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

Nonsingular sorption-desorption behavior of organic compounds has been frequently observed, but it remained unexplained in many cases. In this study we investigate the role slow kinetics may play in nonsingular sorption. Sorption-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 loamy sand-using three different batch techniques. The first technique is the classic decant-refill method where part of the supernatant is replaced with solute-free solution. The second technique is similar; however, part of the supernatant is replaced with solute containing solution instead. In the third technique desorption is induced by successive dilution of the supernatant with time. Nonsingular sorption of chlortoluron was observed independent of the method. We fitted a kinetic two-stage model to the data of the first technique to estimate sorption rate parameters. With this set of parameters, the model predicted the data measured with the second and third technique well. The agreement indicates that nonsingularity phenomena observed by means of the different techniques may be explained by slow sorption kinetics. The ability of the model to describe nonsingular sorption-desorption data was further tested on data sets taken from the literature where nonsingularity remained unexplained or was explained by other causes different from slow kinetics. All phenomena observed in the different studies could readily be explained by the two-stage model.