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Dec 1, 2014 - degradation of diethyl phthalate (DEP). It was found that AC exhibited excellent catalytic ability for decomposition H2O2 and degradatio...
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New Insights into the Mechanism of the Catalytic Decomposition of Hydrogen Peroxide by Activated Carbon: Implications for Degradation of Diethyl Phthalate Guo-dong Fang, Cun Liu, Juan Gao,* and Dong-mei Zhou* Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, P. R. China S Supporting Information *

ABSTRACT: This study investigated the catalytic decomposition of H2O2 by activated carbon (AC) and its implications for degradation of diethyl phthalate (DEP). It was found that AC exhibited excellent catalytic ability for decomposition H2O2 and degradation of DEP. HNO3 modification altered the surface characteristics of AC together with the concentrations and types of AC free radicals (FRs), which further promoted generation of •OH. Positive correlations were found between FR concentration and generation of •OH (R2 = 0.856) and between the proportion of surface-bound hydroxyl groups (C−OH) and the decomposition rate of H2O2 (R2 = 0.776), indicating that FRs in AC were the main contributor to •OH generation, whereas C− OH groups were predominantly responsible for decomposition of H2O2. Electron capturing studies demonstrated that the decomposition reaction likely involves the transfer of FR electrons to H2O2 to induce formation of •OH.

1. INTRODUCTION Activated carbon (AC) has been extensively applied as an environmentally friendly adsorbent in removal of organic compounds from aqueous solutions and as a solid catalyst or catalyst support in catalytic wet peroxide oxidation process1−3 due to its large surface area, well-developed porous structure, and surface functional groups. Nevertheless, the contaminantloaded AC has to be treated as hazardous waste that must be disposed of or regenerated, since adsorption only transfers the contaminants from the aqueous solution to AC, and it does not destroy the contaminants.4 Catalytic decomposition of H2O2 by activated carbon (AC) has been widely used to degrade contaminants and regenerate AC.5−8 The hydroxyl radicals (•OH) and superoxide radical anions (•OOH/O2•−) generated by AC/H2O2 are known to be responsible for degradation of contaminants.9−11 However, the mechanism of •OH generation in the AC/H2O2 system remains elusive. The catalytic activity of AC depends on its characteristics including surface area, porosity, specific surface inertness, and surface functional groups.12,13 Among these characteristics, however, it is difficult to identify the factors responsible for •OH generation in the AC/H2O2 system. For example, Bansal et al.14 found that catalytic decomposition of H2O2 by ACs was described by the following reactions (eqs 1 and 2). AC−OH + H 2O2 → AC−OOH + H 2O (1) AC−OOH + H 2O2 → AC−OH + H 2O + O2

acceptors on the surface of AC is also an accepted mechanism for decomposition of H2O2 into •OH; this process follows a similar pathway as the Fenton reactions16 and proceeds as follows (3)

AC+ + H 2O2 → AC + H+ + HO•2

(4)

However, few studies have provided direct evidence of •OH formation or identified free radicals (e.g., AC+) to support this hypothesis. More recently, Dominguez et al. used the cyclic voltammetry technique to evaluate the catalytic activity of carbon materials (e.g., AC, carbon blacks, and graphites) and found that the exchange current is directly proportional to the catalytic activity of carbon materials for H2O2 decomposition.17,18 Dominguez’s studies provided a new parameter to assess the catalytic activity of AC toward H2O2 decomposition, but the relationship between the exchange current and •OH generation was not clearly elucidated in these publications. Therefore, the main objectives of this study were to investigate the mechanisms of the catalytic decomposition of H2O2 and generation of •OH in the AC/H2O2 system. Electron paramagnetic resonance (EPR) and salicylic acid (SA) free radical trapping methods were used to quantify the FR concentrations in AC and the •OH during the experiments. Diethyl phthalate (DEP, Scheme S1, Supporting Information) was selected as a model phthalate ester (PAE, Scheme S1, Supporting Information) contaminant not only because the mechanism of DEP degradation by •OH has been thoroughly

(2)

