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Measuring metal accessibility in the environment

Sewage can contain harmful metals

Upon leaching, toxic metals may be transported from sewage and waste into soils and sediments and then into the ground water. While some metals are needed for good health, others are harmful at high levels. Humans consume the metals in water and through food. Until recently, there was no common method to measure the levels of metals leached from soils and sediments. Laboratories obtained widely different, incomparable results. A European team came together to establish standardised methods to achieve accurate measurements.


Controlling the amounts of certain metals in the environment has become increasingly important. While it is vital to consume small amounts of many metals to maintain good health, some metals cause particular concern if exposure is above the recommended limit. These metals include cadmium, chromium, copper, nickel, lead and zinc.

Problems with the levels of these metals are often due to industrial waste being dumped in landfill sites and sewage sludge. Regulations establish maximum permissible metal concentration limits. However, measuring the levels of these metals in the environment provides many technical and analytical barriers which must be surmounted if the measurements are to be meaningful and reliable.

The main difficulty is that the bio-availability and mobility of the various metals strongly depends on the chemical forms of the respective metals, which all have different properties. Solubility will differ dramatically. This is important when considering how much of each metal is transferred from soil and sediments into the ground water and/or plants. It is much more important to be able to measure the levels of metals which become accessible to the food chain, or are easily mobilised, rather than determining total metal content of soils.

The availability of the metals to the ground water and to the food chain mainly depends on how strongly the metal is attached to the soil or sediment. If the metals are tightly bound on the soil particles, they are essentially unavailable to plants and animals. But if the metals are superficially bound to the particles, they find their way into the water supply more easily. The water is consumed by humans, or alternatively metals may be taken up by plants and thus the metals enter the food chain. This may present a serious health risk.

These factors makes the environmental analyst's job crucial as well as challenging. In order to assess the environmental risk, he or she must be able to mimic the way that water in nature leaches out metals from soils and sediments and how plants assimilate them. As soils and sediment in nature have very different particle sizes and composition, this task is not an easy one.

Determining a common method

Many laboratories, including environmental regulatory institutes and universities investigating the behaviour of metals, must perform measurements of "accessible" metal contents in the same way if they want to be able to meaningfully compare the results of their studies. Different tests will obviously yield different results. Standardised procedures to be used by all laboratories are therefore essential.

For this reason, a project was launched within what is now the Standards, Measurements and Testing programme of the European Commission. About 40 European laboratories worked together to harmonise metal extraction measurement procedures. They would first go through a number of interlaboratory studies to perfect common procedures. These studies brought geochemical and soil analysts together to perform and compare their metal extraction procedures.

Vast numbers of variables to control

It was important for the partners to consider following variables amongst others:
  • Sample preparation before the extraction
  • Chemical reagents to use to extract the metals from soils and sediments
  • Concentration of extraction solutions to be used
  • Sample shaking
  • Temperature
If laboratories used the protocol differently, they would get different results. So, the team performed each step of the standardised procedure and compared their results through interlaboratory studies. As they completed each step, they met to discuss their results and identify any errors. They returned to the laboratory bench to repeat the step, correcting the errors. This process was repeated until all the laboratories' results agreed.

This collaborative approach is the only reliable way to harmonise a procedure that can produce comparable results. A method that is to be applied internationally has to be tested by many laboratories before being adopted as a standardised method.

Tools to confirm the accuracy of measurements

At the end of this study work, the group agreed on which procedures provide the best results. It was time to develop Certified Reference Materials to be used with the measurement method. The materials are three sewage sludge amended soils which the group prepared and analysed for extractable metal content, using the standardised procedures (single and sequential extraction methods).

The materials had to be carefully selected to contain a certain amount of metal contamination. They were specially prepared and rigorously tested so that each of them was homogeneous and stable enough to remain at the same composition over several years. The laboratories measured the metal content using the standardised procedure and established the "certified" values.

Other similar projects are underway to improve methods for the leaching of heavy metals from other sludges, wastes, water pipes and construction materials. A newsletter about the work around Europe targets over 350 institutes to disseminate information.

Using standard methods and reference materials enables chemists to verify that they are performing accurate measurements. They can also be sure that they can compare their results to those obtained elsewhere or at different times. This in turn means that agriculture institutes and universities are able to evaluate the environmental risks and effects of the various metals. They then have a sound basis to suggest regulation limits to policy makers. The legislation may be modified as more research is completed, and it must always be enforced to prevent harmful and toxic chemicals from entering ground water and food supplies. For testing laboratories to do this effectively, they too benefit from the use of commonly agreed procedures and the use of soil and sediment reference materials. Consequently, European citizens are better protected from health risks.

For reference, the CRMs described are CRM 483, 484, 600 (sewage sludge amended soils) and CRM 601 (sediment).



Project Title:  Extractable Trace Metal Contents in Soil and Sediment

Programmes: Standards, Measurements and Testing
Contract Reference:  

Cordis DatabaseFor more information on this project,
go to the Cordis Database Record