Environ. Sci. Technol. 2009, 43, 2831–2837
Computerized Pathway Elucidation for Hydroxyl Radical-Induced Chain Reaction Mechanisms in Aqueous Phase Advanced Oxidation Processes KE LI* AND JOHN CRITTENDEN Department of Civil and Environmental Engineering, Arizona State University, P.O. Box 875306, Tempe, Arizona 85287-5306
Received July 23, 2008. Revised manuscript received January 16, 2009. Accepted January 20, 2009.
The radical reaction mechanism that is involved in advanced oxidation processes is complex. An increasing number of trace contaminants and stringent drinking water standards call for a rule-based model to provide insight to the mechanism of the processes. A model was developed to predict the pathway of contaminant degradation and byproduct formation during advanced oxidation. The model builds chemical molecules as graph objects, which enables mathematic abstraction of chemicals and preserves chemistry information. The model algorithm enumerates all possible reaction pathways according to the elementary reactions (built as reaction rules) established from experimental observation. The method can predict minor pathways that could lead to toxic byproducts so that measures can be taken to ensure drinking water treatment safety. The method can be of great assistance to water treatment engineers and chemists who appreciate the mechanism of treatment processes.
Introduction
number of organic compounds produced annually that potentially could end up in drinking water, (2) analytical detection of these compounds in water has improved, and (3) perceived or actual human and aquatic health effects of these compounds in water are likely to increase. Using a computer algorithm to generate reaction pathways offers one solution. Two approaches, namely, matrix rearrangement and logical programming, have been used to generate reaction pathways. Several models have successfully replicated gas phase combustion, pure organic synthesis, and petroleum chemical production systems (17-22). Although these models provide guidelines and some useful algorithms for generation of radical reaction pathways, they deal with different elementary reactions and often focus on major products that are found in nonaqueous phases. In water treatment, major pathways and minor pathways need to be examined because minor byproducts may be very toxic. We have developed a computerized pathway generator that can predict degradation pathways and the fate of byproducts generated by the hydroxyl radical-initiated chain reactions in aqueous phase AOPs. This paper describes the first module of the generator, which focuses on the algorithm of generating pathways. The algorithm uses the concept of object-oriented programming, in which each molecule is constructed as an object with specific chemical properties and behavior. The molecular objects behave according to elementary reaction rules based on experimental knowledge. The molecular objects and reaction rules are implemented on the basis of mathematic graph theory. The model is validated by comparing the degradation mechanism of methane and trichloroethylene (TCE) under the attack of hydroxyl radicals with experimental observations.
Advanced oxidation processes (AOPs) have been used for decades to destroy many organic contaminants in water (1). However, AOPs are mechanistically complex in nature. The complexity and diversity of the structures of byproducts make it difficult and expensive to experimentally investigate the degradation pathways of each contaminant and the fate of the intermediate byproducts, which may pose potential risks to human health (2, 3). As drinking water standards become more stringent and analytical methods become more sensitive, researchers worldwide are paying increasing attention to the formation and fate of byproducts in treatment processes. Great efforts have been made to study the degradation mechanisms of organic contaminants under hydroxyl radical attack (refs 4-16 and references therein). A general pattern for contaminant degradation can be written as follows Nonetheless, it is a challenge to apply this knowledge to other contaminants because of the complexity of the chain propagation for different chemical structures. Accordingly, a need exists to develop rule-based models capable of providing heuristics of advanced oxidation byproducts and their potential health effects because (1) there are a large
Materials and Methods
* Corresponding author phone: (480) 965 3171; fax: (480) 965 0557; e-mail:
[email protected].
FIGURE 1. Skeleton of the modules in the computer generation of reaction pathways.
10.1021/es802039y CCC: $40.75
Published on Web 03/10/2009
2009 American Chemical Society
Figure 1 illustrates the logic for the computer reaction network generator. The reaction network generator consists of three interrelated modules: the reaction pathway generation module, the rate constant estimation module, and the module that generates and solves the ordinary differential equations (ODEs).
