MECHANISM OF SLOW DESORPTION OF ORGANIC-COMPOUNDS FROM SEDIMENTS - A STUDY USING MODEL SORBENTS

The desorption kinetics of PCBs and chlorohenzenes have been studied a t 5, 20, and 60 degrees C for model sorbents in which either micropore diffusion (zeolite, montmorillonite, and XAD-8) or organic matrix dif fusion/entrapment (rubbery polyacetal and glassy polystyrene) could oc cur. Also, a sediment was studied whose organic matter (OM) had been c ompletely removed. All sorbents exhibited slow desorption (rate consta nts (1-5 x 10(-3) h(-1)). The sediment without OM showed significantly smaller slowly desorbing fractions (factor 3-8) than the original sed iment (about 6% OM). Sorbent-water distribution ratios of the mi cro p orous sorbents and the sediment without OM were 10-100 times lower tha n the ones of the original sediment. So, although the presence of both mineral micropores and/or OM can result in slow desorption behavior o f organic compounds from soils and sediments, OM is more important for slow desorption than mineral micropores in sediments with more than a bout 0.1-0.5% OM. The sorption and desorption parameters measured for the sorbents were compared to the ones measured for sediment. This ana lysis showed that the observations for XAD-8 tin which slow desorption is assumed to be caused by slow diffusion along hydrophobic pore wall s) were most similar to the ones for the sediment, indicating that dif fusion through pores in the organic matter or pores coated with organi c material play roles in slow desorption.