Lead is an environmental contaminant of concern both in the lithosphere and hydrosphere. It is a non-essential element that is poisonous to both humans and animals. Its health risk has been known for thousands of years, with lead poisoning documented in ancient civilisations.
Lead, particularly in soil, lends its analysis to a variety of spectroscopic techniques ranging from on-site surveying to isotope ratio analysis for potential source identification.
Sources of environmental lead
Lead has been commonly used for thousands of years. Its natural deposits are widespread and the metal itself is easy to extract, easy to work with and is highly malleable. It is ductile, easy to smelt and resistant to corrosion. Much of the former Roman Empire in Europe was plumbed using lead pipes (Latin name plumbum , symbol Pb), and its use continued through to the early 1900s.
The main natural sources of lead are from the minerals Galena (PbS), Anglesite (PbSO4) and Cerrussite (PbCO3). While these compounds have low solubility, reducing the mobility of lead in the environment, lead does have a strong affinity to sorb to manganese and iron oxides, and organic matter. This limited mobility means the soil tends to act as a significant sink for lead.
Urban surveys of soil have reported lead levels significantly higher when compared to the surrounding rural areas. While its rural use in angling shot and lead bullets should be noted, the higher urban levels may be due to:
• Industrial activities such as mining, milling, smelting and refining
• Atmospheric deposition in roadside dust from leaded gasoline and brake linings
• Household and electrical goods, e.g. solder in tin cans pre 1980s, ceramic glazes, crystal glass, newsprint, toys, and painted goods
• Coal dust and ash deposition
Soil background surveys carried out by the USGS and the BGS, for example, map lead concentrations in surface soils. These clearly indicate a link between surface soil lead concentrations, background geology and anthropogenic activity.
Concentrations of lead in the environment
Concentrations of lead in soil vary widely, as do the target and/or intervention limits set by national agencies. In the UK there is no formal soil guideline value (SGV) for lead, with previous values of 450mg/kg for residential use and 750mg/kg for industrial use withdrawn in 2009. The Dutch target and intervention limits remain at 85mg/kg and 530mg/kg respectively.
Within the UK, the national background survey showed that while the mean concentration was found to be 129.4mg/kg the range was from less than 1mg/kg to 35 930mg/kg with higher concentrations associated with natural baseline features and urban centres. With such a range it is difficult to determine whether a high value is natural background or otherwise, and often other lines of evidence need to be sought when risk assessing lead in the environment rather than just comparison with a recognised limit.
Theoretical modelling of lead movement within the environment considers pH, cation exchange capacity and phosphate concentration (phosphate is added to water to control plumbosolvency in lead pipes), however, overall lead has a low solubility and lower solubility forms are likely to persist in soil. It is therefore important to consider lead not simply as total lead, but also as bioaccessible and in turn – bioavailable – lead. The particle size distribution of soils contaminated with lead will affect both its solubility and bioaccessibility, particularly where the lead is soil bound, and a kind of enrichment can occur where lead concentrates predominately in the finer particles due to a higher surface area to volume ratio.
Typical analysis of lead is simply total lead analysis of a homogenised soil, but there are other test methods that may prove useful and provide additional scientific information.
On site analysis with XRF
On site, lead concentrations can be determined using portable X-ray fluorescence spectrometers (XRF). These are small and easy to use, the baby brother of large, laboratory scale instruments used routinely for the determination of metals in solid materials; in fact the BGS soil background survey lead results were determined using laboratory XRF. Critically in this case the test sample was a pre-dried and homogenised soil sample.
When using the portable instrumentation, it is directed at the test sample area, and by bombarding the sample with x-rays, a subsequent fluorescence is detected which is characteristic of electron shell changes for the elements present. Heavier elements (such as lead) tend to give better responses than the lighter smaller elements making this technique an ideal candidate for on-site lead testing. Hot spot areas can be easily identified without the need for laboratory testing, and often sources of contamination can be tracked and identified.
However, the homogeneity of the sample tested is critical, as the technique is non-invasive and is only representative of the material targeted. Lead is known to fractionate and even certified homogenised samples are known to have bimodal distributions of lead, compared to the normal distribution of repeat analysis expected. This is illustrated in Figures 1 and 2.
While portable XRF presents a useful tool, laboratory testing of a prepared homogenised sample will inevitably give results that are more representative of the sample tested.
Laboratory elemental analysis
Laboratory testing for total lead in soil firstly involves sample preparation to obtain a homogenised sample, typically this will be drying the sample and then some form of homogenisation process (grinding, crushing as appropriate). The test soil can then either be tested by non-invasive XRF, or more commonly, chemically extracted with the resulting solution analysed by Inductively Coupled Plasma – Optical Emission Spectroscopy (ICP-OES). This technique involves the solution being aspirated through an argon plasma (around 8000 K) that causes excitation of the elements present, which then subsequently relax back to their ground state emitting light of a characteristic wavelength. The spectrometer detects this, and like the XRF, the intensity of the signal relates to the concentration of the element present. Lead is ionised efficiently in the plasma (around 97 %) and therefore this method is very efficient.
This technique lends itself to total analysis (either Hydrofluoric acid or Aqua regia extraction), as well as the determination of lead in solutions used as part of physical fractionation, geochemical fractionation or bioaccessibility studies.
A useful tool in assessing lead contamination in soil is to determine whether it is present in the finer or coarser factions of the soil. Lead well bound in a stable coarse fraction may present a lower risk to animal and human health, compared to lead that is the finer fraction. Finer fractions of soil are more easily transported to the hydrosphere and atmosphere, presenting a greater inhalation and ingestion risk. A targeted approach looking at physical fractionation is useful where lead has been used in its metallic form, such as in ammunition or as angling weights. Both of these may be subject to some form of weathering or physical breakdown, but often hot spots can be identified and often the coarse fraction can be removed onsite based on size exclusion.