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This paper focuses on the reaction generation module shown in Figure 1. The input for this module consists of the initial reactants and chemical additions as “graphs” and reaction rules, which specify the chemical transformations. The generator then checks the structure of a reactant and applies all reactions possible for the reactant, which leads to the product graphs. New products will be reactants for the next iteration. The recursive generation process enumerates all reaction possibilities and products and byproducts. Information Input. Simplified molecular input line entry system (SMILES) notation is used to represent molecular structures as a linear string of symbols representing atoms with their atomic symbol and bonds (-, single; ), double; and #, triple) (23). Representation of Molecules as Graph Objects. A graph is a mathematic abstraction of a substance, which may be a molecule or, in this case, a reaction rule. According to graph theory, graphs for reactants and products are usually denoted as GR and GP, respectively. The transformation of bonds and atoms, called reaction rules, is denoted as GT. A reaction can be simply depicted as GR + GT ) GP. Manipulation of GR and GT can enumerate all possible reaction pathways. One drawback of traditional models is the huge redundancy due to the fact that mathematical operations do not necessarily follow chemistry rules and laws. For example, 2 + 3 is a valid mathematical operation, yet it is impossible to add a double bond to a carbon atom that already has three other bonds. To avoid the loss of chemistry information, the molecular graph is built as objects. An object is a computational term that refers to an instance that can process information and communicate with other objects. The information in an object is called the data, and the way to process and communicate the information is called the method. Data for Molecular Graph Objects. The data in a molecular graph include atom names, free valence of atoms, carbon chain length in the molecule, bonds and connectivity between atoms, unique root of the molecule, arbitrarily assigned indices of atoms in the molecule, and a string code that contains all of the atoms in lower levels in the tree hierarchy of the molecule. Methods for Molecular Graph Objects. The methods of a molecular object determine the behavior it can perform. A molecular object has more than 40 methods that perform reactions, integrity and structural maintenance, and interfaces. The Supporting Information provides a list of major methods in each category. Details of graph theory and related data structure can be found in the literature (ref 24 and references therein). Implementation of Molecular Graphs. Molecular graphs are implemented as canonical trees in the algorithm. The canonical tree of a molecule contains a special node, called the root, and other nodes as a branch, which denotes a group of connected nodes, or a leaf, which is an individual node. The order of the leaves and branches is determined according to the summary of the atoms’ weights and the lexicographical order of the atoms’ denotations. The root can be an actual atom if a unique atom can be identified or a dummy node if no such atom exists. For example, the carbon atom of methane can serve as root, yet a dummy root needs to be added for ethane. Generation of a Reaction. The algorithm generates reactions by recursively performing the following three steps: (1) search the graph of reactants to find reacting subgraphs, e.g., the C-H bond for H abstraction, (2) identify the reaction rules that match a graph and modify the graph accordingly to form a products graph, and (3) check the canonicity of the product and add it to a reactants pool if it is new. The example below shows the general idea of reaction generation. The reacting graph includes all atoms and bonds that will be changed in a reaction and excludes all remaining ones 2832
