Sources of Microplastics

A wide variety of polymers are used to make a range of plastic products. Almost all of the common polymers have been recognized as microplastic particles. These include polyamide (PA) (nylon), polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl alcohol (PVA), polyvinyl chloride (PVC), polyethylene terephthalate (PET), etc. However, a single polymer can be used in a variety of applications. Therefore, any information on the type of polymer found as a microplastic particle may not be particularly beneficial in identifying the source of the particle [16]. Microplastics can be divided into two major categories depending upon their potential sources and usage: primary and secondary microplastics.

Primary Microplastics

Primary microplastics are the particles that are manufactured for use as microplastic particles (<5 mm in size). The release of primary microplastics into the environment may occur because of the spillage of pre-production pellets (~4 mm in diameter) or powders (>1 mm in diameter). Primary microplastics are also used as shot-blasting media during the cleaning of softer metals like aluminum and for removing paint or rust from machinery, engines and boat hulls [19,20,21]. The use of exfoliant microbeads (typically 250 pm in diameter) as microplastic scrubbers in cosmetic products like exfoliant hand cleansers and facial scrubs also comprise primary sources of microplastics in the environment [6,16].

Secondary Microplastics

Microplastics that result from environmental fragmentation of larger items of plastic debris such as packaging, ropes and fishing nets are termed secondary microplastics. A combination of physical, chemical or biological processes can reduce the structural integrity of plastic debris over time and lead to fragmentation [19]. The quantiles of secondary microplastics broadly reflect the quantities of larger, identifiable items of plastic debris collected during routine monitoring [16]. A substantial input of fibers from textiles has been found in residues from sewage treatment plants on land [17] and at former sewage sludge dumping grounds in the marine environment [18]. Such fibers may enter the environment as particles that are already microplastic in nature. However, since these were not manufactured as microplastics, they are generally classified as secondary microplastics.

 
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