Sorptive Capacities of Nonpolymeric Plant Lipids for Hydrophobic Chemicals Determined by Passive Dosing.

Vegetation plays an important role in the partitioning, transport, and fate of semivolatile hydrophobic organic chemicals (HOCs) in the environment. Leaf/air partition ratios ( Kleaf/air) of HOCs are highly variable for different plant species. The differences cannot be fully explained by the fraction of lipids in the leaves or the thickness of the cuticle. Our goal was to elucidate the importance of nonpolymeric lipids in determining Kleaf/air. To do this, we extracted organic matter from 7 plant species using solvents that do not extract the polymeric lipids cutin and cutan, to yield extractable organic matter (EOM). We used passive dosing to determine the partition ratios of selected HOCs between the EOM of the leaves and our reference lipid, olive oil ( KEOM/olive oil). In addition, we measured analogous partition ratios for three lipid standards. Proton nuclear magnetic resonance (NMR) spectroscopy was used to characterize the composition of lipids. Differences in KEOM/olive oil of two polychlorinated biphenyls and four chlorinated benzenes were below a factor of 2 in the plant species studied, indicating that the reported differences in Kleaf/air are not caused by differences in the sorptive capacities of nonpolymeric lipids or that our EOM is not representative of all nonpolymeric leaf lipids.

A passive dosing method to determine fugacity capacities and partitioning properties of leaves.

The capacity of leaves to take up chemicals from the atmosphere and water influences how contaminants are transferred into food webs and soil. We provide a proof of concept of a passive dosing method to measure leaf/polydimethylsiloxane partition ratios (Kleaf/PDMS) for intact leaves, using polychlorinated biphenyls (PCBs) as model chemicals. Rhododendron leaves held in contact with PCB-loaded PDMS reached between 76 and 99% of equilibrium within 4 days for PCBs 3, 4, 28, 52, 101, 118, 138 and 180. Equilibrium Kleaf/PDMS extrapolated from the uptake kinetics measured over 4 days ranged from 0.075 (PCB 180) to 0.371 (PCB 3). The Kleaf/PDMS data can readily be converted to fugacity capacities of leaves (Zleaf) and subsequently leaf/water or leaf/air partition ratios (Kleaf/water and Kleaf/air) using partitioning data from the literature. Results of our measurements are within the variability observed for plant/air partition ratios (Kplant/air) found in the literature. Log Kleaf/air from this study ranged from 5.00 (PCB 3) to 8.30 (PCB 180) compared to log Kplant/air of 3.31 (PCB 3) to 8.88 (PCB 180) found in the literature. The method we describe could provide data to characterize the variability in sorptive capacities of leaves that would improve descriptions of uptake of chemicals by leaves in multimedia fate models.

Mass transfer of hydrophobic organic chemicals between silicone sheets and through plant leaves and low-density polyethylene.

Plant leaves play an important role in the fate of hydrophobic organic contaminants (HOCs) in the environment. Yet much remains unknown about the permeability of leaves by HOCs. In this pilot study we measured (i) the kinetics of mass transfer of three polycyclic aromatic hydrocarbons (PAHs) and six polychlorinated biphenyls between a spiked and an unspiked sheet of polydimethylsiloxane (PDMS) in direct contact with each other for 24 h and (ii) kinetics of mass transfer of two PAHs through leaves and low-density polyethylene (LDPE) in a passive dosing experiment by inserting these matrices between the two sheets of PDMS for 48 h. The kinetics of mass transfer of fluoranthene between PDMS sheets in direct contact were a factor of 12 slower than those reported in the literature. The kinetics of mass transfer of fluorene and phenanthrene through leaves were within the range of those previously reported for 2,4-dichlorophenoxyacetic acid through isolated cuticles. Our results provide a proof-of-concept demonstration that the passive dosing method applied in this study can be used to measure the mass transfer coefficients of organic chemicals through leaves. Key recommendations for future experiments are to load the PDMS at the highest feasible concentrations to avoid working at analyte levels close to the limit of detection, to keep the leaves moist and to minimize potential pathways for contamination of the PDMS sheets by exposure to laboratory air.