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FIGURE 2. Illustration of pattern match for HCl elimination. The numbers on the tree in step (1) represent the indices of the atom in the molecular tree. X represents any atom. that are unchanged. In the example, the reaction graph of the reactants is the chlorohydrin group. As shown in Figure 2, the algorithm recursively searches the canonical tree of the reactant to match the chlorohydrin subtree. Note that the subtree is also canonical. The algorithm starts from the root and traverses the tree using the depth-first search technique, meaning it searches all descendants of a node before touching any of its siblings (nodes that share a same parent), as depicted in steps 1-5 of Figure 2. Once a pattern is matched, the algorithm will then operate on the tree to transform it into a product according to the reaction rules. For the molecular tree above, the algorithm needs to break the C2-Cl6 and O8-H12 bond and then build another bond between C2 and O8 (as shown in step 6). The resulting tree represents an aldehyde. The product is then checked by the canonicity algorithm and added to a product pool if it is unique. The reaction is then stored by the algorithm in the format below X{C[C(ClClH)HH]C[ClHO(H)]} f C[C(ClClH)C(H ⁄ ⁄ O)HH] + ClH (XE) (1) where XE is the notation for the HCl elimination. The line code represents the reaction below Cl2H3C2-HCl(•OH) f Cl2H3C2-CHO + HCl
(2)
For ease in reading, the predicted reactions will be presented as general chemical formulas instead of line code. Reaction Rules in Aqueous Phases. Hydroxyl radicalinduced chain reactions are mainly responsible for the degradation of organic compounds. They are also the most
complex part of the kinetic mechanism. Reactions with other oxidants, such as H2O2 and O3, and direct UV photolysis are also important for some contaminants but are not the scope of this paper. Bimolecular reactions between most radicals were ignored because they are in trace amounts and have lifetimes too short for a biradical collision. One exception is the peroxyl radicals, which generally have longer lifetimes and larger sizes. Bimolecular reactions of peroxyl radicals account for a large portion of their degradation (16). The propagation mechanism of radicals that generally exist in AOPs is briefly summarized below. The mechanisms discussed here are far from complete. Rather, this is a list of the most commonly agreed on mechanisms according to experimental observations. By all means, this model is not aimed at discovering new mechanisms but rather applying existing general principles to enumerate reaction pathways. 1. Hydroxyl Radicals. Hydroxyl radicals attack organics either by abstracting a hydrogen atom, i.e., H abstraction, or by addition to an unsaturated bond. 2. Carbon-Centered Radicals. The main mechanism for carbon-centered radicals is the formation of peroxyl radical by the addition of dissolved O2 onto the carbon radical center. 3. Peroxyl Radicals. Bimolecular termination through a tetroxide structure is the main mechanism for the degradation of the peroxyl radical. This process has been extensively studied and summarized by von Sonntag and Schuchman (16). Although there are still unknowns about this process, the following four pathways are generally observed. Reaction 3d is thought to be subordinate.
Another important reaction of the peroxyl radical is the elimination of HO2•/O2•-, which yields CdO or CdC double bonds as shown in the example below (25).
Other reactions of the peroxyl radicals such as electron transfer, H abstraction, and double-bond addition were ignored for peroxyl radicals because of their insignificance in the AOP system (26, 27). 4. Oxyl Radicals. Because of the polarity of the water molecule, β-scission and the 1,2-shift of oxyl radicals are accelerated significantly. First-order rate constants for the two types of reactions have been reported to be greater than 106 s-1 (28, 29). Therefore, they almost completely exclude other mechanisms such as H abstraction, disproportionation, and recombination. The latter three mechanisms are typical in either organic solutions or the gas phase but not important in the aqueous phase (16, 30). 5. Chlorine Radicals. Many chlorinated olefins and their degradation byproducts can eliminate chlorine radicals through photolysis and/or H2O-associated decay. Chlorine radicals can then attack the parent compound or byproducts and initiate chain reactions leading to the decay of these species. Addition to unsaturated bonds is an important mechanism for the chlorine radical, according to the byproducts detected for the degradation of TCE in the photolysis system with or without H2O2 (7). Hydrolysis. Hydrolysis plays an important role in the degradation of some small molecules in the advanced
