Abstract
Polycyclic aromatic hydrocarbons (PAHs) have aroused public concerns over the years owing to the wide variety of their emission sources from both natural and anthropogenic activities, their prevalence, persistence, and ubiquity in our environment, as well as the adverse effects they may pose to human health. With the rising concerns over the safety, ubiquity, and possible linkages between PAHs and public health risks, the evaluation of their population exposure is a crucial means to comprehend the potential risks they may pose to human health. Human biomonitoring (HBM) studies have been extensively conducted by measurements of urinary mono-hydroxylated metabolites of PAHs (OH-PAHs) as biomarkers for personal PAH exposures. Still, they render public health assessments difficult due to the arduous acquisition and analysis of sufficient numbers of biological samples representative of a population. Besides, only a snapshot of the exposure status can be attained without a long-term and large-scale sampling of biological samples, which is resource-intensive and time-consuming.Wastewater-based epidemiology (WBE) is regarded as an innovative alternative when compared to typical HBM approaches. WBE is based on the determination of biomarkers collectively excreted by a community or a population into wastewater for the monitoring community-wide abuse of drugs or population exposure to environmental contaminants such as pesticides, flame retardants, phthalate esters, and bisphenol A. Potential advantages of using this approach include (1) acquisition of data indicative of the entire population in near real-time, (2) elimination of the need to acquire samples from any individual subjects, (3) possibility of generation of time-series and community-wide data, and establishment of status or trends regarding the data, and (4) possibility of making intra- and inter-community comparisons. WBE may provide real-time exposure data, which in turn may reflect the health status of a small population or a community.
In this work, we aimed to use OH-PAHs in municipal wastewater as population exposure markers for PAHs by WBE. The selection of biomarkers for the monitoring purpose was based on the detection and quantitation prevalence of OH-PAHs in human urine from the review of different HBM studies. Only OH-PAHs frequently detected in human urine were selected, including OH-naphthalenes, OH-fluorenes, OH-phenanthrenes, and 1-OH-pyrene, which generally correspond to low-molecular-weight (LMW) parent PAHs. Additionally, 1-OH-acenaphthene and OH-anthracenes were included in this study as they are also representatives of LMW-PAHs. In addition, parent PAHs are all listed as priority PAHs by the US Environmental Protection Agency. On the contrary, other OH-PAHs corresponding to parent PAHs of higher molecular weights, such as chrysene and benzo[a]pyrene, were not included because they are scarcely detected in human urine.
In this project, an analytical procedure for the determination of PAHs and OH-PAHs in municipal wastewater was developed, optimized, and validated. Our method includes clean-up and pre-concentration of the target compounds from wastewater samples using solid-phase extraction. Identification and quantification of the target analytes were carried out using a gas chromatograph coupled to a mass spectrometer (GC–MS). The developed method was validated based on linearity, range, limit of detection (LoD), and limit of quantitation (LoQ), percentage recovery, and precision. Calibrations curves with up to twelve points prepared for the target analytes were linear throughout the investigated concentration range of 0.5 to 2500 µg/L. Satisfactory determination of coefficients, R2 values in general higher than 0.99 were achieved. Instrumental LoDs and LoQs were in the range of ppt to low ppb levels for the target analytes. Acceptable recoveries were attained for the selected OH-PAHs, with recoveries ranging from 67.6% to 125.7% and 52.3% to 110.4% from blank spikes and matrix spikes, respectively. The repeatability (intra-day precision) and intermediate precision (intra-day precision) of the OH-PAHs were below 6.5% and 15.5%, respectively.
Furthermore, the assessment of the stability of the selected biomarkers in wastewater is vital as their concentrations at the sewage treatment plant will be used to back-calculate the corresponding levels of these biomarkers at their sources. Errors associated with their quantitation in wastewater due to their transformation and/or degradation inside the sewer system should be corrected. In this regard, two experiments were designed to establish stability models and to investigate the in-sewer stability of the PAHs and OH-PAHs in influent wastewater under different temperatures and storage conditions. Stability models were generated based on non-linear regressions with exponential, linear, or quadratic functions. The first stability study was carried out at the original pH of the wastewater samples (pH ≈ 7) at two temperatures to explore the in-sewer stability of the tested compounds by imitating typical winter (18°C) and summer (28°C) temperatures in Hong Kong. In general, degradation (which was more significant at 28°C than at 18°C) was observed in all the compounds examined. Our results show that the noticeable degradation of some compounds including 1-OH-naphthalene (-47.8% at 28°C), 2-OH-naphthalene (-68.4% at 28°C), 2-OH-anthracene (-38.9% at 28°C), phenanthrene (-43.2% at 18°C and -48.3% at 28°C), and pyrene (-35.6% at 18°C and -32.5% at 28°C) can occur in case of a long in-sewer retention time of 12 hours, indicating that their in-sewer degradation should be taken into account to reduce errors in the estimation of their initial concentrations when entering the sewer systems. The second stability experiment was carried out under different storage conditions to assess whether significant degradation or formation would occur during sampling and storage. Under the typical condition of sample collection by an automatic water sampler (i.e., storage of water samples at 4°C throughout the 24-h collection period), the stability issues of several compounds are noteworthy. Significant degradation of 9-OH-phenanthrene (-35.6%), 1-OH-anthracene (-45.2%), 2-OH-anthracene (-39.5%), and naphthalene (-42.8%), and formation of 1-OH-pyrene (+16.2%) was observed. Our findings suggest that considerable degradation or formation of the analytes during sampling should be considered to reduce errors in the estimation of their concentrations at the wastewater source. Using the stability models developed, the percentage changes of the analytes due to in-sewer degradation and transformation during sampling and storage can be corrected. These findings are useful for optimizing the WBE approach for the back-calculation of the target biomarkers at wastewater sources in future studies.
