Atrazine, isoproturon, mecoprop, 2,4-D, and bentazone adsorption onto ironoxides

Iron oxides are important components influencing the adsorption of various inorganic and organic compounds in soils and sediments. In this study the a dsorption on iron oxides of nonionic and ionic pesticides was determined as a function of solution pH, ionic strength, and pesticide concentration. Th e investigated iron oxides included two line ferrihydrite, goethite, and le pidocrocite. Selected pesticides comprised atrazine (6 chloro-N-2-ethyl-N-4 -isopropyl-1,3,5-triazine-2,4-diamine), isoproturon [3-(4-isopropylphenyl)- 1,1 dimethylurea)] mecoprop [(RS) 2(4-chloro-2-methylphenoxy)propionic acid ],2,4-D (2,4 dichlorophenoxyacetic; acid), and bentazone [3-isopropyl-1H-2, 1,3-benzothiadiazin-4-(3H)-one 2,2-dioxide]. The adsorption of the nonionic pesticides (atrazine and isoproturon) was insignificant, whereas the adsor ption of the acidic pesticides (mecoprop, 2,4-D, and bentazone) was signifi cant on all investigated iron oxides. The adsorption capacity increased wit h decreasing pH, with maximum adsorption reached close to the pK(a) values. The addition of CaCl2 in concentrations from 0.0025 to 0.01 M caused the a dsorption capacity to diminish. The adsorption of bentazone was significant ly lower than the adsorption of mecoprop and 2,4-D, illustrating the import ance of a carboxyl group in the pesticide structure. The adsorption capacit y on the iron oxides increased in the order: lepidocrocite < goethite < two -line ferrihydrite. The maximum adsorption capacities of meco prop and 2,4- D on goethite were found to be equivalent to the site density of singly coo rdinated hydroxyl groups on the faces of the dominant {110} form, suggestin g that singly coordinated hydroxyl groups are responsible for adsorption. D ifferences in adsorption capacities between iron oxides can be explained by differences in the surface site density of singly coordinated hydroxyl gro ups. The maximum measured adsorption capacity of mecoprop on two-line ferri hydrite was equivalent to 0.2 mol/mol Fe.