The Reactivity and Reaction Pathway of Fenton ...

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The Reactivity and Reaction Pathway of Fenton Reactions Driven by Substituted 1,2-Dihydroxybenzenes Pablo Salgado,†,‡ Victoria Melin,†,‡ Yasna Durán,†,‡ Héctor Mansilla,‡ and David Contreras*,†,‡ †

Centro de Biotecnología, and ‡Facultad de Ciencias Químicas, Universidad de Concepción, Barrio Universitario s/n, Casilla 160-C, Concepción, 4070386, Chile S Supporting Information *

ABSTRACT: Fenton systems are interesting alternatives to advanced oxidation processes (AOPs) applied in soil or water remediation. 1,2-Dihydroxybenzenes (1,2-DHBs) are able to amplify the reactivity of Fenton systems and have been extensively studied in biological systems and for AOP applications. To develop efficient AOPs based on Fenton systems driven by 1,2-DHBs, the change in reactivity mediated by different 1,2-DHBs must be understood. For this, a systematic study of the reactivity of Fenton-like systems driven by 1,2-DHBs with different substituents at position 4 was performed. The substituent effect was analyzed using the Hammett constant (σ), which has positive values for electronwithdrawing groups (EWGs) and negative values for electrondonating groups (EDGs). The reactivity of each system was determined from the degradation of a recalcitrant azo dye and hydroxyl radical (HO·) production. The relationship between these reactivities and the ability of each 1,2-DHB to reduce Fe(III) was determined. From these results, we propose two pathways for HO· production. The pathway for Fenton-like systems driven by 1,2-DHBs with EDGs depends only on the Fe(III) reduction mediated by 1,2-DHB. In Fenton-like reactions driven by 1,2-DHBs with EWGs, the Fe(III) reduction is not primarily responsible for increasing the HO· production by this system in the early stages.

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INTRODUCTION

Fe(III) + H 2O2 → Fe(II) + HOO· + H+

Advanced oxidation processes (AOPs) encompass several methods for the chemical or photochemical oxidation of molecules.1,2 AOPs performed at near ambient temperatures constitute a promising technology for wastewater treatment.3,4 Although different reaction systems exist for these processes, all are based on the production of reactive oxygen species (ROS), mainly hydroxyl radical (HO·).5,6 This radical is extremely unstable and reactive (E°(HO·/H2O) = 2.8 V/SHE7) and is consequently able to react quickly with different organic compounds, leading to mineralization of the substrate.8,9 The production of HO· from the reduction of hydrogen peroxide (H2O2) catalyzed by Fe(II) is known as the Fenton reaction: reaction 1.10 The reaction between the Fe(III) produced in (1) and H2O2 is known as the Fenton-like reaction 2 and involves the formation of hydroperoxyl radical (HO2·).11 The Fenton-like reaction represents the limiting step in this redox system because it is 3 orders of magnitude slower than the Fenton reaction. Both Fenton and Fenton-like reactions participate, at the same time, in a redox system.

k = 0.01 mol L−1 s−1

where the reaction 1 k value is taken from ref 12 and the reaction 2 k value is taken from ref 13. Fenton and Fenton-like systems are popular AOPs due to their oxidation power, low toxicity, moderate cost, and simple operation.9,14 However, these systems are highly dependent on the reaction conditions. For example, the pH in the system can change the reaction rate by changing the iron speciation.15,16 The Fenton reaction is limited at acidic pH to avoid the oxidation of Fe(II) to Fe(OH)3 or Fe2O3.17 Several ligands can enhance the production of reactive species by Fenton and Fenton-like systems, of which 1,2dihydroxybenzenes (1,2-DHBs) have been studied in different systems such as metabolic pathways in biological systems18−22 and AOPs for water and wastewater treatment.23−26 1,2-DHBs form complexes with Fe(III) with a pro-oxidant or antioxidant activity that is related to the coordination number, which is pH dependent.27 These complexes keep the iron in solution, but Received: Revised: Accepted: Published:

Fe(II) + H 2O2 → Fe(III) + HO· + HO− k = 76 mol L−1 s−1

(1) © XXXX American Chemical Society

(2)

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October 26, 2016 January 26, 2017 March 8, 2017 March 8, 2017 DOI: 10.1021/acs.est.6b05388 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

