SORPTION OF NAPROPAMIDE ON CLAY AND SOIL IN THE PRESENCE OF DISSOLVED ORGANIC-MATTER

Dissolved organic matter (DOM) can affect the distribution of solutes between solution and sorbed phases and the availability and environmental fate of the solutes. Batch sorption isotherm techniques were used to evaluate solute-solute and solute-sorbent interactions that control the effects of DOM on the sorption of a nonionic, moderately polar organic solute by solid sorbents. The sorption of napropamide (2-(α-naphthoxy-N,N-diethyl propionamide) by Na+-, Cu2+-, and Al3+-montmorillonite decreased when dissolved humic acid derived from peat (peat-DHA) was present in the slurry. For Na-montmorillonite, the effect of DOM on sorption was reduced when a dialysis membrane prevented contact between the DOM and the clay. This suggests that competition for sorption sites on the clay between DOM and the pesticide contributed to decreased napropamide sorption. The extent of the DOM effect was also dependent on the concentration and the source of DOM added. In contrast to the montmorillonite system, the effect of DOM on napropamide sorption by soil was observed only when the dialysis membrane was present or when the dissolution of native organic matter from the sorbate surface itself was enhanced by increasing the pH of the slurry system. These results demonstrate that the effect of DOM on the sorption of nonionic pesticides by soils and sediments can be a function of the association of DOM with pesticides in the solution phase, interactive forces of both DOM and the pesticide with the sorbent surface and the nature of the sorbent surface.