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Metals in the environment are in a delicate balance, many are essential for plant and animal life and thus support a healthy varied ecosystem; while others are toxic even at very low concentrations. Where contamination arises, usually from industrial activity, this situation may be altered and it is crucial to understand the concentrations of the contaminants present, and the proportional concentrations of their more or less toxic species. Fortunately, different forms of spectroscopy are on hand.
There are some metals where they are essential for life in one form, but highly toxic in another – a case in point is Chromium. In the environment, Chromium exists predominantly in two valence states, 3+ (trivalent, Cr(III)) and 6+ (hexavalent, Cr(VI)). Trivalent chromium is an essential trace nutrient and has roles in the action of insulin and the storage of carbohydrate, fat and protein in the body; indeed a deficiency has been linked to alterations to an individual’s glucose tolerance factor and may therefore be a potential contributor to diabetes. Hexavalent chromium is highly toxic and mutagenic, and although not yet a recognised carcinogen in an aqueous form, it has been shown to cause allergic contact dermatitis. In a natural balance, it can be anticipated that the essential and toxic elements in the environment are in a delicate equilibrium forming a normal distribution. For essential elements this can be illustrated with a dose response curve.
Where contamination arises, however, usually from industrial activity, this delicate balance is altered and it is crucial to understand the concentrations of the contaminants present, and the proportional concentrations of their more or less toxic species.
Speciation
Continuing to use chromium as an example, speciation is defined as “the distribution of an element among defined chemical species in a system”, this means of the total chromium present, how much is present as Cr(III) and how much is present as Cr(VI). Other metals with environmentally significant different species are Arsenic – with arsenite (As(III)) and arsenate (V) compounds having quite different toxicities; and mercury – elemental mercury, inorganic mercury compounds and organo mercury compounds (e.g. methyl mercury, ethyl mercury).
It was only through the development of advanced spectroscopic techniques that the different species present were first identified, and now routine analyses are heavily dependent on these techniques.
Chromium
As mentioned previously – environmentally significant species of chromium are trivalent and hexavalent. Hexavalent chromium is an environmental contaminant of significant fame, with the feature film Erin Brockovich (2000) telling the story of an incident in the USA where hexavalent chromium was used in a cooling tower system to fight corrosion and the corresponding waste water discharged to unlined ponds, resulting in percolation of Cr(VI) into the ground water. Chromium (VI) produces brightly coloured pigments and has been used for hundreds of years in a wide range of paints, dyes and coatings. It has also been used in wood preservatives and anti-corrosion coatings. The way in which this species can reach the environment is therefore quite wide and environmental contamination is a significant risk.
It is fairly commonplace to test for total chromium in environmental samples (waters and soils), and in many cases a test for hexavalent chromium is also carried out. The most routine method utilises 1,5-diphenylcarbazide which reacts with Cr(VI) present in a solution (water or soil extract) to form a highly coloured solution. The intensity of the colour is measured by visible spectroscopy at a wavelength of 540 nm. Many laboratories are equipped with a continuous flow analyser which carries out this analysis as an automated process, and allows the detection and quantitation of hexavalent chromium alongside other routine analytes such as cyanide compounds.
Other options are also available which typically give a greater sensitivity in clean matrices such as ground and drinking waters, typically these couple ion chromatography with a highly sensitive detector. The ion chromatography element effectively separates the chromium species meaning that a more sensitive and less selective detector can be used. Such detectors include conductivity, and inductively coupled plasma – mass spectrometers (ICP-MS) as well as a more sensitive 1,5-diphenylcarbazide alternative.
Arsenic
Arsenic has become one of the prime research elements in water (as defined by the WHO) due to the number of countries who have arsenic levels above the 10 ug/l WHO guideline value for drinking water1. Bangladesh is a well published study area where naturally high levels of arsenic in ground and surface water are having major effects on human health2. Other parts of the world with significant geothermal or volcanic activity also have a similar problem, such as in Argentina and even Mexico and the western USA.
