Article pubs.acs.org/est
Acetonitrile and N‑Chloroacetamide Formation from the Reaction of Acetaldehyde and Monochloramine Susana Y. Kimura,†,§ Trang Nha Vu,† Yukako Komaki,†,§ Michael J. Plewa,‡,§ and Benito J. Mariñas*,†,§ †
Department of Civil and Environmental Engineering, ‡Department of Crop Science, and §Science and Technology Center for Advanced Materials for the Purification of Water with Systems, Safe Global Water Institute, University of Illinois at UrbanaChampaign, Champaign, Illinois 61801, United States S Supporting Information *
ABSTRACT: Nitriles and amides are two classes of nitrogenous disinfection byproducts (DBPs) associated with chloramination that are more cytotoxic and genotoxic than regulated DBPs. Monochloramine reacts with acetaldehyde, a common ozone and free chlorine disinfection byproduct, to form 1-(chloroamino)ethanol. Equilibrium (K1) and forward and reverse rate (k1,k−1) constants for the reaction between initial reactants and 1(chloroamino)ethanol were determined between 2 and 30 °C. Activation energies for k1 and k−1 were 3.04 and 45.2 kJ·mol−1, respectively, and enthalpy change for K1 was −42.1 kJ·mol−1. In parallel reactions, 1-(chloroamino)ethanol (1) slowly dehydrated (k2) to (chloroimino)ethane that further decomposed to acetonitrile and (2) was oxidized (k3) by monochloramine to produce Nchloroacetamide. Both reactions were acid/base catalyzed, and rate constants were characterized at 10, 18, and 25 °C. Modeling for drinking water distribution system conditions showed that N-chloroacetamide and acetonitrile concentrations were 5−9 times higher at pH 9.0 compared to 7.8. Furthermore, acetonitrile concentration was found to form 7−10 times higher than Nchloroacetamide under typical monochloramine and acetaldehyde concentrations. N-chloroacetamide cytotoxicity (LC50 = 1.78 × 10−3 M) was comparable to dichloroacetamide and trichloroacetamide, but less potent than N,2-dichloroacetamide and chloroacetamide. While N-chloroacetamide was not found to be genotoxic, N,2-dichloroacetamide genotoxic potency (5.19 × 10−3 M) was on the same order of magnitude as chloroacetamide and trichloroacetamide.
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A recent study11 designed to elucidate some of the formation mechanisms of chloroacetamide and chloroacetonitrile has shown that these two N-DBPs can form from the reaction of monochloramine with chloroacetaldehyde (Figure 1, 1a−7a), a DBP formed during primary disinfection with free chlorine. Chloroacetaldehyde (1a) and monochloramine (2) react relatively quickly to reach pseudoequilibrium with the carbinolamine 2-chloro-1-(chloroamino)ethanol (3a). This carbinolamine then undergoes dehydration to the imine 1-chloro-2(chloroimino)ethane (4a) that quickly decomposes to form chloroacetonitrile (5a). Carbinolamine 3a is also oxidized by monochloramine to form the previously unreported N-DBP N,2-dichloroacetamide (6a), which, although it was found to be stable in water, it was also observed to decompose into chloroacetamide (7a) when water samples are treated with common quenchers.11 Toxicity tests showed that cytotoxicity of N,2-dichloroacetamide was similar in order of magnitude to that of chloroacetamide. However, its genotoxicity still remained to be determined. Although most N-DBP occurrence studies have shown that dichloroacetamide and dichloroacetonitrile concentrations are
INTRODUCTION
The formation of nitrogenous disinfection byproducts (NDBPs) in disinfected drinking water is of growing interest because three classes of N-DBPs (haloacetamides, haloacetonitriles, and halonitromethanes) were found to be more cytotoxic and genotoxic than regulated organic DBPs, i.e., trihalomethanes and haloacetic acids.1−3 Although haloacetonitriles (HANs) and haloacetamides (HAcAms) are produced from chlorination and chloramination some studies have shown that HANs4 and HAcAms5,6 are formed at slightly higher levels in waters disinfected with chloramines compared to those treated with the more common disinfectant free chlorine. This is of concern because due to regulations for trihalomethanes