Stereoisomeric Patterns of Brominated Flame Retardants and Pharmaceutically Active Compounds in Biotic and Abiotic Samples from the Pearl River Estuary, South China

Abstract

The continuously increasing worldwide production and consumption of chemical products has led to expanding contamination of the natural environment, with still largely unknown impacts on human and ecological health. Once in the environment, these chemicals are usually referred to as “micropollutants”. Two classes of organic micropollutants, brominated flame retardants (BFRs) and pharmaceutically active compounds (PhACs), have recently been of high research interest due to widespread occurrence.

BFRs are the most frequently worldwide applied among substances serving as flame retardants. Many BFRs are used as additives, and they can be readily released from consumer goods. In general, BFRs exhibit hydrophobicity and semivolatility. The vast majority of them persist in the environment, tend to bioaccumulate, show inherent toxicity in organisms, and are subject to long-range atmospheric transport. Accordingly, several BFR technical mixtures in common use in the past two decades, such as 1,2,5,6,9,10-hexabromocyclododecane (HBCD), have been listed as “Persistent Organic Pollutants” under the Stockholm Convention for global elimination. This has led to the application of “novel” compounds like 1,2-dibromo-4-(1,2-dibromoethyl)cyclohexane (TBECH) as replacements. However, sharing certain commonalities with HBCD, these substitutes have been ubiquitously found in a wide range of environmental compartments/matrices, especially biota.

Unlike BFRs, PhACs consist of diverse categories of comparatively polar compounds. Despite their broadly short half-lives, numerous PhACs can persist in the environment due to continual disposal and partial resistance to biodegradation, and consequently enter aquatic ecosystems with unchanged biological activities (or active metabolites). For these reasons, PhACs are regarded as “pseudo-persistent” micropollutants which can exist at a concentration chronically toxic to non-target organisms. Specifically, wastewater treatment plants (WWTPs) usually act as the initial reservoir for PhACs.

Many of these chemicals have intrinsic chirality/stereoisomerism. Enantiomers often exhibit different environmental fates and toxicological/pharmacological activities in organisms, despite their identical physicochemical properties present in a symmetric environment. Although significant progress in the development of stereoisomeric separation methods has been achieved in recent years, there is still a lack of understanding of the environmental risks of chiral micropollutants. Notably, many kinds of chiral BFRs and chiral PhACs, have shown species-dependent stereoselectivity from biologically mediated processes. Based on our previous monitoring studies, two resident marine mammal species from the South China Sea, finless porpoises (Neophocaena phocaenoides) and Indo-Pacific humpback dolphins (Sousa chinensis), are susceptible to exposure to high levels of several BFRs, while certain types of PhACs identified in Hong Kong’s sewage are suggested to pose a potential risk to certain receptor organisms. However, stereoisomerism of these chemicals have not been taken into consideration, especially regarding their environmental fates and potential risks.

In this study, to understand the stereoselective fate and distribution of chiral BFRs and PhACs released from municipal discharges, sewage from tanks of sequential treatment processes among four WWTPs within two types of treatment, namely chemically-enhanced primary treatment (CEPT) and secondary (biological) treatment with traditional activated sludge (TAS), was examined seasonally for the studied BFRs and PhACs, including HBCD and TBECH, as well as atenolol, metoprolol, venlafaxine, and chloramphenicol. Initial influent, degritted/settled sewage, non-disinfected effluent, and final effluent were obtained if available. We observed high levels (> 1,000 ng/L) of Σatenolol (the sum of stereoisomers) in influent during the whole sampling period, whereas Σmetoprolol exhibited substantial concentrations (> 200 ng/L) throughout each treatment process. This indicates the large quantities of these two beta-blockers constantly dispensed in Hong Kong’s municipal wastewater. Σvenlafaxine and Σchloramphenicol were detected at relatively low levels ranging from 24.9 to 228 ng/L, with higher amounts observed in the dry season. Compared to TAS, the removal efficiencies of CEPT were insufficient (< 60%) for the studied PhACs. Atenolol, metoprolol and venlafaxine, commonly marketed as racemates, exhibited variations in enantiomeric fractions (EFs) among samples. The distomers of atenolol and venlafaxine, i.e. (+)-atenolol and (‒)-venlafaxine were the predominant species, while a racemic pattern were generally observed with regard to metoprolol enantiomers. In this case, (+)-atenolol could cause a significantly greater ecological impact than its antipode. Sold as a single eutomer (R,R-para-stereoisomer), chloramphenicol might undergo a possible preferential elimination in freshwater sewage during sewage treatment. For BFRs, the mean concentrations of ΣHBCD and ΣTBECH in influent ranged from 1.73 to 37.6 ng/L and from 0.311 to 1.60 ng/L, respectively, which present the low end of the global range. Nearly 70% of the studied BFRs were removed after sewage treatment. On average, α-HBCD and α-TBECH were predominant within their own groups of diastereomers among samples. The enantiomeric profiles of both HBCD and TBECH appeared to remain stable among the different WWTPs. For α-HBCD enantiomeric pairs, in the wet season, racemization occurred to the non-racemic influent treated.

