Background
The behaviour of a chemical is not random, it is governed by the structure and physical and chemical properties of the substance. An understanding of these properties is essential for predicting the environmental impact of the chemical, designing robust experimental tests, for environmental exposure modelling and classifying the hazard of the chemical.
Physico-chemical properties are the inherent characteristics of a substance determined by its molecular structure. These include molecular properties, such as molecular weight and dipole moment, and bulk properties, such as solubility, vapour pressure and the octanol-water partition coefficient of a chemical.
Environmental fate properties, which include the Henry’s Law Constant, the bioconcentration factor, sorption coefficients and degradation rate constants and half-lives, describe what happens to a chemical after its release into the environment and provide an understanding of how it will distribute between air, water, and soil, be accumulated into organisms and how long it will persist in different environmental compartments. Persistence is covered in another component of this module.
Useful properties
Several critical properties act as primary indicators of a chemical's behaviour in the environment:
Water Solubility is the measure of how much of a substance will dissolve in water at saturation. Highly soluble compounds tend to be more mobile in aquatic systems whereas substances with low solubility tend to be less mobile.
Vapour Pressure is a measure of the tendency of a molecule to escape into the gas phase. A high vapour pressure indicates a strong likelihood that the chemical will evaporate into the atmosphere.
The Octanol-Water Partition Coefficient is a cornerstone of environmental chemistry. It is a measure of how a chemical distributes itself between n-octanol (an organic solvent that it immiscible in water) and water. Chemicals with a high Kow are known as hydrophobic (water hating) chemicals and are typically less soluble and less mobile and are more likely to be bioaccumulative and more toxic than low Kow (hydrophilic) chemicals.
The Henry’s Law Constant describes the partitioning of a chemical between air and water, indicating how easily a chemical volatilizes from water or soil and plant surfaces into the air.
Sorption Coefficients describe how a chemical "sticks" to solids like soil, sediment, or sewage sludge. A substance with a low sorption coefficient in soil is more prone to leaching into groundwater while a chemical with a high sorption coefficient in sewage sludge will likely be absorbed to sewage sludge during wastewater treatment and could, therefore, be released to agricultural fields when the sludge is applied to land as a fertilise. Sorption is often influenced by environmental pH and the organic carbon content of the soil.
Bioconcentration Factor (BCF) is a critical metric representing distribution of a chemical between the tissue of in an organism (like fish) and the surrounding water at steady state. A high BCF suggests the chemical will accumulate in the food chain.
The Acid/base dissociation constants (pKa) is used to understand the dissociation of chemicals that are weak acids and weak bases. The pKa is the pH value at which the molecule is 50% in the ionised form and 50% in the non-ionised form. For weak acids, as the pH drops away from the pKa, the substance will become less ionised. The converse is true for weak bases. As the environmental behaviour of the unionised and ionised form of molecules can be very different, the fate of weak acids and bases in heavily influences by the pH of the environment.
Sources of these properties
Information of the properties of a new chemical will typically be obtained experimentally as part of the safety assessment process. The OECD, for example, provide standardised guidelines on testing methods for determining all of the important physico-chemical and environmental fate characteristics of as substance.
For many existing chemicals, information on the properties of the substance can be obtained from online databases such as DrugBank, the Pesticide Properties Database and the Veterinary Products Database. Material Safety Datasheets that are produced by chemical suppliers will also often provide information on many of the properties.
Where property data are not available, predictive models such as in the USEPA’s EPISUITE system which is freely downloadable from the internet or on online systems such as CompTox, can be used to estimate the properties of a chemical from the chemical structure. The performance of these models is however variable with models for properties such as the Kow or solubility generally working quite well while models for sorption behaviour and Henry’s law constant work less well.
Use in hazard assessment
Regulatory systems, such as EU REACH and risk assessment guidelines for pharmaceuticals and biocides, require a hazard assessment for environmental safety purposes. These assessments involve the comparison of data on the environmental properties and effects of a chemical with thresholds to classify substances in terms of their persistence, bioaccumulation and toxicity. Chemicals that end up being classified as PBT (Persistent, Bioaccumulative, and Toxic), vPvB (Very Persistent, Very Bioaccumulative), PMT (Persistent, Mobile, and Toxic) are of particular concern. Under REACH, for example, if a safer alternative is available, then there should be a move towards substituting the substances of concern. Where a safer alternative is not available then emissions and release should be minimised.
Physico-chemical and fate properties are used to support this hazard assessment. For example, the octanol-water partition coefficient can be used to classify for bioaccumulation potential when experimental data is lacking and soil sorption coefficient are used in the mobility assessment of a substance.
Recorded lecture
Alistair Boxall from the University of York gives an overview of some of the important chemical physico-chemical and fate properties that we use in environmental hazard and risk assessment.
Key Reading
Card, M.L. etval. (2017) History of EPI Suite™ and future perspectives on chemical property estimation in US Toxic Substances Control Act new chemical risk assessments. Environ. Sci. Processes Impacts, 19, 203–212. doi.org/10.1039/C7EM00064B
ECHA (2023) Guidance on Information Requirements and Chemical Safety Assessment. Chapter R.11: PBT/vPvB assessment.
National Academies (2014) A Framework to Guide Selection of Chemical Alternatives: Chapter: 5 Physicochemical Properties and Environmental Fate. Washington, DC: The National Academies Press. doi.org/10.17226/9406.
Useful websites
EPI Suite™-Estimation Program Interface
PPDB: Pesticide Properties DataBase
VSDB: Veterinary Substances DataBase