Elucidating the Elementary Reaction Pathways and Kinetics of

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Environmental Modeling

Elucidating the Elementary Reaction Pathways and Kinetics of Hydroxyl Radical-Induced Acetone Degradation in Aqueous Phase Advanced Oxidation Processes Divya Kamath, Stephen Peter Mezyk, and Daisuke Minakata Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00582 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 23, 2018

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Environmental Science & Technology

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Elucidating the Elementary Reaction Pathways and Kinetics of Hydroxyl Radical-Induced Acetone

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Degradation in Aqueous Phase Advanced Oxidation Processes

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Divya Kamath1, Stephen P. Mezyk 2, and Daisuke Minakata*1

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1

Department of Civil and Environmental Engineering, Michigan Technological University

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2

Department of Chemistry and Biochemistry, California State University, Long Beach, CA, 90840.

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*Corresponding author. Phone: +1-906-487-1830; fax: +1-906-487-2943, 1400 Townsend Drive,

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Houghton MI, 49931, U.S.

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Email address: [email protected]

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ACS Paragon Plus Environment


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Abstract

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Advanced oxidation processes (AOPs) that produce highly reactive hydroxyl radicals are promising

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methods to destroy aqueous organic contaminants. Hydroxyl radicals react rapidly and non-selectively

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with organic contaminants and degrade them into intermediates and transformation by-products. Past

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studies have indicated that peroxyl radical reactions are responsible for the formation of many

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intermediate radicals and transformation by-products. However, complex peroxyl radical reactions that

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produce identical transformation products make it difficult to experimentally study the elementary

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reaction pathways and kinetics. In this study, we used ab initio quantum mechanical calculations to

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identify the thermodynamically preferable elementary reaction pathways of hydroxyl radical-induced

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acetone degradation by calculating the free energies of the reaction and predicting the corresponding

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reaction rate constants by calculating the free energies of activation. In addition, we solved the ordinary

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differential equations for each species participating in the elementary reactions to predict the

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concentration profiles for acetone and its transformation by-products in an aqueous phase UV/hydrogen

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peroxide AOP. Our ab initio quantum mechanical calculations found an insignificant contribution of

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Russell reaction mechanisms of peroxyl radicals, but significant involvement of HO2• in the peroxyl

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radical reactions. The predicted concentration profiles were compared with experiments in the literature,

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validating our elementary reaction-based kinetic model.

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Introduction

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The identification of trace organic contaminants in natural waterways1,2 and during water and wastewater

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treatment processes3-5 has raised public concerns about the uncertain adverse effects these contaminants

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may pose to human health and ecosystems.6-8 Because of the increasing plans for wastewater reuse, the

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next generation water treatment infrastructure systems will face the challenge of dealing with these trace

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organic contaminants.9

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Advanced oxidation processes (AOPs), which produce highly reactive hydroxyl radicals (HO•) at

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room temperature and atmospheric pressure, are promising methods that can destroy a wide variety of

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organic contaminants.10-11 HO• rapidly and non-selectively reacts with most electron-rich sites on organic

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contaminants to initiate a series of radical-involved chain reactions that lead to various intermediates and

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transformation by-products.12,13 As a result, the intermediates and transformation by-products formed for

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a number of organic contaminants have been studied, and the degradation pathways of the organic

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contaminants have been proposed.14-19 Based on these experimentally identified reaction pathways and the

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literature-reported rate constants, some kinetic models have been developed to predict the time-dependent

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concentration profiles of a target organic contaminant and the transformation by-products.20-22

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Although a number of past experimental studies and kinetic models have revealed the major

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reaction pathways for some compounds, the fate of the transformation by-products has not yet been

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elucidated. In general, a parent organic compound is transformed into alcohols, ketones, aldehydes, and

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carboxylic compounds.14 The initial HO• reactions with aliphatic compounds, alkenes, and aromatic

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compounds form a carbon (C)-centered radical by an abstraction of H-atom or a hydroxycyclohexadienyl

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radical23-25 by an addition to a unsaturated C-C bond of alkenes and benzene rings, and this radical further

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reacts with the molecular oxygen dissolved in water to produce peroxyl radicals.

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compounds contain sulfur-, nitrogen-, or phosphorus–atom, a two-centered-three electron adduct is

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formed and this further reacts with a molecular oxygen to produce a peroxyl radical. Peroxyl radicals

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When organic


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undergo uni- and/or bimolecular decays to produce intermediate radicals (e.g., alkoxyl radicals) and

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transformation by-products (e.g., alcohols, ketones, and aldehydes). The transformation by-products

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further undergo HO• reactions to form carboxylic acids (Figure 1).24,25

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(Figure 1 goes here)

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While experimental and theoretical gas phase studies have been performed26-29 on the peroxyl radical

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reaction mechanisms, the complex peroxyl radical reactions that produce identical intermediate radicals

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and transformation by-products are difficult to experimentally study in an aqueous phase. The elementary

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reaction mechanisms have been postulated with possible transition state structures supported by water

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molecules. Furthermore, a very limited number of overall reaction rate constants for environmentally

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relevant aqueous phase peroxyl radical reactions that appeared in AOPs have been reported in the

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literature.30,31 Little kinetic information is available for the elementary reactions. Consequently, the

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majority of kinetic models only use estimated rate constants (k = 1.0 ×108 M-1s-1~2.0 ×109 M-1s-1) for the

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multiple pathways in the peroxyl radical bimolecular decay.20-22,32,33 For similar reasons, the alkoxyl

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radical reaction mechanisms (i.e., H atom shift or beta-scission of an oxyl radical) and the reaction rate

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constants have not well been incorporated in kinetic modeling and the mechanistic contribution to the

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overall decay of peroxyl radicals is not well understood. Because peroxyl and alkoxyl radicals are the key

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to understanding the formation of the transformation by-products, mechanistic studies are needed to

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reveal the elementary reaction pathways and their kinetics. Previously developed models that lacked

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these elementary reaction mechanisms were not able to predict some important transformation products

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(e.g., formaldehyde and glyoxylic acid from acetone degradation in UV/H2O2 AOP) that were identified

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by experiments.20

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Ab initio and density functional theory (DFT) quantum mechanical (QM) calculations are robust

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tools to identify elementary reaction pathways because they simulate the single reaction step for each

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reactant and calculate the reaction energy using statistical thermodynamics.34 QM calculations have been

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used to identify the aqueous phase HO•-induced reaction pathways and the kinetics by calculating the free

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react 35-37 act energy of the reactions, ∆Gaq,calc . and the free energy of the activation, ∆Gaq,calc , respectively. The

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direct calculation of the aqueous phase reaction rate constants has been limited to a small organic

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compound but it still has the uncertain solvation effects and requires the large computational power to

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obtain the reliable barrier energies.38 For example, it is required to have an accuracy of ±0.4 kcal/mol of

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act to predict the reaction rate constants within a difference of a factor of 2 from the experimental ∆Gaq,calc

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value using a conventional transition state theory.39 Thus, Minakata et al. developed series of the linear

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act free energy relationships (LFERs) that relate the experimental k to the theoretically calculated ∆Gaq,calc

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using an implicit solvation model for the HO• reaction and other radical reactions that occur in aqueous

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phase AOPs.23,31,40.41 The LFERs are useful for estimating the rate constants of elementary reactions for a

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wide variety of organic compounds with an accuracy of predicting the rate constants within a factor of

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five from experimental values, but they have never been used for the entire degradation pathway of one

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organic compound.

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In this study, we use QM calculations to identify the HO•-induced elementary pathways of

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acetone degradation and to predict the reaction rate constants. The transformation by-products (e.g.,

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pyruvic aldehyde, pyruvic acids, carboxylic acids, and glyoxylic acids) that are formed during the acetone

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degradation are also found in the pathways of other aliphatic and aromatic compounds with diverse

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structures.42 Therefore, elucidating the acetone degradation pathway and its kinetics will be helpful for

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understanding the degradation of many other compounds. Furthermore, the acetone degradation pathway

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has been studied experimentally, and the major and minor transformation by-products have been

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identified in a UV/H2O2 AOP.16-17 Based on the experimentally identified pathways, a computer-based

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kinetic model was developed to predict the degradation and formation of major by-products.20 It is noted

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that the previous kinetic model contained some lumped reaction pathways and only estimated rate

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constants for peroxyl and alkoxyl radical reaction mechanisms found in the previous literatures.

107

Therefore, our elementary reaction-based kinetic model can be compared to previous findings.

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108

We used three steps to develop our elementary reaction-based kinetic model. We first calculated

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react to identify the thermodynamically preferable elementary reaction pathways. Second, we ∆Gaq,calc

110

act calculated ∆Gaq,calc and used them to predict the reaction rate constants. Finally, we numerically solved

111

the ODEs to obtain the concentration profiles for acetone and its transformation by-products. These

112

profiles were compared to those that were obtained in past experiments and kinetics studies.

