Microplastics part 2: Sources

Below is an overview of the sources of microplastics:

1. Degradation and Fragmentation

Larger plastic debris already present on the ocean surface or on beaches may undergo degradation and ultimately fragment. Degradation is a process by which a polymer undergoes a chemical change that significantly reduces its average molecular weight (Andrady 2011). Plastics, also known as synthetic polymers (Moore 2008) can undergo degradation through the actions of sunlight (photodegradation) and high temperatures (thermodegradation) (Andrady 2011). Some microorganisms have been found to degrade plastics, a process called biodegradation (Sivan 2011).
Oxidation via UV-B radiation from the Sun is especially effective at degrading certain polymers, such as Low and High Density Polyethylene (LHDE and HDPE respectively), polypropylene and nylon (Andrady 2011).
Plastic debris on beaches degrades quicker than that found on the surface of the water because the temperature is higher, which accelerates the process (Andrady 2011). By reducing the molecular weight of plastic, degradation renders plastic brittle and thus it fragments (Browne et al. 2007). These newly formed microplastics are then easily carried into the ocean through wind and wave action (Andrady 2011).

2. Plastic resin pellets

Resin pellets are the raw material from which some plastics are made (Cole et al. 2011). These pellets are generally less than 1mm in diametre and may be accidentally released during manufacturing or ocean transport (Ogata et al. 2009).

3. Industrial and domestic cleansers

Both industrial and domestic cleaning products are also a source of microplastics. Amazingly, these contain a range of synthetic compounds from polyethylene and polystyrene, which measure less than 1mm in diametre, to polyester and acrylic, which range between 0.25 and 1.7mm (Brown et al 2007). These particles make their way to wastewater treatment facilities via household or industrial drainage systems. Being so small, they are not caught by the filtration system and thus end up in the ocean (Cole 2011).
According to Fendall and Sewell (2009) polyethylene microplastics are found in the majority of facial cleanser nowadays. These products are often marketed as "micro-beads", "microbead formula" or "micro exfoliates". The authors tested four supermarket-bought facial cleansers for the size of these micro-beads and found that they varied in both size and shape [Figure 1]. The modal size (the most frequent size found) of the micro-beads in three out of the four cleaners was smaller than 100μm (i.e. 0.1mm), suggesting that these are small enough to easily enter the oceans via wastewater treatment facilities.

Figure 1. Microplastics particles in four different supermarket-bought facial cleansers seen under the microscope with 500μm scale bar for reference. Note the different shapes: granular particles (g), ellipses (e) and threads (t) (Fendall and Sewell 2009)

4. Fibres

Synthetic compounds are also found in clothing (Browne et al. 2007). Browne et al (2011) reported that a single item of clothing may shed more than 1900 fibres during one wash cycle. These fibres make they way through to wastewater treatment facilities but similarly to micro-beads, they are too small to be intercepted and thus end up in the ocean.

Next up, the effects of microplastics!

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