Production of Hydroxyl Radical via the Activation of Hydrogen

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Production of Hydroxyl Radical via the Activation of Hydrogen Peroxide by Hydroxylamine Liwei Chen, Xuchun Li, Jing Zhang, Jingyun Fang, Yanmin Huang, Ping Wang, and Jun Ma Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 14 Aug 2015 Downloaded from http://pubs.acs.org on August 14, 2015

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Environmental Science & Technology

Production of Hydroxyl Radical via the Activation of Hydrogen Peroxide by Hydroxylamine ⊥

Liwei Chen,*,† Xuchun Li,§ Jing Zhang,ǁ Jingyun Fang, Yanmin Huang,† Ping Wang,† and Jun Ma*,‡ †

College of Biology and the Environment, Nanjing Forestry University, Nanjing 210037, China

§

School of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou

310018, China ǁ

College of Architecture and Environment, Sichuan University, Chengdu 610065, China

⊥

School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou

510275, China ‡

State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of

Technology, Harbin 150090, China * Corresponding Authors (Liwei

Chen)

Phone:

+86-025-85427479;

fax:

+86-025-85418873;

e-mail:

[email protected] (Jun Ma) Phone: +86-451-86282292; fax: +86-451-8628-3010; e-mail: [email protected].

ACS Paragon Plus Environment

1


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TOC/Abstract art

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ABSTRACT

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The production of hydroxyl radical (HO•) is important in environmental chemistry. This study

5

reports a new source of HO• generated solely from hydrogen peroxide (H2O2) activated by

6

hydroxylamine (HA). Electron paramagnetic resonance analysis and the oxidation of a HO•

7

probe benzoic acid were used to confirm the production of HO•. The production of HO•

8

increased with increasing concentrations of either HA or H2O2 as well as decreasing pH. The

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second-order rate constant for the reaction was (2.2 ± 0.2) × 10−4 M−1 s−1. HO• was probably

10

produced in two steps: the activation of H2O2 by protonated HA and then reaction between the

11

H2O2 and the intermediate protonated aminoxyl radical generated in the first step. Such two-step

12

oxidation can possibly be ascribed to the ionizable hydroxyl moiety in the molecular structure of

13

HA, as is suggested by comparing the reactivity of a series of HA derivatives in HO• production.

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The results shed light on a previously unknown source of HO• formation, which broadens the

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understanding of its role in environmental processes.

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INTRODUCTION


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The hydroxyl radical (HO•) is a most reactive species which can oxidize most organic

18

substrates unselectively.1,2 It has been widely employed to degrade an enormous variety of

19

recalcitrant and toxic contaminants in advanced oxidation processes (AOPs) such as the Fenton

20

reaction, radiolysis, UV/H2O2 purification, O3 oxidation and electrochemistry.3,4 The

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mechanisms of HO• generation and its involvement in redox reactions with contaminants in the

22

environment have been investigated intensely for decades.2–5 HO• is usually produced via metal-

23

involved reactions such as the Fenton reaction or using energy-input systems as in H2O2/UV

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purification.6,7 However, metal-independent and energy-input free production of HO• has been

25

reported occasionally,8–12 an intriguing and attractive finding.

26

The mechanism of metal-independent production of HO• is still under discussion, but it has

27

been established that halogenated quinones and some reducing organic species such as ascorbic

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acid, phenols, reduced humic acid and biochar containing hydroquinone and semiquinone

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moieties can enable the generation of active radicals with oxidants.8–12 One of the underlying

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mechanisms involves nucleophilic attack of halogenated quinones by H2O2, their product then

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decomposes homolytically to produce HO•.8 Another mechanism involves electron transfer.

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Ascorbic acid undergoes a two-step oxidation by H2O2 to yield HO• and dehydroascorbic acid

33

with the formation of an intermediate ascorbyl radical.9 And hydroquinone moieties in reduced

34

humic acids are known to reduce O2 to form H2O2 and semiquinones, which then react with the

35

H2O2 to generate HO•.10 Semiquinone-type persistent free radicals in biochar can react directly

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with H2O2 by single-electron transfer and reduce it to HO•.11 Moreover, phenols can activate

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persulfate to form sulfate radicals, which are then transformed to HO• with hydroxide at alkaline

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pH or water at lower pH.12 The common feature linking ascorbic acid, phenols, reduced humic


