Nonsingular sorption of organic compounds in soil: The role of slow kinetics

Nonsingular sorption-desorption behavior of organic compounds has been freq uently observed, but it remained unexplained in many cases. In this study w e investigate the role slow kinetics may play in nonsingular sorption. Sorp tion-desorption experiments were carried out with the herbicide chortoluron (3-(3-chloro-p-tolyl)-1,1-dimethylurea) and two soils-a silt loam and a lo amy sand-using three different batch techniques. The first technique is the classic decant-refill method where part of the supernatant is replaced wit h solute-free solution. The second technique is similar; however, part of t he supernatant is replaced with solute containing solution instead. In the third technique desorption is induced by successive dilution of the superna tant with time. Nonsingular sorption of chlortoluron was observed independe nt of the method. We fitted a kinetic two-stage model to the data of the fi rst technique to estimate sorption rate parameters. With this set of parame ters, the model predicted the data measured with the second and third techn ique well. The agreement indicates that nonsingularity phenomena observed b y means of the different techniques may be explained by slow sorption kinet ics. The ability of the model to describe nonsingular sorption-desorption d ata was further tested on data sets taken from the literature where nonsing ularity remained unexplained or was explained by other causes different fro m slow kinetics. All phenomena observed in the different studies could read ily be explained by the two-stage model.