Environ. Sci. Technol. 2009, 43, 724–729
Changes in NOM Fluorescence Caused by Chlorination and their Associations with Disinfection by-Products Formation P A O L O R O C C A R O , * ,† FEDERICO G. A. VAGLIASINDI,† AND GREGORY V. KORSHIN‡ Department of Civil and Environmental Engineering, University of Catania, Viale A. Doria 6, 95125, Catania, Italy, Department of Civil and Environmental Engineering, University of Washington, Seattle, Washington 98195-2700
Received July 14, 2008. Revised manuscript received November 14, 2008. Accepted November 20, 2008.
Relationships between the formation of disinfection byproduct (DBPs) and changes of the fluorescence of natural organic matter (NOM) in chlorinated water were quantified using two fluorescence indexes. They were defined as the change of the wavelength that corresponds to 50% of the maximum intensity of fluorescence (∆λem0.5) and the differential ratio of fluorescence intensities measured at 500 and 450 nm (∆(I500/I450)). Although variations of chlorine doses, reaction times and temperatures affected the kinetics of chlorine consumption and DBPs release, correlations between chlorine consumption, concentrations, and speciation of trihalomethanes, haloacetonitriles, haloacetic acids and, on the other hand, ∆(I500/I450) and ∆λem0.5 values remained unaffected by chlorination conditions and, to some extent, NOM properties. These results allow developing a fluorescence-based approach to monitor DBPs formation in drinking water.
Introduction Chlorination of drinking water is widely used worldwide because it is effective against most pathogens, economical, and provides residual disinfecting capacity downstream from the treatment site. The main drawback of chlorination, aside of its limited ability to control Giardia and Cryptosporidium, is that chlorine reacts with natural organic matter (NOM) to generate disinfection byproduct (DBPs) (1, 2). Among >600 individual halogen-containing DBPs species have been identified in chlorinated water, trihalomethanes (THMs) and haloacetic acids (HAAs) are considerably more prominent than any other DBPs. For that reason and due to their toxicity and carcinogenicity, total THMs (TTHM) and five HAAs species (HAA5) are regulated in many countries (1, 3). Among unregulated DBPs, several species occur at much lower concentrations than those of THMs and HAAs, but their toxicities are higher (1, 4). Recent studies have determined that their toxicities follow a halonitromethanes > haloacetamides . haloacetonitriles . haloacetic acids > trihalomethanes sequence (1, 4-7). The nature of halogen atoms * Corresponding author phone: +39 095 7382729; fax +39 095 7382748; e-mail:
[email protected];
[email protected]. † University of Catania. ‡ University of Washington. 724
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incorporated in DBPs also affects their toxicity which follows an I . Br . Cl order. Predicting and monitoring the formation of both major and minor but potentially highly toxic DBPs is difficult due to the complex pathways of their formation and analytical challenges. Several absorbance-based surrogate parameters are useful for these purposes. They include specific absorbance of light at 254 nm (SUVA254) (8, 9), molar absorbance of organic carbon at 280 nm (280) (10), ratio of absorbances at 250 and 365 nm (A250/A365) (11) and differential absorbance at wavelengths near 272 nm (∆A272) (12, 13). In addition to the absorbance-based approaches, fluorescence spectroscopy can be used to monitor NOM halogenation. Fluorescence is a highly sensitive typically interference-free method that yields information on NOM nature and reactivity (8, 14, 15). Fluorescence excitation-emission matrices (EEMs) reveal the existence of mostly aromatic fluorophore groups (that are part of but distinct from a larger group of NOM chromophores) related to allochtonous or autochtonous NOM sources. A fluorescence index (FI) defined as the ratio of NOM emission intensities at 450 and 500 nm and obtained with an excitation of 370 nm was developed and used to ascertain contributions of dissimilar sources of NOM (15). FI is also correlated with NOM aromaticity and molecular weight. Limited studies have been carried out to examine effects of chlorination on NOM fluorescence (16-19). They have demonstrated that several parameters, for instance the position of the wavelength corresponding to 50% of fluorescence intensity compared to that at its maximum (λem0.5), FI, the emission lifetime and apparent size of NOM molecules undergo consistent changes during chlorination. Changes of some of these parameters, notably those of λem0.5 are well correlated with the formation of individual DBPs (18, 20). Despite these efforts, effects of chlorination on the fluorescence of NOM that can, in principle, be used to track DBPs formation have not been fully ascertained. Accordingly, this study examined in detail the behavior of fluorescencebased parameters and quantified their associations with chlorine decay, DBPs generation, and speciation. Special attention was given to the effects of variations of chlorine dose, reaction time, temperature, and NOM properties on the performance of fluorescence indexes developed to probe DBPs formation.
