Sorption-desorption behavior of atrazine in soils irrigated with reclaimed wastewater

A time-dependent sorption-desorption study of atrazine (2-chloro-4- ethylamino-6-isopropylamine-s-triazine) was performed to reveal its behavior in wastewater- and freshwater-irrigated soils. Atrazine sorption affinity to the freshwater-irrigated soils was significantly higher than to their wastewater-irrigated counterparts. The calculated C-normalized partition coefficient (K_OC) values for the freshwater-irrigated soils were 70.7 and 34.2 L kg^-1 organic C and the values for the wastewater-irrigated soils were 52.6 and 29.9, respectively, for the two tested soils. These differences did not result from atrazine interactions with dissolved organic matter (DOM) in the wastewater-irrigated soils. All desorption isotherms exhibited hysteresis. Lower desorption was exhibited for the 14-d sorption-desorption experiments, even though the sorption-affinity parameters were similar for the 2- and 14-d sorption periods. Higher desorption hysteresis was observed for the longer sorbate-sorbent contact time for both soils, with desorption decreasing for decreased atrazine loading. The similar atrazine-sorption parameters obtained for the two sorption periods and the increasingly difficult desorption with increasing contact time suggest that the sorption process occurs in two stages. First, sorption occurs to readily available sites; at a later stage, the sorbed molecules interact with the soil organic matter (SOM) or the SOM-mineral complexes such that their desorption ability is significantly hindered. The higher sorption potential and the lower desorption ability of the freshwater-irrigated soils suggest that the SOM which develops in the wastewater-irrigated soils delays or hinders atrazine interaction with sites that are naturally present in these soils.