Environ. Sci. Technol. 2009, 43, 4381–4385
Ubiquitous Occurrence of the Artificial Sweetener Acesulfame in the Aquatic Environment: An Ideal Chemical Marker of Domestic Wastewater in Groundwater IGNAZ J. BUERGE,* HANS-RUDOLF BUSER, MAREN KAHLE, ¨ LLER, AND MARKUS D. MU THOMAS POIGER Plant Protection Chemistry, Swiss Federal Research Station (Agroscope), CH-8820 Wa¨denswil, Switzerland
Received January 14, 2009. Revised manuscript received March 18, 2009. Accepted March 25, 2009.
Artificial low-calorie sweeteners are consumed in considerable quantities with food and beverages. After ingestion, some sweeteners pass through the human metabolism largely unaffected, are quantitatively excreted via urine and feces, and thus reach the environment associated with domestic wastewater. Here, we document the widespread occurrence of four sweeteners in the aquatic environment and show that one of these compounds, acesulfame, meets all of the criteria of an ideal marker for the detection of domestic wastewater in natural waters, particularly groundwater. Acesulfame was consistently detected in untreated and treated wastewater (12-46 µg/L), in most surface waters, in 65% of the investigated groundwater samples, and even in several tap water samples (up to 2.6 µg/L) from Switzerland. The sweetener was not eliminated in wastewater treatment plants (WWTPs) and was quite persistent in surface waters, where concentrations increased with population in the catchment area and decreased with water throughflow. The highest concentrations in groundwater, up to 4.7 µg/L, were observed in areas with significant infiltration of river water, where the infiltrating water received considerable discharges from WWTPs. Given the currently achieved detection limit of ≈0.01 µg/L, it is possible to trace the presence of g0.05% wastewater in groundwater.
Introduction Groundwater is an important drinking water resource and deserves a high level of protection (1). Contamination by xenobiotic chemicals and pathogenic microbes may originate from various sources such as agriculture, households, industry, and traffic. Knowledge of the relevant sources is therefore necessary to take efficient measures against groundwater pollution. For source apportionment, markers have proved useful. An ideal marker should be source specific, released to the environment in sufficient quantities, reflect contamination in a quantitative sense, and should be amenable to rapid and sensitive analysis (2, 3). Bacterial markers are commonly used to trace domestic wastewater but have disadvantages such as limited source * Corresponding author phone: ++41 44 783 6383; fax: + +41 44 780 6341; e-mail:
[email protected]. 10.1021/es900126x CCC: $40.75
Published on Web 05/07/2009
2009 American Chemical Society
specificity, time-consuming analysis, and relatively short survival in natural waters (e.g., refs 2, 4, 5 and refs therein). Alternatively, several chemical markers have been suggested for domestic wastewater (2, 3, 5-10), including constituents and metabolites of pharmaceuticals, personal care products, household chemicals, food, and beverages. For application in groundwater, where minimal sorption to subsurface material and sufficient stability are other important criteria, currently proposed markers have their limitations. At present, the most promising markers are the pharmaceuticals carbamazepine and crotamiton (8, 10-12). Little attention has been paid to ingredients in food and beverages with respect to occurrence and fate in the aquatic environment. Artificial low-calorie sweeteners, for example, are regularly consumed in considerable quantities. Chemical structures, global consumption data, sugar equivalence values, and organoleptic detection thresholds of four important sweeteners are listed in Table 1. Cyclamate was banned in the United States in 1970 but is registered in the European Union (EU) and many other countries. After ingestion, these four sweeteners pass through the human metabolism largely unaffected, are quantitatively excreted via urine and feces (Table 1), and thus reach the environment associated with domestic wastewater. The sulfoamide sweeteners acesulfame, cyclamate, and saccharin are anionic compounds at typical pH values of natural waters and are therefore expected to be quite mobile in the subsurface. No data has been found in the literature on their occurrence in the aquatic environment. Sucralose, a chlorinated disaccharide, has recently been detected in domestic wastewater and natural waters (13) and was shown to be persistent in wastewater. Aspartame, another important sweetener, was assumed to be quickly biodegraded in WWTPs (aspartame is the methyl ester of a dipeptide). The four sweeteners acesulfame, cyclamate, saccharin, and sucralose were therefore selected for this study with the aim to investigate their occurrence and fate in wastewater, surface waters, groundwater, and drinking water from Switzerland, and finally, to evaluate their suitability as markers for domesticwastewaterwithparticularemphasisongroundwater.
