The Study of Microplastics (MPs) Pollution in Hong Kong's Rivers

Abstract

The global prevalence of microplastic (MP) pollution in aquatic ecosystems, from coastal regions to the open sea, has garnered increasing attention. These MPs invariably find their destination in the oceans. Initial studies predominantly focused on the spatial distribution of MPs within marine environments. Notably, Hong Kong has emerged as a hotspot for MP pollution. The western waters of Hong Kong experiences significant MP discharge from mainland China, whereas the eastern waters exhibits high pollution levels due to local discharge.

Rivers serve as the primary conduits for transporting MPs from land to the ocean, carrying thousands of tons into the ocean annually. The dynamics and mechanisms of this transportation are crucial. However, limited studies examine MP pollution in Hong Kong’s rivers. A comprehensive study was conducted to address this gap The first phase of this study assessed the MP pollution in Hong Kong’s rivers during the dry season. Four major rivers spanning various regions of Hong Kong were selected. Three of them are situated in urban areas, namely Shing Mun River (SM), Lam Tsuen River (LT), and Tuen Mun River (TM), whereas Silver River (SR) is in a rural area. Significantly higher suspected microplastic (SMP) abundance was found in TM (53.80 ± 20.67 items/L) compared to the other rivers (Tukey test, p-value <0.05), followed by SM (24.57 ± 18.49 items/L), LT (22.58 ± 24.85 items/L), and SR (15.34 ± 11.89 items/L). A higher level of SMP was found downstream in nontidal rivers (LT and SR), but it was evenly distributed in tidal rivers (TM and SM). Furthermore, the SMP abundance was positively correlated with the built-area ratio in the dry season (r² = 0.6779, p < 0.001).

The second study investigated the impact of rainfall on the MPs pollution in the four selected rivers. Sampling was conducted during the wet season, and the MP abundance was compared to the levels recorded during the dry season. Similar SMP pollution levels were recorded for both seasons (dry season: 29.53 ± 24.35 items/L; wet season: 27.95 ± 28.86 items/L). The tidal river SM experienced a significantly higher SMP abundance during the wet season, while TM observed a lower SMP abundance (Tukey test, p < 0.05). Nontidal rivers LT and SR showed higher SMP abundances during the dry season compared to the wet season. Among them, SM and LT had significantly higher SMP abundances downstream (Tukey test, p-value < 0.05). The importance of the built-area ratio diminished under the influence of rainfall (r² = 0.3084, p < 0.05). The study has demonstrated the significance of rainfall in transferring MPs from rivers to the ocean.

This study also examined the influence of tidal movement on the distribution of MPs in Shing Mun River (SM), a typical tidal river in Hong Kong. Sampling was conducted upstream and downstream in SM during the dry season. The abundance and composition of MPs, including shape, colour, and polymer type, were compared in different phases of a tidal cycle. A total of 1,728 particles were counted in this study, with only 582 particles confirmed as MPs (30.56%) via micro-FTIR analysis. Among them, MPs consisted of fibres (29.76%), fragments (60.09%) and microbeads (0.58%) only. The MP abundance was significantly higher downstream (14.26 ± 11.14 items/L) compared upstream (5.28 ± 4.53 items/L) throughout all the tidal stages, but not correlated with the water level. The saltwater intrusion during flood tides affected the forms but not the abundance of MPs. Downstream (SM6) obtained a slightly higher MP abundance during ebb tide (17.03 ± 13.05 items/L) than flood tide (12.41 ± 9.62 items/L), whereas upstream (SM2) obtained a slightly higher MP abundance during flood tide (5.79 ± 4.97 items/L) than ebb tide (4.46 ± 3.76 items/L). Non-metric multidimensional scaling (nMDS) analysis showed separate sources of MPs, which differed in shape, colour and chemical composition, in SM2 and downstream during tidal changes. During the early flood tide, SM2 was affected by the harbour and the river, with seawater intruding upstream, while SM6 was affected by the harbour only. The MP composition changed by the saltwater intrusion, with no specific distribution pattern in shape composition during the mixing process. During the beginning of the ebb tide, saltwater flushed back towards the harbour, with SM2 being less influenced by the sea when compared with SM6. By the end of the ebb tide, the shape composition of MPs in both locations was affected mainly by the river. Consequently, the MP shape composition at the end of the ebb tide was different from the beginning of the flood tide in both locations. The flushing direction of MPs corresponded with the tidal flow, causing MPs to remain in the tidal river.

