Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/26/2006
Publication Date: 1/9/2007
Citation: Li, H., Pereira, T., Teppen, B., Laird, D.A., Johnston, C.T., Boyd, S. 2007. Ionic Strength-Induced Formation of Smectite Quasicrystals Enhances Nitroaromatic Compound Sorption. Journal of Environmental Science and Technology. 41(4):1251-1256. Interpretive Summary: The mechanisms that control adsorption of neutral organic compounds on clay minerals in soil environments is very complex. Previously we have discovered that the mineralogy of the clay, the surface charge density of the clay, and the nature of cations that are retained on a clay's surfaces all have a big influence on adsorption of organic compounds by clays. Also previous research has shown that properties of the neutral organic molecules have a big influence on adsorption. In this study we discovered that salt concentration has a big influence on adsorption of neutral organic molecules when the salt is potassium chloride, however salt concentration has little effect when the salt is calcium chloride. We also discovered that potassium chloride concentration has a big influence on the swelling and coagulation of clay particles in a suspension whereas the concentration of calcium chloride has little effect on the swelling and coagulation of the clay particles in a suspension. Thus our findings indicate that both swelling and coagulation of clay particles are major factors controlling the potential for clays to adsorb neutral organic molecules. 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 neutral organic compounds when they are used as pesticides or released into soil environments for other reasons.
Technical Abstract: The influence of ionic strength on nitroaromatic compound sorption from water by K+- and Ca2+-saturated smectite (SWy-2) was examined. The results indicated that sorption of 1,3-dinitrobenzene by K-SWy-2 increased up to 2.2 times as KCl ionic strength increased from 0.01 to 0.30 M. In contrast, sorption by Ca-SWy-2 at CaCl2 ionic strengths of 0.015 M and 0.30 M remained the same. The "salting-out" effect on the reduction of 1,3-dinitrobenzene aqueous solubility within this ionic strength range was <1.5 % relative to the solubility in pure water. This reduction in solubility is insufficient to account for the observed increase of sorption on K-SWy-2 induced by increasing KCl ionic strength. X-ray diffraction patterns of clay suspensions indicated the formation of quasicrystal structures as KCl ionic strength reached 0.10 M; the sorbed 1,3-dinitrobenzene further enhanced the formation of quasicrystals and simultaneously induced shrinkage of clay interlayer distance to an optimal distance of ~12.5 Å for adsorption. A reduction of light absorption of K-smectite suspensions in the presence of 1,3-dinitrobenzene was observed confirming that the intercalation of 1,3-dintrobenzene further reduced the interlayer distance of quasicrystals. Overall, we attributed the observed sorption enhancement to the formation of better-ordered clay quasicrystals and shrinkage of K-smectite interlayer distance rather than the "salting-out" effect. Increasing KCl ionic strength promotes the creation of quasicrystals which concomitantly increases the amount of interlayer surface area thereby facilitating the intercalation of nitroaromatic compounds.