The Optimization Of Passive Dosing Methods And Chronic Toxicity Assessment For Hydrophobic Organic Compounds Using Chironomus Dilutus

Aquatic toxicity testing is required for aquatic risk assessment.As hydrophobic organic compounds?HOCs?have low water solubility,their concentrations in water tended to decrease with increasing exposure time,inducing uncertainty in aquatic risk assessment.Passive dosing method used polymer materials as a“source”of HOCs in water,and the chemicals in polymers were available to compensate the losses of HOCs in toxicity testing system.As a result,passive dosing techniques help maintain HOCs concentrations in water during toxicity testing.Polydimethylsiloxane?PDMS?is the most commonly used polymer material for passive dosing.So far,PDMS passive dosing has been successfully used for studying environmental behavior and biological effects of HOCs.Most passive dosing studies were conducted in small testing systems with short exposure time,although long term exposure in large testing systems was more environmentally relevant.Moreover,there was no method for quantifying the capacity of PDMS to buffer constant chemical concentrations in water.Therefore,the present dissertation examined the potential to use PDMS passive dosing in long-term and large-system tests and quantified the buffering capacity of PDMS passive dosing for HOCs.In addition,sediment is an important part in aquatic ecosystem.The HOCs tend to accumulate in sediment,which may cause secondary pollution to water.In order to evaluated the toxicity of HOCs more accurately,one of the target compounds which was used in PDMS passive dosing test,namely DDT,was selected to evaluate its chronic toxicity to Chironomus dilutus in sediment.Firstly,the loading and releasing procedures for the PDMS passive dosing method were established,and the time for loading and releasing HOCs to and from PDMS were both 24 h.We measured the partitioning coeffients between PDMS and water(K_PW)for common HOCs?PAHs,PCBs and OCPs?and found a good relationship between K_PWW and octanol-water partitioning coefficents?KOW?:log K_PW=1.03 log K_OW+0.47?r²=0.94?,indicating the feasibility of passive dosing method for a variety of chemicals.However,water concentrations of target compounds decreased with increasing time for most HOCs when PDMS passive dosing was applied in the long-term?168 h?and large-system testing?300 mL?.Comparatively,the reduction of HOCs concentrations in passive dosing system were slower than those using traditional active dosing,which showed that passive dosing partially compensate the lost chemicals in water.The decrease of water concentrations for low molecular volume compounds was possibly the results of volatilization.For high molecular volume compounds,the reduction may be related to the small desorption rates of HOCs from PDMS and small molecular diffusion coefficients of HOCs in water.Secondly,six compounds were selected as the representative HOCs and the impact of passive dosing conditions on the buffering capability of PDMS membranes were studied,with seven groups of testing conditions,including PDMS membrane mass,loading concentrations of HOCs,surface areas and specific surface areas of PDMS membranes,and PDMS to water volume ratios?PWRs?.The rate constants for the reduction of water concentrations during exposure in different testing conditions were evaluated and PWR was found the determinant of buffer capacity of PDMS.The minimum PWRs for maintaining constant water concentrations were estimated for test chemicals and they were 0.0073,0.011,0.0072,0.013,0.099,and 0.011 for fluorene,phenanthrene,anthracene,fluoranthene,pyrene,and DDT,respectively.Therefore,the minimum PWR of 0.013 was chosen to maintain water concentrations for HOCs with log K_OW less than 6.91.The PWRs were increased with the increasing log K_OW of the target compounds except for DDT.Thirdly,the full life-cycle toxicity of sediment-bound DDT to Chironomus dilutus was assessed.Although DDT has been banned 30 years ago,DDT is ubiquitous in sediment due to its hydrophobicity and persistence,which poses significant risk to benthic organisms.C.dilutus is an important benthic organism and it plays an important role in aquatic food chain.Therefore,it is imperative to evaluate long-term toxicity of DDT in aquatic ecosystems.DDT degrades as contaminated sediment aging,and C.dilutus are therefore exposed to a combination of DDT and its degradation products DDD and DDE.Using DDT and its degradation as a dose metric accounts for the presence of the degradation products to evaluate the toxicity of DDT to C.dilutus.In addition,the toxicity of DDD and DDE differed from that of the parent compound.Predicted toxic units of DDT and its degradation in porewater were utilized to distinguish the toxicity from DDT and that of DDD and DDE.The results showed that the 5%and 50%lethal concentrations?with 95%confidence intervals?of DDT and its degradation products to C.dilutus were 3.03?0.24–12.0?,0.76?0.05–2.49?,0.11?0.02–0.31?nmol/g organic carbon?OC?and 334?165–568?,21.4?11.2–34.3?,7.50?4.61–10.6?nmol/g OC after 10-,20-,and 63-d exposures,respectively.In addition,the 5%and 50%effect concentrations of DDT and its degradation were 1.68?0.72–2.96?,0.07?0.01–0.22?,0.34?0.01–1.30?nmol/g OC and 20.0?15.0–25.3?,7.13?4.10–10.5?,and8.92?3.32–15.1?nmol/g OC for growth,emergence,and reproduction,respectively.A toxicity spectrum was established to visually summarize chronic effects of DDT and its degradation to midges.While the concentrations of DDD in sediments were lower than DDT,it was the predominated toxicity contributor to midges except for the two highest concentrations in acute toxicity testing.To improve the accuracy of sediment risk assessment of DDT,composition of DDT and its degradation should be considered.Lastly,species sensitivity distributions?SSD?of sediment-bound DDT to aquatic organisms were constructed on the basis of chronic toxicity assessment of C.dilutus.Environmental quality thresholds of sediment-bound DDT to aquatic organisms was estimated using SSD.Six commonly used models were used to fit the SSD curves.The results showed that Gompertz mode was the best fitting model.Acute and chronic concentrations to protected 95%organisms in the environment?HC5?were estimated as 6.48?3.38–10.2?and 0.24?0.018–1.32??g/g OC,respectively.The acute to chronic ratio of DDT to aquatic organisms was approximately 27.In addition,Acartia pacifica and C.dilutus were sensitive to DDT,and they could be used as appropriate organisms for assessing the toxicity of sediment-bound DDT.The concentrations of DDT and its degradation products in some soil and sediment in the Pearl River Delta were higher than the HC5 value deduced in the present study,which may pose potential risks to organisms.In summary,the present dissertation deducted the minimum PDMS to water volu-me ratio for maintaining constant water concentration using passive dosing methods.Full life-cycle toxicity assessment of sediment-bound DDT on C.dilutus and species sensitivity distributions of sediment-bound DDT to aquatic organisms were also evalu-ated,which may provide useful guidance for ecological risk assessment of HOCs.