| Sorption to natural solid is an underlying process affecting the transport, degradation, and biological activity of organic pollutants in the environment. Desorption of many organic pollutants from natural sediments has been observed to be biphasic, containing reversible and irreversible compartments. A fraction of the adsorbed contaminants desorbs reversibly, as expected, but there exists a second and distinct fraction that is resistant to desorption. Irreversible sorption of organic pollutants has attracted more attention in the past years because the irreversible behavior affected the sediment quality criteria, remediation efficiency and the potential danger of pollutants in natural water body.The overall objective of this research has been to explain what causes the desorption of chemicals in the sediments to be resistant and to understand the consequences with respect to sediment clean-up and how-clean-is-clean. In this research, eight sediments covering a wide range of organic carbon content from 0.54 to 36% were selected as sorbents, while phenanthrene and pentachlorophenol (PCP) were selected as model contaminants. Equilibrium sorption experiments and kinetics sorption process were carried out to invest the sorption properties of phenanthrene and PCP in sediments. For modeling the dynamic sorption process, compartment model was applied. Based on the conclusions of modeling for dynamic sorption experiments, correlation analysis between the sorption parameters and the sediment properties was carried out to find out the main fractions affected the irreversible sorption behaviors. The followings were the main results.The sorption capacity of sediment was determined by the organic carbon content of sediment. In relatively low concentration range, the isotherms of phenanthrene and PCP were linear, while in relatively high concentration range, the isotherms were nonlinear, which can be described with Freundlich equation. The partition coefficient (normalized to organic carbon) logKoc for phenanthrene was 4.18+2.98, and for PCP was 2.87+1.50. However, pH values of sediments can inflect the logKoc of PCP. The relation of logKoc and pH was logAToc=-0.2129pH+4.431, r2=0.9049, While thelogKoc of phenanthrene wasn't inflected by pH. That was caused by the PCP slightly ionization in water. The ratio of ionized PCP in total amount was high at high pH, which decreased the sediment sorption capacity. When methanol and model contaminants were coexistent, no competitive sorption was observed, that suggested the partition process in sorption course.Sorption of model pollutants into sediments were biphasic, a fast sorption phase followed by a slow sorption phase. The equilibrium in fast phase reached in 8hrs, while equilibrium in slow phase would cost more than 20d. Likewise, desorption were biphasic, too, containing a fast phase no more than 8hrs, in which a small portion model pollutants adsorbed in sediment desorbed into aqueous phase quickly, however, the other portion seemed to be resistant to desorption. The results of multi-cycle sorption/desorption experiments showed that the irreversible sorption amount of phenanthrene for all sediments were larger than that of PCP. Hypothesis was presented that phenanthrene and PCP molecules sorbed into solid phase were captured by fulvic acid of sediment, which induced irreversible sorption. Therefore, conclusion was drawn that lipid fraction maybe was the main fraction to affect the hysteresis phenomenon, i.e., the higher the lipid percentage, the greater the HI value.From the results of modeling based on the compartment model, the amount of irreversible phase increased with the sorption time, while the reversible phase decreased, at the premise of equilibrium of aqueous phase concentration with total solid phase adsorbed amount. The modeling data was agreement to the experimental data in a rational range, which suggested the feasibility of compartment model for slow sorption process. But the hypothesis that the increased amount in irreversible phase comes from...
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