Environ. Sci. Technol. 2000, 34, 1721-1728
Breakpoint Chemistry and Volatile Byproduct Formation Resulting from Chlorination of Model Organic-N Compounds CHII SHANG, WOEI-LONG GONG, AND ERNEST R. BLATCHLEY III* School of Civil Engineering, Purdue University, West Lafayette, Indiana 47907-1284
Aqueous solutions containing six model organic-N compounds (glycine, cysteine, asparagine, uracil, cytosine, and guanine) were subjected to chlorination at various chlorine (Cl) to precursor (P) molar ratios for 30 min. Chlorine residuals were determined by both DPD/FAS titration and the MIMS (Membrane Introduction Mass Spectrometry) method to evaluate breakpoint chlorination behavior, residual chlorine distributions, and byproducts. DPD/FAS titration was found to yield false-positive measurements of inorganic combined chlorine residuals in all cases. The breakpoint chlorination curve shape was strongly influenced by the structure of the model compound. Cyanogen chloride was found to be present as a byproduct in all cases, and the yield was strongly dependent on the Cl:P molar ratio and the structure of the compounds, with glycine being the most efficient CNCl precursor. Six byproducts other than cyanogen chloride were also identified. Free chlorine measurements by DPD/FAS titration and MIMS were in good agreement. This finding, together with the results of previously conducted research, suggests that both methods are capable of yielding accurate measurements of free chlorine concentration, even in solutions that contain complex mixtures of +1-valent chlorine compounds.
Introduction Chlorination is a well-developed and widely used process for disinfection. In aqueous solutions, free chlorine reacts rapidly with nitrogenous compounds to form other chlorine species (such as inorganic and organic chloramines) many of which have been reported to have less or no inactivation potential against microorganisms (1, 2). Chlorination of ammoniacal solutions has been extensively studied and well defined (3). However, in many chlorination applications, reactive nitrogen may exist predominantly in the form of organic-N. The chemistry and implications of the chlorination of organic-N compounds are less clear than those of Cl:NH3 interactions and are of concern (4, 5). These compounds may exert high chlorine demand (5); diminish disinfection potential (6, 7); interfere in residual chlorine analysis (8); and act as precursors in the formation of disinfection byproducts (DBPs). Amines and some nitrogen heterocyclic aromatic compounds have been found to be very reactive with chlorine (5). The mechanisms of the reactions between these compounds and chlorine to form N-chloramines and * Corresponding author phone: (765)494-0316; fax: (765)4961107; e-mail:
[email protected]. 10.1021/es990513+ CCC: $19.00 Published on Web 03/28/2000
2000 American Chemical Society
N,N-dichloramines have been suggested to be similar to those between ammonia and chlorine. Decomposition of the N-chloramines has been found to be related to the concentration of reactants and the structure of such compounds (9) wherein the R-N-chloroamino acids lead to the formation of corresponding aldehydes or ketones, ammonia, and chloride ion, while R-N,N-dichloroamino acids lead to the formation of corresponding chloraldimines as intermediate products, and nitriles, aldehydes, ammonia, carbon dioxide, and chloride ion as end products (10, 11). In some cases, chlorination of ring nitrogens has led to the loss of aromaticity through substitution reactions or ring cleavage (12, 13). Byproducts, including cyanogen chloride, dichloroacetonitrile, dichloropropanal, chloroform, chloral hydrate, and trichloroacetic acid, were found at Cl:N molar ratios of 8:1 and 15:1 (14). Cyanogen chloride (CNCl) (CAS #506-77-4) is a volatile, colorless gas and is only slightly soluble in water (15). It is highly toxic, even at very low concentration. CNCl causes irritation of eyes and nose, the respiratory tract, bronchi and trachea hemorrhages, and pulmonary edema. The theoretical threshold of toxicity of cyanogen chloride in aqueous solution to rainbow trout was calculated to be 0.08 ppm (16). Cyanogen chloride was found as a byproduct when solutions containing cyanide and nitrogen compounds (such as amino acids, nitriles, nucleic acids, and humic acids) as well as drinking water were chlorinated or chloraminated and has been listed as a tentative candidate for regulation in Stage 2 of the Disinfection/DBP Rule (17). The membrane introduction mass spectrometry (MIMS) method, which has been developed for residual chlorine analyses in our previous work (18), employs a membrane interface to directly introduce aqueous samples to a mass spectrometer. An important advantage of the method is the lack of any requirement for sample workup, thereby simplifying analysis and minimizing changes in chemical content. Essentially unambiguous quantification and characterization of the inorganic chlorine residuals as well as byproduct identification have been demonstrated (18, 19). However, the MIMS method is blind to such common chlorination byproducts as N-chloroamino acids and chloroacetates since the membranes will not transmit charged particles or compounds of low volatility. Traditionally, the DPD/FAS titrimetric method has been used as the standard method when differentiation of mono-, di-, and trichloramine is required. The differentiation in this method is based on the difference of reaction rates of inorganic chlorine compounds with the DPD indicator and potassium iodide (20). However, titrimetric methods are known to be susceptible to interference by many compounds, including organic chloramines (18). In this research, MIMS and DPD/FAS titration were applied in parallel to evaluate breakpoint chlorination behavior, chlorine residual distribution, and byproduct formation from chlorination of aqueous solutions of six model organic-N compounds.