The different species of arsenic have different toxic effects. While the inorganic species arsenite (As(III)) and arsenate (As(V)) in ground water have caused tremendous epidemic poisoning across the globe, organic arsenic species such as monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) are common metabolites found in the human body and are considered to be less toxic. Of the inorganic species, arsenate interferes biologically by replacing phosphate and interfering with numerous biochemical pathways; whereas arsenite may react with critical thiol proteins in the body inhibiting their activity.
In soils, the following forms are often found: As (III), As(V), MMA, DMA, arsenobetaine and arsenocholine. Of these forms, arsenate is the major component. It is also known that microbial activity in soil may generate organo arsenic species such as MMA and DMA.
In the aquatic environment the inorganic species prevail and their distribution is largely dependent upon the pH and redox state of the environment. It is therefore critical to effectively extract the species in their natural state from the sample and for this reason sampling using solid phase extraction is the method of choice. This involves passing the sample water through a cartridge column to which the arsenic present binds, the column is then flushed with two different solutions eluting the species into two different sub samples. These sub samples can then be analysed for total arsenic – typically by a technique such as ICP-MS.
The speciation of arsenic in soils is more complex and is a subject covered by a vast amount of research. In simple terms, firstly the species need to be effectively extracted from soil and for this a complex mixture of inorganic buffers are typically used. The resulting solution containing the various species of arsenic is then typically passed through a liquid chromatography column which is coupled to an ICP-MS.
This process is somewhat more complex than the more straightforward analysis of aqueous samples, and in many cases the use of a soil leachate as an indicator of the soil arsenic species may be a useful approach to consider as arsenic speciation in soil has very limited commercial availability worldwide.
However, all is not lost for those who may wish to evaluate arsenic in soils further as there are sequential extraction methods which are relatively widely used and are commercially available. These use different solvents to assess where within a soil matrix the arsenic is bound/released from and as a result infer its availability rather than its speciation. The most available would be arsenic leached into porewater, and the least available would be highly mineral bound.
These methods do not require any form of chromatographic separation to be used, the different chemical extractions themselves produce different fractions which are each analysed for total arsenic. Depending on the method used, this can give a range of information on the arsenic species present and how labile they are.
Mercury
There are many potential environmental sources of mercury including mining, explosive manufacture, dye manufacture, dentistry and even herbicides. These activities can release mercury in to the environment in a variety of forms: elemental mercury, inorganic mercury (Hg(I), Hg(II)) and organo-mercury species (e.g. methyl mercury, ethyl mercury etc.).
Inorganic mercury salts are soluble in water and this allows them to be readily absorbed into the body through the gastrointestinal tract. Once absorbed they can cause severe kidney damage, however they cannot cross the blood-brain barrier easily and as a result exposure to inorganic mercury tends to cause little neurological damage unless there is prolonged or heavy exposure.
Conversely, organo (methyl) mercury is a form of toxic mercury which bioaccumulates through the food chain and the primary exposure route for humans is through the consumption of fish. Organic methyl mercury can cross the blood-brain barrier and is a neurotoxin. Mercury poisoning of this type was seen in Japan, in the Minamata bay area – and hence this kind of mercury poisoning is widely known as Minamata disease.
Elemental mercury (Hg0, liquid mercury) is a different form of mercury with a different pathway into the human body – typically through vapour inhalation into the lungs.
In the hydrosphere, levels of interest have been defined according to the total concentration of mercury, rather than the species or form present. For example the Environmental Quality Standard for mercury, (as set out in the latest update of the daughter directive) is 0.05 ug/l3. While this limit is low, it is analytically achievable, and often the instrumentation of choice is cold vapour – atomic fluorescence spectroscopy (CV-AFS), although ICP-MS may also be used. When the CV-AFS unit is configured to analyse for mercury the instrumentation is both highly selective and highly sensitive.
