Assessment of Environmental Fate of Plastic-Additives: Styrene Oligomers in Polystyrene = 플라스틱 첨가제의 환경거동 연구: 폴리스티렌 내 스티렌올리고머

다국어 초록 (Multilingual Abstract)

Plastics have been massively used worldwide since the 1950s, leading to a large inflow of waste into the environment. The plastic waste could transport by various routes (e.g., runoff, river water., etc.) and accumulate in the marine environment. Plastic wastes contain various hazardous pollutants, particularly additives with high concentrations, making the wastes a vector of environmental pollutants. Thus, understanding the environmental fate of the additives is important in environmental risk assessment. Here, the total environmental fate of styrene oligomers (SOs) was investigated to represent the plastic additives because SOs are inevitably contained in polystyrene (PS), primarily found in the coastal environment. In addition, SOs are the xenoestrogens and the indicators of PS, making the importance of their investigations. Recently, SOs are concerned as environmental pollutants, because SOs are persistent in beach sediment. However, recent investigations on SOs are mostly focused on their environmental monitoring, causing a limited understanding of their life history to understand their total environmental fate. Therefore, present studies were targeted to investigate the total life history of SOs, including their source, transport (including discard from the source and transport in the environment), and environmental distribution.
In order to understand the life history of SOs, it is important to investigate their sources for developing the database. As the previous studies reported, environmental SOs could be classified into two types: SOs contained in PS particles and leached SOs, indicating their initial conditions are total SOs content (TC) and leachable SOs content (LC), respectively. Thus, the TC and LC of SOs in various PS products were determined in the first study (chapter 2). For the measurement, various PS products of three forms, including expanded polystyrene (EPS), extruded polystyrene (XPS), and solid PS, were collected from Incheon, South Korea. The investigation collected various usage PS products of three forms, including expanded PS (EPS), extruded PS foam (XPS), and solid PS. Methylene chloride and n-hexane were selected to optionally extract TC and LC of SOs in PS products, respectively, based on the Hansen solubility parameters. Both the results of TC and LC showed that SOs levels and compositions in EPS are different from those in XPS and solid PS, resulting from the different manufacturing methods of EPS to other PS. The LC of SOs accounted for 31.6 ± 11.2% of TC in EPS, 84.2 ± 7.8% in XPS, and 70.1 ± 22.6% in solid PS. As a porous material, XPS showed the highest leaching potential, followed by solid PS. The lowest leachable level was observed in EPS, although its high porosity. It could be because massive leachable SOs had been released from EPS during the expanded process. Mostly, the LC in individual PS products was significantly lower than TC. Thus, the estimated result might be overestimated if using TC of SOs in their leaching study or risk assessment. In addition, based on SOs in beach sediment being primarily affected by PS debris, the indicate-ability of SOs to PS debris for quantitative analysis was likely confirmed.
It is known that environmental SOs contaminations are affected by their leaching from PS debris, and only the leached SOs were bioavailable fractions in the environment. Due to the lack of understanding, the leaching kinetics of targeted SOs from PS to artificial seawater was investigated in the second study (chapter 3). Based on the Biot number (Bi), the leaching of SOs from PS is estimated to be limited by internal diffusion. Thus, the internal diffusion coefficient in the model was measured by laboratory experiments. The experimental diffusion ratio of SOs could be well-fitted by a linear curve, indicating the diffusion coefficient was well-explained with calculation. Then, the leaching kinetics of SOs from PS to seawater was modeled using the measured diffusion coefficient and other estimated parameters. The leaching model of SOs from PS to seawater was verified with leaching experiments. It showed that the measured mass fraction remaining at the PS phase was similar to that predicted in the model simulation with an assumed single characteristic length, indicating it was useful to assign a single length dimension in the desorption model even for PS pellets. In addition, the leaching half-life of SOs from PS to seawater was estimated using the model, showing the leaching rates of styrene dimers were very slow while trimers almost could not be leached.
