Environmental Fate: Tracing the Journey of Chemicals in our Ecosystems
Environmental fate pertains to the behaviour and ultimate destiny of chemicals in the environment, encompassing their interactions, transformations and potential toxicity. Comprehending the intricacies of chemicals within an environment is crucial to provide insights into the risk they may pose, enabling policymakers to create well-informed decisions regarding their regulation and utilisation of them. This article aims to provide a broad overview of the factors influencing environmental fate.
Environmental Chemistry | Nargiss Taleb
Introduction
Chemistry plays an elemental role in our incredibly complex world, shaping the interactions and processes that occur within ecosystems and within ourselves. It is thus important to understand how factors like composition, structure, and properties play into the intricate workings of our environment. Environmental fate describes the behaviour of a chemical in the environment and thus what the ultimate destiny of it will be. Recognising the significance of comprehending environmental fate lies in its ability to provide insight into the potential hazards it may present to both humans and the environment. It allows policymakers and those in charge of regulating these chemicals to understand what is safe and what is not.
As synthetic chemistry advances, we discover more and more chemicals that have the potential to contribute to our way of life. More than 100 million in fact, are currently registered in the Chemical Abstracts Service (CAS), with about 4000 new ones registered every day [1]. In Aotearoa, there are more than 30,000 chemicals approved for nationwide use [2]. So how do regulators decide which of these millions of chemicals are allowed on the market? Well, it’s all about environmental fate.
Figure 1. Relationships between physicochemical properties and their relationship to environmental fate, biological and ecological processes, and toxicity [3].
Physicochemical Properties
The structure and thus the inherent physical and chemical properties heavily influence the fate of chemicals (Figure 1.). Characteristics including but not limited to molecular weight, polarity, solubility, and volatility, all help create a chemical identity allowing for scientific understanding of its behaviour in different environments [3]. Solubility for example, determines the ability of a chemical to dissolve, whether this be in water or other liquid mediums. Scientists use the solubility product constant (Ksp) in which a higher value means more solid has dissolved into an aqueous phase [4]. The amount in which a chemical is soluble is not only important but to what phase would it partition to in the environment. Here scientists will use a term called Kow, the octanol-water partition coefficient, which acts as a gauge to assess the relative lipophilicity (fat solubility) and hydrophilicity (water solubility) of a substance [5]. In the environmental fate context, this is important as it dictates whether a chemical will accumulate in fat tissues of organisms, partition into sediments or has the ability to be transported through water bodies. Volatility refers to the tendency for a chemical to evaporate. This is often simply determined by the chemical’s vapour pressure or boiling points. Here, molecular weight and the strength of the intermolecular forces between molecules heavily influence whether a chemical will turn into its gaseous phase. The relevance of this is that volatile chemicals can undergo long-range transport through the atmosphere, having the potential to disperse and deposit in very diverse environments.
It is important to note that it is not solely the physicochemical characteristics of the chemical itself that determine where a chemical may end up but also its interactions with the environmental conditions it is exposed to. Temperature, humidity or pH are a few of the relevant factors that play a role.
Environmental Fate
Earth, air and water – three of the four elements of nature, but also the potential locations for these chemicals to end up in. Chemicals in the environment can undergo various transportation mechanisms, ultimately determining their distribution in the environment. These mechanisms include air, water, soil, biological and human-mediated transport. Air transport shows air currents carrying volatile compounds, allowing for their distribution through global wind networks. This poses the potential risk of contamination far from a chemical source. Water transport moves chemicals through water bodies such as rivers and lakes, which feed into oceans and seas. This mode of transport is significant for water-soluble compounds, which can be carried far distances simply by water currents. Soil transport involves processes such as leaching, where water infiltrates the earth, allowing for the chemical to become mobile and percolate through layers in the soil profile [6]. This is where surface interactions of chemicals, such as their ability to adsorb onto a surface, are significant, determining their ability to hold onto the solid soil particles rather than penetrating the soil further. Biological transport refers to living organisms, both flora and fauna. Flora (plants) can uptake chemicals from the soil through their roots or from the air through their leaves before translocating these to various other parts of the plants. Particular animal (fauna) species that show migratory practices also have the potential to change the geographic scale of chemical distribution. The last mode of transport is through anthropogenic means. Humans can physically move chemicals through goods transportation or events like unintentional spills.