In the lithosphere soil and indeed sediments may be considered as a sink for mercury and in this case the Environment Agency have differentiated the forms when deriving and arriving at their soil guideline values.
Fortunately this Environment Agency publication also recognised that most laboratory analytical methods are not able to distinguish organo-mercury from the total mercury and therefore a standard practice has evolved to use the total mercury analysis as a screening technique such that the speciation into the forms listed in the table are only required where the mercury levels are sufficiently high to cause concern in relation to the proposed site use (residential, allotment or commercial).
When speciation is required then there are a few options available, but all rely on the same basic principles. Firstly the mercury species in the soil sample need to be extracted using a suitable solvent, then the mercury species separated, and then finally detected.
The solvent mixes used may vary depending on the separation technique and detector employed, however they will typically comprise some acid (either nitric or hydrochloric) and a water miscible organic solvent such as ethanol. Once the mercury in the soil sample has been extracted in to a sample aliquot, this is then analysed by one of a variety of complex hyphenated analytical techniques.
Ion Chromatography (IC) or High Performance Liquid Chromatography (HPLC) may be coupled to gas chromatography with a mass spectrometer detector (GC-MS), or coupled to an ICP-MS. The chromatographic element of the instrumentation separates the mercury into the component species and then the mass spectrometer element (MS) detects and quantifies the mercury present. These arrangements are quite complex and rely upon the matching of many parameters across several instrument components which are not typically configured to work together. For this reason, the use of a simpler HPLC-AFS coupled technique has emerged as the more elegant and slightly more robust method.
HPLC-AFS utilises the same HPLC chromatographic separation technique, but then couples this to the highly sensitive and selective CV-AFS detector. The result is a well resolved separation of the different mercury compounds present with a response in signal from the CV-AFS system which entirely due to the mercury present.
While there are commercially available offerings for mercury speciation, this element is another example of where fractionation analysis may be of assistance, providing information as to not only the likely species types present, but also their relative availability and leachability from a soil matrix.
One of the more widely used is a method developed by Bloom et. al., (2003)5 which uses five different solutions to extract fractions containing different mercury compounds. This is summarised in the table below.
The chemistry used is quite straightforward and the final analysis of the extractions may be carried out by a range of spectroscopic techniques such as ICP-MS or CV-AFS. In most cases this is a much more cost effective solution and provides an indication of the mercury compounds which may be present, in particular the organo-complexed fraction (F3) picks up the methyl mercury and the strongly complexed fraction (F4) picks up elemental mercury (Hg0).
Conclusion
In the case of both arsenic and mercury the use of fractionation analysis can assist in making an informed decision as to availability of these metals from the soil and in turn the likely risk. Some information is available as to the likely species present also. Indeed, in many cases it may be impossible to determine the speciation of a metal, and in these cases it is useful to do fractionation instead – which is the classification of an analyte or group of anlaytes according to their chemical properties.
While the three metals considered in this article are a good case study of how the species of a metal has a marked effect on its toxicity, these are only a few examples. There are many other metals that are of interest environmentally and these may be present in a variety of species. Each of these species may present different toxicity risks and different transport pathways, although in most cases these are only just starting to be subject to study. Of these, vanadium is possibly the most interesting due to its wide range of oxidation states and quite complex chemistry. Some work has been done modelling partition coefficients in order to assess the likely movement of vanadium into the aquatic environment from contaminated soils, but it is possible that either speciation studies and/or fractionation work may be of assistance in understanding the associated risk.
All metals in the environment are in a delicate balance and the essential elements for plant and animal life exist in a range of forms and species, some of which may be toxic. In order to better understand the status of these in the lithosphere or hydrosphere a range of speciation methods using complementary spectroscopic techniques has been established, with chromium being perhaps the most widely used and understood; however analytical options for the speciation of other metals such as arsenic and mercury are becoming more well-known and it is likely to only be a matter of time before these analyses become far more routine.
