Environ. Sci. Technol. 2000, 34, 246-253
Historical Trends of N-Cyclohexyl-2-benzothiazolamine, 2-(4-Morpholinyl)benzothiazole, and Other Anthropogenic Contaminants in the Urban Reservoir Sediment Core H I D E T O S H I K U M A T A , * ,† YUKIHISA SANADA,† HIDESHIGE TAKADA,† AND TAKASHI UENO§ Tokyo University of Agriculture and Technology, Fuchu, Tokyo 183-8509, Japan, and Japan Atomic Energy Research Institute, Naka, Ibaraki 319-1195, Japan
A new potential molecular marker, N-cyclohexyl-2benzothiazolamine (NCBA), was discovered in various environmental matrices (i.e., road dusts, runoff- and riverwater particles, river sediments, aerosols) taken in Tokyo, Japan. Concurrent determination of this compound together with 2-(4-morpholinyl)benzothiazole (24MoBT), previously proposed marker of road dust, demonstrated that both compounds are widely distributed in the urban environment (∼ng/g to ∼µg/g), derived from vehicle tire tread, and transported in the environments in the same way. To assess utilities of NCBA and 24MoBT as molecular markers for vehicle-derived contaminants, these compounds were analyzed in a sediment core from the Chidorigafuchi Moat of Imperial Palace, situated in the center of Tokyo. Remarkable is that NCBA existed at higher concentrations than 24MoBT near the surface (0-6 cm depth) and bottom parts (16-24 cm depth) of the region where BTs were detected but lower in the middle parts (6-16 cm depth). Dating of the core by using 137Cs and tetrapropylenebased alkylbenzenes (TABs) revealed the two changeovers coincide well with changes in the production history of vulcanization accelerators containing the compounds. The dated downcore profile of ∑BTs (sum of 24MoBT and NCBA) showed positive correlation with the traffic data in Tokyo Metropolitan Area. These results indicate the usefulness of NCBA and 24MoBT as time markers for recent sections of sediment cores and as molecular markers for reconstructing the history of traffic-induced contamination.
Introduction Anthropogenic chemicals have been deposited since the onset of industrial synthesis and/or the increase in human * Corresponding author phone: +81-426-76-6793; fax: +81-42676-5354; e-mail:
[email protected]. Present address: Laboratory of Environmental Chemistry, Faculty of Life Sciences, Tokyo University of Pharmacy and Life Sciences, Hachioji, Tokyo 192-0392, Japan. † Tokyo University of Agriculture and Technology. ‡ Japan Atomic Energy Research Institute. 246
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FIGURE 1. Molecular structures of (a) 2-(4-morpholinyl)benzothiazole (24MoBT) (MW ) 220.30; CAS: 4225-26-7), an impurity of the vulcanization accelerator, OBS; (b) 2-morpholinothiobenzothiazole (MW ) 252.36; CAS: 102-77-2), a major component of the vulcanization accelerator, OBS; (c) N-cyclohexyl-2-benzothiazolamine (NCBA) (MW ) 232.10; CAS: 028291-75-0), an impurity of the vulcanization accelerator, CBS; (d) N-cyclohexyl-2-benzothiazolesulfenamide (NCBA) (MW ) 264.17; CAS: 95-33-0), a major component of the vulcanization accelerator, CBS. OBS and CBS are names of vulcanization accelerators defined in the Japan Industrial Standard (JIS K 6202-1979). activities and can serve as geochronometers for recent deposition in reservoir sediments (1). Use of the historical record approach of measuring molecular markers in undisturbed sediments to document the release of anthropogenic chemicals to environment has been reported by some researchers (1-3). Since the beginning of the massive vehicular transportation in the catchments, various vehiclederived materials including tire debris are thought to have deposited, and, therefore, the molecular marker for vehiclederived pollutants is expected to be detected in sediment layers corresponding to a time period when the traffic activity is occurring. 