The kinetics and reversibility of the partitioning of polychlorinated biphenyls between air and ryegrass.

Lolium multiflorum (ryegrass) was contaminated with technical mixtures of polychlorinated biphenyls (PCBs) via the gas phase and the subsequent elimination of the PCBs from the vegetation was studied under field conditions. There was a pronounced decrease in the concentrations of the di- through to the pentachlorinated congeners over the 240-h elimination period. For many of the congeners the elimination was nearly complete, suggesting that the partitioning of these compounds from the gas phase into ryegrass is largely reversible. The elimination followed first order kinetics. The elimination half-lives were linearly proportional to the plant/air equilibrium partition coefficients (K(PA)) of the PCB congeners and ranged from 22 h for PCBs 8 + 5 to 87 h for PCBs 84 + 101. For the hexa- through to the octachlorinated congeners no significant decrease in the concentration of the vegetation was observed during the elimination experiment. The elimination was described well using a two-resistance model of PCB desorption which indicated that elimination of the di- and trichlorinated PCBs was limited by transport within the plant itself while elimination of the higher chlorinated congeners was limited by transport from the plant surface into the atmosphere. Furthermore, the results demonstrate that if elimination of higher chlorinated PCBs from ryegrass via biological or photochemical degradation occurs, then it is very slow.