Mercury Transformation Research Articles

Effects of humic acid on transport and transformation of mercury in soil-plant systems

The influence of humic acid (HA) on the transport and transformation of mercury (Hg) in soil was studied. No available Hg could be detected (<2.5 μg kg−1) in alluvial soil when the content of HA-carbon (HA-C) was higher than 0.2 g kg−1 although a large amount of Hg (8 μg kg−1) was applied to the soil. The available Hg decreased with the increase of HA in purple soil (r=0.735). There are significant correlations between HA concentration and organic Hg in the tested soils (r=0.974 for the purple soil and r=0.979 for the alluvial soil). The increase of HA results in decrease of Hg absorbed by plant from the soil. A loss of Hg from soil caused by microbes was observed.

On the Gas Phase Reactions Between Volatile Biogenic Mercury Species and the Nitrate Radical

Tropospheric mercury is dominated by gas phase species. In this paper, the gas phase reactions between the nitrate radical and volatile biogenic mercury species have been investigated. An upper limit for the gas phase rate coefficient for reaction between elemental mercury and NO3-radicals was determined to 4 × 10−15 cm3 molecule−1 s−1 by using the fast flow-discharge technique. The reaction between dimethyl mercury and NO3, previously shown to be rapid, has also been studied in the laboratory with respect to product distribution using FT-IR. The result from the product study is consistent with a transformation of dimethyl mercury into inorganic, divalent mercury. All carbon delivered as dimethyl mercury was transformed into formaldehyde, methanol and methyl peroxynitrate. Hg was observed as a minor (≈2%) product. By exclusion, HgO is proposed as the mercury-containing product. Thus, the reaction between dimethyl mercury and the nitrate radical is excluded as a source of monomethyl mercury species in the atmosphere.

Detection of the merA gene and its expression in the environment

Bacterial transformation of mercury in the environment has received much attention owing to the toxicity of both the ionic form and organomercurial compounds. Bacterial resistance to mercury and the role of bacteria in mercury cycling have been widely studied. The genes specifying the required functions for resistance to mercury are organized on the mer operon. Gene probing methodologies have been used for several years to detect specific gene sequences in the environment that are homologous to cloned mer genes. While mer genes have been detected in a wide variety of environments, less is known about the expression of these genes under environmental conditions. We combined new methodologies for recovering specific gene mRNA transcripts and mercury detection with a previously described method for determining biological potential for mercury volatilization to examine the effect of mercury concentrations and nutrient availability on rates of mercury volatilization and merA transcription. Levels of merA-specific transcripts and Hg(II) volatilization were influenced more by microbial activity (as manipulated by nutrient additions) than by the concentration of total mercury. The detection of merA-specific transcripts in some samples that did not reduce Hg(II) suggests that rates of mercury volatilization in the environment may not always be proportional to merA transcription.

A new procedure for the speciation of mercury in water based on the transformation of mercury (II) and methylmercury (II) into stable acetylides followed by HPLC analysis

AbstractConversion of mercury(II) and methylmercury(II) species dissolved in water into di(phenylethynyl)mercury and methyl(phenylethynyl) mercury takes place in satisfactory yield under alkaline conditions by stirring the aqueous solution with phenylacetylene at room temperature. Mercury speciation is simply obtained by HPLC analysis of the two organometallic species. The presence of heavy metals such as copper(II), zinc(II), cadmium(II) and lead(II) in concentrations 10000 times higher than mercury is tolerated, while little interference is displayed by humic acids and cysteine. Seawater samples can also be analysed following a properly adapted procedure.

Bioaccumulation of Methylmercury and Transformation of Inorganic Mercury by Macrofungi

The terrestrial biogeochemical cycle of mercury between humus and fungi was investigated in a mercury-contaminated former mining area of Germany. We used a variable volume extraction procedure and aqueous sodium boron tetraethyl derivatization to determine accurately and precisely methylmercury bioconcentration factors for fungi. The methylmercury and total mercury content for fungi carpophores ranged from 0.08 to 7.94 μg/g of Hg as methylmercury and from 6.2 to 144.0 μg/g of inorganic Hg, respectively. The levels in humus material were between 0.01 and 0.09 μg/g of Hg as methylmercury and 15.8 and 140.2 μg/g of inorganic Hg, respectively. The bioconcentration factors obtained are generally lower than 1 for total mercury and between 3.0 and 199.0 for methylmercury. By an in vitro methylation experiment with two axenic cultures of Coprinus comatus and Coprinus radians, we could show for the first time that these saprophytic macromycetes are able to methyl ate mercury.

