The evolution and fate of waste plastics in landfills subject to physical and biochemical processes - implications for microplastics

Plastics, renowned for their versatility and extensive applications, play a fundamental role in contemporary living due to their affordability, lightweight composition, and insulating properties. Nonetheless, a substantial volume of plastics becomes plastic waste (PW), with a significant portion finding its way to landfills. Microplastics (MPs), a recently identified group of pollutants with dimensions below 5 mm, raise apprehensions about potential toxicity and their ability to transport other contaminants like heavy metals through adsorption and dissociation mechanisms. The issue of MPs pollution and its environmental impact remains a prominent and pressing subject of discussion. Landfills contain significant amounts of PW and MPs. However, the contributions of various PW fragmentation processes to the quality and quantity of MPs in landfills are unclear. Therefore, the degradation and fragmentation of PW and the generation of secondary MPs in both simulated and real landfills were studied. In this study, both untreated and degraded LDPE and EPS pieces mixed with sand to simulate landfilled MSW, which experienced one-dimensional abiotic compression under vertical stress of 100-800 kPa for 1-300 days. The generated MPs were stained and quantified with a fluorescent microscope. The numbers and masses of the fragmented MPs increase with the increasing compression stress and duration following linear or exponential trends. EPS has a lower stiffness than LDPE, thus generates more MPs under the same compression conditions. Stress-dependent and time-dependent fragmentation mechanisms are distinguished, the former is driven by sand-plastic porosity reduction and the latter is due to microscopic interfacial creep with minimal porosity reduction. Most of the mechanically fragmented MPs have diameters <100 µm. The MPs size distributions follow an established power-law model, which are dependent on stress, duration, porosity reduction, and fragmentation mechanism. Furthermore, a higher quantity of MPs was generated from the degraded LDPE and EPS in the 112-day simulated landfill bioreactors compared to the raw LDPE and EPS under the same compression conditions. these results serve as conservative estimations on long-term MPs generation in real landfills, which provide confirmative and quantitative evidence to support the previous studies reporting the varied MPs abundances and properties within landfills. In addition to mechanical factors, the chemical process and biodegradation in landfills, may cause release of MPs as well, yet the generation and impacts of MPs within landfills remain unclear. In this study, identical compositions of municipal solid waste (MSW) were loaded into five laboratory-scale bioreactors, with three of them supplemented with 5.0 mg of polypropylene (PP) MPs per kg of dry MSW. The significantly slower rates of chemical oxygen demand (COD) decreasing and the rate of total nitrogen (TN) increasing in the leachate samples from the PP MPs added reactors than the control reactors were observed. After 120 days, the average COD values were measured as 33,525 and 38,383 mg/L, while the average TN values were 238 and 143 mg/L, respectively, in the leachate samples of the control and PP MPs added reactors. The abundance of LDPE MPs, EPS MPs, and PP MPs in leachate increased 1.5 to 12 times after the 120-day simulated landfill degradation and exhibited linear increases with time. The average LDPE MPs concentrations in control and PP MPs added reactors after 120 days reached 243 and 427 items/L, respectively. The addition of PP MPs enhanced LDPE degradation, while EPS MPs generation remained unaffected by PP MPs in the simulated biodegradation. Notably, PP MPs underwent degradation during this process, resulting in smaller-sized PP MPs. This study offers compelling direct proof of the increasement and potential risks of MPs in landfills which is crucial for devising effective waste management strategies, minimizing their release. To examine the presence and fate of MPs in real landfill leachate, refuse, and PW, this study investigated across the distinct real landfills. In total, 7 leachate samples and 5 refuse samples from different MSW landfills were investigated. MPs were found in all the samples. The results indicate the identification of over 10 types of MPs in both refuse and leachate samples, with the majority of MPs were fragment-shaped MPs, exhibiting a size of less than 100 μm. The abundance of MPs in refuse and leachate samples ranged from 60 to 1500 items/g and from 50 to 3000 items/L, respectively. The correlations between MPs in refuse and the characteristics of landfill properties such as refuse age, depth, and moisture content were found. The implications of landfill characteristics and the destiny of MPs in diverse conditions were discussed. The study shows that the generation, accumulation, and release of MPs in landfills is a long-term process. Further research on MPs in landfills and the pathways of PW degradation need investigation. Overall, this thesis provides the scientific fundamentals for MPs and PW in landfill conditions, enriches the database of a broader range of MPs in landfills globally, and proposes removal strategies and management strategies associated with landfill sites to mitigate MPs contamination and forecast potential MPs-related pollution.