Distribution and fate of polybrominated diphenyl ethers (PBDEs) in Hong Kong mangrove wetland
Student thesis: Doctoral Thesis
Polybrominated diphenyl ethers (PBDEs) are widely used as flame retardants in polymeric materials such as furnishing foam, rigid plastics and textiles. Concerns about PBDEs increased in the past few decades due to their ubiquitous presence and persistence in the environment and potential toxicity, including disruption of thyroid hormones, neurotoxicity and developmental effects on wildlife and humans. Some PBDE congeners haves been recognized as persistent organic pollutants (POPs). Data on the prevalence of PBDEs in Hong Kong mangrove wetland and their fate in this ecosystem is limited. This PhD study aims to investigate the distribution of PBDEs in Hong Kong mangrove sediments and the fate of PBDEs in mangrove wetland. Sediment samples collected from five mangrove swamps in Hong Kong and the eight measured PBDE congeners, including BDE-28, -47, -99, -100, -153, -154, -183 and -209 were found in all mangrove sediments, indicating that these pollutants were widespread in Hong Kong mangrove wetlands, with relatively high concentration of PBDEs found in Mai Po located in Deep Bay, the northwestern New Territories in Hong Kong. The concentrations of BDE-209 and ΣPBDEs (defined as the sum of all targeted PBDE congeners except BDE-209) ranged from 1.53 to 75.9 ng g-1 and from 0.57 to 14.4 ng g-1, respectively. In general, the concentrations of BDE-209 were 1-2 orders of magnitude higher than the other congeners detected in the same sediment. The contribution of BDE-209 to total PBDEs ranged from 36.0 to 93.7%. Among other PBDEs (targeted PBDE congeners except BDE-209), slightly different compositions were detected among samples collected from different locations, probably due to different contamination sources. In all mangrove swamps, except Tai O, ΣPBDEs concentrations showed a common trend of landward > seaward > mudflat, which might be related to the different contents of total organic matter (TOM) in the sediment. Significant correlations were found between ΣPBDEs and TOM in sediment in each mangrove swamp. However, the concentration of BDE-209 did not show the same trend as ΣPBDEs. The depth profile of PBDEs in sediment cores collected from six layers in Mai Po showed that the input of PBDEs increased year by year in this mangrove swamp. A microcosm study was carried out in a greenhouse to investigate the fate of PBDEs in mangrove swamp. Spiking of BDE-47 and BDE-209 individually at two contaminations, each at 200 ng g-1 and 5000 ng g-1 dry-weight in sediment. More than 90% of BDE-47 spiked into the sediment was lost after seven months of the experiment; however, the concentration of BDE-209 did not show any significant changes. Debromination of BDE-47 was observed in the sediment; low-brominated PBDEs, including BDE-28, -17, -15, -8 and -4/7, were found and quantified. The concentrations of BDE-47 and the low-brominated BDEs of sediment in treated with mangrove plants (Kandelia obovata and Avicennia marina), were significantly higher than those without plants. Uptake of BDE47 and low-bromine BDEs by mangrove plants was observed. The concentrations of PBDEs in A. marina were higher than K. obovata although both plant species had comparable bio-concentration factor (BCF) to PBDEs and the lower brominated PBDEs showed higher BCF in both mangrove trees. Spiking of BDE-47 and BDE-209 at two contaminations did not cause any significant effects on the dry weights and the root/shoot ratio of K. obovata and A. marina. Subsurface sediments from five mangrove swamps, two fresh water ponds and one marine area in Hong Kong were collected to test the potential intrinsic ability to biodegrade or transform PBDEs under aerobic and anaerobic conditions. In aerobic sediment slurry, the concentrations of BDE-47, -153 and -209 in the sediment did not have any significant changes at the end of 60 days incubation and no debromination was observed. On the contrary, all sediment slurries under anaerobic condition showed good intrinsic debromination abilities for BDE-47 and -153. Debromination products ranged from hexa- to mono-BDEs but the half-lives, debromination abilities and pathways of these congeners varied among sediments collected from different locations. Generally, mangrove sediments showed higher debromination efficiency than fresh water and marine sediment. After 90 days, debromination of BDE-209 was not significant in the sediment from Mai Po mangrove under anaerobic condition, indicating that deca-BDE was very persistent in the sediment. Static sorption study showed that Freundlich adsorption isotherm was the best model to describe the static sorption behavior of BDE-47 in air-dried mangrove sediment slurries collected from Mai Po and Sha Tau Kok. The former sediment had higher sorption capacity, which might be due to its higher TOM and clay content than Sha Tau Kok sediment. The sorption of BDE-47 in the TOM-free sediment was much lower than that in the original sediment. TOM in sediment could enhance sorption and reduce the BDE-47 availability in the aqueous phase. The sorption capacity of freshly collected sediment where microorganisms were still active was lower than the air-dried one, suggesting that the microorganisms in mangrove sediment would enhance bioavailability of BDE-47. Sorption behavior of BDE-47 in Mai Po sediment slurry was not significantly affected by salinities ranging from 5 to 25 ‰. The present study, for the first time, reveals contamination status and fate of PBDEs in Hong Kong mangrove wetland. Microbial debromination was the most important process in the fate of hexa- to mono-BDE in the anaerobic sediment. However, Deca-BDE was mainly adsorbed on the sediment and very persistent.
- Environmental aspects, China, Mangrove ecology, Hong Kong, Polybrominated diphenyl ethers