An experimental study on the rate of reaction between mercury vapour and gaseous nitrogen dioxide

The importance of nitrogen dioxide for oxidation of gaseous mercury in the atmosphere and in combustion flue gases was investigated. The reaction rate was studied by a stopped flow technique in a temperature interval from 20 °C to 900 °C and at surface-to-volume ratios of 1.8 and 4 cm−1. The reaction exhibits a first-order dependence with respect to mercury and a second-order dependence with respect to NO2 in the temperature range from 20 °C to 200 °C. From 250 °C to 500 °C, the NO2 dependence is lost and a partly heterogeneous reaction is proposed. Above 500 °C, no reaction was detectable. The Hg-NO2 reaction appears to have little significance for the atmospheric chemistry of mercury nor for the transformation of mercury in combustion processes, It might, however, have some importance on the oxidation of elemental mercury in aging combustion plumes.

Biotic and abiotic mercury methylation and demethylation in sediments

Inorganic mercury (Hg(II)) methylation and methylmercury (MeHg) demethylation may occur in the water column, sediment-water interface and subsurficial sediment of aquatic ecosystems. These transformations involve mainly microbial mechanisms, although abiotic methylation may play a more important role in the water compartment. The relative importance of biotic versus abiotic mechanisms of methylation has not been determined however, and abiotic demethylation remains unknown. Little quantitative information is available on the role of bacterial activity in mercury transformations. It has been reported that at least 16 genera of aerobic and anaerobic microorganisms are able to methylate HG(II), and that a greater number are able to demethylate MeHg. Nevertheless, not all populations of these species are capable of methyl- and demethyl-transformations. The actual concentration of MeHg in the aquatic environment is regulated by the relative production and decomposition rates. This, in turn, depends on the availability of Hg(II), MeHg, and bacteria as well as on the physico-chemical properties of the sample. The objective of this study was to compare mercury methylation and demethylation rates in sediment samples with and without active bacterial populations. We therefore performed experiments to follow bacterial evolution during the course of Hg(II) methylation and MeHg demethylation in sediment slurries containingmore » both sterile and non-sterile sediments.« less

Mercury-cycling in surface waters and in the atmosphere ? Species analysis for the investigation of transformation and transport properties of mercury

The river Elbe has been one of the most contaminated rivers with regard to mercury for many years. In 1991 a length-profile has been measured for mercury and methylmercury (CH3Hg+) from Obristvi, Czech Republic, to the German bight. Total mercury has been measured by cold vapor atomic absorption spectrometry (CVAAS). The organo mercury compounds have been separated by high performance liquid chromatography (HPLC) connected on-line to an atomic fluorescence spectrometer (AFS) by a continuous flow-system. Total mercury up to 120 mg Hg+/kg and CH3Hg+ concentrations up to 130 μg CH3 Hg+/kg could be detected in special sites. The formation of CH3Hg+ in sediments can be caused besides the methylation of mercury, by sulphate reducing or methanogenic bacteria and transmethylation reactions with organometals. Atmospheric mercury concentrations have been measured at three different European sites. Samples have been collected on goldcoated glass balls or on quartz wool, respectively. After thermal desorption mercury has been determined using the two step amalgamation technique with AFS detection. Compared to natural background concentrations of total gaseous mercury (TGM), slightly increased levels could be detected at a rural site in Germany. This increase can probably be explained by long-range transport processes. Within the vicinity of a inactivated mercury production plant high concentrations of up to 13.5 ng/m3 particle associated mercury (Hgpart) have been detected. Consequently, dry deposition of mercury in the particulate form can intensify the total deposition flux close to Hg-emitting sources.

Mercury cycling and effects in freshwater wetland ecosystems

AbstractThis literature review borrows from diverse fields because of the paucity of freshwater wetland studies on mercury cycling and effects. Peat cores provide an excellent means of dating mercury deposition temporal patterns. Conclusions about cycling suggest that a biogeochemical model would prove useful for evaluating wetland processes of mercury transformation and accumulation. Mercury methylation and the association of mercury with organic matter require additional research. Wetlands trap and release mercury, and its association with organic matter seems to affect the release rate. At high exposure, usually associated with laboratory studies or waste discharges, a variety of biotic toxic responses are observed. Predator species accumulate mercury predictably, and residue‐effect relationships seem useful for an index of ecologic damage. More definitive conclusions require additional research to define the ecosystem properties that affect mercury transfer to wetland predators.

Microbial transformation of mercury(II): I. Isolation of microbes and characterization of their transformation capabilities

Using immobilized yeast cells as energy and nutrient source, microbial consortia could be isolated in the presence of mercury (HgCl 2). Most of the 37 strains isolated from the mixed population exhibited high tolerancy against Hg(II). In fixed bed experiments the bacterial consortia retained 82–99% of the total mercury influent over a period of 4–7 weeks. Experiments with resting cells clearly reveal that passive (physical) processes are not responsible for the high mercury retention observed in fixed beds. Instead, as shown by experiments with growing microbes, Hg(II) is reduced to elemental Hg. Hence, the resistance of the isolated strains against HgCl 2 is based on a detoxification mechanism which involves the transformation of Hg(II) to the less toxic Hg 0.