and haloacetic acids7 some U.S. water utilities are switching from free chlorine to alternative disinfection schemes involving the use of ozone as primary disinfectant in the treatment plant followed by chloramines to provide a residual in the distribution system.8 The nitrogen of haloacetonitriles and haloacetamides can originate from the reaction of either chlorine or chloramine with organic nitrogen (e.g., amino acids) that is present in source waters.9,10 However, recent research has shown that greater than 70% of the nitrogen contained in haloacetonitriles and haloacetamides is incorporated from 15N-labeled monochloramine that reacted with various naturally occurring and model NOM.5,10 © 2015 American Chemical Society
Received: Revised: Accepted: Published: 9954
April 13, 2015 June 14, 2015 July 13, 2015 July 13, 2015 DOI: 10.1021/acs.est.5b01875 Environ. Sci. Technol. 2015, 49, 9954−9963
Article
Environmental Science & Technology
Figure 1. Nitrile and N-haloacetamide formation pathway from the reaction of aldehydes and monochloramine.
higher than their chloro- and trichloro- analogs, 6,12,13 chloroacetamide and chloroacetonitrile are more genotoxicity than its di- and trihalogenated counterparts.1,2 While this trend varies with the halogen attached to the α-carbon (Cl, Br, I), it raises the importance of characterizing occurrence, toxicity, and chemical formation studies of DBPs of concern. Furthermore, the nonhalogenated N-DBPs acetamide and acetonitrile that could form through similar reactions as haloacetamides and haloacetonitriles have yet to be characterized. Characterizing their formation and toxicity is important because acetamide and acetaldehyde have been included in the U.S. EPA Draft Contaminant Candidate List 4,14 a database of compounds selected for potential regulatory development, and acetonitrile is formed from acetaldehyde. Formaldehyde and acetaldehyde, the two most predominant aldehydes produced by ozonation15−17 and free chlorine,18 have been shown to react with monochloramine to produce nitriles19,20 (Figure 1, 1b−5b and 1c−5c). Monochloramine reacts with formaldehyde (1b) in a fast and reversible reaction to form carbinolamine N-chloroaminomethanol (3b). 3b slowly undergoes acid/base catalyzed dehydration to form Nchloromethanimine (4b), the rate-limiting step in the reaction pathway. 4b subsequently decomposes to hydrogen cyanide (5b), which further reacts with monochloramine to produce cyanogen chloride.19 A similar reaction pathway was proposed for monochloramine and acetaldehyde to form acetonitrile (Figure 1, 1c−5c);20 however, intermediate products were not confirmed, and kinetic rate constants were not characterized. Drawing a parallel with the oxidation reaction of the carbinolamine formed from monochloramine and chloroacetaldehyde11 (Figure 1), it is proposed that 3c is also oxidized by monochloramine to produce N-chloroacetamide (6c). Therefore, the reaction between acetaldehyde and monochloramine, and the toxicity of 6c needs to be investigated. Furthermore, previous studies have only characterized reaction constants at only one temperature limiting the applicability of the kinetic model to the range of temperatures relevant to most drinking water facilities. The main objectives of this study were to identify reaction intermediates and products and determine equilibrium and rate constants for the overall reaction scheme started by reacting the disinfectant monochloramine and the common DBP acetaldehyde. The study was designed to elucidate the formation pathways and kinetics of two major reaction products, previously reported acetonitrile, and newly identified (in this study) N-chloroacetamide. The cytotoxicity and genotoxicity of the new DBP N-chloroacetamide were also determined and compared to those of other DBPs of concern. A kinetic model
for the formation of acetonitrile and N-chloroacetamide as a function of pH and temperature was also developed and applied to illustrate the formation of the two N-DBPs and a common intermediate carbinolamine species under drinking water treatment and sampling conditions.
