Mercury Transformation Research Articles

Gas-phase transformations of mercury in coal-fired power plants

Gas-phase transformations of mercury in coal-fired power plants

Transformations of elemental mercury to inorganic and organic forms in mercury and hydrocarbon co-contaminated soils

There are many industrial sites, such as gas processing plants, that are contaminated with both mercury and hydrocarbons. These sites tend to be localized but can have very high concentrations of mercury in the soil and heterogeneous distribution of hydrocarbons. The original form of mercury in many cases was elemental mercury from broken manometers. Over time the mercury has become redistributed within soil and has undergone chemical transformations into new forms. The forms of mercury will govern the chemical behavior and the availability of the mercury to biological receptors. The availability of the mercury is important as it will govern the risk associated with the contaminated soil and will also determine the effectiveness of any attempts at remediation. In the present study a chemical extraction protocol was used to determine the forms of mercury in soil originally contaminated by spillage of elemental mercury and petroleum hydrocarbons. Chemical extractions have been used in the past to determine the forms of mercury in uncontaminated soils and several researchers have used them to study contaminated soils. However, to date, no researchers have studied the forms of mercury in soils following years of weathering of elemental mercury after a spill. This study shows that decades after the original spill the elemental mercury has transformed and is dominantly (up to 85%) associated with soil organic matter, and to a lesser extent the mineral fraction of soil.

The photochemical reduction of divalent mercury and methyl‐mercury

In this paper, the photochemical transformation of divalent mercury ions and methyl‐mercury in water solutions was studied under simulated sunlight in a laboratory setting. The results indicated that the Hg2+ in solution can be reduced to Hg0 under irradiation, which was subsequently released to gaseous phase. Natural organic matter, such as fulvic acid (FA), can influence the reduction of Hg2+. The reduction rate of Hg2+ in the presence of FA was enhanced in the initial period. Methyl‐mercury can also be photochemically reduced to gaseous Hg0. Both the irradiation and FA can greatly accelerate the transformation process in solution. This study was of significance in understanding the transformation of divalent mercury and methyl‐mercury at the water‐atmosphere interface.

Evaluating mercury transformation mechanisms in a laboratory - scale combustion system

Mercury speciation measurements during injections of 10 μg/m 3 Hg 0(g) into a 42-MJ/h combustion system containing gaseous O 2–Ar- and O 2–N 2-rich mixtures indicate that 43 and 55% of the Hg 0(g) spike was transformed rapidly (<0.1 s) to Hg 2+ X(g) within a refractory-lined heat exchanger where gas temperatures decrease from ≈620 to 200°C. O 2(g) is the probable Hg 0(g) oxidant (i.e. X=O 2−). The apparent formation of HgO(g) involves a heterogeneous reaction with adsorbed Hg 0 or O 2 on refractory surfaces or a Hg 0(g)–O 2(g) reaction catalyzed by corundum (Al 2O 3) and/or rutile (TiO 2) components of the refractory. The potential catalytic effects of Al 2O 3 and TiO 2 on Hg 0(g) oxidation were investigated by injecting Al 2O 3 and TiO 2 powders into ≈650°C subbituminous coal (Powder River Basin, Montana, USA) combustion flue gas. On-line Hg 0(g) and total mercury measurements indicate, however, that Al 2O 3 and TiO 2 injections were ineffective in promoting the formation of additional Hg 2+ X(g). Apparently, either the chemically complex flue gas hindered the catalytic effects of Al 2O 3 and TiO 2, or these compounds are simply not Hg 0(g) oxidation catalysts.

A sensitivity analysis on the atmospheric transformation and deposition of mercury in north-eastern USA

This paper presents the results of a sensitivity analysis on the factors that affect dry and wet deposition of atmospheric mercury (Hg), using a regional scale air quality model. Simulations were conducted for the north-eastern USA during a summer week and a winter week in 1997. Simulation results for the summer week and the winter week in general showed similar responses to changes in emission, environmental conditions, and alternative chemical mechanisms. Reduction of the ambient concentrations of soot or ozone was shown to reduce the wet deposition of Hg. When averaged over the summer and the winter week, the total deposition to the simulation domain would be reduced by 26% by reducing Hg emission from anthropogenic sources within the domain by 50%. For individual grids, however, only locations near local sources obtained noticeable reductions in ambient concentration and wet deposition due to the influence of re-emission from the natural surfaces and regional/global scale transport. The reduction in deposition would reach 36% if all Hg(II) emitted from anthropogenic sources were attached to particles. The total deposition was predicted to decrease by 22% when the gas phase Hg(II)–Hg(p) partitioning was included in the model. Only small changes in total deposition were observed by including the gas-phase ozone-Hg(0), reaction and the aqueous phase chlorine-Hg(0), reaction, and by lowering ambient concentrations of Hg(II) and Hg(p) at the upper lateral boundaries. During the summer week, Hg(II) deposition contributed 40% or more to the total deposition. The contribution increased to 70% in the winter week.