Experimental Section Chemicals. Acesulfame K (6-methyl-1,2,3-oxathiazin-4(3H)one-2,2-dioxide potassium salt, purity, 99%), N-cyclohexylsulfamic acid (98%), and saccharin (1,2-benzisothiazol-3(2H)one-1,1-dioxide, 99%) are from Fluka, Buchs, Switzerland; sucralose (1,6-dichloro-1,6-dideoxy-β-D-fructofuranosyl-4chloro-4-deoxy-R-D-galactopyranoside, 99.6%) is from Chemos, Regenstauf, Germany, and sucralose-d6 (98%), used as an internal standard, is from Toronto Research Chemicals, North York, Canada. Water Samples. Flow-proportional 24 h composite samples of untreated and treated wastewater were obtained from 10 municipal WWTPs in the region of Zurich, Switzerland. Surface water was collected from four rivers (flowproportional 7-d composite samples taken between June and October 2008) and eight lakes (grab samples taken in February and March 2008) in the Swiss Midland region and from a remote alpine lake. Details on WWTPs and surface waters are described in refs 9 and 14. Groundwater samples (n ) 100) were obtained from an annual routine survey of the Official Food Control Authority of the Canton of Zurich (taken between August and November 2008). Locations of and further information on groundwater pumping stations are available at www.grundwasser.zh.ch (in German). Raw and finished (ozonation) drinking water samples were received VOL. 43, NO. 12, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. General Information on Investigated Artificial Sweeteners
a Sugar equivalence values are relative to sucrose (16). b Organoleptic detection threshold is for young subjects (17). Global consumption is for 2005 (16). d Values are for the unchanged parent compound (18-20). e Approximately 90% of the population excrete less than 1% of a daily dose of cyclamate as cyclohexylamine (21, 22). c
from the Water Supply Authority, Zurich and the Lake Waterworks Appital, Wa¨denswil, Switzerland. Further tap water samples were taken at public places in the lower Glatt valley. Online SPE-Liquid Chromatography-Tandem Mass Spectrometry. The sweeteners were analyzed with LC-MS/ MS after online solid-phase extraction. Online SPE and chromatographic conditions were identical to those described for nicotine derivatives (9), except that two stacked SPE cartridges were used for SPE, and the LC gradient was started at 100% 1 mM ammonium acetate. Retention times were as follows: acesulfame, 11.0 min; saccharin, 13.2 min; sucralose, 15.4 min; and cyclamate, 15.7 min. The mass spectrometer (API 4000 triple quadrupole mass spectrometer, Applied Biosystems, Foster City, CA) was equipped with a turbo ion spray source, operated in negative mode (ion spray voltage, 4 kV, 500 °C) and, for trace analysis, selected reaction monitoring (SRM) with the following ion transitions: acesulfame, m/z 162f82 with a collision energy of 19 eV (and for confirmatory purposes, m/z 162f78, 40 eV); cyclamate, 178f80, 33 eV (178f96, 30 eV); saccharin, 182f106, 25 eV (182f62, 32 eV); sucralose, 395f359, 15.5 eV (397f361, 15.5 eV); and sucralose-d6, 401f365, 15.5 eV (Table S1 of the Supporting Information). Prior to analysis, 5 mL aliquots of samples were fortified with internal standard (100-500 ng). Untreated wastewater was filtered (0.45 µm Chromafil RC-45/25, Macherey-Nagel, Du ¨ ren, Germany). Concentrations were determined from peak area ratios relative to the internal standard and with standard addition to account for matrix effects. Data on precision, matrix effects, and limits of detection are included in the Supporting Information. Activated Sludge Incubation. Biodegradability of the compounds in activated sludge was investigated under laboratory conditions. Untreated wastewater (0.8 L) from the primary sedimentation basin of WWTP Wa¨denswil was mixed with return sludge (0.8 L), thus in a similar ratio as under typical operating conditions. The suspension was stirred at 15 °C in the dark and aerated with water-saturated, compressed air through a glass frit. The sweeteners were present in wastewater at high enough concentrations so that additional spiking was not necessary. Periodically, 5 mL samples were removed, fortified with the internal standard, filtered (0.45 µm Chromafil RC-45/25), and analyzed by online SPE-LC-MS/MS as described above. Total incubation time was 7 h, which is in the range of typical residence times of wastewater in activated sludge basins in Swiss WWTPs. A sterile control was run in parallel. To achieve sterile conditions, the mixture of activated sludge and wastewater was autoclaved at 121 °C for 20 min. 4382
