Organic pollutants adsorbed on microplastics: Analytical methodologies and occurrence in oceans
Graphical abstract
Introduction
Plastic pollution has become a global problem because this debris appears worldwide. Approximately three quarters of rubbish in seas and oceans are plastic [1]. If the size of this plastic waste is smaller than 5 mm, it is considered microplastics (MPs). Primary MPs can enter oceans directly through wastewater discharges or be transported by rainwater or wind from land [2]. Secondary MPs are also generated in oceans by macroplastics fragmentation.
Microplastics can be removed from water, but the costs to perform such elimination today are mostly unaffordable [3]. Given their size, they can be ingested by aquatic organisms (or can hinder their breathing pathways) and enter the food chain [4]. Almost 700 aquatic species are known to be affected by MPs [5]. This pollution type is not limited to oceans as other environmental matrices can also become contaminated. MPs have been found in freshwater, and also in solid samples like sediment, compost, sewage sludge or soil irrigated with reclaimed water from wastewater treatment plants [6]. However, most samples have been taken on beaches and from oceans, which is the reason why we focused our review on the MPs sampled in these places.
Another cause of concern is related to MPs’ capacity to act as vector of other contaminants and to transport them across the planet. Pollutants associated with MPs can be divided into two groups: a) additives and polymeric raw materials that originate from plastics; b) chemicals adsorbed from surrounding media [7].
There are mainly five types of plastics that are frequently detected in the aquatic environment: polyethylene (PE); polystyrene (PS); polypropylene (PP); polyamides (PA); polyvinyl chloride (PVC). They all have different surface characteristics and varying degrees of crystallinity. Depending on the types of MPs, they can adsorb chemical compounds by different processes controlled mainly by the physicochemical properties of both the MP and the pollutant. These processes are affected by the temperature and salinity in the environment [4]. Several authors have concluded that MPs are capable of adsorbing different types of hydrophobic organic pollutants, such as polycyclic aromatic hydrocarbons (PAHs), hexachlorocyclohexanes (HCHs), polychlorinated benzenes (PCBs) [8], perfluoroalkyl substances (PFASs) [9], and even bisphenol analogues (BPAs) [10], which exhibit polarities (expressed as log Kow) that range from 3.3 to 9 in most cases. Although the majority of studies have focused on the presence of priority organic pollutants on MPs, recently some studies reveal that some emerging pollutants in water can also be adsorbed by MPs [11].
The adsorption process depends on the polymer type, characteristics such as density and crystallinity, the surrounding environment, and the pollutants that are present. Moreover, this process occurs through both physico-chemical adsorption and pore-filling.
As aquatic organisms often confuse MPs with food, the adsorbed pollutants could be transferred to tissues after digestion processes and enable their bioaccumulation in the trophic chain. Batel et al. studied the transfer of benzo[a]pyrene adsorbed to MPs from Artemia nauplii to zebrafish (Danio rerio) to study a possible transfer through the trophic chain. These authors showed that the compound can be desorbed in fish intestines [12]. A study by Wardrop et al. demonstrated that fish (Melanotaenia fluviatilis) that eat food with microbeads spiked with polybrominated diphenyl ethers (PBDEs) exhibited significantly higher concentrations of (PBDEs) [13]. In the same context, Rochman et al. studied the exposure of Oryzias latipes to MPs with adsorbed chemicals. They ran three experiments in which they fed fish fodder without plastics, virgin plastics or spiked with PCBs, PAHs and PBDEs. An altered gene expression was observed in the male fish fed spiked MPs, and also in the female fish fed virgin and spiked MPs [14].
In another study, Bakir et al. investigated the ability of PVC and PE to adsorb and desorb 14C-DDT, 14C-phenanthrene, 14C-perfluorooctanoic acid and 14C-di-2-ethylhexyl phthalate at different pHs and temperatures to simulate the conditions inside the stomach of fish. It is noteworthy that, unlike other molecules present in MPs only by the result of the adsorption process, phthalates are additives that are already present in plastic materials. The authors found that desorption under these conditions was up to 30-fold higher than in seawater [15]. However, the possible extent of bioaccumulation at higher trophic chain levels remains unknown [5].
To date, the bulk of literature reports and reviews has focused on studying the presence, characteristics, quantity, distribution, sampling techniques and biological risks of MPs [3,5,[16], [17], [18]]. Several papers report the sorption behaviour of pollutants on MPs under controlled laboratory conditions [19,20].
However as MPs are referred to as contamination vectors, it is necessary to establish suitable procedures to determine the contaminants adsorbed to MPs and to achieve their reliable quantification.
This paper critically reviewed the analytical methodologies developed and applied to extract and determine adsorbed organic compounds on MPs. This work summarises current knowledge from papers published mainly after 2010 and indexed by Scopus. Fig. 1 clearly illustrates the rising number of papers on pollutants adsorbed on MPs. However, our work focuses on the analytical methodologies applied to determine the organic pollutants adsorbed on MPs.
According to the information reported in the reviewed papers, the objective of many of them was to study the occurrence of pollutants more qualitatively, or their adsorption, distribution, etc., but they are not analytical works. This is why extraction and determination procedures are not well documented in many of them. This review shows this lack of information, and researchers are encouraged to verify the analytical parameters of their methods before providing results. We found that information on the procedures in some papers was generally lacking, and we attempted to compare the different methodologies applied as best as possible.
Finally, the worldwide occurrence and distribution of organic compounds found on MPs in oceans is reviewed and the results obtained from different geographical areas are compared.
Section snippets
Analytical methodologies
The most frequently analytical methodologies to have been applied for many studied compounds adsorbed on MPs are presented in Table 1.
Occurrence of organic pollutants on MPs
Microplastic debris is a recognised global environmental contamination problem. Its presence has been reported close to densely populated areas, in remote regions and in different marine environments. MPs can be transported over very long distances by the action of wind and ocean currents. Given their physico-chemical characteristics, MPs can adsorb the organic compounds present in the environment and, therefore, may act as a pollution vector of these chemicals for marine organisms. As the role
Conclusions
After reviewing studies about the adsorption of pollutants to MPs, we found that most works focus on priority organic pollutants like PCBs, DDTs, HCHs and PAHs, better known as environmental contaminants. However, there is almost no data available on contaminants of emerging concern, such as pharmaceuticals and personal care products (PCPs).
For the extraction of pollutants from MPs, conventional liquid–solid extraction techniques are usually preferred, although some new methods are also
Conflict of interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work has been funded by Project MAC 2/1.1ª/265 (IMPLAMAC) Cooperation Programme INTERREG MAC 2014-20. Sergio Santana-Viera and Sarah Montesdeoca-Esponda thank the University of Las Palmas de Gran Canaria (Spain) for their grants.
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