Many articles have been written regarding pollution by heavy metals in this and other journals and we all have perception of heavy metals. But what is a heavy metal? Firstly let’s look at the dictionary definition of a metal: An element which is shiny, malleable and ductile, is a good conductor of heat and electricity the atoms of which readily loose electrons to form positive ions (cations).
So what makes a heavy metal?
This is a lot more difficult to define. In environmental reports and scientific papers heavy metals have a connotation of being toxic to the environment but no standard widely accepted definition exists.
In 1936 Bjerrum defined a heavy metal as any metallic element with a density greater than 7 grams per cubic centimetre, which includes Fe, Co, Cd, Cu, Au, Hg….. but not antimony arsenic chromium and selenium. Others have defined heavy metals by using what would appear to be arbitrary densities ranging from >3.5 up to >7.0.
Other attempts have been made by using the element’s atomic number many giving >20 or metals with an atomic number between 21 and 92 which takes in just about all the stable metals except Li, Be, Na, Mg, and Al.
If we were to define heavy metals by their implied toxicity this definition has also to define the organism to which it relates and has to take into account that some ‘heavy metals’ are also essential elements. Epstein in 1965 defines the essential elements for plant growth and included in his list some heavy metals namely Fe, Mn, Zn, Cu, Mo and Ni. Also in the list of beneficial elements for plant growth are included cobalt and selenium.
For example heavy metals are not toxic to all organisms. Some mining areas have been designated as Areas of Special Scientific Interest due to the adaption of organism to deal with the highly acidic and/or metal laden waters and spoil heaps of the mines.
Alpine penny-cress is an example of a plant which can happily grow in the lead and zinc rich soils of a spoil heap.
Its growth on tailings in the Snowdonia National Park of Wales has caused the Environment Agency additional concerns when dealing with the legacy of mine tailings as these sites have been designated Sites of Special Scientific Interest. Where the ideal treatment method of leaching from the tailing would be to cap or seal the mine waste to prevent metals leaching into local rivers and streams but this could damage the habitat of this rather special plant so novel solutions will have to be sought to satisfy both sides of the environmental requirements.
In other regions specialised Lichens are able to grow on what would normally be considered as highly polluted rock outcrops and specialised bacteria are able to thrive in the acidic metal laden adit waters from disused mines.
Although it is commonly taken that heavy metals are toxic some are required in trace quantities for the normal operation of some biochemistry pathways. An example is Vitamin B12 which is vital for the production of the red blood cells by the bone marrow but actually contains one atom of cobalt at the very centre of the molecule. Although no examples of cobalt deficiency in humans have been reported as causing anaemia it is known for cattle to suffer from Co deficiency due to low concentrations in the pastures on which they graze.
“heavy metals are still in use today and will be with us for some time to come and so will the legacy of their use”
More commonly, as iron is also considered to be a heavy metal, the most common form of anaemia is caused by iron deficiency or the inability of the body to take up iron in the diet. Copper deficiency causes another form of anaemia and also gives rise to neuro degeneration in humans and the WHO recommends an adult to have around 0.9mg of CU in their dietary intake. A deficiency of zinc causes hair loss and skin lesions it is also claimed that increasing zinc intake can lessen the symptoms of the common cold although there are differing views on the efficacy of zinc supplements.
Uses of the ‘Heavy Metals’
Without the discovery of the techniques for producing metals and alloys, such as copper then bronze (originally a naturally occurring alloy of copper and arsenic the arsenic later being replaced by tin) Heavy Metal Monitoring and then iron and the tools and artefacts made from these materials the human race could not have developed into the technologically driven society that we have today.
Modern uses of heavy metals include:
• Arsenic in the form of gallium arsenide is used in high speed semiconductor manufacture and in light emitting diodes and laser diodes. Arsenic was also used until quite recently as a wood preserver as chromed copper arsenate but was banned by the USA and European Union around 2004
• Copper for electrical wire and domestic plumbing (now being replaced by polymers)
• Nickel and chromium in the manufacture of stainless steels
• Mercury is used in fluorescent lamps and even in the modern compact fluorescent lamp known as the energy saving light bulb • Cobalt used in super alloys batteries, and colorants
• Lead still used as the main storage battery in road vehicles
• Chromium used for plating, manufacture of stainless steels and heat resistant materials
• Nickel has a use in NiCad batteries which are being phased out, main use is in the production of stainless steels
So like it or not the heavy metals are still in use today and will be with us for some time to come and so will the legacy of their use.
