Departamento de Química Física, Facultad de Química, Universidad de Santiago de Compostela, Santiago de Compostela, Spain.
Chemosphere. 2010 Mar;78(11):1403-8. doi: 10.1016/j.chemosphere.2009.12.063. Epub 2010 Jan 18.
Anionic pesticides are adsorbed on the mineral oxide fraction of the soil surface but considerably less on the organic fraction, so that the presence of organic matter causes a decrease in the amount of pesticide adsorbed, and may affect the mechanism of adsorption. In the present study we investigated the adsorption of the weak acid pesticide MCPA on the surface of goethite and of humic acid-coated goethite, selected as models of the mineral oxide fraction and organic components present in soil systems. Adsorption of the anionic form of the pesticide on goethite fitted an S-type isotherm and the amount adsorbed increased as the ionic strength decreased and the pH of the medium decreased. Application of the charge distribution multi site complexation model (CD-MUSIC model) enabled interpretation of the results, which suggested the formation of inner and outer sphere complexes between the pesticide and the singly-coordinated surface sites of goethite. Less pesticide was adsorbed on the humic acid-coated goethite than on the bare goethite and the pattern fitted an L-type isotherm, which indicates a change in the mechanism of adsorption. Simplified calculations with the CD-MUSIC model enabled interpretation of the results, which suggested that the pesticide molecules form the same type of surface complexes as in the previous case.
阴离子型杀虫剂被吸附在土壤表面的氧化矿物部分上，但在有机部分上的吸附量要少得多，因此，有机质的存在会导致吸附的杀虫剂减少，并可能影响吸附机制。在本研究中，我们研究了弱酸性杀虫剂 MCPA 在针铁矿表面和腐殖酸覆盖的针铁矿上的吸附，这两种物质分别作为土壤系统中氧化矿物部分和有机成分的模型。阴离子形式的杀虫剂在针铁矿上的吸附符合 S 型等温线，随着离子强度的降低和介质 pH 值的降低，吸附量增加。应用电荷分布多点络合模型（CD-MUSIC 模型）可以对结果进行解释，结果表明，在针铁矿的单配位表面位点与杀虫剂之间形成了内球和外球络合物。与裸露的针铁矿相比，腐殖酸覆盖的针铁矿吸附的杀虫剂较少，等温线符合 L 型，这表明吸附机制发生了变化。使用 CD-MUSIC 模型进行简化计算可以对结果进行解释，表明杀虫剂分子形成了与前一种情况相同类型的表面络合物。
