Occurrence Characteristics,Risk Implication,and Air-Soil Exchange Behavior Of Perfluoroalkyl Substances In Soil And Tree Bark

Perfluorinated organic compounds（PFASs）are a type of persistent organic pollutants that are artificially synthesized and all hydrogen atoms connected to carbon atoms are replaced by fluorine atoms.Due to its environmental persistence,bioaccumulation,and long-distance migration ability,it has been widely detected in the atmosphere,ocean,soil,sediment,surface water,and organisms,thus receiving high attention.Soil is a complex and porous system consisting of three phases,solid,liquid,and gas,with poor fluidity and a certain adsorption effect on PFASs.It is the"sink"and"secondary source"of PFASs.The atmosphere is an important migration pathway for PFASs,and bark,as an excellent passive air sampler,can be used to evaluate long-term exposure levels of air pollution.The occurrence characteristics and exchange behavior of PFASs in soil and atmosphere directly determine their migration fate and ecological risks in the environment.In this study,soil and bark samples along the banks of typical watersheds in the Yangtze River Delta（the Huangpu river,the Taihu lake）were collected,and a method of ultrasonic extraction solid phase extraction ultra high performance liquid chromatography tandem mass spectrometry（UPLC-MS/MS）was established to qualitatively and quantitatively analyze PFASs in the samples.Based on the pollution residue levels of PFASs,the ecological risks within the region were evaluated,and the fugacity model was used to elucidate the direction and flux of soil air exchange in PFASs within the region.The research results provide data support and scientific reference for the future regulation of PFASs pollution.The specific content Is as follows:（1）The total amount of PFASs in the soil along the Huangpu river and the Taihu lake is 1.71～64.4 and 1.59～29.6 ng/g,respectively.Short-chain（C<8）perfluorocarboxylic acids（PFCAs）were the main contributors to pollution（79.9%and69.2%）.Spearman correlation analysis showed that the concentration of long-chain（C≥8）PFASs was significantly positively correlated with organic carbon content（TOC）（p<0.05）,among which the concentration of long-chain（C≥8）PFCAs compounds was significantly positively correlated（p<0.05）,while the concentration of medium and short-chain PFASs（C<8）was significantly positively correlated only in the Taihu region（p<0.05）.The PFOS/PFOA ratio indicates that the PFASs pollution in the soil of the Huangpu river and the Taihu lake is from mixed emission sources.The PFHp A/PFOA ratio indicates that the PFASs in the soil of the Taihu lake may mainly come from long-distance atmospheric transport,and the PFOA/PFNA ratio indicates that the PFASs in the soil of the Huangpu river mainly come from industrial emissions.According to three non carcinogenic exposure models of pollutants,the daily exposure levels of∑PFASs in adults are 0.24 ng/kg bw/day and 0.22 ng/kg bw/day.The main exposure mode of PFASs in soil is skin contact（about 90%）,followed by ingestion（about 9%）and particle inhalation（about 1%）.PFASs exposure in about 23%of soil samples in the Huangpu river is a high risk to human health,while all sites in the Taihu lake are low risk.（2）The total amount of PFASs in bark along Huangpu river is 8.71～74.4 ng/g,and in bark along the Taihu lake is 10.2～183 ng/g.The concentration of PFCAs is significantly higher than that of PFSAs,and long-chain（C≥8）PFASs are the main contributors to pollution（75.5%and 62.8%）.Spearman correlation analysis showed a significant positive correlation（p<0.05）between short-chain（C<8）PFASs in tree bark.Principal component analysis showed that the long-chain（C≥8）PFCAs and PFASs of tree bark samples in the two regions may have similar sources,while the monomer sources of short-chain PFASs are diverse.The PFOS/PFOA ratio indicates that the PFASs in the bark come from multiple mixed emission sources,the PFHp A/PFOA ratio indicates that the PFASs in the bark of the Taihu lake may come from atmospheric transport in other regions,and the PFOA/PFNA ratio indicates that about 35%of the bark samples in the Huangpu river come from industrial emissions.In Huangpu river and the Taihu lake,the HQ（Hazard quotient）value of PFOA is3.32×10^-7～2.01×10^-4 and 1.33×10^-7～2.03×10^-5,while the HQ values of PFOS is6.53×10^-8～1.12×10^-6 and 4.17×10^-8～4.15×10^-7 indicates that there is no significant risk of non carcinogenic effects,indicating that inhalation of PFOS and PFOA in both regions is a low exposure risk.（3）The overall range of fugacity scores of PFASs homologues in the Huangpu river is 3.27×10^-6～0.97.The average fugacity score of short chain（C<8）PFASs at each point is 9.94×10^-2～0.62,while the average fugacity fraction of long chain（C≥8）PFASs is 1.08×10^-3～0.14.More than 91%of PFASs have an average fugacity fraction of less than 0.3,indicating a trend of PFASs mainly settling from the atmosphere into the soil in the Huangpu river.The total settling flux range of PFASs at each sampling point in the Huangpu river area is 7.29×10^-3～3.14 ng/（m²·h）;The average exchange flux range of short chain（C<8）PFASs homologues is 0.04～0.19 ng/（m²·h）;Long chain（C≥8）homologous exchange flux range is 3.05×10^-3～0.32 ng/（m²·h）.The fugacity fraction of PFASs homologues in the the Taihu lake ranges from 2.00×10^-5to0.94.The average fugacity score range of short chain（C<8）PFASs at each sampling point is 4.15×10^-3～0.47,while the average fugacity fraction range of long-chain（C≥8）PFASs is 2.98×10^-2～0.94.More than 94%of PFASs have an average fugacity fraction of less than 0.3,indicating that PFASs in the region mainly settle from the atmosphere into the soil.The total sedimentation flux of PFASs at each sampling point in the Taihu lake ranges from 0.03 to 1.76 ng/（m²·h）（mean 0.59）.The average exchange flux of short chain PFASs homologues ranges from 0.02 to 0.10 ng/（m²·h）.The average exchange flux range of long-chain homologues is 1.26×10^-3～0.65 ng/（m²·h）.