Although the leaching of SOs could be estimated with the model in the above study, inevitable uncertainties remain in environmental simulation due to the effect of environmental factors, particularly weathering. Thus, the effect of UV weathering, the most important environmental weathering, on the SOs leaching from PS was evaluated by laboratory experiment in the third study (chapter 4). For simulating the UV weathering in land and seawater, PS pellets were treated with a xenon lamp in air and seawater conditions over 6 months. Then, the surface texture and oxidants of weathered PS pellets were determined by SEM, XPS, and FTIR analysis. The results showed that the weathered PS pellets with air and seawater conditions varied with UV exposure, indicating the change of parameters in the leaching model, such as radios, diffusion coefficient, and partition coefficient between the pellets and seawater. However, the remaining fraction of SOs in the PS phase measured in the leaching experiment showed that those of weathered PS were similar to fresh PS pellet. It was likely because the weathered fraction of the pellet only accounted for under 0.1% of the total particle in this experiment. Few effects of UV weathering on the low leaching rate of SOs were not consistent with the high concentration of SOs determined in previous studies (Hong et al., 2016; Lee et al., 2017; Yoon et al., 2017; Tian et al., 2020), likely resulting from the micro- or nano-size PS particles generated in weathering. After weathering experiment, only the parent plastics were investigated in this study, although the leaching rates of SOs from small size particles were magnitude orders higher than the parent pellet. It was estimated that each time the particle size is halved, the leaching rate increases by approximately 3 times. Overall, the weathering may increase SOs leaching due to the generated micro-particles, although the parent particles were almost unaffected. In addition, environmental SOs might be derived from the micro- or nano-size size PS, particularly the EPS considering it was easily fragmented into pieces by weathering.
SOs was in concern as environmental pollutants recently, and were reported at high concentrations in the coastal environment. Although the environmental SOs were suspected to be derived from PS debris, it still not clearly investigated. Therefore, the final study investigated the environmental origin and fate of SOs with their environmental monitoring (chapter 5). For the objective, sediment samples were collected around the Shihwa Lake, including four regions: salt marsh, offshore, lake, and urban creeks. The sampling sites include suspected sources of SOs, where PS debris may be accumulated, and the downstream regions. The measured SOs were highest in creaks upstream of the estuary, confirming the suspected sources, such as urban regions (e.g., industry and resident regions) and landfills. Interestingly, beach sediment, which was reported as the source of SOs, showed lower SOs concentrations than benthic sediment, indicating beach could not be their main source. Furthermore, the EPS buoy particles and their leachate were investigated to identify the origins of environmental SOs. The measured composition patterns of SOs in beach sediment were similar to EPS particles, while those in benthic sediment were similar to SOs in leachate. This result implies that only the beach sediment can be directly affected by EPS debris accumulated on the beach as their SO-containing particles are embedded in the sediment. In contrast, the SOs in benthic sediment may be affected by the continuous input of leached SOs from PS products/waste inland. On the other hand, the leached SOs may transport through rivers, and diluted SOs after leaching is caused by water mixing, sorption/desorption, degradation, and evaporation. Overall, the results imply that caution should be taken in assessing the ecological risk of plastic additives as their bioavailable fraction can be erroneously overestimated if plastic particles are included in the analyzed samples.
Based on the above studies, the composition patterns of SOs in their lifetime steps, including source, leaching matrix (seawater), and sediment (which accumulate SOs in the environment). The results rejected the well-known suspicion that the composition patterns of chemicals in the environmental matrix must be similar to their sources. In addition, the leached SOs from PS into the environment could be estimated by the developed model. However, uncertainties inevitably remained because the model was predicted by the assumed parameters (e.g., particle size, PS form, the thickness of diffusion boundary layer, etc.). In addition, since only leached SOs are bioavailable, the results imply using ingested total SOs in risk assessment may overestimate the ecological risk because part of them can be the fraction contained in PS. Finally, the result of the source investigation confirmed that SOs in beach sediment could quantitatively indicate PS debris, referring to the reported data. Therefore, the results obtained in this study are very useful in source appointment of environmental SOs, their risk assessment, and developing PS quantification method.