Boehm titration results showed that basic groups (C−OH) on the surface of AC favored H2O2 decomposition, while acidic groups (e.g., COOH) inhibited H2O2 decomposition.15 However, reactions 1 and 2 would not contribute to formation of •OH or explain the mechanism of •OH production in the AC/H2O2 system. In addition to the above direct H2O2 decomposition reaction, electron transfer via electron donors/ © 2014 American Chemical Society

AC + H 2O2 → AC+ + OH− +• OH

Received: Revised: Accepted: Published: 19925

October 22, 2014 November 26, 2014 December 1, 2014 December 1, 2014 dx.doi.org/10.1021/ie504184r | Ind. Eng. Chem. Res. 2014, 53, 19925−19933

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Table 1. Experimental C1(s), Binding Energy (eV), and Chemical State Assignments for AC and Modified AC Samples sample

C1 (284.6 eV), C (aromatic)

C2 (285.2 eV), C (aliphatic “defects”)

C3 (286.1 eV), C− OH

C4 (287.0 eV), CO

C5 (288.5 eV), COOH; COOC

AC−X AC−X−N2 AC−X−N4 AC−W AC−W−N2 AC−W−N4 AC−S AC−S−N2 AC−S−N4

74.3% 70.9% 66.8% 82.5% 70.3% 69.1% 78.9% 50.7% 43.0%

8.18% 11.1% 12.3% 11.6% 20.8% 13.7% 6.45% 34.2% 42.0%

9.31% 5.26% 4.54% 3.52% 2.60% 1.98% 5.40% 1.88% 0.681%

3.85% 6.00% 7.68% 1.44% 4.07% 7.92% 5.29% 6.62% 7.47%

4.35% 6.68% 8.67% 0.981% 2.24% 7.31% 3.96% 6.64% 6.85%

AC were quantified by EPR using a single crystal of ruby (1.5 × 1.5 × 1.5 mm) doped with 3.69 × 1015 Cr3+/mg as a standard according to previous studies.23,24 2.4. Degradation Experiments. Batch experiments were conducted in the dark in 40 mL brown serum bottles containing 20 mL of reaction solutions and sealed with Teflon Mininert valves. Briefly, 20 mg of AC was dispersed into 19 mL of DEP solution (0.25 mM) at 25 °C, and 1.0 mL of 0.4 M H2O2 (the final measured concentration was 18 mM, pH 6.8) was then quickly added to initiate the reaction. Mixtures were kept horizontally shaking at 150 rpm at 25 °C. Control experiments without AC or H2O2 were also conducted under the same reaction conditions. At a designed time point, 2.0 mL of ethanol was added to quench the reaction. Subsequently, the suspension was centrifuged and filtered through 0.22 μm membrane filters, and the DEP in the supernatant was analyzed using HPLC (Aglient1260, USA). Centrifuged AC particles were collected and divided into two subsamples. One subsample was freeze dried (Christ LD-1-2, Germany) for EPR analysis of FR concentration after reaction, while the other subsample was extracted by methanol to quantify adsorbed DEP in the AC particles. The recovery rate for extraction of DEP from AC was in the range of 80−100%. The pH in all experiments was adjusted to 6.8 by 0.1 M NaOH and HClO4 prior to degradation reaction. Adsorption experiments were carried out under the same reaction conditions in the absence of H2O2 to investigate the adsorption of DEP on AC. Experiments to probe the production of •OH from AC/H2O2 were performed under the same reaction conditions depicted in Text S2, Supporting Information. The analytical methods used in this study are presented in Text S3, Supporting Information.

investigated in previous studies19,20 but also because PAE was extensively detected in the soil, water, and sediment.21,22

