Microplastics in the Environment- how do we measure them?

Last month, I welcomed high school geography students from across Aberdeenshire to a series of geoscience workshops exploring the issue of plastic pollution. During the sessions, students stepped into the role of environmental scientists, working with real samples collected from Aberdeen Bay as part of my current research.

As Plastic Free July draws to a close, his month's blog examines at how scientists sample and measure microplastic pollution in the environment. Although the process is difficult and time-consuming, it plays a vital role in understanding the scale of pollution, and provides the evidence needed to inform policies that protect our environment.

Figure 1: Sample site on Aberdeen Beach showing visible plastic pollution.

Sampling

The first step of understanding microplastic distribution in the environment is to plan a sampling strategy and collect soil samples. My research aims to understand the transfer of microplastics from the land into the ocean, so sample sites across the Aberdeen Bay foreshore and offshore were selected. Beach samples were easy to access and collect, and myself and research assistant Dayna spent two days traversing the length of the Bay from Balmedie in the North to the mouth of the River Dee in the South. We took note of any visible macroplastic waste (e.g. Figure 1) and ensured that any possible contamination was avoided by using metal sampling equipment, wearing clothing made of natural materials, and standing upwind of the sample site.

Offshore sampling was more challenging, and we chartered a survey boat to take us out for a full day of sediment collecting. This involved various stops aound the bay to release a sediment grab overboard to collect sediments from the seabed up to 70 metres below (Figure 2). We had some rotten luck on the day, with a change of port increasing the transit time, a big swell, some motor issues. As a result, it was a pretty rough day for sea sickness - the things I do for science...! Luckily, it was not all bad. We had good sediment recovery at all sample sites, and were able to enjoy seeing the Aberdeen wind farm up close (Figure 2).

Figure 2: Offshore sampling using a sediment grab.

Sample Processing

Once samples are collected, it is back to the lab to process them in preparation for microplastic counting. This is the most time consuming step and requires a great deal of care to avoid contamination at the various steps:

1. Sample Drying: The first step is to slowly dry the collected sand and mud samples. We did this in an oven at a low templerature for 72 hours. A low templerature is required so that any plastics are not melted and damaged by the drying process.

   
   

2. Normalisation of weight: All sediment samples are weighed out to 50g so that they can be directly compared.

   
   

3. Organic Content Removal: Since plastics have a similar density to organic material such as vegetation or seaweed, the organic material should be removed. This was done by bathing the sediment in hydrogen peroxide for 24 hours. This step can produce some spectacular reactions!

   
   

4. Clean and Dry: After 24 hours, it is time to clean the sediment samples in filtered water and then it is back to the oven for a slow dry.

   
   

5. Density Separation: Finally, it is time to extract the microplastics from the sediment. We did this by mixing the sediment sample in a saline solution of zinc chloride. Since rock minerals are more dense than plastic, the sediment sinks to the bottom of the liquid, leaving any microplastics on the surface. This surface solution is siphoned off and filtered leaving the microplastics to be counted (Figure 3).

Figure 3: Late afternoon sun hitting my filtration set in the lab.

So, I hope that you can see, it is quite time-consuming to seperated those microplastics from the sediment. Each step requires patience and presicion to avoid damaging the microplatics, or contaminating the sample with more microplastics. The lab work must be done in a very clean enviornment, in a fume cupboard, and samples should be covered with foil at all times. The researcher must wear latex gloves, a cotton lab coat to avoid contamination also.

Microplastic Counting & Characterisation

The final step is to count the microplastics in each sample, describe them (shape, colour, size) using an optical microscope. There are various different classes of microplastic - the most common of which are outlined in Figure 4 below. They are: (a) Nurdles or pellets - primary plastics used in manufacture of plastic goods; (b) Fragments - pieces of plastic broken down from larger pieces; (c) Fibres - pieces shed from lager plastic pieces - typically sourced from clothing or fishing nets; and (d) Films - sourced from wrappers and other packaging. .

Finally, it is important to determine their composition and origin. We did this using FTIR Spectroscopy. I am still assessing the results - and comparing the microplastic count to the sediment charater and organic content readings. Hope to share more on this soon.

Figure 4: Most common catagories of microplastic.

It is a huge amount of work to monitor microplastics in the environment. This research would not be possible without the support of the Carnegie Foundation, to whom I am immensely grateful for the support. This research will give a snapshot of the microplastic pollution across the Aberdeen Bay area, but to be effective, such research requires repeated surveys over time to assess the changing flux of plastics from the land into the ocean with changing seasons and policy. Much work is still to be done, but there is actions that we can all take to make a difference to the volume of plastic reaching our oceans including:

1. Join a beach clean or collect waste when you are out for a walk. Plastics remian in the environment for hundreds of years, breaking down into smaller and smaller pieces. By collecting larger plastic waste from beaches, we can stop the cycle of these plastics breaking down to microplastics and entering our oceans.

2. Purchase clothing made of natural fibres (cotton and wool). Synthetic fabrics shed microplastic fibres during wear and washes that can enter our ocean via the waste water system. Natural fibres such as cotton and wool break down in the environment.

3. Reduce single-use plastic consumption. Choosing reusable bottles, bags, and food containers reduces the amount of plastic entering the environment, where it can eventually break down into microplastics.

These three simple but effective actions that can help protect our oceans, and hopefully result in me finding less microplastics in future sediment samples!

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