Experimental Section Most of the experiments were carried out using water samples taken at the inlet and outlet of water treatment plant (WTP) that treats the water from Ancipa reservoir (also named Lake Sartori). This 20 million m3 artificial reservoir located at 950 m above the sea level in the middle-east part of Sicily (Italy) is used for water supply. The Ancipa WTP can treat up to 1000 L/s (currently ca. 500 L/s) using a sequence of coagulation, flocculation, and settling, intermediate chlorination, sand filtration. The DOC concentrations and SUVA254 values of Ancipa Inlet samples taken in December 2005 (denoted as Ancipa Inlet D) and March 2006 (denoted as Ancipa Inlet M) were 2.9 mg/L and 2.8-2.9 L · mg-1 · m-1, respectively. The pH values of the above samples ranged from 7.9 to 8.0, their alkalinity was 155-170 mg/L as CaCO3, and hardness was 140 mg/L as CaCO3. The DOC and SUVA254 values in Ancipa Outlet sample were 2.0 mg/L and 1.8 L · mg-1 · m-1, respectively. Experiments were also conducted using samples from the Potomac River and McMillan and Dalecarlia WTPs that 10.1021/es801939f CCC: $40.75
2009 American Chemical Society
Published on Web 01/05/2009
FIGURE 1. Normalized fluorescence emission spectra for chlorinated Ancipa Inlet D water at pH 7.0, varying reaction parameters: (a) chlorine to DOC ratio 1.50 mg/mg, reaction times from 10 min to 3 days and temperature 20 °C; (b) chlorine to DOC ratio 0.75 mg/mg, reaction times from 10 min to 4 h, and temperature 34 °C. utilize this water. In raw Potomac water sample (denoted as “Potomac Inlet”) taken from the Potomac River at the Great Falls intake point (located 10 miles upstream from the Dalecarlia Reservoir), DOC was 2.7 mg/L and SUVA254 was 2.4 L · mg-1 · m-1. Samples of McMillan and Dalecarlia water from the sedimentation tanks (denoted as MS and DS for McMillan and Dalecarlia, respectively) and filtered plant effluents (denoted as MF and DF for McMillan and Dalecarlia plants) were also taken. Their DOC values were 1.6, 1.7, 2.0, and 2.0 mg/L for MS, MF, DS, and DF samples, respectively. SUVA254 values were 1.7, 1.3, 1.8, and 1.5 L · mg-1 · m-1 for MS, MF, DS, and DF samples, respectively. The average pH, alkalinity, and hardness of treated Potomac Inlet water were 7.6, 74, and 114 mg/L, respectively. The concentration of bromide in all water samples used in this study was λmax) and denoted as λem0.5 (18) exhibited a blue shift that increased at higher reaction times or chlorine concentrations or temperatures, as demonstrated by the normalized fluorescence emission spectra for chlorinated Ancipa Inlet water (Figure 1). Normalized emission spectra for chlorinated Potomac water are reported in the SI section (Figure S1). The shift of λem0.5 in both chlorinated water sources was similar to that described previously for Alento River water (18). To quantify the magnitude of the shift, the differential λem0.5 values denoted as ∆λem0.5 (t) ) λem0.5 (t ) 0) - λem0.5 (t) were calculated. The behavior of the ratio of emission intensities at 500 and 450 nm, denoted as I500/I450 and its change calculated as ∆(I500/I450) (t) ) I500/I450 (t ) 0) - I500/I450 (t) were also examined. The I500/I450 ratio defined above is similar to the FI introduced in prior research (15, 22) except that I500/I450 values in this study were measured using excitation at 320 nm while in prior publications a 370 nm excitation wavelength VOL. 43, NO. 3, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Correlations between ∆(I500/I450) values and speciation of THMs (a) and HANs (b). Chlorinated Ancipa Inlet D water at pH 7.0, chlorine to DOC ratios from 0.25 to 2.00 mg/ mg, reaction times from 10 min to 3 days, and temperatures from 3 to 34 °C.