목차 (Table of Contents)

• Chapter 1. General introduction
• 1.1 Research Background 1
• 1.2 Objective 5
• Chapter 2. Profiles of SOs in PS product
• 2.1 Introduction 10
• 2.2 Material and methods 12
• 2.2.1 Sample collection 12
• 2.2.2 Analysis of SOs in PS products 15
• 2.2.3 Quality assurance/quality control (QA/QC) 17
• 2.2.4 Statistical analysis 19
• 2.3 Results 20
• 2.3.1 Total content and leachable content of SOs in PS products 20
• 2.3.2 SOs isomer patterns in PS 26
• 2.3.3 Comparison between total and leachable content of SOs in PS 30
• 2.3.4 Classification of PS products by composition patterns 32
• 2.3.5 HBCD in PS products 34
• 2.4 Discussion 35
• 2.4.1 Environmental implication on SOs 35
• 2.4.2 Preliminary quantification method of PS debris with SOs 38
• 2.5 Conclusion 40
• Chapter 3. Leaching kinetics of SOs from PS to seawater
• 3.1 Introduction 42
• 3.2 Material and methods 43
• 3.3.1 Materials and chemicals 43
• 3.3.2 Film stacking method in measurement of diffusion coefficient 44
• 3.3.3 Leaching experiment of SOs from PS pellet to artificial seawater 46
• 3.3.4 Leaching kinetics model 47
• 3.3 Results and discussion 49
• 3.3.1 Diffusion coefficient of SOs in PS film 49
• 3.3.2 Simulation of SOs leaching 53
• 3.3.3 Validation of SOs leaching model 58
• 3.4 Environmental implications of SOs leaching model 60
• Chapter 4. Effect of PS weathering on SOs leaching from PS to seawater
• 4.1 Introduction 61
• 4.2 Material and methods 63
• 4.3.1 Sample collection and experimental setting 63
• 4.3.2 Characteristics of weathered PS pellets 64
• 4.3.3 Leaching experiment of SOs from fresh/weathered PS to artificial seawater 65
• 4.3 Results and discussion 66
• 4.3.1 PS weathering under UV 66
• 4.3.2 SOs leaching during PS weathering 75
• 4.4 Implication on SOs leaching in the environment 78
• Chapter 5. Environmental Distribution of Styrene Oligomers (SOs) Coupled with Their Source Characteristics: Tracing the Origin of SOs in the Environment
• 5.1 Introduction 81
• 5.2 Material and methods 84
• 5.2.1 Study area and sampling for sediment 84
• 5.2.2 Analysis of chemicals in sediment 87
• 5.2.3 Determination of SOs in EPS particle and its leachate 89
• 5.2.4 Quality assurance/quality control (QA/QC) 90
• 5.2.5 Statistical analysis 92
• 5.3 Results 93
• 5.3.1 Occurrence of SOs in sediment 93
• 5.3.2 Spatial dilution of sedimentary SOs 100
• 5.3.3 Change of composition profile with distance 102
• 5.3.4 SOs in EPS buoy particles and their leachates 104
• 5.3.5 Principle component analysis (PCA) to identify SO origin 106
• 5.3.5 Index for SO source characterization 108
• 5.4 Discussion 110
• 5.4.1 Potential sources of SOs 112
• 5.4.2 Environmental dynamics of SOs 114
• 5.4.3 Implications to environmental risk assessment of additives 115
• 5.5 Conclusion 117
• Chapter 6. Environmental implications of this study
• 6.1 Source identification based on life history of SOs during source, releasement, and environment 119
• 6.2 SOs leaching inventory 126
• 6.3 Implications on SOs used as the indicator of PS 127