MECHANISMS FOR ADSORPTION OF ORGANIC BASES ON HYDRATED SEMECTITE SURFACES

Authors: Laird, David; Fleming, Pierce

Submitted to: Journal of Society of Environmental Toxicology and Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/22/1999
Publication Date: N/A
Citation: N/A

Interpretive Summary: The fate of pesticides and other organic contaminates in soils depends to a large extent on how the organic compounds react with clay and organic matter in the soils. Compounds that bind tightly to surfaces of soil particles are not likely to be leached to ground water and thus pose little risk to the environment. An understanding of how organic molecules bind to soil particles is therefore very important for predicting whether an organic compound will be mobile or immobile in soils. We found that a class of organic compounds, known as organic bases, bind to soil clays by two different mechanisms. In acid soils, the organic bases become positively charged and the positive charge on the bases is attracted to negative charge sites on the clay surfaces. In neutral soils, the organic bases are uncharged and must compete with water for places on the clay surfaces. This research will help scientists to better understand and predict the fate of organic contaminants in soils.

Technical Abstract: Mechanisms by which organic bases are adsorbed on hydrated smectite surfaces were investigated. Three CA-saturated reference smectites (Otay, SPV, and Panther Creek) were dispersed in distilled water containing 5 micro moles of pyridine or 3-butylpyridine. The pH was adjusted between 7.5 and 3 using 0.01 M HCl. After a 2 h equilibration the amount of pyridine or 3-butylpyridine adsorbed on the clay and the amount of Ca desorbed from the clay were determined. Negligible amounts of pyridine were adsorbed by the Ca-smectites in the neutral systems (pH greater than 7), however most of the added pyridine was adsorbed on the smectites in the acidified systems (pH less than 5). Equivalent amounts of Ca were desorbed from the clays indicating that pyridine was adsorbed as a protonated species by cation exchange. By contrast, 40 to 90% of added 3-butylphridine was adsorbed on the smectites at neutral pHs while only small amounts of Ca were desorbed. The results suggest that 3-butylpyridine is initially retained by hydrophobic bonding between the alkyl side chain of the molecule and hydrophobic nanosites located between the charge sites on smectite surfaces. Surface acidity catalyzed protonation 1 to 1.5 pH units above the pKa of the bases.