2-(4-Morpholinyl)benzothiazole (24MoBT; Figure 1a), a minor component of a vulcanization accelerator OBS, has recently been evaluated as a possible marker of tire debris (4-6). OBS consists mainly of 2-morpholinothiobenzothiazole (Figure 1b) and was one of the most widely used vulcanization accelerators, used predominantly in tire tread rubber in the past (1970s and 1980s). In addition to 24MoBT, this study will deal with N-cyclohexyl-2-benzothiazolamine (NCBA; Figure 1c), contained in a vulcanization accelerator, CBS. CBS consists mainly of N-cyclohexyl-2-benzothiazolesulfenamide (Figure 1d) and is currently used predominant vulcanization accelerators, mainly for tire tread rubber. This is analogous to 24MoBT contained in OBS, and, therefore, NCBA is expected to be useful as a molecular marker of vehicle-derived pollution. Also the temporal change in the use of the vulcanization accelerators (i.e., OBS vs CBS; (4)) could be inscribed in recent sediment cores. The objectives of this study are (i) to investigate NCBA in environmental samples and to reveal its distributions and behaviors in the environment around Tokyo and (ii) to demonstrate the utilities of 24MoBT and NCBA (BTs) as markers for dating recent sections of sediment cores and for reconstructing the release history of vehicle-derived particulate matter and contaminants. To achieve the second objective, we employed a molecular marker approach; corroborating downcore profiles of BTs with that of other time markers (i.e., linear- and tetrapropylene based-alkylbenzenes: LABs and TABs) as well as the radionuclide cesium 137, a conventional geochronometer (e.g., refs 3 and 7). LABs and TABs have been used to synthesize linear alkylbenzenesulfonates (LAS) and branched-chain alkylben10.1021/es990738k CCC: $19.00
2000 American Chemical Society Published on Web 12/08/1999
FIGURE 2. An overview of study area, Tokyo Metropolitan Area (a) and the Imperial Palace of Japan (b). Sampling location for the sediment core sample is indicated by a closed circle. zenesulfonates with tetrapropylene-based alkyl chains (ABS), which are widely used synthetic detergents. Unsulfonated LABs and TABs residues in detergents are introduced into an aquatic environment as a result of the use of those detergents and subsequent disposal (8). Large-scale production of LABs started in the late-1960s in Japan when they were introduced as replacements for TABs (8). Therefore, LABs and TABs are considered as time markers for recent sections of sediment cores, of which utilities have been well investigated by some researchers (e.g., refs 1, 2, and 8). Polycyclic aromatic hydrocarbons (PAHs), of which some are known to be carcinogenic, are also investigated in this study. Traffic is recognized as one of the major sources of environmental PAHs, of which contribution to sedimentary PAHs has been investigated by many researchers (e.g., refs 9-12). The contribution of vehicular PAHs could change historically depending on the traffic intensity. However, the historical trend of PAHs is complicated by changes in the use of fuels or emission controls for power, residential heating, and industrial activities as well as that for vehicular emissions. Therefore, concurrent determination of PAHs with traffic-derived markers (i.e., BTs) in the sediment core may allow us more complete characterization of increases in traffic and contamination.
