Technical Expertise > Environmental Fate

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Fugacity modelling - Compartmental models
Degradation and Persistence

Fugacity modelling - Compartmental models

The fate and behaviour of a substance in the environment can make all the difference between whether or not it is available to exert any harmful effects. This is often a key factor determining the difference between 'hazard' and 'risk'.

For example, a substance that is toxic to fish (and therefore hazardous) may have properties which mean that it evaporates very readily and is rapidly degraded in the atmosphere - and therefore will never accumulate in water in the wider environment and fish will not be exposed or at risk.

Fugacity-based compartmental models exist which use chemical properties, release rates and degradation rates to determine the distribution of a substance in environmental models of varying complexity. Behaviour of a substance passing through a waste water treatment plant can also be modelled in a similar way.

These modelling processes are integral to risk assessment and can also be useful in other contexts, such as OECD HPV SIARs.

Degradation and Persistence

Degradation is of extreme importance in environmental fate modelling of a chemical since this will affect the development of distribution equilibria. The fate of any chemical released into the environment will depend on how it degrades and for this reason, degradation rates in the environmental compartments form a key input of fugacity models. Persistence (i.e. resistance to degradation) in the environment is a key factor determining long-term exposure.

Degradation under environmental conditions may proceed by many routes:

Standard test methods exist for measuring biotic and abiotic degradation in aqueous conditions. In particular there are several standard methods for screening microbial biodegradation, and these focus on modelling degradation in a wastewater treatment plant.

In the context of EU approaches and methods, two key concepts exist. Ready biodegradability is a property by which a substance is degraded rapidly (t1/2 <28 days), under aerobic conditions, by a mixed population of WWTP-type micro-organisms known as 'activated sludge'. The conditions are unfavourable in terms of microbial population and other organic nutrient sources for the microbes. Hence a substance that has been determined to be readily biodegradable under these conditions can be relied upon to be degradable in the environment under most conditions.

An inherently biodegradable substance is one which passes an inherent biodegradability test. Inherent biodegradability is normally assessed over a longer time period and may use more favourable test conditions. In the past some authors have used this term to represent 'potential' or 'intrinsic' properties.

Biodegradation can be complete or partial. Complete degradation results in the mineralisation of organic chemicals, breaking them down into carbon dioxide and water, and compounds of any other atoms present (e.g. nitrogen, sulfur). Partial degradation can result in the production of a stable by-product. Micro-organisms may need to adapt to the presence of a chemical before they begin to utilise it and break it down.

Abiotic degradation via hydrolysis is measured in the context of varying pH and temperatures.

Atmospheric degradation rate (by reaction with hydroxyl radicals) is usually estimated, though many measurements are published in the open literature, particularly by Atkinson.

Persistence in an environmental 'compartment' (i.e. water, sediment, soil or the atmosphere) is sometimes referred to in the context of environmental fate. For instance, a very highly volatile substance may not be 'persistent' in surface water because of volatilisation. However, unless degradation occurs in the atmosphere, the substance does persist in the environment at large.

The rate and extent of degradation in all environmental compartments, and by the various biotic and abiotic mechanisms, depend on the characteristics of the chemical, and can vary enormously. Knowledge and/or prediction of chemical characteristics and the degradation processes and rates involved enable the assessment of environmental fate. It is however possible to determine whether a substance will completely mineralise in a short time, or persist in the environment for a very long time.

Images:
Alex Drahon
Diane Wilkin