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acids and biochar is the possession of ionizable hydroxyl moiety or a relevant C-centered radical

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which can participate in the production of HO•.9–12

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Hydroxylamine (HA), a widely used reductant and antioxidant,13 contains ionizable hydroxyl

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moiety and can undergo electron transfer or H-atom abstraction.14–17 Previous work in our

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laboratory has indicated that it can accelerate Fe(II)/Fe(III) redox cycles to enhance the

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generation of reactive radicals dramatically in a Fenton system or an Fe(II)/peroxymonosulfate

45

system.18,19 Considering its similar reducing activity and the ionizable hydroxyl group, it is

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hypothesized that HA may activate H2O2 to form HO• without a transition metal. Radical species

47

are very probably produced as intermediates in the electron transfer process between H2O2 and

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HA,20 though the mechanism of HO• production involving HA and H2O2 is still unknown.

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The objectives of this study were to explore the potential of the reaction between HA and H2O2

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as a new source of HO• and to reveal the mechanism involved. Electron paramagnetic resonance

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(EPR) technique and the oxidation of a HO• probe benzoic acid (BA) were used to illustrate the

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HO• production. The effects of pH and the concentrations of HA and H2O2 on HO• production

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were explored, and the potential pathways for HO• generation will be discussed. By studying

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HO• production by H2O2 with a series of HA derivatives (HAs), the role of ionizable hydroxyl

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moiety in the molecular structure of the HAs during the process was revealed and a plausible

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mechanism can now be proposed.

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MATERIALS AND METHODS

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Materials. All of the chemicals listed in Text S1 of the supporting information were used as

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purchased without further purification. Stock solutions of HA and its derivatives were all freshly

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prepared using deaerated Milli-Q water (deaerated using nitrogen).


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Experimental Procedures. The EPR experiments were conducted at room temperature on a

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Bruker A200 300E instrument. Solutions of H2O2, HA, the spin-trapping agent 5,5-dimethyl-1-

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pyrroline N-oxide (DMPO), transition metal ions, HO• scavengers or iron chelators were mixed

64

and deaerated with nitrogen. They were buffered at the desired pH with perchloric acid or acetate

65

for the detection of HO•. The possible influence of perchloric acid on the system was negligible

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(Figure S1 and Text S2 in the supplementary information). After mixing for about one minute (±

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3 s), the sample solution was transferred into a 100 µL capillary tube, which was then fixed in

68

the cavity of the EPR spectrometer. Ethyl N-hydroxycarbamate (a derivative of HA) was

69

similarly mixed with H2O2 for the detection of relevant aminoxyl radicals.21

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All of the other experiments were carried out in triangular flasks with constant stirring using a

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PTFE-coated magnetic stirrer in Milli-Q water adjusted to pH 3.0 with perchloric acid and

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deaerated with nitrogen throughout the experiments. The concentration of dissolved oxygen

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(DO) in the solution was below the detection limit of a DO meter (less than 0.31 µM). The

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solutions’ temperature was maintained at 25 ± 0.5 °C. All of the experiments were performed at

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least in duplicate. In each run, HA, BA, perchloric acid or tert-butanol were mixed in the desired

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dosages and the experiment was switched on by adding the desired dosage of H2O2. The pH

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changed less than 0.1 unit during the experiments. In order to avoid the interference of nitrogen

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in the detection of N2O generated in the system, argon (Ar, 99.99 %) instead of nitrogen was

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used to remove oxygen in the solution before the initiation of the reaction. The flasks were

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totally sealed to prevent the escape of N2O from the solution and to guarantee the oxygen-free

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conditions during the process.

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Sample Analysis. The concentrations of BA, m-hydroxybenzoic acid (MHBA), o-

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hydroxybenzoic acid (OHBA) and p-hydroxybenzoic acid (PHBA) were determined on high


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performance liquid chromatography (HPLC, Dionex Ultimate 3000) equipped with a reverse-

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phase C18 column. The pH was measured with a Denver Instrument Ultrabasic 10 pH meter.

