HgII Species Research Articles

Non-Equilibrium Plasma Oxidation of Elemental Hg

Atmospheric mercury, specifically oxidized mercury (HgII), is the largest anthropogenic pool of mercury in the atmosphere. This type of mercury can accumulate in the food chain after undergoing methylation and entering ecosystems through wet and dry deposition. It is important to evaluate the level of wet and dry deposition, but measurements of atmospheric mercury have been biased due to insufficient consideration of measurement uncertainty and metrological traceability, especially for low ambient concentrations of gaseous HgII species.To address this issue, a calibration method for HgII species has been presented based on atmospheric pressure non-equilibrium plasma oxidation of Hg0 to HgII. Hg0 is generated by reducing HgII in aqueous solution by SnCl2 and aeration, then oxidized to different HgII species using He plasma with traces of different reactive gases. The non-equilibrium plasma oxidation efficiencies, with corresponding expanded standard uncertainty values, were evaluated using a highly sensitive radiotracer. The efficiencies were found to be 100.5 ± 4.7% (k = 2) for 100 pg of HgO, 96.8 ± 7.3% (k = 2) for 250 pg of HgCl2, and 77.3 ± 9.4% (k = 2) for 250 pg of HgBr2. The presence of HgO, HgCl2, and HgBr2 was confirmed by temperature-programmed desorption quadrupole mass spectrometry (TPD-QMS).The results demonstrate the potential of non-equilibrium plasma oxidation for reliable calibration of ambient air measurement instrumentation, leading to a better understanding of the sources, transport, and fate of this toxic pollutant in the environment. Knowing the chemistry and composition of atmospheric HgII species is crucial for assessing the extent of dry and wet deposition and mitigating the negative impacts of mercury pollution on human health and the environment.

Calibration Approach for Gaseous Oxidized Mercury Based on Nonthermal Plasma Oxidation of Elemental Mercury.

Atmospheric mercury measurements carried out in the recent decades have been a subject of bias largely due to insufficient consideration of metrological traceability and associated measurement uncertainty, which are ultimately needed for the demonstration of comparability of the measurement results. This is particularly challenging for gaseous HgII species, which are reactive and their ambient concentrations are very low, causing difficulties in proper sampling and calibration. Calibration for atmospheric HgII exists, but barriers to reliable calibration are most evident at ambient HgII concentration levels. We present a calibration of HgII species based on nonthermal plasma oxidation of Hg0 to HgII. Hg0 was produced by quantitative reduction of HgII in aqueous solution by SnCl2 and aeration. The generated Hg0 in a stream of He and traces of reaction gas (O2, Cl2, or Br2) was then oxidized to different HgII species by nonthermal plasma. A highly sensitive 197Hg radiotracer was used to evaluate the oxidation efficiency. Nonthermal plasma oxidation efficiencies with corresponding expanded standard uncertainty values were 100.5 ± 4.7% (k = 2) for 100 pg of HgO, 96.8 ± 7.3% (k = 2) for 250 pg of HgCl2, and 77.3 ± 9.4% (k = 2) for 250 pg of HgBr2. The presence of HgO, HgCl2, and HgBr2 was confirmed by temperature-programmed desorption quadrupole mass spectrometry (TPD-QMS). This work demonstrates the potential for nonthermal plasma oxidation to generate reliable and repeatable amounts of HgII compounds for routine calibration of ambient air measurement instrumentation.

Self-cementation of the alkali-activated volcanic tuff coupling with thiol-functionalized expanded perlite that enhances the solidification and stabilization of the mercury-contaminated soil

HgII contaminants tend to be soluble in the anoxic sediments, which would potentially deteriorate the traditional solidification/stabilization (S/S) process. The thiol-functionalized expanded perlite (thiol-Exp-p) was prepared and employed in the self-cementation of the alkali-activated volcanic tuff (VT) as both lightweight aggregate and adsorbent to enhance the S/S of the Hg-contaminated soil. The cementation experiments were comprehensively researched in a systematical design. The experiments of adsorption, self-cementation, coupling, and stability were subsequently configured and implemented to investigate the feasibility of the coupling system and to optimize the whole S/S process. Both the pseudo-first-order kinetic model and the intraparticle diffusion model were appropriate in fitting the dynamic adsorption of HgII to thiol-Exp-p. CaO among the three alkali components had the most significant influence on the self-cementation of VT. The factors including the curing temperatures, the grinding times of VT, and the mass ratios of fly ash (FA) to binder all affected the coupling S/S process of HgII-spiked soil. The compressive strengths gradually decreased over the dosage of soil samples ranging from 10% to 70% in the solidified blocks in the stability experiments with the leaching concentrations contrarily elevating in the range. The addition of thiol-Exp-p to the self-cementation of VT not only reinforced the stabilization of HgII species in the solidified sample but also positively influenced the solidification process.