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EXPERIMENTAL SECTION
Reagents and Solutions. Acetaldehyde (≥99.5%), potassium phosphate monobasic (>99%), sodium hydroxide (≥97%), acetonitrile (GC grade ≥99.5%), and acetamide (>99%) were purchased from Sigma-Aldrich (St. Louis, MO). Sodium bicarbonate (>99.7%), perchloric acid (Fluka 70% puriss), ammonium chloride (≥99.5%, ACS reagent), sodium thiosulfate anhydrous (≥98%), methylene chloride (Optima grade), and sodium hypochlorite solution (5.65−6.00% Laboratory grade) were purchased from Fisher Scientific (Pittsburgh, PA). General biological reagents were purchased from Fisher Scientific Co. (Itasca, IL) and Sigma-Aldrich Co. (St. Louis, MO) unless otherwise noted. Acetaldehyde stock solutions of 1−2 M were prepared daily by diluting acetaldehyde in oxygen-free nanopure water (≥18 MΩ·cm) and held for several hours to allow for depolymerization. Acetaldehyde was then standardized spectrophotometrically (λmax = 277 nm, 7.2 M−1cm−1 at 25 °C).21 Monochloramine (N/Cl molar ratio of 1.1:1) solution preparation and standardization are described elsewhere.11 Spectrophotometric data confirmed that free chlorine was below detection under all conditions investigated. Acetamide, acetonitrile, phosphate, carbonate, sodium hydroxide, and sodium thiosulfate stock solutions were prepared in nanopure water. Acetamide and acetonitrile standard solutions for gas chromatography/mass spectrometry (GC/MS) analyses were prepared by diluting acetamide or acetonitrile stock solution into buffered solutions similar to those used for the kinetic experiments. N-chloroacetamide was prepared by slow addition of sodium hypochlorite solution to a highly stirred acetamide solution (N/ Cl molar ratio of 1.01:1) at pH 9.14 ± 0.02.22 Absorption spectra were taken over time as shown in Figure S1 of Supporting Information (SI). N-Chloroacetamide concentration was calculated by subtracting the remaining sodium hypochlorite at 6 h from the initial concentration determined spectrophotometrically (λmax = 292 nm, 350 M−1cm−1). NChloroacetamide molar extinction coefficients were calculated as described in SI Text S7. Concentrated N-chloroacetamide was analyzed several hours after preparation by low and high resolution mass spectrometry (LRMS and HRMS) to confirm the formation of N-chloroacetamide. Mass to charge fragments 9955
DOI: 10.1021/acs.est.5b01875 Environ. Sci. Technol. 2015, 49, 9954−9963
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Environmental Science & Technology Table 1. Summary of Conditions for Acetaldehyde and Monochloramine Kinetic Experiments exp set
p[H+]
[NH2Cl]0 (mM)
CT,CH3CHO (mM)
buffer
CT, Buffer (mM)
T (°C)
SX-1 SX-2 SX-3 SX-4 SX-5 SX-6 SX-7 SH-1 SH-2 SH-3 SH-4 SH-5 SH-6 SH-7 SH-8
7.8 7.8 7.8 7.8 7.8 7.8 7.8 7.78−7.85 7.53−7.59 5.67−9.41 6.52−7.87 6.19−7.87 9.01−9.67 8.93−9.78 9.8
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
10−90 10−90 10−90 10−90 10−90 10−90 10−90 10−90 10−70 10 10 10 10 10 10
phosphate phosphate phosphate phosphate phosphate phosphate phosphate phosphate phosphate phosphate phosphate phosphate carbonate carbonate carbonate
20 20 20 20 20 20 20 20 20 20 20 20 20 20 10−35
2 10 15 18 20 25 30 18 25 10 18 25 18 25 25