Article

Scheme 1. Mechanism of Fe(III) Reduction by 1,2-DHB

only monocomplexes [Fe(1,2-DHB)]+ can reduce Fe(III), increasing the reactivity of Fenton systems at acidic pH up to pH 5.5 20 In this monocomplex a tautomeric valence equilibrium is achieved,28 after that Fe(II) is released (Scheme 1). Thus, 1,2-DHBs increase Fe(III) reduction, which is the limiting step in the Fenton redox system (2).29 To develop an efficient AOP based on Fenton systems driven by 1,2-DHBs that can be applied in water or soil remediation, the reactivity change mediated by different 1,2-DHBs must be understood. Therefore, a systematic study of the reactivity of Fenton-like systems driven by 1,2-DHBs with different substituents at position 4 was performed. The substituent effect was analyzed using the Hammett constant (σ), which takes on positive values for electron-withdrawing groups (EWGs) and negative values for electron-donating groups (EDGs). The reactivity of each system was determined through the degradation of a recalcitrant azo dye (methyl orange, MO) and HO· production. The relationship between these reactivities and the ability of each 1,2-DHB to reduce Fe(III) was evaluated.

was determined 20 s after initiating the reaction (Figure S2 in the SI). Hydroxyl Radical Production. HO· was detected by a DMPO spin-trapping method using EPR spectroscopy.31 The final concentrations in the systems were 50 × 10−6 mol·L−1 1,2DHB, 20 × 10−3 mol·L−1 DMPO, and 1.0 × 10−3 mol·L−1 H2O2 and Fe(NO3)3. The reactions were initiated by adding Fe(III). Samples were subsequently transferred via syringe to an AquaX capillary in a Bruker EMX microinstrument. The EPR spectra of the DMPO−OH adduct was recorded every 15 s on the X band (∼9 GHz). The amount of DMPO−OH adduct produced was considered proportional to the height of the second peak in the adduct spectra (Figure S3 in the SI). All decay plots were normalized and adjusted to pseudo-first-order kinetics (3) according to Contreras et al.,32 and a pseudo-firstorder constant (kp) and the initial signal (I0) was determined for each system. ln(I ) = −k pt + ln(I0)

(3)

Reduction of Fe(III). Reduced Fe(III) was quantified in a spectrometric method by measuring the levels of Fe(II) formed at different reaction times as a colored complex with ferrozine (λmax = 562 nm).33 The reduction of Fe(III) by each 1,2-DHB was determined at pH 3.4. The final concentrations in the systems were 4.0 × 10−4 mol·L−1 Fe(NO3)3 and 2.0 × 10−5 mol·L−1 1,2-DHB. The kinetics was assessed in a stopped-flow apparatus. The kinetics data were processing with UV−vis ChemStation software. The pseudo-first-order constant (kred) was determined. Determination of the Redox Potential of 1,2-DHB. The redox potential of each 1,2-DHB was determined by cyclic voltammetry using a method modified from Contreras et al.34 The final concentration was 5.0 × 10−3 mol·L−1 1,2-DHB, which was prepared in an aqueous solution at pH 3.0. The ionic strength was adjusted to 1.0 mol·L−1 KNO3. Electrochemical measurements were performed on a computer controlled by a CHI1207A potentiostat (CH Instruments, TX, USA) using a 20 mL glass chamber with a three-electrode system. The working electrode was carbon graphite (3 mm in diameter), an Ag/AgCl electrode was used as the reference electrode, and a platinum wire was used as the auxiliary electrode. The instrumental parameters were as follows: Einitial = −0.4 V, Emax = 0.8 V, Emin = −0.4 V, and scan rate = 0.2 V/s. From the anodic (Epa) and cathodic (Epc) potential, the standard potential (E°) was estimated in eq 4.

