TR 117 – Understanding the relationship between extraction technique and bioavailability

Science Newsflash

New ECETOC report addresses bioavailable residues in soils and sediments

In environmental risk assessment, chemical residues in soils and sediments are considered insignificant if they are bound to the solid matrix and hence are not available to plants and soil organisms. Improved understanding of the mechanisms of binding which can contribute to the rationale for defining appropriate extraction methods is important as well as the threshold where extractive techniques start to destroy the sample matrix. The aim of this ECETOC Report was to develop a standard framework for extraction methods which associates the extractable fractions with both a level of bioavailability and appropriate test organisms for the environmental compartment. It was considered vital to identify and define appropriate key terms, such as the residue categories (dissolved, rapidly desorbed, slowly desorbed, irreversibly desorbed and assimilated) as well as the terms bioavailable and bioaccessible, which are aligned with the various residue types within the framework model.

The framework provides a logical and reasoned sequence of extractions to enable the quantitation of the dissolved and rapidly 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 represent the best currently available approaches to determine each residue pool, is provided. Many of the extraction steps are conservative, providing enhanced extraction from the matrix than would be available to organisms in the environment.

The report is linked to ECETOC Technical Report no. 118 which provides expert guidance on how to incorporate non-extractible residues (NER) into environmental risk assessment schemes.

Summary

It is vitally important in understanding the relationship between extraction techniques and bioavailability that there is a common understanding and agreement on the relevant definitions of so-called "extractable" and "bound" residues. Previous attempts have been made to rigorously define these terms, though without relating them in a robust way to the physical and biological processes taking place in the biological compartment of interest. It has therefore been proposed that extraction methods are contextualised inrelation to the insitusorption and desorption processes and then, in turn, to bioavailability andbioaccessibility.

Revised definitions of the various sorption processes are provided including an indication of the binding energies for certain attractive forces between chemical moieties and the environmental matrix. "Bound" or better covalently bound residues which refer to a defined chemical entity were conceptual differentiated from the irreversibly sorbed residues as a whole. The latter are an unquantifiable mixture of defined chemical entities and natural products into which radiolabelled atoms have been assimilated. This Task Force has developed a model illustrating the key in situ processes. A number of different sampling approaches and extraction methods have been proposed for dissolved concentrations, as well as rapidly and slowly desorbed residues. Irreversibly bound residues are extremely difficult to extract without using more severe solvents or conditions which would inevitably alter the properties and structure of the matrix.

The current state of knowledge regarding the nature (and strength) of binding forces involved in the sorption process has been reviewed which assists in the rationale for defining appropriate extraction methods for the various residue pools. The interactions are grouped into three main categories, namely physical and chemical adsorption, covalent binding and sequestration/ entrapment. Some chemical moieties with different functional groups or side chains can bind to soil organic matter using mixed modes of action. Predicting the likelihood a chemical will form irreversibly sorbed residues based on structural alerts could be a useful tool in environmental risk assessment. However the subject is a very complex one and our current understanding is insufficient to permit such predictions at present.

Many of the techniques currently employed for extraction of a range of chemicals classes are based on maximising the recovery of a compound and its metabolites from the environmental matrix. By employing organic solvents at elevated temperatures and pressures or even harsher conditions still, it is possible to remove irreversibly sorbed residues, which would not be released under typical environmental conditions. These approaches destroy the organic matrix and result in an overestimation of the releasable fraction of irreversibly sorbed residues. A proposed extraction strategy has been based on extraction and quantitation of the dissolved and rapidly 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 represent the best currently available approaches to determine each residue pool, is provided. It is evident these may alter the matrix of interest but nevertheless represent the most robust methodology. When this extraction framework is applied using a considered and rational methodology, it will provide a conservative evaluation of bioaccessible residues. Using an intelligent extraction regime, it is possible to obtain robust laboratory data to assess the bioavailable fraction in an environmental matrix.

With the extraction framework established, further validation is recommended using a series of model compounds. Ideally the selected compounds would cover a range of predicted binding strengths and properties and be representative of various chemical segments. The evaluation would preferably be performed using ¹⁴C-labelled products dosed to soils and sediments according to current OECD guidelines. The objective would be to challenge the framework by progressing through the extraction regime using different extractions techniques and conditions, aimed at obtaining the respective dissolved, rapidlydesorbed, slowly desorbed and irreversibly sorbed fractions and validating these against appropriate bioassays.