MD). Equilibrium constant K1 for different temperatures was determined from absorption spectra taken between 200 and 400 nm. Dehydration and decomposition of 1-(chloroamino)ethanol to its final products was monitored at conditions specified for experimental sets SH-3 to SH-7. Reaction rates k2 and k3 were obtained from absorption measurements at 210, 220, and 243 nm at which N-chloroacetamide, 1-(chloroamino)ethanol, and monochloramine have respective significant absorbance. Data analysis and derivation of rate constants are explained in the results section. The potential occurrence of carbonate catalysis was also tested (SH-8). Because acetaldehyde was always used in excess, its absorption at the monitored wavelengths was subtracted from all experimental absorbance data. Mass Spectrometry. Acetonitrile and (chloroimino)ethane were recovered by solid phase microextraction (SPME, SigmaAldrich, St. Louis, MO) and analyzed with a GC/MS Model 6850/5975C (Agilent Technologies, Santa Clara, CA). To illustrate its formation, 0.2 mM acetaldehyde and 1 mM monochloramine were reacted at pH 7.53 for 1 h before extraction. A 10-mL sample was mixed with 2.85 g of sodium sulfate in a 15 mL vial. The headspace was extracted for 30 min at 25 °C with a 85 μm Carboxen/PDMS fiber (Sigma-Aldrich, St. Louis, MO), desorbed at 250 °C for 1 min, and passed through a DB-624 column (Agilent J&W, Santa Clara, CA) at 1 mL/min. The oven was maintained at 35 °C for 2 min and ramped at 10 °C/min to 230 °C. The sample was ionized with electron impact technique and scanned from 30 to 250 amu. Acetamide was identified by liquid−liquid extraction followed by GC/MS analysis. Samples with volume of 10 mL were first quenched by 1.5 mM sodium thiosulfate followed by pH adjustment to 7.8 for immediate extraction. Solutions used for calibration curves were spiked with acetamide standard solutions that were freshly prepared daily. Calibration solutions and samples were mixed with 1 mL ethyl acetate, 2.85 g sodium sulfate, and 6 mg/L 1,2-dichloropropane as internal standard for 2 min followed by a 3 min separation phase. Extracts were then injected to the GC/MS inlet at 230 °C in splitless mode. Samples were monitored using both scan (30 to 200 amu) and select ion monitoring modes for internal standard (m/z 63, 41.1) and acetamide ions (m/z 59, 44.1). Reaction products were extracted without quenching from a 500 mL batch reactor containing 19.5 mM monochloramine
produced from LRMS and electron impact ionization pertaining to N-chloroacetamide are shown in SI Figure S2. HRMS measured mass was 92.9979 with mass error of 0.2 mDa and mass accuracy of 2.2 ppm, compared to calculated value of 92.9981. Control experiments showed that N-chloroacetamide was stable for up to 6 h at pH 9.1, and 23 h at pH 3.1. Solutions for kinetic experiments were prepared with nanopure water at specific pH, total buffer concentration of 0.02 M, and ionic strength of 0.1 M. Potassium phosphate monobasic or sodium bicarbonate was added as buffer, followed by ionic strength adjustment with sodium perchlorate. Concentrated perchloric acid and 10 M sodium hydroxide solutions were used for pH adjustment. pH and temperature measurements were taken with a Thermo Electron Orion ROSS Ultra pH electrode (Thermo Fisher Scientific, Waltham, MA) and Accumet temperature (Fisher Scientific, Pittsburgh, PA) electrode connected to an Accumet AB15 Plus pH meter (Fisher Scientific, Pittsburgh, PA). Electrode calibration was performed with commercial 4, 7, and 10 pH standards. Actual hydrogen ion concentrations were corrected with H+ activity coefficient γ = 0.85 (or log γ = −0.07) calculated with the extended Debye−Hückel/Güntelberg expression using the equation [H+] = 10(0.07‑pH). Kinetic Experiments. Reaction rates and equilibrium constants were obtained from experiments with conditions specified in Table 1. Forward and reverse reaction rate constants of acetaldehyde and monochloramine to produce intermediate 1-(chloroamino)ethanol at different temperature conditions were obtained from experimental sets SX-1 to SX-7. Fast reactions were performed in a Stopped Flow Spectrophotometer Model SX20 (Applied Photophysics, London, U.K.) operated in single-mixing mode. Increasing concentrations of acetaldehyde were mixed with monochloramine and monitored at 243 nm in a 1 cm cell path-length