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TECHNICAL REPORT
Ground Water Quality

Adsorption of Pesticides onto Quartz, Calcite, Kaolinite, and -Alumina

^a Dep. of Geology and Geotechnical Engineering, Technical Univ. of Denmark, Building 204, DK-2800 Lyngby, Denmark
^b Dep. of Geochemistry, Geological Survey of Denmark and Greenland, Thoravej 8, DK-2400 Copenhagen, Denmark

Corresponding author (igglc{at}pop.dtu.dk)

Received for publication October 20, 1999. The fate of pesticides in aquifers is influenced by the small but not insignificant adsorption of pesticides to mineral surfaces. Batch experiments with five pesticides and four minerals were conducted to quantify the contributions to adsorption from different mineral surfaces and compare adsorption characteristics of selected pesticides. Investigated mineral phases included quartz, calcite, kaolinite, and -alumina. Selected pesticides comprised atrazine (6-chloro-N²-ethyl-N⁴-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]. Specific surface area and mineral surface charge proved to be important 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 was added as an electrolyte, a detectable adsorption of mecoprop and 2,4-D was also measured on kaolinite (which exhibits a negative surface charge), probably due to formation of Ca–pesticide–surface complexes. Adsorption of the uncharged pesticides (atrazine and isoproturon) was detected only on kaolinite. The lack of adsorption on -alumina indicates that the uncharged pesticides have a greater affinity for the silanol surface sites (=SiOH) than for the aluminol surface sites (=AlOH) in kaolinite. No measurable effect of ionic strength was found for the uncharged pesticides. The results indicate that quartz and calcite play a smaller role than clay minerals.

Abbreviations: HPLC, high performance liquid chromatography • SEM, scanning electron micrograph • TOC, total organic carbon

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