FIGURE 3. Correlations between ∆(I500/I450) values and concentrations of CH (a), CHCl2Br (b), and BCAN (c). Chlorinated Ancipa Inlet D water at pH 7.0, chlorine to DOC ratios from 0.25 to 2.00 mg/mg, reaction times from 10 min to 3 days, and temperatures from 3 to 34 °C. was utilized. It is to be recognized that ∆λem0.5 values help quantify the shift of the entire NOM emission band, while the ∆(I500/I450) index is more indicative of its contraction. Both indices are affected by changes in molecular weight and/or aromaticity of NOM (14, 16, 17). Like the differential absorbance (12, 13), the intensity of the ∆(I500/I450) and ∆λem0.5 indexes increased with chlorine dose, reaction time and temperature. Further examination showed that ∆(I500/I450) and ∆λem0.5 parameters were strongly correlated (R2 > 0.90) with DBPs concentrations. This is demonstrated in Figure 2 for TTHM, total of nine HAAs species (THAA) and total haloacetonitriles (THAN, defined as the sum of dichloroacetonitrile, bromochloroacetonitrile, dibromoacetonitrile and trichloroacetonitrile) formed in Ancipa Inlet water at varying chlorine doses, reaction times and temperatures and plotted vs corresponding ∆(I500/I450) values. Similarly strong correlations existed between the concentrations of TTHM, THAA, THAN, and ∆λem0.5 values (SI Figure S2). The fact that the experimental data for each DBPs class collapsed, similarly to the behavior of these compounds in 726
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the differential absorbance domain (13), into a single data set when represented vs their corresponding respective ∆(I500/ I450) or ∆λem0.5 values supports a conclusion that the changes of these fluorescence indexes are indicators of the degree and outcome of NOM halogenation. This was confirmed by the observation that strong relationships existed between these fluorescence indexes and the concentrations of individual DBPs, as shown in Figure 3 for chloral hydrate (CH), CHCl2Br, bromochloroacetonitrile (BCAN) formed in Ancipa Inlet water. Similarly, high correlations were also found between these DBPs concentrations and ∆λem0.5 values (SI Figure S3). SI Table S1 summarizes all data demonstrating the strength of correlations between DBPs concentrations and ∆(I500/I450) or ∆λem0.5 indexes and demonstrates that they can be used for examining the formation of individual halogenated DBPs species at varying chlorination conditions. The R2 values reported in SI Table S1 correspond to mostly power best-fit functions. The type of function that provides the best fit for DBPs vs ∆(I500/I450) and/or ∆λem0.5 relationships is deemed to reflect the nature of DBPs formation mechanisms but further discussion of this issue goes beyond the scope of this paper. Because brominated DBPs are more toxic and carcinogenic (1, 5), relationships between the speciation of THMs, HAAs, and HANs and fluorescence indexes were investigated. Prior research has demonstrated that the relative contributions of brominated THMs and HANs decrease with the increase of chlorine dose, reaction time or temperature and these effects can be predicted via ∆A272 index (13). Similar correlations were observed in this study for the THMs or HANs speciation and, on the other hand, ∆(I500/I450) and ∆λem0.5 indexes (Figure 4 and SI Figure S4, respectively). The contributions of brominated THMs and HANs decreased as ∆(I500/I450) or ∆λem0.5 values increased indicating the prevalence of bromination in the initial phases of halogenation of Ancipa water NOM, as observed for ∆A272 index (13). This
FIGURE 5. Correlations between ∆(I500/I450) values and speciation of (a) selected HAAs species; (b) dihalogenated and trihalogenated HAAs species. Chlorinated Ancipa Inlet D water at pH 7.0, chlorine to DOC ratios from 0.25 to 2.00 mg/mg, reaction times from 10 min to 3 days, and temperatures from 3 to 34 °C. effect is indicative of the higher kinetic activity of bromine species compared to chlorine. It can be utilized to quantify the prevalence of bromination pathway but this option is contingent on the development of an unambiguous mechanistic NOM halogenation model (23) and will be explored elsewhere. The HAAs group was predominated by dihalogenated and trihalogenated HAAs, with