Triphasic desorption of highly resistant chlorobenzenes, polychlorinated biphenyls, and polycyclic aromatic hydrocarbons in field contaminated sediment

Pollutants in aged field sediments seem to differ from spiked sediments in their chemical and biological availability. Biphasic desorption is often used as an explanation. In the present study, desorption kinetics and partitioning of chlorobenzenes (CBs), polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons (PAHs) in long term field contaminated sediment cores and top layer sediment were measured by gas-purging. Desorption from sediment was deduced to be triphasic: fast, slowly, and very slowly desorbing fractions were distinguished. In both the sediment core and the top layer sediment no detectable fast fractions were present for all the compounds studied, so these were estimated as upper limits from the desorption curves. This observation coincided with very high in situ distribution coefficients for several PCBs and PAHs: 10-1000 times higher than literature values for short contact time experiments. Rate constants were (3-8) x 10(-3) h(-1) for slow desorption and (0.16-0.5) x 10(-3) h(-1) for very slow desorption. In some cases only a very slowly desorbing fraction was detectable. Desorption from field contaminated sediments with extended contact times may not be readily estimated from laboratory experiments in which contaminants have contact times with the sediment: in the order of weeks.