at constant target temperature for up to 50 s. Five replicates for each concentration and each temperature condition were performed. Equilibrium constants and carbinolamine extinction coefficients were determined from absorption spectra from experimental sets SH-1 and SH-2. Slow reactions were performed in batch reactors maintained at constant temperature with a Water Recirculator Model 9501 (PolyScience, Niles, IL). Samples were taken over time, placed in 1 cm quartz cuvettes and analyzed with a UV−vis Spectrophotometer Model 2550 (Shimadzu Scientific Instruments, Columbia, 9956
DOI: 10.1021/acs.est.5b01875 Environ. Sci. Technol. 2015, 49, 9954−9963
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and 40 mM acetaldehyde at pH 9.6 after 1.5 h with 800 mL of methylene chloride. The solvent was reduced with a Hei-VAP Precision Rotary Evaporator (Heidolph, Schwabach, Germany) and redissolved in water for analysis. Reaction products and Nchloroacetamide HRMS and LRMS were obtained with a Double Focusing Mass Spectrometer Micromass Model 70VSE (Waters Corporation, Milford, MA) with 70 eV positive electron impact and positive electrospray ionization. The solid insert probe and ion source temperatures were set at 30 and 200 °C, respectively. The emission current was 100 μA. Modeling Software. Micromath Scientist 3.0 (St. Louis, MO) was used to obtain rate constants by fitting experimental data to the kinetic model using a nonlinear simplex algorithm available in the software. The simplex method is an iterative procedure that locates the solution by minimizing an objective function as detailed elsewhere.23 The standard error for each fitted constant was also obtained. Model simulations under different conditions were also obtained with this software. Dissociation constants (Ka) for acid and bases used in this study were obtained from literature at ionic strength of 0.1 M.24 Mammalian Cell Cytotoxicity and Genotoxicity. Chronic cytotoxicity and acute genotoxicity of this newly identified DBP was tested using Chinese Hamster Ovary (CHO) cells, line AS52 clone 11−4−8 (SI Text S1).25 A 10 mM or 25 mM stock of N-chloroacetamide was freshly prepared by slow addition of equal volumes of sodium hypochlorite into acetamide solution in slight excess (0.2%) buffered with 14 mM NaHCO3 at pH 9.3. During this process, pH of the solution was maintained constant at 9.3 by adding a small amount of 3 mM sodium hydroxide solution. The formation of N-chloroacetamide was monitored spectrophotometrically. After the reaction was complete, pH was adjusted to ∼7.5 and the N-chloroacetamide solution was kept at −4 °C for up to 1 week. Ham’s F12 medium (Mediatech, Inc., Manassas, VA) was added with this N-chloroacetamide solution, which was then filter sterilized through 0.22 μm filter. For the cytotoxicity assay, F12 medium was supplemented with 5% fetal bovine serum, 1% antibiotic−antimycotic solution (10 units/mL penicillin G sodium, 10 μg/mL streptomycin sulfate, 25 μg/mL amphotericin B, 0.85% saline; Invitrogen, Carlsbad,CA), and 1% L-glutamine. Chronic cytotoxicity was determined by exposing CHO cells to increasing concentrations of N-chloroacetamide for a period of 72 h. The resulting cell density for each concentration was measured with 8−16 replicates and expressed as the mean percentage of the concurrent negative control. Acute genotoxicity was measured by exposing CHO cells to N-chloroacetamide in serum-free F12 medium for 4 h. Induced genomic DNA damage was quantified using single cell gel electrophoresis (SCGE) assay (SI Text S2).26,27 The percentage of DNA in the tail (% tail DNA) was the primary measurement as the DNA genomic damage since it bears a linear relationship to break frequency, and is relatively unaffected by threshold setting.28,29 Genotoxicity of N,2-dichloroacetamide (preparation of the chemical was described elsewhere)11 was also measured and compared to Nchloroacetamide. Statistical and nonlinear regression analyses were performed with Sigmaplot 11.0 (Systat Software Inc., San Jose, CA) to calculate the concentration that induced a reduction of cell density to 50% of the negative control (LC50) and a 50% tail DNA. Detailed procedures for both the cytotoxicity and genotoxicity assays are described elsewhere.1,3,30