2. MATERIALS AND METHODS 2.1. Materials. The chemicals used in this study are described in Text S1 (Supporting Information). 2.2. Characterization of Activated Carbon. Three types of commercial activated carbon with different physicochemical properties were obtained from Sigma-Aldrich (AC-S, catalog no. 242276, USA), XFNANO Materials Tech (AC-X, catalog no. XF026, Jiangsu, China), and China National Medicines Corporation Ltd. (AC-W, Beijing, China). AC samples were thoroughly washed with deionized water, dried at 105 °C for 12 h, and then stored in a desiccator. Cleaned and dried AC samples were modified with HNO3 to change their surface functional groups by stirring 10 g of AC in 200 mL of 66% HNO3 at 90 °C for 2 and 4 h. Subsequently, modified AC samples were washed repeatedly with deionized water until the pH stabilized and dried at 105 °C for 12 h. Modified AC samples are denoted as AC−X−N2 (AC−X for 2 h), AC−X− N4 (4 h), AC−W−N2 (AC−W for 2), AC−W−N4 (4 h), AC−S−N2 (AC−S for 2h), and AC−S−N4 (4 h). The Brunauer−Emmett−Teller (BET) specific surface areas, average pore sizes, and pore volumes of the samples were determined by multipoint N2 adsorption−desorption using a Micromeritics ASAP2010 accelerated surface area and porosimetry system (Micromeritics Co., USA). Morphologies were examined by scanning electron microscopy (SEM; HitachiS3400N II, Japan). To obtain detailed information regarding surface functional groups, AC samples were characterized by Fourier transform infrared (FTIR; Nicolet380, Thermo Fisher Scientific, USA) spectroscopy and X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe, UIVAC-PHI, Japan). Binding energies for XPS spectra were calibrated by setting C to 1 s at 284.6 eV. The Boehm titration method was applied to determine the concentrations of the surface O functionalities of AC samples. 2.3. Electron Paramagnetic Resonance Studies. To identify the concentrations and types of FRs in AC, a Bruker EMX 10/12 spectrometer (Germany) was applied at a resonance frequency of 9.77 GHz, a microwave power of 20.02 mW, and a modulation frequency of 100 kHz (modulation amplitude of 1.0 G, sweep width of 100 G, time constant of 40.96 ms, sweep time of 83.89 s, and receiver gain of 1.0 × 103). The types of FRs were identify by the g factors of the EPR spectra calculated by the following equation ΔE = hν = gμBB0, where the μB is the Bohr magneton, B0 is the magnetic field, ΔE is the change in energy, h is Planck’s constant, and ν is the frequency of the radiation.23 The concentrations of FRs in

3. RESULTS AND DISCUSSION 3.1. Characterization of AC Samples. SEM images of AC samples revealed amorphous structures with highly heterogeneous surfaces (Figure S1, Supporting Information). To examine the effects of HNO3 oxidation on AC surfaces, modified AC samples were analyzed by FTIR spectroscopy and Boehm titration. Oxygen-containing functional groups such as −OH, CO, and phenolic−OH (bands at 3440, 1610, and 1190, respectively) were dominant in the FTIR spectrum of AC without HNO3 oxidation (Figure S2a, Supporting Information). After HNO3 treatment, the intensities of the phenolic− OH and CO of the modified AC−X samples increased significantly (Figure S2b, Supporting Information). Furthermore, a new group of bands at 1710 cm−1, characteristic of ketone groups (e.g., CO−C), appeared in the FTIR spectra of modified AC−X. Boehm titration results suggested that three groups of organic acids (carboxylic, lactonic, and phenolic) 19926

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Figure 1. Catalytic decomposition of H2O2 by AC and modified AC: (a) kinetics of H2O2 decomposition by AC−X and AC−X treated by HNO3 for 2 and 4 h (AC−X−N2 and AC−X−N4, respectively), (b) pseudo-first-order fitting for H2O2 decomposition, (c) pseudo-first-order H2O2 decomposition rate constants (kH2O2) of AC and modified AC samples, and (d) changes in the trapped [OH] in the AC/H2O2 system determined using salicylic acid methods. Reaction conditions: [AC−X] = [AC−X−-N2] = [AC−X−-N4] = 1.0 g/L; [H2O2]0 = 18 mM; [salicylic acid ]0 = 5.0 mM; T = 25 °C; pH = 6.8.