Mercury transformations in coal combustion flue gas

Mercury chlorination [i.e., formation of HgCl 2(g)] is generally assumed to be the dominant mercury-transformation mechanism in coal combustion flue gas. Other potential mechanisms involve mercury interactions with ash particle surfaces where reactive chemical species, oxidation catalysts, and active sorption sites are available to transform Hg 0(g) to Hg 2+X(g) (e.g., where X is Cl 2 or O) as well as Hg 0(g) and HgCl 2(g) to particulate mercury, Hg(p). Results from an investigation of Hg 0(g)–O 2(g)–HCl(g) and Hg 0,2+(g)–HCl(g)–CaO(s)-fly ash interactions in a 42-MJ/h combustion system are consistent with the following mechanisms: mercury chlorination, catalysis of mercury oxidation by Al 2O 3(s) and/or TiO 2(s), and mercury sorption on a calcium-rich (25.0 wt.% CaO) subbituminous coal fly ash. Additions of 50 and 100 ppmv of HCl(g) and ≈12.6 wt.% of CaO(s) to the subbituminous coal combustion environment inhibited Hg(p) formation, primarily via a change in ash surface chemistry and a decrease in particle surface area, respectively.

Distribution and transformation of elemental mercury in the St. Lawrence River and Lake Ontario

Elemental Hg (Hg0) is a volatile species that is responsible for water-to-air transfer of Hg in Lake Ontario and the St. Lawrence River. We conducted two cruises in 1998 to identify spatial and temporal patterns in Hg0 levels in these systems and performed field and laboratory experiments on redox transformations of Hg. Elemental Hg concentrations were higher in Lake Ontario than in the St. Lawrence River. At stations in Lake Ontario, Hg0 levels were higher at the bottom of the water column than at the surface, whereas they were homogeneous throughout the water column of the river. Elemental Hg concentrations were generally higher in July than in September and, in contrast with experiments on inland lakes, were relatively constant during the day except for a narrow peak at sunrise. Field and laboratory experiments showed that photoreduction of Hg(II) in St. Lawrence River water was substrate limited and was influenced by visible and ultraviolet radiation. Pseudo first-order kinetics best described photoreduction, with k values between 1 and 2·h-1.

Gas-phase transformations of mercury in coal-fired power plants

Because mercury enters the food chain primarily through atmospheric deposition, exposure models require accurate information about mercury emission rates and mercury speciation from point sources. Since coal-fired power plants represent a significant fraction of the anthropogenic emissions of mercury into the atmosphere, the speciation of mercury in coal-fired power plant flue gas is currently an active topic of research. We have demonstrated that the assumption of gas-phase equilibrium for mercury-containing species in coal-fired power plant exhaust is not valid at temperatures below approximately 800 K (500°C). Chlorine-containing species have been shown to be the most important for oxidation of elemental mercury in the post-combustion gases. The conversion of HCl to Cl 2 in the flue gas of a coal-fired power plant is kinetically limited. Kinetic calculations of the homogeneous oxidation of elemental mercury by chlorine-containing species were carried out using global reactions from the literature. The levels of mercury oxidation, while of comparable magnitude to field observations, are still below the 40% to 80% oxidation typically observed in field measurements.

Modeling Aquatic Mercury Fate in Clear Lake, Calif.

This paper describes the construction and application of a numerical model representing mercury transformations and bioavailability in the aquatic environment of Clear Lake, Calif. The initial application described here provides the basis for more detailed fate and transport modeling of the lake. Results show that total Hg and MeHg concentrations in the lake may be reasonably modeled as functions of sediment total Hg. Calibrated rates of methylation and benthic exchange compare favorably with rates observed in the field and at other lakes. The model suggests the importance of accurately defining a biochemically “available” fraction of total Hg, and demonstrates that even a relatively small amount of available Hg in lake sediments can account for high levels of potentially toxic methylmercury in the water and sediment bed. Indications are that Clear Lake is experiencing a net loss of Hg at a rate of less than 2%/year. At such a remediation rate, mercury concentrations in the lake would meet current water quality criteria in a little over 200 years.

