in the Cl• experiments (Figure S9). It is unlikely that this MIF is imparted by a radical pair mechanism linked to restricted movement of radicals relative to one another (cage effect). Unless high pressures are used, cage effects do not occur in the gas phase.93 It is arguable that if the Hg0 oxidation occurs partly on particulates formed in the experiments or on reactor walls, MIE may arise due to feasibility of prolonged radical pair encounters. Nevertheless, our Hg + Cl • experimental observations do not align to any known characteristics of MIE impact on Hg stable isotope systematics.44 This yet unidentified mass-independent effect may be involved in the Br• system but to a comparatively low extent. It is not surprising that MIF triggered by Hg gas-phase oxidation involving radical species may not entirely be explained by mechanisms applicable to terrestrial Hg isotope fractionation. In fact, for atmophilic elements with three or more stable isotopes (O and S), for which MIF can be tested and gas-phase chemistry plays a central role, considerable mass-independent anomalies (Δ17O, Δ33S, and Δ36S) have been observed in atmospheric samples of oxygenated molecules and particlebound anions (O3, CO2, CO, H2O2, BrO, N2O, NOy, HOx, SO42−, NO3− etc.). These mass independent isotope effects are produced during photolysis and to lesser extent radical reactions: and in most cases with major source regions in the upper atmosphere.94 Atmospheric Relevance and Comparison to Field Observations. To present our data in the context of Hg isotope fractionation associated with transitions in Hg speciation and field observations, these processes and observations are shown graphically in Figure 5. The observations of our study may partially link gas phase Hg0 oxidation by atomic halogens produced photochemically to some previously reported Hg-MIF signatures in atmospheric samples. For instance, AMDE-deposited HgII is depleted in odd Hg isotopes with moderate MIF (Δ199Hg down to −1.2‰),60 which is in accordance with the negative direction of MIF imparted in the products during halogen atom oxidation (Figure 3). However, the diagnostic ratios Δ199Hg/Δ201Hg of 1.07 (air-snow)60 and 1.26 (snow-frost flowers)95 observed during Arctic AMDEs differ substantially from this study. Noticing that the NVE-driven MIF is increased at lower temperature (magnitude being inversely proportional to absolute T91), MIF induced during Br• driven oxidation appears nevertheless too mild to fully account for the AMDE observations. Gas-phase Cl• chemistry has been assigned a diminutive role in AMDEs when quantified,16 and regardless of

Figure 4. Δ199Hg/Δ201Hg in Hg0 (filled circles) and HgII (filled squares) during oxidation by Cl and Br atoms. Solid lines are the corresponding fit on the Hg0 pool from reduced major axis regression. Slopes are given within ±2σ. Here, R represents Pearson productmoment correlation coefficients. Dashed lines indicate fractionation during aqueous HgII photoreduction (light blue) and aqueous MeHg photodegradation (blue).

Δ201Hg with reactant and product data from Cl and Br experiments separately. Since both x and y variables contain error, reduced major axis regression is applied and yields slopes of the individual linear fit of 1.89 ± 0.18 (2σ, r = 0.964) and 1.64 ± 0.30 (2σ, r = 0.833). The latter slope that applies for the Hg + Br• experiments agrees with that of NVE during theoretical and laboratory studies. According to Yang and Liu,83 NVE-driven MIF in products HgCl2 and HgBr2 relative to Hg0 F


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Figure 5. Schematic illustration showing some of the major biogeochemical processes of Hg in the earth’s surface reservoirs and their associated Hg isotope fractionations together with corresponding isotopic composition observations with focus on the atmosphere. References: Observations in nature (HgII, dark green font) A ŠtrokS42 (seawater); B Gratz,58 Sherman,60,95 Chen61(precipitation); C ChenS43(riverine water); D Carignan (lichen),97 Demers (litter, foliage),56 Yin (foliage, crop tissue, lichen)S44, Blum (lichen)S45; E Sherman (snow);60 F Sherman (frost flowers, brine);95 G Liu (fresh-water sediment)S46; H Rolison,57 ZambardiS47 (particle-bound Hg); Observations in nature (Hg0, purple font) I Gratz,58 Sherman,60,95 Rolison,59 Demers,57 Yins44 (ambient air), Fu89(air); J Demers (within-canopy air),56 K Sun (anthropogenic emissions)S48; Process-driven signatures (HgII, blue font) M Stathopoulos (•OH initiated aqueous oxidation);88 N Zheng (photoreduction, L = [COO]22−);47 P Koster van Groos (diffusion)S49 Q this study (gas-phase Cl/Br- oxidation); Process-driven signatures (Hg0, red font) R Bergquist,46 Zheng54,55(photoreduction, L = DOC, OR); S Zheng (photoreduction L = SR),55 T Sherman (photoreduction, L = X);60 U this-study (gas-phase Cl/Br- oxidation); Additional data: V Humphries (Hg-driven nucleation of new particles)S50 (Note: the refs S42−S50 are listed in SI References without prefixes).

