1634
Langmuir 1999, 15, 1634-1639
Spectroscopic Study of Sorption of Nitrogen Heterocyclic Compounds on Phyllosilicates Sandip Chattopadhyay†,‡ and Samuel J. Traina*,§ Environmental Science Graduate Program and The School of Natural Resources, The Ohio State University, 2021 Coffey Rd., Kottman Hall, Columbus, Ohio 43210 Received May 22, 1998. In Final Form: December 14, 1998 The present study focused on understanding the sorption characteristics of acridine (AcN) and acridine9-carboxylic acid (AcNCOOH), two typical nitrogen heterocyclic compounds (NHCs), on well-characterized phyllosilicates (hectorite, saponite, and muscovite). Results presented in this article show that the degree of sorption of NHCs on phyllosilicates was dependent on the nature of the participating sorbates and sorbents. Sorption of the selected NHCs was pH-dependent, with maximum sorption occurring at low pH conditions, especially at pH < pKa of the NHC. Though sorption of the cationic forms of the NHCs on clays was preferred, neutral, zwitterionic, and anionic species of NHCs also sorbed on the clay surfaces. Spectroscopic studies have shown that sorbed NHC molecules formed clusters on clay surfaces, which acted as templates for molecular aggregation. Finally, we have also found that the clay surfaces promoted protonation of neutral AcN molecules at low sorbate concentrations.
Introduction The environmental chemistry of NHCs has received considerable attention recently due to the presence of many of these substances in industrial and agricultural waste streams and due to their potential impact on human health.1 In the absence of sorption processes, groundwater is particularly susceptible to contamination from NHCs.2 Fortunately evidence exists that many NHCs (e.g. acridine orange, quinoline, proflavine, methylene blue, rhodamine 6G) can readily sorb on clay minerals,3-10 providing some protection against groundwater contamination. Researchers11,12 have shown that sorption reactions to clay surfaces can significantly alter the bioavailability of sorbed species, increasing their environmental half-lives. O’Loughlin11 observed that microbial degradation of 2-methylpyridine attenuated due to sorption on smectites, while Smith et al.12 found that surface-bound quinoline was not susceptible to microbial degradation, unlike quinoline in aqueous solution. * Corresponding author. † Environmental Science Graduate Program. ‡ Present address: R.S. Kerr Environmental Research Laboratory, USEPA, 919 Kerr Research Dr., P.O. Box 1198, Ada, OK 74820 § The School of Natural Resources. (1) Stuermer, D. H.; Ng, D. J.; Morris, C. J. Environ. Sci. Technol. 1982, 16, 582. (2) Leenheer, J. A.; Stuber, H. A. Environ. Sci. Technol. 1981, 15, 1467. (3) Cohen, R.; Yariv, S. J. Chem. Soc., Faraday Trans. 1 1982, 80, 1705. (4) Zachara, J. M.; Ainsworth, C. C.; Cowan, C. E.; Thomas, B. L. Environ. Sci. Technol. 1987, 21 (4), 397. (5) Zachara, J. M.; Ainsworth, C. C.; Felice, L. J.; Resch, C. T. Environ. Sci. Technol. 1986, 20 (6), 620. (6) Schoonheydt, R. A.; Cenens, J.; de Schryver, F. C. J. Chem. Soc., Faraday Trans. 1 1986, 82, 281. (7) Cenens, J.; Schoonheydt, R. A. Clays Clay Miner. 1988, 36 (3), 214. (8) Traina, S. J.; Onken, B. M. J. Contam. Hydro. 1991, 7, 237. (9) Tapia Este´vez, M. J.; Lo´pez Arbeloa, F.; Lo´pez Arbeloa, T.; Lo´pez Arbeloa, I.; Schoonheydt, R. A. Clay Miner. 1994, 29, 105. (10) Lo´pez Arbeloa, F.; Tapia Este´vez, M. J.; Lo´pez Arbeloa, T.; Lo´pez Arbeloa, I. Langmuir 1995, 11, 3211. (11) O’Loughlin, E. J. Adsorption and biodegradation of 2-methylpyridine in aqueous suspensions of specimen clay minerals. Master’s Thesis, The Ohio State University, 1991. (12) Smith, S. C.; Ainsworth, C. C.; Traina, S. J.; Hicks, R. J. Soil Sci. Soc. Am. J. 1992, 56, 737.
