Sorption of organic compounds of varying hydrophobicities from water and industrial wastewater by long- and short-chain organoclays
Ludmila Groisman a, Chaim Rav-Acha a,*, Zev Gerstl b, Uri Mingelgrin b
b a Research Laboratory of Water Quality, Ministry of Health, P.O. Box 8255, Tel-Aviv 61080, Israel Institute of Soil,Water and Environmental Sciences, The Volcani Center, ARO, P.O. Box 6, Bet Dagan 50250, Israel
Received 29 December 2002; received in revised form 5 February 2003; accepted 19 February 2003
Abstract It is generally assumed that short-chain organoclays are better sorbents for non-ionic organic compounds than are the longchain organoclays. We believe that the above statement is correct only forrelatively hydrophilic or slightly hydrophobic compounds and that the opposite should be true for highly hydrophobic compounds, namely, that long-chain organoclays are expected to be superior to the short-chain analogs for sorption of highly hydrophobic materials. To verify our hypothesis, the sorption of six compounds with a range of log Kow values from 2.5 to 6 was studied ontetramethyltriammonium-(TMA) and octadecyltrimethylammonium-(ODTMA) bentonite, representing short- and long-chain organoclays, respectively. Compounds with low or medium hydrophobicities (log Kow 2.5 – 3.8) were more strongly sorbed on the short-chain organoclay, whereas the more hydrophobic compounds (log Kow 5.2 – 6.1) were better sorbed on the long-chain organoclay, in agreement with our hypothesis. The efficacyof organoclays in removing organic pollutants from the effluent of a pesticide producing plant was evaluated. Pesticide sorption from the wastewater was studied on both types of organoclays. It was found that both organoclays were able to remove organic pollutants from industrial wastewater, but solute uptake by short-chain organoclays was strongly depressed by competition, while long-chainorganoclays were only slightly affected, if at all, by the presence of competing solutes in the industrial wastewater. D 2003 Elsevier B.V. All rights reserved.
Keywords: Short-chain organoclay; Long-chain organoclay; Organoclays; Sorption; Hydrophobicity; Industrial wastewater
1. Introduction Organoclays are clay minerals in which the inorganic exchangeable cations have been replaced by organiccations, such as organic quaternary ammoni-
* Corresponding author. E-mail address: email@example.com (C. Rav-Acha). 0169-1317/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2003.02.001
um or pyridinium cations, thus, modifying the clay’s surface properties from hydrophilic to hydrophobic. Organoclays have a strong capacity to sorb non-ionic compoundsfrom aqueous solutions (e.g. Sheriff et al., 1987). Quaternary ammonium organoclays may be divided into two groups depending on the structure of the organic cation and the mechanism of sorption (Lo et al., 1998). The first group, called adsorptive organo-
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clays, includes clays that contain short-chain quaternary ammoniumions, such as tetramethylammonium or trimethylbenzylammonium. Sorption on this type of organoclays is characterized by Langmuir-type isotherms, which are commonly associated with specific sorption sites. The second group of organoclays, called organophilic organoclays, is composed of clays that contain long-chain quaternary ammonium ions, such as hexadecyltrimethylammonium ordi-dodecyldimethylammonium. Sorption by this group is characterized by linear isotherms over a wide range of solute concentrations. It is generally believed that, despite the more bulky organic phase of the long-chain organoclays, the short-chain sorptive organoclays sorb non-ionic organic compounds more effectively (Smith et al., 1990; Smith and Galan, 1995; Dentel, 1996). This view is based on series of studies,...