Experimental Section Solution Preparation. In all cases, solutions were prepared from reagent grade chemicals (model organic-N compounds from Sigma and sodium hypochlorite, 4%, from Aldrich) or stock solutions (7.14 mM). Dilution to target aqueous-phase concentrations was accomplished with distilled, deionized water. Free chlorine stock solutions and standard solutions of inorganic chloramines were prepared in the same manner as described in ref 18. Standard solutions of CNCl for calibration curve establishment were prepared daily by serial VOL. 34, NO. 9, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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dilution from a CNCl standard solution (2000 mg/L) (Protocol Analytical Supplies, Inc.) to 100-10-3 mg/L. The MIMS Method. The system was based on a modification of an HP 5892 benchtop GC/MS (Hewlett-Packard Co.) containing an HP 5972A Mass Selective Detector (MSD) equipped with both electron (70 eV) ionization (EI) and chemical (methane) ionization (CI). Mass spectrum scan mode (45 e m/z e 300) coupled with both EI and CI was used to identify possible DBPs at high chlorine and organic-N concentrations, while selected ion monitoring (SIM) mode coupled with EI was used for quantification of inorganic chlorine residuals and CNCl at low concentrations. Medical/ pharmaceutical silicone tubing (Baxter), 0.25 mm ID, 0.47 mm OD, 60 mm contact length, was used as the membrane interface for the research described herein. Other details of the configuration and setup of the MIMS system and operational conditions can be obtained from ref 18. Concentrations of inorganic chlorine residuals and CNCl were determined by comparison of ion abundance measurements with those developed from a series of standard solutions. Inorganic chloramines and CNCl were analyzed directly, while free chlorine was estimated by the difference of inorganic monochloramine concentrations before and after sample ammonification (18). Ions at m/z 53, 87, 119, and 61 amu were monitored for quantification of inorganic mono-, di-,and trichloramine and CNCl, respectively. When possible, compound identifications by MIMS were confirmed by analysis of standard solutions. Standard solutions of cyanogen chloride, dichloroacetonitrile in acetone (from AccuStandard), and chloroform (stabilized with ethanol) (from Sigma-Aldrich), developed by addition of pure compounds to distilled/deionized water, were subjected to MIMS analyses. Experimental Procedures. Chlorination experiments were carried out in well-mixed, glass-stoppered flasks in the dark using 200 mL of 0.01 M phosphate buffer solution (PBS) (pH ) 7.0). For each experiment conducted at high concentration, aliquots of organic-N compounds were weighed and added to achieve a concentration of 17.86 mM. Solutions were chlorinated at chlorine:precursor (Cl:P) molar ratios of 0.4, 3.2, and 6.4 for 30 min, which is a characteristic time for chlorine contact chambers and commonly used time scale in previous works involving chlorine:nitrogen interactions. The resulting solutions were subjected to MIMS/CI or MIMS/ EI to investigate byproducts. For each experiment conducted at low concentration, aqueous solutions were freshly prepared prior to experiments by addition of 1.0 mL of model compound stock solutions to 200 mL of PBS to achieve a target concentration of 0.036 mM. Aliquots of standardized sodium hypochlorite (NaOCl) stock solution were then added to the flasks. The experiments were repeated for initial chlorine:precursor (Cl:P) molar ratios of 0.4-3.2 or to the point at which significant free residual chlorine was observed. Concentrations of chlorine residuals were determined 30 min after chlorine addition by DPD/ FAS titration and MIMS/EI, while concentration of CNCl was determined by MIMS/EI initiated immediately after samples were collected from reaction vessels.