Environmental Impacts of Heavy Metals
Rivers which run through established or historically industrialised areas have a legacy of metals deposited in the silts in the bed of the river, industrial sites have land contaminated by them and fossil fuelled power stations will emit huge quantities of metals into the air in 2006 the largest power station in England, according to Environment Agency published records, released 46kg of cadmium, 269kg chromium, 2250 kg of lead, 234kg of nickel and 687kg of selenium into the atmosphere.
The Environment Agency of England and Wales has the unenviable task of mapping the point sources of discharges of polluted mine water from the 1300 or so mines in Wales some of which are thousands of years old.
“the largest power station in England released 46kg of cadmium, 269kg chromium, 2250 kg of lead, 234kg of nickel and 687kg of selenium into the atmosphere in one year”
Landfill is another source of metals entering the environment, although now rapidly changing, our throwaway society has discarded thousands of tons of zinc carbon and alkaline batteries into landfill sites.
We have devised ingenious methods of removal of metals from the environment, reed beds are very effective at this although what happens to the reeds when they reach the end of their useful life? Are they put as feedstock to a smelter or placed in a toxic landfill? Likewise the industrial processes for removing metals from atmospheric and water borne discharges are they recycled or again land filled?
Chemical Phosphate removal processes in waste water treatment has generally used iron salts (a heavy metal) to precipitate out the phosphate the sludge created in this process ends up in the sludge produced by the plant.
Monitoring for metals
Generally heavy metals are analysed by discrete sampling of waters either manually or by automatic samplers or as a composite over time. Each sampling method has drawbacks discrete samples only provide information regarding the sampled source at the exact time of sampling no data can be extrapolated from the result for concentrations before or after the time of sampling. A 24 hour composite sample provides an average of the analyte’s values over the sampling period but cannot provide any detail on transients or the duration of peaks in analyte concentration. Only online monitoring can truly provide a picture of the temporal variation of the target analyte. Once these samples are taken they need to be stabilised by the addition of acid which ensures the metals do not precipitate out or adhere to the internal walls of the container this is usually carried out by the addition of a small quantity of nitric acid and to be analytically correct a sample of deionised water should also be treated with the same quantity of acid and run as a blank to take into account the metal concentration of the acid itself especially if a non analytical grade is used.
“generally heavy metals are analysed by discrete sampling of waters either manually or by automatic samplers or as a composite over time”
On site methods
Unless some form of sample digestion is applied on site testing can only provide concentration of the labile metal. Generally onsite methods break down into categories:
These tests are simple to use and require the user to compare the colour developed on the section of a piece of paper/plastic with that on a colour chart. This presumes the user to have good colour vision and assumes all the tests are viewed under the same light source. In the view of the author these tests have limited value as the results obtained can only be considered to be indicative, they are open to subjective assessment and not compared against a verifiable standard. Also many do not have the sensitivity at the very low levels required for environmental assessments.
In these tests a colour is developed by the reaction between the target metal in the sample and usually an organic molecule resulting in the production of a chromophore the concentration of which is determined by its absorption of light at a specific wavelength/s the degree of attenuation of light being related to the concentration of the target metal. These test kits come in two forms
• 1- for use in a visual comparator having similar drawbacks as the test strip as they rely on the operators colour vision, consistency of light source and diligence of the operator
• 2- for use with a photometer, in this application the colorimetric methods shed their subjectivity. Modern systems have preprogrammed blanks and or standards and can, if used correctly, provide a very useful tool
However both colorimetric methods rely on the optics of the measuring instrument and the cell, cuvette or vial to be clean and free of optical irregularities for example scratches which are common on multi use cells where they have not been adequately protected during storage or transit.
Colorimetric systems in general do not operate well in turbid or coloured samples and can be prone to give false readings if the matrix contains interferences unknown to the user.
ICP and its derivative methods are generally considered to be the ‘gold standard’ for the determination of metals in water samples with Atomic Absorption also being a commonly used laboratory method. ICP-MS (Inductively Coupled Plasma- Mass Spectrometry) is a high precision tool for the laboratory which was first introduced in the early 1980s and has been taken on as the tool of choice for laboratory elemental analysis. The instrument uses argon gas, the atoms of which are excited by high energy radio frequency to produce a plasma (atoms where the electrons are no longer bound to the atom) this is known as the ICP torch. The sample is then introduced into this plasma as an aerosol which rips the molecules apart into the component atoms which in turn become ionised within the plasma these ions are then passed into the second section of analyser, the mass spectrometer which separates the ion by their mass/charge ratio allowing not only the identification of the metal but also the isotope of the metal.
