#02 2022

Microplastics in aquatic systems: vectors of metal pollutants?

By Aldwin Ndhlovu, Taryn Swartbooi, Gavin Rishworth and Lucienne R. D. Human, SAEON Elwandle Node

Pollution threatens water quality in estuaries, with anthropogenic activities being the major contributors to pollution in aquatic systems. Plastics enter coastal water bodies from agricultural, industrial and domestic wastewater activities by being washed into rivers. 

When plastics reach aquatic environments, they start to break down into small pieces and these small plastic materials do not completely decompose. They continue to break down into smaller fragments, resulting in secondary microplastics which can change and form new physical and chemical properties.

Due to their small size, microplastics pose a threat to aquatic organisms (organisms found in oceans, lakes, rivers and estuarine systems) through ingestion and through their respiratory systems. Their small size means there is a real risk that filtering animals, such as mussels (bivalves), fishes and whales, will consume microplastics.

Microplastics are often transported through the food chain to higher trophic level organisms such as fish, where they may be consumed by humans, especially in areas where there is a reliance on subsistence fishing, where they can be a health risk if they reach toxic levels.

South Africa has started implementing mitigation of pollutants and their effects on the environment and people by introducing the Carbon Tax Act 15 of 2019. The Carbon Tax Act gives effect to the polluter-pays principle aimed at reducing pollution by ensuring that large emitters take the negative adverse costs into account in their future production.

South Africa is also supporting a mandate of the United Nations Environment Assembly (UNEA 5.2) on the draft resolution on an internationally legally binding instrument on plastic pollution. The objectives of the instrument are to establish as necessary targets, definitions, methodologies, formats and obligations. These objectives are aimed at promoting national action plans, which are then used to prevent, reduce and remediate plastic pollution. This is tailored to local and national circumstances and the characteristics of specific sectors and support regional and international cooperation and coordination. Assessing and monitoring the relationship between metals and microplastics will strengthen this mandate and ensure that informed management decisions are taken in managing metal and microplastic pollution in estuarine systems.

Potential vectors for metal pollutants 

Microplastics have direct adverse effects on aquatic biota and have been touted as potential vectors for metal pollutants due to their affinity to metals. This affinity to metals adds another pathway of metal pollution to estuarine organisms.

There is very little research focusing on the occurrence and concentration of metal contaminants attached to microplastic surfaces. Hence there is need for studies to investigate this topic of microplastics being vectors of metal pollutants. The hypothesis that ‘microplastics transfer metal pollutants to aquatic animals’ is rapidly gaining attention. When fully explored, it will help in assessing the risk of pollutants and in the management of aquatic systems.

Here at SAEON, we hope to critically evaluate this hypothesis by setting up experiments and collecting data in estuarine systems. Samples collected will be analysed to unpack the relation between microplastics and metal pollutants by means of the Total Reflection X-ray Fluorescence Spectroscopy TXRF (Bruker Germany) and Fourier Transform Infrared (FTIR).

The TXRF and FTIR instruments are located at the DSI/NRF SAEON Coastal Biogeochemistry Platform and at the microplastics laboratory at the SAEON Elwandle Node.

Figure 4. Left = different sizes of microplastics collected from the ocean (© Samuel Bollendorff – Tara Expeditions Foundation) https://journals.openedition.org/factsreports/docannexe/image/5290/img-2.jpg; and Right = projected total municipal plastic waste generated across Africa between 2015 and 2060 (Lebreton and Andrady, 2019) https://www.sciencedirect.com/science/article/pii/S0048969721052475

Figure 1. Microplastics pathways (Issac and Kandasubramanian 2021) https://link.springer.com/article/10.1007/s11356-021-13184-2

Figure 2. How microplastics end up in fish (Mallik et al. 2021) https://www.sciencedirect.com/science/article/pii/S0048969721015011

Figure 3. Dr Aldwin Ndhlovu operating the Fourier Transform Infrared (FTIR) in the microplastics laboratory