a prominent increase of trihalogenated HAAs at higher reaction time (SI Figure S5), chlorine doses or temperatures. Not only the concentrations but also the speciation of HAAs groups were determined to be function of the differential fluorescence. Figure 5a shows the relationships between ∆(I500/I450) values and HAAs speciation for selected HAAs species formed in chlorinated Ancipa Inlet water, while Figure 5b shows these relationships for the contribution of dihalogenated and trihalogenated HAAs. The performance of ∆λem0.5 index to quantify the HAAs speciation was similar (SI Figure S6). Examination of the association between ∆(I500/I450), ∆λem0.5 and chlorine consumption in Ancipa Inlet water showed the existence of trends similar to those described above for individual DBPs (Figure 6). Although kinetically chlorine consumption was more prominent at higher chlorine doses, reaction times or temperatures (13), correlations between ∆(I500/I450) and ∆λem0.5 and chlorine consumption were not affected by variations of reaction conditions. The existence of strong correlations between chlorine consumption and both ∆(I500/I450), ∆λem0.5 fluorescence indexes and differential absorbance parameters (typically quantified as ∆A272) confirms that differential fluorescence of chlorinated NOM provides additional information concerning DBPs formation mechanisms (13, 24). Effects of NOM character on the relationships between DBPs formation and spectroscopic indexes are of significant interest because both NOM concentration and reactivity may
FIGURE 6. Correlations between chlorine consumption and differential fluorescence indexes: (a) ∆(I500/I450); (b) ∆λem0.5. Chlorinated Ancipa Inlet D water at pH 7.0, chlorine to DOC ratios from 0.25 to 2.00 mg/mg, reaction times from 10 min to 3 days, and temperatures from 3 to 34 °C. change due to seasonal variations of water chemistry and/or NOM removal or alteration at WTP. SI Figure S7 presents relationships between concentrations of selected DBPs and ∆A272 values for chlorinated inlet and outlet Ancipa waters and Potomac inlet, MS, MF, DS, and DF waters. The behavior of the DBPs concentrations plotted vs the ∆(I500/I450) and ∆λem0.5 indexes (Figure 7 and SI Figure S8) was different from that of differential absorbance because, when represented vs the differential fluorescence indexes, TTHM and THAA concentrations in all these water apparently formed a single data set. Although this result needs to be confirmed for a wider range of water chemistries, it indicates that the use of the differential fluorescence offers benefits for monitoring and control of DBPs formation in waters with significant short or long-term variations of NOM content or properties. The different patterns observed for the relationships between DBPs and differential absorbance and, on the other hand, the ∆(I500/I450) and ∆λem0.5 indexes measured in chlorinated water with different NOM properties (Figure 7, SI Figures S7 and S8) can be explained by specific manifestations of NOM transformations caused by chlorine in absorbance and fluorescence. The differential absorbance quantifies with high precision changes of NOM aromaticity caused by the incorporation of halogens in the predominantly aromatic attack sites. Because NOM has different concentration and properties of the halogen attack sites in raw and treated waters, this affects the functional dependence of DBPs concentrations vs ∆A272 values. In contrast with that, TTHM and THAA data obtained for both raw and treated waters formed a reasonably coherent data set when represented vs ∆(I500/I450) or ∆λem0.5 (Figure 7 and SI Figure S8). This could be due to the involvement of more complex mechanisms, notably the oxidative and/or incorporative degradation of NOM fluorophores by halogen species and structural transformations of NOM molecules. Prior research indicates that the evolution of the emission spectra during the halogenation VOL. 43, NO. 3, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 8. Correlations between I500/I450 (a) or λem0.5 (b) values and apparent aromaticity. Chlorinated Ancipa Inlet D water at pH 7.0, chlorine to DOC ratios from 0.25 to 2.00 mg/mg, reaction times from 10 min to 3 days, and temperatures from 3 to 34 °C.