agreement with the FTIR spectroscopy and Boehm titration results. 3.2. Catalytic Decomposition of H2O2 and Generation of •OH from AC/H2O2. As shown in Figure 1a, H2O2 concentration decreased rapidly from 18 to 6.7 mM, 18 to 10 mM, and 18 to 9.9 mM for AC−X, AC−X−N2, and AC−X− N4, respectively. The kinetics of H2O2 decomposition over AC can be described by a pseudo-first-order rate equation (Figure 1b, R2 > 0.96); similar results were also observed in other types of AC and modified AC (Figure S5, Supporting Information). Figure 1c shows that after oxidation by HNO3 for 4 h the pseudo-first-order rate constant for H2O2 decomposition (kH2O2) decreased markedly from 0.0039 to 0.0024 min−1 for AC−X, from 0.0030 to 0.0019 min−1 for AC−W, and from 0.0032 to 0.0018 min−1 for AC−S. This suggested that HNO3 treatments inhibited catalytic decomposition of H2O2 by AC, which was consistent with previous studies.26 This inhibition resulted from the increase in acidic groups on AC surfaces generated by HNO3 oxidation, which negatively affects decomposition of H2O2. The above results suggested that both AC and modified AC catalyzed decomposition of H2O2 but with different catalytic activities. Among the free radicals formed in the AC/H2O2 system, • OH is the most reactive and can most efficiently degrade contaminants. Therefore, the salicylic acid (SA) method was used to indirectly quantify the •OH generated by AC/H2O2.27 As shown in Figure 1d, the trapped [OH] (in μM) was 225 for AC−X−N4, 258 for AC−W−N4, and 522 for AC−S−N4, whereas it was only 67, 125, and 48 for AC−X, AC−W, and AC−S, respectively. These results demonstrated that HNO3modified AC samples produced more •OH radicals than

increased markedly in AC specimens after HNO3 treatment (Table S1, Supporting Information). The total acidity increased sharply from 136 to 753 μmol/g for AC−X, 78.1 to 684 μmol/ g for AC−W, and 118 to 713 μmol/g for AC−S after HNO3 treatment for 4 h. The results indicated that treatment with HNO3 greatly increased the number of oxygen functional groups in AC samples, in agreement with the FTIR results. To obtain more detailed information regarding surface functionalities present on the outer surfaces of AC, XPS was used to analyze AC and modified AC (Table 1, Figures S3 and S4, Supporting Information). The C 1s photoelectron spectrum of AC included five signals attributed to the aromatic carbon groups (C1, C−C), aliphatic “defects” (C2), hydroxyl group (C3, C−OH), carbonyl group (C4, CO), and carboxylic/ ester/lactone group (C5, OC−O).17 In the spectra of the AC samples treated for 4 h with HNO3 the proportions of the C1 and C3 peaks were reduced significantly from 74.3% to 66.8% and 9.31% to 4.54% for AC−X, from 82.5% to 69.1% and 3.52% to 1.98% for AC−W, and from 78.9% to 43.0% and 5.40% to 0.681% for AC−S, respectively, indicating loss of volatile compounds and a resultant increase in surface hydrophilicity. In contrast, the proportion of the C2 peak increased markedly from 8.18% to 12.3%, 11.6% to 13.7%, and 6.45% to 42.0% for AC−X, AC−W, and AC−S, respectively. According to the literature, the defects peak can be attributed to carbon atoms in different states, and the relative percentage of this peak (aliphatic defects) depends on the degree of disorder in the carbonaceous material.25 Similarly, the C4 and C5 peaks also increased significantly, suggesting the rapid increase in carboxylic and lactonic groups during HNO3 oxidation, in 19927

dx.doi.org/10.1021/ie504184r | Ind. Eng. Chem. Res. 2014, 53, 19925−19933

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Figure 2. EPR spectra of AC and modified AC samples (0.02 g): (a) EPR spectra of untreated AC samples, (b) EPR spectra of AC−S treated with HNO3 for 2 and 4 h, (c) EPR spectra of AC−W treated with HNO3 for 2 and 4 h, and (d) EPR spectra of AC−X treated with HNO3 for 2 and 4 h.

