Soil Leaching Behavior of the Herbicide Clopyralid Supported On Chitosan-Intercalated Montmorillonite.

Organoclays have been shown to be good adsorbents for different classes of pesticides. They have been proposed to remove pesticides from aqueous solutions and also as supports to design pesticide controlled-release formulations. Organoclays prepared by replacing the original inorganic cations of clays with non-toxic, natural organic cations are of particular interest to minimize the environmental impact of the adsorbent once incorporated into natural ecosystems for practical applications.

Chitosan (CH), poly-β (1,4)-2-amino-2-deoxy-D-glucose, is a natural cationic polymer primarily found in the exoskeletons of crustaceans and cell walls of fungi. Previous work has demonstrated that CH readily intercalates into Wyoming montmorillonite and that the resulting montmorillonite-CH bionanocomposite displays high affinity for the acidic herbicide clopyralid, although only at pH levels where the anionic form of the herbicide (pK_a= 2.1) and the cationic form of CH (pK_a= 6.3) predominate (i.e., pH= 3-5). This is because adsorption of clopyralid on CH-intercalated montmorillonite occurs through an anion exchange mechanism on positively charged CH sites at the montmorillonite surface.

In this work, the ability of chitosan-intercalated montmorillonite to act as a carrier of the herbicide clopyralid, to reduce clopyralid leaching losses after soil application, was evaluated. For this purpose, clopyralid was pre-adsorbed on CH-intercalated montmorillonite, added to 20 cm length x 3 cm i.d. hand-packed soil columns, and its leaching profile was compared with that of the free (water-dissolved) herbicide. The results showed that surface application of clopyralid supported on CH-intercalated montmorillonite led to reduced leaching of the herbicide in both an acidic soil (pH= 4.0) and a slightly alkaline soil (pH= 7.5). In contrast, when the herbicide was incorporated into the top 1 cm soil layer, herbicide leaching was reduced only in the acidic soil. This behavior was attributed to rapid release of the herbicide from CH-intercalated montmorillonite at high soil pH levels, an effect that was particularly noticeable when the herbicide is soil incorporated. Surface application vs soil incorporation and soil pH were thus identified as important factors determining the leaching behavior of clopyralid supported on CH-intercalated montmorillonite.

Acknowledgment: Spanish Project AGL2008-04031-C02-01, Andalusian Government Project P07-AGR-03077,Researc Group AGR246 and FEDER funds.