increase the amount and change the properties of the natural SOM. For instance, irrigation with wastewater A time-dependent sorption-desorption study of atrazine (2-chlorofor a period of 25 yr increased the organic C level in a 4-ethylamino-6-isopropylamine-s-triazine) was performed to reveal its behavior in wastewater-and freshwater-irrigated soils. Atrazine soil by 50% (Friedel et al., 2000). In addition, Gonzalezsorption affinity to the freshwater-irrigated soils was significantly Vila et al. (1995) reported that irrigation with olive mill higher than to their wastewater-irrigated counterparts. The calculated effluents increased and altered the composition of the C-normalized partition coefficient (K OC) values for the freshwatersoil lipid fraction. In another report, changes in the irrigated soils were 70.7 and 34.2 L kg Ϫ1 organic C and the values for chemical properties of fulvic acid were obtained after the wastewater-irrigated soils were 52.6 and 29.9, respectively, for irrigation with wastewater (Boyd and Sommers, 1990). the two tested soils. These differences did not result from atrazine Application of sewage sludge has also been reported to interactions with dissolved organic matter (DOM) in the wastewaterincrease the amount of aromatic compounds in the irrigated soils. All desorption isotherms exhibited hysteresis. Lower SOM, due to enhanced microbial activity and degradadesorption was exhibited for the 14-d sorption-desorption experition of readily degradable DOM and SOM fractions ments, even though the sorption-affinity parameters were similar for the 2-and 14-d sorption periods. Higher desorption hysteresis was (Kalbitz et al., 2003). observed for the longer sorbate-sorbent contact time for both soils, Hydrophobic organic compounds (HOCs) tend to with desorption decreasing for decreased atrazine loading. The similar sorb to soils mainly via interactions with SOM. Many atrazine-sorption parameters obtained for the two sorption periods reports have shown a positive correlation between the and the increasingly difficult desorption with increasing contact time organic C content and the sorption potential of HOCs suggest that the sorption process occurs in two stages. First, sorption (Means et al., 1980; Garbarini and Lion, 1986; Kleineioccurs to readily available sites; at a later stage, the sorbed molecules dam et al., 1999). Therefore, SOM is considered to siginteract with the soil organic matter (SOM) or the SOM-mineral nificantly affect the fate of HOCs in the environment. complexes such that their desorption ability is significantly hindered. The ability of SOM to sorb and, more importantly, to The higher sorption potential and the lower desorption ability of the desorb HOCs in the soil profile significantly affects the freshwater-irrigated soils suggest that the SOM which develops in the wastewater-irrigated soils delays or hinders atrazine interaction with transport of these pollutants toward groundwater. In sites that are naturally present in these soils. addition to the amount of SOM, the sorp...

The soil lipid fraction can play an important role in the sorption of organic compounds. In this study, the impact of the lipid fraction of freshwater-and wastewater-irrigated soils on the sorption of non-and relatively polar compounds was assessed. Lipid analyses revealed a clear difference between the two lipid fractions. The lipid extract from the wastewater-irrigated soil was consistent with mainly straight paraffinic chain materials; the lipid extract from freshwater-irrigated soil, on the other hand, exhibited stronger signals of aromatics, double bonds, ester, ether, and methyl, in addition to a smaller contribution from methylene protons. Our data suggest that lipid removal induced a stronger increase in the soil's sorption affinity for solutes capable of polar interactions such as atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) and chlorotoluron (N9-(3-chloro-4-methylphenyl)-N,N-dimethylurea) as compared to phenanthrene. Moreover, the level of increase in sorption affinities due to lipid removal was much higher for the freshwater-irrigated soil than for its wastewater-irrigated counterpart, even though the level of lipids in the freshwater-irrigated soil was half that in the wastewater-irrigated one (6 vs. 11% of the total organic C). The higher level of polar functionalities, such as ether and ester moieties, in the lipid fraction from the freshwater-irrigated soil suggests that these extractable compounds compete successfully with the polar solutes (atrazine and chlorotoluron) for specific binding sites in the soil organic matter (SOM). It appears that the composition of the lipid fraction may be a key consideration in unraveling the sorption of organic molecules in soils.

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Nonequilibrium sorption plays an active role in the transport of organic contaminants in soil. We applied a two-stage, one-rate model (2S1R) and a new, nonlinear variant (2S1RN) of this model to examine the effects of wastewater irrigation on the sorption kinetics of atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) in soil. The models were applied to previously published sorption-desorption data sets, which showed pronounced deviations between sorption curves and desorption curves (sorption-desorption hysteresis). Moreover, the slopes of the desorption curves decreased with decreasing concentration. Different treatments had been used, and two experimental time steps (2 and 14 d) were used. Treatments considered were lipid removal, fulvic and humic acid removal, and untreated soil. The 2S1R model was unable to reproduce the observed type of hysteresis, but the 2S1RN model, which assumes that the sorption-desorption process follows a power function relationship, was able to reproduce the observed type of hysteresis. Visually, applying the new model improved the model fits in all test cases. Statistically, as tested by an extra sum of squares analysis, the new model performed significantly better in 50% of all test cases. According to an example simulation, the choice of the sorption model has a considerable impact on the prediction of atrazine transport in soil.