Article
RESULTS AND DISCUSSION
Reaction Pathway and Mass Spectrometry Results. Previous research has shown that monochloramine attacks aldehydes by nucleophilic addition on the slightly positive carbonyl carbon to form carbinolamines.11,19,20 Carbinolamines can further dehydrate to imine which then decompose to nitriles. In this study, the reaction pathway initiated by monochloramine and acetaldehyde as shown in Figure 1 (1c−6c) was confirmed by identifying the intermediate and reaction products with mass spectrometry. SPME/GCMS total ion chromatogram (TIC) confirmed the presence of acetonitrile and (chloroimino)ethane as shown in SI Figure S3. The intermediate 1-(chloroamino)ethanol was not observed, likely due to dehydration at the inlet to (chloroimino)ethane.11,19 Because (chloroimino)ethane standard is not commercially available, the compound was identified by analyzing the m/z fragments from the mass spectrum pertaining to the compound’s chromatogram peak (SI Figure S4). The acetonitrile mass spectrum was compared to prepared standards. N-Chloroacetamide was also found to be a product from the reaction of monochloramine and acetaldehyde. Results for Nchloroacetamide standard and reaction products analyzed by LRMS with positive electrospray ionization (ESI+) are presented in SI Figures S5 and S6, respectively. ESI+ [M + H]+ ions that pertain to the N-chloroacetamide standard include mass signals at 94 and 96 with a relative abundance of 100 to 32, signaling the presence of one chlorine atom in the molecule (SI Figure S5). Additionally, traces of acetamide were observed with [M + H]+ ion of 60. Acetamide might have been produced from the decomposition of N-chloroacetamide throughout the sample preparation process and/or analysis. [M + H]+ ions observed in the N-chloroacetamide standard were also observed in reaction samples, confirming the formation of N-chloroacetamide from the reaction of acetaldehyde and monochloramine and the partial transformation of N-chloroacetamide into acetamide during sample preparation/analysis (SI Figure S6). Sodium thiosulfate was added to reaction samples to quench monochloramine31 and N-chloroacetamide, where the latter is transformed to acetamide, and was further analyzed with GC/MS. Acetamide formation was illustrated with the reaction of 10 mM acetaldehyde and 1 mM monochloramine at pH 9.5 (SI Figure S7). Acetamide peak increased over time supporting that Nchloroacetamide is being formed from the reaction (Figure 1, 1c−6c). Another possible explanation is that acetamide could form via acetonitrile hydrolysis. However, control experiments of 1 mM acetonitrile in the presence of 1 mM monochloramine show that acetonitrile hydrolysis is negligible under these conditions (SI Figure S8). Therefore, results suggest that Nchloroacetamide formation follows a similar reaction pathway proposed elsewhere,11 where oxidation of carbinolamines by monochloramine produces N-haloacetamides. In this case, the carbinolamine 1-(chloroamino)ethanol is oxidized by monochloramine to form N-chloroacetamide. Reaction Constants. Equilibrium and kinetic constants were determined in two sections. The first section includes the fast reaction of monochloramine with unhydrated acetaldehyde to form and equilibrate with 1-(chloroamino)ethanol32−35 as shown in Figure 1(1c−3c). Where, unhydrated acetaldehyde is 45% of the total acetaldehyde in aqueous solutions at 25 °C (see SI Text S3). Temperature effects and carbinolamine molar 9957