Results and Discussion Chlorination at High ConcentrationsByproduct Identification. Chlorination was performed using high concentrations of chlorine and model organic-N compounds to aid in the identification of reaction products and for purposes of determining candidate compounds for quantification by MIMS at low concentrations later in this study. Six model organic-N compounds were selected as representative amino acids (glycine, cysteine, and asparagine) and nucleic acid bases (uracil, cytosine, and guanine). These compounds are either present as pure compounds in waters (21) or are 1722
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FIGURE 1. Illustration of the structures of the model organic-N compounds and possible byproducts after chlorination. present as the fundamental building blocks of cell tissues. Model compound selection was also based on their structural characteristics (refer to Figure 1). Glycine contains the simplest structure among the model compounds and is the most abundant amino acid in natural water as well as in cellular materials (21, 22). Cysteine contains a reduced organic sulfur group, which is likely to react rapidly with chlorine. Asparagine, which contains two amine groups, provides a relatively high density of chlorine receptors and has been found to be released from bacterial cells after chlorination (22). Uracil and cytosine are both pyrimidine-based nucleic acid bases but differ in that cytosine contains one amine group attached to the heterocyclic aromatic ring. Guanine, a purine-based nucleic acid base, contains two nitrogenheterocyclic rings and an amine group. Figure 1 provides a summary of the byproducts identified (or tentatively identified) in this study. Representative EI or CI spectra from these experiments are presented in Figure 2. All mass spectra illustrated in Figure 2 are at a Cl:P molar ratio of 3.2. Figure 2a presents the EI spectrum of the product mixture from glycine chlorination. The formation of CNCl was suggested by the existence of peaks at m/z 61 (CN35Cl•+) and 63 (CN37Cl•+); the naturally occurring chlorine isotopic ratio (3:1) was evident in the EI spectrum. This spectrum showed good agreement with the spectrum developed from CNCl standard solutions and a spectrum in ref 16. CNCl formation from glycine chlorination was evident at all Cl:P molar ratios. The formation of CNCl from glycine chlorination was further confirmed with MIMS/CI (Figure 2b). Strong peaks at m/z 62 and 64 at 3:1 abundance ratio were attributed to the protonated molecular ions 35ClCNH•+ and 37ClCNH•+, respectively, corresponding to CNCl. The low energy available
FIGURE 2. Representative spectra of product mixtures after 30 min of chlorination of organic-N compounds at Cl:P molar ratio of 3.2 and pH 7. Off-scale peaks are indicated by arrowheads (v). for transfer in the CI mode made it unlikely that these peaks were attributable to fragmentation of other (larger) compounds. Similar results were observed at other Cl:P ratios. Small CI peaks clustered at m/z 80 and 82 (3:1 ratio) were attributed to N-chloroformamide with molecular weight of 79.5. Other small CI peaks were unidentified because the available information regarding abundance and m/z could not be assigned to specific compounds. For all model compounds except glycine, CNCl was identified as a byproduct only at Cl:P molar ratios of 3.2 and higher; at the Cl:P molar ratio of 0.4, CNCl was not detected (not shown) in the high concentration experiments. In all
cases of chlorination of model organic-N compounds, the relative abundance of CNCl showed good agreement with the results obtained later in the experiments conducted in the low-concentration range (Figure 3). In the case of cysteine (Figure 2c), a small CI peak cluster (m/z 110-112) at a ratio of 9:6 indicated the formation of protonated dichloro compounds (presumably, dichloroacetonitrile with molecular weight of 109.5). A peak at m/z 114 may also have been present but would have been indistinguishable from noise (abundance e 100). CI analyses of standard solutions of dichloroacetonitrile supported the compound identification. A strong CI peak observed at m/z 59 was attributed to the VOL. 34, NO. 9, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Residual chlorine and CNCl concentrations as a function of Cl:P ratio after 30 min of exposure to chlorine at pH ) 7. For each Cl:P ratio, residual chlorine was measured by DPD/FAS titration (left bar) and MIMS/EI (right bar). Note that the CNCl concentration scale in Figure 3a is substantially different from the other cases. possible formation of a corresponding protonated oxalaldehyde (C2H2O2), with a molecular weight of 58. Other compounds which satisfy this criterion (C3H6O, C4H10, and C2H6N2) would appear to be less likely to form as byproducts of chlorination of these organic-N compounds. However, the mass spectra from the MIMS system used in this research did not allow for exclusion of these candidates. A summary of the conditions and findings of confirmatory tests performed by MIMS on aqueous standards is provided in Table 1. 1724