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Results and Discussion Occurrence in WWTPs. In untreated wastewater, the four sweeteners were consistently detected at concentrations of 2-65 µg/L, typically in the order of cyclamate ≈ acesulfame > saccharin > sucralose (Table 2). Per capita loads (concentrations normalized for wastewater throughput and population) indicated fairly constant inputs of sweeteners to wastewater (mean values of 11, 10, 3.9, and 1.5 mg person-1d-1, respectively, Table 2). The loads of cyclamate and saccharin tended to be higher on cooler days and strongly correlated with each other. In Switzerland, these compounds are primarily used as blends in tabletop sweeteners, which are usually consumed with hot beverages. In treated wastewater, concentrations and loads of acesulfame and sucralose were comparable to those in untreated wastewater, whereas cyclamate and saccharin were found at clearly lower levels, corresponding to mean elimination efficiencies of 99 and 90%, respectively (WWTPs with activated sludge process, Table 2). Incubation experiments with activated sludge from a typical WWTP confirmed the persistence of acesulfame and sucralose (no degradation within 7 h of incubation at 15 °C). Dissipation of cyclamate and saccharin followed first-order kinetics with DT50 (time for 50% dissipation) values of ≈20 and 90 min, respectively, and elimination efficiencies of 99 and 78% after 3 h, respectively. No elimination was observed in sterile controls. Occurrence in Surface Waters. The artificial sweeteners were analyzed in samples from various Swiss lakes. During winter, these lakes are typically well-mixed, laterally and vertically, so that samples from the outlets can be considered representative of the whole water body. Acesulfame, cyclamate, and saccharin were found in concentrations ranging from below detection in a remote mountain lake up to 2.8, 0.13, and 0.18 µg/L in Greifensee, respectively, which is a lake with a densely populated catchment area. Generally, concentrations increased with population (P) and decreased with the mean water throughflow (Q, in m3 d-1, dilution). The ratio P/Q expresses the actual anthropogenic burden to a lake by domestic wastewater (7, 9). In fact, for acesulfame, cyclamate, and saccharin, good linear correlations were found between concentrations and P/Q (Figure 1, r2 ) 0.93, 0.95, 0.88, respectively), indicating that the compounds reflect contamination by domestic wastewater not only in a qualitative, but also a quantitative sense. Similar correlations have previously been reported for caffeine, nicotine derivatives, and other compounds (7, 9). Per capita loads in the outlets of these lakes can be estimated from the slopes of the regression lines in Figure 1 (7, 9) (8.1 ( 1.7, 0.39 ( 0.13, and 0.52 ( 0.16 mg person-1d-1 for acesulfame, cyclamate, and saccharin, respectively). For acesulfame and saccharin, per capita export loads from lakes
Not detected; for LODs, see Experimental Section. c a Flow-proportional 24 h composite samples. WWTPs with activated sludge process. b Influent, 9500 m3 d-1 in effluent. Samples < limit of detection (LOD) were considered using 0.5 × LOD as the concentration value. d
1.6 ( 0.6 -5 ( 13 0.37(()0.43 90 ( 14 11 ( 6.7 11 ( 4.2 -9 ( 28 10 ( 3.4 per capita load (mg person-1d-1), mean ( SDd elimination (%), mean ( SD
30 24
30
17
September June 18 May 7 June 23 September June 24 September April 28 September September Gossau Horgen Ma¨nnedorf Meilen Regensdorf Richterswil Uster Wa¨denswil Wetzikon Zurich
0.09(()0.12 99 ( 1
3.9 ( 1.7
1.5 ( 0.6
6.2 2.7 2.9 4.5 3.1 8.8 5.2 2.0 5.2 4.1 n.d. 0.27 0.50 3.2 0.42 0.33 0.68 1.5 1.8 0.51 18 3.9 7.1 7.1 14 10 18 4.8 16 12
5.8 2.2 2.8 3.5 3.2 9.1 5.5 2.0 5.2 3.3
effluent (µg/L)
30
11800 22200 10200 23400 15500 12900 36400 20000 26000 410000
2560 9700 3850 10400 5730 4150 15250 10160b 7610 150100
43 15 19 22 34 23 42 12 36 29
46 15 25 28 14 27 46 16 45 39
56 10 18 18 46 20 65 15 39 34
n.d. n.d. n.d. 0.82 n.d. 0.11 0.20 0.45 0.27 n.d.
