Influence of long contact times on sediment sorption kinetics of spiked chlorinated compounds

The desorption kinetics of hexachlorobenzene (HCB) and 2,4,4'-trichlororbiphenyl (PCB 28) spiked to a field sediment were studied using a gas-purge technique. A contact time of up to 1,461 d was used to assess long-term changes in desorption kinetics. Purge-induced desorption experiments lasted from 300 to more than 4,000 h. Fast-, slow-, and very-slow-desorbing fractions could be distinguished. The desorption patterns changed with contact time from mainly fast- and slow-desorbing fractions toward the domination of slow- and very-slow-desorbing fractions. The desorption pattern for HCB after a contact time of 1,461 d became comparable to previously reported desorption patterns observed for in situ contaminants. An additional spike of HCB, PCB 28, 2,4,6-trichlorobiphenyl, and 2,2',4,5'-tetrachlorobiphenyl applied to the sediment purged for more than 4,000 h showed that very slow sites were accessible for these compounds within a few hours. Only very small fast-desorbing fractions could be detected after a contact time of just 48 h. These results indicate that domination of very slow desorption is caused not only by long contact times but, perhaps, also by the accessibility of specific sites within the sediment matrix.