Photochemistry of oxidized Hg(I) and Hg(II) species suggests missing mercury oxidation in the troposphere

Mercury (Hg), a global contaminant, is emitted mainly in its elemental form Hg0 to the atmosphere where it is oxidized to reactive HgII compounds, which efficiently deposit to surface ecosystems. Therefore, the chemical cycling between the elemental and oxidized Hg forms in the atmosphere determines the scale and geographical pattern of global Hg deposition. Recent advances in the photochemistry of gas-phase oxidized HgI and HgII species postulate their photodissociation back to Hg0 as a crucial step in the atmospheric Hg redox cycle. However, the significance of these photodissociation mechanisms on atmospheric Hg chemistry, lifetime, and surface deposition remains uncertain. Here we implement a comprehensive and quantitative mechanism of the photochemical and thermal atmospheric reactions between Hg0, HgI, and HgII species in a global model and evaluate the results against atmospheric Hg observations. We find that the photochemistry of HgI and HgII leads to insufficient Hg oxidation globally. The combined efficient photoreduction of HgI and HgII to Hg0 competes with thermal oxidation of Hg0, resulting in a large model overestimation of 99% of measured Hg0 and underestimation of 51% of oxidized Hg and ∼66% of HgII wet deposition. This in turn leads to a significant increase in the calculated global atmospheric Hg lifetime of 20 mo, which is unrealistically longer than the 3-6-mo range based on observed atmospheric Hg variability. These results show that the HgI and HgII photoreduction processes largely offset the efficiency of bromine-initiated Hg0 oxidation and reveal missing Hg oxidation processes in the troposphere.

A potential route for photolytic reduction of HgCl2 and HgBr2 in dry air and analysis about the impacts from Ozone

Among various gaseous oxidized mercury species, HgCl2 and HgBr2 are known to be more recalcitrant to direct photolysis under radiations. In this study, we examined a possible route to reduce HgCl2 and HgBr2 in the dry air under UV, visible and solar radiations with different levels of irradiances. Potential impact of O3 on the reductions of HgCl2 and HgBr2 were discussed. The laboratory results showed that if without any radiation, O3 would not influence the chemical conversions between HgCl2/HgBr2 and Hg0. However, if with involvement of radiation, a rapid reduction of HgCl2 and HgBr2 was observed, resulting in a significant production of Hg0 in the air. The magnitudes of reduction reactions heavily depended on the frequency of radiations as well as the irradiances. UV light had the strongest capacity to promote photoreduction of HgCl2 and HgBr2 even at relatively low irradiances. A relationship analysis indicated that the reduction of HgCl2 and HgBr2 was possibly related to the O3 concentrations in the air. We proposed that O3 might be involved in productions of the unstable intermediate HgIX (X = Br, Cl), which has a quicker photolysis rate than the HgIIX2. Our results suggest that the impacts of solar radiation and O3 on reductions of HgII species in the atmosphere are worth of being investigated theoretically and in both laboratory and field studies in the future, since it might be an important but lesser-known part in the atmospheric Hg chemistry.

Photodissociation Mechanisms of Major Mercury(II) Species in the Atmospheric Chemical Cycle of Mercury

AbstractMercury is a contaminant of global concern that is transported throughout the atmosphere as elemental mercury Hg0 and its oxidized forms HgI and HgII. The efficient gas‐phase photolysis of HgII and HgI has recently been reported. However, whether the photolysis of HgII leads to other stable HgII species, to HgI, or to Hg0 and its competition with thermal reactivity remain unknown. Herein, we show that all oxidized forms of mercury rapidly revert directly and indirectly to Hg0 by photolysis. Results are based on non‐adiabatic dynamics simulations, in which the photoproduct ratios were determined with maximum errors of 3%. We construct for the first time a complete quantitative mechanism of the photochemical and thermal conversion between atmospheric HgII, HgI, and Hg0 compounds. These results reveal new fundamental chemistry that has broad implications for the global atmospheric Hg cycle. Thus, photoreduction clearly competes with thermal oxidation, with Hg0 being the main photoproduct of HgII photolysis in the atmosphere, which significantly increases the lifetime of this metal in the environment.

