Environ. Sci. Technol. 1988, 22, 978-981
Influence of Humic Substances on the Formation of Chlorinated Polycyclic Aromatic Hydrocarbons during Chlorination of Polycyclic Aromatic Hydrocarbon Polluted Water Staale Johnsen" and Ingrid Susann Gribbestad st
Center for Industrial Research, P.O. Box 350, Blindern 0314, Oslo, Norway, and Department of Chemistry, University of Trondheim, 7055 Dragvoll, Norway
Chlorinated polycyclic aromatic hydrocarbons (PAH) are present at nanogram per liter levels in lake water. Some of these compounds are known to be mutagenic in the Ames Salmonella test. The PAH compounds fluorene, anthracene, fluoranthene, and benzo[a]pyrene were dissolved in lake water with low humus content and in humus water with 9.17 mg of total organic carbon/L, followed by sodium hypochlorite chlorination at different concentrations. Reaction of PAH and formation of chlorinated PAH were measured by cyclohexane extraction of the samples 3 days after chlorination and gas chromatography/mass spectrometry analyses of the extracts. The PAH-chlorine reaction was found to be dependent upon the concentration of free active chlorine in the water, and the presence of humic substances was found to affect the formation of chlorinated PAH. Chlorinated PAH were formed in the lake water samples of fluoranthene and benzo[a]pyrene, but no chlorinated PAH were detected in the presence of humic substances.
Introduction Surface waters often contain relatively high concentrations of humic substances, and the use of such sources for human water supplies may prove troublesome with regard to both technical and health aspects. Treatment of aquatic humic substances with strong chemical oxidants such as ozone or chlorine affects the color and molecular size distribution of the humic substances (1-4) and may lead to formation of mutagens (5-7). Due to atmospheric long transport, background levels of polycyclic aromatic hydrocarbons (PAH) have been found even in remote areas (8-10). Several sources of PAH, both natural and anthropogenic, are known, the most important one being combustion of organic material (11). Interactions between PAH and aquatic humic substances have been described by several authors (12-16), and different types of bindings are involved in these processes. Even though PAH consist of chemically stable aromatic ring systems, their reactivity varies substantially. For many of the PAH reactions, an oxidation step is rate determining. For addition reactions with PAH, Wheland's concept of localization energy has proved extremely useful (17). On the basis of this theory, the reactivity of the different positions in the ring system of a PAH compound can be determined (18). The rate of bromination for some nonsubstituted PAH has been studied by Altschuler and Berliner (19). The rate constant was found to vary considerably for the compounds studied, anthracene being the most reactive of these. This result indicates that the reaction between selected PAH and oxidants, such as free active chlorine (FAC1) defined as the equilibrium mixture of HOC1 and OC1- formed by chlorination of water, is selective. Furthermore, the presence of aquatic humic substances during this reaction may affect the PAH-FAC1 +Center for Industrial Research. *Universityof Trondheim. 978
Environ. Sci. Technol., Vol. 22, No. 8 , 1988
reaction, since interactions between PAH and humic substances and between FACl and humic substances may occur. Previous work has indicated that the presence of humic substances influences the recovery of PAH from chlorinated PAH-polluted water samples (20). Humic substances have also been found to be able to catalyze certain photochemical and redox reactions (21). In this study we have investigated the recovery of PAH and also the formation of chlorinated PAH during chlorination of PAH-polluted water in the presence and in the absence of humic substances. Four different PAH compounds were chosen for the experiment: fluorene, anthracene, fluoranthene, and benzo[a]pyrene, on the basis of the following criteria. Fluorene and fluoranthrene are expected to be of low reactivity in the PAH-FAC1 reaction (18, 19),but both compounds show interesting differences in recovery from chlorinated lake and humus water (20). Anthracene is, on the other hand, expected to be one of the most reactive PAH compounds and is included for this reason (18,19). None of these compounds are known as possible mutagens or carcinogens. The last compound, benzo[a]pyrene, is known as a potential mutagen and carcinogen and is also one of the most well-studied compounds in environmental research. The formation of chlorinated and oxygenated PAH during chlorination of PAH-polluted water has been shown by several authors (22,23). Some chlorinated derivatives of pyrene and benzo[a]pyrene have been found to show high mutagenic activity in the Salmonella TA-100 test (24). The presence of chlorinated PAH at the nanograms per liter level in lake water has been shown by Shirashi and co-workers (25). Taking all these facts into account, the formation of chlorinated PAH and the possible effect of humic substances on this during water disinfection are of importance from a health perspective.
