Sorption Behavior And Penetration Process Of Typical Organic Pollutants Onto Plant Cuticles

Aerial surfaces of the primary parts of plants are covered by a hydrophobic film named cuticle. Plant cuticles are made up of waxes, cutin, cutin as well as polysaccharides, with three distinctive compartments named epicuticular wax (EW), cuticle proper (CP), and cuticular layer (CL), respectively. The rate of compounds transfer through the plant cuticles is believed to be the dominant limiting factor in the uptake of organic contaminants, which affects the transport of organic pollutants and food safety. In this dissertation, sorption and diffusion characteristics of organic compounds with plant cuticles were reviewed, and then it is noted that some problems need to be further resolved, such as the dose-effect of organic pollutants sorption onto plant cuticles, and the diffusion process of organic pollutants were unclear. To resolve these problems, selected cuticle of plant leaves and fruits were isolated, and characterized by SEM, FTIR, NMR, TD-DTG and elemental analyzer. Sorption behavior of organic pollutants onto plant cuticular fractions was investigated, and sorption compartment model of plant surface was proposed. Penetration process of organic pollutants across the plant cuticle was studied, and the distribution properties of organic pollutants in plant cuticle were discussed during penentration. To further examine the sorption and penetration properties of plant cuticle, leaf samples of three tree species were collected for measurement of polycyclic aromatic hydrocarbons (PAHs) in plant leaf cuticles and inner leaf tissues. The main original conclusions of this work are drawn as follows:(1) Sorption mechanism of organic pollutants onto plant cuticles was elucidated, and a modified lipid-model was suggested to precisely predict plant uptake. The main storage medium of plant cuticle is not attributed to extractable lipids but polymeric lipids, for their higher content (0.8-29folds) and sorption capacity (1.4-5.9folds) in comparison with extractable lipids. The higher sorption coefficients (Kd) of polymeric lipids was attributed to its liquid-like nature with Tg≈-44℃, whereas epicuticular waxes exhibit solid-like nature with Tg-44C. To accurately preditct the sorption behavior, the content of polymeric lipids should be determined and incorporated to the typtical lipid model, and the sorption coefficient (Kd=fwaxKwax+fcutinKcutin) was contributed by both extractable lipids (fwaxKwax) and polymeric lipids (fcutinKcutin).The predicted Kd by the modified lipid model was quite similar to the deterimined value. The significant role of polymeric lipids in plant uptake of organic pollutants was further demonstrated by actual data that total PAHs concentrations of plant were highly related with their polymeric lipids content in both leaf cuticles and the inner tissues.(2) Interaction mechanism of plant cuticular fractions with organic pollutants, and the role of aromatic cores and aliphatic moieties in organic pollutants sorption were elucidated. It is found that cellulose components play a regulating role in the relative contribution of aromatic cores and aliphatic moieties to sorption of plant cuticular fractions. With the coexistence of the amorphous cellulose, the sorption of aromatic C was completely inhibited, so the nonpolar aliphatic moieties (cutin and waxes) dominated the sorption of cuticle. As a result, a linear positive correlation of Kd with the content of aliphatic moieties of cuticle was observed. With the consumption of the amorphous cellulose under acid hydrolysis, sorption capacities of the de-sugared fractions were dramatically enhanced, controlled by the exposed aromatic cores (cutan) and the aliphatic moieties. Nonlinear and competitive sorption isotherms of desugar fractions were observed.(3) The penetration mechanism of organic pollutants across plant cuticle was illustrated, and a new "Three Compartments" penetration model was proposed. It is found that plant cuticle exhibited a linear penetration curve with the decreasing concentration gradient between donor and acceptor aqueous solution. It is proved that the driving force of the diffusion is neither the concentration gradient in the solution nor the concentration gradient of sorbed pollutants on plant cuticle. The penetration process of organic pollutants through plant cuticles can not be simulated by Fick’s law. Based on the penetration characteristics of organic pollutants and its sorption properties,"Three Compartments" penetration model was suggested to describe the penetration process of cuticular fractions. In the process of penetration from epicuticular wax (EW) through cuticle proper (CP) to cuticular layer (CL), the limited diffusion process occurs on the interface of CP with CL. Penetration flux decreased with the increasing sorption capacity of cuticle. The diffusion of organic pollutants across plant cuticle was reversible, and the pseudo-phenomena of "active transport" of organic pollutants happened on the interface of CP with CL due to the strong affinity of cuin and wax.