Competitive effects on the slow desorption of organic compounds from sediments

Experiments were carried out concerning competition effects in slow desorption. Excess lab-added 1,2,4-trichlorobenzene (TCB) decreased the slowly desorbing amounts of aged in situ polychlorinated biphenyls (PCBs) and chlorobenzenes in sediment from Lake Ketelmeer (KM), The Netherlands, after 14 d of TCB-sediment contact time (factor of l.2–2.7). Another experiment showed that the presence of large quantities of in situ oil (15 g/kg) and polycyclic aromatic hydrocarbons (PAHs; l.5 g/kg) strongly inhibited the formation of slowly desorbing amounts of lab-added pentachlorobenzene, PCB-30 and PCB-65. These results imply that (i) freshly sorbed contaminants can influence the desorption behavior of aged contaminants within weeks, and (ii) PAH, oil, PCBs and chlorobenzenes show competition for the same slow sorption sites in sediment. So, the occurrence of competitive effects on slow desorption indicates that the number of slow sorption sites is limited. This is in accordance with the recent finding that part of the sorption of organic compounds in soils and sediments occurs in a nonlinear way, the slowly desorbing fractions showing this nonlinear sorption.