title: Adsorption of paraquat on goethite and humic acid-coated goethite.
abstract: Adsorption of cationic pesticides in soils is generally attributed to mineral clays and organic matter components. However, iron oxides may also contribute to such adsorption or affect it by associating with other components. Using goethite and humic acid as models for iron oxides and organic matter respectively, we studied the adsorption of the cationic pesticide paraquat on goethite and humic acid-coated goethite. At pH 4.0 the adsorption on goethite was not significant, and at pH 10.0, although the surface of the oxide was negatively charged, much less pesticide was adsorbed than on mineral clays. At this pH the adsorption of paraquat decreased as the ionic strength increased, and application of the charge distribution multisite complexation model (CD-MUSIC model) enabled interpretation of the results. At pH 4, the adsorption of paraquat on the humic acid-coated goethite was similar to the adsorption on mineral clays, but was considerably less than the adsorption on humic acid in solution. The lower adsorption on solid organic matter is attributed to a decrease in the number of "active" binding sites on the humic acid as a result of the binding to iron oxide.
authors: Departamento de Química Física, Facultad de Química, Universidad de Santiago de Compostela, E-15782 Santiago de Compostela, Spain.
authors: Ana
authors: A
authors: Iglesias
authors: Rocío
authors: R
authors: López
authors: Dora
authors: D
authors: Gondar
authors: Juan
authors: J
authors: Antelo
authors: Sarah
authors: S
authors: Fiol
authors: Florencio
authors: F
authors: Arce
mesh_terms: D000327:Adsorption; D001665:Binding Sites; D005290:Ferric Compounds; D006540:Herbicides; D006812:Humic Substances; D006863:Hydrogen-Ion Concentration; D058085:Iron Compounds / Q000737:chemistry*; D008903:Minerals / Q000737:chemistry*; D010269:Paraquat / Q000302:isolation & purification*
chemical_list: D005290:Ferric Compounds; D006540:Herbicides; D006812:Humic Substances; D058085:Iron Compounds; D008903:Minerals; C094886:goethite; C000499:ferric oxide; D010269:Paraquat