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Materials and Methods

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All of the QM calculations were performed with the Gaussian 09 revision D.02 program43 using the

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Michigan Tech high-performance cluster “Superior” and homemade LINUX workstations. The

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Gaussian-4 theory (G4)44 was used to optimize the electronic structures and calculate the frequencies in

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both the gas and aqueous phases. The aqueous phase structures and frequencies were obtained using an

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implicit polarizable continuum model [universal solvation model (SMD)]45. Previously, we verified the

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combination of G4 with the SMD model by successfully applying it to other aqueous phase radical-

120

involved reactions.31 The detailed calculation procedures for the transition state search, the aqueous phase

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free energies of activation and reaction, and the associated computational methods are found in the text of

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act Supporting Information (SI). The theoretically calculated ∆Gaq,calc values at 298 K were used to predict

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the reaction rate constant, k, based on the previously developed LFERs.31,40,41 For the acetone degradation

124

reactions, we used the LFERs for the elementary reactions including: (1) H abstraction from a C-H bond

125

by HO• 41; (2) molecular oxygen addition to a carbon-centered radical31; and (3) peroxyl radical uni-

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/bimolecular decays31 (see SI for additional details). For the reaction mechanisms (i.e., β-scission of a

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carbon-centered radical; H-shift of a carbon-centered radical; HO2• reaction; alkoxyl radicals, H2O2

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reaction, and hydrolysis) with rate constants that could not be estimated from the LFERs, we estimated

129

act them based on our ∆Gaq,calc values and the reported experimental k for similar reactions when available.

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The detailed description about the estimation is given in each reaction mechanism under Results and

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Discussion. Once the elementary reaction pathways and the corresponding reaction rate constants were

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identified, we numerically solved the ODEs at non-steady-state condition and non-constant pH by

133

modifying the original UV/H2O2 kinetic model46 with an addition of elementary reactions and

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corresponding reaction rate constants for the subsequent reactions after the initial HO• reaction with

135

acetone and compared to the experimental results reported by other researchers in the literature.17 The SI

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summarizes the experimental conditions. The sample deviation (SD) was calculated as shown in equation

137

(1) to evaluate the discrepancy.

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 1  N i  Cexp, j − Ccal, j  SDi =   ∑ N − 1 Cexp, j  i  j=1 

139

where i indicates the species, N is the total number of data points of species, Cexp,j and Ccal,j are the

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experimental and calculated concentrations of species i, respectively, and the j is the set of all times for

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which experimental data are available.

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Results and Discussion

143 144

Overall Results

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act react and ∆Gaq,calc values, the predicted rate constant, kcalc, and the experimentally obtained rate ∆Gaq,calc

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constants, kexp. The reaction numbers for preferred reaction pathways are underlined in bold. The

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optimized ground and transition state structures for each elementary reaction pathway are given in the SI.

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Figure 2 compares the predicted concentration profiles of acetone, H2O2, and 8 other major and minor

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transformation products to the profiles that were obtained via experiments reported by other researchers in

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the literature 17. Overall trend of major species by this model was satisfactory. The SD value was 0.23

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(0.15 of SD value obtained by previous kinetic model20) for H2O2, 0.35 for acetone (0.21), 0.30 for

(

)

   

2

(1)

Table 1 summarizes the identified major elementary reaction pathways, the theoretically calculated

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pyruvic aldehyde (0.23), 0.28 for acetic acid (0.39), 0.34 for pyruvic acid (0.49), 0.52 for oxalic acid

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(0.47), 0.51 for formic acid (0.40), 0.87 for formaldehyde (not available), 0.52 for hydroxyacetone (not

154

available), and 0.52 for glyoxylic acid (not available). The SD values obtained in this study are

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equivalent or better than those that were obtained by previous kinetic model20. Notably, that previous

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kinetic model did not include formaldehyde and glyoxylic acid but our elementary reaction based model

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was able to predict those concentration profiles reasonably well without estimating the reaction rate

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constants by fitting with the experimentally obtained time-dependent concentration profiles. It should be

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noted that the concentration profile of formaldehyde was still not consistent with the experimental. In the

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previously developed model20, the elementary reaction pathway for the hydration of

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formaldehyde was not considered. Instead, a lumped reaction pathway: HCHO + H2O →

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HCOOH via HO• with the estimated constant, 3.41×108 M-1s-1, was assigned in the kinetic

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model. The same was true for other aldehydes such as acetaldehyde. In our elementary reaction-

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based kinetic model, we considered the hydration of HCHO for the production of methylene

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glycol, CH(OH)2, which partially undergoes a much slower dimerization to dimethylene glycol,

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HOCH2OCH2OH62. The pKa value (-3.36) of HCHO hydration was well known and the kinetic

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rate constants were measured for base-catalyzed (3.24 ×106 M-1s-1), neutral (10 M-1s-1), and acid-

168

catalyzed (5.37 ×103 M-1s-1) hydrations63,64. In our simulated environment at acid pH, we used

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the acid-catalyzed hydration of HCHO. However, the predicted HCHO concentration profile was

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not consistent with the experimental observation (Figure 1) due to the potential missing

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elementary reactions for the formation of formaldehyde in the reaction time after 20 minutes. Minor

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discrepancies of concentration profiles for acetic acid, oxalic acid, and glyoxyalic acid may come from

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the ignorance of the photolysis of intermediates. The missing elementary reaction pathways and/or the

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inaccuracy of reaction rate constants from the LFERs may also cause the minor discrepancies for acetone,

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acetic acid and glyoxylic acid. The accuracy of LFERs was reported as the difference of factor of five

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from experimental rate constants. For example, the kexp values of HO• with acetone vary from 8.3×107 M-

177

1 -1

s to (2.1±0.6)×108 M-1s-1 12,18, whereas our predicted kcalc value from the LFER was 7.5×107 M-1s-1. The

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act estimated error resulting from the calculation of ∆Gaq,calc values with G4 and a SMD solvation model is

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±2.0 kcal/mol for neutral compounds and ±5.0 kcal/mol for ionized compounds, respectively44,45. Thus,

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the rate constants for ionized species of intermediates acids may cause relatively larger errors to the rate

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constant estimation than neutral compounds.

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Stefan and Bolton (1999) also performed a numerical kinetic simulation using a simple finite-

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difference method to solve the coupled differential equations from very simplified 15 reaction pathways

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with the estimated reaction rate constants.17 While the majority of these reactions included the HO•

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reactions with experimentally observed transformation products, one formation reaction of acetonyl

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peroxyl radical and the experimentally determined branching ratio of the peroxyl radical were used to

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simulate the profiles of byproducts. Overall trend of major species by this model was satisfactory.

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However, the kinetic simulations were unable to predict the decay of acetic acid, oxalic acid, and

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glyoxyalic acid due to the ignorance of the photolysis of intermediates. In this model development, the

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branching ratios of acetonyl peroxyl radical decays were determined based on the identification of

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products (e.g., pyruvic aldehyde, hydroxyacetone, formaldehyde, and acetic acid) (Figure S1). However,

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as was emphasized in Introduction, many other elementary reaction pathways are involved in the

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formation of these products and many of those elementary reactions have common products (Figure S2).

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Our methodology relying on the validated consistent quantum mechanical calculations provides

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elementary reaction mechanisms that may be very difficult to identify by experiments. Comparison of the

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branching ratios of acetonyl peroxyl radical’s decay between this and our models will be given in the

197

following part.

198 199

(Table 1 goes here)

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Hydroxyl Radical Reactions and Molecular Oxygen Additions

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In the presence of an excess dose of hydrogen peroxide in the bench-scale batch experiment16-17, the

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degradation of the target test compound, acetone, was only induced by HO•. In 200-300 nm wavelength,

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hydrogen peroxide at 15.6 mM of initial concentration absorbs the majority of photons (83.1%) as relative

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to acetone at 1.02 mM of initial concentration and the photolysis of acetone (4.7%) can be ignored.16 The

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initial H atom abstraction from one of the C-H bonds in acetone by HO• for the formation of a C-centered

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radical (reaction 1) followed by the addition of a triplet state molecular oxygen (reaction 2) for the

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act formation of a peroxyl radical are well known. Our theoretically calculated ∆Gaq,calc values and the

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estimated rate constants are presented in Table 1. The peroxyl radicals (acetonylperoxyl radical,

210

•

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reactions.24,25 In the following sections, detailed discussions on the elementary reaction mechanism and

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the rate constant prediction will be provided.

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Peroxyl Radical Reactions

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Disproportionation Reactions of Peroxyl Radicals

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When two peroxyl radicals come into contact via a head-to-head termination, a short-lived intermediate,

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tetroxide, is postulated to produce.24,25 In the presence of two explicit water molecules, we obtained 5.7

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act kcal/mol for ∆Gaq,calc (reaction 3). Two reaction rate constants for this reaction have been

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act experimentally measured and reported in the literature.32,47 Based on the ∆Gaq,calc for •OOCH2COCH3

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and the previously developed LFER31, this rate constant was estimated to be 7.9 × 108 M-1s-1. This value

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is very close to the experimentally determined overall kexp values of 8.0 × 108 M-1s-1 47and (7.3±1.3) × 108

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M-1s-1 32. In the following part, we investigated the •OOCH2COCH3 decay.

