ENVIRONMENT
their characteristics – such as particle size and the exact nature of the polymer. Analysing the samples must be done with high precision, to ensure there is no cross-contamination – because everything from clothing to skincare products could be a ‘false’ source of microplastic particles. ‘We have to treat samples as if we’re dealing with trace metals, or radio-isotope contamination,’ says Lampitt. All sample manipulation and preparation is done in a cleanroom – while wearing appropriate clothing. The next step is to create simple models that determine which materials are getting into the sea, how they get there and where they end up. ‘It’s quite straightforward and is hardly even science – but more like doing budgets,’ Lampitt says. Research into the potential harm
caused by microplastics, though, will be more ‘nuanced’ – and take longer. ‘Microplastics could cause significant harm, but we don’t yet know enough – or have any proof,’ he says.
River transport Just as rivers supply the sea with water, so they also act as a source of pollution. Researchers at the Helmholtz Centre for Environmental Research (UFZ) in Germany have found that 10 large rivers are responsible for transporting 90% of plastic waste into the sea (Environmental Science & Technology; doi: 10.1021.
acs.est.7b02368). The team collected pre-published data on plastics in rivers and collated it with upstream sites of ‘mismanaged’ plastics waste – municipal waste that is uncollected. ‘The more mismanaged plastic
waste there was, the more you found in the river,’ says Christian Schmidt, of the hydrogeology department at UFZ. ‘There was an empirical relationship between the two.’ Eight of these 10 rivers are in Asia,
while the other two are in Africa. All of them flow through areas of high population. The Yangtze in China, for instance, was the main polluter – and is host to around 500m people. ‘Countries like India and China have seen huge economic growth – and now use large amounts of plastic food packaging and bottles – but have limited waste collection systems,’ he says.
The data include both microplastic
and ‘macro’ plastics – but microplastics data dominate ‘because
scientists are more interested in that’, says Schmidt. While it is important to
measure how much plastic is in the environment, Schmidt believes that the next step of his research will be more important: understanding the journey the plastics make from the river to the sea. ‘We want to know how long it takes for them to move through the river system – and right now we have no idea of the answer,’ he says. Some basic facts are known – such as the fact that large rivers transport plastics more quickly than small ones. ‘Are plastics transported only by the flow? It’s possible, but unlikely,’ he says, adding that plastics may get ‘stuck’ on the way – such as by settling in sediment – before being ‘remobilised’ later. Understanding how long plastics stay in the river system will be important for any action that is taken to manage it, he says. It would also be used to measure the success – or otherwise – of efforts to reduce plastic waste upstream.
Small measures One of the main problems with quantifying microplastics is the delicate and laborious nature of the task. However, a new technique may speed up the detection process – especially for the very smallest particles. Researchers at Warwick University in the UK have used dye molecules to find microplastics as small as 20microns. The Nile Red dye preferentially
stains hydrophobic species – which includes many types of plastic, say the researchers (Environmental Science & Technology; doi: 10.1021.
acs.est.7b04512). ‘The dye fluoresces depending on its environment,’ says Joseph Christie-Oleza, of the school of life sciences at Warwick University. ‘In hydrophobic conditions, it will be intense green.’ This is particularly effective
for plastics such as polyethylene, polypropylene, polystyrene and nylon, but less so for more ‘hydrophilic’ materials like PVC and PET. However, these can still be detected by adjusting the detection settings. ‘A critical step was to find the
correct settings so that only plastics fluoresce in the sample, and not “natural” contaminants like algae,’ says co-researcher Gabriel Erni- Cassola, who wrote the software script for the process. ‘What took time was
Microplastics levels in the Baltic Sea have remained constant for 30 years, in both fish and water samples, says DTU Aqua
Researchers in Denmark have reported finding that levels of microplastics in the Baltic Sea have remained constant over the past three decades, despite rising levels of plastics production and use
90%
Proportion of the plastic waste found in our seas that comes from just 10 large rivers, according to researchers in Germany
finding the correct thresholds to pick up as many polymers as possible.’ The technique may help to find much of the ‘missing plastics’ that are almost impossible to detect. Many studies have reported a decrease in the abundance of plastics particles below a certain size, says Christie- Oleza. ‘It’s not what you’d expect from a fragmentation pattern,’ he says. ‘Instead, you’d expect to see an exponential increase in smaller particles.’ The reason, he says, is the sheer difficulty in detecting these particles – especially when using techniques like manual separation and counting. ‘Our technique is more objective,’ he says. ‘If it fluoresces, it gets detected’. In the study, the researchers set a maximum particle size of 1mm and a lower threshold of 20microns – though they admit: ‘There are probably even more below this size’. At these smaller sizes, it can be
hard to determine plastic samples from natural substances like chitin. However, a ‘separation’ phase – which includes using chitinase enzymes to digest the chitin – helps to make the sample as ‘pure’ as possible before measurement. Once microplastics have been quantified, they could later be analysed by Raman spectroscopy to determine their exact nature. Doing every sample would take a long time, but analysing just a few would determine the prevalence of certain types of plastic. For all the uncertainty and debate
over how much plastic is in the sea – and what harm it can do – one thing is clear: future research is likely to focus more on the plastics that we can’t see, rather than the stuff we can.
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SABRINA BEER
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