Sorption of polar and nonpolar aromatic organic contaminants by plant cuticular materials: Role of polarity and accessibility

In both forest and agricultural soils, plant derived cuticular materials can constitute a significant part of soil organic matter. In this study, the sorption of nonpolar (naphthalene and phenanthrene) and polar (phenol and 1-naphthol) aromatic organic pollutants to aliphatic-rich cuticularfractions of green pepper (Capsicum annuum) (i.e., bulk (PC1), dewaxed (PC2), nonsaponifiable (PCS), nonsaponifiable-nonhydrolyzable (PC4), and dewaxed-hydrolyzed residue (PCS)) were examined to better understand the influence of polarity and accessibility on their sorption behavior. The polarity and structures of cuticular fractions were characterized by elemental analysis, Fourier transform infrared spectroscopy, and solid-state ¹³C NMR. The sorption isotherms fit well to the Freundlich equation. Sorption of the tested organic compounds to PC4, which had more condensed domains, was nonlinear (Freundlich N_s values of 0.766-0.966). For naphthalene and phenanthrene, the largest sorption capacity (K_oc) occurred in PC5, which contained the highest paraffinic carbons (63%) and the lowest polarity: ∼2 and ∼3 times higher than the respective carbon-normalized octanol-water partition coefficient (K_OWC), indicating that PCS was a powerful sorption medium. For phenol and 1-naphthol, the largest K_oc values occurred in PC4 with polar aromatic cores: ∼17 and ∼7 times higher than the respective K_owc, suggesting that PC4 was much more accessible and compatible to polar aromatic pollutants than nonpolar aromatic pollutants. There was little or no correlation of K_oc with either aliphatic or aromatic components of the tested aliphatic-rich sorbents because the polarity and accessibility apparently played a regulating role in the sorption of organic contaminants.