Following the ECETOC workshop on “Significance of bound residues in environmental risk assessment” in 2009, two Task Forces were set up to (1) understand the relationship between extraction technique and bioavailability and (2) develop interim guidance for the inclusion of non-extractable residues in environmental risk assessment. The goal of the first Task Force was to address knowledge gaps in the relationship between bioavailability and extraction technique with regards to bound and non-extractable residues with the ultimate goal being the development of a standard framework for intelligent extraction strategies. A number of residue ‘categories’ were defined (dissolved, readily desorbed, slowly desorbed, irreversibly sorbed and incorporated) as well as the terms bioavailable and bioaccessible which were aligned with each type of residue within the framework model. It was decided to differentiate residues termed ‘reversibly bound’ into those ‘readily desorbed’ and ‘slowly desorbed’. This differentiation was based on the solvent strength necessary to extract each type of residue and led to the development of the extraction regime to tie in with the framework model.
A proposed extraction strategy has been based on extraction and quantitation of the dissolved and readily desorbed fraction (for the bioavailable residue) and in addition the slowly desorbed fraction (for the bioaccessible fraction). A selection of appropriate extraction solvents and parameters, which will not result in destruction of the organic matrix, is proposed. When this extraction framework is applied using a considered and rational methodology, it will provide a conservative evaluation of bioaccessible residues.
An important consideration in predicting the behaviour of chemicals entering the soil is understanding their interaction with the soil matrix. To be able to better predict the chemical dynamics once a chemical enters the soil, it is necessary to understand the processes which govern these interactions. Generally, chemicals which were most strongly associated with the soil (and least bioaccessible) were either covalently bound to the soil, or physically sequestered and trapped in soil pores. Other interactions which were shown to lead to NER or slowly desorbed residues included ionic and ligand exchange. Chemicals were also shown to interact with the soil matrix via Van der Waals forces, hydrophobic partitioning, charge transfer complexes and hydrogen bonds, these interactions are generally thought of as weaker and most likely to lead to desorbable residues. The various interactions studied (and their bond strength ranges) were aligned with the extraction regime and framework model.
One of the major issues of particular concern with regards to environmental risk assessment is the future re-release of NER. It was found that physical processes such as freeze-thaw and wet-dry cycling can cause the release of sequestered residues via the breakup of the soil matrix and soil organic matter (SOM). Additionally, chemical and biological processes such as microorganism metabolism and pH changes have been found to cause the release of NER. The current literature suggests that the amounts of NER released do not pose an environmental risk, however, it was identified that further research is necessary in this area, especially with regards to release caused by physical processes, on which very few studies exist.
In conclusion, the issue of non-extractable residue formation and release is a very complex one. The interaction of chemicals released to the environment with the soil is reliant on a number of factors, not least of all the nature of the soil. Soil organic matter is a key component of soil, this complex soil constituent and the potential interactions it may have with chemicals is not very well understood and needs further research. However, this Task Force has developed a framework model and extraction scheme (ECETOC, 2013a). It is expected that work in this area of research will greatly increase over the coming years as environmental risk assessment becomes an increasingly important issue.