The purpose of geochemical fractionation is to better understand how metals are bound within the lithosphere and to give an idea as to their mobility. The soil is treated with different solvents in a sequential manner to extract a metal (eg. Lead) bound within a particular soil fraction. A variety of methods exist, but for lead typically the exchangeable, carbonate bound, Fe-Mn oxide bound and residual fractions are determined.
A soil with lead mostly in the exchangeable fraction presents the higher environmental risk – with lead readily transported through the soil. The second most easily transported is the carbonate bound lead; in this case reduction in soil pH may release the lead into the environment. Fe-Mn oxide bound lead is less easily mobilised and the residual fraction is the lead remaining in the soil after the other extractions have been carried out. This residual lead can be considered less mobile and present a lower environmental risk.
Bioaccessibility and bioavailability
Bioavailability can alter health risk estimates; consequently, its importance in risk assessment cannot be underestimated. Bioavailability is defined as the degree to which a substance is absorbed and becomes available to the target tissue, without first being metabolised. Bioavailability is an in vivo measurement. The bioavailable concentration of a metal does not correlate with the total concentration – it is a function of a variety of factors including species, geochemical fraction and sample matrix.
Bioavailability is not to be confused with bioaccessibility – the degree to which a chemical is released from soil into solution and thereby becomes available for absorption when that soil is ingested and undergoes digestion. Bioaccessibility of metals in soil can be measured in vitro (in the laboratory) using methods which have been proven to yield data that correlates to bioavailability.
Bioaccessibility testing is carried out in the laboratory to simulate the effect of the human digestive system on a contaminated soil and determine how much of a toxic contaminant is accessible to the body.
The sample preparation method for bioaccessibility testing is critical. Samples are dried and passed through a 250 µm sieve; the sample is not crushed. All bioaccessibility testing methods are based on the physiology of a small child and it is critical that the preparation method replicates the manner in which they may ingest the soil, and therefore the finest fraction which is likely to adhere to their hands is used for testing. A crushed sample may yield false data for a variety of reasons, including the crushing action yielding surfaces for interaction which may not normally be accessible, the dilution of a high metal concentration in the finer fraction with crushed stones, or even vice versa. It is therefore also important that the ‘total’ measurement is undertaken on the same portion of the sample as the bioaccessibility testing.
The general rule of thumb is that the bioaccessible fraction is greater than the bioavailable fraction, but less than the acid extractable and total concentrations of a metal. The bioaccessibility also relates to the fraction of the soil to which the metal is bound.
There are two recognised methods for determining lead bioaccessibility – the Simplified Bioaccessibility Extraction Test (SBET) and the Bioaccessiblity Research Group Europe (BARGE) Unified Bioaccessibility Method. The SBET method is designed for waste materials and is a simple low pH hydrochloric acid and glycine extraction, whereas the BARGE method is more complex and more physiologically based. In both methods, metals are assessed in the fasted state and each produces aqueous extract solutions that can be analysed for lead by ICP-OES.
There are clear recommended guidelines on minimum reporting requirements in order to reassure the end user of the data of the quality of information. With these recommendations adhered to, along with laboratories using the standard methods (rather than in house variations), bioaccessibility testing could become more widely accepted and recognised as a viable laboratory method for enhancing site investigations and risk assessments.
Looking at lead analysis in terms of its isotopic abundance is perhaps also useful for environmental risk assessments, and may be used for determining likely sources of lead. Lead has four main stable natural isotopes: 204 (approx. 1.4%), 206 (approx. 24.1%), 207 (approx. 22.1%) and 208 (approx. 54.2%) with trace levels of 210. Synthetic lead with a mass of 205 may also be found. Three of these are products of natural radioactive decay of uranium-238, uranium-235 and thorium-232 (lead-206, 207, 208 respectively). For this reason the natural isotopic abundance of lead isotopes varies throughout the world.
In terms of the analysis a sample is extracted in acid similarly to ICP-OES analysis, but the final determination is performed by Inductively Coupled Plasma – Mass Spectroscopy (ICP-MS). This is similar in principle to ICP-OES, except that analytes are determined by their mass to charge ratio. Lead can be determined as a total of all isotopes (in order to not discriminate for natural variation in isotopes), or it may be determined for each isotope – isotope ratio analysis.
In principle, a comparison of lead isotope ratios 204/206, 206/207 and 207/208 can produce a ratio indicative of a particular source of lead. An example of this may be tetra-ethyl lead (used as an anti-knock additive in gasoline); where the majority of lead used was obtained from mines in Australia. This has a particular ‘lead fingerprint’ that may be linked to roadside dust lead concentrations and isotopes.
This tool is often used in archaeology, and occasionally in human health studies, but is an emerging technique beginning to be used in environmental analysis.
Lead is an element that lends itself to a variety of different spectroscopy methods, each with a different merit in the field of environmental analysis. For soils, both physical and chemical fractionation and specific extraction can yield specific information about the soil matrix the lead is present in, and tools even exist now to potentially identify the source chemically, rather than through extensive sampling and analysis protocols.
Each technique may be applicable to one site, or they may be used separately and discretely to provide information of benefit and use to the environmental consultant and researcher alike. Isotopic analysis may be applied to a whole range of matrices – waters, plant material and animal matter to name but a few. While this method is in its infancy environmentally, it is an established technique and it can be expected that numerous studies are yet to emerge detailing lead isotope analysis in the environmental sector.
Published: 11th May 2016 in AWE International