Photodissociation Mechanisms of Major Mercury(II) Species in the Atmospheric Chemical Cycle of Mercury.

Mercury is a contaminant of global concern that is transported throughout the atmosphere as elemental mercury Hg0 and its oxidized forms HgI and HgII . The efficient gas-phase photolysis of HgII and HgI has recently been reported. However, whether the photolysis of HgII leads to other stable HgII species, to HgI , or to Hg0 and its competition with thermal reactivity remain unknown. Herein, we show that all oxidized forms of mercury rapidly revert directly and indirectly to Hg0 by photolysis. Results are based on non-adiabatic dynamics simulations, in which the photoproduct ratios were determined with maximum errors of 3%. We construct for the first time a complete quantitative mechanism of the photochemical and thermal conversion between atmospheric HgII , HgI , and Hg0 compounds. These results reveal new fundamental chemistry that has broad implications for the global atmospheric Hg cycle. Thus, photoreduction clearly competes with thermal oxidation, with Hg0 being the main photoproduct of HgII photolysis in the atmosphere, which significantly increases the lifetime of this metal in the environment.

Atomistic View of Mercury Cycling in Polar Snowpacks: Probing the Role of Hg2+ Adsorption Using Ab Initio Calculations

Photochemical oxidation of atmospheric elemental mercury (Hg0) promotes reactive oxidized Hg (HgII) adsorption on particles and deposition to the polar snowpack. The deposited Hg either returns to the atmosphere via photochemical reduction or remains in the snowpack depending on the strength of adsorption. In this study, we performed ab initio calculations to understand the atomic-level cause of the fate of adsorbed Hg by determining the adsorption affinity for Hg2+, the simplest form of HgII, of barite, halite, muscovite, illite, and ice-Ih as potential adsorbents. The adsorption affinity was estimated by calculating the energy required to dissociate adsorbed Hg2+ from the adsorbents. The results reveal that Hg2+ is stable on the surfaces of the selected adsorbents, except barite, but is prone to photodissociation under solar ultraviolet radiation. This mild adsorption is expected to contribute to the bidirectional exchange of Hg between the atmosphere and the polar snowpack. Thus, this theoretical approach can provide complementary perspectives on polar Hg dynamics beyond the limitations of field and laboratory experiments. Further studies on more complicated and realistic adsorption models with different HgII species and adsorbent surfaces having diverse defective structures are required to better comprehend air–snow Hg cycling in the polar regions.

Mercury Stable Isotope Fractionation During Coal Combustion in Coal-Fired Boilers: Reconciling Atmospheric Hg Isotope Observations with Hg Isotope Fractionation Theory.

Mercury (Hg) stable isotope is a useful tool to understand the transformation of atmospheric Hg. The observation on the enrichment of heavier isotopes in gaseous elemental Hg (GEM) relative to oxidized HgII species in atmosphere cannot be convincingly explained by isotope fractionation of Hg redox processes. This review shows that the large Hg isotope mass dependent fractionation (MDF) in coal-fired boilers is one of the underlying reasons. The reported Hg isotope data of feed coals and their combustion products are first summarized to give a general overview of how Hg isotopes fractionate before Hg discharge from coal-fired boilers. Then, predictive MDF models are discussed to simulate δ202Hg values of different Hg species in coal combustion flue gases. The discharged GEM is predicted to have the highest δ202Hg followed by gaseous HgII and particulate-bound HgII, which is in consistent with the observed MDF pattern of atmospheric Hg species.