Surface sediments collected from Hong Kong’s coastal areas were assessed to elucidate the potential local sources of hydrophobic micropollutants. ΣHBCD was detected in all sediment samples, with the highest level of 239 ng/g dry weight (dw) present in one specific sample collected in Southern Waters. Sediment from Victoria Harbor had a relatively high ΣHBCD concentrations ranging from 0.47 to 47.9 (mean: 13.0) ng/g dw, while those from Deep Bay had much lower ΣHBCD levels ranging from 0.91 to 6.58 (mean: 2.59) ng/g dw. ΣTBECH in sediment occurred at very low levels of roughly 10 times lower than those of ΣHBCD, but a similarity was observed in the input pathway between HBCD and TBECH to the local coastal areas. Both γ-HBCD and α/β-TBECH (the sum of α-and β-diastereomers) predominated in sediments, exhibiting technical-like patterns. No enantioselectivity was found for either HBCD or TBECH enantiomeric pairs.

Marine fish inhabiting Hong Kong waters, some of which are prey for marine mammals, were investigated to evaluate the environmental behaviors of HBCD and TBECH stereoisomers in these local fish species. Levels of ΣHBCD varied greatly among species, and the highest mean level of 93.0 ng/g lipid weight (lw) was found in tissues (pooled whole-body samples) of ovate sole (Solea ovata), akind of benthos, possibly due to a high potential to accumulate HBCD from sediments and/or a low capacity to eliminate this chemical in vivo. Fish samples with higher ΣHBCD concentrations were largely found from species which are numerically important prey for dolphins rather than porpoises. ΣTBECH detected in fish occurred at relatively low levels with mean values approximately 10 times lower than those of ΣHBCD. While α-HBCD was the most abundant HBCD diastereomer in all samples with a mean proportion of 75.8%, no clear diastereomer-predominant pattern could be found for TBECH across the fish species. However, compared to the diastereomeric compositions of TBECH in sediments, significantly decreased proportions of α-and β-TBECH were accompanied by an elevation in the overall percentages of γ-and δ-TBECH in the fish samples. With an apparent fluctuation in racemic deviations in terms of enantiomeric profile, a significant enrichment of (−)-α-HBCD and (−)-γ-HBCD as well as E1-β-TBECH and E1-δ-TBECH was observed across the fish species. In particular, for HBCD diastereomers, using exposure assessed from achiral concentration measurements would overestimate the toxicity to the investigated marine fish.

Finless porpoises (Neophocaena phocaenoides) and Indo-Pacific humpback dolphins (Sousa chinensis) were selected as targets for the investigation of perennial bioaccumulation. Blubber of these stranded marine mammals was analyzed. Concentrations of ΣHBCD in porpoises and dolphins ranged from 97.7 to 6,260 ng/g lw and from 445 to 45,800 ng/g lw, respectively, while those of ΣTBECH were around two orders of magnitude lower. A significantly increasing trend of ΣHBCD levels was found in dolphins over the past decade with an estimated annual rate of 8%, suggesting a continuous consumption of tHBCD in the Pearl River Delta region.The diastereomeric profiles in blubber exhibited an absolute predominance of α-HBCD with mean proportions of over 90% in both mammal species, while the four TBECH diastereomers appeared to be similarly distributed in proportion. An enantioselective enrichment of (−)-enantiomers of α-, β-, and γ-HBCD was found in most samples of both porpoises and dolphins. Interestingly, the body lengths of porpoises negatively correlated with the enantiomer fractions of α-HBCD, probably due to maternal transfer. Significant racemic deviations were observed for α-, γ-, and δ-TBECH enantiomeric pairs. Our findings suggest that the investigated region might be a contamination “hotspot” for HBCD possibly together with TBECH as a potential replacement.

In conclusion, two classes of organic micropollutants with chirality were investigated and found to be prevalent in the Pearl River Estuary, South China. For chiral PhACs, a certain variability in enantiomeric distribution could be observed to a different extent among various compounds during sewage treatment process. For chiral BFRs, one consistent trend across samples is found to be the shift in HBCD stereoisomeric profile with trophic level that tends to favor the accumulation of α-over β-and γ-diastereomers as well as the enrichment of (‒)-over (+)enantiomers. This finding in turn enlightens us to investigate the possible mechanisms behind the similar stereoselective changing patterns between TBECH and HBCD found in the environment. To our knowledge, this was the first study to reveal the occurrence of inactive chloramphenicol stereoisomers in the environment as well as the presence of TBECH in marine mammals from the Asia-Pacific region. This was also the first report documenting the enantiomeric patterns of all parachloramphenicol stereoisomers as well as TBECH enantiomers in environmental matrices. Clearly, more toxicological information on these chiral BFRs and PhACs, especially TBECH stereoisomers, is in urgent need to better assess their actual environmental risks.