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

Lolium multiflorum (ryegrass) was contaminated with technical mixtures of p olychlorinated biphenyls (PCBs) via the gas phase and the subsequent elimin ation of the PCBs from the vegetation was studied under field conditions. T here 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 p artitioning 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 partitio n 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 w as observed during the elimination experiment. The elimination was describe d well using a two-resistance model of PCB desorption which indicated that elimination of the di- and trichlorinated PCBs was limited by transport wit hin the plant itself while elimination of the higher chlorinated congeners was limited by transport from the plant surface into the atmosphere. Furthe rmore, the results demonstrate that if elimination of higher chlorinated PC Bs from ryegrass via biological or photochemical degradation occurs, then i t is very slow. (C) 2000 Elsevier Science B.V. All rights reserved.