Experimental Section Sample Collection. The study was conducted in Tokyo, in the central part of Japan Islands. Tokyo, together with
adjacent areas (Tokyo, Saitama, Kanagawa, and Chiba prefectures; Figure 2a), constitutes the Tokyo Metropolitan Area, which is one of the most urbanized areas in the world (13). A sediment core sample was taken on June 6, 1997 from the Chidorigafuchi Moat, one of the moats surrounding the Imperial Palace located in the central part of the Tokyo (Figure 2b). The moat of the Imperial Palace was chosen because artificial reservoirs having little watershed drainage area would clearly record the release history of anthropogenic compounds to the system, especially when it is situated in the highly urbanized and/or industrialized area. The average water depth and surface area of the Chidorigafuchi Moat is 1.0 and 66 521 m2, respectively. Water in the Palace Moat is mostly derived from groundwater (ca. 91 000 metric tons/ year: (14)) and precipitation (typically 1700 mm/year in Japan (15), including street runoff water from the Tokyo Expressway running above the moat. The water depth at the sampling site was ca. 2 m. The sample was taken with a normal gravity coring equipment, immediately sliced at 2-cm intervals, and then transferred into polypropylene containers and stored at -30 °C until analysis. Bulk densities were determined on the basis of measured water content by weighing duplicated subsamples (i.e., 2 mL) of individual sections before and after drying. A small portion of each sediment was made carbonate-free (decalcified using hydrochloric acid) and was analyzed for total organic carbon (TOC) and total nitrogen (TN) by dry catalytic oxidation on NCS2500 (Fisons InstruVOL. 34, NO. 2, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Downcore profiles of nitrogen (a) and carbon content (b), C/N ratio (c), and dry bulk density (d) from sediments of the Chidorigafuchi Moat. ment SpA, Milan, Italy). Radionuclide activities were determined in Bq/kg (dry weight) by γ spectroscopy (661.66 keV for 137Cs; 30.07-year half-life) on well type Ge detectors. Details were described elsewhere (3). In addition to the sediment core sample, road dust, particles in street runoff water and in river water, river sediment, and atmospheric aerosol samples were also collected and analyzed to reveal 24MoBT and NCBA distributions in a wide variety of environmental compartments. Some of the samples used in this study were collected in 1989 (16, 17) and others were in 1993 and 1994 (4). Sediment Characteristics. The dark gray to black color of the sediments indicated anoxic conditions. It is also indicated by a strong smell of hydrogen sulfide during core sectioning. The core showed a rhythmical succession of dark/ light laminae that would provide evidence for nondisrupted lamination and, hence, for absence or negligibility of events, such as dredge, which would largely and discontinuously disturb the vertical profile of the analytes. The negligibility of disrupted sedimentation is also provided by the downcore profile of dry bulk density, which revealed an almost linear increase with depth (Figure 3). Downcore profiles for TOC, TN, organic carbon to nitrogen (C/N) ratio, and the dry bulk density are also illustrated in Figure 3. Concentrations of TOC and TN represented gradual decrease with depth. The C/N ratio, on the other hand, discontinuously changed at the depth of 24-26 cm from the surface of the sediment, indicating some drastic changes have occurred in input and/ or autochthonous production.
Analytical Procedures Chemicals. Running standards of 24MoBT and NCBA were prepared from vulcanization accelerators (i.e., OBS and CBS, respectively) used in the rubber industry and identified by GC/MS analyses. The vulcanization accelerators OBS and CBS were kindly offered by Ohuchi Shinko Kagaku Inc. (Tokyo, Japan) and Bridgestone Inc. (Tokyo, Japan), respectively. To confirm NCBA identification, it was synthesized by refluxing 2.4 g of 2-chlorobenzothiazole (Aldrich Chem. WI) at 100 °C for 3 h with 7.2 g of cyclohexylamine (Aldrich) (18). The reaction mix was rinsed with dilute HCl, neutralized, and extracted with dichloromethane. Recrystalization from n-hexane gave a solid, with mp 106-107 °C. IR spectra (1590, 1550, 1448 cm-1) and mass spectra (Figure 4a) confirmed it is NCBA (lit. mp 103-104 °C: (19), 105-107 °C: (18); IR 1590, 1550, 1448 cm-1: (20); M + 232: (21)). The NCBA running standard, extracted from a vulcanization accelerator CBS, has the same mass spectrum as synthesized NCBA (Figure 4a,b). An intense ion of m/z 150 is typical for N-substituted benzothiazoles with ethyl or higher N-substituent and is probably because of the McLafferty re248