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Figure 2d presents the results of asparagine chlorination. In addition to the peak cluster at m/z 62 and 64 attributable to CNCl, three additional CI clusters were also observed. The peak clusters located at m/z 125-127-129, with a ratio of 9:6:1; m/z 110-112, with a ratio of 9:6; and m/z 89-91, with a 3:1 ratio, suggested the possible formation of N,Ndichloroaminoacetonitrile, dichloroacetonitrile, and N-chloroiminoethanenitrile, respectively. The formation of dichloroacetonitrile was confirmed by EI and CI analyses of a standard solution. The EI spectrum of this mixture (not
TABLE 1. Summary of Conditions and Findings in Confirmatory Tests by MIMS spectrum mode
concn in standard aqueous soln (mg/L)
scan m/z rangea
characteristics ions (m/z) in spectra
related figure in article
0.001-1.0
40-140
25
65-120
Figure 2a,e Figure 2b-d,f Figure 2e
dichloroacetonitrile chloroform
CI scan EI scan
25 25
70-115 45-135
61:63 ) 3:1 62:64 ) 3:1 74:76 ) 3:1 82:84:86 ) 9:6:1 110:112:114 ) 9:6:1 83:85:87 ) 9:6:1 48:50 ) 3:1 47:49 ) 3:1
in accordance with results
dichloroacetonitrile
EI scan CI scan EI scan
Figure 2c,d,f Figure 2e
in accordance with results in accordance with results
compound cyanogen chloride
a
comparison
in accordance with results
Chosen to maximize ability to detect peaks of interest.
shown), also supported the identification of dichloroacetonitrile as a byproduct of asparagine chlorination, with good relative abundance of the primary peak cluster (74-76 amu) at a 3:1 ratio and a second peak cluster (82, 84, 86 amu) at a 9:6 ratio, as described in ref 23. A CI peak at m/z 59 was also shown as a product of asparagine chlorination. CNCl and dichloroacetonitrile were also found as byproducts of uracil chlorination (Figure 2f). The yield of the EI peak (Figure 2e) cluster at 83-85-87 amu (9:6:1 ratio) indicated the formation of an unidentified polychloro compound (possibly, chloroform). This compound identification was supported by EI analyses of a standard solution containing chloroform. Chloroform formation has also been reported from chlorination of aqueous uracil solutions by ECD/GC in the literature (14). CNCl and dichloroacetonitrile were the only two byproducts detected from chlorination of cytosine, while CNCl was the only compound detected from chlorination of guanine (not shown). CNCl was detected directly by MIMS/EI with no evidence of interference by other (chlorinated) compounds. It is possible that other products were generated but were not detected by MIMS, due to either low permeability to the membrane or fragmentation in the ion source leading to insufficient abundance or information of characteristic ions. Thus, only CNCl and inorganic chlorine compounds were monitored by the MIMS method in chlorination experiments at low concentrations. Chlorination at Low Concentrations. For purposes of discussion, the terms breakpoint and persistent chloramine residual require definition. The term “breakpoint” will refer to the condition wherein chlorine consumption by model organic-N compounds is just satisfied 30 min after chlorine addition, such that free residual chlorine is detected at Cl:P ratios above this value. The term “persistent chloramine residual” will be used to describe the sum of titrable combined residual after the breakpoint, which includes NH2Cl, NHCl2, NCl3, and organic chloramines. Figure 3 presents the results of chlorination of solutions containing amino acids at different Cl:P ratios. The typical breakpoint curve shape was evident for chlorine residuals determined by DPD/FAS titration from glycine chlorination (Figure 3a). In contrast, markedly different behavior was evident in chlorine residuals as determined by MIMS. In particular, no residual chlorine was observed in the MIMS measurements until Cl:P g 1.6. Since no ammonia was added in the solution, it was likely that the “apparent” residual chloramines determined by DPD/FAS titration were attributable to the formation of organic chloramines (or other compounds), possibly monochloroglycine and dichloroglycine that are in ionic form at pH 7 and have been reported to respond as residual chloramines (24). Therefore, the legends (Mono, Di, Tri, and Free) used in Figures 3 and 4 denote the compound(s) that responded in DPD/FAS titration as measured “apparent” chlorine species, while they represent actual NH2Cl, NHCl2, NCl3, and HOCl + OCl- measured