Mercury transformations in resuspended contaminated sediment controlled by redox conditions, chemical speciation and sources of organic matter

Mercury (Hg) contaminated sediments can be significant sources of Hg in aquatic ecosystems and, through re-emission processes, to the atmosphere. Transformation and release of Hg may be enhanced by various sediment perturbation processes, and controlling biogeochemical factors largely remain unclear. We investigated how rates of Hg transformations in pulp-fiber enriched sediment contaminated by Hg from chlor-alkali industry were controlled by (i) transient redox-changes in sulfur and iron chemistry, (ii) the chemical speciation and solubility of Hg, and (iii) the sources and characteristics of organic matter (OM). Sediment-bottom water microcosm systems were exposed to four combinations of air and nitrogen gas for a total time of 24 h. The treatments were: 24 h N2, 0.5 h air + 23.5 h N2, 4 h air + 20 h N2 and 24 h of air exposure. As a result of these treatments, microcosms spanned a wide range of redox potential, as reflected by the dissolved sulfide concentration range of ≤0.3–97 µM. Four different chemical species of inorganic divalent Hg (HgII) and methyl mercury (MeHg), enriched in different Hg isotope tracers, were added to the microcosms: 201Hg(NO3)2(aq), 202HgII adsorbed to OM (202HgII-OM(ads)), 198HgII as microcrystalline metacinnabar (β-198HgS(s)) and Me204HgCl(aq). Microcosm systems were composed of bottom water mixed with sediment taken at 0–2, 0–5 and 0–10 cm depth intervals. The composition of OM varied with sediment depth such that compared to deeper sediment, the 0–2 cm depth-interval had a 2-fold higher contribution of labile OM originating from algal and terrestrial inputs, serving as metabolic electron-donors for microorganisms. The potential methylation rate constant (kmeth) of Hg tracers and net formation of ambient MeHg (MeHg/THg molar ratio) increased up to 50% and 400%, respectively at intermediate oxidative conditions, likely because of an observed 2-fold increase in sulfate concentration stimulating the activity of sulfate reducing bacteria with a capability of methylating HgII. Due to differences in HgII water-sediment partitioning, kmeth varied by a factor of 11–70 for the different isotope-enriched Hg tracers. The chemical form of HgII was a major controlling factor for kmeth and its response to the resuspension-oxidation of the system. The β-198HgS(s) tracer had the lowest kmeth and it was mainly constrained by redox-driven HgII solubility. The 202HgII-OM(ads) tracer showed an intermediate value on kmeth. It was controlled by both HgII solubility and availability of electron donors and acceptors, regulating bacterial activity. The 201Hg(NO3)2(aq) tracer had the highest value on kmeth which was limited mainly by bacterial activity. The kmeth was up to a factor of 3 higher in the 0–2 cm sediment depth-interval than in 0–5 and 0–10 cm intervals due to a larger contribution of labile OM in the 0–2 cm sediment. Reduction of HgII to Hg0 followed by volatilization exclusively occurred at high sulfidic conditions in the top 0–2 cm sediment. Aromatic moieties of terrestrial OM, present mainly in the top sediment, is suggested to control the reduction of HgII. The Hg0 volatilization rate constant for the 202HgII-OM(ads) tracer exceeded that for β-198HgS(s) by one order of magnitude. Our results suggest that contaminated sediments posing a high risk for reactivation of legacy Hg following transient redox resuspension events are characterized by depletion of sulfate in the sediment porewater prior to resuspension, predominance of HgII species with solubility exceeding that of crystalline β-HgS(s), and conditions promoting in situ formation and/or import of labile OM from algal and terrestrial sources.

Mercury Methylation Rates for Geochemically Relevant HgII Species in Sediments

Monomethylmercury (MeHg) in fish from freshwater, estuarine, and marine environments is a major global environmental issue. Mercury levels in biota are mainly controlled by the methylation of inorganic mercuric mercury (Hg(II)) to MeHg in water, sediments, and soils. There is, however, a knowledge gap concerning the mechanisms and rates of methylation of specific geochemical Hg(II) species. Such information is crucial for a better understanding of variations in MeHg concentrations among ecosystems and, in particular, for predicting the outcome of currently proposed measures to mitigate mercury emissions and reduce MeHg concentrations in fish. To fill this knowledge gap we propose an experimental approach using Hg(II) isotope tracers, with defined and geochemically important adsorbed and solid Hg(II) forms in sediments, to study MeHg formation. We report Hg(II) methylation rate constants, k(m), in estuarine sediments which span over 2 orders of magnitude depending on chemical form of added tracer: metacinnabar (β-(201)HgS(s)) < cinnabar (α-(199)HgS(s)) < Hg(II) reacted with mackinawite (≡FeS-(202)Hg(II)) < Hg(II) bonded to natural organic matter (NOM-(196)Hg(II)) < a typical aqueous tracer ((198)Hg(NO(3))(2)(aq)). We conclude that a combination of thermodynamic and kinetic effects of Hg(II) solid-phase dissolution and surface desorption control the Hg(II) methylation rate in sediments and cause the large observed differences in k(m)-values. The selection of relevant solid-phase and surface-adsorbed Hg(II) tracers will therefore be crucial to achieving biogeochemically accurate estimates of ambient Hg(II) methylation rates.

