Accumulation Characteristics And Exposure Risk Of PAHs In Air-Leaf Vegetable-Soil System

Polycyclic aromatic hydrocarbons?PAHs?are toxic persistent organic pollutants with carcinogenicity,teratogenicity and gene mutation and can be widely found in environmental media such as the atmosphere,water,and soil.PAHs in environmental media not only directly expose to the human beings and cause health risks,but also can migrate and transform between the atmosphere,water,soil and organisms,and then affect human health through the food chain.Many studies have focused on the environmental behavior and health risks of PAHs in polluted environments,but the behavior of PAHs in different media and their cancinogenic risk still need to be further studied.Therefore,this study explored the factors affecting the accumulation of PAHs in leaf vegetables from the atmosphere and soil.The effects of greenhouse planting condition,atmospheric dry and wet deposition,and sludge application in soil on the absorption and accumulation of PAHs in vegetables were studied.The contribution of vegetables to the accumulation of PAHs from different pathways was simulated and analyzed.The comprehensive health risks caused by the atmosphere,soil and the ingestion of vegetables under different polluted conditions were calculated.This study had important practical significance for effectively reducing the pollution levels of PAHs in vegetables and ensuring food safety and reducing the health risks caused by PAHs exposure.In addition,it can provide a scientific basis for reducing the pollution of PAHs in the environment and for rationally planning agricultural land.Thanks to the support of the National Natural Science Foundation of China?No.41271472?and the Shanghai Natural Science Foundation Project?No.12ZR1409000?.Taking the common leafy vegetables in Shanghai as the research object,using the methods of physical geography,environmental ecology,environmental engineering and environmental geochemistry,through field investigation and analysis,field micro-area vegetable cultivation,experimental simulation of dry and wet deposition in vegetable greenhouse and pot experiment of sludge application,the accumulation characteristics and exposure risk of PAHs in air-leaf vegetable-soil system were systematically studied.Vegetables,soil,atmosphere particles,air,domestic sludge and dust were collected.The concentrations of PAHs in these media were determined by gas chromatography/mass spectrometry?GC-MS?.The effects of vegetable greenhouses on PAHs absorption and accumulation in soil-vegetable systems were investigated.The influences of atmospheric dry and wet deposition and sludge application in soil on PAHs accumulation in vegetables were studied.The sources of PAHs in vegetables were discussed.Lifetime cancer risk model was used to analyze the health risks of PAHs for different age groups and gender groups under different experimental conditions.The main conclusions are as follows:?1?The PAHs concentrations in farmland soils and vegetables near typical industrial areas in Shanghai were greatly affected by industrial types,relative location and wind direction.Leaf vegetables had the strongest accumulation effect of PAHs in the environment,followed by stem and fruit vegetables,and root vegetables had the lowest PAHs concentrations.Greenhouse could reduce the levels of PAHs in soils and vegetables.PAHs in vegetables and soil both inside and outside the greenhouse,the particulate and dissolved PAHs in the wet deposition were all dominated by 2-3 rings.PAHs in vegetables were mainly from the process of petrochemical products in Wujing Chemical Plant and the coal combustion in thermal power plants.The main source of PAHs in soil was related to the traffic pollution of Zifeng Road.The particulate PAHs in wet deposition mainly came from the high-temperature pyrolysis of petrochemical products and coal combustion.Combustion is related to soot emissions from chemical plants and thermal power plants.The sources of dissolved PAHs in wet deposition were complex,including high-temperature pyrolysis of petrochemical products,coal combustion and oil spills.?2?Under different wet deposition conditions,the concentrations of PAHs in vegetables and soils were significantly different,with the highest concentrations of soils in the seedling stage of vegetable and the highest concentrations of vegetables in mid-term growth stage.The wet deposition patterns didn't increase the absorption of PAHs in vegetables and soils significantly,but might hinder the absorption of PAHs by vegetables,especially those in foliar spray group.Low-ring PAHs were dominant in all vegetables.The characteristic ratio method showed that some PAHs in soil mainly came from high temperature combustion of grass,wood and coal,and PAHs in vegetables also came from high temperature combustion of coal,which was mainly the external pollution carried by rainwater in the process of wet deposition simulation.?3?Under different dry deposition conditions,the concentrations of PAHs in vegetable leaves,roots,surface soils,atmospheric particles and air were significantly different.The concentrations of PAHs in vegetable leaves was higher than those in roots,while the average concentration of PAHs in soil was higher than those in vegetable leaves and roots.The 2-3 ring PAHs were dominant in vegetable leaves,roots and gas phase,while 4-6 rings were dominant in surface soil,dust and atmospheric particulate samples.Different vegetable types also had significant effects on the accumulation of PAHs.The concentrations of PAHs in spinach were the highest,while the concentrations in Shanghai Green and Chinese cabbage was less.The concentrations of PAHs in the middle growth stage of vegetables was higher than those in the seedling stage and the mature stage.The concentrations of PAHs in leaves of Shanghai Green and lettuce increased regularly with the increase of dust intensity,but the concentrations of PAHs in spinach and Chinese cabbage decreased with the increase of dust intensity,which