Mechanism of slow desorption of organic compounds from sediments: A study using model sorbents

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