Microplastics are small and barely visible pieces of plastic that are between 0.1 µm and 5 mm in size, and known to pollute the environment through man-made activities.
They have been widely studied and are known to be products of fragmentation of large pieces of plastics that have deteriorated in quality after long periods of exposure and shedding.
They cause a lot of pollution, and have grabbed global attention as one of the major causes of marine pollution which is due to exponential increases in production of plastics for beneficial uses.
Coppock et al. (2017) cited several studies that reported the following statistics which are related to production and deposition of microplastics:
• within the past 75 years, plastic production drastically increased from 1.5 million tonnes/year to 322 million tonnes/year, worldwide.
• in the year 2010 alone, between 4 million and 12 million tonnes of plastic were estimated to have been deposited in the marine environment from land-based activities.
• there have been reports of widespread debris of microplastics in deep seas, open oceans, at the poles, and at shorelines in many parts of the world.
Also, Miller et al. (2017) cited several studies which reported the following:
• in the year 2015, over 300 million metric tonnes of petroleum-based plastics were produced worldwide. Most of this quantity contained 6 main types of plastics: polyethylene terephthalate, polypropylene, polyethylene, polyvinyl chloride, polyurethane, and polystyrene.
• the rate of annual production of plastics would lead to a cumulative production of 33 billion metric tons by the year 2050; this implies that there would be mass production and increase in the quantity of microplastics along shorelines, and in oceans.
• between 4.8 and 12.7 metric tons of plastic debris is being deposited in oceans each year; this quantity of pollution creates the possibility for organic contaminants such as polycyclic aromatic hydrocarbons, carcinogenic polychlorinated biphenyls, polybrominated diphenyl ethers and toxic metals to enter the marine food web.
Negative impact of microplastics on marine environment
Among different categories (megaplastics, macroplastics, mesoplastics, microplastics and nanoplastics) of microplastics that cause marine plastic pollution, microplastics are of major concern because they are readily and easily consumed by organisms living in marine environments. In fact, microplastics can be consumed by mussels, crabs, oysters, fish, and even humans.
Ingestion of microplastics has been found to have negative effects on food intake, and decrease the amount of energy needed for reproduction and growth of aquatic species. Microplastics are sources of chemical contamination because they contain additives and plasticizers added during production of plastics.
Studies have revealed that plastic litter/debris contains harmful pollutants that are about a million times more than pollutants in seawater; the intake/digestion of this could result in negative impacts on marine life.
Brief review of existing separation techniques for extraction of microplastics from marine environments
Several researches showed that the principle of density flotation is commonly employed in separating less dense plastic polymers from denser sediment particles, and different types of high density salt solutions could be employed in extracting microplastics from marine sediments; however, existing methods have been known to show a number of drawbacks:
• they are expensive.
• their extraction efficiencies are low.
• they are not compatible with sediments that are very fine.
• they are complex and inhibited by impracticalities or inefficiencies.
One major challenge has been the unsettling of settled sediments which occurs at periods when floating microplastics are being removed. Typically, this has led to low extraction efficiencies and caused extractions to be repeated. In some other methods, several steps need to be taken in order to extract microplastics; such steps might require the use of equipment that are only suitable for extracting coarse sediments.
An example of a method that could require several steps is the elutriation step which allows clogging to occur whenever very fine sediments are used. Owing to this reason, it is important to promote a cheap and convenient method that uses zinc chloride (ZnCl2), as highlighted in the study carried out by Coppock et al. (2017).
Proposed extraction method, and stages employed
Coppock et al. (2017) constructed a prototype of the popular Munich Plastic Sediment Separator (MPSS). Typically, the original MPSS isolates microplastics above a shut-off valve and achieves 95.5% recovery rates on microplastics that are less than 1 mm in size. The typical MPSS is 1.75 meters high, made of stainless steel, and was initially designed to take large quantities of sediment (6 kg); however, MPSS is quite expensive to produce and is less feasible and portable, especially when numerous replicates of small sediment samples have to be used.
In order to cut down expenditure and achieve equal or higher recovery rates, a cheap, small-scale and portable microplastic extraction unit was constructed and designed to mirror/mimic the original MPSS. This prototype was used to assess the viability and financial cost involved in extracting microplastics using 3 high-density salt solutions: sodium chloride (NaCl), sodium iodide (NaI) and zinc chloride (ZnCl2).
It was observed that this method is less expensive, and can be used on a wide range of sediment types to extract microplastics from marine sediment samples. Before assessments and comparisons were made between the 3 methods, and a conclusion was arrived at—that the study involving zinc chloride (ZnCl2) is cheap and can be used for optimal extraction of microplastics—the following stages were employed in the method highlighted by Coppock et al. (2017):
(1) Three separate media were prepared for floatation.
(2) A prototype sediment-microplastic isolation (SMI) unit was constructed.
(3) The SMI unit was cleaned, purged and primed.
(4) Extraction of microplastics was carried out on 3 different sediment samples: each contained NaCl, NaI and ZnCl2, respectively.
After construction and application of the prototype unit using the stated method, the following were observed:
(1) Although saturated sodium chloride (NaCl) solution is cheap and adequate enough to extract low density plastics from marine sediments, employing it in the constructed unit wouldn’t help to estimate the exact quantity of abundant microplastics found in a marine environment containing microplastics that are highly dense.
(2) Although sodium iodide (NaI) solution can be used to prepare higher density sediment samples than NaCl, it is the most expensive option for preparing high density samples; based on this, it was eliminated from the sediment trial samples prepared for extraction in the study.
(3) Zinc chloride (ZnCl2) solution is the cheapest and most inexpensive method that can be used to prepare high density samples (greater than 2 g/cm3) for extraction. For this reason, ZnCl2 was used to quantify microplastics in a number of previous studies.
It can be observed that for high density samples involving NaI, fine sediments in the SMI method were in suspension; as a result, the method was deemed to be inefficient for optimal extraction of microplastics.
In comparison with sodium chloride (NaCl)—which cannot be used efficiently to extract highly dense microplastics—and sodium iodide (NaI)—which costs quite much to use—results show that zinc chloride (ZnCl2) is the cheapest and most appropriate solution that can be employed in floating microplastics when using SMI unit to make extractions from marine sediments.
In summary, the method involving ZnCl2 is the best one for extracting microplastics because it is cheap, convenient and can recover up to 95.8% in a single step—a bit higher than the recovery rate for the original MPSS (which is 95.5%).
Also, the method involving the use of ZnCl2 would likely gain universal acceptance because it can fit into budgets. With further researches in this area, it might be possible to obtain 100% extraction of microplastics in the near future.