Transformations of mercury species in the presence of Elbe river bacteria

AbstractThe influence of Elbe river bacteria isolated from suspended particulate matter (SPM) on dynamic species transformation of mercury was investigated. Experiments were carried out in the presence of bacteria (batch cultures) and in sterile tapwater as a control. For the methylation of inorganic mercury ions by bacteria several cofactors are under discussion. In this work, methylcobalamin, methyl iodide and S‐adenosylmethionine were tested as biogenic methyl donors and trimethyl‐lead chloride, trimethyltin chloride and dimethylarsenic acid as abiotic methyl donors. Transmethylation reactions as examples of abiotic methyl transfers have higher effectiveness in the formation of methylmercury (CH3Hg+) than methylation with biogenic compounds. This result was observed in batch cultures as well as in sterile water. SPM‐bacteria inhibit methyl transfer to mercury(II) ions. This is not only due to passive adsorption processes of mercury(II) to bacterial cell walls; methylmercury is also decomposed very rapidly by SPM‐bacteria and is immobilized as mercury(II) by the cells.

Methylation of inorganic mercury: Study of susceptible sites in an urban area in India

Abstract The biogenic transformation of inorganic mercury into methyl mercury by the interaction of selected substrates (sewage sludge, normal soil, TEL ‐contaminated soil, pond sediment, river sediment and cow dung) with mercuric chloride in closed vessels has been studied here. The methyl mercury evolved was measured in each case by allowing its passage through a solution of nitric acid, and estimating the Hg(II)ions spectrophotometrically using dithizone. The transformation into methyl mercury vas found significantly enhanced in presence of carbohydrates(glucose and starch) and carbohydrate‐bearing factory effluents such as those of distillery and rice mill. The methylation capacities of the substrates have been found in the following order: Sewage sludge > TEL‐contaminated soil > Cow dung> Pond sediment? River sediment> Normal soil. The relative susceptibilities of the sites of occurrence of these materials in an urban area to transform anthropogenically released mercury into much more toxic methyl mercury vapours have been discussed.

Occurrence of methylated tin and dimethyl mercury compounds in a mangrove core from Sepetiba Bay, Brazil

AbstractAnalyses of organotin and organic mercury compounds have been performed in a sediment core from Sepetiba Bay, Brazil, in order to investigate possible methylation pathways in a mangrove environment. The results have revealed that the physico‐chemical conditions existing in this type of environment (high organic inputs, anaerobic conditions, microbial activity, etc.) account for high methyltin concentrations (mono‐, di‐ and trimethyltin) in the sediments, which are dependent upon the total load of metal released (e.g. from anthropogenic sources). Furthermore, the presence of dimethylmercury and not monomethylmercury in the samples demonstrated a new pathway of transformation of mercury in the environment: this compound, thought to be unstable in sediment, is assumed to be stabilized by a conjunction of factors, such as high sulphide levels, anoxic conditions and constant inputs of methane into the medium.

The effect of Huanghe River runoff on the occurrence, transportation and speciation of mercury in the Huanghe Estuary and adjacent sea.

Mercury content of the sediments and water in the area of the Huanghe Estuary and adjacent sea was measured for several species. The content values were found to be among the lowest levels in the world estuaries. The distributions, more or less homogeneous in dry season while exhibiting a gradient during the flood season, show the main role of the Huanghe runoff, having a heavy load of silt. Also observed were the transformation of inorganic mercury into organic species during the mixing between fresh and saline waters, rectilinear correlation of sediment mercury to its median diameter, and the stratification of mercury species at the near estuary.

Volatilization of mercury compounds by methylmercury-volatilizing bacteria in Minamata Bay sediment.

Minamata Bay has been heavily polluted by high mercury concentrations which gave rise for a long time to methylmercury poisoning, Minamata disease (Kutsuna 1968; Irukayama 1977). The mercury still exists in the sediments of the Bay. The population of mercury-resistant bacteria in the sediments of Minamata Bay is larger than that in the sediments of other marine environments. The mercury-resistant bacteria isolated from a marine environment have been found to transform organic and inorganic mercury compounds into mercury vapor. The mercury-resistance confirmed in various bacterial genera has been shown to be plasmid-mediated volatilization. However, there has been little definitive information on the volatilization of organic mercury by the bacteria living in the mercury-polluted environment. It is important to know what bacterial transformations of mercury have been taking place and how the mercury-resistant bacteria may be playing a role in the mercury cycle in the marine environment of Minamata Bay. The object of the present study is to clarify the characteristics of the methylmercury-volatilizing bacteria in the sediments of Minamata Bay and of the volatilization of various mercury compounds by these bacteria.