A regional scale modeling study of atmospheric transport and transformation of mercury. II. Simulation results for the northeast United States

This paper presents the results of a simulation study of the transport, transformation, and deposition of atmospheric mercury (Hg) in the northeastern United States for 5 consecutive days in summer 1997, using the newly developed regional scale air quality model described in part I. Hourly ambient concentrations, in-cloud transformations, and deposition fluxes were predicted for each of the three mercury species: Hg(0), Hg(II), and Hg(p). The simulation results showed that surface level Hg concentrations over the land varied diurnally with the boundary layer structure. The Hg concentrations over the coastal ocean water were strongly influenced by the concentrations over land because of atmospheric advection by the prevailing westerly winds in the region. Over 90% of the ambient air concentration of Hg was in the form of Hg(0) vapor. Approximately half of the total Hg in wet deposition originated from ambient Hg(0), while the predominant species in dry deposition was Hg(II). The simulation predicted more dry deposition ( 31 kg day −1 ) than wet deposition ( 18 kg day −1 ) over the region for the simulation period. The re-emission of Hg(0) from land and water surfaces ( 46 kg day −1 ) was of the same magnitude as the total anthropogenic emission ( 55 kg day −1 ) within the region. Both precipitating and non-precipitating clouds were shown to be important media for in-cloud transformation of Hg(0). It was predicted that transformation and redistribution caused a net gain of Hg(p) at the upper troposphere during rain events. Over the region of simulation, the amount of Hg(0) transformation in non-precipitating clouds was 30% higher than in precipitating clouds. In co-existing non-precipitating clouds, the amount of Hg(0) transformation in the lower and upper troposphere was approximately equal.

A regional scale modeling study of atmospheric transport and transformation of mercury. I. Model development and evaluation

A three-dimensional regional scale air quality model was developed to study the atmospheric transport, transformation and deposition of mercury (Hg) by formulating and incorporating mercury chemistry, cloud processes, and air–surface exchanges into the framework of Sarmap Air Quality Model (SAQM). Three mercury species were included: elemental mercury Hg(0), divalent mercury Hg(II), and particulate mercury Hg(p). Precipitating clouds, co-existing non-precipitating clouds, and fair weather clouds were considered in modeling the in-cloud transformation processes. A formulation of bi-directional air-surface exchange of elemental mercury was used for emission from, and dry deposition to, natural surfaces. Preliminary evaluation of the model was conducted by comparing six major weekly output variables, including ambient Hg concentrations and Hg concentration in precipitation, with corresponding measurements at eight monitoring stations in Connecticut for a summer week and a winter week. Model predictions of surface-level gaseous Hg concentrations were close to measured levels, agreeing to within 12% on average, about half the estimated error in measurements. The predicted Hg concentrations in precipitation were 50% higher than measured values on average, slightly lower than the estimated 60% error in measurements. The model was shown to be capable of predicting hourly concentrations and deposition fields of the three Hg species as well as in-cloud transformation of Hg(0) by each of the three cloud types, and useful in analyzing the effects of various controlling factors on the transport and transformation of Hg species in the atmosphere.

Distribution and transformation of elemental mercury in the St. Lawrence River and Lake Ontario

Distribution and transformation of elemental mercury in the St. Lawrence River and Lake Ontario

99/01170 Interaction of carbon dioxide with methane on oxide catalysts : Krylov, O. V. et al. Catal. Today, 1998, 42, (3), 211–215

99/01170 Interaction of carbon dioxide with methane on oxide catalysts : Krylov, O. V. et al. Catal. Today, 1998, 42, (3), 211–215

99/01165 Effect of the chemical activity of carbonaceous materials on reforming of coke oven gas : Zubilin, I. G. Koks Khim., 1998, (3), 16–18. (In Russian)

99/01165 Effect of the chemical activity of carbonaceous materials on reforming of coke oven gas : Zubilin, I. G. Koks Khim., 1998, (3), 16–18. (In Russian)

Two-phase model of mercury chemistry in the atmosphere

A hybrid kinetic and equilibrium model simulating the conditions in clouds is developed to assess mercury chemistry in the atmosphere. Simulation of an air parcel containing cloud droplets is used to examine the effects of a number of physical and chemical parameters on the evolution of dissolved divalent mercury [Hg(II)] concentration profiles. Sensitivity analysis on each parameter is performed and the resulting Hg(II) profiles are plotted. The modeled steady-state Hg(II) levels are then compared to the measured values in precipitation. It is found that gaseous-phase oxidation of elemental mercury by ozone contributes a significant fraction of dissolved Hg(II) in the droplets, and that aqueous phase radical (i.e. OH and HO 2) play an important role in mercury transformations. S(IV) is an important reductant for aqueous-phase Hg(II). However, the contribution of S(IV) in mercury chemistry is limited to the cases when aqueous phase S(IV) is present at a concentration to maximize HgSO 3 formation. After the system reaches a steady state, HO 2 is the only reductant to balance the Hg(II) production by various oxidation pathways in both phases. Based on the simulation results, it is suggested that more oxidation pathways should be identified to better describe mercury chemistry in the atmosphere.

