Improving bioaccessibility testing for contaminated soils

Soils are a key part of everyday life. We are constantly exposed to small amounts of soil or soil-derived dust from the land around us, including the built environment, agricultural land and recreational areas such as playgrounds and playing fields. Soils that have been affected by pollution from sources such as vehicle emissions, industry or other human activities may contain harmful substances. Understanding how these contaminants can affect people is an important step in assessing and managing potential health risks.

When assessing contaminated soils for potential human health risks, laboratories often determine the total concentration of potentially toxic elements such as lead, arsenic and cadmium. However, the total concentration alone does not indicate how much of these contaminants can dissolve within the human digestive system and therefore become available for absorption.

This is when bioaccessibility testing becomes important. Bioaccessibility measures the fraction of a contaminant that can dissolve in the gastrointestinal tract, providing a more realistic estimate of human exposure than considering the total concentration of elements in the soil alone. As a result, it has become a key tool in contaminated land risk assessment.

One of the most widely used laboratory methods is the unified BARGE (Bioaccessibility Research Group of Europe) method or UBM (Figure 1), which simulates digestion using synthetic saliva and gastric and intestinal fluids. The concentration of metals released into these fluids is measured to estimate the fraction that could become available for absorption following accidental soil ingestion.

Traditionally, UBM extractions use a heated water bath to maintain samples at body temperature. Although effective, water baths require regular filling and cleaning, occupy valuable laboratory space and can limit the number of samples we can test at one time. This project investigated whether a heated rotor, which uses a heated air chamber rather than a water bath, could provide a more practical alternative.

Tests carried out with the heated rotor produced comparable analytical precision for the majority of the nine elements studied. Statistical analysis showed no significant difference in variance for most elements, although chromium and antimony demonstrated improved precision using the heated rotor, while cobalt and lead (Figure 2) showed increased variability. Mean bioaccessibility values agreed well for several elements, including arsenic (Figure 3), cadmium and nickel, with differences generally below 20 per cent. In contrast, lead and zinc showed the largest differences, with gastrointestinal bioaccessibility differing by 52 per cent and 194 per cent, respectively. These differences are consistent with the known difficulties of lead and zinc, as their solubility is strongly influenced by pH, precipitation reactions and complexation with components of the synthetic digestive fluids.

A further investigation examined whether changing the solid-to-liquid ratio could improve the alignment between the two methods. Increasing soil mass moderately improved the gastric-phase agreement for lead (Figure 4), reducing the difference from 43 per cent to approximately 20 per cent, depending on the ratio used. However, substantial differences remained during the gastrointestinal phase, demonstrating that intestinal chemistry and background zinc contamination had a greater influence on the results than sample loading alone.

The project also characterised ten soil reference materials representing a range of geological settings. These showed distinct bioaccessibility behaviours linked to differences in mineralogy, contamination sources and soil composition. The reference materials provide valuable quality-assurance tools when selected to match the soil type under investigation (Table 1).

Overall, this study demonstrated that using a heated rotor system offers a practical alternative to the traditional water bath, providing improved laboratory efficiency through simplified operation and increased sample throughput while maintaining comparable analytical performance for most elements. The findings also highlight the importance of developing matrix-matched reference materials for bioaccessibility testing and continuing to optimise these methods to improve the reliability of environmental risk assessments.

About the author

David King (BGS)