In Situ Investigation Into Surfactant Effects On The Environmental Behavior Of Gaseous Parent And Alkylated PAHs Adsorbed Onto Crop Leaf Surfaces

The exchange of PAHs on the atmosphere-plant interface is very active.PAHs absorbed by plants can be transmitted and bioaccumulation to the animals and human by the food chain.Therefore,an increased understanding of the environmental behaviors of parent and alkylated PAHs between the air and vegetation has significant implications for crop safety and risk assessments.Moreover,under natural conditions,PAHs that are adsorbed onto crop leaf surfaces commonly coexist with many other substances?e.g.,surfactants?.The plasticizing effect of surfactant can not only decrease the viscosity and tortuosity of epicuticular wax,but it can also increase the porosity of the cuticle,enhancing the accessibility of plant cuticle to sorbate and offering more adsorption sites and specific interaction domains on the outer surface.However,the impact of surfactants on the environmental behavior of parent and alkylated PAHs by living crop leaf surfaces remain unclear so far.In this study,the uptake and clearance of gaseous parent and alkylated PAHs by living crop leaf surfaces,as well as the effects of surfactant,were investigated in situ using fiber-optic fluorimetry,and the possible mechanisms were discussed.Main results were summarized as followings:1.A set of fiber-optic fluorimetry system was utilized for the in situ quantitative detection of Phe,3-MP and Retene that was absorbed onto the maize and soybean leaf surfaces.Experimental results showed that the linear dynamic ranges for the in situ determination of the Phe,3-MP and Retene adsorbed onto the leaf surfaces of maize/soybean were 2.4-1200/3.5-1000?1.3-1000/1.1-1100 and 2.6-985/3.7-925 ng/spot,with the detection limits of0.20/0.60?0.24/0.41 and 0.32/0.36 ng/spot,respectively.The relative standard deviations were less than 5.3%/4.6%?n=9?.The experimental recoveries for the maize/soybean were 98.2-102.7%/97.1-105.2%?90.3-105%/99.2-102.3%and 83.3-107.7%/96.6-104.2%,respectively.These results clearly confirmed the feasibility of the fiber-optic fluorimetry for the in situ determination of adsorbed Phe,3-MP and Retene.2.The uptake of Phe,3-MP and Retene by maize and soybean leaf surfaces,as well as the effects of CTMAB and SDBS,were investigated in situ using fiber-optic fluorimetry.Experimental results showed that the uptake achieved equilibrium between the air and crop leaf surfaces within the120-h monitoring periodit and that neither of the two surfactants have any effects on the time required to achieving adsorption equilibrium.There is inter-chemical and inter-species variability in terms of both the time required reaching equilibrium and the EAC.The EAC of the parent and alkylated PAHs for each of the three crops‘leaf surfaces increased with the number of alkyl substitutions on the aromatic ring.For any given PAHs,the EAC values followed the sequence of soybean>maize,which was dominantly controlled by their leaf surface polarity index??O+N?/C?.The presence of CTMAB and SDBS increased the EAC of Phe,3-MP and Retene in the three crops‘leaf surfaces by 5.3-25.7%,due to the plasticizing effect induced by the surfacesorbed surfactants,and the enhancement degree was closely associated with leaf-wax content and lg K_OW values of PAHs.3.The established fiber-optic fluorimetry method was employed to in situ investigate effects of the CTMAB and SDBS on the clearance of the Phe,3-MP and Retene adsorbed individually onto the leaf surfaces of maize and soybean.Experimental results showed that the clearance of the parent and alkylated PAHs from the living crop leaf surfaces included a rapid and a slow phase,and both of them followed first-order kinetics.The effects varied significantly with surfactant types primarily in terms of the elimination rates and the final residues of PAH chemicals.With increasing CTMAB and SDBS concentrations,volatilization rate constants of both parent and alkylated PAHs initially decreased at fast rates and then at more moderate rates later on,resulting from the plasticizing effect of surfactants adsorbed on crop leaf surfaces.In addition,the photolysis rate constants decreased with the presence of SDBS but increased with the presence of CTMAB.Overall,the total clearance rates of parent and alkylated PAHs adsorbed onto living crop leaf surfaces were inhibited by the presence of CTMAB and SDBS.