Environ. Sci. Technol. 2007, 41, 863-869
Free Radical Chemistry of Disinfection Byproducts. 2. Rate Constants and Degradation Mechanisms of Trichloronitromethane (Chloropicrin) S. KIRKHAM COLE,† W I L L I A M J . C O O P E R , * ,‡ R O B E R T V . F O X , § PIERO R. GARDINALI,| STEPHEN P. MEZYK,⊥ BRUCE J. MINCHER,# AND KEVIN E. O’SHEA| Civil and Environmental Engineering Department, Old Dominion University, Kaufman Hall, Norfolk, Virginia 23529, Urban Water Research Center, Department of Civil and Environmental Engineering, University of California, Irvine, California 92697-2175, Idaho National Laboratory, Chemical Sciences Department - Chemistry, P.O. Box 1625, Idaho Falls, Idaho 83415-2208, Department of Chemistry & Biochemistry, Florida International University, University Park, 11200 SW 8th Street, Miami, Florida 33199, Department of Chemistry and Biochemistry, California State University at Long Beach, 1250 Bellflower Boulevard, Long Beach, California 90840, and Idaho National Laboratory, Radiation Physics Group, P.O. Box 1625, Idaho Falls, Idaho 83415-7111
Absolute rate constants for the free-radical-induced degradation of trichloronitromethane (TCNM, chloropicrin) were determined using electron pulse radiolysis and transient absorption spectroscopy. Rate constants for hydroxyl radical, •OH, and hydrated electron, eaq-, reactions were (4.97 ( 0.28) × 107 M-1 s-1 and (2.13 ( 0.03) × 1010 M-1 s-1, respectively. It appears that the •OH adds to the nitro-group, while the eaq- reacts via dissociative electron attachment to give two carbon centered radicals. The mechanisms of these free radical reactions with TCNM were investigated, using 60Co gamma irradiation at various absorbed doses, measuring the disappearance of TCNM and the appearance of the product nitrate and chloride ions. The rate constants and mechanistic data were combined in a kinetic computer model that was used to describe the major free radical pathways for the destruction of TCNM in solution. These data are applicable to other advanced oxidation/reduction processes.
The LC50 of TCNM was ranked second of 11 cyanohydrins when tested using house flies (0.49 µM) and the ninth most lethal (LC50 ) 7.91 µM) for the lesser grain borer (14). Its use as a World War I chemical agent (15) confirmed this chemical as a potent mammalian cytotoxin and genotoxin, being 32.6 times more cytotoxic than the EPA-regulated dichloroacetic and trichloroacetic acids (16). TCNM is metabolized to toxic thiophosgene in mammals (17), and other studies have indicated genotoxicity from 0.3 to 3 µg L-1 (18). Bacterial mutagenicity studies indicated that TCNM was mutagenic but not toxic on addition of glutathione (19). Limited studies have been reported for the treatment of TCNM. Photolysis of aqueous TCNM solutions produced phosgene (20), and under cryogenic conditions nitrosyl chloride was obtained (21). Xenon light (solar simulator) degraded 1 mM TCNM at 25 °C and pH 7 to CO2, Cl-, NO3-, and NO2- with a half-life of 31.1 h, whereas dark controls were stable in solution for 240 h (22). Ultrasound degraded 99% of a 10 µM TCNM solution with recovery ratios for Cland inorganic nitrogen (NO3-, NO2-) at 72 ( 1 and 91 ( 2%, respectively (23). However, no studies of TCNM free radical chemistry in aqueous solution have been reported. The objective of this study was to explore the utility of advanced oxidation/reduction processes for treatment of TCNM in water. Of particular interest were the kinetics and mechanisms of the hydroxyl radical (•OH) and hydrated electrons (eaq-) reactions. We have combined our data into a kinetic computer model to gain further insight into the free-radical-induced degradation chemistry for this HNM.
