Enumeration of the constitutional isomers of environmentally relevant substituted polycyclic aromatic compounds

It's time to reevaluate the list of priority polycyclic aromatic compounds: Evidence from a large urban shallow lake

Monitoring only 16 priority PAHs (Pri-PAHs) may greatly underestimate the pollutant load and toxicity of polycyclic aromatic compounds (PACs) in aquatic environments. There is an urgent need to reevaluate the list of priority PACs. To determine which PACs deserve priority monitoring, the occurrence, sources, and toxicity of 78 PACs, including 24 parent PAHs (Par-PAHs), 49 alkylated PAHs (Alk-PAHs), 3 oxygenated PAHs (OPAHs), carbazole, and dibenzothiophene were investigated for the first time in Lake Chaohu sediments, China. Concentrations of ∑Par-PAHs, ∑Alk-PAHs, and ∑OPAHs ranged from 35 to 165, 3.4-26, and 7.7-26 ng g-1, respectively. Concentrations of 16 Pri-PAHs have decreased by 1-2 orders of magnitude compared to a decade ago, owing to the effective implementation of PAHs emission control measures. Comparisons with the sediment quality guidelines indicated that 16 Pri-PAHs have negligible adverse effects on benthic organisms. Positive matrix factorization (PMF) model results showed that coal combustion was the major source of PACs (accounting for 23.5 %), followed by traffic emissions (23.4 %), petroleum volatilization (21.9 %), wood/biomass combustion (18.2 %), and biological/microbial transformation (13.1 %). The toxicity of PACs was assessed by calculating the BaP toxic equivalent concentrations (TEQBaP) and toxic units. It was found that Par-PAHs were the predominant toxic substances. In addition, monomethyl-BaPs, OPAHs, BeP, and 7,12-DMBaA should be prioritized for monitoring due to their noticeable contributions to overall toxicity. The contributions of different sources to the toxicity of PACs were determined based on PMF model results and TEQBaP values, which revealed that combustion sources mainly contributed to the comprehensive toxicity of PACs in Lake Chaohu sediments.

Predicting the Occurrence of Substituted and Unsubstituted, Polycyclic Aromatic Compounds in Coking Wastewater Treatment Plant Effluent using Machine Learning Regression

Organic contaminants such as polycyclic aromatic compounds (PACs) occurring in industrial effluents can not only persist in wastewater but transform into more toxic and mobile, substituted heterocyclic products during treatment. Thus, predicting the occurrence of PACs and their heterocyclic derivatives (HPACs) in coking wastewater is of utmost importance to reduce the environmental risks in water bodies that receive industrial effluents. Although HPACs can be monitored through sampling and analysis, the characterisation techniques used in their analyses are costly and time-consuming. In this study, we propose 3 distinct kernel-based machine learning (ML) models for predicting PACs including substituted HPACs and alkylated PACs occurring in coking wastewater. By using routinely measured wastewater quality data as input for our models, we predicted the occurrence of 14 HPACs in the final effluent of a coking wastewater treatment plant. Support Vector Machine based regression model (SVR) used for HPAC prediction showed the highest R2 of 0.83. Performance assessment of SVR model showed a mean absolute logarithmic error (MALE) of 0.46 and root mean square error (RMSE) of 0.073 ng/L. Comparatively, K-Nearest Neighbor and Random Forest models showed lower R2 of 0.75 and 0.76 respectively for HPAC prediction. Feature analysis attributed the superior predictability of SVR model likely to its higher weightage (81%) towards dissolved organic carbon and total ammonia as input variables. Both these variables could capture the underlying secondary PAC transformations likely occurring in the treatment plant. Partial dependence plots predicted that ammonia levels higher than 120 mg/L and DOC levels of 50-60 mg/L were likely linked to higher HPACs occurring in the final effluent. This work highlights the capability of kernel-based ML models in capturing nonlinear wastewater chemistry and offers a tool for monitoring trace organic contaminants released in coking effluents.

