Exposure Of Graphene To Aquatic Organisms:Bioaccumulation,Depuration And Trophic Transfer

Since its first synthesis of graphene in 2004,graphene have drawn great attention for the outstanding physicochemical properties and promising applications in chemistry,physics,biology,material and electronics areas.With the growing industrial applications of graphene and its derivatives,they will be inevitably released into the environment during the production,usage and disposal processes,subsequently become an emerging contaminant and lead to the potential negative effects on biological systems and human health.As one of the essential parts in the environment and health risk assessment,exposure process of pollutants in organisms should be investigated to confirm the real exposure dose in the actual environment,which is responsible for the reliability of results obtained in the labatory study.The lack of knowledge about the uptake,depuration and trophic transfer of pollutants will lead to the differential and uncertainty of measured toxicological data by misunderstanding the real exposure dosage.Thus,the first step to assess the ecological risk of graphene in the aquatic environment should be exploring the behavior of its uptake,depuration and trophic transfer in the aquatic organisms.The lack of reliable quantification methods makes it difficult to quantify graphene family nanomaterials(GFNs)in complex matrices and biological samples,which lead to the deficiency in the GFNs exposure studies of aquatic organisms.In this thesis,a microbial food chain constituted by bacterial(Escherichia coli)and protozoa ciliate(Tetrahymena thermophila)and an aquatic food chain constituted by invertebrate(Daphnia magna)and vertebrate(Danio rerio)are built to investigate the uptake,depuration and trophic transfer of graphene in the aquatic organisms.Main research contents and results are as follows:(1)graphene was synthesized by means of graphitization and exfoliation of sandwich-like FeP04/dodecylamine hybrid nanosheets,and 14C was incorporated in the synthesis.This method presents the solution for the difficulty in the quantifying of graphene in the complex matrices for the first time,14C-labeled few layer graphene(14C-FLG)was spiked to artificial freshwater(AF)and the uptake and depuration of FLG by Daphnia magna were assessed.After exposure for 24 h to a 250?g/L suspension of FLG,the FLG concentration bioaccumulated in the organism was nearly 1%of the organism dry mass.These organisms excreted the FLG to clean artificial freshwater and achieved roughly constant body burdens after 24 h depuration periods regardless of the initial FLG exposure concentration.Addition of algae and humic acid to water during the depuration period resulted in release of a significant fraction(>90%)of the accumulated FLG,but some still remained in the organism.Accumulated FLG in adult Daphnia was likely transferred to the neonates.The uptake and elimination results provided here support the environmental risk assessment of graphene and the graphene quantification method is a powerful tool for additional studies.(2)The uptake,biodistribution and depuration of FLG in the vertebrate was deeply explored by exposing adult zebrafish in the suspension of 14C-FLG.After exposure for 48 h to a 250?g/L suspension of FLG,the FLG concentration in the organism was as high as 48 mg/kg fish dry mass.However,as soon as the fish were transferred to clean medium,they immediately excreted the accumulated FLG to the water and almost all of their body burdens were depurated after 12 h depuration periods.The mainly retaining of FLG in the gut but not a chance of entering the internal environment may be an explanation for the rapid depuration,which have confirmed in the biodistribution study.About 98.3%of FLG taken up were distributed in the intestinal tract,while the rest were detected in the gill.The presence of NOM in the suspension increased the body burden,which is 2.5-fold higher than the condition with the absent of NOM.Furthermore,the interaction between NOM and graphene induced the longer retaining in the organisms.However,FLG penetrating through the intestinal wall and intruding the other tissues still cannot observed with the presence of NOM,In addition,though the bioaccumulation of FLG in the intestinal tract was observed,the abundance and diversity of the intestinal microflora was steady,while the FLG may aggregate on the membrane of bacteria and influence the viability of cells.(3)A microbial food chain constituted by bacterial(Escherichia coli)and protozoa ciliate(Tetrahymena thermophila)and an aquatic food chain constituted by invertebrate(Daphnia magna)and vertebrate(Danio rerio)are built to simulate a complete aquatic ecosystem.The association of FLG on bacteria was investigated and the results indicate that the rapid and impressive capability of bacteria to absorb graphene.FLG can also be taken up by T.thermophila in a rapid rate.After exposure to 250?g/L graphene suspension,the body burden reached a plateau and tend to be in pseudosteady-state.The log BCFs of each measured time interval was 4.7-6.1,indicating that the graphene was "very bioaccumulative" in the T.thermophila.In the trophic transfer of FLG from bacteria to protozoa,the body burden of FLG in the T.thermophile was much less than that of direct exposure in medium.Although the trophic transfer happened through the model food chain,the BMFs at each measured time interval were calculated between 2.12 and 53.85,indicating the occurrence of biomagnification.In the trophic transfer of FLG from invertebrate to vertebrate,the FLG was transferred to the fish from Daphnia through the dietary uptake.And the data have shown that the trophic transfer may be one of the essential route for the bioaccumulation of graphene.However,the BMF was calculate to be 0.009(BMF<1),indicating the nonoccurrence of biomagnification as well.Comparing to the direct exposure,the graphene bioaccumulated through trophic transfer may harder to be depurated by the organism.Aithough the FLG was mainly distributed in the intestinal tract of fish,the radioactive signal can be detected in the liver and filet,which may be one of the explanation to the harder depuration of FLG accumulated through trophic transfer.