Nonionic organoclay: A ‘Swiss Army knife’ for the adsorption of organic micro-pollutants?
Graphical abstract
Introduction
The anthropogenic input organic micro-pollutants hydro-systems generates severe human health risks and impacts on aquatic ecosystems [1]. Despite the establishment of strict regulations on industrial discharges and the improvement of specific treatments for both wastewater and drinking water, numerous organic products, incriminated to be responsible of several cancers, are accumulated at significant concentrations in the water [2], [3].
Clay minerals have been for a long time known for their outstanding adsorption properties. However, although being used in drinking water treatment, these layered materials turned out to be ineffective for the sequestration of persistent nonpolar hydrophobic contaminants [4], [5], [6], [7], [8], [9], [10]. The intercalation of cationic surfactants, through ion exchange with the inorganic cations, switches the chemical nature of the starting layered material from hydrophilic to hydrophobic. Moreover, the resulting organoclay composite has a wide opening of its interlayer space that significantly improve the adsorption of numerous organic compounds [4], [5], [6], [7], [8], [9], [10]. Nevertheless, the improvement of the adsorb mainly depends on both the chemical nature and the structural organization of the intercalated surfactants. Thus, surfactants showing a long alkyl chains such as hexadecyltrimethylammonium (HDTMA) create an appropriate organic environment within the inorganic frame for the adsorption of alkanes whereas modifiers such as benzyl decyltrimethylammonium (BDTA) show an excellent affinity with aromatic compounds [10].
While numerous works focused on the improvement of the sequestration of organic contaminants by using cationic organoclays, the study on the adsorption properties of organoclays synthesized with unconventional nonionic surfactants showed far much less concern which is even more surprising in regards to the major interests that nonionic surfactants show: (i) a biodegradability and a no toxicity [11]; (ii) a good thermal and chemical stabilities [12], [13]; (iii) a preservation of the exchangeable inorganic cations after adsorption onto clay minerals, conferring a dual hydrophilic/hydrophobic character [14], [15], [16], [17], [18]; (iv) a still possible cation exchange of the resulting nonionic organoclay with both organic and inorganic cations [13], [16], [17], [19]; and (v) aggregates of which structural arrangement depends on the surfactant state in aqueous solution expanding at wide openings the interlayer space of the layered material [13], [17], [19], [20].
Thus, this work focuses on the study of the adsorption properties of nonionic organoclay using the tri-ethylene glycol mono n-decyl ether (C10E3) and compare them to those of the starting clay mineral (Mt) and a cationic organoclay. The selected BDTA cationic surfactant combines a long alkyl chain and an aromatic ring of which affinity with organic compounds was shown in various works. It should be also noticed that in order to get realistic and economical viable organoclay materials, only a monolayer of surfactant was confined within the interlayer space of Mt. Such organoclay arrangement, where the surfactant undergoes severe effects of confinement, shows a maximal thermal stability and prevents any unwanted surfactant release. The adsorption properties of the three sorbent materials were studied by focusing on three water micro-pollutants: (i) benzene; (ii) dimethyl phthalate; and (iii) paraquat,. If the adsorption of benzene and paraquat on both clay minerals and cationic organoclays has been the subject of numerous studies focusing on the behavior of the adsorption isotherms, details on the evolution of the interlayer space determined by XRD and FTIR analyses are still scarce. Moreover, no study has been focused on the sequestration of these two pollutants by a nonionic organoclay as well as the adsorption of phthalate. Results obtained in this study with the use of a set of complementary techniques (XRD, FTIR, and adsorption isotherms), beyond serving as only data for the effectiveness comparison of the three different sorbents will be useful to identify the interactional mechanisms involved in the adsorption of organic micro-pollutants.
Section snippets
Clay mineral
Wyoming sodium montmorillonite (Mt), obtained from the Source Clay Minerals Repositery, University of Missouri (Columbia, MO) was used in this study as a starting material. This Mt clay mineral shows a cation exchange capacity (CEC) of 80 meq per 100 g clay, and a BET specific surface area of 35 m2 g−1 (condensation of a non-polar N2 gas) as well as a 660 m2 g−1 accessible surface area determined through the adsorption of ethylene glycol due to the solvation of Na+ cations in Mt for polar molecules
Organoclays properties
The C10E3Mt and BDTAMt organoclays were characterized by XRD, TEM and FTIR analyses.
Both 00l diffraction patterns of the prepared organoclays shift to lower angular values underlining the expansion of the interlayer space at 14 and 13.5 Å for dehydrated BDTAMt and C10E3Mt respectively in contrast to 9.7 Å for the untreated dehydrated NaMt (Fig. 1). As expecting with the concentrations of the prepared surfactant solutions, where C10E3, below the cmc is only in monomers form, and for BDTA below 1
Conclusions
The chemical modification of clay minerals via the intercalation of cationic surfactants for the preparation of environmental organoclay obviously improves the efficiency of the adsorption of numerous chemicals [10], [14]. Nevertheless, this observation is only correct for a restraint range of hydrophobic chemicals where cationic organoclays mainly excel, due to their hydrophobic character, but show some limits for other organic compounds [35]. Worse, cationic organoclays proved to be an
Associated content
Additional figures on Freundlich and Dubinin–Radushkevish (D–R) fits of the adsorption isotherms of the micro-pollutants on the three sorbents are included.
Acknowledgments
We are grateful to the Canon Foundation in Europe for their support in this research, which allowed the first author to be hosted in laboratory of the Professor Ogawa at the University of Waseda last year 2013 for 6 months where some experiments were performed.
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