Microbead-Beating Extraction of Polycyclic Aromatic Compounds from Seabird Plasma and Whole Blood

Seabirds are widely regarded as an invaluable bioindicator of environmental health. Matrices including eggs and feathers have been used as non-lethal means to assess contaminant burdens. We have developed a new approach for extraction of polycyclic aromatic compounds (PACs) from seabird plasma and serum based on automated microbead-beating homogenization and extraction. Commercially available bovine serum and plasma were purposely fortified with a suite of PACs separately at three dosing levels, placed inside a custom-made stainless-steel tube containing ceramic microbeads, and subjected to an extraction process using a Precellys tissue homogenizer. Tubes were shaken forcefully in three-dimensions, facilitating high mass-transfer of PACs from the matrix into the hexane extraction solvent. The accuracy of the method ranged from 55 to 120% and limits of detection and quantitation ranged from 0.1 to 8 and 0.2 to 27 pg/μL, respectively. The method exhibited good repeatability with both inter- and intra-day repeatability < 30%. The developed method represents an effective and efficient approach to extraction of PACs from important biological matrices.

Combustion of crude oil during in-situ burning can introduce polycyclic aromatic compounds (PACs) into small-scale freshwater systems

In-situ burning (ISB) is the controlled combustion of an oil slick to remove large quantities of spilled oil from the aquatic environment. Prior to employing ISB as a remediation technique, an oil slick must often be corralled by physical or chemical means to achieve a sufficient thickness (typically >1 mm) for ignition. While ISB is an effective means to remove oil mass, less is known about the potential for ISB to mobilize polycyclic aromatic compounds (PACs) into the aquatic environment. The PACs are primary contaminants of concern in crude oil due to their environmental persistence and toxicity. We examined the potential for ISB to mobilize PACs into underlying waters in a series of small-scale burns conducted across a gradient of oil slick thicknesses (0-7 mm). Concentrations of PACs in underlying waters were evaluated and compared to reference conditions using an equivalent gradient of oil slick thicknesses that were not ignited. At thinner slick thicknesses (i.e. 0 - 4 mm) ISB enhanced the mobilization of total PACs, likely a result of heat transfer to underlying waters; this effect increased as slick thickness increased. Among thicker slicks (i.e. 4 - 7 mm), pyrogenic PACs became more prevalent and greater concentrations of 4-ring PACs were detected in underlying waters. The potential for PAC mobilization needs to be considered in scenarios where ISB may be the only viable oil spill remediation option (e.g. wetlands, marshes, or where oil is entrained) and in shallow systems susceptible to temperature changes.

Bioaccumulation and toxicokinetics of polycyclic aromatic compounds and metals in giant floater mussels (Pyganodon grandis) exposed to a simulated diluted bitumen spill

Oil spills constitute a major risk to the environment and the bioaccumulation potential of the derived oil constituents will influence their impact on aquatic biota. Here we determined the bioaccumulation potential and toxicokinetic parameters of polycyclic aromatic compounds (PACs) and various selected metals in the giant floater mussels (Pyganodon grandis) following experimental oil spills in a freshwater lake. Specifically, these mussels were exposed ex situ for 25 days to water contaminated with naturally weathered diluted bitumen (dilbit), a form of oil commonly transported through pipelines. We detected greater concentrations of total PAC in mussels (∑PAC₄₄) exposed to dilbit-contaminated water (25.92-27.79 μg g^-1 lipid, n = 9, at day 25 of the uptake phase) compared to mussels from a control with no exposure to dilbit (average of 2.62 ± 1.95 μg g^-1 lipid; ±SD, n = 17). This study demonstrates the importance of including alkylated PACs when assessing the impacts of an oil spill as the concentration of alkylated PACs in mussel tissue were an order of magnitude higher than their parent counterparts. However, metal accumulation in dilbit-exposed mussels did not exceed the unexposed controls, suggesting no excess metal accumulation by mussels from a 25-day dilbit exposure. From first-order one-compartment models, mean uptake rate constants (0.78-18.11 L g^-1 day^-1, n = 29) and bioconcentration factors (log values from 4.02 to 5.92 L kg^-1, n = 87) for the 29 individual PACs that accumulated in mussels demonstrated that some alkylated PACs had greater bioaccumulation potential compared to their parent PAC counterpart but for the most part, alkylated and parent PACs had comparable BCF values. Results from this study also demonstrated that giant floater mussels could be used to biomonitor PAC contamination following oil spills as PACs accumulated in mussel tissue and some were still detectable following the 16-day depuration phase. This study provides the largest, most comprehensive set of toxicokinetic and bioaccumulation parameters for PACs and their alkylated counterparts (44 analytes) in freshwater mussels obtained to date.