Some drawbacks of ICP-MS
Most manufacturers recommend a sample total dissolved solids content of less than 0.2% as the solids will eventually clog the portion of the analyser separating the ICP torch and the mass spectrometer therefore sample with a high total dissolved solids content will require dilution. Mass overlap, this is where a compound formed within the plasma torch has a very similar mass to a target analyte examples are 75As is interfered with by 40Ar35Cl where the actual mass difference is only 0.00963. There are high resolution versions of ICP-MS instruments that are able to resolve these but generally they have several drawbacks 1 – they tend to cost a lot more than a standard ICP-MS, 2 – they are more complex and 3 – there is a trade off between the resolving power of the analyser and the sensitivity of the system.
Graphite Furnace Atomic absorption (GFAA)
In this measurement process the sample is vaporised within a graphite furnace (usually a graphite tube) heated to around 3000oC and hot enough to vaporise the sample breaking the chemical bonds of the compounds producing free ground state atoms. When UV light is passed through the vaporised sample the atoms absorb at specific wavelengths. The absorption bands of the ‘vapour’ are measured using a monochromator and often a photomultiplier the metal being identified by the absorption wavelength(s) and the concentration of the element calculated from the degree of absorption compared to a calibration curve made from standards
This measurement method also is not without interferences. There are chemical, spectral and ionisation interferences. The sample matrix generally has the largest effect which can be removed by digestion or sample dilution .spectral interferences occur when another atom in the sample also absorbs at the same wavelength and ionisation interferences occur when atoms become completely ionised within the furnace reducing the concentration of ground state atoms able to absorb light.
Anodic Stripping Voltammetry (ASV)
This analytical technique requires relatively simple technology and relies upon the oxidation potential of the target metal/s in aqueous solution.
The method involves two stages; firstly the metal ions in the solution are reduced to the elemental form onto a negatively charged working electrode, the time required for this stage being dependent upon the target concentration. The voltage on the working electrode is then ramped in the positive direction toward zero, at a fixed rate. Upon reaching its ionisation potential, the metal is oxidised from the surface of the working electrode and returns to the solution as the metal ions. The flow of electrons resulting from this oxidation process produces a current which is directly proportional to the concentration of the metal.
Samples containing organics which may chelate the heavy metals can be treated using a digestion system which releases the metals into solution enabling the determination of the total metal. It is also possible just to measure the labile ions in the sample if no digestion is applied.
The technique of ASV applies itself well to both laboratory, portable and online applications as it does not require expensive infrastructure, pure gasses or highly trained staff and modern voltammeters can now be battery operated enabling real time measurement during site surveys. This negates any problems with degradation of the sample during storage and transportation to a laboratory and also permits re-sampling of outliers to the data set.
“online analysers employing ASV provide real-time data on the concentration of target metals”
Online analysers employing ASV provide real-time data on the concentration of target metal/s enabling concentration profiles to be established and out of range concentrations of metals to be investigated.
The example given below shows the measurement of chromium in the outfall of sewerage treatment works having a source of chromium discharging into the sewer network.
The graph above shows the data downloaded from a Cogent Environmental on line heavy metal analyser measuring chromium at the outlet of a Municipal Waste Water Treatment Works in the UK and clearly shows a peak of nearly 65ppb of chromium being detected in the outlet waters of the plant. The true magnitude and duration of the incident is unlikely to have been detected by routine sampling of the works and was later discovered to have been caused by the industrial site cleaning down storage tanks before a summer shut down period.
Interferences to voltammetric analysis Matrix interference is the main obstacle to overcome as mentioned above, metals have an affinity to organic compounds which tend to chelate the ions and render them unavailable to the method. This is usually overcome by digesting the sample with a mineral acid and subjecting the solution to intense UV irradiation. Heavy Metal Monitoring However in some applications this ability of organics to chelate the metal ion is used to remove interferences by the addition of a metal specific chelant and then applying the technique to the chelated metal which requires the opposite polarity to operate and is called CSV (Cathodic Stripping Voltammetry).
Disparities of standards
There are some interesting problems facing water companies. Under the European Water Framework Directive, Environmental Quality Standards (EQS) are set for the quality of water in the natural environment. The EQS for copper being set at between 1 and 28 microgrammes per litre depending upon the hardness of the water being based on the toxicity of copper upon invertebrates. In potable water the main source of copper is household plumbing and fittings and in the UK the Drinking Water standard for copper is 2 milligrammes per litre, 71 times the highest EQS standard. So it is therefore possible that a municipal waste water treatment plant in the UK receiving 100% household waste water could exceed its consent limit on copper. This also means that you shouldn’t put drinking water into a river!
Cogent Environmental Ltd
184 Cambridge Science Park
CB4 0GA United Kingdom
Tel: +44(0)1223 395450
Email: [email protected]
Published: 10th Dec 2009 in AWE International