Common plastics degradation in coastal environments

Resumen

According to recent studies, only 9% of plastic waste is recycled, 12% incinerated and 79% is accumulated in landfills and dumps or littered in the environment. Once in the environment, plastic waste is degraded through factors working together or in sequence. They cause degradation, defined as a process that causes the material to lose its original properties and fragmentation. In the environment, the main factors influencing the degradation of plastics are the type of polymer, abiotic processes, and biotic factors. Photodegradation, thermal degradation, oxidative degradation, hydrolytic degradation, and mechanical disintegration contribute to abiotic degradation; while biodegradation is a degradation process initiated or propagated by microorganisms such as bacteria, fungi, protozoa, algae, which act by mechanical, chemical and / or enzymatic means. According to the literature reviewed, the environment where plastic waste from anthropogenic sources is subjected to environmental conditions that favor its degradation and the production of microplastics seems to be the coastal one. In order to increase knowledge on the degradation of plastics in this environment, in situ experiments were conducted in sub coastal environments to test the degradation of six commonly used types of plastics and the possible factors responsible for the degradation were investigated. The degradation in the urban air environment has also been studied and compared with that in the coastal environment. The types of plastic used were polystyrene (PS), polypropylene (PP), high density polyethylene (HDPE), low density polyethylene (LDPE), polyethylene terephthalate (PET) and polyvinyl chloride (PVC). The sub-environments selected were a) a lagoon, a natural coastal environment with low hydrodynamic energy, b) a port environment, a heavily anthropized coastal environment with low hydrodynamic energy, and c) a fluvial environment near the mouth, a natural transition environment with low hydrodynamic energy. They have been identified in Italy, in Goro (Ferrara) and in the area of Siracusa. Before proceeding with the experiment that forms the basis of this thesis, I participated in a similar but smaller scale experiment already started along the southern shore of the Choptank River, a tidal sub-estuary of the Chesapeake Bay, Maryland (USA). This experiment was performed using two types of plastics, HDPE and PS, to test how intertidal and subtidal exposure regimes under contrasting hydrodynamic, erosive versus depositional conditions, affected their fragmentation and degradation. Plastics were collected after environmental exposures at 4, 8, and 43 weeks and analyzed for change in mass, algal biofilm growth, and imaged by petrographic and electron microscopy (SEM-EDS). Significant surface erosion was evident on both polymers and was more rapid and more extensive with PS. Degradation of PS was responsive to intensity of hydrodynamic activity, and was greater at intertidal depths, highlighting the critical role played by photo-oxidation in the coastal zone, and suggesting that algal biofilms may slow degradation by playing a photo-protective role. To proceed with the experiment, testing racks were built to allow the exposure of all types of plastics in the form of strips, so that they were individually traceable and equally exposed for each selected environment. Once built, the testing racks were suitably installed in the Goro lagoon and in the “Porto Piccolo” of Siracusa at intertidal and subtidal depths, at the Ciane River in Siracusa, in semi-floating and submerged conditions, and on a terrace of a building at Siracusa. Sampling of the plastic strips was performed after 4, 8, 12, 16, 20, 28 and 36 weeks of exposure in each environment. At each sampling time point, total mass change and mass change after washing with hydrochloric acid were measured; from week 4 to week 28 samples chlorophyll a accumulation were measured. The 12- and 28-week exposure samples were also observed using the scanning electron microscope (SEM) and were subjected to leaching testing. Moreover, plastic strips exposed for 28 weeks in the lagoon and port environments were subjected to dissolution by acid attacks. Subsequently, factorial ANOVA was performed individually for each environment to assess the influence of plastic type, depth zonation, and deployment time, on apparent plastic mass change, biofilm mass accumulation, and Chla accumulation. Furthermore, ANOVA was performed for the data of apparent mass change, fouling mass and chlorophyll a, considering the factors of location, zonation, deployment time and plastic type for the port, lagoon and river environments. The study showed that the rate of degradation and the type of degradation strongly depend on the environment to which the plastics were exposed. Greater UV radiation, higher temperatures and the absence of fouling are the causes that have led to greater degradation in the air environment than in the aquatic environment. Among the aquatic environments tested in this study, the one that caused the greatest degradation is the port one, followed by the lagoon one and finally the river one. In the latter, however, no significant degradation was found. The agents that contributed to the degradation are many: exposure to UV rays, environmental temperature, water salinity, accumulation of fouling, oxygen availability, hydrodynamic energy. Beyond UV radiation, considered the most influential factor on degradation, fouling also played an important effect. In fact, it mainly carried out a role by shielding the plastic from UV radiation. It was also seen how the degradation was influenced by the depth of deployment. In intertidal/semi-floating conditions, in fact, due to the greater UV radiation, the greater thermal stress and the greater hydrodynamic energy, the plastic strips have undergone greater degradation compared to the subtidal/submerged conditions. The type of plastic also affected the rate of degradation. PS was the one most subject to degradation in all environments, showing mostly fragmentation. In the air environment, there was a greater degradation for PP, PET and PVC plastics and minimal degradation for LDPE and HDPE. In the lagoon and port environment, on the other hand, there was a greater degradation for PET and PVC and gradually decreasing for LDPE, HDPE, and PP. In the river environment, even if it was present differentiated by type of plastic, the degradation was not found significantly. Leaching test and dissolutions did not produce relevant results.