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The concentration of DO was measured by a DO meter (Rex Electric Chemical, JPSJ-605). The

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H2O2 concentrations were measured by a colorimetric method using peroxidase and N,N-diethyl-

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p-phenylenediamine (DPD) on a PE Lambda 25 UV-vis spectrometer.22 The HA samples were

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derivatized to acetone oxime using acetone and were subsequently extracted using methyl tert-

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butyl ether (MTBE). The acetone oxime concentrations were measured using a Shimadzu GC-

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2010 gas chromatograph with a flame ionization detector (FID) and N2 as the carrier gas.23 Ion

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chromatography (Dionex ICS-900) was employed for the detection of NO2− and NO3−. The

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concentration of dissolved N2O was analyzed using gas chromatograph (Agilent 7890) equipped

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with an electron capture detector (ECD) and a headspace sampler.19,24 Additional details of the

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sample analysis procedures are presented in Text S3 of the supplementary information.

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RESULTS AND DISCUSSION

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HO• Production. Figure 1a shows typical EPR signals of DMPO-OH during the reaction of

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HA with H2O2. The typical DMPO-OH spin adduct (aH = aN = 14.9 G), which has a specific

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quartet spectrum with peak height ratios of 1:2:2:1, indicates the formation of HO•.25 It should be

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noted that a similar DMPO-OH signal may arise from the decomposition of DMPO-OOH and

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the nucleophilic addition to DMPO in the presence of Fe(III) or Cu(II).26,27 Additional analysis

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was therefore carried out to confirm the production of HO•. Dimethyl sulfoxide (DMSO), azide

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and formate react with HO• to generate methyl radicals (CH3•), azidyl radicals (N3•) and carbon

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dioxide anion radicals (CO2−•), respectively, as shown in equations (eqs. 1–3). These

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intermediate radicals can also be captured by DMPO to yield the corresponding DMPO-CH3 (aH

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= 23.4 G, aN = 16.4 G), DMPO-N3 (aH = aN = 14.7 G) and DMPO-CO2− (aH = 19.3 G, aN = 15.9


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G) spin adducts (eqs. 4–6).26 DMSO, azide and formate were therefore adopted in the EPR study

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to verify the formation of HO•. The spin adducts of DMPO-CH3, DMPO-N3 and DMPO-CO2−

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were generated instead of DMPO-OH when excessive DMSO, azide or formate was present in

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the system, accompanied by a marked decrease in or disappearance of the DMPO-OH spin

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adducts (Figure 1b). These EPR results indicate the generation of HO• in the system.

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HO• + (CH3)2SO → CH3SO2H + CH3•

(1)

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HO• + N3− → OH− + N3•

(2)

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HO• + HCO2− → H2O + CO2−•

(3)

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CH3• + DMPO → DMPO-CH3

(4)

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N3• + DMPO → DMPO-N3

(5)

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CO2−• + DMPO → DMPO-CO2−

(6)

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Further evidence comes from the inhibition effect of tert-butanol (TBA), which is a strong

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HO• scavenger (kTBA, HO• = 7.6 × 108 M−1 s−1),28 on the competition of HO• with BA, the HO•

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probe. Figure S2 shows almost complete inhibition of BA oxidation by 0.1 M TBA, suggesting

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the dominant contribution of HO• to BA oxidation in the system. Moreover, the m-

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hydroxybenzoic acid (MHBA), o-hydroxybenzoic acid (OHBA) and p-hydroxybenzoic acid

123

(PHBA) are the primary intermediates, so their detection during the oxidation of BA also helps

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confirm the involvement of HO• in the system.29,30 As is shown in Figure S3, MHBA, OHBA

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and PHBA were all detected, further supporting the generation of HO•.

126

Possible Influence of Trace Transition Metal Ions. Trace transition metal impurities in the

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reagents may possibly have enhanced the generation of HO•. As can be seen from Figure S4,

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only iron could notably enhance the intensity of the HO• signal. This was readily excluded by

129

using ferrozine and ferene as iron chelators.26 Almost complete disappearance of the DMPO-OH


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signal in a traditional Fenton system when ferrozine or ferene is present, confirming their

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excellent iron complexing ability, shown in Figure S5. In contrast, no significant decrease in the

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intensity of the HO• signal was observed in HA-H2O2 reactions in the presence of sufficient

133

ferrozine or ferene (Figure 2), indicating that trace iron was not responsible for the HO•

134

production. The generation HO• by the reaction of HA and H2O2 is indeed metal-independent.

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Effect of pH. Figure 3 presents the profiles of DMPO-OH spin adducts at pH ranging from 2.0

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to 6.0. The intensity of the HO• signal decreased with increasing pH, and it almost disappeared at

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pHs higher than 6.0.