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MATERIALS AND METHODS Reagents. Ferric nitrate nonahydrate (Fe(NO3)3·9H2O), MO, potassium fluoride (KF), 3-(2-pyridyl)-5,6-diphenyl-1,2,4triazine-p,p′-disulfonic acid (ferrozine), 4-morpholineethanesulfonic acid (MES), potassium nitrate (KNO3), 5,5-dimethyl1-pyrroline N-oxide (DMPO), 1,2-benzendiol (catechol), 4tertbutylcatechol, 4-ethylcatechol, 4-methylcatechol, 3,4-dihidroxibenzylamine, noradrenaline, caffeic acid, dopamine, 4chlorocatechol, adrenaline and 3,4-dihydroxybenzonitrile were purchased from Sigma-Aldrich. Nitric acid (HNO3), 30% H2O2, hydrocaffeic acid, 3,4-dihydroxybenzaldehyde, 3,4-dihydroxybenzoic acid, and 4-nitrocatechol were purchased from Merck. All reagents were used without additional purification. General Procedure. All reagent solutions were prepared in the dark under an argon atmosphere. The ionic strengths of all solutions were adjusted to 0.10 mol·L−1 with KNO3. All experiments were performed at 20 ± 0.1 °C in triplicate (n = 3). The pH of each solution was adjusted to 3.4 with HNO3 prior to the experiments using a Thermo Scientific Orion 3-Star pH meter. This pH value was selected because is the optimal pH value observed for Fenton systems driven by 1,2-DHB.2,30 All the experiments were performed at pseudo-first-order conditions (1,2-DHB/Fe(III) molar ratio 1:20). A UV−vis diode array spectrophotometer (Agilent 8453) coupled to a stopped-flow system (applied photophysics RX2000) was used for spectrophotometric measurements. The spectra (190−1100 nm) were recorded every 0.1 s for 20 s. Oxidation of MO. The degradation of MO (λmax = 499 nm, Figure S1 in Supporting Information, SI) at pH 3.4 in the Fenton-like system driven by different 1,2-DHBs was followed spectrophotometrically. The final concentrations in the systems were 1.0 × 10−6 mmol·L−1 1,2-DHB and 20 × 10−6 mol·L−1 Fe(NO3)3, MO, and H2O2. The reaction was initiated by adding Fe(III). The kinetics data were analyzed with UV−vis ChemStation software. The pseudo-first-order constant (kobs)

E° = Epa − Epc

(4)

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RESULTS AND DISCUSSION Oxidation of MO. The oxidation of MO was performed for 20 s under pseudo-first-order conditions. The 15 1,2-DHBs shown in Table 1 were utilized. Their structures are shown in Figure S4 in the SI. The pseudo-first-order rate constant (kobs) was determined for each assayed system (Table S1 in the SI). The oxidation ability of each Fenton-like system driven by a 1,2-DHB was B

DOI: 10.1021/acs.est.6b05388 Environ. Sci. Technol. XXXX, XXX, XXX−XXX


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changes depending on whether the substituent on 1,2-DHB is an EWG or EDG. A possible explanation for the rate processes is a dual reaction mechanism, with the overall rate constant kobs being given by the sum of two rate constants kl and k2. After adjusting these results to eq 6, described by Exner,37 these kinetics constants are 0.1440 and 0.9998, respectively.

Table 1. 1,2-DHBs Utilized in This Study and Their Hammett Parameters Hammett parameters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

1,2-DHBs

σm

σp

Σσ = (σm + σp)

4-tert-butylcatechol 4-ethylcatechol 4-methylcatechol 3,4-dihydroxybenzilamine hydrocaffeic acid catechol norepinephrine caffeic acid dopamine 4-chlorocatechol epinephrine 3,4-dihydroxybenzaldehyde 3,4-dihydroxybenzoic acid 3,4-dihydroxybenzonitrile 4-nitrocatechol

−0.10 −0.07 −0.07 −0.03 −0.03 0.00 0.11 0.14 0.23 0.37 0.36 0.35 0.37 0.56 0.71

−0.20 −0.17 −0.15 −0.11 −0.07 0.00 0.09 0.09 0.17 0.23 0.30 0.42 0.45 0.66 0.78

−0.30 −0.24 −0.22 −0.14 −0.10 0.00 0.20 0.23 0.40 0.60 0.66 0.77 0.82 1.22 1.49

log kobs = log(k110 ρ1σ + k 210 ρ2 σ )

