The toxic effects of mercury and its compounds were proven many years ago, and major instances of mercury poisoning are well documented. Two of the most well known incidents are the Minamata Bay area of Japan, due to fish contamination from industrial waste in the 1950s; and secondly, the derivation of the phrase ‘mad as a hatter’, when mercury compounds were used to cure animal pelts in the production of hats during the 19th Century.
So there is no doubt that mercury and its compounds present ‘significant possibility of significant harm’ (SPOSH) to human health. The different forms of mercury can present different risks, however, and thus the determination of speciated mercury can help in assessing the potential risks presented by mercury in contaminated sites, and possibly save significant costs in site remediation.
Mercury generally exists in one of three phases: elemental, inorganic, or organic.
In its elemental form, mercury is a dense, silvery white metal, sometimes known as quicksilver, and exists as a moderately volatile liquid at room temperature. Mercury readily forms amalgams with a variety of other metals including sodium and zinc, although not the lighter transition metals such as iron.
The inorganic compounds exist in both the mono and divalent states – mercurous and mercuric respectively, with the latter being more soluble in water and therefore more toxic.
A large number of organic monomethylmercury compounds with the general formula, CH3HgX compounds are known, and can be synthesised by the action of sodium amalgam or Grignard reagent and mercuric chloride, HgCl2, with halogenated hydrocarbons. They are most likely to be found in soil as a result of natural microbial transformation of inorganic mercury, a process known as methylation.
The most important source of mercury is the naturally occurring mineral cinnabar (HgS, or mercuric sulphide), but global production has fallen over the last 50 years, with its main usages including recovery of gold and silver ores, alkaline batteries, dental amalgams, paints, pharmaceuticals, thermometers, and agriculture. Many of these applications are now phased out of Western countries, due to toxicity and disposal issues. As a chemical element, however, mercury released into the environment will remain there indefinitely, and although it may be converted to various forms over time, it will never disappear.
Mercury and its compounds are highly toxic to animal life, and because of the persistence of mercury and its compounds in the environment, contamination can travel over long distances, transported by atmospheric and ocean currents. Mercury is regarded as a global pollutant and is controlled under the United Nations Economic Commission for Europe (UNECE) Heavy Metals Protocol.
Elemental mercury is not readily digested or absorbed through skin contact, but the vapour released is easily absorbed – more than 80% of inhaled vapour is absorbed into the bloodstream, and repeated exposure to levels as low as 20 µg/m3 will produce damage to the Central Nervous System (CNS), with symptoms ranging from slurred speech, loss of motor control, and impaired cognitive skills. The calculated Tolerable Daily Intake (TDI) for inhaled mercury is 0.06 µg/kg of body weight/day.
Inorganic mercury, particularly in the mercuric form (the most soluble and stable form), is easily absorbed by the body after ingestion, but affects the kidneys rather than the CNS, causing potential kidney failure, depending on the exposure dose and timescales. The TDI for inorganic mercury is 2 µg/kg of body weight/day, so it is less toxic than the vapour from elemental mercury.
Where organic mercury is concerned, the brain is the key target for the toxic effects of ingested methylmercury, with similar symptoms as those for inhaled elemental mercury. Organic mercury compounds tend to bioaccumulate and can have a particularly significant toxic effect on aquatic species. Low levels of mercury contamination in lakes can lead to high concentrations in insects, fish and birds, building up to very toxic contamination in various parts of the ecosystem, as demonstrated in the Minamata case. The TDI for methylmercury is 0.23 µg/kg of body weight/day, therefore more toxic than inorganic mercury, but not as toxic as inhaled elemental mercury.
Excessive exposure to mercury and its compounds may affect the brain, digestive system, eye, heart, kidney, lung, nervous system, reproductive system, skin, and the unborn child. Symptoms include ataxia, numbness in the hands and feet, general muscle weakness, narrowing of the field of vision and damage to hearing and speech. In extreme cases, insanity, paralysis, coma and death follow within weeks of the onset of symptoms.
Soil Guideline Values (SGVs)
In 2009, the Environment Agency published a Science Report SCO 50021, entitled Soil Guideline Values for Mercury in Soil, and these are based on estimates representative of exposure of young children, because they are more likely to have higher exposures to soil contaminants. The document details the background, uses and toxicology of mercury, and states that analysis should be performed to provide values for three forms of mercury – elemental, organic, and inorganic.
The main sources of contamination in soils have been mining and smelting, burning of fossil fuels, industrial production of sodium hydroxide and chloride, and waste incineration, although mercury also occurs in trace amounts in fertilisers used on farmland.
Normal background values for mercury in soil are usually very low, in the range of 0.02 to 0.06 mg/kg, so elevated levels are due to anthropogenic activity, mostly from aerial emissions, discharges, or burying of waste. The UK Soil and Herbage Survey (2007) found background concentrations in rural soils from 0.07 to 1.22 mg/kg, and in urban soils from 0.07 to 1.53 mg/kg.
SGVs for the three chemical forms of mercury differ significantly from each other for two reasons:
• Human toxicology differs for each of the three main forms, with elemental mercury and methylmercury being the most toxic by inhalation and oral routes respectively • Elemental mercury and monomethylmercury forms are much more volatile than inorganic mercury and therefore vapour inhalation is more likely to contribute to exposure Soil guideline values (SGVs) are therefore provided for all three forms:
The soil matrix is assumed to be a sandy loam with a TOC value of 6%.
For elemental mercury, the contribution to exposure from dust is a minor pathway for the residential scenario.
