Studies On The Uptake,Transformation And Migration Of Organophosphorus Esters(OPEs) In The Soil-plant Systems

Organophosphate esters(OPEs)have been widely used in various industrial and electronic products as alternative flame retardants,since the production and use of polybrominated diphenyl ethers(PBDEs)and polychlorinated biphenyls(PCBs)banned in many countries.However,similar to PBDEs,OPEs were organic chemicals with properties of persistence and long-distance transport,also it had potential toxicity to plants,animals,and human beings.Soil OPEs can enter the human body through the food chain,so the environmental behavior of OPEs in soil has attracted lot of attention from the researchers.To date,studies on OPEs mainly focus on their adsorption-desorption behavior of OPEs in the soil system,the composition of parent compounds or their metabolites.However,only a small part of the studies involved the uptake,translocation,and distribution of OPEs in the soil-plant system.Pot experiment were investigated in the laboratory to study the effects of chemical properties(like structure,hydrophobicity,and substituent species),plant cultivars,soil type,and Cu on the uptake,distribution,and transport behavior of OPEs.In addition,studies were also focus on how the rainfall,soil type,and the presence of plant influenced the migration behavior of OPEs.Target compounds in this study were include five Alkyl-OPEs[triethyl phosphate(TEP),triisopropyl phosphate(Ti PP),tripropyl phosphate(Tn PP),tri-n-butyl phosphate(Tn BP)and tris(2-ethylhexyl)phosphate(TEHP)],two Aryl-OPEs[triphenyl phosphate(TPHP)and trimethylphenyl phosphate(TMPP)],three Chlorinated-OPEs[tris(2-chloroethyl)phosphate(TCEP),tris(2-chloroisopropyl)phosphate(TCEP)and tris(1,3 dichloro-2-propyl)phosphate(TDCPP)],and two Hybrid-OPEs[Tris(2-butoxyethyl)phosphate(TBOEP)and 2-ethylhexyl diphenyl ester(EHDPP)];The tested plants were carrot,zucchini,soybean,lettuce,tomato,and corn.Krasnozem soil and Margalitic soil were chosen.The main conclusions are as follows:(1)The effects of compound structure,plant cultivars and the presence of Cu on the uptake and distribution of twelve OPEs in six cultivars were studied by pot experiment.The results showed that bioaccumulation of OPEs in roots and shoots varied among plant cultivars,indicting different uptake and translocate ability by plants.Generally,OPEs were preferentially enriched in carrot roots and shoots,followed by zucchini,soybean,lettuce,and tomato,with the lowest concentrations observed in the corn.OPEs with electron-ring substituents(ER-OPEs,TPh P,TMPP,TCPP,and TDCPP)exhibited higher potential for root uptake than did OPEs with open-chain substituents(OC-OPEs,TEP,Ti PP,Tn PP,Tn BP,TEHP,TCEP,TBOEP,EHDPP),indicating the plant root had stronger uptake ability to ER-OPEs.It could be attributed to the interaction between the electron-rich structure of ER-OPEs with the charged root surface,which promoted the uptake of ER-OPEs by plant roots.There was no significant difference between the content of ER-OPEs and OC-OPEs in the plant shoot,indicated the compound substituents had no significant effect on the transport process of OPEs.In treatment of OPEs with Cu,the content of OPEs in the roots of carrot,zucchini,soybean and the shoots of tomato and corn were increase,indicating that Cu altered the uptake and accumulation of OPEs.After exposed with Cu,root concentration factor(RCF)of ER-OPEs was still higher than that of OC-OPEs,and Cu slightly reduced the distinct difference between the uptake ability of OC-OPEs and ER-OPEs.This was explained by the interactions of Cu with the electron-rich structure of ER-OPEs,which suppressed the sorption of ER-OPEs on the root surface.Both in solo OPEs and OPEs+Cu treatment,the logarithms of octanol-water partition coefficient(log K_ow)of OPEs was negatively correlated with their translocation factor(TF)(p<0.001),indicating that hydrophobicity was an important fact controlling the transport process of OPEs from roots to the aboveground parts.(2)The uptake and translocation of four OPEs(Tn BP,TCEP,TPh P,and EHDPP)with different substituent groups by corn were studied.It was tried to find how chemical treatments level,soil type and Cu governing the uptake,distribution,and transport process of OPEs in corn.The contents of four OPEs in corn roots and TCEP in shoots were increased with the treatment levels in soil.However,the concentrations of Tn BP,TPh P,and EHDPP in the shoot showed limited change.TCEP was the dominated chemicals in the corn tissues.Root concentrations of OPEs in the plants cultivated in the Krasnozem soil were relatively lower than those from Margalitic soil(p<0.05),which might be related to the higher dissolved organic matter(DOM)and root lipid content from Margalitic soil.DOM could compete with the organic chemicals for adsorption sites on the soil particle surface,or combine with organic chemicals to form soluble complex,thus promoting the mobility of OPEs in Margalitic soil,finally increasing the root uptake of OPEs.Root lipid was also a key factor affecting the uptake of hydrophobic compounds,higher root lipid content tend to have greater uptake capacity.In the OPEs+Cu treatments,Cu increased the concentration of OPEs in plant roots and shoots,and the content were increased gradually with the treatment levels of Cu in soil.However,the increment of OPEs in plants cultivated from Krasnozem soil were higher than that from Margalitic soil,and it might due to the toxicity of Cu and destruction in corn.Higher content of bioavailable Cu in Krasnozem soil along with the distinct damage to corn demonstracted stronger Cu toxicity resulted in more accumulated OPEs in the damaged plants.A positive correlation between RCF values of OPEs(except TCEP)and the root Cu content was abserved,and TF decreased with the inhibited plant biomass,indicating the heavy metals played a more significant role on the uptake and translocation of higher hydrophobic compounds.(3)The composition,distribution,and transport of Alkyl-OPEs,Aryl-OPEs,Chlorinated-OPEs,and Hybrid-OPEs in soil at different depths,leachate,and plant tissues were analyzed under the simulated rainfall from the South China,and to explore the leaching and migration behavior of OPEs by the influence of soil type,rainfall,and the presence of plants.Plants significantly reduced the concentration of OPEs in the leachate,and it might be attributed to the photosynthesis and root adsorption,which reduced the total amount of leachate in the soil column and delaying the transport of OPEs from the top soil.The concentrations of Ti PP,Tn PP,and TCEP were relatively high,which might be due to the relatively high solubility and hydrophilicity of these three chemicals,therefore promoting their migration in the soil.Content of OPEs in leachate from Krasnozem soil was higher than that of the Margalitic soil,probably due to the high sorption between soil organic matter and organic chemicals in the Margalitic soil.The adsorption of organic matter on the Margalitic soil reduced the migration of OPEs,thus hindering the flow of OPEs in the leachate.The concentration of OPEs in topsoil(5?15 cm)was negatively correlated with the log K_ow of the compounds,indicating the hydrophobicity controlled the migration process of OPEs in soil,and their migration was relatively high.The content of OPEs in soil with soil depth(20-40cm)generally decreased with the soil depth.The concentrations of Chlorinated-OPEs in the roots,stems,leaves and grains were the highest,indicating the stronger accumulated ability by plants.The results of this study deepened understanding the migration of OPEs,and their uptake and translocate behavior under the Cu in the soil-plant system.In addition,it provides a theoretical basis to control the soil pollution and evaluate the OPEs ecological risk by crops.