The adsorption of glyphosate by several soils and clay minerals was investigated with a high-performance liquid chromatographic method. Freundlich adsorptive capacities (K) were determined as 138,115, and 8, respectively, for the clay minerals montmorillonite, illite, and kaolinite; and 76, 56, and 33, respectively, for Houston clay loam, Muskingum silt loam, and Sassafras sandy loam. Adsorption by the soils appeared to be related to the clay content and the cation-exchange capacities of the soils. Adsorption of glyphosate by illite was found to be independent of pH. The loss of solution protons as revealed by the increase in pH of the glyphosate solutions equilibrated with illite suggested the existence of a cation-exchange reaction. Saturating montmorillonite with various cations increased glyphosate adsorption in the order Na+ < Ca2+ < Mg2+ < Cu2+ < Fe3+. The complexation of glyphosate by cations released from cation-saturated clays via a cation-exchange reaction with solution protons was proposed as an adsorption mechanism. The formation of a new absorbance band ( ,^= 226 nm) in the UV spectra of glyphosate solutions (pH 4) equilibrated with Cu2+-montmorillonite provided additional evidence for complex formation between glyphosate and cations.The adsorption of the herbicide glyphosate, lV-(phosphonomethyl)glycine, by soils was first reported by Sprankle et al. (1975) who found that [14C]glyphosate adsorbed more readily to a clay loam soil than a sandy loam. Additions of cations to bentonite clay were shown to increase the adsorption of glyphosate in the following order: Ca2+ < Mn2+ < Zn2+ < Mg2+ < Fe3+ < Al3+. The authors postulated that the cations assisted in forming new adsorption sites on the clay minerals. Hensley et al. (1978) also studied the effects of cations on the behavior of glyphosate in soils and concluded that the inactivation of glyphosate by Fe3+-and Al3+-saturated soils was caused by the chelation of glyphosate. Shoval and Yariv (1979), utilizing Fe3+-and Al3+-saturated montmorillonite, investigated several glyphosate-cation complexes by infrared
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