Partitioning and photodegradation characteristics of HBCD in aqueous systems and its behavior in marine and terrestrial organisms

Abstract

To better understand the fate of hydrophobic organic contaminants (HOCs) in nature, the interactions between HOCs and organic matter in aquatic and terrestrial environments should be clarified. Once HOCs are emitted, chemical and biological reactions such as sorption, photodegradation, and biodegradation occur until they fall into the soil, river, or ocean. For these reasons, the bioavailability and toxicity of unidentified behavior of HOCs should be determined and potential harm to humans due to exposure should be quantified. Hexabromocyclododecane (HBCD) was designated as a persistent organic pollutant (POP) at Stockholm Convention since 2013. HBCD is a brominated flame retardant (BFR) and is the world’s third-most used member of this class. However, despite the prevalence and significance of HBCD in the environment, its photodegradation characteristics and sorption interactions with dissolved or particulate organic matter in the aqueous environment is not known. Therefore, preliminary studies to learn the behavior of HBCD were performed in an aqueous system. Furthermore, on the basis of the results presented, the environmental fate of HBCD was observed in biota, including fish and plants. 1. The partitioning characteristics of HBCD to Aldrich humic acid (AHA) were investigated in comparison to the absence of AHA and homogeneous condition (i.e., saline buffer and organic solvent). HBCD diastereoisomers show different van der Waals interactions with AHA, and therefore the diastereoisomers partitioned quite differently to the AHA, so they might become differently distributed in the hydrosphere. α-, β-, and γ-HBCD diastereoiosmers all showed a tendency to aggregate in water in the absence of AHA (namely, low concentration in supernatant), but in high-concentration AHA solution, α-HBCD did not partition into AHA, β- and γ-HBCD had partitioned strongly into it. Therefore, AHA influences the distributional characteristics of HBCD in the aqueous environment. 2. Photosensitized degradation of HBCD in AHA solution was examined concurrently with measurements of reactive oxygen species (ROS), including 1O2, O2-∙, and ∙OH. Under simulated solar light in the presence of AHA, the α-, β-, and γ-HBCD diastereoisomers showed distinct photodegradation characteristics; α-HBCD was not identified during photodegradation of γ-HBCD. As the level of AHA increased, the inhibition of photodegradation of HBCD decreased, especially those of β- and γ-HBCD. Among three HBCD diastereoisomers, the photodegradation of α-HBCD was most severe due to its weak binding affinity to AHA and relatively high water solubility. In a HBCD technical mixture, the decreased photodegradation rate of α-HBCD might be due to isomerization of γ-HBCD to α-HBCD or to inhibition of photodegradation as a consequence of sorption of HBCD to AHA. Photodegradation products of HBCD (e.g., TBCD, PBCD, OH-TBCD, and OH-PBCD) were identified using MRM (multiple reaction monitoring) transitions. Because humic acid is ubiquitous in the aquatic environment, some generation of photodegradion products can be expected to some extent. 3. In-depth research into the fate and distribution of HBCD was performed with various fish and marine invertebrates. Biota can metabolically eliminate or metabolize organic pollutants and the low levels of pollutants might be highly related with the metabolic capabilities of it. In this study, 37 species of marine organisms (8 fish and 29 marine invertebrates) were studied to develop understanding of the distributional properties of HBCD and it was reclassified based on the metabolic capacity, swimming ability (e.g., nekton and benthos), feeding habits (e.g., filter feeder, deposit feeder, raptorial feeder, and browser), and feeding modes (e.g., carnivore and herbivore). In general, ∑2HBCD (i.e., α- and γ-HBCD) and the diastereoisomeric fraction of α-HBCD showed phylum, class, and species-specific properties. 4. In a terrestrial environment, the fate of HBCD was estimated using tobacco (N. benthamiana) and the uptake and translocation of each HBCD diastereoisomer were compared in plants cultured on AHA amended soil. In addition, water content (%), biomass (g), ∑HBCD, and diastereoisomeric fraction of α-HBCD were scrutinized to examine the effects of AHA along with the HBCD exposure. Effects of AHA presence on uptake of HBCD in the soil-plant systems were particularly noticeable in plants cultured on soil with 0.01% AHA; the value of concentration factor that shows bioaccumulation from the root to the shoot was quite different in plants in HBCD contaminated soils than from plants in from uncontaminated soils.