Experimental Section PAH compounds were dissolved in the water samples and then chlorinated. After 3 days of storage at 4 "C, the samples were extracted and analyzed for PAH and chlorinated PAH content. The 3-day storage is assumed to be similar to the time from chlorination in a water plant until the water reaches the consumer. Synthesis of Chlorinated PAH. In order to obtain standard chlorinated PAH compounds for quantitative and qualitative analyses, anthracene, fluoranthene, and benzo[a]pyrene were chlorinated under controlled conditions, and the reaction products were isolated and purified according to the method described by Shirashi et al. (25). In addition, a 9-chlorofluorene standard was obtained from ICN Biomedicals Inc. The synthesized compounds were analyzed by gas chromatography/mass spectrometry (GC/MS) and in one case (anthracene) by proton nuclear magnetic resonance (lH NMR). The instruments used were H P 5890/5970 GC/MS and Bruker 400 mHz NMR. A quantitative reference standard mixture was made from these products.
0013-936X/88/0922-0978$01.50/0
0 1988 American Chemical Society
Chlorination Experiments-Lake Water and Aquatic Humic Substances. One of the drinking water sources of the city of Trondheim, Norway, Jonsvatnet, was used as a control water source. This lake has a low total organic carbon (TOC) content approximately 1 mg of TOC/L, and the color is below 10 mg of Pt/L. The aquatic humic substances used for the experiments were sampled during Fall 1985 from a bog, Hellerudmyra, outside the city of Oslo. This source has been well described and characterized during the last 20 years (26). Prior to the experiments, 1L of each water type was analyzed for free active chlorine, PAH, and chlorinated PAH according to the procedures described below in order to investigate the possible background levels. Immediately after sampling, the water was membrane filtered through 0.45-pm filters to remove particles. The TOC concentration of the aquatic humic substances was 9.17 mg/L after filtration. In order to keep pH at a constant level during the chlorination, all samples were buffered to pH 7.2 by the addition of 10 vol % of 1/15 M phosphate buffer (20). To avoid microbiological growth and possible transformation reactions prior to chlorination, 0.02 wt % of sodium azide was added (20). The water solutions of PAH were made in triplicate by the addition of 20 pL of an acetone solution of each compound to 1-L volumes of water, followed by rapid stirring for 2 h. Concentration of PAH was in the range 2-10 ppb with benzo[a]pyrene having the lowest concentration since the water solubility of this compound is approximately 3.5 ppb (27). Aliquots of sodium hypochlorite solution were then added to yield concentrations of 0 (blank), 1.0, 2.5, 5.0, and 10 mg of FACl/L for lake water samples, 1.0 mg/L corresponding to 0.019 mmol of FACl/L. Substantially higher initial FACl concentrations of 0 (blank), 10,20,30, and 40 mg/L were necessary in the humus water due to reactions of FACl with humic substances. FACl was measured as the sum of (OC1-) and (HOC1) by Heilige Neocomparator, DPD-A. This gave a total of 20 samples, each with three parallels. After the chlorination, the samples were stored in darkness at 4 “C for 3 days. The products formed during chlorination of aquatic humic substances may interact with both PAH and chlorinated PAH and thereby affect the extraction efficiency of these. In order to investigate this possibility, some of the lake water and humic water was chlorinated to a residual FACl concentration of 5 mg/L after 3 days of storage. The excess FACl was removed by addition of 0.1 g of ascorbic acid, and a mixture of the actual PAH and chlorinated PAH were added to the water according to the same procedure as above. Triplicate samples were made in each case and stored as above. Workup and Analytical Procedures. Prior to the analytical workup, the residual FACl concentration in the samples was measured. The 1-L sample parallels were extracted twice with 50 mL of cyclohexane, each time for 2 h. Internal standards for the quantitative analyses, 3,6-dimethylphenanthreneand /3’,/3’-biphenyl, were added to the cyclohexane prior to extraction. The organic layers were separated from the water and combined to one sample. These samples were dried with anhydrous sodium sulfate for 2 h, and the solvent was removed by evaporation under a gentle stream of highly purified nitrogen, leaving a total volume of approximately 1 mL for each sample. The analyses were performed by selected ion monitoring GC/MS, with a Hewlett-Packard 5890/5970 GC/MSD with a 30 m X 0.33 mm H P SE-54 fused-silica capillary column. Response factors were calibrated every day by running authentic standards prior to the sample analyses. Identification of the compounds was done by recognition