Methods and Materials Solutions of TCNM (Aldrich, >99.4%) were prepared in ultrapure deionized filtered water (g18 MΩ) that had a measured TOC content below 13 µM. Kinetic measurements on these solutions were performed using the linear accelerator/absorption spectroscopy system at the Department of Energy Radiation Laboratory, University of Notre Dame (24). The radiolysis of water gives a distribution of transient and stable products according to the eq (25, 26)
H2O '[0.28]•OH + [0.06]H• + [0.27]eaq- + [0.05]H2 +
Introduction Disinfectants used in water treatment, such as chlorine, chloramines, and ozone, react with dissolved natural organic matter to form a range of disinfection byproducts (DBPs) * Corresponding author phone: (949)824-5620; fax: (949)824-3672; e-mail:
[email protected]. † Old Dominion University. ‡ University of California, Irvine. § Idaho National Laboratory, Chemical Sciences Department Chemistry. | Florida International University. ⊥ California State University at Long Beach. # Idaho National Laboratory, Radiation Physics Group. 10.1021/es061410b CCC: $37.00 Published on Web 01/04/2007
(1). Many DBPs are regulated under the Safe Drinking Water Act (2). One class of DBPs that has been the focus of recent reports is the halonitromethanes (HNMs) (3-5). Of the HNMs, trichloronitromethane (Cl3CNO2, chloropicrin, TCNM) is the most common (6, 7), being formed in disinfected waters containing nitrite (8-10). Both ozonation and chlorination affect TCNM concentrations (11), and it has been found as high as 6.5 nM when preozonation was followed by chloramination (12, 13). TCNM has also been introduced to water systems through various commercial applications (see the Supporting Information).
2007 American Chemical Society
[0.07]H2O2 + [0.27]H3O+ (1)
where the numbers in brackets are the G-values (yields) in µmol J-1. Absolute dosimetry for kinetics measurements was based on the transient absorbance produced in N2Osaturated 1.0 × 10-2 M KSCN solution at λ ) 475 nm (G ) 5.2 × 10-4 m2 J-1) with doses of 3-5 Gy per 2-3 ns pulse (27). The oxidation of TCNM by reaction with the hydroxyl radical gave no significant transient absorbance over the range 260-800 nm. The radical rate constant determination was therefore performed using SCN- competition kinetics, VOL. 41, NO. 3, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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863
by monitoring the changes in absorption of the (SCN)2•transient at 475 nm in the competition (25): •
OH + TCNM f products
(2)
OH + SCN- (+SCN-) f OH- + (SCN)2•-
(3)
•
This competition can be analyzed to give the expression
[(SCN)•2 ]o [(SCN)•2 ]
)1+
k2[TCNM]
(4)
k3[SCN-]
where a plot of [(SCN)2•-]o/[(SCN)2•-] against the concentration ratio [TCNM]/[SCN-] gives a straight line of slope k2/k3. Based on the rate constant for hydroxyl radical reaction with SCN-, k3 ) 1.1 × 1010 M-1 s-1 (25), the k2 rate constant is readily determined (28). To isolate the reaction of •OH with TCNM, solutions were presaturated with N2O, which quantitatively converts hydrated electrons, eaq-, and hydrogen atom, •H, to •OH (25):
eaq- + N2O + H2O f N2 + OH- + •OH 9
k5 ) 9.1 × 10 M •
H + N2O f •OH + N2
-1
s
-1
(5)
k6 ) 2.1 × 106 M-1 s-1 (6)
For the determination of the reaction rate of TCNM and hydrated electrons, solutions were sparged with N2 to remove the dissolved oxygen and had 0.50 M tert-butanol added to selectively scavenge the formed hydroxyl radicals and hydrogen atoms: •
OH + (CH3)3COH f •CH2(CH3)2COH + H2O
k7 ) 6.6 × 108 M-1 s-1 (7)
•
H + (CH3)3COH f •CH2(CH3)2COH + H2
k8 ) 1.7 × 105 M-1 s-1 (8)
Hydrated electron kinetics were directly observed at 700 nm. A 60Co γ-irradiator, Shepherd 109-68 at a dose rate of 122 Gy min-1 (Fricke dosimetry) was used for all steady-state studies. Experiments were performed using nominally 1 mM TCNM solutions in aerated (2.00 × 10-4 M O2 measured) ultrapure water, in sealed 40 mL glass vials with no headspace. These TCNM solutions were irradiated at doses of 1.2, 2.4, 3.6, 6.1, and 8.5 kGy by varying the time of irradiation. The concentration of TCNM in 60Co irradiated samples was determined using a Finnigan DSQ Ultra-Trace GC/MS system. An internal standard, 1, 2-dichloropropane, was first added to 1 mL of the aqueous solution, and then liquidliquid extraction using 1 mL portions (2 × 500 µL aliquots) of MTBE was performed. Helium was used as a carrier gas, and compounds were eluted using an oven program of 45 °C to 150 °C at 15 °C min-1. The minimum detection limit (MDL) was 180 ( 20 nM. Chloride and nitrate product ions were analyzed using a Dionex DX-500 ion chromatograph with MDL of 0.3 µM and 0.2 µM, respectively. Analytical error for Cl- and NO3- was (0.02 and (0.01 µM, respectively. The kinetic model used was MAKSIMA-CHEMIST (2931), which was coded with the basic reactions of water radiolysis and the appropriate reaction rates for TCNM and byproducts (25). This model was used to conduct a sensitivity analysis of all components of the destruction mechanism, allowing us to estimate unknown reaction rates and to closely 864