2.0040.29 In this study, the FR g factors were obtained from EPR spectra using simulations carried out with Bruker WinEPR Acquisition software. The obtained g factors were 2.0027, 2.0029, and 2.0032 for AC−W, AC−X, and AC−S, respectively (Table 2), suggesting that carbon-centered free radicals were

unmodified AC. Figure 1c and 1d shows that HNO3 treatments inhibited decomposition of H2O2 but increased formation of • OH in AC/H2O2. This suggested that generation of •OH is not a unique pathway for consumption of H2O2; thus, other H2O2 decomposition reactions unrelated to •OH formation occur in the AC/H2O2 system. The mechanisms of these processes are discussed in the follow sections. 3.3. Determination of Free Radicals in AC and Modified AC. To further elucidate the mechanism of •OH generation in the AC/H2O2 system, EPR coupled with 5,5dimethyl-1-pyrroline N-oxide (DMPO) as the spin-trapping agent was used to detect •OH in aqueous solutions.28 Surprisingly, no DMPO−OH signal was detected in the supernatant of the AC/H2O2 system, possibly due to the high adsorption of DMPO−OH by AC, resulting in an aqueous concentration of DMPO−OH too low to be detected by EPR. However, a narrow singlet, typical of organic free radicals, was observed for AC powders (Figure 2). Peak intensities of the singlet signal were 1800 au for AC−W, 1000 au for AC−X, and 600 au for AC−S (Figure 2a). These results suggested that the AC samples contain free radicals (FRs), although their FR concentrations varied markedly. FR concentrations increased sharply when AC was treated by HNO3 (Figure 2b−d); increases in FR peak intensity of nearly 11 times for AC−W, 10 times for AC−X, and 89 times for AC−S were observed after treatment with HNO3 for 4 h. The g factor of an EPR spectrum is a useful parameter for identifying the types of free radicals. The g factors of carboncentered radicals are known to be less than 2.0030, while combinations of carbon- and oxygen-centered radicals have − factors in the 2.0030−2.0040 range, and oxygen-centered radicals such as semiquinone radicals have g factors exceeding

Table 2. Concentrations, g Factors, and Line Widths of Free Radicals in AC and Modified AC Samples in the Absence and Presence of H2O2 FRs concentration (1017 spins/g) sample

g factor

line width (Gauss)

AC

AC/H2O2

AC−X AC−X−N2 AC−X−N4 AC−W AC−W−N2 AC−W−N4 AC−S AC−S−N2 AC−S−N4

2.0029 2.0043 2.0046 2.0027 2.0045 2.0046 2.0032 2.0054 2.0056

5.5 3.1 3.0 5.9 3.1 3.2 4.7 2.2 2.1

4.30 26.7 31.2 7.58 48.5 35.2 4.50 140 178

2.45 13.4 12.5 5.87 27.6 16.2 1.38 75.2 80.1

dominant in AC without HNO3 treatment. In contrast, the g factor increased significantly in the HNO3-treated samples to 2.0043 and 2.0046 for AC−X−N2 and AC−X−N4, 2.0045 and 2.0046 for AC−W−N2 and AC−W−N4, and 2.0054 and 2.0056 for AC−S−N2 and AC−S−N4, respectively, suggesting that oxygen-centered free radicals were predominant in modified AC specimens. Furthermore, the ΔHp‑p (peak to peak width) decreased significantly after 4 h of HNO3 treatment from 5.5 to 3.0 G for AC−X, from 5.9 to 3.2 G 19928

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Figure 3. Catalytic degradation of DEP in the AC/H2O2 system: (a) degradation kinetics of DEP, (b) pseudo-first-order rate constants of DPE degradation (kobs) with different types of AC and modified AC, (c) changes in kobs in the presence of different scavengers (1,4-benezequinone, KI, and methanol), and (d) calculated inhibitory efficiency of different scavengers. Reaction conditions: initial concentrations [H2O2]0 = 18 mM; [ACs] = [ACs-HNO3-2h] = [ACs-HNO3-4h] = 1.0 g/L; [DEP]0 = 0.25 mM; [1,4-benezequinone] = [KI] = 10 mM; [methanol] = 50 mM; T = 25 °C; pH = 6.8.

