title: Probing the specific sorption sites on montmorillonite using nitroaromatic compounds and hexafluorobenzene.
abstract: The objective of this work was to test two possible sorption mechanisms of organic chemicals to montmorillonite: n-π electron-donor-acceptor (EDA) interaction with lone electron pairs of siloxane oxygens (n-donors) and complexation with exchangeable cations. Batch sorption experiments were performed for 1,3-dinitrobenzene, 1,4-dinitrobenzene (π-electron acceptors and cation binders), and hexafluorobenzene (π-electron acceptor only) to homoionic montmorillonites in water or hexane. For all three sorbates, the aqueous sorption affinity showed large cation dependency (Cs(+) > K(+) > Na(+)), wherein sorption of hexafluorobenzene to Cs(+)-montmorillonite was the strongest (K(d) in the order of 10(4) L/kg). Change of the solvent media from water to hexane generally favored sorption, indicating suppressive effect by cation hydration. Cosorption of 1,4-dinitrobenzene prominently decreased sorption of 1,3-dinitrobenzene to all cation-exchanged montmorillonites; however, hexafluorobenzene caused strong competition only to Cs(+)-montmorillonite. Furthermore, complexation of exchangeable cations by 18-crown-6 ether dramatically suppressed sorption of 1,3-dinitrobenzene to K(+)-montmorillonite in water and all cation-exchanged montmorillonites in hexane, but not to the rest. The contrast patterns of binary competitive sorption between nitroaromatics and hexafluorobenzene indicated they sorbed to different sites on montmorillonite. It was proposed that sorption of hexafluorobenzene was dominated by n-π EDA interaction, while sorption of nitroaromatics was dominated by cation-polar interaction.
authors.affiliation: State Key Laboratory of Pollution Control and Resource Reuse/School of the Environment, Nanjing University, Jiangsu 210093, People's Republic of China.
authors.forename: Xiaolei
authors.initials: X
authors.lastname: Qu
authors.forename: Yingjie
authors.initials: Y
authors.lastname: Zhang
authors.forename: Hui
authors.initials: H
authors.lastname: Li
authors.forename: Shourong
authors.initials: S
authors.lastname: Zheng
authors.forename: Dongqiang
authors.initials: D
authors.lastname: Zhu
mesh_terms: D000327:Adsorption; D001546:Bentonite / Q000737:chemistry*; D004136:Dinitrobenzenes / Q000032:analysis / Q000737:chemistry*; D004785:Environmental Pollutants / Q000032:analysis / Q000737:chemistry*; D005466:Fluorocarbons / Q000032:analysis / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D012995:Solubility
chemical_list: D004136:Dinitrobenzenes; D004785:Environmental Pollutants; D005466:Fluorocarbons; D001546:Bentonite; C053052:1,4-dinitrobenzene; C003005:hexafluorobenzene; C017906:3-dinitrobenzene