oxidation processes. Four structures have been observed to hydrolyze rapidly in AOP systems, aldehydes, carbonyl chlorides (-C(O)Cl), alkyl radicals with β-halogens (-C•-C-X), and chlorine-containing small alkenes and alkynes. Hydrolysis of aldehydes starts from the formation of hydrated molecules (4, 13, 31). The hydrogen atom attached to the carbon atom carrying the hydroxyl functional group can then be abstracted, and the resultant carbon-centered radical reacts with O2 to form a peroxyl radical. The peroxyl radical then decomposes into a carboxyl group. R-CHO f R-CH(OH)2R-C•(OH)2 f R-(OH)2C-OO• f R-COOH (5) The hydrolysis of carbonyl chloride also leads to carboxyl groups (5, 16, 32). The rate constant for this type of reaction is not yet well-known; however, it should be at least on the same order as aldehyde hydrolysis. Other species undergoing hydrolysis include alkyl radicals, containing β-halogens (33), and small molecules, containing chlorine and unsaturated carbon bonds such as Cl2CdO (34), HCtCCl, and ClCtCCl. Reaction with H2O2. In general, the reaction between H2O2 and contaminants cannot compete with radical reactions because they are too slow. One exception is glyoxylic acid, which has a rate constant as high as 105 L mol-1 s-1 (16). OHC-COOH + H2O2 f HO-O-CH(OH)COOH f HCOOH + H2O + CO2 (6) Termination. In order to reduce the redundancy of generated reactions, two criteria have been used to terminate the generation process. The first criterion limits the carbon chain length of the product to less than double the chain length of the parent compounds. This is based on the fact that most reactions in the hydroxyl radical-induced oxidation system, except the peroxyl radical combination, reduce the length of the carbon chain. The second criterion checks for the generation of terminating species. Terminating molecules are single carbon species that lead to inorganic species in the aqueous phase, such as HC(OCl), Cl2CdO, and •COOH. Their degradation mechanisms are built into the pathway generator, and no reaction generation will be attempted when they are produced. Currently, degradations of formic acid, oxalic acid, HC(OCl), Cl2CdO, and •COOH are built into the generator as shown in the Supporting Information. Complexity Setting. Although many possibilities exist for each type of species, normally there are one or two prevailing pathways. For example, a carbon-centered radical could undergo hydrogen and chlorine abstraction, double-bond addition, β-scission, hydrolysis, O2 addition, etc., but if it contains a Cl(OH)C functional group, it will eliminate HCl swiftly. Because of the high rate constant for this reaction, other mechanisms may be neglected. Yet because some minor pathways may lead to toxic byproducts, these pathways need to be enumerated. This is handled by a complexity setting in the algorithm. Each of the reaction mechanisms is associated with a priority flag, which enables an experienced chemist to set the priority of the mechanism, according to the target compounds of interest. At a low complexity setting, only mechanisms with high priorities will be considered, while a higher complexity setting considers lower priority mechanisms. The low complexity setting would give a compact pathway set for kinetic modeling for the fate of byproducts through prevailing mechanisms. The high complexity setting serves two purposes: (1) to explore reaction pathways for specific byproducts of special interest (e.g., some plausible toxic byproducts known based on experience or experimental observations of similar target compounds), and (2) to generate all possible byproducts for screening. VOL. 43, NO. 8, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Settings for Two Complexity Levels complexity 1 radicals react with organic molecules H abstraction addition addition chlorohydrin peroxyl radical peroxyl radical hydrolysis
HO•, Cl• (addition only)
added methyl radical
no abstraction from oxygen-bound hydrogen only to carbon-carbon unsaturated bonds exclude abstraction HCl elimination excludes O2 addition unimolecular exclude bimolecular pathways three bimolecular reactions, i.e., reactions 3a-c exclude other mechanisms
same same no exclusion no exclusion no exclusion reactions 3a-d no exclusion
TABLE 2. Number of Generated Species and Reactions for a Few Sample Compounds complexity 1
TCE methane ethane
complexity 2
species
reaction
species
reaction
35 11 25
41 17 78
87 84 827
384 402 35641
At low complexity, the algorithm only considers the prevailing pathways for a radical. This imposes the most stringent restriction on the reaction possibilities. For the sake of flexibility, the detailed settings may be adjusted according to the experiences of the researcher and compounds of interest. Table 1 lists the setting for two complexity levels. Table 2 shows the number of generated species and reactions for the degradation of three compounds under two complexity settings. The number of reactions may increase exponentially as the number of carbon atoms increase for complexity level 2. For the same compounds, the number of species and reactions increases significantly as the complexity level goes up. Case Study for Methane. The