Two values for ρ were determined from the plot in Figure 1. For the Fenton-like systems driven by 1,2-DHBs with EWGs, ρ = 0.2145, and for the Fenton-like systems driven by 1,2-DHBs with EDGs, ρ = −2.8076. The magnitude of ρ indicates that the mechanism of the oxidation of MO in Fenton-like systems driven by 1,2-DHBs with EDGs is most affected by the change in the substituent and that the limiting step is probably highly dependent on the ability of the hydroxyl groups to donate electron density. Conversely, the mechanism of the oxidation of MO in Fentonlike systems driven by 1,2-DHBs with EWGs is relatively less influenced by the substituent. The positive value for ρ in these systems indicates that the limiting step in the mechanism includes an increase in the electron density of the transition state or intermediate.38 Hydroxyl Radical Production. Several reports have emphasized the importance of HO· in the oxidizing ability of Fenton systems. For the Fenton-like system driven by 1,2DHBs, HO· production was determined from EPR measurements. The decay kinetics of each studied system was fit to a pseudo-first-order equation, obtaining values for the pseudofirst-order constant (kp) and initial signal (I0) of the DMPO− OH adduct ( Table S2 in the SI). The studied systems showed different abilities to produce HO· (Figure 2). The kinetics parameter I0 is related to the Hammett constant in a similar manner to that observed for MO oxidation (Figure 3A. According to Contreras et al.,32 this parameter is proportional to the initial amount of HO·. In this way, I0 linearly increases in systems driven by 1,2-DHBs with EWGs and linearly decreases in systems driven by 1,2-DHBs with EDGs, observing a minimum in I0 when the Fenton-like system is driven by catechol. A linear relationship was observed between I0 and kobs from MO oxidation (Figure 3B; r = 0.9842). Therefore, although the mechanism of the Fenton reaction differs based on the 1,2DHB substituent, the HO· is the main oxidizing species responsible for MO oxidation. Fe(III) Reduction. To understand the deviations in the obtained Hammett equation, the ability of each 1,2-DHB to reduce Fe(III) was studied. Fe(II) production from Fe(III) reduction mediated by 1,2-DHB has been considered essential to the increased oxidizing ability of Fenton-like systems driven by 1,2-DHBs. The reduction rate constants (kred) were obtained from the kinetics profiles of Fe(III) reduction (Table S2 in the SI). A quasi-linear relation was obtained between log(kred/kred ° ), where kred is the rate constant of Fe(III) reduction mediated by different 1,2-DHBs and kred ° is the rate constant of Fe(III) reduction mediated by catechol (unsubstituted), and the Σσ values of the substituents on 1,2-DHB (Figure 4A). This trend suggests an increase in Fe(III) reduction by 1,2-DHBs with EDGs and a decrease with 1,2DHBs with EWGs. The ρ constant for this reduction reaction is −0.6701 (r = 0.9839), which indicates a loss of electron density in the aromatic ring during the limiting step of the reaction, according to Exner.37 The redox potential of 1,2-DHB (Table

significantly different. The Hammett eq 5 was used to determine whether the substituent on 1,2-DHB had a direct influence on the observed changes in reactivity of the system, where kobs is the rate constant for X substituent and kobs ° is the rate when X = H (catechol). The reaction constant (ρ) is a measure of the sensitivity of the reaction to electronic effects and is independent of the substituent.35 The Hammett parameters (σ) for meta (σm) and para (σp) substituents are defined from this equation. If the substituent is an EWG on the aromatic ring, σ is greater than 0. If the substituent is an EDG, σ is lower than 0. The absolute values of σ indicate the relative capacity of the substituent to withdraw or donate electron density to the aromatic ring. log

kobs o = σρ kobs

(6)

(5)

Considering that 1,2-DHBs have two hydroxyl groups and one substituent, the effect of this substituent was evaluated by Σσ, which includes the effect in the meta (σm) and para (σp) position. Figure 1 shows the dependence of log (kobs/k°obs) on the Hammett constant (∑σ), and a nonlinear Hammett relationship with a concave upward deviation was observed. According to the literature,36,37 this type of deviation indicates that the mechanism of the oxidation of MO in a Fenton-like system

Figure 1. Hammett plot for MO oxidation (kobs) in Fenton-like systems driven by different 1,2-DHBs. C



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Figure 2. Hydroxyl radical production by Fenton-like systems driven by different 1,2-DHBs. (A) Pseudo-first-order constant (kp) and (B) initial signal (I0) determined from decay kinetics.

Figure 3. Relationship between the (A) initial signal of HO· production (I0) and Hammett constant and the (B) pseudo-first-order rate constant for MO oxidation (kobs) and I0.

Figure 4. (A) Hammett plot of Fe(III) reduction (kred) by different 1,2-DHBs. (B) Relationship between the Hammett constants of different 1,2DHBs with the standard redox potential (E°).

Proposed Pathways for HO· Production. According to our results, more than one pathway can produce HO· in Fenton-like systems driven by 1,2-DHBs. Although several publications indicate the reduction of Fe(III) as the main mechanism by which 1,2-DHBs promote the Fenton reaction, it is remarkable that the relationship obtained between ∑σ and MO oxidation is different than the relationship between ∑σ and the reduction of Fe(III) (kred). While the first relationship follows a concave upward deviation (Figure 1B), the second has a linear relationship with a negative slope (Figure 4A). Despite this disagreement, the MO degradation is closely related to the