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The pKa values for HA are 5.96 and 13.74.31,32 When the pH is less than 5, the majority of the

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HA is in its protonated form (H3NOH+), it is mainly in its unprotonated form (H2NOH) at pHs in

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the range of 7.0 to 12.0. The second-order rate constants for the reactions of HO• with H3NOH+

141

and H2NOH are ≤ 5.0 × 108 M-1 s-1 and 9.5 × 109 M-1 s-1, respectively,33 so the scavenging of

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HO• by HA increases with increasing pH. In addition, some researchers have proposed that the

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protonated form of the hydroxylamino radical, the one-electron transfer intermediate of HA with

144

a pKa of about 4.2, reacts much faster with H2O2 than the unprotonated hydroxylamino radical to

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produce HO•.17,33 Under strongly acidic conditions, most of the hydroxylamino radical is

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protonated, and thus the HO• production increased with decreasing pH. The pH might therefore

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affect HO• generation mainly through controlling the species distribution of HA and the

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intermediate hydroxylamino radical.

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Effect of HA and H2O2 Concentrations. The EPR results presented in Figure S6 show that

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the concentration of HO• increased with increasing concentrations of either HA or H2O2,

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indicating the involvement of the two reagents in the HO• generation. The reaction between

152

H2O2 and H3NOH+ was studied under acidic conditions by purging with N2 constantly to


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minimize the influence of side reactions such as oxidation of unprotonated HA by HO•. Under

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those conditions the k was determined to be (2.2 ± 0.2) × 10−4 M−1 s−1 (Text S4 and Figure S7 of

155

the supplementary information), which seems relatively low.

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Role of the Molecular Structure of the Hydroxylamines. The contribution of the series of

157

hydroxylamine derivatives (HAs, shown in Figure 4) to HO• generation with H2O2 was

158

investigated. Figure 5 shows the individual DMPO-OH spin adduct in each HAs-H2O2 reaction.

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HO• production was observed in the case of N-methyl hydroxylamine (N-methyl HA), N, N-

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dimethyl hydroxylamine (N, N-dimethyl HA), N-tert-butyl hydroxylamine (N-tert-butyl HA), N-

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benzyl hydroxylamine (N-benzyl HA) and N, O-dimethyl hydroxylamine (N, O-dimethyl HA).

162

N-methyl HA, N, N-dimethyl HA and N-tert-butyl HA were even more efficient than HA in

163

producing HO•, but N-benzyl HA and N, O-dimethyl HA were much less reactive. And no HO•

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was detected under the same conditions with the O-substituted HAs e.g. O-methyl HA, O-tert-

165

butyl hydroxylamine (O-tert-butyl HA) and O-benzyl hydroxylamine (O-benzyl HA). N-

166

substituted HAs showed much greater HO• generation than the O-substituted HAs in the

167

presence of H2O2. This suggests that the production of HO• by H2O2 and HAs is probably related

168

to the –OH group in the structure of HAs.

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Mechanism of HO• Generation. An intermediate aminoxyl radical (H2NO•) or its isomer

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hydroxylamino radical (•HNOH) is formed during the oxidation of HA by a one-electron oxidant

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or an H-atom abstractor.34 Considering that the bond dissociation energy (BDE) of O-H in HA

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(75 to 77 kcal M−1) is weaker than that of N-H (81 to 82 kcal M−1),34,35 H2NO• is more easily

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generated and stable than •HNOH.34 When HO• is generated by the reaction of HA and H2O2

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through one electron transfer under acidic conditions, H2O2 presumably reacts with protonated

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HA to produce HO• and aminoxyl radical via eq. 7. To verify this hypothesis, ethyl N-


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hydroxycarbamate, an HA derivative, was reacted with H2O2 instead of HA to generate a more

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intense EPR signal of its corresponding aminoxyl radical.21 Despite its short lifetime,35 the EPR

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signal shown in Figure 6 verified the generation of aminoxyl radical (aN =7.2 G, aN-H = 11.4 G,

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aC-H = 0.9 G) in the system,21 supporting the pathway presented in eq. 7.

