Sorption equilibrium and kinetics of hydrophobic nonionic organic pollutants in streams

The objectives of this research were to evaluate the effect of dissolved natural organics on the observed apparent sorption and desorption equilibria of hydrophobic non-ionic organic pollutants, to estimate the sorption and desorption kinetics and to examine the importance of sorption kinetics on the predictions of pollutant distribution in the transverse mixing zone of an ideal stream with a point source.;1,4-Dichlorobenzene was selected as the model hydrophobic non-ionic organic pollutant. Sediments collected from Lake Austin were fractionated into 10 classes with different particle size distributions, organic carbon contents, and organic carbon compositions. The sorption and desorption equilibria in batch-type experiments were quantified by both headspace analysis and liquid-phase analysis utilizing conventional phase separation methods. The sorption and desorption kinetics were observed by gas-purging method with an experimental apparatus which can be operated in both closed and open modes.;The experimental results indicated that dichlorobenzene is sorbed on organic matter both dissolved and associated with sediment particles. This finding implies that the solid concentration effect and desorption hysteresis are probably caused by the sorption of sorbate on non-separable sorbents. Another finding was that the fulvic acid fraction has higher affinity for dichlorobenzene than humic acid and the humin fraction. A model reflecting the sorption of dichlorobenzene on dissolved organics successfully described the sorption coefficients measured by liquid-phase analysis with assumptions for the composition of dissolved organics.;The experimental results for sorption kinetics with different sorbent types showed that the sediments have two types of sorption sites with different sorption and desorption rates, which can be approximated by first-order rates. The two types of sorption sites can be distinguished by the organic fractions such as the fulvic acid fraction with a relatively rapid rate constant and humic acid and humin fractions with a slower rate constant. As a result, the overall kinetics were described by a two step, parallel first-order rate model.;A pollutant transport model with the two step, parallel sorption kinetic model was developed to simulate the pollutant distribution in the transverse mixing zone of an ideal stream, based on the weight-averaged properties of sorbents. The simulation results were compared with other types of transport model, adopting some simplification for sorption kinetics or sediment characteristics. The comparison indicated that the proper description of both sorption kinetics and sediment characteristics is very important in modeling the pollutant transport.