Binding mechanism of 2,4-dichlorophenoxyacetic acid by organo-clays

Abstract

The interaction of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) with decylammonium-montmorillonite (C10M) and decylammonium-vermiculite (C10V) was studied by measuring 2,4-D adsorption at different pH levels, and by preparing herbicide-organic clay complexes which were characterized by x- ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). The adsorption of 2,4-D at different pH values indicated that the undissociated form was preferentially adsorbed on C10M, whereas the anionic form was adsorbed by C10V. The adsorption of 2,4-D was much greater for C10V than for C10M. X-ray diffraction diffractograms and FT-IR spectra of C10M did not show any changes after exposure to 2,4-D, indicating adsorption on external surfaces by weak lyophilic, aromatic ring-alkylchain(tail), bond. The d001 values of the two C10V samples expanded after 2,4-D exposure from 2.1 to 2.9 nm and from 2.4 to 3.3 nm, indicating interlayer adsorption of the herbicide. The FT-IR spectrum of herbicide-C10V complex showed the presence of a small amount of the molecular form of 2,4-D weakly adsorbed on external surfaces by lyophilic (ring-tail) bonding, but most of the herbicide molecules were adsorbed at the interlayer spaces by hydrogen bonds between carbonyl group (ν(C=0) at 1634 cm-1) of 2,4-D and ammonium group of the interlayer organic cations. Some 2,4-D molecules in C10V complex appeared to be dissociated as suggested by two bands at 1590 and 1330 cm-1, which corresponded to the unsymmetrical and symmetrical vibrations, respectively, of the COO- groups.