pathway for methane degradation was generated to validate the model. Because of its low solubility in water, the oxidation of methane by hydroxyl radicals in the aqueous phase has not been studied as intensely as in the gas phase reaction (35-37). The radiolysis study by Schuchmann and von Sonntag (37)was chosen to validate the pathway generator, where N2O/O2 was used to quench hydrated electrons and H atoms. Generated Pathway at Complexity Level 1. The model generated 17 reactions leading to the formation of four byproducts, including formaldehyde, hydrogen peroxide, formic acid, and methanol. The radical intermediates include hydroxyl radicals, methyl, methyl peroxide, methyloxyl, hydrated methyl, and methyl hydroperoxide radicals. Methyl hydroperoxide (14% yield) and dimethyl peroxide (2% yield) were not predicted because of the limit on the complexity setting. The pathways that lead to these byproducts and intermediates are consistent with those reported in the literature as discussed below. The degradation of methane starts from the abstraction of an H atom by •OH as in reaction 25. The generated methyl radical reacts with O2 to form a methylperoxyl radical, as in reaction 26. Methylperoxyl radicals cannot undergo unimolecular reactions such as HO2• elimination. Instead, they recombine into tetroxide that decays through the four mechanisms discussed in Peroxyl Radicals, as in reaction 3. Formaldehyde, methanol, and hydrogen peroxide are produced during the decay of methyl peroxyl radicals, as shown in reactions 27-29. This agrees with mechanisms reported in the literature (37). In a water solution, the methoxyl radical formed by the bimolecular decay of the methyl peroxyl radical undergoes 2834
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swift rearrangement into alkyl radicals as shown in reaction 30. This process is well-established in the literature (16, 28, 29). The hydroxymethyl radical then reacts with O2 and yields a hydroxymethyl peroxyl radical (32). This reaction is near diffusion controlled (k ) 4.9 × 109 M-1 s-1 (38)). Another well-known process for the decay of a hydroxyl methyl radical in an oxygen-free solution is a reaction with H2O2 (39). It is ignored because of the relatively low rate constant. •
CH2OH + H2O2 f CH2O + OH-+ •OH (6 × 104 M-1 s-1) (7)
The hydroxymethyl peroxyl radical decays mainly through unimolecular elimination of HO2•/O2•- and produces formaldehyde (33). As discussed in Reaction Rules in Aqueous Phases, HO2• and O2•- elimination are not differentiated for species with the OH group R to the peroxyl group. H2O2 is formed mainly through the bimolecular decay of methyl peroxide radicals. Schuchman et al. (37) also proposed the formation of H2O2 by bimolecular decay of hydroxymethyl peroxyl radicals, but this process cannot compete with the first-order HO2•/O2•- elimination process. Formaldehyde can hydrate to a considerable extent in an aqueous solution and eventually leads to the formation of formic acid as reported in the literature (40). In the absence of O2, H2O2 also reacts with hydrated formyl radicals to form formic acid at a rate constant of (3.5 ( 1.2) × 106 L mol-1 s-1 (41). However, this reaction would be suppressed by the O2 reaction in the case of advanced oxidation where O2 is generally abundant. HC•(OH)2+ H2O2 f HCOOH + HO•+ H2O
(8)
Decay of methanol also starts from H abstraction by •OH. Generally, abstraction occurs at the C-H bond because the O-H bond is stronger than the C-H bond. This leads to the formation of hydroxymethyl radicals, which decay through peroxyl radicals and form formic acid. At this level, abstraction from the O-H bond is neglected. Although the methoxy radical has been observed in the radiolysis of methanol, it accounted for only 7% of the methanol degradation and, thus, is neglected at the low complexity setting (42). Formic acid is a terminating species, and its decay pathways are built according to the literature (43). Because complexity level 1 does not allow the fourth mechanism for peroxyl decay (reaction 3d) or the cage recombination of alkoxyl radicals, dimethyl peroxide was not predicted. There are no reports indicating the importance of peroxide formation in peroxyl radical systems other than methane radiolysis, where dimethyl peroxide was detected and accounted for 2% of the methyl peroxyl radical yield. The other species that was not predicted is methyl hydroperoxide. Although its formation by the disproportionation of hydroxymethyl peroxyl radicals via reaction 9 is possible, it is widely assumed that organic hydroperoxide is formed by the reaction of the organic peroxyl radical with HO2•/O2•- (37).