OOCH2COCH3, in this case) are known to undergo two major reactions: uni- and bimolecular

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Bimolecular Decay of Peroxyl Radicals

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If a peroxyl radical has an α-C-H bond (i.e., a primary or secondary peroxyl radical), an irreversible

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tetroxide decay occurs by a self-induced tetroxide homolysis. Our QM calculations found five major

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elementary reaction pathways for the bimolecular decay of •OOCH2COCH3: (1) the formation of two

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alkoxyl radicals and 3O2 (reaction 4); (2) the formation of H2O2 and two aldehydes (reaction 5) named the

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Bennett reaction65,66; (3) the formation of one aldehyde, an alcohol and 3O2 (reaction 6) named the Russell

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reaction67; (4) the formation of a trioxide (reaction 7), and (5) the formation of HO2• and one aldehyde

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act (reaction 8). For the reaction 4, we obtained a ∆Gaq,calc value of 5.5 kcal/mol in the absence of explicit

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act water molecule. Even though we included several explicit water molecules, the ∆Gaq,calc values did not

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significantly change because water molecules do not assist the formation of transition state. Based on the

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previously developed LFER31, this kcalc was determined to be 9.59 × 108 M-1s-1. The reaction 4 producing

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react two alkoxyl radicals (•OCH2COCH3) in the triplet state and 3O2 resulted in a ∆Gaq,calc value of -9.1

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kcal/mol and this is the thermodynamically preferable pathway. In contrast, the formation of a singlet

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react oxygen, 1O2, as well as 2 singlet-state alkoxyl radicals resulted in a ∆Gaq,calc value of 18.5 kcal/mol,

236

which indicated this thermodynamic pathway is not favorable, consistent with the experimentally

237

identified pathways24,25,47.

238

act For the second pathway, we obtained a ∆Gaq,calc value of 10.3 kcal/mol for the formation of

239

H2O2 and two pyruvic aldehydes (i.e., CH3COCHO) (reaction 5). This reaction is known as the Bennett

240

reaction.65,66 We included three explicit water molecules in this calculation, and obtained a value of 4.3

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act kcal/mol for ∆Gaq,calc . The water molecules formed hydrogen bonds with the oxygen in the carbonyl

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functional group and the other water molecules, and this stabilized the peroxyl radical, and reduced the

243

act value of ∆Gaq,calc . The transition states found in the absence and presence of the explicit water

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244

molecules (SI) were consistent with the ones that were previously postulated in the literature24 and

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involved two five-membered rings among the 2 oxygen atoms of 3O2, the oxygen atom of the aldehyde,

246

and the α-carbon and hydrogen of the aldehyde. The estimated kcalc based on the LFER is 2.81 × 109 M-1

247

s-1. Pyruvic aldehyde is the primary degradation intermediate and the concentration was peaked at around

248

10 minutes followed by a gradual decay17. Thus, the fast bimolecular decay of acetonyl peroxyl radical to

249

produce pyruvic aldehyde is consistent with the experimental observation. The formation of H2O2 was

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also postulated via the formation of two six-membered rings with the support of two explicit water

251

molecules.24 Our calculations found that the H abstraction from the α-hydrogen of the peroxyl radical by a

252

act water molecule requires a value of 26.1 kcal/mol for ∆Gaq,calc . We found similar transition states for the

253

conformers, and all the values were approximately the same. This finding indicates that the water-

254

assisted H2O2 production does not compete with the electrocyclic process for the formation of H2O2 and

255

two pyruvic aldehydes. This is possibly because the H atom abstraction from the C-H bond at the α-

256

position of the peroxyl radical requires a much larger energy than the abstraction by the inner oxygen of

257

react the tetroxide. The ∆Gaq,calc value for the second pathway that yields H2O2 and two pyruvic aldehydes is -

258

110.2 kcal/mol, which indicated that this pathway is a highly exothermic reaction because of the

259

significant hydration of H2O2 in water. The third tetroxide decay pathway is called the Russell mechanism,67 and the reaction yields an

260 261

alcohol and an aldehyde via a cyclic transition state (reaction 6). We calculated a value of 31.5 kcal/mol

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act for ∆Gaq,calc in the absence of explicit water molecule(s) for the acetonyl peroxyl radical bimolecular

263

decay that yields hydroxyacetone, CH3COCH2OH, and a triplet state of pyruvic aldehyde. This transition

264

state indicates that one of the terminal hydrogen atoms shifts to the terminal oxygen atom on the other

265

acetonyl peroxyl radical, and the O-O bond of the peroxyl radicals is simultaneously broken to produce

266

3

267

also be produced, but this has not been confirmed in an aqueous phase.24 The inclusion of one water

O2. Because of the spin conservation, a singlet carbonyl, an alcohol and a singlet state of oxygen can

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act molecule did not significantly reduce the value of ∆Gaq,calc (i.e., 29.4 kcal/mol) because this reaction is

269

not supported by any explicit water molecule(s). We also used other DFT methods and basis sets (i.e.,

270

M06-2X/cc-pVDZ and M05/cc-pVDZ) that are relevant to multi-reference states68 and investigated the

271

act Russell mechanism. Regardless of the inclusion of several explicit water molecules (1-3), the ∆Gaq,calc

272

act values did not significantly change (being ~40 kcal/mol for ∆Gaq,calc at the level of M06-2X/cc-pVDZ).

273

Similar observations have been reported by several theoretical studies in the gas phase.26-28 Zegota et al.

274

(1986)47 conducted a γ-radiolysis-based product study and concluded that the Russell mechanism for

275

acetonyl peroxyl radical bimolecular decay is minor (≈20%), and the significant amount of pyruvic

276

aldehyde formation came from the previously described Bennett mechanism (≈40-45%). It is noted that

277

these γ-radiolysis experiments were conducted in N2O/O2-saturated basic solutions of acetone

278

(pH 9.5-11.3) where HO2• rapidly dissociates to O2•- (pKa = 4.8 69). Thus, the HO2• reaction with

279

acetonyl peroxyl radical was not accounted. More detailed explanation will be given in Overall

280

act Reaction Pathway. Like other theoretical studies in the literature, our ∆Gaq,calc value for this pathway

281

indicates that the Russell reaction does not compete with the other peroxyl radical bimolecular decays,

282

which indicates that there may be other reaction pathways that contribute to the initial rapid formation of

283

hydroxyacetone observed by previous experiment17. We also investigated the Russell reaction

284

mechanism at the triplet state resulting from the intersystem crossing from the singlet state because this

285

pathway has been suggested for the gas phase methylperoxyl radical reactions29 or other alternative

286

reaction mechanisms (see the detailed discussion below) to the minor Russell pathway. In this case, our

287

act theoretically calculated ∆Gaq,calc value required for the triplet state Russell mechanism was 12.5

288

kcal/mol, and the k value was estimated to be 6.7 × 105 M-1s-1.

289 290

The fourth pathway is to produce a trioxide (CR3OOOH) and an aldehyde (reaction 7) because this pathway had the lowest energy barrier for the gas phase reaction of HO2• with CH3OO• to form

13



291

HOOOH and HCHO.28,29 The presence of these trioxides has not been experimentally identified because

292

of the high reactivity of trioxide with water to produce an alcohol. Our calculation found a value of 25.6

293

act kcal/mol for ∆Gaq,calc in the absence of explicit water molecule, and this energy did not change

294

significantly in the presence of explicit water molecule. Thus, the fourth pathway does not significantly

295

contribute to the aqueous phase tetroxide decay. The other possible mechanism is the formation of HO2•,

296

an alkoxyl radical (•OCH2COCH3), and a pyruvic aldehyde via a self-induced tetroxide decay (reaction 8).

297

act The calculated ∆Gaq,calc value was 16.7 kcal/mol in the absence of explicit water molecules. With two

298

act explicit water molecules, the ∆Gaq,calc values substantially decreased to 12.3 kcal/mol due to the

299

hydrogen bonds that formed with the surrounding water molecules. We were not able to identify the

300

act aqueous phase transition state in the presence of three explicit water molecules and thus the ∆Gaq,calc

301

value was estimated to be ~ 7 kcal/mol based on the gaseous-phase transition state and the

302

act value. ∆Ggas,calc

303

Subsequently, the HO2• readily reacts with •OCH2COCH3 and •OOCH2COCH3 in the solvent cage

304

via secondary reactions on the triplet state potential energy surface28,29 to produce the hydroxyacetone that

305

was the early transformation by-product from the acetone decay (reactions 9 and 10). We obtained values

306

act of 6.1 kcal/mol and 8.4 kcal/mol for ∆Gaq,calc for the reaction of HO2• with the acetonyl peroxyl radical

307

and alkoxyl radical, respectively. We did not develop an LFER for the HO2•/O2•- reactions, but the HO2•

308

rate constants were estimated to be 1.2 × 107 M-1s-1 (reaction 9) and 1.0 × 106 M-1 s-1 (reaction 10),

309

act respectively, based on the similar HO2•/O2•- reactions and calculated ∆Gaq,calc values. Schaefer et al.

310

(2012)32 estimated the reaction rate constants for HO2•/O2•- with the acetonyl peroxyl radical as 1.0 × 108

311

M-1s-1 and 1.0 × 109 M-1s-1, respectively, but they did not specify the reaction products. Bothe et al.

312

(1983) estimated the rate constant for the reaction of HO2• with an ethanol-derived peroxyl radical

14


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Page 15 of 39


313

[CH3CH(OH)O2•] to be of the order of 107 M-1s-1 for the formation of an ultimate product, acetic acid,

314

under steady-state irradiation conditions.70 A similar mechanism was examined via a theoretical

315

calculation of the singlet-state CH3CH2O2•,28,29 and the proposed self-induced acetonyl peroxide decay

316

was supported by Khursan (2014)71. He proposed a cyclic reaction mechanism for the formation of an

317

alcohol as an alternative pathway to the Russell mechanism. Further verification of our proposed

318

pathway will be discussed in the kinetic modeling. The formation of a triplet oxygen (3O) from the

319

reaction of HO2• with acetonyl peroxyl radical is also supported by the postulated pathway: the formation

320

of a triplet carbonyl 72, alcohol, and triplet ground state oxygen from the cyclic reaction of acetonyl

321

peroxyl radical24. One may wonder the opposite trends of estimated reaction rate constants and the

322

react values for the proposed reactions, Russell reaction, and bimolecular reactions of oxyl radicals ∆Gaq,calc

323

(reactions 6, 9, and 12) to those observed based on a linear free energy relationship (i.e., proportional

324

trend between kinetic rate constants and free energies of reactions)73. For many fast radical reactions,

325

kinetics often overrun thermodynamics74 and we also observed the similar opposite tread for chlorine-

326

radical reactions75.

