Characterization,Bioconcentration And Ecological Risk Assessment Of Organophosphate Esters And Phthalates In Farmlands

Organophosphate esters（OPEs）and phthalates（PAEs）are two typical semi-volatile organic pollutants which are ubiquitous in the environment.As a substitute flame retardant for polybrominated diphenyl ethers and polychlorinated biphenyls,OPEs are widely used in industrial production.As plasticizers and additives,PAEs are widely used in plastic products,including agricultural mulch film,greenhouse film,etc.This resulted in the detection of high concentrations of PAEs in farmland soil,which has aroused widespread concern worldwide.Rice and corn are two major crops in China,and their pollution status is directly critical to the food security and people’s health.Therefore,it is of great significance to investigate the bioconcentration,migration,transformation and ecological risks of typical organic pollutants in rice and corn.This study took the rice fields and corn fields in the suburbs of Benxi City,Liaoning Province as the research areas,and investigated the contaminations during the complete growth periods of these two crops（taking rice as an example:seedling period,greening period,jointing period,tillering period,heading period,filling period,maturity period and Idle period）.The content,composition,distribution,bioconcentration,temporal variations and potential ecological risks of OPEs and PAEs in farmland soil and plant tissues were measured,and their environmental behavior and plant uptake by rice and corn were also studied.The results of the study are as follows:Here,we analyzed 15 paddy soils（rhizosphere soil/bulk soil）,13 maize soils（rhizosphere soil/bulk soil）,and 23 rice tissue samples（root/stem/leaf/species）and 18 maize tissue samples（root/stem/leaf）during the complete growth period of rice and corn in the suburbs of Benxi City,Liaoning Province.The results showed that the total OPE concentrations of paddy soil were in the range of 197-440 ng/g dw.The total OPE concentrations of maize soil were in the range of 142-243 ng/g dw.The total PAE concentrations of paddy soil were 718-2240 ng/g dw;while the total PAE concentrations of corn soil were 1270-2840 ng/g dw.The concentrations of OPEs in rice ranged from 0.0580 to 0.180 mg/kg dw;while the concentrations of PAEs in rice ranged from 2.28 to 16.3 mg/kg dw.The concentrations of OPEs in corn ranged from 0.165to 0.695 mg/kg dw;while the concentrations of PAEs in corn ranged from 0.663 to 17.9 mg/kg dw.In the paddy ecosystem,the concentrations of OPEs and PAEs in rhizosphere soil were significantly higher than those in bulk soil,indicating that rice roots may activate and/or accumulate OPEs and PAEs in the soil.In the cornfield ecosystem,the concentrations of PAEs in the rhizosphere soil were significantly higher than those in the bulk soil,and the difference in the content of OPEs in the rhizosphere and bulk soils were insignificant,and there was no statistically significant difference.In this study,a step-by-step extraction method was used to extract OPEs and PAEs in the soil to obtain the pollutant concentration levels of the active adsorption component and the stable binding component,respectively.The active adsorption component can be used to characterize the bioavailable component,while the stable binding component can characterize the bioavailable state.For both OPEs and PAEs,it was found that the concentrations of active adsorbed pollutants in the rhizosphere soil was significantly lower than those of the non-rhizosphere soil,which indicated that the roots of rice and maize may adsorb OPEs and PAEs in soil.It showed obvious absorption and degradation effects.The concentrations of OPEs and PAEs in the surface soil of the paddy soil profile were the highest,and their concentrations gradually decreased with the increase of soil depth.As rice grown,the concentrations of OPEs and PAEs in rice body gradually increased.The highest levels of OPEs were discovered in leaves,followed by roots and stems.The contents of OPEs and PAEs in rice grains were relatively low,but there was also a phenomenon of enrichment in rice grains with the growing season.The content of PAEs was the highest in roots,followed by leaves and stems,which may be related to the use and residue of agricultural mulch.The content of OPEs and PAEs in maize fluctuated greatly with the growth period,and the fluctuating trend was similar to that of the OPEs and PAEs concentrations in the corresponding rhizosphere soil,indicating an influence by pollution sources.A significant positive correlation was found between the bioconcentration coefficient（log BCF）of OPEs and PAEs in rice and corn and their octanol-water partition coefficients(log K_ow).Meanwhile,a significant positive correlation was also found between the root transfer coefficient（log TF）of chemicals and their octanol-air partition coefficients(log K_oa).A hazard assessment suggested potential medium to high risks from tricresyl phosphate（TMPP）and di-n-butyl phthalate（DNBP）for the agricultural soils in this study,especially for TMPP.Although the ecological risks of OPEs and PAEs in the rhizosphere soils were relatively lower than those in the bulk soils,the relevant risk could still endanger human health via oral intake of these plants.The daily dietary intakes of OPEs and PAEs via vegetable and rice consuming were estimated,and the result suggested a higher exposure risk via ingestion of leafy vegetables than rice.