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The protonated form of the hydroxylamino radical (pKa ≈ 4.2) was reported to react with H2O2

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much faster than the unprotonated hydroxylamino radical to produce HO• and nitroxyl

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(HNO).17,33 However, aminoxyl radical has been proved to be the dominating active intermediate

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in one-electron oxidation of HA aforementioned.34 Thus, we proposed that the protonated form

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of aminoxyl radical would react with H2O2 much faster to produce HNO and HO• via eq. 8.

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H2O2 + H3NOH+ → H3NO•+ + HO• + H2O

(7)

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H2O2 + H3NO•+ → HNO + HO• + H2O + H+

(8)

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The aminoxyl radical and HNO, oxidation intermediates of HA, were ultimately converted to

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nitrogen and various nitrogen oxides (e.g. N2O, NO2− and NO3−) depending on the conditions.34

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The generation of gaseous products (N2 and N2O) mainly depends on the pH. The dimerization

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of unstable HNO to form N2O is favored in strongly acidic conditions with the rate constant of

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(4.5 ± 2.7) × 109 M−1 s−1 (eq. 9).36-38 N2 could be formed by the dimerisation/cleavage of H2NO•

192

at pHs higher than 4.0 with a second-order rate constant of (2.8 ± 0.5) × 108 M−1 s−1 (eq.

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10).17,20,34 In addition, reactions involving excess H2O2 might account for the production NO3−,

194

and NO2− is probably formed as an intermediate (eqs. 11–15).17 N2O was (Figure S8) determined

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in the system at pH 3.0 to verify the formation of HNO indirectly, as HNO is unstable and it

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spontaneously dimerizes and subsequently dehydrates to form N2O with a high rate constant.38

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Figure S8 in the supplementary information also shows the concentration of NO2− and NO3−

198

generated in the HA and H2O2 reaction. The generation of NO2− and NO3− was not so evident,


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and the gaseous products N2O increased with longer reaction time, supporting the formation of

200

HNO and the two-step oxidation pathway for HO• generation (eqs. 7–8).

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2HNO → N2O + H2O

(9)

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2H2NO• → N2 + 2H2O

(10)

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H2O2 + HO• → HO2•/O2•− + H2O

(11)

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H2NO• + O2•− → NO2− + H2O

(12)

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HO• + NO2− → NO2• + OH−

(13)

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NO2• + NO2• → N2O4

(14)

207

N2O4 +H2O → NO2− + NO3− +2H+

(15)

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According to the former two-step oxidation hypothesis, the theoretical stoichiometric ratio of

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HA consumption to H2O2 depletion is 0.5 not only for protonated HA but also for the

210

intermediate protonated aminoxyl radical reacting with H2O2 (eqs. 7−8). In order to estimate the

211

actual stoichiometric ratio in the system, BA was employed to scavenge HO•. The amount of HA

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consumed was then much lower than that of H2O2 (Figure 7a). The slope in Figure 7b shows that

213

the stoichiometric ratio of HA consumption to H2O2 depletion was about 0.638, supporting the

214

statement on the two major steps (eqs. 7−8) involved in the reaction between HA and H2O2.

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Figure 7a also shows the relationship between the oxidized BA and H2O2 depletion during the

216

process. More than 95 % of the depleted H2O2 was transformed to HO• to attack BA within 24 h.

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However, the ratio of oxidized BA to H2O2 depletion gradually decreased during the process.

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This may be because H2O2 competes with HO• to form NO2− and NO3− (eqs. 11–15 and Figure

219

S8),17 and some of the HO• is consumed by the HA for the second-order rate constants of BA

220

and HA with HO• are 4.3 × 109 M−1 s−1 and ≤ 5.0 × 108 M−1 s−1, respectively.1,34 Furthermore, the

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second-order kinetic constants for HO• attacking OHBA and PHBA are 2.7 × 1010 M−1 s−1 and


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8.0 × 109 M−1 s−1, respectively,1,30 suggesting that HBAs are more reactive with HO• than BA.

223

The total amounts of MHBA, OHBA and PHBA only accounted for about half of the observed

224

BA oxidation (Figure S3), so those primary products (OHBA, MHBA and PHBA) formed in the

225

system further compete with BA for HO• and yield other intermediate products as the reaction

226

progresses.39 Thus, the estimated concentration of HO• at 1.49 × 10−16 M based on the BA

227

degradation kinetics (Figure S9) ought to be lower than the actual one in the system.