Effects of acidification on mercury methylation, demethylation, and volatilization in sediments from an acid-susceptible lake.

The effect of experimental acidification on mercury methylation, demethylation, and volatilization was examined in surficial sediment samples from a weakly buffered northern Wisconsin lake. All mercury transformations were measured with radioisotopic tracers. Acidification of sediment pH with H2SO4, HCl, or HNO3 significantly decreased 203Hg(II) methylation. Acidification of pH 6.1 (ambient) sediments to pH 4.5 with either H2SO4 or HCl inhibited methylation by over 65%. The decreased methylation was due to the increased hydrogen ion concentration because methylation was not affected by concentrations of Na2SO4 or NaCl equimolar to the amount of acid added. Inhibition of methylation was observed even after prolonged acidification of sediments to pH 5.0 for up to 74 days. Acidification of sediments to pH 5.5, 4.5, and 3.5 with HNO3 resulted in a near complete inhibition of methylation at each pH. Similarly, the addition of equimolar amounts of NaNO3 resulted in a near complete inhibition of methylation, indicating that the inhibition was due to the nitrate ion rather than to the acidity. Demethylation of methyl mercury was not affected by pHs between 8.0 and 4.4, but sharply decreased below pH 4.4. Volatilization of 203Hg(II) from surface sediments was less than 2% of methylation activity and was not significantly different from that in killed sediments. This study indicated that acidification of sediments inhibits mercury methylation and that the observed increase in the mercury burdens in fish from low pH lakes is not due to increased production of methylmercury in sediments.

Mercury transformations by heterotrophic bacteria isolated from cinnabar and other metal sulfide deposits in Italy

Abstract Several mercury‐resistant bacteria were isolated from sites in Italy that surrounded natural mercury deposits. Bacterial strains resistant to 5 mg mercuric chloride per liter reduced Hg2+ to elemental mercury by an inducible process. One bacterial strain was resistant to methylmercuric chloride and produced elemental mercury when incubated with several organomercurial compounds. No methylmercury was detected in bacterial cultures growing in the presence of 2 mg mercuric chloride per liter. Mercury reduction is the major mercury transformation in mercury‐resistant bacteria isolated from these environments.

A non-equilibrium approach to modeling toxic metal speciation in acid, aquatic systems

A general algorithm (NONEQUI) was developed to simulate the kinetics of sorption and cation exchange processes among heavy metals and substrates in acid (pH < 6.5) lake or stream environments. Because the model is formulated in a kinetic framework, both fast and slow (relative to both the time frames of biological metal uptake and adsorption as well as hydrological residence time) metal substrate reactions can be simulated without incurring computational errors due to the assumption of chemical equilibrium at all times. The model is useful in that it predicts temporal and spatial concentrations of both bound and unbound metals. This is particulary important because it is commonly the unbound species of a metal (e.g., a free divalent cation of cadmium, Cd 2+) that is its most toxic form. The model was developed for predicting the fate and transport of any divalent cation including mercury. Important transformations of mercury were included that other cations do not significantly experience: methylation, demethylation, volatilization, and humic acid reduction of Hg 2+ to Hg 0. The latter reaction has potential for significantly affecting the fate, transport, and effects of heavy metals in waters of the southeastern U.S.

Mercury Circulation in Aquatic Environment. Part 3: Mercury Transformations in Sediments

Two types of sediments, one of them highly polluted by the pulp and chemical industry and the other one polluted by the urban type of sewage, have been analysed for their content of methylmercury and inorganic mercury. The rate of the formation of methylmercury in sediment from inorganic mercury (II) and the decomposition of methylmercury and phenylmercury have been investigated using 203mercury compounds. It has been found that the rates of both processes depend on the experimental conditions. The quickest formation of methylmercury has been observed under the Procedure C which simulates the natural conditions. The lower the rate of demethylation of methylmercury the higher the equilibrium concentration of methylmercury which has been found in sediments.

Non-enzymatic reduction of organomercurial salts in biological systems

Mercury redox transformation reactions in nature may be biological (enzymic) or nonbiological (nonenzymic). Biological mercury transformation has usually been explained in terms of the action of a number of specialized enzymes. While nonbiological transformation has been explained in terms of the action of certain biological products such as methylcobalamin (methyl vitamin B/sub 12/), methionine, ascorbate, humic acid and some proteins. It has been shown that mercuric salts (Hg/sup + +/) can be reduced and alkylated by the action of ethylene and acetylene (two known reducing agents in air), and it has been established that mercuric salts are reduced by propene and other higher molecular weight unsaturated hydrocarbons (olefins).