DOI: 10.1021/acs.est.5b01875 Environ. Sci. Technol. 2015, 49, 9954−9963
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stable with lower equilibrium constants than chloroacetaldehyde followed by formaldehyde.37 Temperature Effects. The forward and backward reaction rates, k1 and k−1, dependence on temperature plotted in SI Figure S10 are described with the Arrhenius equation (S13) detailed in SI Text S5. The activation energies for k1 and k−1 were 3.04 and 45.2 kJ mol−1, which correlates with the higher k1 value (lower activation energy) and the lower k−1 value (higher activation energy), respectively. The corresponding values for constant A were 83.4 M−1 s−1 for k1 and 2.05 × 107 s−1 for k−1. Equilibrium constants in Table 2 are plotted against the corresponding (1/T) values in SI Figure S10. The enthalpy (ΔH°) and entropy (ΔS°) were calculated from the slope and intercept obtained by linear regression of the lower plot in SI Figure S10 to eq S14. The resulting values of ΔH° and ΔS° were −42.1 kJ mol−1 and −103.2 J K−1 mol−1, respectively. Molar Extinction Coefficients and Equilibrium Constant. 1-(Chloroamino)ethanol molar extinction coefficients and equilibrium constant K1 were determined using a previously published method.11 Reaction absorbance spectra were taken at 30 s with increasing concentrations of acetaldehyde at near neutral pH (experimental sets SH-1 and SH-2 in Table 1). Under these conditions, no measurable decomposition of 1(chloroamino)ethanol took place. Because acetaldehyde reacted in excess of monochloramine, the acetaldehyde absorbance spectra were subtracted from each reaction spectra taken at equilibrium. The resulting absorbance spectra could be expressed as follows:
extinction coefficients where also determined in this section. The second section characterizes the decomposition of 1(chloroamino)ethanol to produce N-chloroacetamide and acetonitrile. Acid/base and carbonate catalysis, and temperature effects were evaluated for 1-(chloroamino)ethanol decomposition. 1-(Chloroamino)ethanol Formation. The forward and reverse reaction rates k1 and k−1 were determined using the chemical relaxation method19,36 detailed in Text S4 of SI. Experimental results (absorbance over time) from sets SX-1 to SX-7 were fitted to eq 1: (Abs − Abse ) = (Abs0 − Abse )exp( −t /τ )
(1)
to obtain Abse, Abs0, and (1/τ) where, Abse and Abs0 are the absorbance at equilibrium and at t = 0, and (1/τ) is a constant. The forward and reverse reaction rate constants were determined by plotting (1/τ) against unhydrated acetaldehyde [B]0 concentrations according to (1/τ) = (−k1[B]0 + k−1) where, k1 is the slope and k−1 is the interception at [B]0 = 0 found in SI Figure S9. Equilibrium constant K1 is calculated from k1 and k−1 with the expression: K1 =
[CH3CH(OH)NHCl] k1 = k −1 [NH 2Cl][CH3CHO]
(2)
where [CH3CH(OH)NHCl], [NH2Cl], and [CH3CHO] are the concentrations of 1-(chloroamino)ethanol, monochloramine, and unhydrated acetaldehyde at equilibrium. Results are shown in Table 2. The kinetic rates k1 and k−1 increase with
Aλ , e = εNH2Clλ[NH 2Cl]e + εCH3CH(OH)NHClλ
Table 2. Acetaldehyde and Monochloramine Reaction Constants at Different Temperatures T (°C) 2 10 15 18 20 25 30 a
k1 (M−1s−1)a 22.1 22.6 23.7 23.2 24.4 24.3 24.9
± ± ± ± ± ± ±
0.295 0.117 0.116 0.162 0.140 0.212 0.189
k−1 (s−1)a
K1 (M−1)b
± ± ± ± ± ± ±
379 254 178 151 134 98 69
0.058 0.089 0.133 0.153 0.182 0.247 0.360
0.004 0.002 0.002 0.003 0.003 0.005 0.004
[CH3CH(OH)NHCl]e
(3)