Whereas a significant amount of study has examined the macroscopic uptake of NHCs by hydrated clays, limited studies have been conducted to provide molecular insight into the mechanisms responsible for NHC sorption. The objective of the present study is to probe the sorption characteristics of simple NHCs on different phyllosilicates (layered silicates) by investigating the physical and chemical properties of the sorbates and sorbents. Layered silicates are made up of sheets of Si(O,OH)4 tetrahedra linked with M2-3(OH)6 (M ) Al, Mg) octahedra. A 2:1 clay lattice is formed by a combination of an octahedra sheet with two tetrahedra sheets. Isomorphous substitutions in the tetrahedral and octahedral sheets lead to different clay structures, and replacement of the resident atom by one of lower valence produces negative charge in the lattice. We intended to prove that the clay particles act as templates in the formation of stable NHC clusters or aggregates, and the stability of these aggregates and the degree of sorption were dictated by the nature of the participating sorbents and sorbates. Macroscopic sorption data was combined with UV-visible and fluorescence spectroscopic measurements to better elucidate the mechanisms of NHC reactions with clay surfaces. Materials and Methods Acridine or dibenzo[b,e]pyridine (Aldrich Chemical) and its carboxylic derivative, AcNCOOH (Aldrich Chemical), of purity 99.9% and 99.7%, respectively, were used as received. Both chemicals were used as received from the vendor. Acridine is an organic base and exists as an organic cation at pH < pKa (pKa ) 5.6).13 Acridine-9-carboxylic acid has two pKa values at 3.0 and 5.0,13 and AcNCOOH molecules exist as zwitterions between pH 3.0 and 5.0. Experiments were conducted at two pH conditions (pH 4.5 and 8.5) to study the sorption characteristics of different ionic forms of the selected NHC molecules on clay minerals. Aqueous concentrations of AcN for all experiments ranged from 6 to 195 µmol L-1, while concentrations of AcNCOOH ranged from 0.5 to 112 µmol L-1. The upper limits of the concentration ranges were dictated by the aqueous solubilities of AcN and AcNCOOH. All NHC solutions were prepared fresh before each experiment. Clay-organic suspensions were prepared by adding measured (13) Albert, A. The Acridines: Physical, chemical, and biological properties and uses, 2nd ed.; St. Martin’s Press: New York, 1966.
10.1021/la980607h CCC: $18.00 © 1999 American Chemical Society Published on Web 02/04/1999
Nitrogen Heterocyclic Compounds on Phyllosilicates amounts of clay suspensions to the desired concentrations of either AcN or AcNCOOH solutions at a particular pH while continuously stirring. Three well-characterized 2:1 phyllosilicates (layered silicates) were selected as model sorbents. They were hectorite (SHCa-1), saponite (SapCa-1), and muscovite (Pelco Mica Sheets no. 54, PELCO Electron Supplies). Clay samples of hectorite and saponite were obtained from the Source and Special Clays Repository of the Clay Minerals Society (MI) in the powdered form. Samples of muscovite were available as sheets, which were ground in a disk mill (Sieb Technik, GMBH, Mu¨lhein-Ruhr, Germany) for the sorption studies. These minerals were chosen for their low Fe contents, so that the fluorescence properties of the probe molecules would not be quenched. The total negative charge of -0.62 lay entirely on the octahedral layer of hectorite.14 The total negative charge on saponite was -0.76, with -0.46 on the tetrahedral layer and -0.30 on the octahedral layer.14 The total negative charge on muscovite was determined as -1.86, with -1.94 on the tetrahedral layer and +0.08 on the octahedral layer. The cation exchange capacities (CECs) of hectorite and saponite were reported as 89.2 and 80.4 cmol kg-1, respectively.14 Both Na- and Ca-exchanged forms of hectorite and saponite were used for this study. Na-exchanged clays were prepared by equilibrating the clay samples in 1 M NaCl solution three times. The clay-NaCl suspensions were stirred for 24 h, followed by centrifugation and resuspension. The size fraction
