Uptake And Accumulation Of Hydrophobic Organic Contaminants Into Two Plants During The Whole Life

Since the first half of the 20th century,chemicals have been widely produced and used in the fields of industry,agriculture,commerce and consumer goods,resulting in a large number of hydrophobic organic contaminants（HOCs）being released into the environment.In addition,some combustion and high temperature processes in nature,industry and daily life also release a large number of HOCs into the environment.Since most HOCs have persistent organic pollutants（POPs）or POPs-like properties（persistence,bioaccumulation,biological toxicity,and long-distance transfer,etc.）,their behavior and fate in the environment have received extensive attention.HOCs can enter the human body through breathing,dietary intake and skin contact,and then cause harm to the human body.Among them,dietary intake plays a very important role in the three ways of human exposure to HOCs.Crops are the most important part of human food,and the toxic and harmful substances such as HOCs in food will pose a major threat to food security.During the growth of food crops,HOCs can be absorbed into the body through atmospheric exchange/sedimentation and rhizosphere absorption.It is of great significance to explore the process of absorption,transfer and accumulation of HOCs in food crops and the impact of HOCs on the growth process of related plants for understanding the fate of HOCs in the environment and evaluating the ecological risk of HOCs.At present,there were many studies on the absorption and translocation of HOCs（such as halogenated flame retardants（HFRs）,polychlorinated biphenyls（PCBs）and phthalate esters（PAEs））by plants,mainly through greenhouse cultivation,closed chamber,spiked soil or hydroponic experiments.However,the simulated conditions,plant growth and plant accumulation may be different from the field conditions.In addition,most indoor experiments basically use short-term exposure and a single source of exposure（such as soil,air,and water）.There were also some studies on the accumulation of organophosphates esters（OPEs）in plants,but most of the related studies focus on individual OPEs congeners,and the absorption characteristics found in the studies are also quite different.In addition,the current research on the absorption of HOCs by plants mainly focuses on a specific stage of plant growth,each stage of plant growth constitutes a life cycle,and the sensitivity of plants at different growth stages to stress was not consistent,so it was limited to evaluate the impact of HOCs on plants only from a single growth stage.Therefore,this study focuses on the soil-plant-atmosphere system in the typical polluted area of HOCs,studying the concentration characteristics of HOCs in the tissues of the two commonly grown crops,peanut and corn,in different growth periods,and investigating the accumulation and distribution characteristics,translocation and transformation rules of HOCs in the two plants.At the same time,the metabolites of OPEs in two plants were also studied to evaluate the accumulation potential of OPEs metabolites.The main research conclusions are as follows:The geometric mean concentrations（GMs）of polybrominated diphenyl ethers（PBDEs）and polychlorinated biphenyls（PCBs）were 22.3 and 11.9 ng/g in peanut and 16.6 and 13.6 ng/g in corn,respectively.BDE209 was the main congener of PBDEs and PCB153 was the dominant congener of PCBs in both plant.Decabromodiphenyl ethane（DBDPE,6.07 ng/g）and dechlorane plus（DPs,6.22 ng/g）also showed significant concentrations in peanut.The plant uptake was initiated from root absorption at the emergence stage but it was subsequently surpassed by leaves absorption from the air since the late seedling stage or early reproductive stage.There was a rapid uptake of lower halogenated HOCs at the early vegetative stages in both species.However,robust uptake of highly halogenated compounds at the reproductive stages suggests a delayed accumulation of them by the plants.PBDE and PCB congener profiles suggest more noticeable tendency for inter-compartment translocation in peanut than in corn during the plant development.The DP and HBCD isomeric compositions in peanut（enriched with syn-DP andγ-HBCD）were different from those in the rhizosphere soil and air,suggesting a more stereoisomer-selective uptake and/or biotransformation in this species compared to corn.The bioaccumulation factors for root-soil and stem-root of these HFRs and PCBs in most cases were<1.The tissue-distributions demonstrated that leaves serve as a significant reservoir of absorbed HFRs and PCBs under the field conditions,because leaf mass accounted for 15%-69%and had large surface area.In general,more HFRs and PCBs were concentrated in vegetative compartment rather than edible parts.Whereas the low concentrations in peanut and corn kernels indicated translocation of most HFRs and PCBs into this compartment was significantly hindered（especially for highly halogenated compounds）.Phthalate esters（PAEs）were the highest concentration of all HOCs.The GMs of PAEs and OPEs in peanut were 1389 and 15.3 ng/g,respectively.Which were lower than those in corn（3711 and 16.6 ng/g）.DIBP was the predominant PAEs congener in plants and soil,follow by DEHP,indicating that there were a lot of PVC sources in the e-waste area;TCPP and RDP were the dominant OPEs.The total concentration of emerging OPEs（EOPEs）and isopropylated and tert-butylated triarylphosphate（ITPs-TBPPs）exceeds the traditional OPEs（TOPEs）in plants in most cases,implying the environmental risks of EOPEs flame retardants.We observed the rapid absorption of DMP,DEP and DBP with relatively low molecular weight（LMW）in the early vegetative stage of the two plants.DIBP,DEHP and BPA（M range 228-390）with relatively high molecular weight（HMW）reach to their highest concentration was later than that of PAEs with LMW,but earlier than that of BDE209 with HMW（M=959）.Like HFRs and PCBs,leaves were also serve as a significant reservoir of absorbed PAEs and OPEs in both plants.However,corn root was the main reservoir of PAEs and OPEs in vegetative stage.Plant roots had a strong ability to accumulate PAEs than HFRs,PCBs and OPEs.The ability of plants to transfer OPEs and PAEs from roots to stems were weak.Correlation analysis showed that there was a positive correlation between log K_PAand log K_OA（especially in peanut）,indicating that the distribution of OPEs and PAEs in leaf and air was determined by the value of K_OA.The GMs order of OPEs metabolites in the main tissues of the two plants were in the order of root>stem>leaf.By comparing the root concentration factors and translocation factors values of the two plants,it was found that BCP with lower polarity was more likely to be accumulated in roots,while DPHP with higher polarity was more likely to be accumulated in stems.BCP in peanut was likely to come from the metabolism of parent compounds to OPEs,while other metabolites in plants may come from the root absorption of parent compounds of OPEs in soil（which is not detected in air）,the combination of plant internal metabolism and the direct absorption of surrounding environment.The potential risk of BCP in corn kernel may have exceeded its parent TCP.