Influence Of Particle Size On Bioavailability And Toxicity Of Cypermethrin In Sediment

With rapid development of economy in recent years,more and more contaminants have been discharged into aquatic environment,which resulted in serious deterioration to aquatic ecosystem.Sediment is an important component in aquatic environment,thus understanding sediment toxicity is critical for assessing ecological risk of aquatic environment and it also becomes a hot topic globally.Hydrophobic pollutants tend to deposit in sediment after entering aquatic environment through surface runoff.Sediment particles have different composition,texture and size,and thus they may settle to the bottom at different velocity and distance.As a result,bottom sediment is composed by different particle sizes,resulting in different adsorption/desorption behavior for sediment-bound contaminants,as well as different bioavailability,toxicity and ecological risk.However,the knowledge of the effect of sediment particle size distribution on bioavailability and toxicity of highly toxic pollutants is limited.The current thesis studied the influence of sediment particle size on bioavailability and toxicity of sediment-bound contaminants using cypermethrin,a pesticide with high toxicity to benthic organisms as a representative pollutant.Sediment properties were assessed including total organic carbon(TOC)content,composition,texture,surface area,pore volume and adsorption capacity.The impact of sediment particle size on desorption,bioavailability and bioaccumulation potential of cypermethrin to benthic invertebrates were investigated by consecutive Tenax extraction,solid-phase microextraction(SPME)and bioaccumulation testing using Lumbriculus variegatus.Finally,two model species,Chironomus dilutus and Hyalella azteca,were used to assess the effect of sediment particle size distribution on the toxicity of cypermethrin.Firstly,a field collected sediment was wet seived to five particle size fractions,namely<20,20-63,63-180,180-500 and>500?m and the results of sediment characterization showed that TOC contents in the five fractions were 1.78±0.02%,0.817±0.12%,1.04±0.12%,1.66±0.15%and 7.99±1.8%,respectively.The composition and texture of the five sediment fractions can be classified into four types,including clay with porous surface(<20?m),silt with most of smooth sandy loam and few of porous clay(20-63 and 63-180?m),sand with smooth surface(180-500?m)and biological material(>500?m).Surface area,pore volume and adsorption capacity for contaminants increased with decreasing sediment particle size for these fractions.Secondly,biomimetic techniques and bioaccumulation testing were conducted with these sediments to explore the role of sediment particle size on the bioaccessibility and bioavailability of cypermethrin along with the analysis of pore volume and TOC.The rapidly desorption fraction(F_r)increased with increasing pore volume(i.e.,decreasing particle size).The rapidly desorption rate(k_r)increased evidently with increasing TOC contents,suggesting that cypermethrin in sediment with higher TOC content desorbed more rapidly than that in sediment with lower TOC content.In addition,the freely dissolved concentrations in porewater decreased with increasing TOC contents.The particle-scale differences of desorption rate and freely dissolved concentration did not influencebioaccumulation potential of cypermethrin in L.variegatus,suggesting that selective ingestion of fine sediment particles may play an important role in accumulating cypermethrin by worms.Finally,dose-response curves of cypermethrin in various particle size sediments to C.dilutus and H.azteca were established to further evaluate influence of particle size on sediment toxicity.The LC50 values for both species increased with decreasing particle size,indicating cypermethrin in fine sediment exhibits stronger toxicity.While the desorption rate constants and freely dissolved concentrations of cypermethrin in coarse sediment were higher than fine that in fine sediment,as indicated by biomimetic tests,the toxicity of cypermethrin in coarse sediment to benthic organisms was lower,which further indicated that ingesting fine sediment particles might be an important exposure route for benthic invertebrates.Therefore,it is important to quantify the influence of particle size on bioavailability and toxicity of pollutants in sediment toxicity assessment,especially considering contribution of sediment ingestion other than partitioning.