Rate of Access to the Binding Sites in Organically Modified Silicates. 3. Effect of Structure and Density of Functional Groups in Mesoporous Solids Obtained by the Co-Condensation Route

Mercaptopropyl-functionalized silica spheres have been prepared by self-assembly co-condensation of mercaptopropyltrimethoxysilane (MPTMS) and tetraethoxysilane (TEOS) precursors in the presence of a cationic surfactant as templating agent and ammonia as catalyst. Several materials have been obtained by varying the MPTMS content from 5 to 100% in the synthesis medium, giving rise to a wide range of porous solids featuring different functionalization levels (denoted MPS-n%, with n ranging from 5 to 100) and distinct structural order/disorder over different length scales. They were characterized by N2 sorption isotherms, XRD, scanning and transmission electron microscopy, and particle size analysis. Their reactivity in aqueous media was studied with respect to their binding properties toward HgII species by complexation with thiol groups. Total accessibility (expressed on the basis of a 1:1 S/Hg stoechiometry) was demonstrated and quantified for well-ordered materials containing up to 30% MPTMS. Less open s...

Grafted Silicas in Electroanalysis: Amorphous Versus Ordered Mesoporous Materials

AbstractThis work reports the enhanced response of electrodes modified with ordered mesoporous silica‐based materials, as compared to corresponding amorphous materials, in case of open‐circuit accumulation and voltammetric detection of a target analyte. Several porous silica samples grafted with either thiol or ammonium groups, displaying high surface areas with various structural characteristics and different pore sizes, have been examined with respect to their ability to concentrate HgII species. Better accessibility to the binding sites and faster diffusion processes in the interior of grafted silicas have been observed for the ordered samples prepared by micelle‐templating because they contain channels of regular size, which are less tortuous than the irregular pore systems in amorphous materials. When applied in electrochemical detection of HgII after preconcentration, the ordered solids gave rise to voltammetric signals higher by more than one order of magnitude in comparison to the amorphous organically‐modified silicas. This rather new class of materials, which can be easily produced as ordered organic‐inorganic hybrids, is expected to improve the performance of silica‐modified electrodes in various fields of electroanalysis.

Highly dissymmetric chelate coordination of 3,4,7,8-tetramethyl-1,10-phenanthroline to CuI(SR)

Not only AuI and HgII species but also CuI(SR) fragments can bind in a highly dissymmetrical fashion to symmetrical diimine chelate ligands; the 2 + 1 coordination arrangement observed for the metal in two complexes (tmphen)Cu(SR) (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline) is characterised by obtuse angles α[N(1)–Cu–S] > 159° and by two very different distances Cu–N(1) and Cu–N(2).

Ion‐exchange voltammetry of trace mercury(II) at glassy carbon electrodes coated with a cationic polypyrrole derivative. Application to pore‐waters analysis

AbstractGlassy carbon electrodes modified with coatings of poly‐[1‐methyl‐3‐(pyrrol‐1‐ylmethyl)pyridinium], poly‐MPP, were prepared by electrochemical oxidation of the suitable monomer in acetonitrile solutions. The anion exchange properties of the coated electrode were exploited in aqueous solutions for preconcentrating and detecting the anionic complex HgCl42−, which is the prevailing inorganic HgII species in seawater and other chloride media. The partition coefficients for the ion‐exchange equilibrium involved were calculated from voltammetric data and compared with those obtained at electrodes coated with Tosflex, a perfluorinated anion exchanger, and poly(vinylpyridine). The selectivity of the poly‐MPP coated electrodes towards the rejection of copper interference was also studied. The optimization of experimental conditions allowed the development of a differential pulse voltammetric method for the determination of submicromolar mercury concentrations with a detection limit of 0.1 nM. The method was applied to the analysis of mercury in the pore‐waters of tidal sediments of the Lagoon of Venice (Italy), determining HgII concentration values in the 70–80 nM range.