by MIMS. Products from reactions of glycine and free chlorine responded predominantly as “apparent” monochloramine, with smaller amounts of “apparent” dichloramine by the DPD/FAS method. These findings contradict the assertion in the literature that chlorination of organic-N compounds yields compounds that interfere predominantly with dichloramine determination (4). For the conditions of this study (i.e., pH ) 7, 0.01 M PBS), the breakpoint for the reactions between chlorine and ammonia will occur at Cl:P of 1.7 (3). In contrast, the apparent “breakpoint” for this system occurred at Cl:P of 2.8. The shift in the location of the breakpoint was probably attributable to competition for chlorine on the part of carbon and nitrogen in the glycine molecule. Upon analyzing similar aqueous solutions, Morris et al. (referred to in ref 4) found that carbon, and not nitrogen, was oxidized when Cl:P molar ratios were 1.5:1 to 2:1. The oxidized carbon was released as carbon dioxide, while the original nitrogen was still present either as ammonia or as organic nitrogen. However, reaction products that titrated as “apparent” chloramines in this study, presumably in the form of chlorinated organic-N compounds, were found among these Cl:P ratios. The results from the experiments with amino acids indicate that the shift in “breakpoint” location may also be at least partially attributed to the formation of disinfection byproducts. In the case of glycine, the predominant detectable byproduct was CNCl. Relatively small quantities of CNCl were formed for Cl:P less than 1.2; this Cl:P ratio also coincided with the appearance of “apparent” dichloramine according to the DPD/FAS data. As the Cl:P ratio was increased above this value, CNCl production increased, up to the breakpoint (Cl:P ) 2.8). Above the breakpoint, CNCl showed a rapid decline, probably due to increases in the residual free chlorine concentration in the system. Chlorination of glycine solutions yielded significantly greater quantities of CNCl than chlorination of other model organic-N compounds (Figures 3 and 4), a finding that is in agreement with the work of Matsushita et al. (25). This difference is hypothesized to be attributable to the formation of hydrogen cyanide (H-CtN) through the decomposition of chlorinated glycine and an organic substituted nitrile (R-CtN) in the cases of other the model compounds. The hypothesized mechanisms for the formation of these intermediates are similar to the reaction scheme proposed in ref 11. HCN would be expected to react rapidly with residual (+1 valent) chlorine to yield CNCl. The organic substituted nitriles suppressed further reaction with chlorine to form cyanogen chloride. The maximum concentration of CNCl, occurring at Cl:P molar ratio of 3.2, corresponded to conversions of 72% and 11% of glycine and free chlorine, respectively. These findings were consistent with the reaction scheme proposed by Alouini and Seux (11). By extrapolation, these results suggest the formation of CNCl in the range of 20-60 µg/L upon chlorination of typical municipal wastewater effluents, where typical glycine concentrations have been reported in the range of 45-105 µg/L VOL. 34, NO. 9, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Residual chlorine and CNCl concentrations as a function of Cl:P ratio after 30 min of exposure to chlorine at pH ) 7. For each Cl:P ratio, residual chlorine was measured by DPD/FAS titration (left bar) and MIMS/EI (right bar). (21). Interestingly, the concentration of CNCl formed in these experiments conducted at low concentrations was well above the theoretical threshold toxicity limit for rainbow trout (80 µg/L) (26), for the majority of the Cl:P ratios tested. In practical terms, these results also imply that an aqueous solution of glycine at a concentration of 0.036 mM (0.5 mg/L as N) could yield dangerously high concentrations of CNCl for chlorine additions at concentration ranges that may be of significance in some aqueous disinfection applications. Persistent chloramine residuals detected at Cl:P ratios beyond the breakpoint by DPD/FAS were consistently higher 1726
