Sorption of paraquat on clay components in Taiwan's oxisol

The sorption of herbicides in soils is mainly influenced by clay components. The objectives of this study were to evaluate the contribution of clay components on paraquat sorption. The surface soils (0-20 cm) of a Laopi pedon (Fine, mixed, Hyperthermic Typic Hapludox) were separated clays into whole (< 2.0 microm), coarse (0.2-2.0 microm), and fine (< 0.2 microm) fractions with the treatments of removals of organic matter (OM) and free Fe (Fed) oxides. Results indicated that sorption isotherm of paraquat was fitted by the nonlinear Freundlich equation with R2 values ranged in 0.79-0.96, respectively. The shape of paraquat adsorption isotherm on the fine fraction was H-type, but their shapes on the whole and coarse fractions were L-types. The fine clay fractions gave higher contribution on paraquat sorption than the coarse clay fractions identified by their K(f) values. Organic matter associated with fine clay fraction had high CEC contributing to relatively high affinity for paraquat. The DCB treatment created high-affinity sites for paraquat on the fine clay, but had little effect on paraquat sorption for the coarse clay. Chemisorption is the major mechanism for retention of paraquat on clay components, not ion exchange. However, the silicate clay had the highest affinity for paraquat and free Fe compound had the lowest.