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FIGURE 4. EI mass spectra of NCBA in standard solution and ambient sample: (a) an authentic standard of NCBA, synthesized in laboratory, (b) a stock solution extracted from a vulcanization accelerator, CBS, and (c) extracts from a freshwater sediment sample. arrangement (21). LAB mixture consisting of all secondary congeners (no primary congeners) ranging in alkyl chain length from C10 to C14 and TAB mixture ranging in alkyl chain length from C8 to C13 were kindly offered by Mitsubishi Petrochemical Co. (Japan). All other reagents and organic solvents were purchased from the commercial suppliers. Extraction and Fractionation Procedures. The entire extraction and fractionation procedure for analyses of PAHs, alkylbenzenes, and BTs is a combination of previously reported procedures (1, 22, 23). Since the Supporting Information details the procedure, only the outline is described here. The freeze-dried particulate samples were Soxhlet extracted with toluene/methanol (1:1 v/v) for >100 cycles. After spiking with known amounts of PAHs and alkylbenzenes recovery surrogate standards, the extract was concentrated and processed for BTs isolation procedures modified from the method by Kumata (23). The sample extract in toluene/methanol was rinsed twice with a 1:1 mixture of methanol and aqueous base (pH 9) and then liquid-liquid extracted four times with 0.18 M H2SO4 (pH < 2). Four aqueous phases were combined, made basic (pH 9), and back-extracted with DCM. The DCM extracts containing basic compounds were purified through a 5% H2O deactivated silica gel column (23). The purified fraction was collected as the BTs fraction for subsequent GC analysis. The base-free extracts (supernatant) were dried through Na2SO4 (anhydrous), concentrated, and used for PAHs and alkylbenzenes analyses, which is also detailed in the Supporting Information and elsewhere (24). Instrumental Analyses and Quantification. 24MoBT and NCBA (BTs). BTs fractions were analyzed on a HewlettPackard 5890 Series II gas chromatograph equipped with a flame photometric detector (FPD) with a splitless injection. BTs were separated on a HP-35 fused silica capillary column (30 m × 0.25 mm i.d., 0.25 µm film thickness) together with
TABLE 1. 24MoBT and NCBA Concentrations and Their Compositions (NCBA/24MoBT Ratio) Observed in the Urban Environment of Tokyo, Japan 24MoBT
n
concn range
NCBA av ( SD
n 1989
tire tread road dustb inside tunnel outside tunnel runoff spmc
concn range
NCBA/24MoBT ratio av ( SD
range
av ( SD
Samplesa
4
1000-2770
2330g
4
1410-13 800
6040g
0.51-7.13
3.24
5 6 3
60-491 12-289 12-215 (0.73-29)h
293 ( 155 84 ( 108
7 9
87-440 15-73 160-303 (10-41)h
286 ( 148 41 ( 27
0.48-1.45 0.17-1.88 1.41-13.7
1.24 ( 0.26 0.84 ( 0.73
525 ( 404 100 ( 61
3.84-41.3 1.78-17.2
16.9 ( 13.6 5.45 ( 4.55
1993/1994 Samplesd road dust inside tunnel outside tunnel river water spm normal flowe storm flowf river sediment Sumidagawa Tamagawa aerosol
18-99 12-30
36 ( 27 19 ( 7.1
7 9
137-1230 27-224
2 4
bdl 5.2-12 (0.5-1.0)h
7.5 ( 4.0 (0.76 ( 0.24)h
1
NA 22 (4.3)h
8 7 4
2.0-9.0 1.0-5.4 91-210 (5.6-18)i
5.2 ( 2.4 2.5 ( 1.5 149 ( 56 (11 ( 6.2)i
6 2 2
8 10
4.9-16 3.2-5.7 334-701 (16-48)i
4.15 11 ( 5.4 4.5 ( 1.8 518 ( 260 (32 ( 22)i
1.09-3.68 0.59-1.90 2.90-7.70
2.12 ( 0.91 1.25 ( 0.92 5.30 ( 3.39
a Samples were taken in 1989. b Collected by Takada et al. (17). c Collected by Ito et al. (16). d Samples were taken in 1993 and in 1994 (4). e At water flow 20 m3/s. g Calculated by weighting individual concentrations by fractional market share of four commercial suppliers. h ng/L. i pg/m3. j Concentrations are on a unit weight basis (ng/g-dry matter), except for those presented in parentheses. bdl: below the detection limit, NA: not available because it was not analyzed.