FIGURE 7. Correlations between ∆(I500/I450) values and concentrations of TTHM (a), THAN (b), and THAA (c). Chlorinated Ancipa and Potomac waters at pH 7.0, chlorine to DOC ratios from 0.25 to 2.00 mg/mg, reaction times from 10 to 3 days, and temperatures from 3 to 34 °C. is likely to be indicative of the breakdown of NOM molecules (16) and attendant release of DBPs. Alternatively, the observed changes of NOM fluorescence can be associated with preferential consumption of selected types of fluorophores, for instance, dihydro-substituted aromatic groups that form a part of the ensemble of the fluorophores whose collective properties can be estimated using the FI and/or EEM approach (15, 21, 25). This view is in agreement with the point that the FI is likely to be correlated with the aromaticity of NOM (15, 26). Changes of the apparent aromaticity of NOM, calculated in accord with ref 8 as Φ (%) ) 527 SUVA254 + 2.8, plotted vs I500/I450 or λem0.5 values, obtained by the chlorination of Ancipa Inlet D water at varying chlorine doses, reaction times and temperatures in this study (Figure 8), suggest that these fluorescence indexes can be considered as indicators of the destruction of the reactive aromatic groups in NOM. 728
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The above observations are not in conflict with the hypothesis assuming that transformations of the fluorescence spectra of NOM are caused both by the changes of the aromatic fluorophores and also of the conformations and molecular weights of NOM molecules. For instance, the contraction of emission band (i.e., the blue shift of λem0.5) has been shown to be correlated to the decrease of both aromaticity and average molecular weight (16). Therefore, the degradation of the aromatic chlorine attack sites and breakdown of NOM molecules accompanied by the release of DBPs can be hypothesized to occur simultaneously. Although the role of contributions of these two distinct processes in the evolution of the fluorescence spectra of halogenated NOM remain to be ascertained, this study shows that differential fluorescence indexes can be used to track the DBPs formation and speciation in a wide range of water treatment conditions, specifically at varying reaction times, temperatures, chlorine doses and NOM properties. Additional benefits of the use of NOM fluorescence to track DBPs formation are defined by its very high sensitivity, ability to generate information-rich 3D EEM spectra and lack of interference by many species (e.g., chlorine, nitrate, nitrite) whose presence complicates the use of absorbance spectroscopy. This approach can be useful for the development of integrated water systems that can be optimized based on real-time monitoring of water quality parameters. Further examination of the practical aspects of such an approach needs be carried out in the future.
Acknowledgments This study was partially supported by the United States EPA/ Cadmus (grant 069-UW-1) and the Italian Ministry of Instruction, University, and Research (MIUR), through the
program Research Programs of National Interest “Control and Monitoring of Drinking Water Quality”. Views expressed in this paper do not necessarily reflect those of the funding agencies. We are thankful to Professor Mark M. Benjamin (University of Washington) and to Professor Rodolfo M. A. Napoli (University of Napoli “Parthenope”) for support of this study.
Supporting Information Available Details of the experimental and analytical methods, a table summarizing the strength of the correlations discussed in the main manuscript, and eight figures demonstrating changes of the normalized fluorescence spectra of Potomac River water, correlations between ∆λ0.5 values and concentration/speciation of THM, THAN, and THAA groups for Ancipa and Potomac waters, and correlations between concentrations of these groups and intensity of differential absorbance at 272 nm. This material is available free of charge via the Internet at http://pubs.acs.org.
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