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FIGURE 1. Typical kinetics of (SCN)2•- formation at 475 nm for N2O saturated 1.04 × 10-4 M KSCN solution containing 0, 1.55, 3.55, and 5.00 mM chloropicrin at natural pH and 20 °C. (b) Competition kinetics plot for hydroxyl radical reaction with chloropicrin using SCN- as a standard. Solid line is weighted linear fit, corresponding to a slope of 0.00473 ( 0.00027. This gives a second-order rate constant for chloropicrin reaction as (4.97 ( 0.28) × 107 M-1 s-1. predict our measured TCNM removal and reaction byproduct formations in deionized water.
Results and Discussion Kinetic Measurements. The hydroxyl radical reaction rate with TCNM was evaluated using SCN- competition kinetics. The initial TCNM standard was >99.4% pure, and a rate constant of (4.84 ( 0.42) × 107 M-1 s-1 was obtained (32). To ensure that the 0.6% impurity did not significantly affect the rate constant a second standard of 99.8% purity was used in a repeat measurement, giving a rate constant of (4.97 ( 0.28) × 107 M-1 s-1 (Figure 1) confirming the original evaluation. The reaction of TCNM reaction with the eaq- was determined using the rate of change of the hydrated electron absorbance rate of decay at 700 nm (32, 33). Based on these data the bimolecular rate constant for the reaction of eaqwas k7 ) (2.13 ( 0.03) × 1010 M-1 s-1 (Figure 2). Steady-State Irradiations. Steady-state 60Co gamma irradiations were performed for solutions containing 1.13 mM TCNM in aerated deionized water at 20 °C. The measured loss of TCNM, and the corresponding formation of chloride and nitrate ions, is summarized in Table 1. Chloride ion mass balance was nearly achieved (92%) at 8.5 kGy. The nitrate ion recovery was 79% at the highest dose. No other organic or inorganic ions were detectable in these irradiated solutions. Kinetic Model Construction. Based on our measured rate constants, it appears that the reductive processes would be primarily responsible for the destruction of TCNM. The dissociative hydrated electron reaction was expected to proceed via two main reaction pathways (33), giving two different carbon-centered radicals:
e-(aq) + CCl3NO2 f Cl- + •CCl2NO2
(9)
e-(aq) + CCl3NO2 f NO2- + •CCl3
(10)
In our model we assumed a branching ratio of unity for these two reactions, based upon the Cl:NO2 atom ratio in TCNM
O2 + •CCl3 f •OOCCl3
k12 ) 3.3 × 109 M-1 s-1 (36) (12)
Rate constants for peroxyl radical formation are typically in the range (2-4) × 109 M-1 s-1 (37). Peroxyl radicals are generally unreactive in aqueous solution (38); however, •OOCCl can react via electron transfer with an iodide or 3 aromatic thiols (39-41) to give -OOCCl3. In relatively simple solutions it was not clear that such a reaction would occur. Peroxyl radicals also combine to form tetroxides (R3C-O4CR3) which then decompose according to eqs 13-16, (38):
The lack of hydrogen atoms on the carbon of TCNM precludes reactions 13 and 14 from occurring, and because this is a one-carbon compound reaction 16 cannot proceed (38). Therefore, reaction 15 is the only remaining pathway, leading to the formation of three different alkoxy radicals that would subsequently undergo intramolecular rearrangements: FIGURE 2. (a) Typical kinetic decay profiles obtained for the hydrated electron absorbance at 700 nm for the electron pulse irradiated aqueous solution at natural pH and 20 °C containing 0 (0), 0.112 (O), 0.232 (∆), and 0.435 (3) mM chloropicrin, respectively. Curves shown are the average of 15 individual pulses. Solid lines correspond to rate constant fitting with the pseudo-first-order values of 1.71 × 105, 2.64 × 106, 5.26 × 106, and 9.30 × 106 s-1, respectively. (b) Second-order rate constant determination for the reaction of the hydrated electron with chloropicrin. Single-point error bars are one standard deviation, as determined from the average of at least three kinetic traces. Solid line corresponds to weighted linear fit, giving k ) (2.13 ( 0.03) × 1010 M-1 s-1.