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Results Synthesis of Chlorinated PAH. Chlorination of anthracene in acetic acid resulted in the formation of 9,lOdichloroanthracene, identified by lH NMR analyses. The purity of the compound after recrystallization was found to be 99% by GC/MS analyses. lH NMR analyses showed that the 1% of impurity consisted of 9-chloroanthracene. Chlorination of fluoranthene in acetic acid resulted in the formation of a large number of mono-, di-, and trichlorofluoranthene isomers. These were not separated further, but the quantitative composition of the mixture was determined by GC/MS. Finally, chlorination of benzo[a]pyrene in the acetic acid medium resulted in the formation of four compounds, two dichloro- and two trichlorobenzo[a]pyrene derivatives. GC/MS analyses of the mixture showed that the total purity of the two dominating products, one dichloro- and one trichlorobenzo[a]pyrene, was 99%. The remaining 1% consisted mainly of the other isomers. Lake Water and Humus Water Experiments. No FACI, PAH, or chlorinated PAH was detected in the original lake water sample. With regard to the humus water, the PAH content had been determined earlier and could be ignored in comparison with the added amount of PAH (26). Figures 1-4 show the recovery of PAH and the formation of chlorinated PAH in the chlorinated lake water and humus water as a function of initial FACl concentration. The residual FACl concentrations are also given in the figures. For fluoranthene the recovered chlorinated compounds are given as “total chlorinated fluoranthene”. Environ. Sci. Technol., Vol. 22, No. 8, 1988 979
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The precision of the results is given by showing the standard deviation of the replicates on the curves. The recovery of PAH and chlorinated PAH from the lake water and humus water chlorinated prior to the addition of the organic micropollutants was quantitative for all compounds except 9-chlorofluorene. This compound was not detected in any of the samples.
Discussion Synthetic Products. As could be expected from Wheland's concept of localization energy, chlorination of anthracene resulted in the formation of 9,lO-dichloroanthracene. For the other two PAH compounds, fluoranthrene and benzo[a]pyrene, milder reaction conditions would be needed to form a single product rather than a mixture of products. Such syntheses were not found necessary to carry out, since the formation of the mixture rather than single products would be expected under aqueous conditions (see below). Accuracy and Precision of the Analytical Results. Response factors of each compound were calibrated by running a standard mixture of authentic PAH and chlorinated PAH as described above. The results obtained from the analyses of the chlorinated samples were compared io the results from the blank samples analyses to control the extraction efficiency. However, a possible decrease in recovery of the organic micropollutants because of interactions with products from the chlorination of humic substances is not accounted for by the blank samples. The experiment with the addition of PAH and chlorinated PAH to prechlorinated humus water showed that such interactions did not affect the cyclohexane extraction efficiency. Since 9-chlorofluorene was lost in both the chlorinated lake water and humus water samples, it 980
Environ. Sci. Technol., Vol. 22, No. 8, 1988
is likely to assume that this loss was not caused by such interactions. The precision of the results was controlled by the three replicates of each sample, as shown in Figures 1-4. Lake Water Samples. No significant decrease in the recovery of PAH from the nonchlorinated lake water samples (blanks) was observed. This proves that the efficiency of the extraction method is very good, a result that is in accordance with previous experiments (20,26). Some general observations were made from the analytical results of the lake water samples. All of the investigated PAH compounds reacted with FACl, and the recovery of PAH decreased with increasing initial FACl concentration and increasing residual FACl concentration. This proves that the PAH-FAC1 reaction efficiency is FACl concentration dependent. As expected, the reaction between PAH and FACl is selective, i.e., the recovery or reaction efficiency varies for the different PAH compounds. Anthracene is, not surprisingly, the most reactive, but fluoranthene and benzo[a]pyrene were also found to be very reactive. While all anthracene and fluoranthene reacted a t initial FACl concentrations above 2.5 and 5.0 mg/L, respectively, a small but still significant amount of benzo[a]pyrene was recovered even for the highest FACl concentrations. The high reactivity of fluoranthene is perhaps a little surprising with regard to the rates of bromination found by Altschuler and Berliner (19). It is seen from Figure 1 that the reactivity of fluorene is much lower than for the other PAH. Even at high initial and residual FACl concentrations, 10 and 6 mg/L, respectively, 60% of the added fluorene was recovered. 