Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/8/2007
Publication Date: 3/3/2008
Citation: Pereira, T.R., Laird, D.A., Thompson, M., Johnston, C.T., Li, H., Teppen, B., Boyd, S. 2008. Role of Smectite Quasicrystal Dynamics in Adsorption of Dinitrophenol. Soil Science Society of America Journal. 72:347-354. Interpretive Summary: Most negatively charged organic compounds, including many common herbicides, are only weakly adsorbed by soils because surfaces of soil particles are negatively charged, and negatively charged surfaces repel negatively charged molecules. 4,6-dinitro-o-cresol (DNOC) is an exception to this rule. Even though DNOC is negatively charged it is strongly adsorbed by soils. In previous studies, we discovered that DNOC is adsorbed into the interiors of clay minerals as a complex with positively charged potassium ions. This discovery partially explained the high adsorption of DNOC but did not explain how a negatively charged molecule was able to get inside of the negatively charged clays in the first place. In the present study, we discovered that the clays tend to split apart in water and that the DNOC is initially adsorbed on the outside surfaces of the clay minerals. Then, when the clay suspension is dried the clay particle reforms trapping the DNOC on the inside along with potassium ions. This discovery will help scientists to better understand interactions between organic molecules and soils, and will help engineers and regulators to more accurately predict the risk of surface and ground water contamination by negatively charged organic compounds when they are used as pesticides or released into soil environments for other reasons.
Technical Abstract: Understanding sorption mechanisms and processes is critical for predicting the fate of organic pollutants in soils and for development of effective remediation strategies for contaminated soils. Novel X-ray diffraction (XRD) techniques were used to investigate processes associated with the adsorption of 4,6-dinitro-o-cresol (DNOC) on two potassium (K)- and calcium (Ca)-saturated reference smectites (SWy-2 and SAz-1). The objective was to elucidate how DNOC adsorption influences the size and swelling of smectite quasicrystals in aqueous suspensions using a novel suspension XRD technique and compare that to information gained using conventional XRD techniques which required dried oriented clay films. The phenolate form of DNOC, which is predominant in solution with pH>4.4 is primarily adsorbed on external surfaces of K-smectite quasicrystals in aqueous suspensions and may enter the interlayers as a K-DNOC complex when K-smectite quasicrystals coalesce. In Ca-smectite systems with pH>4.4, the phenolate form of DNOC is adsorbed only on external surfaces; however DNOC may become entrapped between substacks within Ca-smectite quasicrystals as the smectite suspension dries to form an oriented smectite film. The neutral form of DNOC (pH>4.4) was adsorbed in the interlayers of the low charge Ca-smectite in aqueous suspension but not in the interlayers of high charge Ca-smectites, apparently due to steric restrictions. Clearly, XRD analysis is a valuable tool for gaining insight into the interactions between smectites and organic molecules, but caution must be used in extrapolating data based on XRD patterns of air-dried and oven-dried smectite samples to processes that may occur in aqueous smectite suspensions.