was related to the photosynthesis of vegetables,the growth rate of different vegetables,leaf morphological structure,leaf surface area ratio and the effect of dry sedimentation particles on leaf stomata or stratum corneum.?4?The cumulative contribution of atmospheric dry deposition to PAHs in soil was larger than that of wet deposition.In different depth of soils,PAHs tended to be enriched in the surface layer influenced by the absorption of vegetable roots and surface dustfall.Organic matter concentrations and particle size composition of soil did not affect the PAHs concentration.The average concentration of gaseous PAHs in the greenhouse was slightly lower than that outside the greenhouse under the dry deposition conditions,while the average concentration of particulate PAHs in the greenhouse was significantly higher than that outside the greenhouse.Pearson correlation analysis showed that the concentrations of gaseous and particulate PAHs in the greenhouse were less influenced by temperature,humidity and atmospheric pressure.The gas-particle partition coefficient of PAHs in the greenhouse showed that the 5-6 ring PAHs were mainly adsorbed on particles,while the 2-4 ring PAHs mainly existed in gaseous form.The partition of PAHs was mainly affected by their physical and chemical properties.?5?Using R statistical analysis software and according to the concentrations of PAHs in different media in the greenhouse,two migration and fate models of PAHs with statistical significance were constructed.The results showed that the PAHs in vegetable leaves were mainly related to the gaseous PAHs,the particulate PAHs and the PAHs in soil.While PAHs in soil mainly came from the atmospheric dry deposition dust in the simulation experiment,followed by the large amount of vegetable rhizomes remaining in the soil.Some PAHs in both granular and gaseous forms would enter the air again by re-suspension or volatilization.The optimized physical model simulation results showed that the PAHs in vegetables mainly came from the absorption of gaseous PAHs by the leaves,accounting for 90.6%of the total PAHs exposure amount in vegetables.The PAHs absorbed by the roots from the soil accounted for 9.4%of the total exposure.And under the dry deposition conditions,4.81 ng/m³ of PAHs in the air originated from the volatilization of PAHs from soil,accounting for 13.2%of the PAHs concentration in the air.The main pathways for different PAH monomers to be absorbed by vegetables were quite different.PAH monomers with log K_AW values between-6 and-1 were mainly absorbed by vegetables through the air-leaf route;and PAH monomers with log K_OW<4 or log K_AW>-1 were mainly absorbed from the soil;the PAH monomers with log K_AW>-3 and log K_OW<5 were more easily volatilized from soil into the air.The PAHs in various media in greenhouses were mainly from oil leakage and coal combustion during coking,followed by wood combustion sources.In addition,some PAHs were derived from transportation sources and diesel-fueled agricultural machinery on farms.?6?The concentrations of PAHs in vegetable leaves under the soil sludge application condition were significantly lower than those under atmospheric dry deposition condition,while the concentrations of PAHs in vegetable roots were significantly higher than those in the latter.When the amount of applied sludge increased,the concentrations of PAHs in vegetable roots and leaves increased significantly.Under the sludge application condition,PAHs could enter the vegetable leaves through the soil-root pathway.The concentrations of PAHs in the topsoil with sludge application would gradually decrease with time,which mainly related to PAHs absorption by vegetables and PAHs volatilization.?7?Physical model was used to calculate the different migration contributions of PAHs in soil.The increase of sludge application in soil is beneficial to promote the absorption of PAHs in vegetables.The concentrations of PAHs in the control group,the sludge application group and double application group were 5.51,9.91 and 10.14ng/m³,respectively.If the concentrations of PAHs in the air of the growing environment was used to simulate,the PAHs absorbed from soil accounted for 9.2%of the total PAHs in the control group.In addition,the concentrations of PAHs volatilized into the air from the soil in the control group,the sludge application group and double application group were 40.83,70.30 and 74.71 ng/m³,respectively.?8?The lifetime cancer risk model results showed that the greenhouse soil,the dry deposition soil in the vicinity of Zifeng Road in Minhang District and the soil in the pot experiment had a potential potential carcinogenic risk to the human body.The carcinogenicity risk of PAHs in soil outside greenhouse was significantly higher than that inside greenhouse.The carcinogenic risk of PAHs mainly came from gastrointestinal exposure,while the carcinogenic risk of PAHs exposed through respiratory tract and skin was lower.The calculation results of the lifetime cancer risk model showed that the naturally grown vegetables near Zifeng Road had potential carcinogenicity risk to all populations,and the risk outside greenhouse was significantly higher than that inside greenhouse.In the three simulation experiments,the carcinogenicity risk caused by vegetables under wet deposition condition was higher than that under dry deposition condition.While the vegetables grown in sludge application soil had no carcinogenicity risk.The health risks of PAHs for children were higher than those for the elderly and adolescents,in which adolescents had the lowest carcinogenicity risk and women had higher carcinogenicity risk than men.The carcinogenicity risk caused by different vegetables was:spinach>Chinese cabbage>lettuce>Shanghai Green.Ingesting spinach could pose potential carcinogenicity risk for children,adults and women,ingesting Chinese cabbage could pose potential carcinogenicity risk for female adults,and ingesting Shanghai Green and lettuce pose no potential carcinogenicity risk to anyone.