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that those detected by MIMS. These data suggest the existence of small amounts of organic chloramines beyond the breakpoint. However, the increase of residual free chlorine with increasing Cl:P ratio was smaller than the increase attributable to chlorine addition, thereby indicating the existence of quantifiable chlorine demand above the breakpoint. Only small quantities of inorganic chloramines were observed by MIMS in the experiments with glycine. At Cl:P molar ratios of 1.6 and higher, the MIMS analyses revealed the presence of NH2Cl, NHCl2, and NCl3, thereby implying
limited C-N bond cleavage, leading to the release of small quantities of NH2Cl or ammonia. This hypothesis is well supported by the mechanisms of the reactions from chlorination of amino acids proposed in the literature (11, 27). NH2Cl or ammonia would then be expected to react with chlorine according to the classical mechanisms of chlorineammonia interactions. Figure 3b presents the results of chlorination of cysteine. Cysteine, which was selected for study because it contained a reduced organic sulfur group that has been reported to be more reactive to chlorine than the amine group (5), responded differently than glycine. Though the “breakpoint” curve shape was observed by DPD/FAS titration, the apparent breakpoint was shifted toward high Cl:P ratios, as compared with other model compounds. No quantifiable residual chlorine was observed for Cl:P molar ratios less than 1.2 by either method. It is likely that chlorine interacted preferentially with reduced sulfur instead of nitrogen. “Apparent” residual dichloramine, possibly in the form of an organic (di)chloramine, together with relatively small quantities of “apparent” monochloramine, were observed by DPD/FAS only after the stoichiometric chlorine demand attributable to reduced sulfur was satisfied (Cl:P of 1.0). The slow increase of apparent residual chloramines before the “hump” in the DPD/FAS data indicated the great tendency of decomposition of chlorinated organic-N compounds to form nontitrable byproducts. Moreover, the concentrations of inorganic chloramines measured by MIMS, which were substantially higher than in the case of glycine, indicated the formation of ammonia (or NH2Cl). These results showed good agreement with the general conclusion in the literature that N-chloro-R-aliphatic amines with a good leaving group attached to the R-carbon appear to be unstable and decompose rapidly to form unstable imines, which subsequently hydrolyze to yield aldehydes and ammonia (5). The breakpoint (at Cl:P of 4.8), which was higher than the case of glycine and also higher than the stoichiometric value based on available sulfur and nitrogen, indicated that chlorine demand by slow oxidation-reduction reactions also took place. Quantifiable CNCl was not observed until chlorine demand by sulfur was satisfied. Thereafter, the formation of CNCl followed the same pattern as in the case of glycine but with a much lower CNCl yield. These data also supported the previously described hypothesis that sulfur was more reactive to chlorine than nitrogen. Figure 3c presents the results of chlorination of asparagine; asparagine was selected as a model compound because it provides a greater density of N-based chlorine receptors than glycine or cysteine. Once again, the classical breakpoint curve shape was evident in the DPD/FAS titration data. However, in this case, higher concentrations of “apparent” chloramines and “persistent chloramine residual” were observed by DPD/ FAS than in the previous two cases. Significant quantities of inorganic chloramines, which accounted for 25-33% of the residual chlorine in the DPD/FAS data, were detected by MIMS for Cl:P ratios of 1.6-3.6. It is likely that the additional amine group provided a greater density of amino-N, such that hydrolysis and release of ammonia (or NH2Cl) were facilitated. Thereafter, ammonia (or NH2Cl) further reacted with chlorine in a fashion consistent with the classical breakpoint curve shape, as observed in the MIMS results. Furthermore, the breakpoint (at Cl:P molar ratio of 4.0) was observed to coincide with the stoichiometry of chlorine substitution from nitrogen, thereby suggesting a strong preference for chlorine to react with nitrogen as opposed to carbon. CNCl formation followed the same pattern as the previous two cases. As in the case of cysteine, only small quantities of CNCl were formed as a result of asparagine chlorination.