The above transportation mechanisms all have the ability to interact with each other to determine the distribution scale and final location of chemicals. It is crucial to understand these processes as they directly influence biological and ecological processes.
Biological and Ecological Processes
Once in these environments, chemicals are able to undergo various biological and ecological processes. This includes but is not limited to degradation, bioaccumulation, and biomagnification. Physical, chemical and biological processes can help to degrade a chemical in the environment. Physical processes, like weathering, change the physical appearance of the chemical, like a reduction in particle size. Chemical degradation involves breaking down a chemical into smaller molecular constituents, most commonly hydrolysis or oxidation [7]. Chemicals can also be broken down by biological organisms such as fungi, bacteria and other microorganisms. These organisms have enzymes capable of metabolising chemicals, which change the toxicity compared to the original substance (detoxification lowers toxicity whilst bioactivation increases it) [8]. Chemicals that persist in fauna (animals) also can bioaccumulate and thus biomagnify chemical concentrations. Bioaccumulation here refers to the process in which chemicals build up in the fat/tissues of organisms. As chemicals bioaccumulate, the result of predator/prey relationships shows chemical concentrations increasing further up the food chain - this is called biomagnification [9]. It is acknowledged that this is a limited list of all the processes and that many other processes and influencing factors can change chemicals in the environment.
Ecotoxicity
The environmental fate of chemicals is closely tied to their toxicity and their potential to have ecological consequences. After undergoing various processes to degrade or biotransform these chemicals, it has the potential to pose toxicity. Different organisms exhibit varying degrees of sensitivity to particular chemicals, the most sensitive species being used by regulators as indicators [10]. These indicators are exposed to varying concentrations of a chemical to evaluate its potential hazard in the environment using toxicity metrics like LC₅₀, EC₅₀ or NOEC.
LC₅₀ refers to the concentration of a chemical that causes mortality in 50% of the test organisms within a specified period [11]. This is typically used for acute toxicity. EC₅₀ is the concentration of a chemical that produces a biological response in 50% of the test organisms [11]. NOEC (no observed effect concentration) is the highest concentration of a chemical where no statistically significant adverse effects are observed, this being a good determinant for the threshold at which toxic effects start to occur [11]. One challenge with using exclusively indicator species data for these metrics is that not all species and other organisms in the ecosystem will display the same sensitivity as the ‘test’ species. So how can we extrapolate the data produced for a small subset of species to help predict the potential impact on the larger ecosystem?
According to the Hazardous Substances and New Organisms (HSNO) Act 1996, a document that Aotearoa uses to determine chemical risk, we can assign these toxicity metrics under physical, health and environmental hazards [12]. Some subcategories include acute/chronic toxicity, mutagenicity, carcinogenicity, reproductive toxicity, and hazardous nature to soil, aquatic or terrestrial organisms. To address the issue of being reliant on indicator species data, an arbitrary ‘safety factor’ can be applied to the toxicity metric (dividing the metric by 10, 100, 1,000 or 10,000) to determine whether the chemical can be considered ‘safe’ [10].
Conclusion
By understanding the factors that govern chemical behaviour, transport, transformations and ecotoxicity, well-informed policies and risk management strategies can be put in place to mitigate their effects. Insight into the environmental fate of chemicals and thus the effective regulation of the chemical market is thus highly important to protect both human and ecosystem health.
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Nargiss is a dedicated early career researcher with a passion for utilizing chemistry to address environmental challenges. Having recently completed her BSc (Hons), she aspires to make scientific contributions through innovative research. Currently employed as a research assistant, she is investigating compostable packaging and the potential for implementing Aotearoa-specific policies.