Biomethylation of mercury(II) adsorbed on mineral colloids common in freshwater sediments

The effects of freshwater sediment components such as kaolinite, montmorillonite and birnessite (δ-MnO 2 ) on the biomethylation of mercury(II) in a synthetic growth medium (M-IIY) were assessed. Additions of kaolinite or montmorillonite to media containing mercuric nitrate [Hg(NO 3 ) 2 ; 12 μg Hg ml -1 ] had no significant effect on either bacterial growth or the production of methylmercury (CH 3 Hg + ). However, whereas the addition of birnessite resulted in only a small (ca 4%) increase in bacterial growth, it also produced a significant decrease (ca 50%) in the production of CH 3 Hg + . Further, it was demonstrated that, with the exception of kaolinite, adsorption of mercury(II) onto the sediment components before they were added to the M-IIY medium decreased its bioavailability, i.e., the amounts of CH 3 Hg + produced from the adsorbed mercury(II) were significantly lower than those produced from equivalent concentrations of Hg(NO 3 ) 2 in the absence of the mineral colloids. In the case of montmorillonite, CH 3 Hg + production was decreased by 21% relative to the control system. Most striking was the case of birnessite, in which no CH 3 Hg + was detected after a 25 h incubation period and only very small quantities of CH 3 Hg + (3-7 ng l -1 ) were present in the medium after 336 h. These data demonstrate that mineral colloids common in freshwater sediments significantly influence the extent of biomethylation of mercury(II) adsorbed on their surfaces. Birnessite, in particular, is a very effective inhibitor of the biomethylation of surface-bound mercury(II). Therefore, it may be possible to reduce the severity of mercury pollution in some aquatic environments by adding a reactive manganese oxide, such as birnessite, to the system and thereby to inhibit the transformation (methylation) of inorganic mercury(II) into the much more toxic CH 3 Hg + species.

Aqueous Photochemistry of Mercury with Organic Acids

ABSTRACT To understand the dynamic chemical changes of mercury taking place in atmospheric water, a more detailed knowledge of the reactions of mercury with organic acids must be obtained. The rate of the above chemical reactions with different Hg(II) species is especially essential for understanding the dominant pathway of mercury transformation in the atmosphere. The objective of this research is to study the rate of photochemical reactions of dissolved divalent mercury with organic acids such as oxalate, acetate, and formate, which are abundant in atmospheric water. Laboratory photochemical experiments with simulated sunlight were conducted to assess the role of homogeneous photochemistry in changing the redox states of mercury in atmospheric water. It is observed that Hg(II) is readily reduced by hydroperoxyl radicals produced by the photolysis of oxalate. The second-order rate constant for the Hg(II)-hydroperoxyl radical reaction was found to be 1.7 x 104 M-1 s-1 when no chloride is present and 1.1 x 104 M-1 s-1 when chloride is present in the system. Modeling of mercury speciation has been performed using MICROQL equilibrium software.

Dissolution of mercury vapor in simulated oral environments

Objective The aim of this investigation was to determine the effects of variables on the dissolution of atomic mercury in simulated oral fluids. This study was based on a hypothesis that the rate of mercury vapor dissolution in synthetic saliva is affected by the oxidation power and stirring of the solution. Method The reference solution was synthetic saliva, and the oxidation power was increased by hydrogen peroxide or decreased by ascorbic acid. Test solutions were exposed to mercury vapor in a sealed container, and the dissolved mercury concentration was determined after exposures ranging from 2 to 48 h. The results were analyzed by ANOVA and Tukey's multiple comparison test (p < 0.05). The effect of stirring was examined for a 24 h exposure. Student t-tests were used to compare this data (p < 0.05). The analysis for mercury was performed by cold-vapor atomic absorption spectrophotometry. Results Mercury dissolved much faster in the highly oxidizing solution with hydrogen peroxide than in the other two solutions, which dissolved mercury at the same rate. In the less oxidizing solutions, the dissolved mercury concentration reached a nearly constant value. Stirring the solution increased the rate of mercury dissolution for the highly oxidizing solution but had no effect on the less oxidizing solutions. Significance The results show the importance of the transformation of mercury from the atomic to the ionic state. When dissolving atomic mercury is not oxidized, it may evaporate and be inhaled, but the dissolution rate becomes low. Under more oxidizing conditions, mercury dissolves faster, but does not generate mercury vapor. Diffusion in the liquid affects the mercury dissolution rate only under highly oxidizing conditions.

97/03250 Catalysis and ecological problems in salty coals thermal decomposition processes

97/03250 Catalysis and ecological problems in salty coals thermal decomposition processes

97/03257 Control of dusting of coal faces in mines using lecithin

97/03257 Control of dusting of coal faces in mines using lecithin