Environ. Sci. Technol. 1998, 32, 350-357 13 sometimes make it the equal of solid acids like zeolites or C NMR Study of Co-Contamination silica-aluminas. Benesi (7-9) measured the Hammett acidity of Clays with Carbon Tetrachloride function H0 for a number of clays; these H0 values range from +1.5 to -8.2 (in comparison to H0 )-12 for 100% ) and Benzene sulfuric acid and H0 5 for pure acetic acid). Therefore, one can expect that certain chemical transformations might occur in/on clays that are similar to what are observed in zeolite TING TAO AND GARY E. MACIEL* systems. Thus, it is of interest to examine what happens Department of Chemistry, Colorado State University, when carbon tetrachloride and benzene are ªco-contami- Fort Collins, Colorado 80523 nantsº in a clay. This kind of information would be useful in a long-term view for understanding chemical transforma- tions of contaminants in soil at contaminated sites. Data on 13 these phenomena could also be useful for designing predic- Both solid-sample and liquid-sample C NMR experiments tive models and/or effective pollution remediation strategies. have been carried out to identify the species produced by Solid-state NMR results, based on 13C detection and line the reaction between carbon tetrachloride and benzene narrowing by magic angle spinning (MAS) and high-power when adsorbed on clays, kaolinite, and montmorillonite. Liquid- 1H decoupling (10), have been reported on a variety of organic sample 13C and 1H NMR spectra of perdeuteriobenzene soil components such as humic samples (11-13). Appar- extracts confirm the identities determined by solid-sample ently, there have been few NMR studies concerned directly 13C NMR and provide quantitative measures of the amounts with elucidating the interactions of organic compounds with of the products identifiedsbenzoic acid, benzophenone, and soil or its major components. Most of the few such reports - triphenylmethyl carbocation (or triphenylmethanol, the have concentrated on soil pesticide interactions (14), al- product of hydrolysis of the carbocation). Montmorillonites though some work has addressed other species, e.g., benzene (15), dimethylsulfoxide (16), trichloroethylene (17), and amine are much more effective than kaolinite in promoting this or alkyl ammonium ion systems (18, 19). To the best of our chemistry, and their reactivity is dramatically enhanced by knowledge, studying chemical transformations in co- 2+ Zn exchange of the montmorillonite. The results can contamination systems on clays by solid-state 13C NMR or be explained by invoking reaction mechanisms of the Friedel- any other technique has not previously been reported, Crafts type. 27Al MAS NMR spectra were obtained to although clays have been widely used as catalysts in organic provide structural information on the clays. syntheses (20). In this paper, we report 13C NMR results (from both solid and liquid samples) on the co-contamination of clays with carbon tetrachloride and benzene. Introduction Experimental Section The release and dispersion of organic contaminants in soil Materials. Three clay samples were used in this study. Ca- have become a matter of increasing concern with respect to montmorillonite and kaolinite were obtained from the Source environmental and health issues (1). Some chlorinated Clay Minerals Repository, University of Missouri at Columbia. organics, such as carbon tetrachloride, are known to cause K-10 montmorillonite was purchased from Aldrich. 13C- ozone depletion and to affect the human central nervous labeled carbon tetrachloride was obtained from Cambridge system, while aromatic compounds, e.g., benzene and its Isotopes Laboratory and was used without further purifica- derivatives, can result in blood dyscrasias, including lym- tion. Benzene (from Aldrich) was dried by refluxing with phopenia, thrombopenia, and pancytopenia; a decrease in calcium hydride and distilled. all types of circulating blood cells; and the development of NMR Spectroscopy. All solid-state 13C MAS NMR spectra aplastic anemia and leukemia (2). Therefore, it is important shown were obtained at 22.71 MHz at room temperature on to understand the detailed chemical/physical behaviors of a Chemagnetics M-90S spectrometer. Kel-F rotors (2.4 cm3 and complex interactions among such organic pollutants in sample volume) were used at a spinning speed of about 2.5 soil. Knowledge of such fundamental chemical/physical kHz. For cross-polarization (CP) experiments, the following detail is a prerequisite to the formulation of reliable models experimental conditions were employed: 1H decoupling, 45 of contaminant behavior for optimizing technology to kHz; CP contact time, 2 ms; delay between