Investigation of Occurrence, Sources, Transport, and Preliminary Environment Risk of Liquid Crystal Monomers in the South China Sea Area

Abstract

Liquid crystal monomers (LCMs), commonly used in liquid crystal displays (LCDs), have recently been identified as emerging contaminants. Given their structures with cyclohexyl/aromatic rings, LCMs are potentially persistent, bioaccumulative, and toxic substances and may pose threats to aquatic ecosystems. The Pearl River Estuary (PRE) and the surrounding coastal region receive organic contaminants from the Pearl River Delta, a hub for LCD panel manufacturing and e-waste processing. However, data on the pollution status of LCMs in the South China Sea (SCS) is currently lacking, and their occurrence, sources, environmental fate, toxicity, and uptake in marine organisms remain largely unknown. Therefore, this PhD study aimed to (1) investigate the occurrence, spatial distribution, and risk assessment of LCMs in marine sediments in PRE; (2) exam the driving factor, daily mass load, and ecological impacts of dissolved LCMs in stormwater and treated wastewater in Hong Kong; and (3) evaluate the tissue distribution, temporal trends and source of LCMs in two marine cetaceans in SCS, along with their toxicity using dolphin cell lines.

First, I tested the hypothesis that LCMs originating from land-based sources would enter the PRE marine environment and accumulate in sediment. A rapid and robust analytical method using GC-Orbitrap-MS was established to detect the LCMs in marine sediment (Chapter 3). In 45 surface sediment samples from PRE, ten out of 39 target LCMs were detected, with concentrations of ΣLCMs ranging from 0.9 to 31.1 ng/g dry weight (Chapter 4). The spatial distribution showed a gradient from inshore to offshore, indicating land-based origins. The concentrations of ΣLCMs were comparable to other legacy and emerging pollutants in the area. Preliminary risk assessment showed that PVB, PPB, and PCFTB as the top three risk contributors.

In Chapter 4, I found that the highest concentrations of LCMs were detected in sediments near a sewage treatment plant in Hong Kong, suggesting wastewater effluents as a major source. Therefore, dissolved LCMs in stormwater and sewage samples in Hong Kong were investigated to understand their environmental behaviors and risks (Chapter 5). The results showed that contamination of LCMs in stormwaters and sewage effluents was widespread, with a concentration of ΣLCMs ranging from 28.26 ng/L to 99.01 ng/L in stormwater and 42.82 ng/L to 86.18 ng/L in sewage effluent, respectively. Among the detected LCMs, three non-fluorinated compounds, namely PPB, PeCHPrB, and MOPrCHB, were the main dissolved LCMs in stormwater and sewage. The annual input of total LCMs was estimated as 17.2 kg from stormwater and 1,220 kg from sewage effluent discharge. Ecological risks were comparable between stormwater and sewage effluents, with PPB and PeCHPrB posing higher ecological risks than other LCMs. This work also highlighted that composition profiles of LCMs in stormwater were region-specific and influenced by land use patterns, while sewage effluent levels of LCMs were affected by population size. Effective stormwater drainage design and connections to sewage treatment plants are necessary to mitigate ecological risks.

LCMs were found to be abundant in the marine sediment, sewage effluents, and surface runoff in the SCS area, a resident habitat for nationally protected marine cetaceans, like the Indo-Pacific humpback dolphin (Sousa chinensis). The existence and ecological risk of LCMs in marine cetaceans are still unknown. A comprehensive study was, therefore, conducted to investigate the bioaccumulation of LCMs in tissues of two resident cetacean species in the SCS (Chapter 6). Herein, target and suspect screenings of LCMs were conducted using tissues of marine cetaceans from stranded individuals of S. chinensis and the Indo-Pacific finless porpoise (Neophocaena phocaenoides) collected during 2007–2021. In total, 40 LCMs were detected in various tissues, including muscle, brain, blubber, liver, and kidney samples from these two species. Additionally, 38 LCMs were quantified, and another 2 LCMs were identified by suspect screening using GC-Orbitrap-MS. The highest levels of LCMs were found in blubber tissues, with the dolphins exhibiting higher concentrations of ΣLCMs (74.38 ±88.15 ng/g; mean ±SD) than the porpoises (61.12–109.38 ng/g). Televisions, Brand A computers, smartphones, and Brand P computers were identified as four primary sources of LCMs found in the cetaceans by positive matrix factorization-multiple linear regression (PMF-MLR). Meanwhile, the transcriptomic analysis through LCM-exposed skin and kidney cells from the Melon-headed whale (Peponocephala electra) revealed that LCMs could lead to DNA damage, cell cycle suppression, and impaired cell division. Overall, this present study provided the first evidence of LCM contamination in various tissues, even in the brain of the two cetacean species, implying their potential threat to marine mammals and even humans via the consumption of contaminated seafood.

Overall, the thesis highlights LCMs as novel emerging contaminants with widespread distribution in the SCS area. Firstly, the presence of LCMs in marine sediment was studied, demonstrating their environmental presence and possible source from wastewater. Secondly, LCMs in urban runoff, including stormwater and sewage, were examined in Hong Kong, revealing high concentrations in industrial and densely populated areas. The high input of dissolved LCMs from these sources resulted in significant ecological risks, as indicated by high-risk quotients for aquatic organisms. Thirdly, the study on two local cetacean species revealed a significant bioaccumulation of LCMs in their tissues, indicating potential threats to these marine mammals. Although this PhD study has revealed the occurrence of LCMs in sediment, stormwater, sewage, and marine mammals in the PRE and SCS, data on LCMs in the broader environment remains limited. Future research should focus on developing rapid analytical methods for LCM detection, especially in biological samples. Additionally, strategies to identify the transformation products of LCMs in both biotic and abiotic media need to be established. This study only conducted a preliminary in vitro toxicity test using dolphin cell lines; thus, the in vivo toxicity of emerging LCMs and their transformation products, as well as their combined risks with other contaminants require further investigation.

Date of Award	12 Sept 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kwan Sing Paul LAM (Supervisor), Mei Yee Kenneth LEUNG (Supervisor) & Yuhe HE (Co-supervisor)