Figure 1: Example of plastic fragments covered with biofilm (visible as a green layer). Image obtained with an optical microscope.

Plastic particles dispersed in the environment are long known to be an environmental issue of concern. More recently, a surprising interaction of plastics with metals have been observed which triggered the interest of the scientific community. However, the different environmental implications of plastic and metals interaction are still hard to reconstruct. This critical review attempts to summarize the state of the art in this field in order to inspire future studies by suggesting an interaction mechanism and a conceptual framework.

Authors

Gilberto Binda is a postdoctoral researcher at the Norwegian Institute for Water Research (NIVA).
Davide Spanu is an assistant professor of analytical chemistry at Insubria University.
Damiano Monticelli is an associate professor of analytical chemistry at Insubria University.
Andrea Pozzi is an associate professor of environmental chemistry at Insubria University.
Arianna Bellasi is a PhD candidate in chemical and environmental sciences at Insubria University.
Roberta Bettinetti is an associate professor of applied ecology at Insubria University.
Stefano Carnati is a PhD candidate in chemical and environmental sciences at Insubria University.
Luca Nizzetto is a Leading Research Scientist at NIVA and a part-time researcher at RECETOX (Masaryk University). 

Highlights

• The emerging issue of the interaction between microplastic and (trace) elements needed a critical review
• Plastic degradation mechanisms and water chemistry play a crucial role in this interaction
• As a basis for future studies, a conceptual framework to model the plastic-element interaction is proposed
• Critical aspects of current experimental approaches are discussed
• Ideas for future experimental work are shared

RESEARCH BRIEF

Plastic litter and microplastic pollution is an environmental issue of concern. A conspicuous fraction of produced plastic waste is mismanaged and results in pollution, with freshwater and estuarine environments being mostly impacted.

It is well known that plastic takes up organic pollutants from the environment. The interaction of plastic with trace elements (including many metals) has however been long overlooked; plastic was in fact expected to have limited interaction with metals owing to its chemical properties (Holmes et al., 2012). This paradigm has been questioned by recent laboratory research that demonstrates a substantial interaction between metal ions and environmental plastics. Several consequences are expected if this interaction is also happening in the environment: plastic can potentially change the natural cycling of nutrients and the environmental fate of toxic metals (Bradney et al., 2019).

A critical review

These findings have opened a new research venue in the plastic pollution assessment, with an exponentially increasing body of literature being published in the last few years.

This review article by Binda et al. (2021) critically analyzes the recent knowledge on this issue (reviewing 50 published papers) and provides perspectives for future research on the interaction of plastic with trace elements. Different facets are analyzed: the main factors affecting the interactions; the conceptual models developed so far to interpret the interactions; the development of a new framework; and the critical discussion of different experimental approaches used in this research field.

Important factors

Both water chemistry on one side, and the characteristics of plastic on the other side affect the interaction between plastic and trace elements. The reviewed studies tried to clarify which factors mainly regulate this interaction.

On the water side, salinity and pH seem to influence the interaction of plastic with trace elements. A low pH value and high salinity decreases the adsorption rate for most of the analyzed metals due to competition with other ions. The role dissolved organic matter plays in this process is still unclear, as studies showed that it can either promote or limit interaction.

On the plastic side, the plastic polymer degradation plays an important role, affecting the surface reactive groups as well as the wettability of particles. The degree of plastic ageing, which can be measured by the density of oxidized functional groups, was identified as a strong determinant of metal attachment on the plastic surface. Last but not least, the colonization of plastic in the environment by biofilms was recently determined as an important factor enhancing such an adsorption (Figure1).

Watch researcher Gilberto Binda explaining this article in a video.

Conceptual models

Based on these proofs, conceptual models of interaction between plastic and (trace) elements have been drawn from studies conducted in controlled environments. Physicochemical models are usually fitted to experimental data in order to apply interaction mechanisms in an environmental context.

However, this canonical approach has some limitations in describing the interaction of plastic with (trace) elements. Experimental results suggest that (trace) elements attach onto plastic surface through a multi-step process. However, the degradation degree and the presence of biofouling, which is common in environmental plastic, determines very different behaviors that are generally not captured by overly simplistic laboratory experiments.

The authors therefore propose a conceptual model of the plastic-element interaction articulated along three different scenarios and at different levels of environmental relevance:

1) Pristine plastic particles allow for very limited interaction with (trace) elements where only the plastic chemical additives at the interface between plastic and water can possibly mediate an interaction;

2) Chemically aged plastic with oxidized polymers that can interact through electrostatic weak attraction with metals;

3) Biofilm-covered plastic which presents a completely altered surface reactivity. Active biosorption processes are likely to happen in the cellular membranes and extracellular polymeric substances (e.g. polysaccharides) produced by the microorganisms in the biofilm which can chelate metal ions.

The chemical paradigm of the lack of interactions between plastic and aqueous metal ions has been questioned by the results of recent laboratory studies, demonstrating a substantial interaction between metal ions and microplastics.

Finally, the review highlights the research needs for the analysis of metals absorbed by plastic particles in environmental samples. It further provides suggestions to improve experimental setups to simulate their interaction:

1) A reliable sampling and processing of environmentally collected particles including specific extraction schemes to separate differently bonded chemicals can lead to a detailed analysis of the adsorbed metals in different phases. For this purpose, state-of-the-art surface techniques (Pořízka et al., 2023) that separate the metals interacting on the plastic surface with the one present inside the polymer – for example in pigments or in stabilizers – should be used.

2) There is a demand to design (de)sorption experiments with a more realistic and well-defined selection of conditions. For example, using realistic trace concentrations of metals, water physicochemical conditions varying in the environmental range, plastic specimens fully characterized for their chemical and/or biological ageing. This would ensure more representative and intercomparable results.

3) A reliable combination of environmental samples analyses, including only the quantification of the adsorbed elements, can contribute to a more realistic reconstruction of environmental conditions in experimental settings.

The full article is available here

References

Binda, G.; Spanu, D.; Monticelli, D.; Pozzi, A.; Bellasi, A.; Bettinetti, R.; Carnati, S.; Nizzetto, L. (2021). Unfolding the interaction between microplastics and (trace) elements in water: A critical review. Water Research, 204, 117637. https://doi.org/10.1016/j.watres.2021.117637

Bradney, L.; Wijesekara, H.; Palansooriya, K. N.; Obadamudalige, N.; Bolan, N. S.; Ok, Y. S.; Rinklebe, J.; Kim, K. H.; Kirkham, M. B. (2019). Particulate plastics as a vector for toxic trace-element uptake by aquatic and terrestrial organisms and human health risk. Environment international, 131, 104937. https://doi.org/10.1016/j.envint.2019.104937

Holmes, L. A.; Turner, A; Thompson., R.C. (2012). Adsorption of trace metals to plastic resin pellets in the marine environment. Environmental Pollution, 160, 42-48. https://doi.org/10.1016/j.envpol.2011.08.052

Pořízka, P.; Brunnbauer, L.; Porkert, M.; Rozman, U.; Marolt, G.; Holub, D.; Kizovský, M.; Benešová, M.; Samek O.; Limbeck A.; Kaiser J.; Kalčíková, G. (2023). Laser-based techniques: Novel tools for the identification and characterization of aged microplastics with developed biofilm. Chemosphere, 313, 137373. https://doi.org/10.1016/j.chemosphere.2022.137373