As illustrated in SI Figure S11 for experimental set SH-1, two isosbestic points were observed with increasing acetaldehyde concentration, suggesting that the only reaction taking place is the formation of 1-(chloroamino)ethanol from acetaldehyde and monochloramine. Although it has been shown that carbinolamines can further react with aldehydes19 (e.g., Nchloroaminomethanol reaction with formaldehyde), SH-1 and SH-2 results show that with increasing acetaldehyde concentration the pair of isosbestic points do not shift suggesting that 1-(chloroamino)ethanol does not further react with acetaldehyde to form 1,1′-(chloroamino)diethanol. A possible explanation is that 1-(chloroamino)ethanol is sterically hindered making the disubstituted species formation a less favorable reaction. Applying monochloramine mass balances at equilibrium, monochloramine concentration is equal to the following:
Measured. bCalculated from k1 and k−1 values with eq 2
increasing temperature. However, the equilibrium constant K1 decreases with increasing temperature. This phenomenon means that at lower temperatures, the equilibrium is pushed toward the carbinolamine, which increases its concentration. Therefore, for a fixed concentration of monochloramine and acetaldehyde, the carbinolamine concentration will be higher for lower temperatures than for higher temperatures. Additionally, it is interesting to compare K1 to equilibrium constants of reactions between monochloramine with other aldehydes. For example, the constant K1 = 98 M−1 obtained for acetaldehyde at 25 °C in this study is lower than values reported at the same temperature for chloroacetaldehyde (K1 = 1871 M−1),11 and formaldehyde (K1 = 660 000 M−1).19 These differences, similar to those for the corresponding aldehyde hydration equilibrium constants (Kh =1.22, 33, and 2000 for acetaldehyde, chloroacetaldehyde and formaldehyde, respectively), are the result of the increased stability caused by the substitution group (CH3 → ClCH2 → H) at the carbonyl carbon. The more electro-donating group will reduce the partial positive at the carbonyl carbon making acetaldehyde more
[NH 2Cl]e = [NH 2Cl]0 − [CH3CH(OH)NHCl]e
(4)
where [NH2Cl]0 is the initial monochloramine concentration, and [NH2Cl]e and [CH3CH(OH)NHCl]e are the monochloramine and 1-(chloroamino)ethanol concentrations at equilibrium. Substituting eqs 2 and 4 into eq 3 and with the simplifying approximation of excess acetaldehyde, we obtain the following: Aλ , e = εNH2Clλ[NH 2Cl]0 + (εCH3CH(OH)NHClλ − εNH2Clλ) [NH 2Cl]0 [CH3CHO]0 (1/K1 + [CH3CHO]0 ) 9958
(5) DOI: 10.1021/acs.est.5b01875 Environ. Sci. Technol. 2015, 49, 9954−9963
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Environmental Science & Technology where, the absorbance spectra is expressed in terms of unknowns K1 and εCH3CH(OH)NHClλ. Eq 5 was fitted to experimental data from sets SH-1 and SH-2 to calculate K1 and εCH3CH(OH)NHClλ for different wavelengths (SI Table S1). The average values of K1 were calculated to be 147 ± 2.4 M−1 at 18 °C and 97.1 ± 1.84 M−1 at 25 °C. Molar extinction coefficients calculated at both temperatures (SI Table S1) did not differ significantly. Therefore, K1 = 147 M−1 was fixed for all experiments performed at 18 °C, and the data were fitted with eq 5 for a second time to determine εCH3CH(OH)NHClλ for different wavelengths (SI Figure S12). Average K1 values were found to agree with results obtained from the analysis of the forward and reverse reaction analysis shown in Table 2. However, because the temperature range investigated for experiment sets SX-1 to SX-7 was broader, K1 values used subsequently are those found in Table 2. 1-(Chloroamino)ethanol Decomposition. Spectra obtained for the reaction of acetaldehyde and monochloramine over time are illustrated in SI Figure S13. The absorbance at 243 nm, at which monochloramine and carbinolamine absorbs the highest, decreased with time, while the absorbance at