For inorganic mercury, the SGV considers the oral, dermal and inhalation exposure routes, but the dermal contribution is considered to be zero.
Inorganic mercury has a strong tendency to form complexes with other anions, such as chloride (Cl-), hydroxide (OH-), and sulphide (S2-), and also humic matter, so is rarely found in soil solutions and is usually present bound or adsorbed to soil minerals, inorganic mineral surfaces, or organic matter.
For methylmercury, all three routes are considered, and although the SGV is based on a child of up to six years of age, it is also protective of an adult of childbearing age.
The matrix and soil conditions are also important, and can impact on the likely uptake and availability of the mercury; for example, elemental mercury is stable in the presence of sulphide species under strongly reducing conditions, but with increasing redox potential, it will precipitate as mercury sulphide. It is therefore necessary to measure other parameters (e.g. pH and soil organic matter) to determine soil conditions when assessing the risk of mercury, as well as measuring the mercury concentrations themselves.
Site specific consideration should be given to peaty or flooded soils, or soils amended with sewage sludge, as these more reducing conditions may trigger increased methlyation (conversion from inorganic mercury).
These widely differing guideline values should impact significantly on the design of site investigations and the implementation of remediation strategies, and the type of analysis requested from the laboratory.
Total mercury analysis
Currently, more than 95% of soil samples for metals, submitted by consultants or contractors to laboratories, are requested for total mercury analysis, usually included in a suite of metals by ICP (Inductively Coupled Plasma emission spectroscopy).
The soils are dried at a temperature between 30° C and 40° C, but this can be as high as 100° C, depending upon the laboratory. The samples are then crushed to a fine powder, approximately 250 microns particle size, weighed into a glass reaction tube and heated to more than 80° C with a 3:1 digestion mixture of concentrated hydrochloric and nitric acids (aqua regia) for one to two hours. After digestion, the samples are filtered, made up to volume, and then analysed by ICP.
If elemental or methylmercury are present in the sample, it is obvious that there will be some volatilisation leading to possibly significant losses of these forms of mercury. Most clients are not aware of this issue.
Mercury speciation analysis
Speciated mercury analysis is performed on the wet, as received soils, so no losses are likely due to the preparative procedures.
Inorganic Mercury (II) and Methyl Mercury
Extraction of samples follows the USEPA Method 3200 guidelines, for ‘Mercury species fractionation and quantification by microwave assisted extraction’.
Samples are homogenised and taken through a two step microwave extraction procedure to take both the extractable and non-extractable mercury (semi and non-mobile) compounds into solution. The use of microwave extraction in sealed vessels prevents the loss of any of the more volatile mercury components. Then the extracted species are separated by HPLC, oxidised to break down the organic complexes, followed by treatment with a reducing agent and analysis by atomic fluorescence spectroscopy. All stages are performed on a continuous ‘on-line’ setup directly linked to an atomic fluorescence detector. Quantification is performed by comparison to a specifically generated calibration curve.
Samples are again tested on an as-received base. Samples are purged with argon, the volatilised elemental mercury is collected on a silica-gold vapour trap, and the collected elemental mercury analysed by atomic fluorescence spectroscopy. Quantification is performed by comparison to a specifically generated calibration curve.
Risk assessment and disposal
Assessors should consider carefully whether there is any evidence for elemental mercury to have been introduced into the soil by anthropogenic activity. This should be completed at the preliminary stage of risk assessment when formulating the outline conceptual model, preferably using the DEFRA and EA Model Procedures for management of land contamination, 2004.
Currently, more than 95% of soil samples submitted for metals are requested for total mercury analysis only, and the resulting mercury figure has to be assumed as a worst case scenario to exist as elemental mercury, with the SGV of 1 mg/kg. So if anything above 1 mg/kg is obtained, all the soil will probably need to be removed or treated if the site is intended for residential use, and the cost of this can be very high.
If speciated mercury testing is undertaken, however, this can save potentially significant costs.
Example of savings
Assume the soil from a residential development site contained, on average, 10mg/kg of total mercury and this was the only contaminant of concern.
If landfill gate prices are taken as being £10 to £15 per tonne for non hazardous waste and up to £80 per tonne for hazardous waste, with an additional considerable tax liability, then it is financially essential to classify the waste correctly.
If this waste was analysed for total mercury only, then one would have to assume that all the mercury found could be present as elemental mercury, exceeding the 1mg/kg SGV trigger. If a speciated mercury analysis showed the elemental mercury to be less than 1mg/kg, however, the material may not have to be removed at all, or could at least be classified as non-hazardous, saving a considerable amount of time and money.
So, for a scenario of say, ten lorry loads of waste (at 20 tonnes per lorry) from a site with 10 mg/kg of total mercury, the cost would be £16,000 for disposal at hazardous landfill (plus the tax) compared with say £2,000 for disposal at a non hazardous landfill, in addition to additional transport costs, resulting in considerable savings.
Depending upon the volumes and site specific circumstances, the savings could potentially be much greater than this.
Mercury is a highly toxic element, but the toxicity varies depending upon the form of the mercury – elemental, inorganic or organic. Soil Guideline Values published by the Environment Agency are very different, depending upon the form of mercury and the end use of the land. Speciating the mercury can provide helpful information for the consultant in understanding the risks to human health of the site, and can potentially save significant disposal or remediation costs. Reference Soil Guideline Values for mercury in soil – Science Report SC050021 / Mercury SGV – Environment Agency
Published: 27th Nov 2012 in AWE International