Adsorption of pesticides onto quartz, calcite, kaolinite, and alpha-alumina

The fate of pesticides in aquifers is influenced by the small but not insig nificant adsorption of pesticides to mineral surfaces. Batch experiments wi th five pesticides and four minerals were conducted to quantify the contrib utions to adsorption from different mineral surfaces and compare adsorption characteristics of selected pesticides. Investigated mineral phases includ ed quartz, calcite, kaolinite, and alpha -alumina. Selected pesticides comp rised atrazine (6-chloro-N-2-ethyl-N-4-isopropyl-1,3,5-triazine-2,4-diamine isoproturon [3-(4-isopropyl-phenyl)-1,1-dimethylurea)], mecoprop [(RS)-2-( 4-chloro-2-methyl phenoxy)propionic acid], 2,4-D (2,4-dichlorophenoxyacetic acid), and bentazone [3-isopropyl-1H-2,1,3-benzothiadiazin-4-(3H)-one 2,2- dioxide]. Specific surface area and mineral surface charge proved to be imp ortant for the adsorption of these pesticides. Detectable adsorption of the anionic pesticides (mecoprop, 2,4-D, and bentazone) was only measured when positive sites were present on the mineral surface. However, when CaCl2 wa s added as an electrolyte, a detect able adsorption of mecoprop and 2,4-D w as also measured on kaolinite (which exhibits a negative surface charge), p robably due to formation of Ca-pesticide-surface complexes. Adsorption of t he uncharged pesticides (atrazine and isoproturon) was detected only on kao linite. The lack of adsorption on alpha -alumina indicates that the uncharg ed pesticides have a greater affinity for the silanol surface sites (= SiOH ) than for the aluminol surface sites (= AIOH) in kaolinite. No measurable effect of ionic strength was found for the uncharged pesticides. The result s indicate that quartz and calcite play a smaller role than clay minerals.