While intensive research has focused on understanding the dynamics of MPs, there remains a dearth of information regarding their association with pollutants. Studies have demonstrated the harmful effects of MPs on aquatic organisms by obstructing food and nutrient ingestion. Moreover, MPs have been found to absorb numerous organic and inorganic pollutants, such as polyaromatic hydrocarbons (PAHs), polychlorinated biphenyl (PCBs), organochlorinated pesticides (OCPs) and heavy metals. Per- and polyfluoroalkyl substances (PFASs) possess hydrophobic fluorinated backbone and hydrophilic terminal function group, exhibiting varying adsorption mechanisms in laboratory and field settings. The fourth part of the thesis explored the behaviour of PFASs associated with MPs, focusing on the highly polluted area of Tolo Harbour. Four sites in the Tolo region, namely Sam Mun Tsai (SMT), Wu Kai Sha (WKS), Lai Chi Chong (LCC), and Tap Mun (TM) were selected. SMT and WKS are located in the inner harbour, whereas LCC and TM are in the outer harbour. Sediments and meso-plastics (2 mm to 10 mm) were collected. Three kinds of polymers (PE, PP, and PS) were also deployed in the study areas to absorb PFASs for three months. Additionally, the adsorption of PFASs (5 µg/L) by polymers (PE and PS) was studied in the laboratory for six days. The extraction of PFASs from plastic using 0.1% NH₄OH at 25°C yielded the highest recovery efficiency (ranging from 64.73% - 89.18%). PFASs concentration (PE: 62.74 ± 2.69 ng/g; PS: 73.01 ± 8.33 ng/g) was found to reach equilibrium within 12 hours in laboratory simulations, necessitating further experimentation to elucidate adsorption mechanisms. In sediment samples, SMT (2.08 ± 1.69 ng/g) accumulated the highest ∑PFASs concentration, followed by WKS (1.15 ± 1.46 ng/g) and LCC (0.25 ± 0.16 ng/g) (Tukey test, p-value < 0.05), but not at TM (1.85 ± 1.70 ng/g). Conversely, no significant differences in mesoplastics concentrations were observed between study sites. WKS (19.32 ± 22.98 ng/g) and LCC (19.37 ± 38.34 ng/g) accumulated higher ∑PFASs concentrations than SMT (13.89 ± 9.85 ng/g) and TM (17.93 ± 22.98 ng/g). Field adsorption experiments with three types of polymers reflected temporal variations in ∑PFASs concentrations. For deployed pellets, only PE showed significant differences in the ∑PFAS concentrations between locations. SMT (0.47 ± 0.24 ng/g) and WKS (0.60 ± 0.64 ng/g) showed significantly lower ∑PFAS concentrations than LCC (5.27 ± 7.11 ng/g) and TM (4.81 ± 5.05 ng/g) whereas no significant spatial differences in the ∑PFAS concentrations were found for PP and PS. The sorption of PFASs between polymers (PE, PP, and PS) for all the study sites were also compared. SMT and LCC showed no significant differences in the concentrations of PFASs among the polymers. In contrast, PS showed significantly higher sorption concentrations than PE in WKS whereas PE showed significantly higher sorption concentration than PS in TM.

The final chapter provides a comprehensive overview of MP pollution in Hong Kong rivers. Pollution levels in Hong Kong surpass those in Japan, Korea, various European countries (such as Italy, and the United Kingdom), and the U.S.A. It is similar to China but lower than Vietnam and India. Rainfall transferred MPs downstream for both tidal and nontidal rivers. In tidal rivers, MPs tended to stay in the river rather than being transferred to the ocean in the dry season. Lastly, MPs were proven to adsorb PFASs, with PS posing a higher risk for accumulation. Consistent monitoring of MP pollution in Hong Kong rivers is recommended for the next decades. Since microorganisms and algae adhered to MPs could potentially alter the adsorption mechanisms for pollutants, further investigations are recommended on the interactions between biofilm and the accumulation of pollutants in MPs.

Date of Award	6 Dec 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Siu Gin CHEUNG (Supervisor)