Characteristics of Organophosphate Esters in the Greater Bay Area and the Pearl River Estuary: Occurrence in Multi-Environmental Media, Sources, Transport and Transformation Behaviors and Environmental Risks

大灣區及珠江河口環境中有機磷酸酯的區域特征:環境賦存,來源,遷移與轉化行為和環境風險

Student thesis: Doctoral Thesis

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Award date7 Sept 2023

Abstract

Organophosphate esters (OPEs) have been widely used in various commercial products, particularly after the ban on the use of some polybrominated diphenyl ethers in 2004. Given the prevalence of OPEs in multi-environmental media and their toxic effects, it is necessary to understand the OPEs’ environmental behaviors and risks for informed environmental management. Notably, transformation products of OPEs may also induce detrimental effects but are scarcely examined and included in risk assessment. Furthermore, the occurrence of OPEs has been reported to be related to anthropogenic activities. Therefore, the present study chose the highly urbanized region—the Greater Bay Area (GBA) and its estuary, the Pearl River Estuary (PRE)—as the study area. The present study mainly aimed to understand OPEs’ emission sources and their transport and transformation mechanisms in the air and water of this urbanized region, to evaluate related environmental risks and prioritize OPEs for regulation, and finally to elucidate the loading of continental OPEs into the ocean through particulate-mediated transport and reveal their environmental fate.

Based on the hypothesis that waste treatment facilities can be the secondary sources of atmospheric OPEs in urban area and that particle size can significantly affect the transport and transformation of particulate OPEs, atmospheric size-fractionated particulate and gaseous samples from seven urban-related settings of Hong Kong (e.g., sewage treatment plant, landfill, reference sites) were collected. The results indicated that OPEs occurred widely in ambient air of Hong Kong, and sewage treatment and landfilling could be direct emission sources of atmospheric OPEs in urban areas. The means of dry deposition velocities of individual OPEs ranged from 0.32 cm s-1 to 1.03 cm s-1. Tris(2-butoxyethyl) phosphate (TBOEP) with much lower dry deposition velocity (mean: 0.39 cm s-1) was not easily removed from the atmosphere by dry deposition and might be subject to a longer-range atmospheric transport; the opposite was true for tris(1-chloro-2-propyl) phosphate and tris(2-chloroethyl) phosphate (TCEP). Regarding the transformation, alkyl-OPE transformation products ubiquitously occurred in Hong Kong’s ambient air. The transformation ratios of TBOEP were dependent on particle size, implying that transformation processes might be affected by particle size. TBOEP sorbed on particles of 0.32−1.0 μm and > 10 μm was more resistant to transformation in the ambient air. Although the inhalation risk of co-exposure to OPEs and transformation products in urban ambient air was low, health risks related to OPEs may be underestimated. It is because they are constrained by the identification of plausible transformation products and their toxicity testing in vitro or in vivo at the current stage.

Apart from the air, water is another significant environmental medium for OPEs. To understand the environmental behaviors and risks of dissolved OPEs in the aquatic environment of the urbanized region, OPEs in riverine outlets were investigated as the first step. With a hypothesis that the tides, season differences, and biogeochemical processes could affect the occurrence and transport of OPEs in riverine outlets, a field-based investigation was conducted on eight major riverine outlets located between the GBA and PRE during the wet season and during the dry season under three different types of tides, i.e., neap, medium, and spring tides. In general, the occurrence of OPEs was strongly influenced by emissions from land-based sources and hydrological conditions with minor influences from the biogeochemical processes. Higher levels of total OPEs were discerned in the eastern riverine outlets than in the western ones. Compared to previous studies, the amounts of traditional OPEs discharged to the eight riverine outlets have declined in recent years, showing lower levels of OPEs than those in many other rivers in China. Regarding the riverine input from the GBA, the riverine inputs of individual OPEs showed seasonal variations. Approximately 45,200 kg year−1 of the total OPEs were discharged from the eight major riverine outlets to the PRE in 2020–2021. The environmental stress index (ESI) was proposed in this study to evaluate the pollution status of OPEs in the PRE. Tributyl phosphate and TCEP, with the highest ESIs, are the two priority OPEs, necessitating further monitoring and control measures in this estuary. The urbanized riverine outlets face greater environmental stress and lower resilience to OPE pollution than other less-urbanized outlets.

Dissolved OPEs in urban contaminated wastewater were investigated to further understand their transport and transformation in the aquatic environment. Due to their prevalence in urban contaminated water, their driving factors should be examined while their related ecological impacts should also consider the toxicity of their transformation products. To address these issues, urban stormwater and sewage effluents were analyzed in Hong Kong during the dry and wet seasons. The results demonstrated that the total OPE concentrations in stormwater were significantly affected by the land use pattern and positively correlated with the area of transportation land. However, the population in the catchments greatly increased with increasing OPE levels in sewage effluents. The OPE loads in the wet season were 2.34 and 1.68 times of those in the dry season through stormwater drains and sewage effluents, respectively. OPEs generally showed relatively higher risk quotients than transformation products; however, approximately 38.2% of the total risk quotients would be increased by including those of transformation products. Combining the results of ecological risk and potential endocrine disruptor screening, bis(2-ethylhexyl) phosphate and diphenyl phosphate might be the priority transformation products for wastewater management and further investigation.

Three works mentioned above have established that OPEs abundantly exist in the atmosphere and in the dissolved phase of surface runoff. Most OPEs could enter the marine environment primarily through atmospheric deposition, surface runoff, and sewage effluent, yet the role of particle-mediated transport in their inputs and loss processes remains poorly understood. Therefore, samples of size-segregated atmospheric particles, suspended particulate matter (SPM) in seawater, and sediments in the PRE were collected and analyzed to fill this knowledge gap. The spatial and vertical distribution patterns of OPEs in SPM were diverse, which could be largely affected by the physicochemical properties of SPM, marine microbial activities, hydrodynamic conditions, and environmental factors. Sedimentation fluxes of the total investigated OPEs at the sampling sites ranged from 87.4 ng m-2 d-1 to 666 ng m-2 d-1 in the PRE. Approximately 24,100 g d-1 and 65,100 g d-1 of particulate OPEs were imported into the PRE through atmospheric deposition and surface runoff, respectively; 83,200 g d-1 of which were exported to the open sea. 4,500 g d-1 and 1,500 g d-1 of particulate OPEs were estimated to be subject to degradation and sedimentation in PRE, respectively.

In conclusion, the present study demonstrates the significant impact of particle size on the atmospheric persistence of OPEs and suggests that health risk assessments should be conducted with concurrent consideration of both parental OPEs and transformation products. In addition, the present study integrated field-based investigations and modelling techniques to refine the ecological health assessment related to OPEs and provided insights into prioritization for OPEs’ regulation in the urbanized region of the PRE and the GBA. The results are useful to the environmental authorities for making informed decision on the source control and regulation of OPEs so as to making the rivers and estuaries cleaner and safer for wildlife and humans.