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Mercury: a poisonous solution

Sigrid Griet Eeckhout from the

Image courtesy of Andraz Cerar / iStockphoto Cerar Andraz Image courtesy of European Synchrotron Radiation Facility in Grenoble, France, investigates what determines the toxicity of mercury compounds – and how X-ray light is helping to solve the mystery.

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Cutting-edge science

etals comprise about 75% of functional groups of various enzymes and living organisms. Biogeochemical Mknown elements. They can and proteins, and so inhibits or nega- processes affect the metal’s atomic form alloys with each other and with tively affects key organic functions. form (speciation), and therefore its non-metals and are widely used, for Mercury is used to extract gold, and solubility, mobility, bioavailability and example, in cars, computers, high- is found in thermometers, dental toxicity. As a rule, the less soluble a amalgam, thermostats, relays, switch- chemical species is, the less mobile ways and bridges. Civilisation was es, barometers, vacuum gauges and and less toxic it is. Therefore, trans- founded upon the metals of antiquity, other scientific apparatus, although forming soluble species to sparingly namely gold, silver, copper, mercury, concerns about its toxicity have led to soluble forms, either in situ or in land- tin, iron and lead. Gold was first dis- mercury thermometers being largely fills after excavation, can lessen the covered around 6000 BC and mercury phased out in clinical environments. impact of hazardous heavy metals on has been found in tombs dating back Mercury is a trace element with both living organisms and the environ- to 1600 BC. The ancient Greeks used natural (native metal, Hg, and ment. mercury in ointments; the Romans cinnabar, HgS) and anthropogenic Micro-organisms can transform met- used it in cosmetics. Since the begin- (man-made) sources. Anthropogenic als by means of oxidation-reduction ning of the Industrial Age, metals sources include agricultural (fungi- or other chemical reactions. One have slowly entered into the environ- cides) and metallurgic (mining and example is another heavy metal, ment, accumulating in soils, sedi- smelting) uses, plastics industries, hexavalent chromium, Cr(VI), which ments and surface waters. refuse disposal and landfills. Most of is a very dangerous, water-soluble Small quantities of many trace metal the mercury in soils, sediments and form of chromium. Ingesting large elements are of ecological interest due surface waters comes from the com- amounts of Cr(VI) can cause stomach to their necessity as nutrients or their bustion of fossil fuels. This volatile upsets and ulcers, convulsions, kid- toxicity as pollutants. Nutrient trace metal can travel over long distances ney and liver damage, various forms elements include magnesium, man- in its gaseous form or by attaching of cancer and even death. Trivalent ganese, copper and zinc, some of itself to small dust particles. Gaseous chromium, or Cr(III), on the other which become toxic at high concen- mercury can remain in the atmos- hand, is an essential trace nutrient trations. Others, including the heavy phere for up to one year before being that helps the body to use sugar, pro- elements such as mercury, cadmium, deposited onto the earth via rainfall. tein and fat. Cr(III) is insoluble in Image courtesy of Klaas Lingbeek-van Kranen / iStockphoto arsenic and lead, are of environmental Once deposited, metals and metal- water. Reducing Cr(VI) to Cr(III) concern due to their high toxicity and loids (elements with properties of using micro-organisms makes it insol- widespread industrial use. Mercury is both metals and non-metals) undergo uble in water, hence limiting its avail- present in the environment at concen- dynamic biogeochemical processes in ability and toxicity w1. trations of less than 0.1%, but is the near-surface environment, which This type of transformation can also extremely toxic because it binds to the is a mixture of rocks, soil, water, air occur the other way around. In soils,

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X-ray techniques (for advanced readers) For a simple explanation of these two techniques, see Capellas (2007).

X-ray absorption spectroscopy (XAS) The XAS spectrum is sensitive to the formal oxidation state (which reflects the number of electrons available X-rays are light with energies ranging from ~500 for binding to other atoms), the co-ordination chem- electron volts (eV) to 500 keV (1 keV is equivalent to istry (e.g., octahedral or tetrahedral co-ordination), 1000 eV). and the distances, co-ordination number and species When you perform an X-ray absorption spectroscopy of the atoms immediately surrounding the selected (XAS) measurement, you vary the energy of the inci- element. dent X-rays. When the energy of the incident X-ray equals the binding energy of a core-level electron X-ray fluorescence (usually 1s electron), the electron is ejected from the atom. The corresponding X-ray absorption spectrum The well-defined characteristic binding energies of an shows a sharp rise, also known as the absorption element are used as a fingerprint in the X-ray fluores- edge (Figure 1). The position of the absorption edge is cence technique. When you change the energy and also determined by the formal oxidation state. For you observe a peak at a particular energy, you know instance, the absorption edge of Cr(VI) occurs at high- that the respective element is present (Figure 2). er energy than the one for Cr(III). The outgoing elec- tron interacts with the surrounding atoms, thus creat- Image courtesy of ESRF ing oscillations in the spectrum beyond the edge. These oscillations provide information on the neigh- bouring atoms. Image courtesy of Sigrid Griet Eeckhout