TABLE 1. Summary of Experimental Results for 60Co Irradiation of TCNM Solutions (1.13 mM) in Ultrapure Water at Doses up to 8.54 kGy and Kinetic Model Results experimental
kinetic model
dose (kGy)
TCNM (mM)
Cl(mM)
NO3(mM)
TCNM (mM)
Cl(mM)
NO3(mM)
0 1.22 2.44 3.66 6.10 8.54
1.13 0.80 0.42 0.17 NMa BMDLb
0.00 0.92 1.67 2.17 2.73 3.13
0.00 0.29 0.44 0.58 0.79 0.89
1.13 0.80 0.54 0.23 c c
0.00 0.98 1.71 2.50 3.13 3.13
0.00 0.26 0.41 0.59 0.95 0.94
a Not measured. b Below method detection limit. c Modeled data are below detection limit.
and the rate constants (25) for hydrated electron reaction with CCl4 (1.3 × 1010 M-1 s-1) and C(NO2)4 (∼4.6 × 1010 M-1 s-1). The reaction of eaq- with tetranitromethane has also been shown to produce the •NO2 radical (34, 35), and, by analogy, a similar reaction might be expected with TCNM. However, the resultant trichloromethyl anion would not be as stabilized as the nitroform anion for tetranitromethane. Furthermore, a sensitivity analysis including this reaction did not improve the accuracy of product predictions, and therefore it was not included. In aerated solutions, these carbon-centered radicals will react with dissolved oxygen to give peroxyl radicals:
O2 + •CCl2NO2 f •OOCCl2NO2
(11)
The unstable product phosgene, COCl2, hydrolyzes to CO2 and HCl (42). Reaction 18, presumably similar to that reported earlier (21), would hydrolyze to CO2, HCl, and HNO2. The •NO2 radical of (17) can also be produced by the reaction of the hydroxyl radical with formed NO2- (43-48):
OH + NO2- f •NO2 + OH-
•
k20 ) 9.6 × 109 M-1 s-1 (20)
This radical would most likely react with the hydrogen atom (43) or with O2•-/HO2 (49)
NO2 + •H f H+ + NO2-
•
k21 ) 1.0 × 1010 M-1 s-1 (21)
NO2 + O2•-/HO2 f H+ + NO2- + O2
•
k22 ∼ 1 × 108 M-1 s-1 (22)
to regenerate nitrite. Some of the formed •NO2 could also react with •OH, leading to the formation of peroxynitrous acid (43, 50, 51): •
OH + •NO2 f ONOOH
k23 ) (4.5 ( 1.0) × 109 M-1 s-1 (23)
ONOOH h ONOO•- + H+ VOL. 41, NO. 3, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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The pKa of ONOOH is 6.5-6.8; therefore, under the acidic conditions of our irradiations we would mostly have ONOOH (43, 50-53). The mechanism for the (acid) hydrolysis of the ONOOH has been summarized (51, 53-55): -
ONOOH f NO3 + H
+
ONOOH f •NO2 + •OH
k38 ) 2.0 × 109 M-1 s-1 (67, 68) (38)
30%
(26)
Cl2-• + O2-• f O2 + 2Cl- k39 < 2.0 × 109 M-1 s-1 (69) (39)
(27)
ONOOH f H+ + NO3-
k28 ) 1.25 ( 0.05 s-1 (51, 53-55) (28)
Our solutions were unbuffered and with increasing radiation, the pH decreased (eq 1). At more acidic pH the NO3- product is favored (54). The •Cl formed in the reactions 18 and 19 might be expected to react as follows, where the forward and back reactions are summarized (56-62):
Cl + Cl- f Cl2-•
not reported
(29)
