Heavy metals have in the past been amenable to traditional 'wet chemistry' analytical techniques. Unfortunately they tend to suffer from a relatively poor detection limit, and as we know, 'the dose makes the poison'. Paul Board, Robertson Laboratories, on modern metal analysis.
Britain's industrial legacy of metalliferous pollution goes back several thousand years. For instance, the Romans sought tin in Cornwall and lead in the Somerset Mendips. More modern applications took off with the industrial revolution: the use of chromium in tanning was introduced as recently as 1858, and leaded petrol was introduced in the 1920s. Up until relatively recently, metals now known as acutely poisonous were used as medication.
We are now far more aware of the effects of so-called 'heavy metals' on both the environment and human health. Strictly speaking, the term 'heavy metals' refers to metals with a high atomic mass or density, such as mercury, lead, cadmium, chromium and plutonium. However, the term is also used rather more loosely to include lighter elements such as arsenic, beryllium and selenium. All have varying degrees of toxicity to various target organisms, and some can be lethal in minute quantities. Lead has probably been the subject of more toxicological investigations than any other substance, let alone just metals.
But as we know, 'the dose makes the poison', and that's where analytical chemistry comes in. Metals have in the past been amenable to traditional 'wet chemistry' analytical techniques, which certainly had, and in certain cases still have their uses. Unfortunately, they tend to suffer from relatively poor sensitivity and can be less than efficient when compared with modern instrumental techniques.
Perhaps the most common form of metals analysis for environmental testing laboratories is the analysis of soils for so-called 'total' metals, using aqua regia digestion followed by instrumental determination of the metals digested (typical instruments being Atomic Absorption Spectrometry (AAS) or Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES)). Aqua regia (so-called because it dissolves the 'king of metals', gold) comprises a mixture of concentrated nitric and hydrochloric acids. Whereas it does not extract all the metals in a soil (a particularly intractable metal being tin, which is perhaps best analysed for by XRF, X-ray Fluorescence), it does pull out most.
The industrialised world (and those downwind of it) does suffer from acid rain - fortunately for us, it isn't that acidic. However, in many cases, such an analysis may overestimate the real risk, dependent upon target organisms. A metal's availability will depend upon a number of factors, including pH, soil organic matter, redox conditions, cation exchange capacity and whatever particular form the metal happens to take. Barium is very toxic in certain forms, for instance, but is relatively innocuous as a sulphate (just as well, given that it is fed to patients in hospitals as a 'barium meal' for internal investigations).
The following examples of cadmium contamination offer a classic illustration of the effects of soil chemistries on the bioavailability of a metal. A localised concentration of cadmium found in domestic garden soils in the village of Shipham in Somerset (up to 470mg/kg, found in studies in the late 1970s) was due to previous zinc mining activities. Yet, due to a relatively high soil pH, there were no obvious adverse health effects found in the group of 500 volunteers (about half of the village population). Compare this with the serious repercussions of cadmium contamination in a paddy rice-growing district in Japan, up river from a zinc and lead mine and smelter. Concen-trations of cadmium in the soils were much lower than those in Shipham (typically 5mg/kg), yet the effects were all too tangible. The contamination resulted in an outbreak of Itai Itai (literally translated as "Ouch Ouch") disease, a severe skeletal disorder. This was due to a completely different cadmium chemistry precipitated by alternating waterlogging and drying out of the paddy soils and a heavy dietary reliance on the rice grown.
Various analytical procedures are available to estimate a metal's bioavailability:
- General soil parameters (such as pH, soil organic matter, and cation exchange capacity).
- Phytotoxicity (toxicity to plants): this can be estimated by measuring the available metal content, using EDTA (Ethylene Diamine Tetra-acetate) as a chelating agent to simulate uptake in plants.
- Impact on groundwater: the Environment Agency's Leach Test protocol estimates impact on groundwater resources by agitating a 1:10 soil/water mix for 24 hours and then analysing the filtered sample.
However, perhaps the pathway of greatest interest for most metals uptake (apart from certain exceptions such as beryllium, where inhalation is a particular risk, or radioactive metals) is the route of human gastrointestinal ingestion. This is often attributable to the pica habit of young children who put things into their mouths that they shouldn't. Robertson Laboratories has recently been providing in vitro studies of uptake of metals via the human gut to provide a more realistic estimate of risk. The Physiologically Based Extraction Test (PBET) was designed around paediatric gastrointestinal tract parameters for a child 2-3 years old, believed to be at the greatest risk from accidental soil ingestion.
The PBET test employs a series of three mixing chambers all at body temperature (37°C), with mixing achieved by the passage of an inert gas supply through the chambers. Stomach contents are mimicked by creating an artificial gastric solution with pH-adjusted de-ionised water, and various acids and enzymes.
Such a test can only be seen as an approximation of what goes on in vivo. Every target individual is unique with varying physiologies and nutritional status, and this test admittedly does not mimic the entire physiological process controlling human uptake of metals (for instance, it has not been designed to simulate metal transport across the intestinal epithelium). However, it is yet another test in the analyst's armoury, and has already proved invaluable in giving more meaningful data to risk assessors.