228

The two-step oxidation hypothesis is further demonstrated by the role of the molecular

229

structure of the HAs (Figure 5). N-substituted HAs with ionizable hydroxyl moiety showed much

230

greater HO• generation than O-substituted HAs. The BDE of the N-H bonds in HAs is 81 to 82

231

kcal M−1, while the BDE of O-H bonds in the N-substituted HAs illustrated here is less than 77

232

kcal M−1.35 Such N-substituted HAs tend to yield aminoxyl radicals (R2NO•) and HO• when

233

reacting with H2O2, and the aminoxyl radicals may yield oxoammonium (RN=O) and HO•

234

through reacting with H2O2 in acidic conditions. Furthermore, the substitution of –OH by

235

methoxyl on the N atom weakens the N-H bond, leading to a low yield of R2NO•.33–35 So N-

236

substituted HAs have much higher reactivity with H2O2 to produce HO• than O-substituted ones.

237

In addition, N, O-dimethyl HA shows insignificant reactivity to produce HO• because O-

238

substituted HAs can undergo only one-step oxidation with H2O2 to yield •NROR and HO•.

239

Consequently, it is hypothesized that the ionizable hydroxyl (–OH) moiety attached to an N atom

240

of HAs is transformed to an =O moiety through the formation of an intermediate –O•. This may

241

then be responsible for the production of HO• with H2O2.

242

Implication. This study has demonstrated a previously unrecognized pathway for the

243

formation of HO• from HA and H2O2. These results highlight the significance of reactions

244

between H2O2 and HA which might be catalyzed rapidly by some transition metals.14–16,38 It is


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245

therefore expected that HO• generation via eqs. 7 and 8 will be greatly enhanced. Indeed,

246

previous work in our laboratory has partially confirmed such a hypothesis in the case of Fenton

247

reactions in the presence of HA.18 H2O2 and/or superoxide radicals (e.g., HO2•) are ubiquitous in

248

aquatic environment,40,41 and the HA-like intermediates can be formed through the

249

biodegradation of nitrogen-containing species (e.g., nitrogen fixation, nitrification and

250

denitrification).35,42 The findings of this study may therefore help to elucidate the redox dynamics

251

of contaminants in natural processes.

252

ASSOCIATED CONTENT

253

Supporting Information.

254

Four texts and nine figures are available with further information about the materials and

255

additional data. This material is available free of charge via the Internet at http://pubs.acs.org.

256

AUTHOR INFORMATION

257

Corresponding Authors

258

* (Liwei Chen) Phone: +86-025-85427479; fax: +86-025-85418873; e-mail:

259

[email protected]

260

* (Jun Ma) Phone: +86-451-86282292; fax: +86-451-8628-3010; e-mail: [email protected].

261

Notes

262

The authors declare no competing financial interest.

263

ACKNOWLEDGMENTS

264

This study was supported by the Natural Science Foundation of China (grants 51408317,

265

21307057 and 51378515) and the Natural Science Foundation of Jiangsu Province (BK20140966


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and BK20130577). Funds from the priority academic program development of Jiangsu higher

267

education institutions (PAPD) are also gratefully acknowledged. The authors sincerely thank

268

Chengyue Sun, Associate Professor Jimin Shen, Jing Zou (Harbin Institute of Technology),

269

Yunxia Sui (Nanjing University), Lei Yu, Qinhua Fan and Qi Li (Nanjing Forestry University)

270

for their technical help with the EPR, GC and ion chromatography analysis.

271

REFERENCES

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(1) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Critical review of rate

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(2) Goldstone, J. V.; Pullin, M. J.; Bertilsson, S.; Voelker, B. M. Reactions of hydroxyl radical

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with humic substances: Bleaching, mineralization, and production of bioavailable carbon

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393

Figure legends:

394

Figure 1. The EPR signals of DMPO spin trapping adducts produced by (a) HA and H2O2, and

395

(b) HA and H2O2 in the presence of HO• scavengers. Reaction conditions: [DMPO]0 = 20.0 mM,

396

[H2O2]0 = 10.0 mM, [HA]0 = 10.0 mM, [scavengers]0 = 500.0 mM, pH = 3.0, 25 °C, DMPO-OH:

397

■, DMPO-CH3: ∆, DMPO-N3: ○, DMPO-CO2: *.