of DEP as a result of formation of more free radicals such as OH. As both the type and the concentration of free radicals would affect the degradation of DEP, free radical quenching studies were used to further identify the dominant radical species in these processes. To quantify the contributions of different free radicals to DEP degradation, inhibitory efficiencies (ψ) were calculated by eq 5

for AC−W, and from 4.7 to 2.1 G for AC−S. The above results indicated that HNO3 treatment changed the types of FRs in AC, likely due to formation of phenolic−OH and aromatic C O groups on the surface of AC, resulting in production of oxygen-centered free radicals such as semiquinone radicals during the oxidation process. To quantify the unpaired electrons in AC samples, the concentrations of FRs in both unmodified and modified AC were determined by EPR. FR concentrations were 7.58 × 1017 spins/g for AC−W, 3.45 × 1017 spins/g for AC−X, and 2.11 × 1017 spins/g for AC−S. After HNO3 treatment for 4 h, FR concentrations increased significantly to 35.2 × 1017, 31.1 × 1017, and 178 × 1017 spins/g for AC−W−N4, AC−X−N4, and AC−S−N4, respectively. Thus, treatment with HNO3 not only increased FR concentration but also changed the type of FRs in AC samples, which would influence the reactivity of AC toward H2O2 to form •OH in the AC/H2O2 system. 3.4. Catalytic Degradation of DEP in the AC/H2O2 System. Figure 3a shows that 98.8%, 85.3%, and 69.6% of DEP (0.25 mM) was degraded within 8 h by 18 mM H2O2 in the presence of 1.0 g/L AC−W, AC−X, and AC−S, respectively, while no significant changes of DEP concentration were observed with H2O2 alone. Similar results were observed in the other modified AC/H2O2 systems. Degradation of DEP was well described by the pseudo-first-order rate equations well (Figure S6, Supporting Information; R2 > 0.95, p < 0.01); kobs values for DEP degradation were 0.0036 and 0.0062 min−1 for AC−X and AC−X−N4, 0.0092 and 0.0104 min−1 for AC−W and AC−W−N4, and 0.0021 and 0.0179 min−1 for AC−S and AC−S−N4, respectively. These results suggested that HNO3 treatment increased the catalytic ability of AC for degradation

•

⎛ k − ks ⎞ ψ% = ⎜ 0 ⎟ × 100% ⎝ k0 ⎠

(5)

where ks and k0 are the pseudo-first-order reaction rate constants of DEP with and without scavengers, respectively. 1,4-Benzequinone (BQ) was used as the •OOH/O2•− quencher since it was found to selectively react with •OOH/O2•− species in previous study.30 With addition of 10 mM BQ, kobs values decreased from 0.0092 to 0.0080 min−1, from 0.0037 to 0.0034 min−1, and from 0.0021 to 0.0019 min−1 for AC−W, AC−X, and AC−S, respectively (Figure 3c). Meanwhile, ψ% was 13.1% for AC−W, 9.2% for AC−X, and 10.9% for AC−S. These results indicated that superoxide radical accounted for 9.0−13% of DEP degradation in the AC/H2O2 system; thus, superoxide radical was not the dominant reactive species for DEP degradation under the current experimental conditions. Potassium iodide (KI) was employed as the scavenger of • OH bound to the AC surface (bound-•OH), while methanol, which has low affinity for solid surfaces, was used as the scavenger of •OH in the reaction solution (solution-•OH).31−33 With addition of 10 mM KI, kobs of DEP degradation decreased significantly from 0.0092 to 0.0071 min−1 for AC−W, from 19929

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Figure 4. Correlations between AC characteristics and the decomposition rate of H2O2 (kH2O2): (a) total acidity vs kH2O2, (b) C−OH percentage vs kH2O2, (c) C defects vs kH2O2, and (d) FR concentration vs kH2O2. Data were obtained from Figure 1 and Tables 1 and S1, Supporting Information.

