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Mechanisms of Acifluorfen Interaction with Humic Acid

Dipartimento di Valorizzazione e Protezione delle Risorse Agroforestali, via Leonardo da Vinci 44, 10095 Grugliasco (Torino), Italy

Shahamat U. Khan and Morris Schnitzer^*

Centre for Land and Biological Resources Research, Research Branch, Agriculture Canada, Ottawa, Ontario, Canada K1A0C6

* Corresponding author.

ABSTRACT

The interaction of the herbicide acifluorfen (AC; 5-[2-chloro-4-(trifluoromethyl)phenoxy]2-nitrobenzoic acid) with a humic acid (HA) extracted from an undisturbed Ah horizon of a Haploboroll, was studied at different pHs. Both the HA and AC were methylated to determine the possible role of O-containing functional groups in the adsorption process. Fourier-transform infrared spectroscopy (FT-IR) was employed to establish the mechanism of interaction. The interaction was affected by the pH of the system and by the methylation treatment, as they changed the polarity of both the HA and AC. A nonpolar environment at low pH or HA after methylation increased retention of AC by the humic materials. Carboxyl and phenolic groups of the HA negatively affect the interaction by repulsing the herbicide, as indicated by the increased adsorption by the methylated HA over the unmethylated HA. The herbicide appeared to interact with the HA by physical adsorption or weak chemical bonding that could readily be broken by extraction with water. The FT-IR spectra suggested limited involvement of H bonding in the interaction processes.

Received for publication September 10, 1996.