S3 in the SI) was significantly dependent on the type of substituent on 1,2-DHB (Figure 4B). 1,2-DHB is easier to oxidize when the substituent has a more negative σ value. Overall, these results suggest that an EDG on 1,2-DHB increases the electron density over the hydroxyl group of the catechol portion, promoting internal electron transfer in the complex [Fe(1,2-DHB)]+, which results in the reduction of iron in the coordination sphere. This result agrees with other reports,39,40 wherein E° is strongly related to the ability of another DHB molecule to reduce Fe(III). D



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Figure 5. Absorption spectra for solutions with Fe(III) (1 × 10−4 mol·L−1) and 1,2-DHB (1 × 10−2 mol·L−1) at the initial time (t = 0 s, continuous lines) and after 60 s (dotted lines): (A) 4-tert-butylcatechol (σ = −0.30) and (B) 3,4-dihydroxybenzonitrile (σ = 1.49).

Scheme 2. Proposed Pathways for Fenton-like Systems Driven by 1,2-DHB with (A) Electron-Donating Group (EDG) and (B) with Electron-Withdrawing Group (EWG)

iron should remain inside of the monocomplex. This is because the tautomeric valence equilibrium is displaced from Fe(II)− semiquinone to Fe(III)−catecholate (Scheme 2B). Thus, it can be concluded that the main iron species available to react with H2O2 are different in the studied Fenton systems, depending on the kind of 1,2-DHB. In Fenton systems driven by 1,2-DHB with EDGs, the H2O2 reacts with free Fe(II) by a conventional Fenton reaction (Scheme 2A). Otherwise, in Fenton systems driven by 1,2-DHB with EWG the H2O2 react mainly with the iron inside the monocomplex which has higher stability because the EWG on the 1,2-DHB. The formation of Fe(III) peroxocomplexes have been described in the literature.16,41−43 In Scheme 2B is postulated a possible pathway to produce HO· from iron-peroxocomplexes. When the monocomplex of Fe(III) is formed with a 1,2DHB with EWG, the H2O2 reacts mainly with Fe(III)− catecholate monocomplex, but also with a few portions of the Fe(II)−semiquinone monocomplex (Scheme 2B). The reactivity of the peroxocomplexes of Fe(II) is expected to be higher than peroxocomplexes with Fe(III) whereby the H2O2 reacts faster with Fe(II) producing HO· and Fe(III). In this way the Fe(II)−semiquinone is consumed and the tautomeric valence equilibrium is displaced. If this pathway is considered, the electron is transferred from the 1,2-DHB to H2O2 through the iron.

amount of HO· produced in each system. Both results indicate that only Fenton-like systems driven by 1,2-DHBs with EDGs depend on the ability of 1,2-DHB to reduce Fe(III) and produce HO·. In addition, a significant difference was observed between the stability of the monocomplex when the substituent was EDG or EWG (Figure S5 in the SI). After 1 min, the spectrophotometric signal of the monocomplex formed from 1,2-DHBs with EDGs decreased by 70.2% (Figure 5A), whereas the monocomplex formed from 1,2-DHBs with EWGs did not significantly change (Figure 5B). This higher stability of the monocomplex with EWGs will allow the reaction of the monocomplex with H2O2 and the subsequent production of HO·. In summary, the monocomplex stability changes depending on the substituent on the 1,2-DHB. This change in the reactivity affects the ability of the system to reduce Fe(III) but not its ability to increase the HO· production. If the tautomeric valence equilibrium is considered the substituent in the 1,2-DHB could be displacing the equilibrium Fe(III)−catecholate/Fe(II)−semiquinone. In this way the EDG displaces the equilibrium from Fe(III)−catecholate to Fe(II)−semiquinone (Scheme 2A), whereby the EDG on the 1,2-DHB, promotes the internal electron transference with the consequent release of Fe(II). Otherwise, the systems of 1,2DHB with EWG show a low amount of free Fe(II) since the E



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ASSOCIATED CONTENT

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* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b05388. Structure and absorption spectrum of MO, structures and E° of 1,2-DHBs employed in this study, absorption spectra of complexes formed at pH 3.4 after 1 min of mixing Fe(III) with 1,2-DHB, and kinetic data obtained from MO oxidation and HO· production (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +56412204601; e-mail: [email protected]. ORCID

David Contreras: 0000-0003-1346-5918 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support for this work was provided by FONDECYT (Grant 1160100), FONDEQUIP (Grant EQM140075), FONDAP Solar Energy Research Center, SERC-Chile (Grant 15110019), CONICYT (Ph.D. Grant 21120966), REDOC-UDEC and UDT-CCTE fellowship (Grant PFT-072).

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