k30 ) (7.8 ( 0.8) × 109 M-1 s-1 (60) (30)
k-30 ) (5.7 ( 0.4) × 104 s-1 (60) Cl2-• + H2O f •ClOH- + H+ + Cl•
Cl + •Cl f Cl2
k31 < 100 s-1 (60) (31)
k32 ) 8.8 × 107 M-1s-1 (56, 59)
(32)
•
Cl + H2O T •ClOH- + H+
k33 ) (1.6 ( 0.2) × 105 s-1 (60) (33)
k-33 ) (2.6 ( 0.6) × 1010 M-1 s-1 (60) •
ClOH- T •OH + Cl-
k34 ) (6.1 ( 0.8) × 109 s-1 (62) (34) 9
k-34 ) (4.3 ( 0.4) × 10 M -
-1
s
-1
(62)
•Cl
The reaction of the eaq and would be expected to proceed at diffusion controlled rates; however, no value has been reported (a rate constant of 5 × 1010 M-1 s-1 was assumed in the model). Radical-radical recombination (32) is not likely to be a major contributor to the loss of the •Cl. Therefore, reaction 30 appears to be the major loss mechanism for Cl radicals, forming Cl2-•. The reaction of the highly unstable •ClOH- would proceed via hydrolysis (36), with subsequent formation of •OH and Cl-. The chemistry of the Cl2-• has been studied extensively and when incorporated into the kinetic model significantly improved the results:
Cl2-• + Cl2-• f Cl- + Cl3-
k35 ) 2.0 × 109 M-1 s-1 (56, 63-66) (35)
Cl3- + H• f Cl- + Cl2-• + H+
k36 ) 3.0 × 1010 M-1 s-1 (67) (36)
866
9
Cl2-• + HO2• f O2 + H+ + 2Cl-
(25)
The overall reaction for the peroxynitrous acid decomposition is therefore
•
k37 ) 8.0 × 109 M-1 s-1 (66, 68) (37)
70%
2•NO2 + H2O f NO3- + NO2- + 2H+
eaq- + •Cl f Cl-
Cl2-• + H• f H+ + 2Cl-
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 3, 2007
Cl2-• + NO2- f •NO2 + 2Cl-
k40 ) 2.0 × 108 M-1 s-1 (70) (40)
Hydrogen Atom Reaction. Another possible reduction of TCNM would be by reaction of the hydrogen atom. Bimolecular reaction rates of the TCNM and H• were not evaluated in this work; however, the following primary reactions
TCNM + H• f •CCl2NO2 + H+ + Cl-
(41)
TCNM + H• f •CCl3 + H+ + NO2-
(42)
were included in our computer model based upon analogous hydrogen atom reaction with CCl4 (3.2-4.4 × 107 M-1 s-1) to give Cl- (71, 72). Recently, it was also shown that hydrogen atoms react with dichloroethanes to only give Cl- in aqueous solution (73). The precedence for reaction 42 is from H• reaction with tetranitromethane (TNM), which occurs with a rate constant of 5.5-26 × 108 M-1 s-1 and gives NO2- (35, 74, 75). Given the large uncertainty in the latter rate constant, reactions 41 and 42 were added to the model using equivalent estimated rate constants of 2.0 × 108 M-1 s-1 for TCNM. Superoxide Anion Reactions. The presence of dissolved oxygen in our 60Co irradiations would also result in the formation of superoxide anion radical (O2•-) through the reactions of hydrated electrons and hydrogen atoms. Sensitivity analysis of our kinetic model revealed that significant formation of O2•- occurred and that the reaction of •NO2 with O2-• had a major influence on the removal rate of TCNM. Additional chemistry involving direct O2-• reaction with TCNM was thus considered. By analogy with the established reaction of O2-• with TNM (35, 74, 75) we propose the