9-Chlorofluorene and 9,lO-dichloroanthracene were not detected in any of the fluorene and anthracene lake water samples. In all chlorinated fluoranthene lake water samples the same chromatographic peak pattern of mono-, di-, and trichlorofluoranthene as found in the synthetic standard mixture was detected. However, most of the peaks were close to the limit of quantification in size, so the precision of the quantitative results is rather low. These results are therefore regarded as qualitative. Both major chlorinated derivatives of benzo[a]pyrene were detected and quantified in the chlorinated lake water samples. As shown in Figure 3, the formed amount of both analyzed benzo[a]pyrene derivatives increased with increasing FACl concentration to a maximum at initial FACl concentration of 5 mg/L. When the FACl concentration increased further, formation of trichlorobenzo[a]pyrene became dominant, while the detected amount of dichlorobenzo[a]pyrene was below the limit of quantification at FACl concentration of 10 mg/L. A mass balance for the added PAH and unreacted and chlorinated PAH was not possible to obtain since sodium hypochlorite treatment resulted in the formation of many oxidized products as well. No other chlorinated PAH derivatives other than the synthesized standards were identified in the extracts of the lake water samples. Effect of Humic Substances. As mentioned earlier, the consumption of FACl was much higher for humus water than that of the low humic substances containing lake water due to reactions between FACl and aquatic humic material. The results of the analyses of the two sample types must therefore be compared with regard to the residual FACl concentration rather than the initial FACl concentration. Figure 4 shows that anthracene is the most reactive of the studied PAH compounds; also in the presence of humic substances, anthracene is the most reactive of any of the chlorinated samples. Since no residual FACl concentrations were observed in the samples with
initial concentrations of 10 and 20 mg/L after 3 days of storage, this indicates that anthracene is more reactive than some of the humic substances. Fluorene shows the same behavior in the lake water and humus water samples. At 2.5 (lake water) and 2.4 (humus water) mg/L residual FAC1, the recovery is 70% and 7570, respectively, which means that fluorene consumed in the FAC1-fluorene reaction is not influenced by the presence of humic substances. The same behavior was observed for benzo[a]pyrene; even at a high FACl concentration, a small amount was recovered. Comparison of Figures 2 and 4 shows that the behavior of fluoranthene differs from lake water to humus water. At a residual concentration of 1.5 mg/L FaCl no fluoranthrene was recovered from lake water, while 65% was recovered from humus water a t 2.4 mg/L residual FAC1. A possible explanation is that humic substances or the remaining products from the reaction of FAC1-humic substances are able to interact with fluoranthene and, thereby, decrease the efficiency of the FAC1-fluoranthene reaction. However, no such interactions were detected by cyclohexane extraction of the prechlorinated lake water and humus water experiments. It should be emphasized that this does not necessarily mean that such interactions do not take place. As can be seen from Figure 4, no chlorinated PAH compounds were detected in any of the humus water samples. Interactions between humic substances and PAH may affect the reaction efficiency of and/or the type of reaction products formed in the FACl-PAH reaction. During chlorination of humic substances the original humic molecules are transformed and degraded into new organic compounds. Interaction between such compounds and chlorinated PAH does not influence the extraction efficiency of the latter and cannot explain the absence of chlorinated PAH. The results of these experiments show that presence of PAH in drinking water sources will result in the formation of some chlorinated derivatives during chlorine disinfection. Some of these chlorinated compounds are strong mutagens and may be of potential hazard to human health. The presence of humic substances during the chlorine disinfection affect the PAH-FAC1 reaction by favoring the formation of products other than chlorinated PAH. Detection and characterization of such products would be very difficult because of their polar nature and the complex matrix in these samples.
Acknowledgments We thank A. Schie-Bergan, University of Trondheim, for help with the 'H NMR analyses. Registry No. Anthracene, 120-12-7; fluoranthene, 206-44-0; benzo[a]pyrene, 50-32-8; 9,10-dichloroanthracene,605-48-1; dichlorobenzo[a]pyrene, 26573-11-5; trichlorobenzo[a]pyrene, 97303-27-0.
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