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Title: SPECTROSCOPIC STUDY OF CARBARYL SORPTION ON SMECTITE FROM AQUEOUS SUSPENSION

Author
item FERNANDES D OLIVEIRA, MAURILIO - PURDUE UNIVERSITY
item ZHU, DONGQIANG - PURDUE UNIVERSITY
item JOHNSTON, CLIFF - PURDUE UNIVERSITY
item PREMACHANDRA, G - PURDUE UNIVERSITY
item TEPPEN, BRIAN - MICHIGAN STATE UNIVERSITY
item LI, HUI - MICHIGAN STATE UNIVERSITY
item Laird, David
item BOYD, STEPHEN - MICHIGAN STATE UNIVERSITY

Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/19/2005
Publication Date: 12/1/2005
Citation: Fernandes D Oliveira, M., Zhu, D., Johnston, C.T., Premachandra, G.S., Teppen, B.J., Li, H., Laird, D.A., Boyd, S.A. 2005. Spectroscopic study of carbaryl sorption on smectite from aqueous suspension. Environmental Science and Technology. 39:9123-9129.

Interpretive Summary: Carbaryl is a widely used insecticide in agriculture, professional turf management and ornamental production, as well as in pest control for residential garden and lawn use. In 1997 an estimated 4.9 million pounds of carbaryl were used to treat over 3 million acres in the United States. At present, there is increased concern about the environmental fate of these compounds because of their high toxicity to both humans and wildlife. We discovered that adsorption of carbaryl by clay minerals varies with both the surface properties of the clay and the type of exchangeable cations on the clay surfaces. Our results indicate that carbaryl reacts simultaneously with both the exchangeable cations and with hydrophobic nanosites on the clay surfaces. This research will help scientists to better understand how organic chemicals such as pesticides react with soils and will thereby help predict the fate of these compounds when they are applied to soils. This research will be of interest to the pesticide manufacturing industry as reactions between pesticides and soils influence the efficacy of the the pesticide. The research may also be of interest to regulators responsible for assessing the risk of pesticides to human health and the environment.

Technical Abstract: Sorption of carbaryl (1-naphthyl-N-methylcarbamate) from aqueous suspension to smectite was studied using Fourier Transform Infrared (FTIR), High Performance Liquid Chromatography (HPLC) (for batch sorption) and quantum chemical methods. The amount of carbaryl sorbed was strongly dependent on the surface charge density of the smectite with more sorption occurring on 'low' surface-charge density smectites (SHCa-1 and SWy-2) compared to the high surface charge SAz-1 smectite. The amount of carbaryl sorbed was strongly dependent on the nature of the exchangeable cation (Ba ~ Cs > Ca > Mg > K > Na > Li for SWy-2 and Cs > Ba > Ca > K > Mg > Na > Li for SHCa-1). FTIR spectra of carbaryl sorbed to the smectites were influenced by both the nature of the clay and by the type of exchangeable cation. The position of the carbonyl stretching band shifted to lower wavenumbers with increasing ionic potential of the exchangeable cation. The shift to lower energy indicates a lengthening of the C=O bond, resulting from the carbonyl group interacting directly with the exchangeable cation. A concomitant shift in the opposite direction was observed for several vibrational bands in the 1355 to 1375 cm-1 region assigned to stretching bands of the carbamate N-C carbonyl and Oether-C carbonyl bonds. The amount of carbaryl sorbed was inversely proportional to the enthalpies of hydration for both alkali metal and the alkaline earth cations. The combined spectroscopic-sorption data presented here indicate that carbaryl sorption is due to both (1) site-specific interactions between the carbamate functional group with hydrated exchangeable cations in the interlayer and (2) the area of hydrophobic siloxane nanosites available in clay interlayers. The area of the hydrophobic siloxane nanosites is inversely proportional to the hydration energies of the exchangeable cations. The experimental results were in agreement with quantum chemical predictions that showed a metal induced increase in the carbonyl bond length upon complexation.