Figure 2: X-ray fluorescence spectrum showing the presence of different elements

Figure 1: X-ray absorption spectrum of a Cr(III) sample (green) and Cr(VI) sample (red). The arrow shows the position of the absorption edge. au= arbitrary unit

Since every atom has core-level electrons with well- Reprinted with permission from Chemical Research in Toxicology. defined characteristic binding energies, the XAS tech- Copyright (2005) American Chemical Society nique is element-specific. This means that you can Figure 3: Micrograph of a soil sample (left), distribution study a chosen element (e.g., mercury) inside hetero- of Cr(total) within the sample (middle) and of the toxic geneous matter, such as a soil consisting of organic Cr(VI) (right). The colour scale reflects the concentra- matter, microbes, minerals, metals and so forth. tion. au= arbitrary unit BACKGROUND

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Cutting-edge science

microscopic organisms can transform environment. Furthermore, since sediments and surface waters. Once the less poisonous, inorganic (non- these mercury-thiol complexes are we know the speciation of the metal, carbon-containing) form of mercury soluble, they can be mobilised and we can look into how to limit its solu- into a poisonous, organic (carbon-con- transported to places where methylat- bility and bioavailability. As the taining) form. In this reaction, called ing bacteria live. world’s population and economies methylation, an atom, usually hydro- The next step is to identify the role continue to grow, especially in devel- gen, is replaced by a methyl group that different sulphur-containing mol- oping countries, the need for metals

(-CH3). As a positively charged ion, ecules frequently found in soil organ- will increase but their use will ampli- + methylmercury (CH3Hg ) readily ic matter play in the transformation of fy the potential for soil and water combines with anions such as chlo- mercury to its poisonous form. contamination. Since this could have ride (Cl-), hydroxide (OH-) or nitrate The use of XAS to characterise the serious implications for human health - (NO3 ). speciation of mercury is quite new. It and environmental quality, environ- Transforming mercury to a is a big step forward compared with mental studies are extremely impor- methylmercury compound produces earlier wet (liquid phase) biochemical tant. a metal form that becomes lipophilic methods and it is the first time that (i.e. can be dissolved in fat) and can such low concentrations of mercury References thus pass through cell membranes, (0.1 gram of mercury per 1000 grams Capellas M (2007) Recovering the blood-brain barrier and placenta. of soil) have been measured. Pompeii. Science in School 6: 14-19. In this organic form it can enter the Deciphering the chemistry of trace www.scienceinschool.org/2007/ food chain and accumulate in fish, metals and metalloids in the environ- issue6/pompeii fish-eating animals and humans. In ment is difficult because natural mate- Skyllberg U, Bloom PR, Qian J, Lin other words, the less poisonous, inor- rials are complex in composition and CM, Bleam WF (2006) Complexation ganic form of mercury, which would structure. With the advent of of mercury(II) in soil organic matter: normally be safely excreted by organ- advanced synchrotron light sources, EXAFS evidence for linear two- isms, is transformed into an organic which provide techniques using coordination with reduced sulfur form, which becomes available and intense X-rays and a greater spatial groups. Environmental Science & poisonous to organisms. resolution, scientists are able to deter- Technology 40: 4174-4180 So how does the less poisonous, mine the forms and distributions of inorganic form of mercury transform metals in heterogeneous systems such This paper was chosen from more into a poisonous, organic form? as soils, plants and mineral-microbe- than 1100 articles in Environmental Researchers from Sweden and the metal interactions. To do this, three Science and Technology as the top USA used synchrotron light at the micro-analytical techniques can be environmental science contribution European Synchrotron Radiation applied together. The micro-X-ray flu- from 2006. Facility (ESRF) to determine the speci- orescence technique (see box) can ation of mercury in natural organic map the distributions of the different Web references matter at environmentally relevant metals and help identify metal associ- w1 – For a discussion of chromium concentrations, using X-ray absorp- ations (Figure 3). Then, the species with reference to the film Erin tion spectroscopy (XAS) techniques hosting the metal (such as clay or Brockovich, see: (see box). mineral) is determined at points of Stevens J (2007) Erin Brockovich. They found that mercury in natural interest on the chemical maps by Science in School 4: 67-69. soil organic matter binds to two micro-X-ray diffraction and micro- www.scienceinschool.org/2007/ reduced organic sulphur groups, XAS. The diffraction pattern shows issue4/erinbrockovich/ mainly thiols (-SH). The thiol group is the inner structure of the material. the sulphur equivalent of the hydroxyl The proportion of each species in the group (-OH) found in alcohols. bulk material is further calculated by Resources Laboratory experiments indicate that linear combination of the different A short explanation of the use of neutral, inorganic mercury-thiol and component species (in other words, synchrotron light at ESRF is avail- mercury-sulphur species in solution by adding up the amounts on the able here: www.scienceinschool.org/ determine the rates of methylation. spectra). 2006/issue1/maryrose#esrf This means that the binding of mercu- In conclusion, X-ray techniques For information about ESRF, see: ry to thiol groups in natural organic using synchrotron light are extremely www.esrf.eu matter makes the element available valuable in determining the forms for the methylating bacteria in the and distributions of metals in soils,

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