Figure 5. Correlations between AC characteristics and generation of hydroxyl radicals (trapped [OH]): (a) total acidity vs trapped [OH], (b) C− OH percentage vs trapped [OH], (c) C defects vs trapped [OH], and (d) FR concentration vs trapped [OH]. Data were obtained from Figure 1 and Tables 1, 2, and S1, Supporting Information.

0.0037 to 0.0029 min−1 for AC−X, and from 0.0021 to 0.0017 min−1 for AC−S (Figure 3a and 3b). Values of ψ% were 23.4%,

20.5%, and 19.6% for AC−W, AC−X, and AC−S, respectively, indicating that surface-bound •OH played an important role in 19930

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formation (Figure 1d, Table S2, Supporting Information). This indicated that FRs or C defects was primarily responsible for generation of •OH in the AC/H2O2 system. Figures 4 and 5 suggest that the surface functional groups such as C−OH controlled the catalytic decomposition of H2O2, while the FRs or C defects accounted for generation of •OH from the AC/ H2O2 system. 3.5.2. Mechanism of •OH Generation from AC/H2O2. Free radicals in biochar (g factor 2.0029−2.0040) can react directly with H2O2 by single-electron transfer and reduce H2O2 to • OH.24 On the basis of their similar g factors, the types of FRs present in AC and modified AC are similar to those in biochar. Therefore, the FRs in AC were hypothesized to mediate generation of •OH in the presence of H2O2, and electron capturing studies were employed to further test this hypothesis. Potassium dichromate (K2Cr2O7) is typically used as an electron scavenger to verify the electron transfer process.33,34 Thus, in this study, K2Cr2O7 was utilized to elucidate the singleelectron transfer mechanism of •OH formation in the AC/ H2O2 system. In the presence of 2.0 mM K2Cr2O7, the FR concentration decreased markedly from 7.58 × 1017 to 2.13 × 1017 spins/g for AC−W, from 3.45 × 1017 to 0.23 × 1017 spins/ g for AC−W, and from 2.11 × 1017 to 0.05 × 1017 spins/g for AC−S (Figure S8a, Supporting Information), indicating that K2Cr2O7 can efficiently scavenge the FRs in AC. K2Cr2O7 produced a greater reduction in FRs than H2O2, suggesting that FRs were more reactive with K2Cr2O7 than with H2O2. In the presence of K2Cr2O7, trapped [OH] decreased rapidly from 212 to 105 μM for AC−W, from 122 to 54 μM for AC−X, and from 95 to 32 μM for AC−S (Figure S8b, Supporting Information), indicating that K2Cr2O7 greatly inhibited formation of •OH in the AC/H2O2 system. This result is explained by the competition between K2Cr2O7 and H2O2 to accept electrons from FRs, which inhibited production of •OH from AC/H2O2. Figure S8, Supporting Information. clearly indicates that the transfer of electrons from FRs to H2O2 was the most probable mechanism for formation of •OH from AC/ H2O2.