measured (+) odd-MIF induced in remaining Hg0 during gasphase oxidation contributes to moderate instantaneous negative Δ199Hg in evading aquatic Hg0 toward more neutral values as observed in atmospheric bulk Hg0. The directions of 200Hg MIF imparted in Hg0 and HgII respectively during Cl• oxidation are in correspondence to field observations speciated atmospheric Hg and HgII in precipitation. However, the small negative diagnostic ratio of Δ200Hg/Δ199Hg for this reaction (Figure S9) is not observed in Hg0 and precipitation data sets from atmospheric studies.57,58 The mismatch of Δ200Hg/Δ199Hg between our study and previous observations of atmospheric samples may suggest strong influence of particulate photoreduction of aqueous HgII during global Hg cycling and/or that Hg0 gas-phase oxidation by O3/oxygenated radicals (•OH, BrO, NO 3 etc.) provides (+) odd-MIF in Hg II products. Furthermore, the tropopause and lower stratosphere is an effective chemical sink of the Hg0 due to the enhancement of UV light-driven chemical processes and oxidants.12,99,100 As suggested by Chen et al.61 and analogous to the major source regions of MIF (e.g., O3 and SO42−), it is conceivable but not yet evinced that stratospheric processes induce strong (odd and even) Hg MIF signatures that are subsequently introduced to the troposphere as aerosols, which may recycle Hg back to the gas-phase when transported downward100 or scavenged and precipitated by deep convective clouds.99 Consequently, there are several lines of evidence suggesting that the role of

stronger MIF, it is unlikely to supply the odd Hg isotope deficiency level. After deposition, HgII can be photoreduced within snow that induces a degree of (−) MIF in the substrate unmatched so far in natural samples (down to −5.08‰).60 The ligand matrix plays a critical role in controlling Hg II photochemical reduction in the environment and its resulting large MIE during this process with (+) MIE being due to aquatic HgII-DOM and HgII−OR photoreduction, whereas aqueous photoreduction of HgII bound to SR-ligands and to constituents of snow are impacted by AMDEs display (−) MIE (cf. Figure 5). It has been hypothesized that atmospheric and oceanic reservoirs might contain complementary MIF signatures,46,96 since aquatic organisms carry large positive Δ199Hg values acquired from microbially produced MeHg that has been extensively photochemically degraded.42 Later, direct measurements of atmospheric vapor-phase Hg (∼Hg0) and precipitation (HgII) have shown circum-zero (slightly negative) and significantly positive odd-MIF, respectively.56−59,61,62 These observations question the importance of aquatic photoreduction as a source of MIF in atmospheric Hg. In a global model of MIF of odd Hg stable isotopes, Sonke97 postulated deliquescent aerosol HgII photoreduction (+) MIE and continental HgII photoreduction on soil, snow, and vegetation surfaces (−) MIE to make up for the inconsistencies. However, air-surface Hg0 exchange of vegetation and forest floor tend to display near-zero (−) MIF.56,98 In contrast, the G


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Environmental Science & Technology atmospheric oxidation of Hg0 should not be under-rated in shaping the stable Hg isotope composition in the atmosphere and interfaced environmental compartments.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b01668. Complementary materials including chemicals and reagents, detailed descriptions of the relative rate and isotope fractionation experiment, separation of reactants and products from the reaction mixture, investigation of the product phase distribution, details of chemical analysis, and figures relating to Hg bulk and isotope fractionation kinetics during Cl• and Br•-initiated oxidation of Hg0 vapor (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +86-851-85895728. Fax: +86-851-85891721. E-mail: [email protected] (X.F.). *Phone: +86-015885096925. Fax: +86-851-85891721. E-mail: [email protected] (J.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Qifan Liu, Kun Li, Ben Yu, Jiubin Chen, Yun Liu, and Huiming Bao for experimental assistance and helpful discussions. This research was funded by the National Science Foundation of China (grant #41273144, 41428301, 41173024), the National Key Basic Research Program of China (No. 2013CB430003), and by the innovation and collaboration program of Chinese Academy of Sciences.

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REFERENCES

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