Differentiating between physical and chemical constraints on pesticide andwater movement into and out of soil aggregates

A laboratory experiment comparing the movement of (H2O)-H-3 and [C-14]isopr oturon into and release from soil aggregates is described. Small aggregates (2.0-2.4 mm) were prepared from a clay topsoil and maintained at three dif ferent initial moisture conditions. A small volume of the radioisotope solu tion was introduced prior to bathing the aggregates in a 2 mM CaCl2 solutio n to represent new rainwater. Whilst the (H2O)-H-3 was imbibed by the air-d ry aggregates, the pesticide did not follow the water but remained on the s urface of the aggregates. This may be related to its sorptive properties an d an excess of sorption sites on the sorbent with respect to the sorbate. I ncreasing the length of exposure of the moist aggregates to [C-14]isoprotur on reduced the initial release of the compound into the bathing solution, p robably due to diffusion (retarded by sorption) into the aggregates. The di ffusion model described by Crank and a non-equilibrium desorption model wer e used to analyse the (H2O)-H-3 and [C-14]isoproturon release curves. This showed that the release of (H2O)-H-3 from the dry aggregates was controlled by diffusion. The release of isoproturon was probably controlled by non-eq uilibrium sorption/desorption from air-dry aggregates and by a combination of non-equilibrium sorption/desorption and diffusion from wet aggregates. ( C) 1999 Society of Chemical Industry.