Frank A.P.C. Gobas1, S. Victoria Otton2, Laura F. Tupper-Ring3 and Meara A. Crawford4
1School of Resource and Environmental Management, Simon Fraser University, Canada, 2 Simon Fraser University, Canada, 3 Simon Fraser University, Canada, 4 Independent consultant, Canada
Thermodynamic activity (also frequently referred to as chemical activity) was first proposed by Lewis (1901, 1907) and has been successfully applied in many areas of science, engineering and medicine to better understand and describe the behaviour of chemical substances and pharmaceutical drugs. In the field of environmental chemistry and toxicology. Thermodynamic activity has also been put forward as a theoretically well founded and practical metric for studying and managing environmental contaminants (Mackay and Arnot, 2011). Thermodynamic activity is a multimedia exposure parameter that provides a metric for comparing, relating and combining exposure and toxicity data from various sources, which is useful for environmental risk assessment and management.
The thermodynamic activity approach can be defined as the expression of chemical exposure and toxicity data in terms of thermodynamic activity with the purpose to (i) study and assess the distribution or fate of chemicals in the environment; (ii) to study and assess the toxicity and modes of toxic action of chemicals; and (iii) to conduct risk assessment and management of chemicals at specific locations and in the general environment (Figure 3.2.1).
Figure 3.2.1: Conceptual diagram illustrating the activity approach as a method for expressing chemical exposure and toxicity data in terms of thermodynamic activity with the purpose to (i) establish exposure pathways of chemicals in the environment; (ii) to study and assess the toxicity and modes of toxic action of chemicals; and (iii) to conduct risk assessment and management of chemicals at specific locations and in the general environment.
While the activity approach has been used for many years in environmental chemistry and toxicology to study the environmental distribution and toxicity of chemicals, it has rarely been used to make management and regulatory decisions. The only documented example to date where the application of thermodynamic activity played a role in environmental decision making was a relatively recent decision by Environment Canada to reverse its original assessment of the chemical Decamethylcyclopentasiloxane (D5) from toxic to non-toxic within the context of the Canadian Environmental Protection Act (Gobas et al., 2015).
Figure 3.2.2 activities (unitless) in waste water treatment plant (WWTP) effluents, ambient water, ambient sediment, plankton, invertebrates, fish, birds, terrestrial mammals and marine mammals from different locations in the Northern hemisphere (black filled circles) in relation to the maximum activity (a=1, red line) and various NOECs (Gobas et al., 2015) in sediment and soil dwelling invertebrate and plant species at 25oC (dashed grey lines).
Figure 3.2.2, which summarises the activity based analysis of D5, shows all environmental concentration and toxicity data, expressed in terms of the dimensionless thermodynamic chemical activity for D5 in a single plot. Figure 3.2.2 illustrates that activities of D5 in the environment range by orders of magnitude and are always less than 1, which is the maximum activity that a liquid chemical like D5 can achieve in the environment. In contrast, activities of D5 associated with reported no-effect concentrations in toxicity tests are well above 1. Activities above 1 cannot occur in the environment and are most likely the result of experimental artefacts in the toxicity experiments due to dosing at concentrations above D5’s solubility in water and/or sorptive capacity in sediments and soils.
The activity approach has also been applied to the environmental risk assessment of the plasticiser di-ethyl-hexyl-phthalate ester or DEHP and mixtures of phthalate esters. This risk assessment illustrates the application of activity to include large numbers of field based concentrations of phthalate esters in various environmental media and data from in-vivo and in-vitro toxicity studies in a comprehensive risk assessment (Tupper-Ring, 2015).
There are many more possible applications of the thermodynamic activity approach. Thermodynamic activity can be used to develop environmental quality guidelines or criteria because of its ability to express medium specific environmental criteria in terms of generally applicable thermodynamic activities. Thermodynamic activity is also a useful method in green chemical design for establishing a maximum ceiling above which a chemical’s thermodynamic activity and corresponding concentrations in the environment cannot go. If the maximum activity of a chemical falls below the apparent toxicity threshold, the chemical (on its own, i.e. in absence of other chemicals with a similar mode of toxic action) cannot exert known toxicity. Activity is also useful for monitoring of risks in the environment (Booij and Smedes 2010; Jahnke et al., 2012, 2014). For example, monitoring data from passive samplers can be expressed in terms of thermodynamic activity and then compared to toxicological data also expressed in terms of thermodynamic activity. Thermodynamic activity can also play an important role in characterising the toxicity of mixtures of chemicals (Reichenberg and Mayer. 2006).
In conclusion, the application of the thermodynamic activity approach provides many opportunities to advance the study and management of chemicals in the environment. The approach complements the adverse outcome pathway analysis by providing a dose metric to link response levels from the initiation event to the whole organism level.