2CH3O2• f CH3O2H + HCOOH
(9)
CH3O2• + HO2• ⁄ O2•-fCH3O2H + O2
(10)
In the radiolytic system, HO2• is produced mainly by the H atom through the reaction below, and its yield can be on the same order as the peroxyl radicals. Thus, reaction 10 is important. However, in the aqueous phase AOP, such as the H2O2/UV process, the source of HO2•/O2•- is the scavenging of the hydroxyl radical by H2O2, which should be limited by an appropriate dosage of H2O2. Therefore, the production of methyl hydroperoxide may not be important. Nevertheless, it will be meaningful to include the HO2•/O2• reactions in the future. Generated Pathway at Complexity 2. The generated degradation pathways for methane contain 84 species and 402 reactions. Dimethyl peroxide is predicted through the reaction below 2CH3-OO• f CH3OOCH3+ O2
•
CH3OH + HO f C H2OH + H2O
(12)
H abstractions by methyl radical (reactions 13 and 14) have not been reported in the literature because the O2 addition mechanism should be the overwhelming mechanism for the decay of CH3•. OHCH3 + CH3• f O•CH3+ CH4
(13)
OHCH3 + CH3• f C•H2OH + CH4
(14)
Methyl hydroperoxide is not predicted because the two mechanisms that lead to its formation involve either cage recombination or HO2• radicals. Neither mechanism is generally observed for most other compounds, and they are not enabled in the current algorithm. They could be enabled for chemicals belonging to a group where these mechanisms are prevailing. Case Study for Trichloroethylene. TCE degradation was predicted using both complexity 1 and 2. With a low complexity setting, 35 species and 41 reactions are predicted that include the decay of TCE and the fate of dichloroacetic acid (DCA), formic acid, phosgene, glyoxylic acid, formyl chloride, oxalic acid, and unstable intermediates. The predicted pathways for TCE and byproduct formation and decay are consistent with the experimental results (7, 44). The only exception is that monochloroacetic acid (MCA) was not predicted, which has been proven to be a byproduct of photolysis instead of the hydroxyl radical mechanism. Because most pathways have been extensively discussed in the literature, the focus in this section is given to several special pathways. The first step is the addition of a hydroxyl radical onto the most substituted carbon atom of the TCE molecule. The electrophilic hydroxyl radical preferably adds onto the carbon atom with one Cl substituent. At a low complexity setting, the pathway generator successfully predicted the selective addition process. With a high complexity setting, addition to both carbon atoms will be predicted. ClHC)CCl2 + •OH f Cl2C•-ClCH(OH)
CH4 + HO• f CH3• + H2O CH3• + O2 f CH3OO• CH3OO• + CH3OO• f 2CH3O• + O2 CH3OO• + CH3OO• f 2HCHO + H2O2 CH3OO• + CH3OO• f HCHO + CH3OH + O2 H3C-O• f H2C •-OH CH3OH + HO• f OHCH2C• + H2O OHCH2C• + O2 f OHCH2COO• OHCH2COO• f HCHO + HO2• HCHO + H2O f HCOOH HCHO + H2O f CH2(OH)2 CH2(OH)2 + HO• f HC•(OH)2 + H2O HC•(OH)2 + O2 f •OO-CH(OH)2 • OO-CH(OH)2 f HCOOH + HO2• HCOOH + HO• f CO2•- + H+ + H2O CO2•- + O2 f CO2 + O2•CO2•- + H2O2 f CO2 + H2O + HO•
25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
(11)
Enabling the abstraction of hydrogen from the O-H bond allows prediction of the pathway that accounts for 7% of the decay of methanol. •
TABLE 3. Predicted Reaction Pathways for Methane Decay
are slower. The pathway generator predicts elimination, instead of O2 addition. Cl2C•-ClCH(OH) f Cl2C•-CHO + H++ Cl-
