title: Ionic strength-induced formation of smectite quasicrystals enhances nitroaromatic compound sorption.
abstract: Sorption of organic contaminants by soils is a determinant controlling their transport and fate in the environment. The influence of ionic strength on nitroaromatic compound sorption by K+- and Ca2+ -saturated smectite was examined. Sorption of 1,3-dinitrobenzene by K-smectite increased as KCl ionic strength increased from 0.01 to 0.30 M. In contrast, sorption by Ca-smectite at CaCl2 ionic strengths of 0.015 and 0.15 M remained essentially the same. The "salting-out" effect on the decrease of 1,3-dinitrobenzene aqueous solubility within this ionic strength range was <1.5% relative to the solubility in pure water. This decrease of solubility is insufficient to account for the observed increase of sorption by K-smectite with increasing KCl ionic strength. X-ray diffraction patterns and light absorbance of K-clay suspensions indicated the aggregation of clay particles and the formation of quasicrystal structures as KCI ionic strength increased. Sorption enhancement is attributed to the formation of better-ordered K-clay quasicrystals with reduced interlayer distances rather than to the salting-out effect. Dehydration of 1,3-dinitrobenzene is apparently a significant driving force for sorption, and we show for the first time that sorption of small, planar, neutral organic molecules, namely, 1,3-dinitrobenzene, causes previously expanded clay interlayers to dehydrate and collapse in aqueous suspension.
authors.affiliation: Department of Crop and Soil Sciences, Michigan State University, East Lansing, Michigan 48824, National Soil Tilth Laboratory, USDA-ARS, Ames, Iowa 50011, USA. lihui@msu.edu
authors.forename: Hui
authors.initials: H
authors.lastname: Li
authors.forename: Tanya R
authors.initials: TR
authors.lastname: Pereira
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Teppen
authors.forename: David A
authors.initials: DA
authors.lastname: Laird
authors.forename: Cliff T
authors.initials: CT
authors.lastname: Johnston
authors.forename: Stephen A
authors.initials: SA
authors.lastname: Boyd
mesh_terms: D000327:Adsorption; D002122:Calcium Chloride / Q000737:chemistry; D003460:Crystallization; D004136:Dinitrobenzenes / Q000737:chemistry*; D009994:Osmolar Concentration; D011189:Potassium Chloride / Q000737:chemistry; D017640:Silicates / Q000737:chemistry*; D012989:Soil Pollutants / Q000737:chemistry; D014961:X-Ray Diffraction
chemical_list: D004136:Dinitrobenzenes; D017640:Silicates; D012989:Soil Pollutants; D011189:Potassium Chloride; C033214:Smectite; C017906:3-dinitrobenzene; D002122:Calcium Chloride