Abstract
Silicate clay minerals, metal oxides and organic matter (OM) are the major colloidal components that determine adsorption behaviors of herbicides in soils. The purpose of this study was to evaluate the role of size distribution and the components of soil clay on the adsorption of 2,4-dichlorophenoxyacetic acid (2,4-D) in a tropical soil. The clay particles of surface soils (0-20 cm) of an Oxisol were separated into whole (＜2.0 μm), coarse (0.2-2.0 μm), and fine (＜0.2 μm) fractions through subsequent treatments of removing OM and free Fe oxides (Fe(subscript d)). The adsorption isotherm data were analyzed based on the Freundlich and Langmuir models. The analytical results indicated that the adsorbed amounts of 2,4-D on coarse clay and fine clay fractions were greater than those on whole clay fractions. When OM is removed, the surface of silicate clay is only coated by Fe oxides, which are positively charged at pH 6-7 and exhibit a stronger affinity for deprotonated 2,4-D. The removal of Fe(subscript d) from the H2O2 treated clay reduced 2,4-D adsorption. This indicates that silicate clays adsorb fewer amounts of deprotonated 2,4-D if the Fe oxides are removed from the surface of silicate clays.
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