TNM + O2-• f O2 + •C(NO2)3- + •NO2
k43 ) 2.0 × 109 M-1 s-1 (43)
following reaction occurs for TCNM
TCNM + O2-• f O2 + •CCl2NO2 + Cl-
(44)
Including this specific reaction in the kinetic model resolved the sensitivity of the reaction of •NO2 with O2-• on the removal of TCNM. By adjusting this unknown rate constant to 1.5 × 108 M-1 s-1 it was possible to increase the level of confidence in the mechanism for TCNM removal and to better approximate chloride and nitrate ion formation. In fact, our model was very sensitive to the rate constant; grossly underpredicting removal of TCNM using a rate constant of 1.0 × 108 M-1 s-1 and overpredicting TCNM removal using a rate constant of 2.0 × 108 M-1 s-1. In future studies this reaction rate will have to be confirmed empirically. Hydroxyl Radical. The oxidation of TCNM by reaction of the hydroxyl radical was initially thought to be of minimal importance in the overall degradation due to its slow rate constant. However, including our measured rate constant for the •OH reaction with TCNM overpredicted the removal of TCNM, although the calculated formation profiles of the product Cl- and NO3- anions were in much better agreement
TABLE 2. Linearized Reaction Mechanism for the Free Radical Destruction of TCNMa eq no. 9 10 11 12 15 15 17 18 19 20 23 28 29 30 -30 35 36 37 38 39 40 41 42 43 44 45 47
reactions TCNM TCNM •CCl NO 2 2 •CCl 3 •OOCCl NO 2 2 •OOCCl 3 tetroxide A tetroxide B tetroxide AB •OCCl NO 2 2 •OCCl NO 2 2 •OCCl 3 COCl2 CONO2Cl2 •OH •OH ONOOH eaq•Cl Cl2-• Cl2-• Cl3Cl2-• Cl2-• Cl2-• Cl2-• TCNM intermediate intermediate TCNM TCNM TCNM •CCl NO 2 2 •OOCCl NO 2 2 tetroxide C NO2•OH ONOOH eaqeaq•OH •NO 2 •NO 2
+ + + + + +
eaqeaqO2 O2 •OOCCl NO 2 2 •OOCCl 3
+ + + + + + +
H2O H2O NO2•NO 2 H2O •Cl Cl-
+ + + + + + +
Cl2-• H• H•
•OH
+ + + + +
H• H• O2-• O2 •OOCCl NO 2 2
+ + + + + + + +
•OH
•NO
2
H2O NO2NO3NO32O2-• HO2•
f f f f f f
f f f f f f f f f f f f f f f
f f f f f f f f f f f f f f f f f
•CCl •CCl
2NO2
3 •OOCCl •OOCCl
+ +
ClNO2-
+ + + + + + + +
O2 O2 •OCCl NO 2 2 •NO 2 •Cl •Cl 2H+ HCl •NO 2
+
NO3-
+ + + + + + +
ClCl3Cl2-• 2ClH+ 2Cl2Cl-
+ + + + +
•OH ONOOH H+ H+ Cl-
+ +
4 ClOH-
+
NO3-
+ + +
OHO2 O2
2NO2 3
tetroxide A tetroxide B 2•OCCl2NO2 2•OCCl3 •OCCl 3 COCl2 CONO2Cl2 COCl2 CO2 CO2 OHONOOH H+ ClCl2-• •Cl ClClH+ O2 O2 •NO 2 intermediate TCNM •CCl 3 •CCl NO 2 2 •CCl 3 •CCl NO 2 2 •OOCCl NO 2 2 tetroxide C 2NO2•NO 2 ONOOH H+ •NO 2 NO32NO3NO2NO2-
+
O2
2ClHNO2
2Cl-
ClNO2O2 2CO2
H+
Kb,c (M-1 s-1)
ref
1.07 × 1010 1.07 × 1010 3.0 × 109 3.3 × 109 1.0 × 108 1.0 × 108 1.0 × 106 1.0 × 106 1.0 × 106 105 (s-1) 105 (s-1) 104 (s-1) 9 (s-1) 9 (s-1) 9.6 × 109 4.5 × 109 1.25 (s-1) 5.0 × 1010 7.8 × 109 5.7 × 104 (s-1) 2.0 × 109 3.0 × 1010 8.0 × 109 2.0 × 109