Figure 4 presents the results from chlorination of aqueous solutions containing nucleic acid bases at different Cl:P ratios. No evidence of the classical breakpoint curve shape was found in the DPD/FAS data. Free residual chlorine was detected consistently by both methods over the entire range of Cl:P ratios for the cases of uracil and cytosine, while it was only detected at Cl:P molar ratios of 3.2 and higher for the case of guanine. These data imply that the chlorine demand of guanine was higher than that of uracil and cytosine. However, neither the mechanisms nor the kinetics of the reactions responsible for this “demand” were elucidated by the analytical methods employed in this research. According to the chemistry of uracil reactions with chlorine proposed by Gould et al. (28), the lower demand by uracil was attributable to the slow reaction rates involved in uracil chlorination. The concentrations of free chlorine and (apparent) chloramines showed monotonic increases with Cl:P ratio according to both analytical methods. As in the case of the of amino acids, chlorinated organic-N compounds from chlorination of nucleic acid bases provided interference in the determination of inorganic chloramines by DPD/FAS titration. Interestingly, only the apparent dichloramine was evident at low Cl:P ratios, while both apparent mono- and dichloramines were evident at high Cl:P ratios in the DPD/FAS data. It has been reported that 5-chlorouracil, 1-, 3-, and 5-chlorocytosine were formed at low Cl:N ratios, while 3,5dichlorouracil and 3,5-dichlorocytosine were formed at high Cl:N ratios (12, 28). Moreover, nonaromatic compounds, which responded as combined residual chorine, have been found to increase progressively with increases in the Cl:N molar ratios, even at Cl:N ratios where concentrations of chlorocytosine and dichlorocytosine gradually decreased to zero (12). As mentioned previously, the distinction of “apparent” mono- and dichloramine by DPD/FAS titration is based only on the difference in reaction rate between chloramine and potassium iodide; as such, the implications of this distinction are less clear in the case of organic chloramines than with inorganic chloramines. Additionally, CNCl and inorganic chloramines were detected at relatively low concentrations by MIMS/EI and increased monotonically with increases in Cl:P ratio, thereby implying limited ring cleavage. By comparing the chlorination curves of uracil and cytosine, it appears that the exocyclic amino group on cytosine represented a more reactive form of nitrogen than the heterocyclic-N in terms of substitution reactions, as indicated by the formation of higher quantities of chloroorganic-N compounds responding as “apparent” chloramines in DPD/FAS titration. This implication shows good agreement with the results in ref 29 but conflicts with the results in ref 12. The exocyclic amine on cytosine also represented a form of (relatively) easily cleavable nitrogen that could be released to yield inorganic nitrogen, thereby leading to the formation of inorganic chloramines, as detected by MIMS/EI. Free chlorine measurements by both DPD/FAS titration and MIMS/EI were in good agreement in all cases. These results, combined with the results of previous investigations involving chlorination of municipal wastewater effluents (18), suggest that both methods are capable of providing accurate measurements of free residual chlorine, even in aqueous samples containing complex mixtures of +1-valent chlorine.
Acknowledgments The authors are grateful to the Purdue Research Foundation for financial support and to D. W. Margerum of the Purdue University Department of Chemistry for his discussion and input. VOL. 34, NO. 9, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Received for review May 4, 1999. Revised manuscript received February 15, 2000. Accepted February 17, 2000. ES990513+
