Andrea E Russell
Professor of Chemistry
University of Southampton
1.Introduction
So enamoured is modern society with wonderfully versatile plastic materials that little consideration has been given to either their disposal or to their potential to become, beneath the radar of environmental oversight, a major pollutant. Few discoveries have better indicated the extent of plastic infiltration into the environment than Victor Vescovo’s distinctive find in the Mariana Trench. His recent seven mile excursion revealed a plastic bag and sweet wrappers at the bottom of this, the deepest part of the ocean, a place so remote that it has only ever been visited by four humans.
Science Cafe welcomed Professor Andrea E Russell of Southampton University. Originally from Michigan, she studied in America, passed through a fellowship at the US Naval Research Lab and arrived on these shores in 1991, ultimately becoming Professor of Physical Electrochemistry at Southampton. Her research includes the use of radiation sources to explore electrocatalysts, particularly those used in PEM fuel cells and to a lesser extent, in a rather roundabout way, microplastics.
2.What are microplastics?
To put plastic usage into context, plastic manufacture increased from 2 x 106 tonnes in 1950 to 300 x 106 tonnes today. It is reckoned that 90% of all the plastic that has ever been produced is still with us1.
The term ‘microplastic’ was coined by oceanographer Richard Thompson in 2004 and refers to any type of plastic of <5mm to >100nm in size (see Fig. 1, Fig. 3A). Plastic particles smaller than this are known as nanoplastics. Microplastics are considered ‘primary’ if they are of the requisite size range prior to entering the oceans and include clothing microfibres discharged by washing machines, nurdles (plastic raw pellets), cosmetic exfoliators and plastic particles used in blasting technology to clean machinery, which as they are reused, can acquire metallic contaminants. Other sources include vehicle tyres.
Primary microplastics are considered to consist of 15-51 trillion units globally with a mass of 93-236 x 103 tonnes. Not only are they found in oceans but there are definite indications that they can also be found to a lesser extent in the atmosphere and in soils.
Secondary microplastics are created by the degradation (physical, photo, chemical & biological) of larger plastic products such as water bottles and the ubiquitous plastic bag. Degradation, usually by fragmentation, is a very slow process.
3.Microplastics in the ocean
Most research on plastic pollution has concentrated on larger plastic items (e.g. plastic bottles, plastic bags) which are easily seen and their despoiling effect on the environment readily appreciated. The first wake-up call for the impact of microplastics came in 2004 with the publication of a paper in Science, ‘Lost at Sea: Where is all the Plastic?’. Oceanographer and lead author, Richard Thompson, sought to quantify plastics in the marine environment. To verify any finds as being plastic, he required a chemist and Andrea became that chemist and a co-author of the paper. Generally, she found that if a specimen looked like plastic, it was plastic, but more accurate verification was required and vibrational spectroscopy, a speciality of Andrea’s, was used.
Fig. 2 Sir Chandrashekhara Venkata Raman (1888-1970)
Nobel Laureate in Physics (1930)
Two types of vibrational spectroscopy, each complementing the other, were utilised. Both measure vibrational energy in the molecules in a material. The first is Raman spectroscopy, based on the Raman effect, discovered by Sir C.V. Raman (see Fig. 2) in 1928, for which he received a Nobel Prize in physics in 1930. When photons are directed at a material, the majority are scattered with the same energy (frequency) as incident photons. Some photons (1 in 107) are scattered at a different, lower frequency (Raman effect). Using intense monochromatic light from a (IR, UV or visible light), Raman spectroscopy shows a diagnostic set of lines for each specimen. It is a good tool to quickly identify materials even in an aqueous state but fluorescence from a sample can detract from its effectiveness.
In contrast, IR spectrometry (700-1000nm) measures light absorbed (not scattered) by vibrating molecules within a sample. Unlike Raman spectrometry, it requires a solid sample and pre-preparation. Raman spectroscopy has the distinct advantage that it can utilise wavelengths comparable to microplastic particle size. Spectroscopy readings are compared against a library of logged materials allowing identification of any sample (see Fig. 3C). Common plastics include polyethylene, PVC, polypropylene, polystyrene and PTFE (see Fig.4).
In ‘Lost at Sea’ the authors attempted to quantify microplastics in the marine environment by analysing estuarine and subtidal sediments around Plymouth followed by investigation of a further seventeen beaches (see Fig. 3B). To assess long-term trends, stored plankton samples from the 1960’s were analysed. These samples were taken originally for plankton analysis only, with no thought to a possible alternative use of looking at microplastic contamination at a later date; a serendipitous discovery. The routes examined (Fig. 3B) were CPR1 Aberdeen to the Shetlands (315km) and CPR2 from Sole Skerry to Iceland (850km). Microplastics archived amongst the plankton samples and showed a significant increase over time (Fig. 3E). The authors went further, keeping various organisms with different feeding modalities representative of the marine ecosystem (amphipods- detritivores, lugworms- deposit feeders and barnacles- filter feeders) and found that all three ingested microplastics.
Microplastics, it seems, are found everywhere in the marine environment.
Other research concurred, showing that microplastics become embedded in marine creatures by respiration as well as ingestion. Zooplankton, for example, ingest plastic beads and excrete them into marine snow. Microplastics can take 14 days to pass through a zooplankton digestive system compared to 2 days for usual food substances.
