Research On Bioavailability Of Sediment-associated Of Aquaculture Zone Hydrophobic Organic Contaminants Based On Their Desorption Kinetics

Hydrophobic organic contaminants（HOCs） in sediment of aquaculture zone have attracted more and more attention due to their threat to the quality of aquatic products and human health. However, conventional evaluation of sediment quality based on total burden of HOCs in sediment can not reflect bioavailability of pollutants, what’s more, they are time-consuming, poor readability and reproducibility in large benthic habitat toxicity test. In the present study, Tenax-based sequential extraction was applied to access the bioavailability of HOCs in sediment according to desorption kinetics of these two typical HOCs pollutants（OCPs and PAHs） in sediment. In parallel two economic benthic organisms Ruditapes philippinarum and Corbicula fluminea were used to explore the relationship between in vivo concentrations and results of desorption kinetics, and to verify the accuracy and feasibility of the method based on desorption kinetics to evaluate the bioavailability of pollutions in sediment. Researching results were summarized as follows.（1） The determined methods were established for two typical HOCs pollutant organochlorine pesticides（OCPs） and polycyclic aromatic hydrocarbons（PAHs） in organisms. The method was described for measuring OCPs in organisms as follows. The solvent mixture of acetone/hexane（V/V=1:1） was used to extract the nine OCPs residues in samples by soxtex system for 24 hours. After being concerntrated, the extract was cleaned up by column chromatography, which is filled by alumina and silica gel. The target analytes were eluted with 30 m L hexane/dichloromethane（V/V=1:1）. The fat in biological sample was dislodged by pertroleum ether saturated with acetonitrile, then analyzed by gas chromatograph-mass spectrometer（GC/MS）. The method was described for measuring PAHs in organisms as follows.The seven PAHs residues in samples were extracted by microwave extraction, with 10 mL solvent mixture of acetone /hexane（V/ V=1:1）, at 80℃, for 50 min. After being concerntrated, the extract was cleaned up by column chromatography, which is filled by florisil and silica gel. The target analytes were eluted with 20 m L dichloromethane, then analyzed by high performance liquid chromatography（HPLC）. The recoveries of the added test on 5, 20, 50 and 10, 50, 100 μg·kg^-1 for OCPs and PAHs, were over 70%, and the relative standard deviations were under 20% in biological sample, respectively. The method detertion limits of nine OCPs and seven PAHs in biological sample were 0.30¹.5μg·kg^-1 and 0.38³.16μg·kg^-1, which were used for detecting OCPs and PAHs in real biological sample.（2） Bioaccumulation kinetics about OCPs with Ruditapes philippinarum: Based on the exposure experiment by use of Ruditapes philippinarum, from the date exposed to the environment polluted by OCPs, respectively in 4, 10, 16, 22, 28, 36, 45 and 56 day sampling, accumulation of OCPs in Ruditapes philippinarum were measured by GC/MS.The results showed that the concentrations of OCPs in organism might trend to temporal equilibrium since the organism exposed 28 day.（3） The results showed that the concentrations of OCPs in Ruditapes philippinarum and Corbicula fluminea were 371.9 6832 and 64.2³507 μg/kg lipid, respectively; the concentrations of PAHs in Ruditapes philippinarum and Corbicula fluminea were 263.6 3500 and 348.2¹667μg/kg lipid, respectively. In parallel, the biota-sediment accumulation factor（BSAF） of OCPs in Ruditapes philippinarum and Corbicula fluminea were within the range of 0.31¹.89 and 0.12².12, respectively. And the biota-sediment accumulation factor（BSAF） of PAHs in Ruditapes philippinarum and Corbicula fluminea were within the range of 0.010⁰.088 and 0.016⁰.210, respectively. Furthermore, there was a poor correlation between two HOCs organic carbon normalized concentrations in sediment and lipid-normalized concentrations in organisms（P>0.05）, indicating that the whole sediment monitoring approach ignores the bioavailability and biological absorption of contaminants in sediment.（4） The Tenax desorption kinetics study showed that the rapidly-desorbing fraction（Frap） of OCPs and PAHs for both seawater and freshwater sediments were about 50% and 40%, respectively, indicating that about 40%-50% HOCs in sediment were bioavailable, and the rate constants for the rapidly-, slowly- and very slowly-desorbing fractions were of the order of magnitude of 10^-1、10-2 and 10-4, respectively. The 6h and 24h-desorbing fraction（F6 and F24） were significantly correlated with the rapidly-desorbing fraction（Frap） in Tenax extraction（P<0.0001）, therefore 6-h and 24-h Tenax extractions can serve as a simple, rapid altemative to evaluate the bioavailability of hydrophobic organic contaminants in sediment, providing a sound basis for assessing aquatic product safety and human health risks associated with its consumption.（5） Tenax desorption kinetics of these HOCs from the tested sediment samples in comparison with their bioaccumulation characteristics in the two model organisms. The results showed that the lipid-normalized concentrations of HOCs in the test organisms were significantly correlated with their concentrations of the rapidly-desorbing fraction in Tenax extraction. The lipid-normalized concentrations of OCPs in the test organisms were significantly correlated with their concentrations of the rapidly-desorbing fraction in Tenax extraction（R2=0.75, P<0.0001）. The lipid-normalized concentrations of PAHs in the test organisms were significantly correlated with their concentrations of the rapidly-desorbing fraction in Tenax extraction（R2=0.66 and 0.62, P<0.001）, and lgBSAF with Frap also showed significant correlation（R2=0.53 and 79, P <0.001）. The results can not only evaluate the bioavailability of sediment associated HOCs, thus provide a reliable basis for the sediment quality and risk assessment; but also can be used to predict bioaccumulation of HOCs in bivalve benthic organisms, therefore provide a more simple and easy method for risk assessment of aquaculture sediments and relevant aquatic products.In conclusion, the technology of Tenax extraction based on desorption kinetics can be used to evaluate the bioavailability of two HOCs（OCPs and PAHs） with different concentrations in the seawater and freshwater sediments, and also can predict the bioaccumulation of the ocean and freshwater economic benthic animals. Therefore, this study not only simplifies the evaluation process of the breeding sediment pollution and bioaccumulation, but also contributes to the migration, transformation and bioavailability of HOCs in sediment. In parallel, to provide the theoretical basis for the environmental management, optimization, quality standards and restoration in contaminated sediment, as well as provide an early warning for the quality and safety risks associated with consumption of aquatic products.