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SOIL CHEMISTRY

Role of Smectite Quasicrystal Dynamics in Adsorption of Dinitrophenol

^a Dep. of Crop and Soil Sciences, Michigan State Univ., East Lansing, MI 48824-1325
^b USDA-ARS, National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011
^c Dep. of Agronomy, Iowa State Univ., Ames, IA 50011
^d Dep. of Agronomy, Lilly Hall, Purdue Univ., 915 W. State St., West Lafayette, IN 47907-2054

^* Corresponding author (david.laird{at}ars.usda.gov).

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. A novel suspension x-ray diffraction (XRD) technique was used to elucidate the influence of the size and swelling of K- and Ca-smectite quasicrystals in aqueous suspensions on adsorption of 4,6-dinitro-o-cresol (DNOC). X-ray diffraction patterns of randomly oriented smectite quasicrystals in aqueous suspensions were compared with diffraction patterns for air-dried and oven-dried oriented films of the same smectites. The XRD patterns reveal that 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 suspensions 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 film. The neutral form of DNOC (pH 4.4) was adsorbed in the interlayers of a low-charge-density Ca-smectite in aqueous suspension but not in the interlayers of the high-charge-density Ca-smectite, apparently due to steric restrictions. Clearly, XRD analysis is a valuable tool for gaining insight into 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 films to processes that may occur in aqueous smectite suspensions.

Abbreviations: Ca-SAz-1, calcium-saturated Arizona montmorillonite • Ca-SWy-2, calcium-saturated Wyoming montmorillonite • DNB, 1,3-dinitrobenzene • DNOC, 4,6-dinitro-o-cresol • K-SAz-1, potassium-saturated Arizona montmorillonite • K-SWy-2, potassium-saturated Wyoming montmorillonite • NACs, nitroaromatic compounds • pK_a, acid dissociation constant • RH, relative humidity • SAz-1, Arizona montmorillonite reference clay (smectite) • SWy-2, Wyoming montmorillonite reference clay (smectite) • XRD, x-ray diffraction