c
sucralose
influent (µg/L) effluent (µg/L)
saccharin
influent (µg/L) effluent (µg/L)
cyclamate
influent (µg/L) effluent (µg/L)
acesulfame
influent (µg/L)
wastewater throughput (m3 d-1) population served sampling date (2008) WWTP
TABLE 2. Concentrations and Loads of Artificial Sweeteners in WWTPs, Canton of Zurich, Switzerlanda
FIGURE 1. Concentrations of sweeteners in Swiss lakes are proportional to the ratio of the population in the catchment area per throughflow of water (9). P/Q is a measure for the actual anthropogenic burden to a lake by domestic wastewater (7). The second abscissa indicates the approximate wastewater burden, derived from the mean per capita wastewater discharge in the Canton of Zurich of 0.5 m3/d (23). were thus similar to those of treated wastewater (Table 2), suggesting that processes such as hydrolysis, photolysis, and biodegradation play a minor role in these lakes. In contrast, for cyclamate, somewhat higher per capita loads were observed in lakes compared to treated wastewater. This is likely due to inputs of untreated wastewater (≈3%, estimated with a simple mass balance approach (9)). In the study area, untreated wastewater primarily stems from combined sewer overflows during rain events. The estimated fraction of sewer overflows is consistent with earlier findings for caffeine and cotinine, two markers with similar high elimination efficiencies in WWTPs (9, 15). In rivers, even higher concentrations of acesulfame were observed (Figure 2). For example, along the course of the river Glatt, where wastewater from 7 additional WWTPs is discharged, concentrations increased from 2.8 µg/L at its source (Lake Greifensee, see above) to 6.0 µg/L at km 18 and 6.9 µg/L at its outlet to the river Rhine (km 35) and were thus proportional to the P/Q values (0.34, 0.56, and 0.63 persons m-3 d, respectively). The mean per capita load of acesulfame in all investigated rivers amounted to 9.5 mg person-1d-1, again similar to that in treated wastewater. Sucralose was only detectable in rivers, not in lakes, despite comparatively high loads in WWTP effluents. The sweetener was only recently registered for use in Switzerland. The late market introduction, quite high water residence times in these lakes (1-15 years), and the relatively high limit of detection (LOD) (0.2 µg/L) explain why sucralose was not found in the investigated lakes. The highest concentrations were again observed in the river Glatt at its outlet to the river Rhine (0.6 µg/L). Occurrence in Groundwater. In groundwater samples from a regular monitoring program in the area of Zurich, only acesulfame was detected, however, at high frequency and sometimes fairly high concentrations. In fact, the sweetener was found in 65 of the investigated 100 samples at concentrations up to 4.7 µg/L (Figure 2). High concentrations were generally observed in aquifers with VOL. 43, NO. 12, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Acesulfame in rivers and groundwater, Canton of Zurich, Switzerland. Sample designation for rivers are Limmat (L), Glatt (G), To¨ss (T), and Sihl (S); the number indicates the distance of the sampling location from the origin of the river in km (for Sihl distance from Sihlsee). For groundwater samples, 35 of 100 samples are below LOD, and the numbers indicate percentile values. Some groundwater samples are linked to the infiltrating river sample upstream of the pumping stations. The second ordinate indicates the approximate wastewater burden, derived from a mean acesulfame concentration of ≈20 µg/L in wastewater and assuming a conservative behavior during infiltration. significant infiltration of river water, where the infiltrating water received considerable discharges from WWTPs, such as in the Glatt and Limmat valleys (14). Links between concentrations in groundwater samples and samples from the infiltrating river upstream of the pumping stations are indicated in Figure 2. The presence of acesulfame in groundwater may thus indicate infiltration of treated domestic wastewater via surface water (indirect inputs) but may also indicate untreated wastewater, e.g., from a leaky sewer (direct inputs). Because of its persistence, acesulfame does not discriminate between treated and untreated wastewater. However, the absence