327

Uni-molecular Decay of the Peroxyl Radical

328

A peroxyl radical undergoes a uni-molecular decay by eliminating HO2•/O2•-. The functional group (R)

329

adjacent to the peroxyl radical function (ROO•) significantly affects the stability of the positive charge

330

act created after the O2•- leaves. We obtained a ∆Gaq,calc value of 27.7 kcal/mol for the O2•- elimination of

331

the acetonyl peroxyl radical (reaction 11). Based on the previously determined LFER31, the first order

332

reaction rate constant was determined to be 0.62 s-1, which is consistent with the experimental rate

333

constant for a similar peroxyl radical uni-molecular decay (i.e., < 1 s-1)30. The lower reactivity of this

334

acetonyl peroxyl radical compared to those that have two alkoxyl groups or α-hydroxyalkyl groups (i.e.,

335

•

336

the methyl functional group.

OOCR1R2OH) and faster uni-decay (i.e., 103-106 s-1) is due to the carbonyl functional group adjacent to

15



337

Another possible uni-molecular decay reaction of peroxyl radicals is to produce a C-centered

338

radical via a cyclic transition state16. The acetonyl peroxyl radical undergoes a cyclic transition state, and

339

the hydrogen of the methyl group is abstracted by the peroxyl radical to form the •CH2COCH2OOH

340

act react radical. We obtained a ∆Gaq,calc value of 35.1 kcal/mol and a ∆Gaq,calc value of 7.6 kcal/mol, which

341

indicated that this reaction is not thermodynamically preferable. This C-centered radical further yields a

342

act ketene (CH2CO) and another C-centered radical (•CH2OOH)76. Based on the calculated ∆Gaq,calc value,

343

this uni-molecular rearrangement has a larger reaction barrier than the competing elimination of O2•-.

344

Guo et al.’s model20 generated reaction pathways based on past experimental observations that solely

345

relied on the hydrolysis of ketene for the formation of acetic acid. Therefore, without the uni-molecular

346

rearrangement of the peroxyl radical, acetic acid was not formed in their model. The elementary reaction

347

mechanisms for the formation of acetic acid will be discussed in a later section.

348

Alkoxyl Radical Reactions

349

As was shown in Reaction 4, the decay of peroxyl radicals produces other active radicals (i.e., alkoxyl

350

radical, •OCH2COCH3). Our calculations found two major reaction pathways: (1) the formation of an

351

alcohol and aldehyde via the Russell type mechanism (reaction 12) and (2) the formation of a C-centered

352

act act radical (reactions 13 and 14). We obtained a ∆Gaq,calc value of 17.0 kcal/mol and a ∆Gaq,calc value of

353

12.0 kcal/mol for the singlet and triplet states of the Russell type mechanism, respectively, in the absence

354

of any explicit water molecule(s). No rate constants have been experimentally measured for the alkoxyl

355

bimolecular decay. Based on our previous investigations on the peroxyl radical bimolecular decay via the

356

Russell mechanism, the alkoxyl radical bimolecular decay is insignificant compared to the uni-molecular

357

decay of the alkoxyl radical discussed below.

358

The alkoxyl radical undergoes either a H shift or β-scission to produce a C-centered radical. We

359

found two pathways for the H shift of CH3COCH2O•: (1) shifting the hydrogen atom from the α-position

16


Page 16 of 39

Page 17 of 39


360

of the alkoxyl radical to the oxygen radical of the alkoxyl radical (1,2-H shift) and (2) shifting the

361

hydrogen atom from the terminal methyl group to the oxygen radical of the alkoxyl radical (1,3-H shift).

362

act act The first pathway required a ∆Gaq,calc of 31.4 kcal/mol, and the second pathway required a ∆Gaq,calc of

363

19.0 kcal/mol in the absence of explicit water molecule(s). When we added 3 explicit water molecules to

364

stabilize the alkoxyl radical, hydrogen bonds formed between the water molecules and the carbonyl

365

functional group, the oxygen of the alkoxyl radical and the hydrogen of the methyl group. We obtained a

366

act of 13.5 kcal/mol and determined that the inclusion of explicit water molecules stabilizes the ∆Gaq,calc

367

act alkoxyl radicals by forming hydrogen bonds, reducing the ∆Gaq,calc value. In the aqueous phase, water

368

act molecules are thought to assist the H atom shift. Therefore, we also calculated the ∆Gaq,calc value for one

369

act and two water molecule-assisted transition states for the 1,2-H shift and obtained ∆Gaq,calc values of 32.4

370

kcal/mol and 45.1 kcal/mol, respectively, for each pathway. The theoretically calculated high reaction

371

barrier for the H shift of the alkoxyl radical does not explain its very fast kinetics (k > 5.0 × 105 s-1).24

372

Konya et al. (2000) proposed the initial formation of a radical anion/hydronium ion (i.e., RCH2O•-/H3O+)

373

pair followed by the collapse to a neutral product pair (i.e., R•CHOH/H2O) for the mechanism of water-

374

assisted 1,2-H shift of alkoxyl radical in the aqueous-phase.77 Our theoretical calculation of this proposed

375

act mechanism (i.e., charge transfer from a water molecule followed by H shift) gave a ∆Gaq,calc value of 0.9

376

kcal/mol and the estimated rate constant was 5.0 × 105 s-1 based on the experimental value78. In this case,

377

the alkoxyl radical coordinated with surrounding water molecule(s) does not sterically hinder the

378

movement of H from the methyl group to the alkoxyl group.

379

The alkoxyl radical also undergoes a β-scission reaction. Our calculations in the absence or

380

act presence of explicit water molecules revealed ∆Gaq,calc values of -0.19 kcal/mol (3 water molecules), 1.4

381

kcal/mol (2 water molecules) and 3.0 kcal/mol (no water molecules). Here, the water molecules stabilize

17



Page 18 of 39

382

the alkoxyl radical by forming hydrogen bonds with the alkoxyl radical oxygen. Compared to the H shift

383

act for the alkoxyl radical investigated above, the β-scission requires a substantially smaller ∆Gaq,calc value.

384

Our extensive literature search found one experimentally measured rate constant for the β-scission of tert-

385

butoxyl radical, •OC(CH3)3, (1.4 × 106 s-1) in the aqueous phase.79 Based on this experimental

386

investigation, we estimated the rate constant to be 1.4 × 106 s-1 for the β-scission of the •OCOCH3 alkoxyl

387

radical, which forms an acetyl radical (i.e., •COCH3) and formaldehyde (reaction 14) whose gradual

388

formation was observed by experiments (Figure 1). The acetyl radical either reacts with 3O2 to produce

389

•

390

reaction 15) or undergoes dissociation to produce CO and •CH3. However, the latter reaction requires a

391

react ∆Gaq,calc of 9.7 kcal/mol, thus is not thermodynamically preferable.

392

Reaction Pathways for the Formation of Acetic Acid

393

Two different experiments16,17,32 reported the acetic acid/acetate concentration increased with an increase

394

in the pyruvic acid and pyruvic aldehyde concentrations during acetone decay, which implies that the

395

formation of acetic acid/acetate results from the decay of either pyruvic acid or pyruvic aldehyde.

396

Therefore, we thermodynamically investigated this pathway to estimate these kinetics. The hydrolysis of

397

ketene was the major reaction that produced acetic acid (reaction 30) in the previously developed model19,

398

but the formation of ketene was not thermodynamically favorable in our QM calculations during the

399

peroxyl radical decay. In the aqueous phase, pyruvic aldehyde hydrolyzes to form α-ketopropanal

400

[CH3COCH(OH)2]. This geminal diol reacts with HO• to produce two types of C-centered radicals:

401

•

402

(reaction 17). The estimated k values for the corresponding reactions are 1.2 × 108 M-1s-1 and 1.5 × 108

403

M-1s-1, respectively, and, thus, these two reactions occur competitively. The latter reacts with 3O2 to

404

produce a peroxyl radical, CH3COC(OH)2OO• (reaction 18), ( ∆Gaq,calc = -6.7 kcal/mol), and the

act OOCOCH3 ( ∆Gaq,calc = -22.6 kcal/mol with an estimated k = 2.5 × 109 M-1s-1 based on the LFER,

CH2COCH(OH)2 ( ∆Gaq,calc = 8.8 kcal/mol) (reaction 16) and CH3CO•C(OH)2 ( ∆Gaq,calc = 8.3 kcal/mol) act

act

act

18


Page 19 of 39


405

estimated k value was 7.4 × 108 M-1s-1 based on the LFER. This peroxyl radical undergoes a uni-

406

molecular decay by eliminating HO2• to form pyruvic acid (CH3COCOOH)42 (reaction 19).

407

The ∆Gaq,calc value is 10.3 kcal/mol, and the k value is estimated to be 8.0 × 103 s-1. Because this peroxyl

408

radical has two hydroxide groups on the α-carbon, the elimination of HO2• rapidly occurs via the support

409

of the hydrogen in the hydroxide, and this pathway is dominant over the peroxyl radical bimolecular

410

decay. The rapid formation of pyruvic acid resulting from acetic acid is consistent with experimental

411

observation; simultaneous formation of both acids (Figure 1). The pyruvic acid dissociates, and is mostly

412

present as pyruvate (CH3COCOO-) at pH above the pKa value (2.5).80 Thus, pyruvate further reacts with

413

HO• via two major mechanisms: (1) H abstraction from the C-H of a methyl functional group to produce a

414

C-centered radical, •CH2COCOO-, ( ∆Gaq,calc = 8.6 kcal/mol) (reaction 20), and (2) HO• attack on the keto

415

functional group to produce an alkoxyl radical, •OC(OH)(CH3)COO-, ( ∆Gaq,calc = 8.9 kcal/mol) (reaction

416

21). The k value for the H abstraction is estimated to be 3.9 × 107 M-1 s-1 based on the LFER. We could

417

not estimate the k value for the HO• attack on the keto functional group of the ketone, but this rate

418

constant should be smaller (≈1.0 × 107 M-1s-1) than that for the H abstraction based on our theoretically

419

obtained ∆Gaq,calc . The alkoxyl radical rapidly undergoes a •COOH elimination to produce acetate. The

420

pyruvate ion also reacts with H2O2 in the dark at 0.11 M-1s-1 16, and the H2O2 reacts with the double bond

421

of the keto group ( ∆Gaq,calc = 31.8 kcal/mol) (reaction 22). Because the rate constant for the H2O2

422

reaction cannot be estimated based on our ∆Gaq,calc , we used experimental rate constants for the kinetic

423

modeling. Following the H2O2 attack, pyruvate is produced via decarboxylation. The pyruvic aldehyde

424

reacts with both HO• and H2O2 via mechanisms that are similar to those for pyruvate and produces

425

acetate. We obtained ∆Gaq,calc values of 7.0 kcal/mol and 4.8 kcal/mol for the H abstraction from the

426

methyl functional group (reaction 23) and the HO• attack on the keto functional group (reaction 24),

act

act

act

act

act

act

act

19



427

respectively. The H2O2 reaction requires a much higher ∆Gaq,calc value (31.8 kcal/mol) (reaction 25).

428

The pathway with the lowest energy barrier is the release of •CHO to produce acetic acid.

429

Overall Reaction Pathways

430

Based on the identified elementary reaction pathways for acetone degradation induced by HO•, the rest of

431

the major elementary reaction pathways identified in this study and their estimated kcalc values are

432

summarized in Table 1. The general scheme is the following: the initial HO• reaction via H abstraction to

433

form C-centered radicals, addition of molecular oxygen to the C-centered radicals to form peroxyl

434

radicals, peroxyl radical uni-/bimolecular decay to form aldehydes, alcohols, hydrogen peroxide, and

435

alkoxyl radicals, and peroxyl radical reactions with HO2•, and β-scission/H shift of alkoxyl radicals to

436

form C-centered radicals, aldehydes, ketones, and carboxylic acids (Figure S9). The aldehydes are

437

rapidly hydrolyzed to form geminal diols. The geminal diols, ketones, and carboxylic acids further react

438

with HO• in the subsequent reactions via the general scheme described above.

439

act

The distinctive difference between the experiment based kinetic model and our elementary

440

reaction based model was that the previously developed kinetic model20 used the bimolecular decay of a

441

peroxyl radical (reaction 6) as the major pathway to produce hydroxyacetone (i.e., k = 108 M-1 s-1) and did

442

not consider the reaction of HO2•. Our QM calculations revealed the insignificant contribution of this

443

Russell mechanism (k = 105 M-1 s-1) and indicated that the HO2• involved reactions are key in the

444

formation of hydroxyl acetone. Figure 3 represents the identified initial pathways of acetone degradation

445

induced by HO• with a focus on the first generation of transformation by-products via acetonyl peroxyl

446

and alkoxyl radical reactions. The reaction rates and their contributions (%) to the overall rate for the

447

decay of acetonyl peroxyl radical and alkoxyl radical were calculated based on the simulated

448

concentration of each species at reaction time 2 minutes and the reaction rate constants determined in this

449

study. First, the formation of alkoxyl radical via pathways 4 and 8 is 20% of overall acetonyl peroxyl

450

radical decay and this contribution is close to the value (15%) determined by experiment17. Second, the

20


Page 20 of 39

Page 21 of 39


451

Bennett reaction via pathway 5 contributes 27% of overall acetonyl peroxyl radical decay (25%17).

452

Finally, while Stephan and Bolton (1999)17 used 60% of acetonyl peroxyl radical decay contribution for

453

the formation of hydroxylacetone via the Russell reaction, our proposed pathway 9 via the reaction with

454

HO2• is 55% of overall peroxyl radical decay.

455


456

During the experiment by Stefan et al. (1996)17 and our numerical simulation, the pH dropped rapidly

457

from the initial pH of 5.9 to 3.6 (3.51 by simulation) at 30 minutes and then increased to 5.0 (4.5 by

458

simulation) at 80 minutes (Figure S3 in SI). The simulated non-dissociated HO2• concentration (4.8 of

459

pKa for HO2•/O2•-)69 was changed from 3.0×10-6 mole/L to 2.5×10-5 at 20 minutes and then dropped

460

rapidly to 3×10-8 at 80 minutes (Figure S4 in SI). The initial high concentration of HO2• is another

461

evidence for the initial formation of hydroxyl acetone resulting from the proposed reaction No. 9. The

462

drawbacks of the Russell mechanism have been previously discussed in the literature.71 The local

463

sensitivity analysis (see SI for detailed analysis) revealed that the acetonylperoxyl radical reaction with

464

HO2• (reaction 9) significantly affected the predicted concentration profile because its reaction rate

465

contribution is greater (approximately 10-9 mol•L-1s-1 reaction rate) than that of the other reactions [e.g.,

466

10-13 mol•L-1s-1 for the bimolecular decay of the peroxyl radical (reaction 6), 10-14 mol•L-1s-1 for the

467

reaction of the alkoxyl radical with HO2• (reaction 10), and 10-25 mol•L-1s-1 for the bimolecular decay of

468

the alkoxyl radical (reaction 12)]. This key reaction was determined to be the hydroxyacetone formation.

469

Although experimental identification of this pathway is challenging, our efforts are underway using a

470

combination of pulse radiolysis with product measurement techniques to experimentally elucidate this

471

pathway.

472 473

Environmental Implications

474

HO• induced elementary reaction pathways and corresponding reaction rate constants. Although

This study highlights the importance of an elementary reaction-based kinetic model based on the

21



475

HO•-induced reactions may not be significant in natural aquatic environment or soils under

476

sunlight irradiation because of the low concentration of HO• (e.g., ~10-16 M), similar reactions

477

occur in water droplets of cloud in atmosphere. In these oxygenated environments, peroxyl

478

radical reactions also play critical roles for the fate of contaminants and other species involving

479

in the processes32,33. Thus, findings from this study can be used to advance our understandings

480

the impacts from subsequent transformation products.

481

Associated Content

482

Supporting information of text for ab initio and DFT quantum mechanical calculations, linear free energy

483

relationships, experimental conditions, reaction pathways, pH and concentration profiles of radical

484

intermediates, comparison of predicted rate constants to experimental values, sensitivity analysis,

485

elementary reactions for UV/H2O2 process, overall core scheme of acetone degradation pathways, and

486

optimized molecular and radical structures in xyz-matrix forms as noted in the text. This material is

487

available free of charge via the Internet at http://pubs.acs.org.

488

Acknowledgements

489

This work was supported by the National Science Foundation Award: CBET-1435926. Any opinions,

490

findings, conclusions, or recommendations expressed in this publication are those of the authors and do

491

not necessarily reflect the view of the supporting organization. The authors appreciate the support from

492

the Michigan Tech HPC cluster ‘Superior’. The authors appreciate helpful comments and suggestions

493

from three anonymous reviewers. Finally, D.M. appreciates John Crittenden at Georgia Tech for allowing

494

us to modify the original AdOx model.

22


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References

496

1. Kolpin, D.W.; Furlong, E.T.; Meyer, M.T.; Thurman, E.M.; Zaugg, S.D.; Barber, L.B.; Buxton, H.T.

497

Pharmaceuticals, hormones, and other organic wastewater contaminants in U.S. streams, 1999-2000:

498

A national reconnaissance. Environ. Sci. Technol. 2002, 36, 1202-1211.

499

2. Schäfer, R.B.; von der Ohe, P.C.; Kühne, R.; Schuüuürmann, G.; Liess, M. Occurrence and toxicity

500

of 331 organic pollutants in large rivers of north Germany over a decade (1994 to 2004). Environ. Sci.

501

Technol. 2011, 45, 6167-6174.

502

3. Westerhoff, P.; Yoon, Y.; Snyder, S.; Wert, E. Fate of endocrine disruptor, pharmaceutical, and

503

personal care product chemicals during simulated drinking water treatment processes. Environ. Sci.

504

Technol. 2005, 39, 6649–6663.

505 506 507

4. Heidler, J.; Halden, R.U. Meta-analysis of mass balance examining chemical fate during wastewater treatment. Environ. Sci. & Technol. 2008, 42(17), 6324-6332. 5. Yoon, Y.; Westerhoff, P.; Snyder, S.A.; Wert, E.C. Nanofiltration and ultrafiltration of endocrine

508

disrupting compounds, pharmaceuticals and personal care products. J. Membrane Sci., 2005, 270(1-

509

2), 88-100.

510

6. Heyes, T.B.; Khoury, V.; Narayan, A.; Nazir, M.; Park, A.; Brown, T.; Adame, L.; Chan, E.;

511

Buchholz, D.; Stueve, T.; Gallipeau, S. Atrazine induces complete feminization and chemical

512

castration in make African clawed frogs (Xenopus laevis). PNAS, 2010, 107(10), 4612-4617.

513

7. NRC, 1999a, Hormonally Active Agents in the Environment, NAP, Washington, D.C.

514

8. NRC, 1999b, Identifying Future Drinking Water Contaminants, NAP, Washington, D.C.

515

9. NRC, 2012 Water Reuse Potential for expanding the nation’s water supply through reuse of

516 517 518

municipal wastewater. The national academy press, Washington D.C. 10. Glaze, W. H.; Kang, J-W.; Chapin, H. D. The chemistry of water treatment processes involving ozone, hydrogen peroxide and ultraviolet radiation. Ozone. Sci. Eng. 1987, 9, 335-352.

23



519

11. Glaze. W. H.; Kang, J-W. Advanced oxidation processes. Test of a kinetic model for the oxidation of

520

organic compounds with ozone and hydrogen peroxide in a semibatch reactor. Ind. Eng. Chem. Res.

521

1989, 28, 1580-1587.

522

12. Buxton, V. B.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Critical review of rate constants for

523

reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (•OH/•O-) in aqueous solution.

524

J. Phys. Chem. Ref. Data. 1988, 17(2), 513-795.

525

13. Herrmann, H.; Hoffmann, D.; Schaefer, T.; Bräuer, P.; Tilgner, A. Tropospheric aqueous-phase free-

526

radical chemistry: Radical sources, spectra, reaction kinetics and prediction tools. ChemPhysChem.,

527

2010, 11, 3796-3822.

528

14. Bolton, J. R.; Carter, S. R. Homogeneous photodegradation pf pollutants in contaminated water: An

529

introduction, Chap. 33, in G. R. Helz, R. G. Zepp, and D. G. Crosby (Eds), Aquatic and Surface

530

Photochemistry, 1994, CRC Press, Boca Raton, FL.

531 532

15. Stefan, M. I.; Bolton, J. R. Mechanism of the degradation of 1,4-Dioxane in dilute aqueous solution using the UV/hydrogen peroxide process. Environ. Sci.& Technol. 1998, 32, 1588-1595.

533

16. Stefan, M.I.; Hoy, A.R.; Bolton, J.R. Kinetics and mechanisms of the degradation and mineralization

534

of acetone in dilute aqueous solution sensitized by the UV photolysis of hydrogen peroxide. Environ.

535

Sic. Technol. 1996, 30, 2382-2390.

536 537 538 539 540 541

17. Stefan, M. I.; Bolton, J. R. Reinvestigation of the acetone degradation mechanisms in dilute aqueous solution by the UV/H2O2 process. Environ. Sci. & Technol. 1999, 33, 870-873. 18. Gligorovski, S.; Rousse, D.; George, C.H.; Herrmann, H. Rate constants for the OH reactions with oxygenated organic compounds in aqueous solution. Int. J. Chem. Kinet. 2009, 309-326. 19. Stefan, M. I.; Mack, J.; Bolton, J. R. Degradation pathways during the treatment of methyl tert-butyl ether by the UV/H2O2 process. Environ. Sci. & Technol. 2000, 34, 650-658.

542

20. Guo, X.; Minakata, D.; Junfeng, N.; Crittenden, J.C. Computer-based first-principles kinetic

543

modeling of degradation pathways and byproduct fates in aqueous phase advanced oxidation

544

processes. Environ. Sci. & Technol. 2014, 48(10), 5718-5725.

24


Page 24 of 39

Page 25 of 39

545 546 547


21. Guo, X.; Minakata, D.; Crittenden, J. On-the-Fly Kinetic Monte Carlo Simulation of Aqueous Phase Advanced Oxidation Processes. Environ. Sci. & Technol. 2015, 49(15), 9230-9236. 22. Guo, X.; Minakata, D.; Crittenden, J. Computer-Based First-Principles Kinetic Monte Carlo

548

Simulation of Polyethylene Glycol Degradation in Aqueous Phase UV/H2O2 Advanced Oxidation

549

Process. Environ. Sci. Technol. 2014, 48(18), 10813-10820.

550

23. Minakata, D.; Song, W.; Mezyk, S.P.; Cooper, W.J. Experimental and theoretical studies on aqueous-

551

phase reactivity of hydroxyl radicals with multiple carboxylated and hydroxylated benzene

552

compounds. PCCP, 2015, 17, 11796-11812.

553

24. von Sonntag, C.; Schuchmann, H-P. The elucidation of peroxyl radical reactions in aqueous solution

554

with the help of radiation chemical methods. Angew. Chem. Int. Ed. Engl. 1991, 30, 1229-1253.

555

25. von Sonntag, C.; Schuchmann, H-P. in Peroxyl radicals, Z. Alfassi, Editor. 1997, John Wiley &

556 557 558 559 560 561 562

Sons: New York. 26. Vereecken, L.; Francisco, J.S. Theoretical studies of atmospheric reaction mechanisms in the trosphere. Chem. Soc. Rev., 2012, 41, 6259-6293. 27. Zhang, P.; Wang, W.; Zhang, T.; Chen, L.; Du, Y.; Li, C.; Lü, J. Theoretical study on the mechanism and kinetics for the self-reaction of C2H5O2 radicals. J. Phys. Chem. A. 2012, 116, 4610-4620. 28. Liang, Y-N.; Li, J.; Wang, Q-D.; Wang, F.; Li, X-Y. Computational study of the reaction mechanism of the methylperoxy self-reaction. J. Phys. Chem. A. 2011, 115, 13534-13541.

563

29. Ghigo, G.; Maranzana, A.; Tonachini, G. Combustion and atmospheric oxidation of hydrocarbons:

564

Theoretical study of the methyl peroxyl self-reaction. J. Chem. Phys. 2003, 118(23), 10575-10583.

565

30. Neta, P.; Huie, R. E.; Ross, A. B. Rate constants for reactions of peroxyl radicals in fluid solutions. J.

566 567

Phys. Chem. Ref. Data. 1990, 19(2), 413−515. 31. Minakata, D.; Mezyk, S.P.; Jones, J.W.; Daws, B.R.; Crittenden, J.C. Development of linear free

568

energy relationships for aqueous phase radical-involved chemical reactions. Environ. Sci. Technol.

569

2014, 48, 13925-13932.

25



570

32. Schaefer, T.; Schindelka, J.; Hoffmann, D.; Herrmann, H. Laboratory kinetic and mechanistic studies

571

on the OH-initiated oxidation of acetone in aqueous solution. J. Phys. Chem. A 2012, 116,

572

6317−6326.

573 574 575 576 577

33. Rodigast, M.; Mutzel, A.; Schindelka, J.; Herrmann, H. A new source of methylglyoxal in the aqueous phase. Atmos. Chem. Phys. 2016, 16, 2689-2702. 34. Gibbs, J.W. Elementary Principles in Statistical Mechanics, 1902 Charles Scribner’s Sons. New York. 35. An, T.; Guo, Y.; Li, G.; Kamat, P.V.; Peller, J.; Joyce, M.V. Kinetics and mechanisms of •OH

578

mediated degradation of dimethyl phthalate in aqueous solution: Experimental and theoretical studies.

579

Environ. Sci. Technol. 2014, 48, 641-648.

580

36. Xiao, R.; Noerpel, M.; Luk, H.L.; Wei, Z.; Spinney, R. Thermodynamic and kinetic study of

581

ibuprofen with hydroxyl radical: A density functional theory approach. Int. J. Quantum. Chem. 2014,

582

114, 74-83.

583

37. Liu, S.; Zhou, X.; Han, W.; Li, J.; Sun, X.; Shen, J.; Wang, L. Theoretical and experimental insights

584

into the •OH-mediated mineralization mechanism of flutriafol. Electrochimica Acta. 2017, 235, 223-

585

232.

586

38. Galano, A.; Alvarez-Idaboy, R. Kinetics of radical-molecule reactions in aqueous solution: A

587

benchmark study of the performance of density functional theory. J. Comp. Chem. 2014, 35, 2019-

588

2026.

589 590

39. Eyring, H.; Gershinowitz, H.; Sun, C. E. The absolute rate of homogeneous atomic reactions.

J. Chem. Phys. 1935, 3, 786−796.

591

40. Minakata, D.; Song, W.; Crittenden, J. Reactivity of aqueous phase hydroxyl radical with halogenated

592

carboxylate anions: Experimental and theoretical studies. Environ. Sci. Technol. 2011, 45, 6057-6065.

26


Page 26 of 39

Page 27 of 39


593

41. Minakata, D.; Crittenden, J. Linear Free Energy Relationships between the Aqueous Phase Hydroxyl

594

Radical (HO•) Reaction Rate Constants and the Free Energy of Activation. Environ. Sci. Technol.

595

2011, 45, 3479-3486.

596

42. Cooper, W.J.; Cramer, C.J.; Martin, N.H.; Mezyk, S.P.; O’Shea, K.E.; von Sonntag, C. Free radical

597

mechanisms for the treatment of methyl tert-butyl ether (MtBE) via advanced oxidation/reductive

598

processes in aqueous solutions. Chem. Rev. 2009, 109(3), 1302-1345.

599

43. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.;

600

Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.;

601

Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.;

602

Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.;

603

Vreven, T.; Montgomery, Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.;

604

Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant,

605

J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J.

606

B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;

607

Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G.

608

A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J.

609

V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision D.1; Gaussian, Inc., Wallingford CT, 2009.

610 611 612

44. Curtiss, L. A.; Redfern, P. C.; Raghavachari, K. Gaussian-4 theory. J. Chem. Phys. 2007, 126, 084108. 45. Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. Universal solvation model based on solute electron

613

density and on a continuum model of the solvent defined by the bulk dielectric constant and atomic

614

surface tensions. J. Phys. Chem. B 2009, 113, 6378−6396.

615 616

46. Crittenden, J.C.; Hu, S.; Hand, W. D.; Green, A. S. A kinetic model for H2O2/UV process in a completely mixed match reactor. Wat. Res. 1999, 33(10), 2315-2328.

27



617

47. Zegota, H.; Schuchmann, M.N.; Schulz, D.; von Sonntag, C. Acetonyl radicals, CH3COCH2O2: A

618

study on the γ-radiolysis and pulse radiolysis of acetone in oxygenated aqueous solutions. Zeitschrift

619

fur Naturforschung B. 1986, 41(8), 1015-1022.

620 621 622 623 624

48. Kraljic, I. In The Chemistry of Ionization and Excitation; Johnson, G.R.A., Scholes, G., Ed.: Taylor and Francis, Ltd.: London, 1967, p 303. 49. Park, S.E.; Joo, H.; Kang, J. W. Photodegradaton of methyl tertiary butyl ether (MtBE) vapor with immobilized titanium dioxide, Sol. Energy Mater. Sol. Cells, 2003, 80(1), 73-84. 50. Ervens, B.; Gligorovski, S.; Herrmann, H. Teperature-dependent rate constants for hydroxyl radical

625

reactions with organic compounds in aqueous solution. Phys. Chem. Chem. Phys., 2003, 5, 1811-

626

1824.

627

51. Monod, A.; Poulain, L.; Grubert, S.; Voisin, D.; Wortham, H. Kinetics of OH-initiated oxidation of

628

oxygenated organic compounds in the aqueous phase: new rate constants, structure-activity

629

relationships and atmospheric implications. Atmos. Environ. 2005, 39, 7667-7688.

630 631 632 633 634 635 636 637 638 639 640 641

52. Bothe, E.; Dessouki, A.M.; Schulte-Frohlinde, D. Rate and mechanism of the ketene hydrolysis in aqueous solution. J. Phys. Chem. 1980, 84, 3270-3272. 53. Marchaj, A.; Kelley, D.G.; Bakac, A.; Espenson, J.H. Kinetics of the reactions between alkyl radicals and molecular oxygen in aqueous solution. J. Phys. Chem. 1991, 95, 4440-4441. 54. Schuchmann, H-P.; von Sonntag, C. Photolysis at 185 nm of dimethyl ether in aqueous solution: Involvement of the hydroxymethyl radical. J. Photochem. 1981, 16, 289-295. 55. Adams, G.E.; Wilson, R.L. Pulse radiolysis studies on the oxidation of organic radicals in aqueous solution. Trans. Faraday Soc. 1969, 65, 2981-2987. 56. Chin, M.; Wine, P.H. Aquatic and Surface Photochemistry, G.R. Helz, R.G. Zepp and D.G. Crosby (eds.), CRC Press, Inc., Boca Raton, FL, 1994, p.85-96 57. Bothe, E.; Schulte-Frohlinde, D. Reaction of dihydroxymethyl radical with molecular oxygen in aqueous solution. Zeitschrift A Naturforschung B. 1980, 1035-1039.

28


Page 28 of 39

Page 29 of 39

642


58. Bothe, E.; Behrens, G.; Schulte-Frohlinde, D. Mechanism of the first order decay of 2-hydroxy-

643

propyl-2-peroxyl radicals and of O2- formation in aqueous solution. Z. Naturforsch., Teil B. 1977,

644

32B. 886-889.

645 646

59. Josimovic, L.R.; Draganic, I.G.; Markovic, V.M.; Glas. Hem. Drus. Radiolysis of aqueous solutions of acetic acid containing oxygen. Beograd, 1976, 41, 75-79.

647

60. Schuchmann, M.N.; Zegota, H.; von Sonntag, C. Acetate peroxyl radicals, •O2CH2COO2-: A study on

648

the γ-radiolysis and pulse radiolysis of acetate in oxygenated aqueous solutions. Z. Naturforsch. B.

649

Anorg. Chem., Org. Chem. 1985, 40B: 215-221.

650 651 652 653 654 655 656

61. Getoff, N.; Schwörer, F.; Markovic, V.M.; Sehested, K.; Nielsen, S.O. Pulse radiolysis of oxalic acid and oxylates. J. Phys. Chem. 1971, 75(6), 749-755. 62. Rivlin, M.; Eliav, U.; Navon, G. NMR studies of the equilibria and reaction rates in aqueous

solutions of formaldehyde. J. Phys. Chem. B. 2015, 119(12), 4479-4487. 63. Guthrie, J.P. Hydration of carbonyl compounds, an analysis in terms of multidimensional

Marcus theory. JACS., 2000, 122, 5529-5538. 64. Greenzaid, P.; Luz, Z.; Samuel, D. N.M.R. study of reversible hydration of aliphatic

657

aldehydes and ketones. Part 3.-Isoropic oxygen exchange of acetone. Trans. Faraday. Soc.,

658

1968, 64, 2780-2786.

659 660 661

65. Bennett, J.E.; Summers, R. Product studies of the mutual termination reactions of sec-alkylperoxy radicals: Evidence for non-cyclic termination. Can. J. Chem. 1974, 52, 1377-1379. 66. Bennett, J.E.; Brunton, G.; Smith, J.R.L.; Salmon, T.M.F.; Wassington, D.J. Reactions of

662

alkylperoxyl radicals in solution. Part 2.- A kinetic and product study of self-reactions of 2-

663

propylperoxyl radicals between 253 and 323 K. J. Chem. Soc. Faraday Trans. I. 1987, 83, 2433-

664

2447.

665 666

67. Russell, G.A. Deuterium-isotope effects in the autoxidation of aralkyl hydrocarbons. Mechanism of the interaction of peroxy radicals. JACS, 1957, 79(14), 3871-3877.

29



667

68. Zhao, Y.; Schultz, N.E.; Truhlar, D.G. Design of density functionals by combining the method of

668

constraint satisfaction with parametrization for thermochemistry, thermochemical kinetics, and

669

noncovalent interactions. JCTC, 2006, 2, 364-382.

670 671

69. Bielski, H. J.; Benon, H. J.; Cabelli, D. E.; Ravindra, L. A.; Alberta, A. B. Reactivity of perhydroxyl/superoxide radicals in aqueous solution. J. Phys. Chem. Ref. Data. 1985, 14, 1041-1100.

672

70. Bothe, E.; Schuchmann, M.N.; Schulte-Frohlinde, D.; von Sonntag, C. Hydroxyl radical-induced

673

oxidation of ethanol in oxygenated aqueous solutions. A pulse radiolysis and product study. Z.

674

Naturforsch. 1983, 38b, 212-219.

675 676 677 678 679 680 681

71. Khursan, S.L. Organic tetroxides and mechanism of peroxy radical recombination. PATAI’s Chemistry of Functional Groups. 2014, John Wiley & Sons, Ltd. 72. Lee, S.-H.; Mendenhall, G.D. Relative yields of excited ketones from self-reactions of alkoxyl and alkylperoxyl radical pairs. JACS, 1988, 110, 4318-4323. 73. Brezonik, P. L. Chemical Kinetics and Process Dynamics in Aqueous Systems; Lewis

Publishers: Boca Raton, FL, 2002. 74. Naumov, S.; von Sonntag, C. standard gibbs free energies of reactions of ozone with free

682

radicals in aqueous solution: Quantum chemical calculations. Environ. Sci. Technol. 2011,

683

45, 9195−9204.

684

75. Minakata, D.; Kamath, D.; Maetzold S. Mechanistic insight into the reactivity of chlorine-

685

derived radicals in the aqueous-phase UV/chlorine advanced oxidation process: Quantum

686

mechanical calculations. Environ. Sci. Technol. 2017, 51(12), 6918-6926.

687 688 689 690

76. Volman, D. H.; Swanson, L. W. The photochemical decomposition of acetone in aqueous solutions of allyl alcohol at 2537 Å. J. Am. Chem. Soc. 1960, 82(16), 4141-4144. 77. Konya, K.G.; Paul, T.; Lin, S.; Lusztyk, J.; Ingold, K.U. Laser flash photolysis studies on the first superoxide thermal source. First direct measurements of the rates of solvent-assisted 1,2-Hydrogen

30


Page 30 of 39

Page 31 of 39


691

atom shifts and a proposed new mechanism for this unusual rearrangement. JACS, 2000, 122, 7518-

692

7527.

693 694 695 696 697

78. Schuchmann, H-P.; von Sonntag, C. Methylperoxyl radicals: A study of the γ-radiolysis of methane in oxygenated aqueous solutions. Z. Naturforschung, 1984, 39b, 217-221. 79. Erben-Russ, M.; Michel, C.; Bors, W.; Saran, M. Absolute rate constants of alkoxyl radical reactions in aqueous solution. J. Phys. Chem. 1987, 91, 2362-2365. 80. Dawson, R.M. Data for biochemical research. Oxford, Clarendon Press, 1959.

698 699

31



700

Figure and Table Captions:

701

Figure 1: Known and unknown reaction pathways of organic compound degradation induced by hydroxyl

702

radicals in aqueous phase AOPs.

703

Figure 2: Predicted concentration profiles for acetone and the major transformation products compared to

704

the experimental observations that were reprinted with permission from 17. Copyright 1999 American

705

Chemical Society.

706

Figure 3: Calculated branching ratio of acetonyl peroxyl and alkoxyl radicals based on the findings in this

707

study.

708

Table 1: List of the identified, major elementary reaction pathways, calculated free energies of activation,

709

and the estimated reaction rate constants for acetone degradation induced by hydroxyl radicals.

710

32


Page 32 of 39

Page 33 of 39


H-atom abstraction from C-H of aliphatic compounds Parent compound + HO

HO• addition to C=C bond of alkenes

O2 addition C-centered Radical

Peroxyl Radical 2σ/1σ* two-center-threeelectron (2c-3e) adduct

Ring opening

HO• interactions with S-, N-, or P-atom-containing compounds

711 712

HO• addition to C=C bond of aromatic compounds

Uni-/bimolecular decay Disproportionation HO2/O2 -

Figure 1

713

33


β-scission, H shift HO2/O2 -

Alkoxyl Radicals Hydrolysis HO

Aldehydes, Alcohol, Ketones, Carboxylic acids


Page 34 of 39

714

1.2

16 Acetone(exp) Acetate(exp)

14

Oxalate(exp)

1

Pyruvic acid(exp)

12

Pyruvic Aldehyde (exp) Acetone (calc)

10

Acetate (calc) Oxalate (calc)

0.6

8

pyruvate (calc) pyruvic aldehyde (calc)

6

H2O2 (exp)

0.4

H2O2 (calc)

4

0.2

2

0

0 0

10

20

30

40

50

60

70

80

Time, minutes

715 0.06

Formic Acid (exp) Hydroxyacetone (exp)

0.05

Glyoxylic Acid (exp)

Concentration, mM

Formaldehyde (exp) Formic acid (calc)

0.04

hydroxyacetone (calc) Glyoxylic acid (calc)

0.03

formaldehyde (calc)

0.02

0.01

0 0

716 717

10

20

30 40 50 Time, minutes

60

70

Figure 2

34


80

H2O2, mM

Concentration, mM

0.8

Page 35 of 39


CH3COCH3 HO• O2 CH3COCH2OO•

r =1.1×10-7 mole/Ls [20%: pathway 4 (17%) and pathway 8 (3%)]

CH3COCH2O• r =5.7×10-8 mole/Ls [28%: pathway 13] •CH(OH)COCH 3

+HO2•

r =1.5×10-7 mole/Ls [72%: pathway 14] •COCH 3

+

r =1.5×10-7 mole/Ls [27 %: pathway 5 (24%) and pathway 8 (3%)]

CH3COCHO

r =3.1×10-7 mole/Ls [55%: pathway 9]

CH3COCH2OH

HCHO

718 719

r =6.1×10-9 mole/Ls [1.1%: pathway 11]

Figure 3

720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740

35


CH2C=C+OCH3 + O2•-


741 742

Table 1

36


Page 36 of 39

Page 37 of 39


Reaction Reaction No. class 1 2

HO radical O2 addition

CH3COCH3 + HO → CH2COCH3 + H2O

CH2COCH3 + O2 → OOCH2 COCH3

-17.4

react

3

OOCH2COCH3 + OOCH2COCH3 → CH3COCH2 OOOOCH2COCH3

7.9 ⋅ 10

8, b

OOCH2COCH3 + OOCH2COCH3 → 2OCH2 COCH3 + O 2

5.5

-9.1

9.6 ⋅ 10

8, b

5

OOCH2COCH3 + OOCH2COCH3 → 2CH3 COCHO + H2O 2

4.3

-110.2

1.4 ⋅ 10

9, b

6

6.7 ⋅ 10

5, b

Peroxyl radicals

5.7

3

3

OOCH2COCH3 + OOCH2COCH3 → CH3COCHO + CH3COCH2OH + O2

12.5 (triplet) -93.4

OOCH2COCH3 + OOCH2COCH3 → CH3COCHO + CH3COCH2OOOH

25.6

-53.3

< 1,

8

OOCH2COCH3 + OOCH2COCH3 → OCH2COCH3 + HO2 + CH3COCHO

~7

-43.7

1.9 ⋅ 10

8, b

9

6.1 (triplet)

0.48

1.2 ⋅ 10

7, c

10

OCH2COCH3 + HO2 → CH3COCH2OH + O 2

8.4 (triplet)

-49.8

1.0 × 10

11

OOCH2COCH3 → CH2=C OCH3 + O2

27.7

-71.3

0.62 s

3

3

OOCH2COCH3 + HO 2 → CH3 COCH2OH + O + O2 3

+

-

OCH2COCH3 + OCH2COCH3 → CH3COCHO + CH3COCH2OH

12 (triplet)

-84.3

2.2 ⋅ 10

13

1,2 H shift

OCH2COCH3 → CH(OH)COCH3

0.9

-23.9

5.5 ⋅ 10 s

β-scission

OCH2COCH3 → COCH3 + HCHO

-0.19

-3.5

1.4 ⋅ 10 s

O2 addition

-34.6

-39.3

2.5 ⋅ 10

9, b

CH3COCH(OH)2 + HO → CH2COCH(OH)2 + H2O

8.8

-26.0

1.2 ⋅ 10

8, a

CH3COCH(OH)2 + HO → C(OH)2COCH3 + H2 O

8.3

-43.0

1.5 ⋅ 10

8, a

-6.7

-9.5

7.4 ⋅ 10

8, b

10.3

-1.5

8.0 ⋅ 10 s

8.6

-16.4

3.9 ⋅ 10

7, a

1.0 ⋅ 10

7, a

15 16 17

HO radical

18

O2 addition

19

Peroxyl radical

20 21 22 23 24 25 26 27

HO radical H2O2 reaction

HO radical H2O2 reaction HO radical

Peroxyl radical

30 31

C(OH)2COCH3 + O 2 → OOC(OH)2COCH3

OOC(OH)2 COCH3 → HO2 + CH3COCOOH -

28 29

COCH3 + O 2 → OOCOCH3

-

CH3COCOO + HO → CH2COCOO + H2O -

CH3COCOO + HO → OC(OH)(CH3)COO -

-

8.9

-

23.1

c

0.11,

7.43 ⋅ 10

CH3COCHO + HO → OC(OH)(CH3)CHO

4.8

-7.2

5.0 ⋅ 10

CH3COCHO + H2O 2 → CH3COO + HCOO + H2O

31.8

3.4

0.2,

CH3COCH2 OH + HO → CH2COCH2OH + H2O

9.0

-25.2

1.0 ⋅ 10

8, a

7.2

-38.3

5.0 ⋅ 10

8, a

1.5

-23.8

3

7, a

OOCOCH3 + OOCOCH3 → 2 OCOCH3 + O 2

8.3 ⋅ 10

9, b

OOCOCH3 + HO 2 → CH3 COOH + O + O2

19.4 (triplet) -10.9

2.0 ⋅ 10

6, c

OOCOCH3 → HO2 + CH2 CO

31.8

16.8

1.82 s

17 8

-1, c

CH2CO + H2O → CH3COOH

32.0

-21.3

44 s

32

β-scission

OCOCH3 → CH3 + CO2

5.4

-14.9

1.0 ⋅ 10 s

50,51

44

52

6 -1, c

33

O2 addition

CH3 + O 2 → OOCH3

-24.0

-28.6

2.8 ⋅ 10

9, b

4.1 ⋅ 10

34

Peroxyl radical

OOCH3 + OOCH3 → 2OCH3 + O 2

6.5

-31.0

3.4 ⋅ 10

8, b

1.8 ⋅ 10

35

Peroxyl radical

OOCH3 + OOCH3 → 2HCHO + H2O 2

6.5

-106.3

3.4 ⋅ 10

8, b

36

OOCH3 + OOCH3 → HCHO + CH3OH + O 2

36.3

-113.6

< 1,

37

6.6 (triplet)

3.0

1.0 ⋅ 10

7, c

38

6.2

-7.0

4.0 ⋅ 10

8, b

Peroxyl radical

3

3

3

3

OOCH3 + HO2 → CH3OH + O + O 2

3

OOCH3 + OOCH2 COCH3 → OCH3 + OCH2COCH3 + O2

3

b

39

OOCH3 + OOCH2 COCH3 → CH3COCHO + CH3 OH + O 2

27.2

-90.9

< 1,

40

OOCH3 + OOCH2 COCH3 → H2O2 + HCHO + CH3COCHO

9.3

-107.3

1.0 ⋅ 10

37

8

6.49 ⋅ 10 , 5.3 × 10

-1, b

Hydrolysis

48,49

c

3

8

8, a

3

7

3.1 ⋅ 10 , 7.0 × 10 0.11

-96.2 -15.0

CH3COCH2 OH + HO → CH(OH)COCH3 + H2O

30

3 -1, b

46.1

Elucidating the Elementary Reaction Pathways and Kinetics of

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