Comparison of different approaches to quantify substituted polycyclic aromatic compounds

Unlike native polycyclic aromatic hydrocarbons (PAHs), quantitation of substituted polycyclic aromatic compounds (PACs) has been a challenge in the environmental industry. The challenge can be attributed in part to the large number of theoretically possible isomers and the lack of authentic standards for quantitation. In addition, the lack of a unified approach to the quantitation of these compounds has led to poor interlaboratory accuracy. Because these compounds are often used for toxicology studies or to delineate sources and fingerprinting, it is vital that a standardized approach to quantify them is established. This study evaluated different quantitation approaches to quantify both 16 individual PACs and 32 groups/clusters of substituted PACs in three standard reference materials (SRM 1944 - New York / New Jersey waterway sediments, SRM 1597 - a coal tar sample and SRM 2779 - Gulf of Mexico crude oil). The methods employed include: (1) external calibration taking into account recovery correction factor for each analyte, (2) an average relative response factor (ARRF) of PACs obtained with a recovery correction, (3) ARRF of PACs obtained using uncorrected peak areas (i.e., no recovery correction), (4) ARRF of PACs calculated by normalization to deuterated PAHs and (5) ARRF of native PAHs to quantify substituted PACs. The evaluation of concentrations of individually substituted PACs from the different quantitative approaches compared to the certified/reference values showed that methods 1, 2 and 3 performed best. The average percentage of compounds that fell within our acceptable limit (±30%) using methods 1, 2 and 3 for SRM-1944, -1597a and -2779 was 87, 75 and 100%, respectively. Using native PAHs to quantify their substituted analogs resulted in data of the poorest quality. Irrespective of the approach used, there were significant systematic errors in measurements on clusters/groups PACs most notably C₁ and C₂-benzanthracenes/ chrysenes/triplenylenes, and C₂- and C₃-dibenzothiophenes being consistently greater than 100% of the stated value. Commerical availability of more substituted PACs will mitigate the biases associated with the quanititation of PAC clusters/groups.

Polycyclic aromatic compounds (PACs) in the Canadian environment: The challenges of ecological risk assessments

Ecological risk assessments (ERAs) of polycyclic aromatic compounds (PACs), as single congeners or in mixtures, present technical challenges that raise concerns about their accuracy and validity for Canadian environments. Of more than 100,000 possible PAC structures, the toxicity of fewer than 1% have been tested as individual compounds, limiting the assessment of complex mixtures. Because of the diversity in modes of PAC action, the additivity of mixtures cannot be assumed, and mixture compositions change rapidly with weathering. In vertebrates, PACs are rapidly oxygenated by cytochrome P450 enzymes, often to metabolites that are more toxic than the parent compound. The ability to predict the ecological fate, distribution and effects of PACs is limited by toxicity data derived from tests of a few responses with a limited array of test species, under optimal laboratory conditions. Although several models are available to predict PAC toxicity and rank species sensitivity, they were developed with data biased by test methods, and the reported toxicities of many PACs exceed their solubility limits. As a result, Canadian Environmental Quality Guidelines for a few individual PACs provide little support for ERAs of complex mixtures in emissions and at contaminated sites. These issues are illustrated by reviews of three case studies of PAC-contaminated sites relevant to Canadian ecosystems. Interactions among ecosystem characteristics, the behaviour, fate and distribution of PACs, and non-chemical stresses on PAC-exposed species prevented clear associations between cause and effect. The uncertainties of ERAs can only be reduced by estimating the toxicity of a wider array of PACs to species typical of Canada's diverse geography and environmental conditions. Improvements are needed to models that predict toxicity, and more field studies of contaminated sites in Canada are needed to understand the ecological effects of PAC mixtures.

