2. Discussion, Environmental Considerations, and Challenges
The literature coverage of this review study certainly evidenced the fact of MPs’ release into water resources, mainly reported in stormwaters and human-related activities, including sport and recreational activities. In recent years, global research has focused on MPs in marine ecosystems, but data on presence, monitoring, and assessment in freshwater environments are still scarce
[16]. This research limitation can closely approach the occurrence, distribution, and chemical composition of MP pollution, mainly in European ponds, aquifers, and water resources
[16]. In this research context, a systematic flow-chart of the presence of MPs in surface waters through land and sea origins is presented in
Figure 3.
Figure 3. Flow-chart of the presence of microplastics in surface waters trough land and sea origins. Source: Modified and enhanced from Galafassi et al.
[15] (p. 133499).
Figure 3 collectively represents that critical and challenging issues for researchers are the conceptualization of MP as a complex, dynamic mixture of land-, sea-, and freshwater-based sources. In the relevant literature, it was shown that polymers and additives, to which organic material and contaminants can successively bind to form an ecocorona, can increase the density and surface charge of particles and change their bioavailability and toxicity. However, there exists a major research gap on ecocorona formulation and in-field applicability in natural water resources
[17]. It is also noteworthy that chronic exposure to MPs is rarely lethal, but it adversely affects individual animals, since MPs reduce food and deplete energy stores, with knock-on effects for fecundity and growth. Therefore, it is important for researchers to discover and comprehend the exact ecological processes that affect altered behaviors, bioturbation, and behavioral changes of carbon flux to the deep ocean
[17].
There is growing evidence that MP contamination is extended even to freshwater ecosystems, a fact that can lead future studies and risk assessments
[7]. Moreover, reporting MPs in freshwater and other water sources suggests that the mechanism of secondary MPs can be delivered by diffuse sources of pollution, especially among rivers and other water bodies of circulating currents, no stagnant waters. Secondary MPs sources in these types of flowing systems are determining the food chain through their intake by fauna, thus, consecutive desorption, fate, and risks around world deserve further research attention both in field and laboratory studies
[5][10]. These studies should explore the ways in which MPs are ingested by freshwater fish and, then, evidencing those factors that influence ingestion and exposure hazardous matter to fish and marine biota
[6].
From a methodological point of view, MPs’ identification, comparison of procedures, limitations, and applicability of supplementary types of analysis such as FTIR, Raman spectroscopy, and thermo-analytical methods are recommended
[9]. In this respect, future research on vibrational spectroscopy for MP detection is anticipated to minimize misidentification of the potential MPs in the environment. The accuracy and the efficiency of natural sampling are factors of utmost importance of the quantitative analytical procedure, rather than the (decreasing) analysis time efficiency. Moreover, it is analytically challenging to solidify the accuracy of identification of potential MPs traces by FTIR and Raman by the type and size of MPs being priority over decreasing analysis time, while considering a 25 mm filter area taking over 7 h to analyze
[9]. There is a wider possible application of FPA-micro-FTIR and it has a feasible application to water and wastewater treatment. Analytical improvements contain the tracing capability, the accuracy, and the time efficiency of FTIR, while electron-multiplying charged coupled device detector (EM CCD) showed a low signal to noise ratio
[9].
Advancements of analytical techniques can be further recommended for degrading (by weathering) MPs to be collected at spectral libraries, enabling the identification and the quantification of such degraded MPs in samples analyzed. Moreover, IR and Raman analysis can be designed and developed for the better preparation of samples, since minimizing the chemical modification of the MPs samples can cause a misinterpretation by the IR spectra
[9]. Another analytical challenge of complementary to μ-Raman spectroscopy is that Py-GC/MS pigment, containing particles, fibers, and sea sediment particles, are all identified as plastics
[18]. From a technical point of view, the foundation of systematic protocols for MPs analysis is also suggested
[9], focusing on: (a) developing water treatment strategies, (b) setting effective limits for MPs, (c) making thorough conventions of what types of water management are linked to what types of MPs, which are both (water- and MPs- types) characterized as hazardous materials
[19].
Another research orientation should provide a better understanding of the MPs-carriers sorption mechanism as well as the desorption behavior under different environmental conditions in water resources
[20]. It is noteworthy that water sensing devices can support clean and safe water bodies
[21], while the adsorption performance of heavy metals by polymeric MPs can better simulate the surface attachment models developed at different kinds of pollutants
[20].
Polymeric MPs can be proven as indicators of emerging environmental pollution, acting as carriers for bacterial colonization and propagation of particularly harmful microorganisms, leading to ecological risks attributed due to high stability, pathogenicity, and stress tolerance of the bacterial communities on MPs
[22]. Therefore, new insights can pave the way for understanding bacterial dynamics on polymeric MPs in urban water environments
[23]. However, future research should confront MPs pollution with susceptibility regarding MPs’ pollution in urban wetlands, stressing out the ubiquitous nature of the prevailing MP fragments, indicating that plastic litter degradation plays a significant role of MPs’ sourcing in urban environments and industrial areas
[11].
It is also imperative that experimental results should be confirmed by model-based studies, investigating the extent to which MP concentrations are negligible, or not, for the overall pollutant uptake of freshwater indicators, such as zooplankton species, with water as an additional uptake pathway
[24]. Such research paths can reveal the contradictions between the level and the severity of MPs presented in water sources, thus, encouraging future studies to:
- (a)
-
further and fully investigate how MPs are considered as dominant anthropogenic pollutants of ecological risk,
- (b)
-
support the primary scope of science and society in tackling such a global environmental issue in the future
[17],
- (c)
-
investigate the ways in which MPs pollution and sediment resuspension in shallow lakes are affecting climate change, eutrophication, and resuspension
[25][26][27], emphasizing the role of MPs in intensifying the two environmental effects of climate change and eutrophication
[28][29][30][31],
- (d)
-
reinvent plastics production under the environmental considerations and the social provisions for radical modes of eco-design plastics production, biodegradable plastics production, as well as a circular thinking of manufacturing production, making the used plastic products able to undergo a second round of use after recovering and recycling. Moreover, legislative framework updating and WWTPs’ adaptation to the aforementioned directions should be shown to be vital tools to endorse those safety regulations of MP pollution decrease in the contexts of circular economy and the employment of effective practices to control the plastic waste crisis
[32][33].