Hydrothermal Synthesis of FeS2 as a High-Efficiency Fenton Reagent

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Hydrothermal Synthesis of FeS2 as a High Efficient Fenton Reagent to Degrade Alachlor via Superoxide Mediated Fe(II)/Fe(III) Cycle Wei Liu, Yueyao Wang, Zhihui Ai, and Lizhi Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09919 • Publication Date (Web): 08 Dec 2015 Downloaded from http://pubs.acs.org on December 15, 2015

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ACS Applied Materials & Interfaces

Hydrothermal Synthesis of FeS2 as a High Efficient Fenton Reagent to Degrade Alachlor via Superoxide Mediated Fe(II)/Fe(III) Cycle Wei Liu, Yueyao Wang, Zhihui Ai*, and Lizhi Zhang*

Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, Institute of Environmental Chemistry, Central China Normal University, Wuhan 430079, P. R. China

* To whom correspondence should be addressed. E-mail: [email protected]; [email protected]. Phone/Fax: +86-27-6786 7535 1

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ABSTRACT In this study, we demonstrate that hydrothermal synthesized FeS2 (syn-FeS2) is highly efficient to catalyze the H2O2 decomposition for the alachlor degradation in a wide range of initial pH (3.2-9.2). The alachlor degradation rate of syn-FeS2 heterogeneous Fenton system was almost 55 times that of commercial pyrite (com-FeS2) counterpart at an initial pH of 6.2. Experimental results revealed that the alachlor oxidation enhancement in the syn-FeS2 Fenton system was attributed to the molecular oxygen activation induced by more surface bound ferrous ions on syn-FeS2. The molecular oxygen activation process could generate superoxide anions to accelerate the Fe(II)/Fe(III) cycle on the syn-FeS2 surface, which favored the H2O2 decomposition to generate more hydroxyl radicals for the alachlor oxidation. It was found that the hydroxyl radicals generation rate constant of syn-FeS2 Fenton system was 71 times that of com-FeS2 counterpart, and even 1-3 orders of magnitudes larger than those of commonly used Fe-bearing heterogeneous catalysts, respectively. We detected the alachlor degradation intermediates with gas chromatography-mass spectrometry to tentatively propose a possible alachlor degradation pathway. These interesting findings could provide some new insights on the molecular oxygen activation induced by FeS2 minerals and the subsequent heterogeneous Fenton degradation of organic pollutants in the environment.

Keywords FeS2; Molecular oxygen activation; Superoxide anions; Fe(II)/Fe(III) cycle; Alachlor

1. INTRODUCTION Alachlor (2-chloro-2’, 6’-diethyl-N-(methoxymethyl) acetanilide) is one of the most common chloroacetanilide herbicide and widely used to control the broadleaf weeds and specific annual grasses in crop fields.1 However, it has been proven to be a carcinogen by the Environmental Protection Agency of the United States, and might cause harmful effects to humans.2 Because of its 2


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high solubility in water (240 mg/L at 25 oC) and extensive usage, alachlor has been widely detected in rivers, reservoirs and even ground water.3, 4 Although there are many methods for the treatment of alachlor, including photocatalytic oxidation,5 electrochemical oxidation,6 ozonation,7 and biological methods,8 they still suffer from strict reaction conditions and/or high energy consumption. Therefore, it is still a challenge to develop more environmental friendly, high efficient and low cost method for the remediation and treatment of alachlor pollution. Fenton reaction, involving the decomposition of hydrogen peroxide with ferrous ions to generate a strong oxidant •OH (E0 = 2.8 V), can oxidize the refractory organic contaminants.9-11 However, the classic homogeneous Fenton reaction often suffers from the rapid precipitation of Fe(OH)3, which disfavors the Fe(II)/Fe(III) cycle and thus terminates the Fenton reaction.12 Thus, low pH of 2.0-4.0 is inevitably required. To overcome this disadvantage of homogenous Fenton reaction, heterogeneous Fenton and Fenton-like systems have been developed. For example, different heterogeneous catalysts such as iron-oxygen compounds,13, 14 Fe-immobilized materials15-17 and other natural iron-bearing minerals,18, 19 were used to catalyze the H2O2 decomposition for the organic contaminants treatment. These catalysts could prevent the precipitation of soluble iron and thus possess a remarkable reusability. