Peracetic acid: Structural elucidation for applications in wastewater treatment

Water Research 168 (2020) 115143

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Peracetic acid: Structural elucidation for applications in wastewater treatment Wesley Pereira da Silva, Thayrine Dias Carlos, Grasiele Soares Cavallini, Douglas Henrique Pereira*
s, P.O. Box 66, 77 402-970, Gurupi, Tocantins, Brazil Chemistry Collegiate, Federal University of Tocantins, Campus Gurupi -Badejo

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 July 2019 Received in revised form 26 September 2019 Accepted 29 September 2019 Available online 30 September 2019

Peracetic acid (PAA) is an oxidizer widely used for the sterilization of equipment in hospitals, pharmaceutical, cosmetic and food industries and also for water and wastewater disinfection. Even with its increasing applications, there have been no previous theoretical studies that explain the experimental results based on its molecular behavior. In this context, this work used calculations based on the density functional theory (DFT) combined with experimental results to elucidate the decomposition mechanisms of PAA for predicting its stability and the possible products generated from its decomposition. The results obtained showed that the protonation of PAA promoted its spontaneous decomposition in acetic acid and molecular oxygen. The hydrolysis mechanism of PAA in acidic medium indicated that the low energy difference involved in the mechanism’s stages is responsible for the equilibrium between PAA and H2O2. The structural and electronic comparison of PAA with H2O2 showed that the OeO bond length of PAA is longer than that of H2O2 and is also weaker, therefore may demonstrate greater efficiency in advanced oxidative processes by photocatalysis. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Peracetic acid Spontaneous decomposition Reaction mechanism Density functional theory

1. Introduction Peracetic acid (PAA) has a wide disinfectant spectrum and is effective against many microorganisms such as bacteria (fecal coliforms, Escherichia coli, Pseudomonas spp., and Salmonella spp.), viruses, and protozoan cysts (Veschetti et al. (2003); Baldry and French (1991); Pechacek et al. (2015); Moor et al. (2016); Koivunen et al. (2005a); Koivunen et al. (2005)b; Luukkonen et al. (2014); Antonelli et al. (2006)). It exhibits a similar or superior efficiency to that of ultraviolet radiation (De Sanctis et al. (2016)) because it is a strong oxidizing agent (Caretti et al. (2003); Kitis (2004); Klenk et al. (2000)), and according to Zhang et al. (2018), PAA has a redox potential of 1.385 V vs. SHE under standard state biochemical conditions (pH 7, 25
C, 101.325 Pa). The main advantage of its application in relation to chlorinated disinfectants is its low potential for the formation of toxic by-products, because chlorinated compounds can, react with organic matter to generate carcinogenic by-products (Kitis (2004); Who (2008); Wu et al. (2010); Richardson et al. (2007).

* Corresponding author. E-mail address: [email protected] (D.H. Pereira). https://doi.org/10.1016/j.watres.2019.115143 0043-1354/© 2019 Elsevier Ltd. All rights reserved.

The applications of PAA in environmental sanitation were first described in the late 1980s (Baldry and French (1989)) and from these results, several other experimental studies have been evaluated with the objective of knowing and predicting the behavior of this oxidant for the treatment of effluents from different sources (Koivunen et al. (2005b); West et al. (2016)). Studies using mathematical models were also developed to optimize the disinfection process (Manoli et al. (2019). In addition to its the potential for disinfection, kinetic studies (Zhang et al. (2018); Zhao et al. (2007); Koubek (1963); Yuan and Van Heiningen (1997), formation of byproducts (Monarca et al. (2004); Crebelli et al. (2005); Dell’erba et al. (2007); Xue et al. (2017)) and ecotoxicity (Henao et al. (2018); Macedo et al. (2019)) are the most evaluated factors in relation to PAA in the environment. These studies have contributed significantly to the consolidation of PAA as a promising substitute for chlorinated disinfectants. Although experimental studies are very important for the safe application of PAA, the understanding of its mechanism of action is directly related to its molecular structure, bond lengths, and other intrinsic molecular characteristics, which can be estimated by theoretical studies. Theoretical studies are intended to explain and prove experimental behavior, or to provide information that may promote or invalidate forms of compound application (Morgon

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(2007)). There are a few theoretical studies on PAA reported in the literature, such as the study of peracids by Langley and NOE (2004), which evaluated the most stable PAA conformation and showed the formation of an intramolecular hydrogen bond, described from ab initio methods. There are several computational methods for theoretical calculations such as ab initio, semi-empirical methods, and density functional theory (DFT). The DFT is currently one of the most used tools in theoretical chemistry because it allows for the study of small, medium and relatively large molecular systems with high accuracy and low computational time (Alfredsson and Hermansson (1999); Morgon (2007)). In this context, considering the increasing use of PAA in the environment and the advances referring to the computational tools that can contribute to explaining and deepening the experimental information, the objective of the present work was to conduct theoretical studies on the possible mechanisms of decomposition of peracetic acid, and to confront them to the results of the experimental oxygen release study and to also compare the electronic and structural properties of PAA and hydrogen peroxide, in order to obtain insights for environmentally relevant applications.

2.2. Evaluation of PAA decomposition by the quantification of dissolved oxygen The decomposition analysis of PAA was performed by quantifying dissolved oxygen (DO) in a solution of PAA (15% PROXITANE® 15121) with a concentration of 1 mg L1 as a function of time. The experiment was carried out using a Digimed oximeter model DM-4 by 4500-O G method (APHA, 1998). The PAA concentration used was selected such that the dissolved oxygen concentration did not exceed the saturation value of the oxygen in the water. The PAA solution was placed in a 300 mL BOD (biochemical oxygen demand) bottle so that there was no interference from the atmospheric oxygen. The readings were performed at 10 min intervals, totalizing to 520 min of evaluation. The assays were performed in triplicate, in pH 5.2 ± 0.1, with monitoring of the temperature and the PAA concentration by using DPD (N,N-dietil-p-fenileno diamina) method with read in l ¼ 515 nm, and hydrogen peroxide by the iron thiocyanate method, in l ¼ 470 nm, both were detected by visible spectrophotometry, using CHEMetrics Vacu-vials ® reagents. 3. Results and discussion 3.1. Conformational analysis

2. Methods and materials 2.1. Computational simulations Theoretical calculations for the decomposition of PAA were performed using the density functional theory (DFT) with the hybrid functional B3LYP. The 6-31 þ G(d,p) (Ditchfield et al. (1971); Hehre et al. (1972); Hariharan and Pople (1973)) basis set was used for all the calculations. To confirm that the optimized structures were at their minimum energy, frequency calculations were employed and no imaginary frequency was found. Some water molecules and hydronium ions were used to simulate the solvent. To correct the error associated with the harmonic frequencies related to the method, the scale factor 0.952 was used. D3 dispersion effect (Grimme et al. (2010)) was employed in all the calculations. All the calculations were performed using the Gaussian 09 program (Frisch et al. (2009)). To characterize the chemical bonding of PAA, hydrogen peroxide (H2O2), and also to understand the nature of their bonds, Quantum Theory of Atoms in Molecules (QTAIM) analyzes were performed (Bader and Essen (1984); Balder (1990)). The topological analyses of the molecules were performed using the QTAIM at the B3LYP/631 þ G(d,p) level. In a QTAIM analysis, we can describe the nature of the bond or the interaction through some parameters such as electronic density (r(r)), Laplacian of electronic density (V2r(r)), kinetic energy (G(r)), potential energy (V(r)) and total electronic energy (H(r)), that is H(r) ¼ G(r)þV(r). According to QTAIM, when two atoms interact with each other, a Bond Path (BP) is formed, the formation of these density gradient trajectories originate from a point located between two atoms known as the bond critical point (BCP). The properties analyzed in these points provide us with information regarding the characteristics of the bond (Bader and Essen (1984); Bader (1990)). Negative values of (V2r(r)) indicate covalent bonds, while positive values indicate intermolecular interactions (Keith et al. (1996); Popelier (1999)). Furthermore, the ratio of the kinetic energy G(r) and the potential energy V(r) in the BCP is used to characterize the nature of the interaction (Keith et al. (1996); Popelier (1999); Kumar et al. (2016)). The AIMALL package was used to perform the QTAIM analyses (Aimall (2017)).

Conformational analysis represents an important aspect in different fields, which are closely related to highly important problems in chemistry, biology, biochemistry (Pereira et al., 2014). For the PAA study, three dihedrals were calculated, Fig. 1a. The torsion angle range between 0
and 360
with intervals of 10
. The dihedral torsion results showed that for dihedral I, Fig. 1b, the methyl rotor presented energy minima at 60
and 180
with maxima at 0
and 120
. The energy difference from the eclipsed to the alternating conformation was small, 0.4 kcal mol1, which showed that the rotation was free at room temperature in this dihedral (RT ¼ 0.6 kcal mol1). Dihedral II was calculated through the OC-OO bonds, Fig. 1c. The results indicate two energy minima at 0
and 180
with maxima at 90
and 270
. The rotational barriers observed were 15.60 kcal mol1 (0
to 90
); 9.72 kcal mol1 (90
e180
); 12.62 kcal mol1 (180
e270
) and 18.46 kcal mol1 (270
e360
). In dihedral III, Fig. 1d, minimum energy values were observed at angles of 0
and 360
and maximum energy values were observed at 180
. The most stable conformation was observed when the rotation angles of the dihedral III (COeOH) presented a proximity of the hydrogen of hydroxyl (-OH) with the oxygen of the carbonyl (-C]O) due to the intramolecular hydrogen bond that forms in PAA. The maximum energy was observed when the dihedral COeOH is in 180
and the height of the rotational barrier was 4.48 kcal mol1. The results found were similar to those theoretically evidenced by Langley and NOE (2004); however, all peracetic acid dihedrals were studied in the present study. It is important to highlight that the rotational barriers were estimated only with rigid geometries. The conformational study of PAA described important information that consolidates the geometry of the PAA molecule and justifies the weak acid behavior due to intramolecular hydrogen bonding. According to the studies of Ando (1992) the pKa of peracetic acid is 8.2 and the same author also evidenced the probability of hydrogen bond formation, which was verified by the conformational analysis study. The weak acid behavior of PAA is important for the oxidation/disinfection process, as it avoids the need for pH adjustment after its application. Experimental studies performed by Stampi et al. (2001), Cavallini, et al. (2013), Koivunen and Heinonen-Tanski (2005b), also demonstrated a small reduction in the pH of the sanitary effluent after using PAA for disinfection,

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Fig. 1. a) Peracetic acid structure, b) dihedral I, c) dihedral II and d) dihedral III.

this behavior can be justified by the conformation of the PAA molecule. 3.2. Mechanism I: spontaneous decomposition by protonation Three mechanisms of PAA (Zhao et al. (2007)) were studied theoretically. Mechanism I is described in Fig. 2 and can be divided into three steps: protonation, formation of an active intermediate, and formation of the final product. The first step is the protonation of the PAA molecule and was evaluated for the three possibilities: a, b and c (Fig. 2, Step 1). The first hypothesis (a) indicated that the structure was stable and that protonation occurred; the second hypothesis (b) demonstrated the breakdown of the PAA molecule into two fragments; and the third hypothesis (c) showed that the proton (Hþ) migrated to the carbonyl oxygen. It was concluded that of the three hypotheses concerning the protonation of PAA, only step (a) can describe the protonation of the peracetic acid. The result of the protonation of the peracids follows the same trend as that of protonation of carboxylic acids (Carey, 2011). The structure formed after protonation reorganizes to form a carbocation on the central carbon of the molecule, which will act as a Lewis acid in the mechanism. The 2nd step represents the attack of the PAA to the carbocation resulting in the formation of an active intermediate and consequently the release of protons (Step 2). In the last step there is the formation of the final products of the spontaneous decomposition, in which two molecules of acetic acid and one molecule of oxygen were formed (Step 3). The relative energy graphs, steps 2 and 3, were evaluated as described in the mechanism (Fig. 2) and the

results for the mechanism are represented in Fig. 3. The starting point of the mechanism refers to the sum of the energies of the molecules, considering that there were no interactions between them, that is, they are apart from each other (isolated molecules). The sum of the energy values of the isolated molecules presented the highest energy and was used to determine the stability of the others, where DE was calculated as: Eisolated molecules - Eother steps. The representation of the second stage is defined as an attack of the PAA molecule on the carbocation, and the results showed that there was a lower energy compared to that of the initial point because of the interaction that occurred between the molecules. Then, the formation of an active intermediate with energy higher than that of the reagents occurred owing to the need for structural reorganization. Finally, the formation of the final products (2CH3COOH þ O2), which corresponds to the lower energy structure of the mechanism, and consequently the more stable structure. The energy difference between the product and the attack step is 22.42 kcal mol1, which showed that the PAA spontaneously decomposes into acetic acid and O2. The formation of acetic acid from the decomposition of PAA justifies the increase in chemical oxygen demand (COD) or TOC after its application even after the total decomposition of the disinfectant. Chen and Pavlostathis (2019) evaluated the decomposition in poultry processing effluents and observed that their decomposition resulted in the equimolar production of acetic acid, confirming the theoretical mechanism described. As for the BOD parameter, no increase is expected as soon as O2 has been released, and thus, the quantification of DO consumption by the bacteria is underestimated. This behavior was observed by

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Fig. 2. Spontaneous decomposition of PAA in acetic acid and O2 at pH < 5.5.

Baldry et al. (1995) and Cavallini et al. (2013), which describes the reduction of BOD as a function of the increase of PAA concentration applied in secondary effluent. In concentrations above 30 mg L1, BOD values below zero were observed due to the high O2 generation, demonstrating that this parameter does not adequately represent the estimation of biodegradable organic matter in these conditions. Thus, after the application of PAA, the BOD parameter is not recommended for the monitoring of biodegradable organic matter in water or wastewater. As an example, Stampi et al. (2001) did not observe an increase in BOD after the application of PAA (1.5e2 mg.L1), while, Lazarova et al. (1998) measured the total organic carbon (TOC) and the total biodegradable organic carbon and observed an increase in both after PAA application. The DO parameter is relevant for evaluating the final quality of the treated effluent; however, at high concentrations, the oxygen saturation in the liquid medium is rapidly reached and the gas is released into the atmosphere, with the use of PAA, supersaturation of the water can also be observed. Fig. 3. Energy profiles obtained for PAA decomposition in acetic acid and O2.

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3.3. Mechanism II: spontaneous decomposition in the pH range of 5. 5e10.2 The second mechanism presents the spontaneous decomposition of PAA in the pH between 5.5 and 10.2 (Fig. 4). According to Koubek (1963) the reaction rate at 25
C reached a maximum at pH 8.2, which was equal to the pKa of PAA. This mechanism consists of two steps: Step 1 is the attack of peracetic anion on PAA and formation of the active intermediate, the Step 2 is the decomposition of the active intermediate and formation of acetic anions, O2 and Hþ products. This mechanism was studied by Koubek (1963), Yuan and Van Heiningen (1997), and Zhao et al. (2007), and according to Zhao et al. (2007) depending on the concentration of Hþ in solution, PAA will be present in the molecular form and there will be no formation of the peracetic anion (higher pH) or formation of hydrogen peroxide (lower pH). Fig. 4 shows that an anionic PAA molecule attacks a PAA neutral molecule forming the active intermediate, which decomposes to generate the acetate anion, oxygen and Hþ. All steps of the mechanism were simulated and the results of the energies involved in each step are represented in Fig. 5. The mechanism II follows the same route as that of mechanism I for obtaining the products. Step 1 indicated the anion attack on PAA, entailing in a significant energetic decrease when compared to the first point. The active intermediate had a higher DE than did the reactants, and finally, the active intermediate decomposes to form the final product. According to the relative energy values obtained, the energy difference between the final reactants and products was 28.85 kcal mol1, demonstrating that the PAA decomposition was spontaneous. As mentioned by Zhao et al. (2007) both the paracetic anion and the acetic anion may not be in the anionic but in the molecular form. From the theoretical calculations using H2O molecules as solvent, and hydronium ions to represent the acidic medium the structures after optimization presented the molecular form, ratifying the information of Zhao et al. (2007). These results help in understanding the work described by Chen and Pavlostathis (2019) and Pedersen et al. (2009), which observed that the higher the initial PAA concentration applied to the treatment of effluents, the greater is its rate of decomposition. This is because a higher concentration of PAA molecules favors the occurrence of mechanism 2, which predicts the decomposition

Fig. 5. Energy profiles obtained for the PAA decomposition in the pH range of 5.5e10.2.

when its molecules react with each other and promote the formation of oxygen and acetic acid, this being the spontaneous reaction at pH 5.5 to 8.2. Mechanism (II) explains an interesting behavior regarding the residual PAA, because at low concentrations, its decomposition will be slow, which contributes to the occurrence of residual concentrations in the liquid medium, which contributes to the inhibition of microbial growth. Koivunen and Heinonen-Tanski (2005a) highlighted the presence of residual concentrations of PAA in the treatment of sanitary effluent and also describe it as an advantage, comparable to the behavior of chlorine. 3.4. Mechanism III: decomposition by hydrolysis Mechanism III corresponds to the study of the decomposition of PAA by hydrolysis in acidic medium and has as final product the formation of acetic acid and H2O2 (Zhao et al., 2007), Fig. 6. This mechanism consists of five steps, initially protonation on the carbonyl oxygen of the PAA molecule occurs (Step 1), followed by a structural resonance resulting in the formation of a

Fig. 4. Spontaneous Decomposition of PAA in the pH range of 5.5e10.2.

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W.P. da Silva et al. / Water Research 168 (2020) 115143

Fig. 6. Hydrolysis of PAA in acidic medium.

carbocation, which is attacked by a molecule of water (Step 2). Subsequently there are three attacks of the reaction medium (Steps 3, 4 and 5) resulting in equilibrium, forming a molecule of acetic acid and hydrogen peroxide. The simulations of the steps were performed and the results are presented in Fig. 7.

Step 0 in Fig. 7 refers to the isolated molecules. Step 1 represents the protonation of the PAA molecule when the molecules approached each other. Steps 2 and 3 presented higher energies than that of the initial step (Step 1) due to the attacks and consequently due to the reorganization of the molecules. The simulations showed that in step 4, the intermediate decomposes to form H2O2 and acetic acid, and the last step is not necessary. From the results obtained, it was possible to infer, after comparing step 1 with step 4, that the energy difference was 1.66 kcal mol1. Thus, the acidic PAA molecule favors the displacement of the reaction toward the formation of acetic acid and hydrogen peroxide, but due to the low energy difference between steps 1 and 4, an equilibrium is established corresponding to the stability of the compound. These results are consistent with the commercial composition of PAA, in which PAA is present as a quaternary equilibrium composed of: H2O (46%), PAA (15%), H2O2 (23%) and acetic acid (16%), as shown in the reaction (1). CH3COOOH þ H2O / CH3COOH þ H2O2

Fig. 7. Energy profile for the reaction mechanisms of PAA hydrolysis with acidcatalysis.

(1)

Regarding the stability of the compound in its quaternary form, it was possible to conclude that commercial PAA allows the exploitation of the properties of both PAA and hydrogen peroxide without interference between the two chemical species, and thus, the oxidative or disinfection processes can be more effective since it deals with the combined performance of two strong oxidants.

W.P. da Silva et al. / Water Research 168 (2020) 115143

It is worth mentioning that Aslari et al. (1992), Lubello et al., 2002,Kitis (2004) and Du et al. (2018) described the superiority of the disinfection/oxidation potential of PAA in relation to hydrogen peroxide, thus, in acid pH (pH < 5.5), the efficiency of PAA would be negatively affected due the decomposition by hydrolysis and formation hydrogen peroxide. 3.5. Evaluation of PAA decomposition by the release of O2 The quantification of the dissolved oxygen (DO) in a solution of PAA and the monitoring of the PAA and H2O2 concentrations allowed for the confirmation of the theoretical results regarding its decomposition and the generation of molecular oxygen, acetic acid and hydrogen peroxide by the mechanisms under study. In Fig. 8, the formation of dissolved oxygen (ln [DO]) with respect to time can be observed, which can be represented by first-order kinetics. The linearity of the data can be best represented by dividing the curve into two steps, the fast step and slow step (Fig. 8). The transition point of the steps was verified in the time of 150 min, during which the PAA concentration remained practically constant for more than 6 h. In the fast step, the initial PAA concentration (1 mg.L1) decreased to 0.41 mg.L1 in the first 150 min, corresponding to the formation of 0.87 mg.L1 of O2. In the slow step, the PAA concentration after 300 min was 0.37 mg.L1 and remained at that concentration until the end of the study (520 min), in this interval there was the formation of 0.1 mg.L1 of O2. The study at pH 5.2 allowed the formation of H2O2 by hydrolysis of PAA. Initially, the sample had a H2O2 concentration of 1.5 mg.L1, after 150 min the concentration increased to 1.8 mg.L1 and remained constant until the end of the study. Thus, it was possible to observe the equilibrium establishment and the coexistence of both oxidants (Du et al., 2018) even at low concentrations, which is justified by the energies described in Fig. 7. Observing the results obtained in Fig. 8, it can be confirmed that, in the reactions between molecules of PAA in the reaction medium (H2O), due to the higher concentration of disinfectant, the was a higher PAA decomposition rate. This fact occurs because there is a greater probability of shock to the molecules (mechanisms I and II). At the start of the reaction, a significant increase in the release of

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dissolved oxygen was observed and as the PAA decomposed, the oxygen release occurred more slowly. The results obtained experimentally also corroborate the studies by Zhao et al. (2007) who also describe an PAA decomposition in acid medium by first-order reaction. In Fig. 8 it was also possible to observe that the data do not fit perfectly with a first-order reaction (R2 ¼ 0.75206). However, when the data were divided into two steps it was possible to observe the linearity of the decomposition at a faster stage (R2 ¼ 0.9341, kobs ¼ 0.0045 min1) and a slower one (R2 ¼ 0.9646, kobs ¼ 0.0006 min1). The persistence of the PAA residual concentration also reinforces the information obtained in the mechanisms. 3.6. Theoretical comparison between PAA and hydrogen peroxide for applicative purposes In this simulation, the structural property related to bond length and the electronic properties based on QTAIM were evaluated for PAA and Hydrogen peroxide. A comparison between the chemical species was carried out to obtain information that could contribute to understanding the effectiveness of the products, because both are strong oxidants. The bonds lengths values of the species studied are shown in Fig. 9. Comparing the two molecules, it was observed that the OeO bond length of the PAA molecule was slightly longer than that of the H2O2 molecule, this behavior explains the results obtained by Bianchini et al. (2002), who described the necessary energies to

Fig. 9. Bond lengths for Hydrogen peroxide (H2O2) and Peracetic acid (CH3COOOH). Data in Å.

Fig. 8. Evaluation of PAA decomposition by the release of O2.

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W.P. da Silva et al. / Water Research 168 (2020) 115143

break the OeOH bond of PAA (38 kcal mol1) and H2O2 (51 kcal mol1). The study by Bach et al. (1996) describes the value of 48 kcal mol1 for breaking the OeOH bond of PAA, which would be even more consistent with the theoretical results of this work as the length of the bonds are very close (PAA 1.427 Å and H2O2 1.443 Å). The same occurred for the OeH bonds. With the information on the species, it is possible to estimate that PAA follows the same mechanism of H2O2 radical generation. The electronic properties of CH3COOOH and H2O2 were evaluated using the Quantum Theory of Atoms and Molecules (QTAIM) (Balder and Essen (1984); Bader (1990); Keith et al. (1996); Popelier (1999); Kumar et al. (2016)). This model is useful in computational chemistry to visualize possible interactions of the atoms of a molecule, as well as to understand the nature of chemical bonds or interactions. Fig. 10 represents the molecules of PAA and Hydrogen peroxide, respectively. In Fig. 10 it is possible to visualize the formation of an intramolecular hydrogen bond in PAA and also a ring critical point. The Bond Critical Points (BCPs) and topological parameter values for the species studied are presented in Table 1. For the systems, three topological properties were analyzed: I) electron density (r(r)), which represents the interaction/bond force at the BCP, and the higher the density value (r(r)) the stronger the interaction/bond; II) laplacian of the electronic density (V2r(r)) that indicates whether the bonds are covalent or not with values of V2r(r) < 0 suggestive of covalent bonds and values > 0 of noncovalent bonds; III) Elipicity (ε(r)), which shows whether the bonds have s or p character, where ε ¼ 0 denotes characteristics of s bonds and ε s 0 denotes predominantly p bond characteristics. From the results of r(r) (Table 1) it was possible to observe that the C5eO6 bond of PAA was the strongest bond (double bond). The electron density of the O7eO8 bond of PAA presented a value of r(r) ¼ 0.284922 au, this value being lower than that of the O1eO3 bond of H2O2, which presented a value of 0.2797995 au. This fact indicates that the OeO bond of H2O2 was stronger than that of PAA, reconfirming the results of Bianchini et al. (2002) and Bach et al. (1996). According to the values of V2r(r), the bonds O6eH9 and C5eO6 were characterized as being non-covalent, and both involved in the intramolecular hydrogen bonding of PAA. The other bonds of PAA and H2O2 molecules have values lower than 0 and are covalent. The bond densities of the PAA molecule when compared to the hydrogen peroxide molecule indicate that the formation of hydroxyl and hydroperoxyl radicals may occur with lower energy demand, and this information is of paramount importance for advanced oxidative processes by photocatalysis. Thus, the time of exposure to radiation or even the exposure to radiation with a longer wavelength, such as solar radiation, could be sufficient for

Table 1 Topological parameters r(r), V2r(r) and ε(r) for PAA and H2O2. Values in atomic units. BCPs

Bonds

r(r)

V2r(r)

ε(r)

CH3COOOH 1 2 3 4 5 6 7 8 9

C1 e H2 C1 e C5 C1 e H3 C1 e H4 C5 e O6 C5 e O7 O6 e H9 O7 e O8 O8 e H9

0.280835 0.261081 0.276790 0.276805 0.411139 0.299375 0.033378 0.284922 0.345031

0.994833 0.657834 0.964983 0.965108 þ0.045467 0.469301 þ0.112734 0.072435 1.997985

0.009314 0.064855 0.010703 0.010715 0.097558 0.018454 0.234158 0.085685 0.041056

O1 e O3 O1 e H2 O3 e H4

0.297995 0.368701 0.368701

0.146782 2.135817 2.135818

0.016770 0.047040 0.047040

H2O2 1 2 3

the formation of hydroxyl radicals by PAA. Corroborating the theoretical results obtained regarding the possible formation of hydroxyl radicals in the process with radiation/PAA, Cai et al. (2017) evaluated the removal of drugs by the UV/ PAA process and attributed the process efficiency to the radicals HO, CH3C(¼O)O and/or CH3C(¼O)O2 and also observed that the UV/PAA process contributed more to the degradation of the drugs than the UV/H2O2 process. Bianchini et al. (2002), Caretti, et al. (2003), Souza et al. (2015) and Rizzo et al. (2019) also reported the contribution of the hydroxyl radicals formed during the PAA/UV process. 4. Conclusions The studies carried out in this work proved the possible routes of peracetic acid decomposition, which helped to understand its stability in the effluent treatment processes. Mechanism I made it possible to identify the steps involved in the spontaneous decomposition of PAA into oxygen and acetic acid due to its protonation, which can be observed experimentally in the PAA decomposition assays by DO quantification. The mechanism II that occurs in the pH range between 5.5 and 10.2 showed that the PAA decomposition depends on its concentration and therefore tends to present spontaneous residual concentrations. Mechanism III indicated that the low energy difference between steps 1 and 4 promoted an equilibrium between the PAA and H2O2 molecules in acid medium conferring stability to the compound. Conformational analysis demonstrated the formation of an intramolecular hydrogen bond in PAA and the comparison of the bond lengths and QTAIM analyzes between the PAA and H2O2 molecules suggest that PAA possibly

Fig. 10. Molecular graphs of PAA and H2O2 generated by QTAIM: Bond critical points are represented by yellow circles, and red circles are the ring critical points. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