(16)
The generated pathways for the decay of Cl2•C-CHO agree perfectly with what is reported in the literature (44), except for reaction 17below. •
OOCH(OH)2 f HCOOH + HO2•
(17)
Gehringer et al. (44)proposed bimolecular decay as shown below. 2•OOCH(OH)2 f 2HCOOH + H2O2+ O2
(18)
The fact that bimolecular radical termination is not enabled in the pathway generator accounts for this difference. The prediction agrees with the common perception that HO2• or O2•- elimination is the main mechanism for the decay of R-hydroxyalkylperoxyl radicals (ref 16and reference therein) because of its high rate constant (e.g., HO2• elimination for HOC(OH)2O2• was reported to be larger than 105 s-1 (45)). Unlike the methylperoxyl radical, which can decay by pathways 3a-c, the radical •OO-Cl2C-CHO does not have a C-H R attached to the peroxyl group, so that it can only undergo pathway 3c to yield the corresponding oxyl radical. 2•OO-Cl2C-CHO f 2•O-Cl2C-CHO + O2
(19)
The β-scission mechanism of oxyl radicals is generated for all possibilities. For example, •OCCl2-CHO can undergo both C-Cl scission, leading to formic acid, and C-C cleavage, leading to glyoxylic acid. •
O-Cl2C-CHO f O)CCl-CHO + Cl•
(20)
•
O-Cl2C-CHO f Cl2C)O + O)C•H
(21)
The formation of DCA starts from the addition of a chlorine radical to the double bond of TCE. O2
ClHC)CCl2+ Cl• f Cl2HC-C•Cl2 98 •OOCCl2-CHCl2
(15)
(22)
The chlorohydrin radical produced in reaction 15quickly eliminates HCl with a rate constant of 105 s-1 (16), which excludes the O2 addition pathway. This is partly because highly chlorinated radicals generally have a longer lifetime than unchlorinated radicals, and their reactions with oxygen
The peroxyl radical does not have an R-H or -OH, so it decays only by bimolecular decay, yielding an oxyl radical and O2. 2•OOCCl2-CHCl2 f 2Cl2HC-CCl2O•+ O2 VOL. 43, NO. 8, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
(23)
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The oxyl radical can undergo a homolytic splitting of the C-Cl bond and produce dichloroacetyl chloride, i.e., Cl2CHC(O)Cl, which hydrolyzes quickly to form DCA. This is the pathway that accounts for the formation of DCA via hydroxyl radical mechanisms (46). H2O
Cl2HC-CCl2O• f Cl2HC-CClO + Cl• 98 Cl2HCOOH + HCl (24) The pathway generator also generates pathways for the degradation of DCA, which are consistent with our previous work (13). The generated pathway for TCE at a low complexity setting is sketched in the Supporting Information. It is similar to the one that has been previously modeled (7). Therefore, the modeling results in the previous paper (7)can be used to compare with water treatment data.
Acknowledgments This work was supported by the Center for Clean Industrial and Treatment Technologies (CenCITT) sponsored by the U.S. Environmental Protection Agency at Michigan Technological University and National Science Foundation award NSF 0607332. Partial support was provided by the Arizona State University Foundation and the Richard Snell Presidential Chair Funds.
Supporting Information Available Definitions of molecular graphs, major methods for molecular objects, terminating and built-in reactions, sketch for TCE degradation pathway generated at a low complexity setting, and an example of input-output for the TCE case study. This material is available free of charge via the Internet at http://pubs.acs.org.
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