Two complementary sides of bioavailability: Accessibility and chemical activity of organic contaminants in sediments and soils

Abstract

Research during the last decade has led to several competing concepts of bioavailability and to many more methods to measure bioavailability. One reason for disagreement is the confusion of two fundamentally different parameters, accessible quantity and chemical activity. The accessible quantity describes a mass of contaminants, which can become available to, for example, biodegradation and biouptake. It can be determined with mild extraction schemes or depletive sampling techniques. The chemical activity, on the other hand, quantifies the potential for spontaneous physicochemical processes, such as diffusion, sorption, and partitioning. For instance, the chemical activity of a sediment contaminant determines its equilibrium partitioning concentration in sediment-dwelling organisms, and differences in chemical activity determine the direction and extent of diffusion between environmental compartments. Chemical activity can be measured with equilibrium sampling devices and, theoretically, is closely linked to fugacity and freely dissolved concentration. The distinction between accessibility and chemical activity is outlined, and the benefits and limitation of both endpoints are provided. Finally, examples of how to measure and apply them are presented.

Uptake and toxicity of polycyclic aromatic hydrocarbons in terrestrial springtails—studying bioconcentration kinetics and linking toxicity to chemical activity

“…It is well established that nominal or total measured concentrations of HOCs in soil are not sufficient to define exposure due to the numerous interactions between the HOCs and the heterogeneous soil matrix [1,2]. The freely dissolved concentration (C free ) takes sorption into account and is therefore frequently perceived as the effective concentration for bioconcentration and toxicity [3][4][5], and the concept is clear and straightforward for aqueous media and sediments.…”

“…However, it seems more challenging to define the exposure of HOCs in soils, particularly in unsaturated soils, where only limited free water is present or when the target organism mainly inhabits the interstitial pore space. In these cases, it might make better sense to define exposure in terms of chemical activity (denoted a) [2].…”

“…The pure subcooled liquid of the HOC was chosen as the reference state (a ¼ 1), and the chemical activity is then defined between 0 and 1 [2]. In this way, chemical activity takes into account losses and matrix interactions that can either increase or decrease the exposure.…”

“…At equilibrium, chemical activity is equal in all compartments (e.g., a soil ¼ a air ¼ a organism ) in contrast to the concentration (e.g., C soil 6 ¼ C air 6 ¼ C organism ). Therefore, a difference in chemical activity between compartments determines the potential, direction, and extent of spontaneous physicochemical processes such as diffusion, partitioning, and chemical reactions [2]. This implies that the diffusive bioconcentration of HOCs in small soil organisms is driven by a difference in chemical activity between the organism and the surrounding environment.…”

Is it possible to increase bioavailability but not environmental risk of PAHs in bioremediation?

“…To this end, the 7 integration of a bioavailability-efficient technology into current bioremediation 8 practices at no additional cost will ensure that the target values for risk reduction and 14 Assuming that 1% of these costs are allocated to PAH-polluted sites comprising 13.3 % 15 of the total number of investigated sites in Europe [55], where bioremediation at a cost 16 of 45 €/m 3 is a realistic alternative to excavation and disposal priced at 200 €/m 3 , the Bioavailability represents the accessibility of a chemical for biotransformation 23 and toxicity. This concept is difficult to simplify and has recently been the subject of 24 intensive discussion and redefinition [67][68][69] left as residuals from bioremediation may still be toxic to specific ecological targets, 7 such as terrestrial invertebrates [73]. Furthermore, under some circumstances 8 bioremediation may even increase bioavailability and risk of PAHs.…”