scans, 1 s. For minimize these contamination problems. direct polarization (DP) experiments (no cross polariza- Recently, we reported (3, 4) a ªship-in-a-bottleº synthesis tion): a 90° pulse length of 5.5 µs, 45 kHz 1H decoupling; + 13 of the triphenylmethyl cation, (C6H5)3C , from carbon repetition time, 5 s. C spectra were externally referenced tetrachloride and benzene inside the supercage of zeolite to liquid tetramethylsilane (TMS) based on substitution of HY. In view of the well-known chemical similarities and the secondary reference, solid hexamethylbenzene (peaks at properties of certain clays and zeolites, this provocative result 16.9 and 132.3 ppm, relative to TMS). Solid-state 27Al MAS prompted our interest in exploring analogous chemistry in NMR spectra of the clays were obtained at 156.4 MHz at clay systems. Clay minerals are primary soil components room temperature on a Chemagnetics CMX Infinity spec- with largely aluminosilicate structures that have large surface trometer, with a 29° pulse length of 0.8 µs and a repetition areas and strong adsorbing properties (5), which markedly time of 0.2 s; Chemagnetics 3.2 mm PENCIL rotors were affect the behaviors of contaminants in soil and groundwater used at a spinning speed of about 18 kHz. 27Al spectra were 3+ systems (6). Along with cation exchange capability, the externally referenced to Al(H2O)6 in 1 M aqueous Al(NO3)3 surface acidity of a clay can be one of its most prominent solution. A Bruker AC-300P spectrometer was used to obtain and remarkable features; the acidic character of a clay can all of the liquid-solution 1H and 13C spectra, at 300.13 and 75.47 MHz, respectively. + * Corresponding author fax: 970-491-1801; e-mail: maciel@lamar. Sample Preparations. Zn2 -Exchanged Clays. All three colostate.edu. clay samples were doped with zinc ion by the following 350 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 3, 1998 S0013-936X(97)00604-4 CCC: $15.00 1998 American Chemical Society Published on Web 02/01/1998 13 1 TABLE 1. Clay-Pollutant Samples Studied and Contents of C6D6 Extracts of CCl4/Benzene/Clay Samples Based on H NMR Analysis weight proportions sample 13 13 13 13 of clay/ CCl4/ weight for TTMSM (C6H5)3 COH (C6H5)2 CO C6H5 CO2H sample Zinc contenta benzene (g/g/g) extraction (0.2 ppm) (7.3 ppm) (7.7 ppm) (8.1 ppm) no. type of clay (wt %) (g)b mol × 10-7 c mol × 10-6 c mol × 10-6 c mol × 10-7 c 1 Ca-montmorillonite <0.0050 1.98/0.0315/0.132 0.176 8.4 0.50 (2.8)d 0.66 (3.7) 0 (0) 2 K-10 montmorillonite <0.0050 2.02/0.0320/0.131 0.174 9.4 0.73 (4.1) 0.82 (4.6) 0 (0) 3 kaolinite <0.0010 2.03/0.0324/0.136 0.170 8.3 0 (0) 0 (0) 0 (0) 4Zn2+-exchanged 2.02 ( 0.02 2.06/0.0328/0.133 0.177 11 2.5 (14) 8.2 (45) 5.7 (31) Ca-montmorillonite 5Zn2+-exchanged 1.86 ( 0.02 2.08/0.0325/0.132 0.172 11 2.3 (13) 11 (64) 2.1 (12) K-10 montmorillonite 6Zn2+-exchanged 0.49 ( 0.02 2.03/0.0318/0.133 0.175 8.3 0 (0) 0 (0) 0 (0) kaolinite a b 13 Zn contents were determined by Huffman Laboratories, Golden, CO. Weight of CCl4/C6H6/clay sample used for C6D6 extraction after 30 days. c d Number of moles of individual components in concentrated C6D6 extracts. The numbers in parentheses correspond to percent yield relative 13 to CCl4. procedure: A 10.0-g portion of the clay was gradually added (22). A tetrahedral layer within a clay mineral consists to a stirred slurry of 1.63 g of zinc chloride in 60 mL of primarily of a sheet of tetrahedral silicons linked via oxygens acetonitrile. The suspension was stirred at room temperature (SiO4) with other silicons. One of the four corners of each for 48 h, and the mixture was filtered and washed with two tetrahedron is shared with an adjacent octahedral layer, which 25-mL portions of acetonitrile. The resulting solid was dried typically contains Al3+ (or Mg2+) at the center and -OH or at 80 °C in an oven under atmospheric pressure (∼645 Torr) -O- at the apices, where -O- occupied apices are corner for 12 h and ground with a mortar/pestle. The Zn contents shared with adjacent tetrahedra and -OH occupied apices of the ion-exchanged clays were analyzed by Huffman are not shared. The general structure of 1:1 clay minerals, Laboratory, Golden, CO, and the results are shown in Table such as kaolin or kaolinite, consists of parallel stacked sheets, 1. each being a composite of one tetrahedral and one octahedral Pretreatment of Clays. Before mixing with an organic layer. The general structure of 2:1 clay minerals, e.g., reagent, each clay sample (as received, as well as Zn2+-ion montmorillonite, consists of parallel stacked sheets, each exchanged) was calcined in vacuum (<10-3 Torr) by (i) first composed of two tetrahedral layers and one octahedral layer.