of saccharin and cyclamate in the investigated groundwater samples suggests that contamination primarily originated from treated wastewater because in treated wastewater their concentrations are 10-100 times lower than in untreated wastewater (Table 2), or else a possible contamination with untreated wastewater occurred some time ago so that the two sweeteners have been degraded in the meantime in or on their way into the groundwater. In any event, detection of saccharin and cyclamate in groundwater would be a strong indication for a recent contamination by untreated wastewater. Occurrence in Drinking Water. Ground- and spring water are important drinking water resources in the Canton of Zurich (≈40% and 20% of total water consumption, respectively; population, 1.3 million). Another ≈40% is treated water from Lake Zurich. As groundwater is often not further purified, similar concentrations of acesulfame can be expected in tap water as in groundwater. Some exploratory analyses were therefore performed with tap water samples from the lower Glatt valley, where the highest groundwater concentrations of acesulfame were observed. In fact, concentrations up to 2.6 µg/L were measured in these tap water samples. Furthermore, acesulfame was also detectable in finished drinking water (0.02-0.07 µg/L) after ozonation of 4384
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lake water. Oxidative elimination, estimated from the comparison between raw and finished drinking water, was ≈85-95%. Concentrations were thus quite high in certain tap water samples but still orders of magnitude lower than those in low-calorie beverages and lower than organoleptic detection thresholds (Table 1). Suitability of Acesulfame as a Chemical Marker for Domestic Wastewater. The present study demonstrates that acesulfame meets all of the above-mentioned criteria for a good marker of domestic wastewater for application in groundwater and surface waters. The synthetic compound is source specific as it is used as sweetener in beverages, food, and certain consumer products such as toothpaste. Industrial or agricultural uses are not known. The correlation between concentrations in lakes and the actual anthropogenic burden (Figure 1) indicates that acesulfame quantitatively reflects contamination by domestic wastewater. The detection limit of ≈0.01 µg/L is low enough for analyses in surface and groundwater and allows detection of g0.05% wastewater contamination (derived from a mean acesulfame concentration of ≈20 µg/L in wastewater and assuming a conservative behavior). Analysis is fast and results are obtained within 2 h or less because no offline preconcentration step is necessary. Acesulfame is not eliminated in WWTPs and is quite persistent in surface waters. The sweetener is also sufficiently persistent and hydrophilic to reach groundwater (high frequency of detection, 65%). The widespread occurrence of acesulfame confirms that, in the study area, many groundwater aquifers are substantially influenced by infiltration of river water, where the infiltrating water received considerable discharge from WWTPs (14). Our data suggest that, in the lower Glatt valley, ≈10-20% of the water at the pumping stations indirectly stem from domestic wastewater.
Acknowledgments We thank the personnel of the WWTPs for wastewater samples, the Office for Waste, Water, Energy, and Air of the Canton of Zurich, Switzerland (AWEL, C. Balsiger), and Labor Veritas, Zurich (C. Ho¨ckelmann) for river samples, the Official Food Control Authority of the Canton of Zurich (A. Besl, D. Bringolf) for groundwater samples, the Water Supply Authority, Zurich (E. Keller, H.P. Kaiser) and the Lake Waterworks Appital, Wa¨denswil (B. Niederberger) for drinking water samples, A. Ba¨chli (Agroscope) for her help in lake water sampling, I. Mu ¨ hlebach (Federal Office for Agriculture, Bern) for consumption data, and M.E. Balmer and O. Daniel (Agroscope) for reviewing the manuscript.
Supporting Information Available Details on the analytical method (ion transitions and collision energies) and data on precision, matrix effects, and limits of detection. This material is available free of charge via the Internet at http://pubs.acs.org.
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