Sorption and conformational characteristics of reconstituted plant cuticular waxes on montmorillonite

Plant cuticular waxes are essential barriers that regulate the transport of water and organic molecules to intact cuticular membranes. They also compose a significant fraction of the recalcitrant aliphatic components of soil organic matter (SOM). In this study, we examined the sorption and desorption of three polycyclic aromatic hydrocarbons (PAHs), naphthalene (NAPH), phenanthrene (PHEN), and pyrene (PYR), by cuticular waxes of green pepper (Capsicum annuum) that had been reconstituted by loading them onto montmorillonite (at four different loadings). The reconstituted wax samples, with and without sorbed PAHs, were characterized by solid-state C-13 NMR to supply the evidence of melting transition. The sorption isotherms fit well to a Freundlich equation. Sorption isotherms were practically linear except for that of PYR sorption to the low-load wax-montmorillonite sample. The organic-carbon-normalized sorption coefficients (K-oc) depended on PAH's lipophilicity (e.g., octanol-water partition coefficient) and increased with increasing wax-load on clay. Desorption was dependent on PAH's molecular sizes and sorbed amounts and on the wax load of the clay. Desorption hysteresis was observed only at high loads of NAPH and PHEN, and it decreased with both increasing wax load and molecular size (i.e. NAPH > PHEN > > PYR). Contributing to hysteresis, the melting transition of the reconstituted waxes after sorbing the PAHs was confirmed by solid-state C-13 NMR data. Upon adsorption, the intensity of the NIVIR peak at 29 ppm (attributed to mobile amorphous paraffinic domains) increased, and a peak at 167 ppm (-COOH) appeared, reflecting the transition of solid amorphous to mobile amorphous domains in the reconstituted waxes. The intensity of melting induced by PAH adsorption decreased with increasing PAH molecular size.