W.P. da Silva et al. / Water Research 168 (2020) 115143

exhibits the same hydrogen peroxide radical generation mechanism. Then, the studies developed in this work contribute to the elucidation of PAA decomposition mechanisms, providing information about its molecular structure, which justifies its behavior in real systems and helps in the in-depth understanding of its stability and reactivity in comparison with peroxide of hydrogen. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements ~o de The authors acknowledge funding from CAPES (Coordenaça Aperfeiçoamento de Pessoal de Nível Superior - Coordination of Improvement of Higher Education Personnel - Brazil), Funding Code 001 CAPES, the UFT New Researchers Program (Programa Novos Pesquisadores da Universidade Federal do Tocantins - PROPESQ/UFT). Pereira DH acknowledges the Center for Computational ~o Engineering and Sciences (Financial support from FAPESP Fundaça  Pesquisa, Grant 2013/08293-7, and Grant 2017/11485de Amparo a 6), the National Center for High Performance Processing (Centro Nacional de Processamento de Alto Desempenho e CENAPAD) in ~o Paulo and UNICAMP (Universidade Estadual de Campinas), for Sa computational resources. The authors also acknowledge the Brazilian company Thech Desinfecç~ ao Ltda. References Aimall, 2017. Todd A. Keith, TK Gristmill software, Overland park KS, USA, (aim.tkgristmill.com). Version 17.11.14. Alfredsson, M., Hermansson, K., 1999. HartreeeFock and DFT calculations of quadrupole coupling constants in water clusters and ice. Chem. Phys. 242 (2), 161e175. Ando, W., 1992. Organic Peroxides. John Wiley & Sons, New York. Antonelli, M., Rossi, S., Mezzanotte, V., Nurizzo, C., 2006. Secondary effluent disinfection: PAA long term efficiency. Environ. Sci. Technol. 40 (15), 4771e4775. APHA, 1998. Standard Methods for the Examination of Water and Waste Water, twentieth ed. American Public Health Association, American Water Works Association and Water Environmental Federation, Washington, DC. Aslari, A., Roques, C., Michel, G., 1992. Bactericidal properties of peracetic acid and hydrogen peroxide, alone ande in combination, and chlorine and formaldehyde against bacterial waer strains. Can. J. Microbiol. 38 (7), 635e642. Bach, R.D., Ayala, P.Y., Schlegel, H.B., 1996. A reassessment of the bond dissociation energies of peroxides an ab initio study. J. Am. Chem. Soc. 118 (50), 12758e12765. Bader, R., 1990. Atoms in Molecules: A Quantum Theory, 1th ed. Oxford University Press. Bader, R.F.W., Essen, H., 1984. The characterization of atomic interactions. J. Chem. Phys. 80 (5), 1943e1960. Baldry, M.G.C., French, M.S., 1989. Disinfection of sewage effluent with peracetic acid. Water Sci. Technol. 21 (3), 203e206. Baldry, M.G.C., French, M.S., Slater, D., 1991. The activity of peracetic acid on sewage indicator bacteria and viruses. Water Sci. Technol. 24 (2), 353e357. Baldry, M.G.C., Cavadore, A., French, M.S., Massa, G., Rodrigues, L.M., Schirch, P.F.T., Threadgold, T.L., 1995. Effluent disinfection in warm climates with peracetic acid. Water Sci. Technol. 31 (5e6), 161e164. Bianchini, R., Calucci, L., Lubello, C., Pinzino, C., 2002. Intermediate free radicals in the oxidation of wastewaters. Res. Chem. Intermed. 28 (2e3), 247e256. Cai, M., Sun, P., Zhang, L., Huang, C., 2017. UV/Peracetic acid for degradation of pharmaceuticals and reactive species evaluation. Environ. Sci. Technol. 51 (24), 14217e14224. Caretti, C., Lubello, C., 2003. Wastewater disinfection with PAA and UV combined treatment: a pilot plant study. Water Res. 37 (10), 2365e2371. ~o. Carey, F.A., 2011. Química Org^ anica, vol. 2. Mc Graw Hill. 7
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