Since the assessment of population exposure to PAHs is a crucial means to comprehend the possible risks they may pose to human health, WBE was applied as an attempt to gather information about human exposure levels of PAHs on a community-wide level. Concentrations of selected OH-PAHs were determined in 24-h composite wastewater samples collected from a sewage treatment plant located in Shatin, a highly developed satellite town in the New Territories of Hong Kong Special Administrative Region, China, with a population of around 630,000 and a daily sewage production of about 250,000 m3 from March 2019 to January 2020. The sampling of wastewater samples was conducted for 11 consecutive months (2 to 6 times a month) to acquire data on concentrations of OH-PAHs at the sewage treatment plant. The estimation of population exposures to PAHs was performed by incorporating the degradation kinetics data, levels of OH-PAHs measured in wastewater, information regarding the wastewater treatment plant (including population served by the plant and the daily wastewater flow), as well as the excretion ratios between excreted OH-PAHs and PAH intake. The mean concentrations of the OH-PAHs at the wastewater source (estimated from the measured concentrations in the aqueous phase of wastewater samples) were in the range of 0.4 to 43.9 ng/L throughout the whole sampling period. These concentrations were used to back-calculate mass loads of PAHs to the studied population within the catchment area. The total daily mass loads of 5 parent PAHs (which were back-calculated from their corresponding OH-PAHs in the aqueous phase of the wastewater samples) were in the range of 82.2 to 365.9 mg per day per 1000 inhabitants residing at the sewage catchment area from Mar 2019 to Jan 2020. Our findings suggested that the sum exposures of five parent PAHs (∑[PAHs]5) ranged from 1.32 to 5.87 µg per kg of body weight per day. The risks associated with PAH exposures were evaluated by comparing the estimated daily exposure levels with the minimal risk levels derived by the Agency for Toxic Substances and Disease Registry. Our results showed that the hazard quotients associated with exposures to the five parent PAHs were ≪1, indicating an acceptable risk when only OH-PAHs in the aqueous phase of the wastewater samples were considered using the WBE approach. However, when considering the estimated concentrations of OH-PAHs bound to the suspended particulate matter in the wastewater samples, the PAH exposure levels of ∑[PAHs]5 estimated were in the range of 2.66 to 13.42 mg per kg of body weight per day. The hazard quotients associated with exposures to fluorene were in the range of 0.1386 to 1.3255, indicating a non-negligible risk and adverse effects from PAH exposures are possible.
In conclusion, this study involves the development of an analytical protocol for the determination of OH-PAHs corresponding to LMW-PAHs in municipal wastewater and the assessment of the stability of the OH-PAHs in wastewater under different conditions and contributes to the monitoring of population exposures to PAHs in a populated area in Hong Kong, China. Results generated from this study may be included to establish status regarding population PAH exposures at a community-wide level and for inter-community comparisons if further studies are done to estimate PAH exposures in other populations. Nonetheless, it should be noted that some uncertainties, such as the possible contribution of OH-PAHs in wastewater by other exogenous sources, were associated with the use of the WBE approach in the estimation of population PAH exposures based on measurements of OH-PAH levels in wastewater as biomarkers of exposure. Additional research is required to investigate other exogenous sources that may contribute to the levels of OH-PAHs in sewage. Moreover, only OH-PAH metabolites corresponding to LMW-PAHs were considered, and only the aqueous phase of the wastewater samples was analyzed in this study. Additional OH-PAHs may be required, and analysis of the particulate should be done for a better assessment. Furthermore, the metabolism of PAHs in humans should be comprehended as the excretion ratios between the OH metabolites and parent PAHs are used to back-calculate PAH mass loads when applying WBE.
| Date of Award | 12 Jul 2021 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Hon Wah Michael LAM (Supervisor) |
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