Microplastics can absorb pollutants and carry these into food chains. With their high surface area to volume ratio, there is particular concern for their potential efficacy as contamination vectors. Humans obtain 6.1% of their protein from seafood and polluted microplastics could introduce poisonous and carcinogenic chemicals into food chains.
Fig. 3. Some results from the paper, ‘Lost at Sea’
A- Microplastic particle.
B- Sampling locations including and other selected beeches and routes sampled by Continuous Plankton Recorder (CPR 1 & 2) to assess microplastic abundance since 1960.
C- Matching IR spectra of a microplastic particle againt pristine spectrum of nylon.
D- Microplastics were more abundant in subtidal habitats than on sandy beaches but abundance was consistent among sites within habitat types.
E- Microscopic plastic in CPR samples revealed a significant increase in abundance when samples from the 1960s and 1970s were compared to those from the 1980s and 1990s. Approximate global production of synthetic fibres overlain for comparison. Microplastics were also less abundant along oceanic route CPR 1 than along CPR 2.
Source: Thompson et al., (2004)
4.Further Research
A key question which, at the moment, appears to be answered in the negative, is whether microplastic consumption is causing harm to humans. Another, connected area of investigation, is whether microplastics, by absorption or adsorption, are a means of transporting poisons into the human food chain.
Andrea has two students working on microplastics research demonstrating the developing trend from (previously) recording and quantifying microplastics (as in 2004) to looking at dynamic effects, interactions and associations of microplastic within ecosystems and environments.
Christina Thiele is looking at the marine environment and possible risks to human health by studying the microplastic body burden of the native oyster Ostrea edulis and how human health might be affected by the consumption of such organisms. These filter feeders are very efficient at processing sediment and therefore a significant consumer of microplastics.
Jessica Stead is looking at microplastics transportation in coastal environments, in particular salt marshes and using plate sampling to investigate interactions within the sea surface micro layer separating the oceanic and atmospheric domains.
5. Dealing with the problem
Research shows that plastics take many decades to breakdown. Compostable plastic, which includes a non-carbon atom within the polymer chain, makes the plastic vulnerable to hydrolysis by acid, alkali or enzymes and can breakdown within 90 days, a considerable improvement on traditional plastics. Similarly, the use of bioplastics made entirely of natural, compostable materials can replace current plastic materials without environmental impacts.
A greater emphasis on recycling is required (see Fig.4). Recycling is variable across different jurisdictions. Sweden has an efficient system, with colour coded sacks for different plastics. For the UK, when better separation becomes profitable, a tipping point for the reuse of plastics will be reached.
Fig.4 There are many different types of plastic and most plastic items will have a plastic code indicating plastic type, general uses and recycling regime. All plastics can be recycled though not all are presently economic to process.
Source: Cohort Hostel https://www.stayatcohort.co.uk/2018/11/26/recycling-plastic/
Whilst bans of specific categories of plastics have been useful (e.g. microbeads banned in the UK from 2018) there is a lack of worldwide coordination in the move away from plastics. However, the European Research Council, together with not inconsiderable US research efforts and significant European funding provide a rough, pan-global approach. A good way forward would be to adopt ‘the polluter pays’ principle. The problem here is, who is the polluter? Possible alternatives include, the oil producer, manufacturer(s) of the initial polymer and the plastic item and/or the consumer?
With a lack of global coordination, local and individual initiatives can work best. Beach Watch2, for example, the Marine Conservation Society’s national beach cleaning and litter survey (to discover trends) and run by dedicated volunteers, is one such effective initiative.
Supermarkets are still, unfortunately, major plastic polluters. Ongoing consumer pressure will provide desirable responses in directing future behaviour of corporate giants. Interestingly, Andrea noted that the recent abolition of free plastic bags in supermarkets in the UK was some way behind Michigan which took similar action way back in the 1970’s.
Other microplastic sources receive less attention. Treated sewage is a significant source of microplastics which escape filtering. On release from sewerage plants into water courses, they eventually find their way into the sea. Where sewage sludge is applied to farmland as agricultural fertiliser, microplastics pollute the soil. Clearly, sewerage systems need to be more exacting in their treatment regimes.
In addition, there are health risks associated with microplastics in industrial environments where microfibres are routinely sprayed (e.g. carpet industry) and these risks are yet to be fully appraised.
6.Conclusions
Microplastics are a major source of marine pollution. So far there is no indication of any specific problems in food chains, but with microplastics likely to increase in foreseeable decades in the marine environment due to both the input of new material and long residence time of existing material, vigilance of the possible effects of microplastics must remain part of the scientific agenda. Care too, must also be taken to monitor soils and workplaces to identify and eliminate undesirable effects of microplastics within these environments.
References
Thompson, R.C., Olsen, Y., Mitchell, R.P., Davis, A., Rowland, S.J., John, A.W.G., McGonigle, D. and Russell, A.E. (2004) Lost at sea: Where is all the plastic? Science, 304, 838-838. (doi:10.1126/science.1094559).
1 ‘What to Do about Plastic Pollution, Science Agenda’, Scientific American, June 2019
2 Beach Watch @ https://www.mcsuk.org/beachwatch/