In the present study, the desorption kinetics of 15 PAHs (two to six rings) from sediments were determined before and after bioremediation in a bioreactor or landfarm. Desorption kinetics were measured with a method in which the water phase was kept PAH-free by Tenax TA beads. For almost all degraded PAHs, rapidly desorbing fractions (desorption rate constants > 0.1 h -1 ) were much smaller after bioremediation than before treatment whereas the slowly desorbing amounts remained unchanged. Thus, mainly the rapidly desorbing PAHs are degraded during bioremediation. The extent of possible PAH degradation could be roughly predicted from the initial rapidly desorbing fraction. For nondegraded PAHs, the rapidly desorbing fractions were substantial (up to 55%) and remained unchanged by remediation. The magnitude of the rapidly desorbing fractions of the nondegraded PAHs suggests that their persistence is due to microbial factors, not bioavailability.
A simple method to determine the availability of sediment-sorbed organic contaminants was developed and validated. For 10 polycyclic aromatic hydrocarbons, 4 polychlorinated biphenyls, and 9 chlorobenzenes in 6 sediments, we measured the fraction extracted by Tenax in 6 and 30 h. These fractions were compared with the rapidly desorbing fractions determined by consecutive Tenax extraction. Extraction by Tenax for 30 h completely removed the rapidly desorbing fraction plus some part of the slowly desorbing fraction. The fraction removed after 30 h was about 1.4 times the rapidly desorbing fraction. The fraction extracted by Tenax after 6 h is about 0.5 times the rapidly desorbing fraction for chlorobenzenes (CBs)/polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs). The rapidly desorbing fraction probably represents the fraction of sorbed organic compound that poses actual risks for transport to (ground) water and determines the uptake by organisms and that can be microbially degraded. Extraction by Tenax for 6 h provides an easy way to address these issues more accurately than does the measurement of total concentrations.
Desorption kinetics were determined for 1 2,4‐trichlorobenzene (TCB), 13‐dichlorobenzene (DCB), and trichloroeth‐ylene (TCE) in a sediment at various concentrations. The desorption data were interpreted with a (nonmechanistic) first‐order three‐compartment model. In this way, separate sorption isotherms could be constructed for rapidly, slowly, and very slowly desorbing sorbate, respectively. Slowly desorbing (rate constant k ∼10−3/h) and very slowly desorbing (k = 10−4 to 10−5/h) sorbate exhibited nonlinear Langmuir‐type sorption, with capacities on the order of 4.6 to 19 and 0.54 to 1.5 mmol/kg organic carbon (OC) and affinity constants of 0.18 to 41 and 32 to 272 L/mmol, respectively. The affinity constants increased with increasing sorbate hydrophobicity. Rapidly desorbing (k = 10−1/h) sorbate showed linear sorption isotherms, with log Koc (octanol–water partition coefficients) of 1.59 ± 0.12 (TCE), 2.03 ± 0.13 (DCB), and 3.13 ± 0.03 (TCB), respectively. These results confirm the hypothesis that desorption is rapid from linearly sorbing organic matter, whereas it is slow and very slow from nonlinearly sorbing sites. Furthermore, the results also demonstrate the applicability of the desorption kinetic method in terms of experimentally separating an overall Freundlich‐like isotherm in linear and nonlinear isotherms.
Abstract-Desorption kinetics were determined for 1,2,4-trichlorobenzene (TCB), 1,3-dichlorobenzene (DCB), and trichloroethylene (TCE) in a sediment at various concentrations. The desorption data were interpreted with a (nonmechanistic) first-order threecompartment model. In this way, separate sorption isotherms could be constructed for rapidly, slowly, and very slowly desorbing sorbate, respectively. Slowly desorbing (rate constant k ϳ10 Ϫ3 /h) and very slowly desorbing (k ϭ 10 Ϫ4 to 10 Ϫ5 /h) sorbate exhibited nonlinear Langmuir-type sorption, with capacities on the order of 4.6 to 19 and 0.54 to 1.5 mmol/kg organic carbon (OC) and affinity constants of 0.18 to 41 and 32 to 272 L/mmol, respectively. The affinity constants increased with increasing sorbate hydrophobicity. Rapidly desorbing (k ϭ 10 Ϫ1 /h) sorbate showed linear sorption isotherms, with log K oc (octanol-water partition coefficients) of 1.59 Ϯ 0.12 (TCE), 2.03 Ϯ 0.13 (DCB), and 3.13 Ϯ 0.03 (TCB), respectively. These results confirm the hypothesis that desorption is rapid from linearly sorbing organic matter, whereas it is slow and very slow from nonlinearly sorbing sites. Furthermore, the results also demonstrate the applicability of the desorption kinetic method in terms of experimentally separating an overall Freundlich-like isotherm in linear and nonlinear isotherms.
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