| Περίληψη: | Plastic pollution is one of the most critical environmental threats and a significant scientific issue due to the variety of negative effects. Million tons of plastics come into the oceans and thus more plastics than fish is estimated to be in oceans worldwide by 2050 (by weight). Covering the biggest part of our planet (~70%), oceans as dynamic systems, transferring plastic litter in global scale as well as accepting plastic pollution from other environments such as terrestrial (e.g. via coastal zones), atmospheric, riverine, etc. Due to the variety of conditions such as plastic sinking in the sea floor, its type, size, fragmentation, degradation etc, it is important to better understand the behavior of plastics in the environment, such as interactions with other pollutants, microorganisms, and other parameters (biological, chemical, geological, and physical).
In the current dissertation, the formation of microplastics, the development, and the characteristics of biofilm on plastic debris and the interaction of micro and macroplastics with an organic pollutant (phenanthrene) in the absence and presence of biofilm in the coastal environment were examined combining field and laboratory experiments. In the current investigation, three different sizes of polymers (powder, pellets and strips) were used to examine the interactions with phenanthrene as well as the examination of biofilm formation on plastics in different shapes and sizes; emphasis was given to a new method for characterizing biofilm on plastic surfaces (DRS).
Cracks on plastic bags surfaces collected from coastal zone, as well as fragmentation of an oxo-degradable plastic bag producing several microscopic particles were observed. Plastic bags (polyethylene) can break into microplastics before major chemical modifications can be detected in the synthetic polymers and mechanical alterations take place during the fragmentation process. As a result (a) of the presence of oxo-degradable substances (for instance starch, the adhesive of the synthetic polymer pieces in oxo-degradable plastic bags, which was added during the production stage), starch degradation was observed, but no chemical alteration of the synthetic polymer and (b) surface oxidation caused by chemical degradation was observed – the formation of C–O type functional groups, leading to H–C and C–C bonds breakage which in turn resulted in segmentation of plastic bags.
Plastic pellets (raw materials) as well as fragments were identified on a second coastal zone. Microplastics which can be observed on coastal zone depend on land use. For instance plastic pellets and fragments on both beaches in Salamina Island were made of PE, as well as EPS originating from fishing activity. In specific, on Psili Ammos beach, higher PE fragment abundances were observed compared to Kanakia, because of industrial activity in the area. For Psili Ammos, most of the PE fragments were degraded, except of a few fresh PE pellets suggesting input from industrial activity as well as some fresh PE fragments from consumer plastic products and their physical breaking down. All PE samples found in Kanakia were degraded because of older input or input originating from distant sources from this site.
Submicroscopic plastic particles (i.e. ~spheres, fibers etc) were also identified in the virgin powder sample. The virgin LDPE powder agglomerate in larger particles, which had neutral surface charge in suspensions based on Zeta-potential measurements, whereas suspensions with LDPE microplastics and microorganisms also include agglomerates of both LDPE and microorganisms that demonstrate high surface charge.
When the microplastics are smaller than the microorganisms, they can attach on the membranes of the microorganisms, while when they are bigger than the microorganisms, they create a biofilm. Microscopy techniques revealed various species of microorganisms on plastic surfaces.
Fluctuations in DRS peaks were observed, i.e., the biofilm patterns can be illustrated. Peaks likely correspond to formation as a result of a number of species which colonized surfaces corresponding to the time and type of plastic. Biofilm develops in a specific way on plastic surfaces; depending on the plastic material, the aquatic environment as well as the humidity conditions (alterations on functional groups of LDPE samples were observed, based on IR spectra which were independent of aquatic environment); IR spectra indicated fluctuation differences mainly on LDPE [in both samples (LDPE and PET), different transmittance (%) values were observed]. DRS is a useful, simple, non-destructive, and low-cost way of studying biofilm growth in plastic waste, without requiring any special treatment for the samples before the analysis.
The different sizes of the aforementioned plastics were tested to understand the interactions with phenanthrene. The experiments were carried out using the three different size-categories of plastics in the absence and presence of microorganisms. Virgin pellet samples in combination with a powder plastic sample (also raw material) and macroplastics (in strip shape) using also different types of plastics showed different sorption behaviour of phenanthrene (in artificial sea water) in the presence and absence of biofilm.
Effective diffusion rate (De/a2) was calculated higher for the virgin samples than samples in the presence of biofilm; among the two different sizes of the virgin LDPE samples, De/a2 was insignificant different (powder: 3.8x10-13 s-1 and pellet: 6.1x10-13 s-1). Pseudo first-order kinetic for different sizes of the same material (virgin LDPE samples) were calculated at 0.338±0.034, 0.074±0.002 and 0.438±0.108 d-1 for powder, pellet and strip shapes respectively (i.e. strip > powder > pellet); virgin PET samples: strip > pellet. Among strip samples, pseudo first-order kinetic: strips with biofilm > virgin strips. Desorption process was also observed (De/a2: desorption > sorption; pseudo first-order kinetic: desorption > sorption). To conclude, the type and size of plastic were observed to be related to the sorption kinetics as well as the presence of the biofilm does affect the sorption kinetics.
The most important conclusions of this study can be summarized as follows: (a) plastics can break into microplastics before major chemical modifications can be detected in synthetic polymers. (b) the type of microplastics observed on the coastal zone depend on land use, (c) when microplastics are smaller than the microorganisms, they can be attached to the membranes of the microorganisms, whereas when microplastics are bigger than microorganisms, the microorganisms are getting attached and then, create a biofilm; develops in a specific way on plastic surfaces, (d) DRS was proven to be a useful tool to examine biofilm on plastic surfaces without the destruction of the sample, and e) the presence of biofilm does affect the sorption kinetics using phenanthrene as a model of organic pollutant; the type as well as the size of plastic were observed to be related to the sorption kinetics.
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