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Sorption of nitroaromatics by ammonium- and organic ammonium-exchanged smectite : Shifts from adsorption/complexation to a partition-dominated process
Roberts, Michael G. (Michigan State University. Department of Crop and Soil Sciences)
Li, Hui (Michigan State University. Department of Crop and Soil Sciences)
Teppen, Brian J. (Michigan State University. Department of Crop and Soil Sciences)
Boyd, Setephen A. (Michigan State University. Department of Crop and Soil Sciences)

Fecha: 2006
Resumen: Nitroaromatic compounds (NACs) are components of munitions commonly found as soil contaminants at military training sites and elsewhere. These compounds pose possible threats to human health and ecological systems. Recent studies indicate that these compounds are strongly retained by smectite clays. The adsorption mechanisms are not fully reconciled, but it is known that the type of exchangeable cation strongly affects NAC affinity for smectites. This study examined the sorption of 1,3-dinitrobenzene, 2,4-dinitrotoluene and naphthalene from water by a smectite clay (SWy-2) saturated with ammonium, tetramethylammonium (TMA), trimethylphenylammonium (TMPA) and hexadecyltrimethylammonium (HDTMA). In all cases, we observed greater sorption of 2,4-dinitrotoluene compared with 1,3-dinitrobenzene. The sorption isotherms for 2,4-dinitrotoluene and 1,3-dinitrobenzene displayed a concave-downward curve for NH-SWy-2 and TMA-SWy-2, whereas the isotherms for sorption of HDTMA-SWy-2 and TMPA-SWy-2 were essentially linear. The magnitude of sorption followed the order: NH-SWy-2 > TMA-SWy-2 > TMPA-SWy-2 > HDTMA-SWy-2 for both compounds. The greater affinity of NACs for NH- and TMA-SWy-2 is due in part to complex formation between the exchangeable cation and -NO groups. These clays also provide near optimal interlayer distances that approximate the molecular thickness of NACs hence promoting the simultaneous interaction of the planar aromatic rings with opposing siloxane surfaces and solute dehydration. Both processes are energetically favorable. In HDTMA-SWy-2, sorption of all solutes is via a partition-dominated process. Solute competition (diminished uptake of one solute in the presence of a second) was observed for TMA-SWy-2 but not HDTMA-SWy-2. This is consistent with an adsorptive mechanism for TMA-SWy-2 and a partitioning mechanism for HDTMA-SWy-2. This study demonstrates that the dominant molecular mechanism of NAC sorption by smectite changes fundamentally from complexation between -NO groups and the exchangeable cation (viz. NH and TMA) to partitioning for a systematic series of ammonium and quaternary ammonium cations in which the locus of positive charge (the central N atom) is progressively shielded by organic moieties of increasing size.
Derechos: Tots els drets reservats.
Lengua: Anglès
Documento: article ; recerca ; publishedVersion
Materia: Munitions ; Nitroaromatic Compounds ; Smectite ; Soil Contamination
Publicado en: Clays and Clay Minerals, Vol. 54, Núm. 4 (2006) , p. 426-434, ISSN 1552-8367

DOI: 10.1346/CCMN.2006.0540403

9 p, 559.7 KB
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