398

Figure 2. Effect of the iron chelating agents ferrozine and ferene on the EPR signals of DMPO-

399

OH spin trapping adducts produced by HA and H2O2. Reaction conditions: [DMPO]0 = 20.0

400

mM, [H2O2]0 = 10.0 mM, [HA]0 = 10.0 mM, [ferrozine]0 = 50.0 µM, [ferene]0 = 50.0 µM, pH =

401

3.0, 25 °C.

402

Figure 3. Effect of pH with acetate buffer on the EPR signals of DMPO-OH spin trapping

403

adducts produced by HA and H2O2 in the pH range of 2.0 to 6.0. Reaction conditions: [DMPO]0

404

= 20.0 mM, [H2O2]0 = 10.0 mM, [HA]0 = 10.0 mM, 25 °C.

405

Figure 4. Chemical structures of the HAs used.

406

Figure 5. EPR signals of DMPO-OH spin trapping adducts produced by H2O2 and HAs.

407

Reaction conditions: [DMPO]0 = 20.0 mM, [H2O2]0 = 10.0 mM, [HAs]0 = 10.0 mM, pH = 3.0, 25

408

°C.

409

Figure 6. EPR signal of aminoxyl radical produced by H2O2 and ethyl N-hydroxycarbamate.

410

Reaction conditions: [ethyl N-hydroxycarbamate]0 = 50.0 mM, [H2O2]0 = 50.0 mM, pH = 3.0, 25

411

°C.


21


Page 22 of 29

412

Figure 7. (a) The concentration changes of reagents and (b) the ratio of HA consumption to

413

H2O2 depletion during the oxidation of BA by HA and H2O2. Reaction conditions: [BA]0 = 0.4

414

mM, [H2O2]0 = 0.4 mM, [HA]0 = 0.4 mM, pH = 3.0, 25 °C.


22

Page 23 of 29


415

416 417

Figure 1. The EPR signals of DMPO spin trapping adducts produced by (a) HA and H2O2, and

418

(b) HA and H2O2 in the presence of HO• scavengers. Reaction conditions: [DMPO]0 = 20.0 mM,

419

[H2O2]0 = 10.0 mM, [HA]0 = 10.0 mM, [scavengers]0 = 500.0 mM, pH = 3.0, 25 °C, DMPO-OH:

420

■, DMPO-CH3: ∆, DMPO-N3: ○, DMPO-CO2: *.


23


Page 24 of 29

421 422

Figure 2. Effect of the iron chelating agents ferrozine and ferene on the EPR signals of DMPO-

423

OH spin trapping adducts produced by HA and H2O2. Reaction conditions: [DMPO]0 = 20.0

424

mM, [H2O2]0 = 10.0 mM, [HA]0 = 10.0 mM, [ferrozine]0 = 50.0 µM, [ferene]0 = 50.0 µM, pH =

425

3.0, 25 °C.


24

Page 25 of 29


426 427

Figure 3. Effect of pH with acetate buffer on the EPR signals of DMPO-OH spin trapping

428

adducts produced by HA and H2O2 in the pH range of 2.0 to 6.0. Reaction conditions: [DMPO]0

429

= 20.0 mM, [H2O2]0 = 10.0 mM, [HA]0 = 10.0 mM, 25 °C.


25


Page 26 of 29

430 431

Figure 4. Chemical structures of the HAs used.


26

Page 27 of 29


432 433

Figure 5. EPR signals of DMPO-OH spin trapping adducts produced by H2O2 and HAs.

434

Reaction conditions: [DMPO]0 = 20.0 mM, [H2O2]0 = 10.0 mM, [HAs]0 = 10.0 mM, pH = 3.0, 25

435

°C.


27


Page 28 of 29

436 437

Figure 6. EPR signal of aminoxyl radical produced by H2O2 and ethyl N-hydroxycarbamate.

438

Reaction conditions: [ethyl N-hydroxycarbamate]0 = 50.0 mM, [H2O2]0 = 50.0 mM, pH = 3.0, 25

439

°C.


28

Page 29 of 29


440

441 442

Figure 7. (a) The concentration changes of reagents and (b) the ratio of HA consumption to

443

H2O2 depletion during the oxidation of BA by HA and H2O2. Reaction conditions: [BA]0 = 0.4

444

mM, [H2O2]0 = 0.4 mM, [HA]0 = 0.4 mM, pH = 3.0, 25 °C.


29

Production of Hydroxyl Radical via the Activation of Hydrogen

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