the degradation of DEP in the AC/H2O2 system. With addition of 50 mM methanol, the values of kobs dropped significantly from 0.0092 to 0.0033 min−1 for AC−W, from 0.0031 to 0.0010 min−1 for AC−X, and from 0.0021 to 0.0007 min−1 for AC−S with corresponding ψ% values of 64.3%, 67.2%, and 68.4%, respectively. The results suggested that solution-•OH was the dominant radical species for DEP degradation in the AC/H2O2 system. 3.5. Mechanism of Catalytic Decomposition of H2O2 and Generation of •OH in the AC/H2O2 System. 3.5.1. Correlations between Characteristics of AC and •OH Generation. The surface functional groups of AC can influence the catalytic decomposition of H2O2 in AC/H2O2.12 Tables S1, Supporting Information, 1, and 2 indicate that the surface functional groups including C (aliphatic “defects”), C−OH, CO, and COOH along with the total acidity and free radicals in AC samples changed significantly during the HNO 3 modification process. The changes in CO and COOH groups were usually consistent with the changes in total acidity, whereas the C−OH group reflected total basicity levels. Therefore, to elucidate the mechanism of the catalytic decomposition of H2O2 by AC and modified AC, four parameters (total acidity, C−OH, C defects, and free radical concentration) were correlated with the H2O2 decomposition rate (kH2O2) and generation of •OH (trapped [OH]). A significant negative correlation was found between the total acidity and kH2O2 with a correlation coefficient (R2) of 0.578 (p < 0.01) (Figure 4a). In contrast, the proportion of C−OH was positively correlated with kH2O2 (Figure 4b; R2 = 0.776, p < 0.001). These results suggested that catalytic decomposition of H2O2 by AC was inhibited by acidic functional groups such as COOH and promoted by basic groups such as C−OH. Figure 4c and 4d shows that there were no significant correlations between C defects and kH2O2 (R2 = 0.271, p = 0.08) or FR and kH2O2 (R2 = 0.263, p = 0.09). Figure 5 presents the relationships between each of these four parameters and trapped [OH]. The correlation between total acidity and trapped [OH] was insignificant (Figure 5a; R2 = 0.317, p = 0.066). In contrast, C−OH was negatively correlated with trapped [OH] (Figure 5b; R2 = 0.600, p < 0.05). This negative correlation can be explained by the fact that C−OH not only drove the decomposition of H2O2 but also consumed •OH radicals, resulting in the reduction of •OH. Furthermore, the correlation between C−OH and trapped [OH] was opposite that between C−OH and kH2O2. These results further suggested that distinct pathways were responsible for decomposition of H2O2 and generation of •OH in the AC/H2O2 system, in agreement with the results shown in Figure 1c and 1d. Positive correlations were found between C defects and trapped [OH] (Figure 5c; R2 = 0.885, p < 0.0001) and FRs and trapped [OH] (Figure 5d; R2 = 0.856, p < 0.001). The C defects and FRs reflect the structural disorder and unpaired electrons of AC. Consequently, it was concluded that higher levels of disorder and higher concentrations of unpaired electrons were associated with more •OH radical production in the AC/H2O2 system. A positive linear correlation was found between C defects and FRs (Figure S7, Supporting Information; R2 = 0.973, p < 0.0001), indicating that the amount of unpaired electrons depended on the structural disorder of AC and that the concentration of FRs reflected the degree of structural disorder. FR concentrations were markedly reduced by addition of H2O2, which was accompanied by •OH

4. CONCLUSIONS This study showed that AC and modified AC are efficient catalysts for catalytic decomposition of H2O2 and generation of • OH from AC/H2O2. It was demonstrated that surface functional groups such as C−OH were the main controlling factor in the decomposition of H2O2, whereas free radicals (FRs) in AC were the main contributor to •OH formation in the AC/H2O2 system. The conclusions were supported by the positive correlations between C−OH and kH2O2 and between FR concentration and trapped [OH]. AC samples were shown to contain free radicals (FRs), and treatment with HNO3 both increased the FR concentrations and altered the types of FRs, which favored generation of •OH from AC/H2O2. The findings of this study provide new insights into the mechanism of catalytic decomposition of H2O2 and generation of hydroxyl radicals by carbonaceous materials (e.g., AC, graphite, carbon blacks) and the properties of HNO3-modified AC.

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ASSOCIATED CONTENT

S Supporting Information *

Characteristics of biochar (SEM, FTIR, element composition, and XPS spectra), SA method for quantification of hydroxyl radicals, and effects of K2Cr2O7 on formation of •OH in the 19931

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AC/H2O2 system. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support of the National Key Basic Research Program of China (No. 2014CB441105), the National Natural Science Foundation of China (No. 21377136, 41401252), the Natural Science Foundation of Jiangsu Province of China (BK20141047), and One Hundred Person Project of the Chinese Academy of Sciences.

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