Identification of halogenated polycyclic aromatic hydrocarbons in biological samples from Alberta Oil-Sands Region

Halogenated polycyclic aromatic hydrocarbons (HPAHs) were identified in biological samples from the Alberta Oil-Sands Region (AOSR) using gas chromatography coupled with high-resolution time-of-flight mass spectrometry (GC-HRTOF-MS) at a resolving power of 25,000. Knowledge of the electron ionization (EI) fragmentation behavior of individual HPAH isomers, achieved by injecting authentic standards in full-scan MS mode, was paramount in identifying a suite of HPAHs in samples from the AOSR. Confirmation of compounds in biological samples was based on the measured mass accuracy (±3 ppm) of 2 characteristic ions prominent in the EI mass spectra of each compound. Numerous compounds were detected in the high resolution total ion chromatogram in liver extracts of 4 biological species from the AOSR: river otter (Lontra Canadensis), northern pike (Esox lucius), lake whitefish (Coregonus clupeaformis) and snails (Gastropod sp.) many of which remain unidentified. Careful examination of the high-resolution accurate mass data suggests that dichloro-anthracene/phenanthrene, bromo-anthracene/phenanthrene and dibromo-fluorene were present in the biological samples. Lipid corrected concentrations of dichloro-PAHs were estimated to be 16.3 ± 11.4 (n = 4) and 5.5 (n = 1) ng/g in lake whitefish and river otter, respectively. Concentrations of mono-bromo-PAHs were an order of magnitude greater in snails (170.5 ng/g) than in northern pike (12.5 ng/g) while concentrations of dibromo-PAHs were 4 times greater in snails than in northern pike. The detection of these compounds in biota implies that these compounds are bioaccumulative. The liver-based biomagnification factor of the dichloro-PAH congener in the river otter/lake whitefish feeding relationship is much smaller than 1 implying that this compound does not biomagnify.

Validation of a simultaneous method for determining polycyclic aromatic compounds and alkylated isomers in biota

RATIONALE

There is a need for a validated method to improve detection limits and simultaneously quantify polycyclic aromatic compounds (PACs, both parent and alkylated homologues) in biota by gas chromatography/tandem mass spectrometry because of their environmental significance. The validation of the method was performed in accordance to the Eurachem Guide to Quality in Analytical Chemistry.

METHODS

Gas chromatography coupled with a triple quadrupole mass spectrometer used in multiple reaction monitoring (MRM) mode was used for detection and quantification. Retention time windows and selective MRM ion transitions were optimized for a suite of PACs. The developed method was validated by comparing our measurements made on a reference material of freeze-dried mussel tissue (Mytilus edulis) with the certified values.

RESULTS

Linearity was observed between 10-1000 pg/μL (PAHs) and 2-500 pg/μL (alkyl-PACs including S-based PACs). The overall mean (±SD) for the limits of detection of 43 PACs studied were 0.305 ± 0.276 and 2.69 ± 1.10 ng/g, respectively. For the 14 certified target analytes, the percent relative error ranged from 1.3 to 33%. With the exception of benzo(a)pyrene, the between-day and within-day repeatability for all target analytes was lower than 15% RSD.

CONCLUSIONS

This is the first report of a fully validated method to simultaneously quantify PACs in biota performed in an ISO accredited laboratory.