a precolumn, Supelco medium-polar fused-silica tube (1 m × 0.53 mm i.d., 0.25 µm film thickness). The GC program used for BTs analyses was as follows: initial temperature at 70 °C, isothermal for 1.5 min, 30 °C/min to 150 °C, 4 °C/min to 260 °C, isothermal for 10 min. Compound identifications were based on GC retention time and on data from electronimpact gas chromatography-mass spectrometry (GC-MS). GC and GC-MS conditions are detailed elsewhere (23, 25). The concentrations of 24MoBT and NCBA in ambient samples were calculated from a calibration curve based on the GCFPD peak height. Benzo[b]naphtho[2,1-d]thiophene (Aldrich) was used as an injection internal standard (ISTD) for the quantification of BTs. The ratio of BTs- to ISTD-peak height was proved to be logarithmically proportional to the sample amounts (C). The curve was obtained from gas chromatographic runs of the mixture of running standards (0.9, 1.5, 2, 4, 8 ppm) on the same day. The linear correlation coefficient (r2) between the logarithm of C and of RBT/RIS was normally >0.998 for both compounds. Since we could not determine the exact purity (assay) of prepared standards, their exact concentrations were calibrated with benzothiazole (Aldrich) on GC-FID, assuming that the FID responses to 24MoBT, NCBA, and benzothiazole are the same. Therefore, 24MoBT and NCBA concentrations presented in this study are calculated in terms of benzothiazole concentrations. Procedural blanks did not display any peaks corresponding to 24MoBT and NCBA on the resultant gas chromatogram. Reproducibility (RSD; n ) 4) of the method, determined by four replicated Soxhlet extractions and analyses of 10-g subsamples of river sediment, was 8.7% for 24MoBT and 12.1% for NCBA. Percent recoveries of 24MoBT and NCBA standards, spiked to duplicated Soxhlet-extracts of another two subsamples (10 g) of the above sample, were 81.2 ( 0.8% (n ) 2) and 75.1 ( 1.2% (n ) 2). The recoveries of blanks spiked with 24MoBT and NCBA ranged from 80 to 140%. Considering the complexity of the analytical procedure and the trace amounts of the targets in a complex mixture, the precision and recovery are acceptable. Figure A-1 in the Supporting Information shows an example of a FPD chromatogram of an environmental sample purified according to the above method, where both 24MoBT and NCBA peaks
are clearly identified without any interfering peak. It also indicates the validity of our method. Limits of detection by the GC-FPD were calculated from the calibration curves to be 0.4 ng-injected 24MoBT (at S/N ) 3) and 0.5 ng-injected NCBA, corresponding to 20 ng-24MoBT and 25 ng-NCBA per sample, as 50 µL was the minimum volume of the final solution just prior to GC injection.
Results and Discussion Identification and Distribution of NCBA in the Urban Environment. It has already been reported that 24MoBT exists in tire tread rubber and in various environmental matrices (i.e., road dust, particles in street runoff water and in river water, river sediment, and atmospheric aerosol) (46). As shown in Table 1, NCBA was detected not only in tire tread rubber but also in most environmental samples where 24MoBT was found. Figure 4c illustrates EI mass spectra of NCBA for the freshwater sediment. Extracts from other types of samples had the mass spectra essentially similar to those shown here. Compared with the spectra of NCBA standard solution (Figure 4a,b), the spectra generated with NCBA in the sediment extract are unambiguous, which confirms the identification of NCBA in the environmental samples. As for NCBA, this is the first report on its existence in environmental samples. The concentrations of NCBA in all the environmental compartment were higher than but of the same magnitude as those of 24MoBT (4), probably reflecting their concentrations in the source material, tire tread rubber (Table 1). However, caution should be taken in handling the tire tread data, since our tire samples were collected in 1989 and hence would not represent BTs compositions in currently used vehicle tires. The temporal changes will be discussed later. In the environment around Tokyo, road dust and aerosol contain high concentrations (hundreds ng/g) of NCBA. NCBA concentrations were higher in road dusts collected from inside tunnels than those from outside tunnels which were diluted by soils and some other nonvehicle derived materials. In river water, storm flow samples had significant amounts of NCBA, while there was no detectable NCBA in normal VOL. 34, NO. 2, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Downcore profiles of (a) 24MoBT (closed square) and NCBA (open square); (b) ∑LABs (closed circle), ∑TABs (open circle), and I/E ratio for the C12LAB homologue (open square); (c) pyro-PAHs; and (d) 137Cs activity from sediments of the Chidorigafuchi Moat. I/E ratio is defined as [6-C12AB+5-C12AB]/[4-C12AB+3-C12AB+2-C12AB]. flow samples. Sedimentary NCBA concentrations were less than the storm flow samples. All these distributions as well as spatial trends of NCBA are similar to those of 24MoBT, indicating that NCBA is introduced into and transported within the urban environment in the same manner as 24MoBT (4). That is, NCBA as well as 24MoBT is first deposited on road surfaces as vehicle tire abrasions which are then incorporated into the aquatic environment via urban runoff. In addition, BTs could be deposited into the aquatic environment from local aerosols. Consequently, NCBA would also be utilized as an indicator of tire wear particles and/or road dusts. Anthropogenic Markers in the Sediment Core. Figure 5 presents the BTs, alkylbenzenes, and pyro-PAHs contents in the sediments from the Chidorigafuchi Moat. Significant amounts of BTs were detected in upper parts of the core (Figure 5a). The concentrations of 24MoBT and NCBA had ranges of 4.2-18 and 5.3-22 ng/g, respectively. These were larger than, but of the same magnitude as, those found in river sediments in Tokyo (e.g., 5.2 ( 2.4 ng/g for 24MoBT and 11 ( 6 ng/g for NCBA in Sumidagawa River; Table 1). 24MoBT and NCBA were first detected at the depth of 22-24 cm, and there was no trace of BTs in the deeper sections of the core. 24MoBT and NCBA revealed different vertical profiles. NCBA profile showed its maximum, 22 ng/ g, at the surface of the core and small peaks at the layer of 6-8 and 16-18 cm depth. On the other hand, 24MoBT profile showed a subsurface maximum: a drastic rise from 5.1 ng/g at 16-18 cm depth to its maximum, 18 ng/g at 10-12 cm depth, followed by a decrease to the depth of 4-6 cm. Levels then increased again to the surface of the core. Remarkable is that NCBA existed at higher concentrations than 24MoBT near the surface (i.e., 0-6 cm depth) and bottom parts (i.e., 16-24 cm depth) of the region where BTs were detected but lower in the middle parts (6-16 cm depth). Also, these sediments contained significant concentrations of alkylbenzenes (∼7.5 µg/g of ∑LABs and ∼1.4 µg/g of ∑TABs; Figure 5b) and PAHs (∑PAHs 7.5-30 µg/g; Figure 5c). The compositions of LABs observed in the core were dominated by homologues with 12, 13, and 11 carbon alkyl chains, which is consistent with that in LAS-type synthetic detergents available in Japan and in environmental samples (26). Detection of alkylbenzenes in the moat sediment would probably be ascribed to some type of surfactants used for windscreen-wiper containing alkylbenzenes and/or that they were derived from the boathouse standing on the moat since 250
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the Chidorigafuchi Moat does not receive influent from sewage (according to the Administration Office of the Outer Gardens of the Imperial Palace, Environmental Agency, Government of Japan). Although individual PAHs concentrations ranged widely from tens to thousands of nanograms per gram of dry weight, generally uniform compositions of PAHs were observed throughout the core, dominated by parental (nonalkylated) 3- to 6-ring species. A ratio of the sum of 3-, 2-, 9-, and 1-methylphenthrenes to phenanthrene (MP/P ratio) ranged from 0.58 to 1.08, indicating their pyrogenic origin. Fluoranthene and pyrene, in particular, were the most abundant parental PAH compounds throughout the core. These PAHs compositions are similar to those observed in Tokyo Bay sediments (27) and throughout the world (12, 28, 29). Dating of the Core Sediment. The date of deposition was estimated by using the depth profile of 137Cs activity in the sediment of Chidorigafuchi Moat (Figure 5d). Activity of 137Cs in aquatic sediment in Japan usually shows almost a unimodal depth profile, of which a peak corresponds to a consequence of atmospheric nuclear weapons testing with a maximum during 1963 (30, 31). Negligible is a sign of the accident of the Chernobyl nuclear reactor in 1986 (3, 32). Hence, around 1963 can be assigned to the maximum at 16-18 cm depth. Assuming a linear sedimentation rate from this layer to the surface of the core, an approximate deposition date for the upper part of the core (i.e.,
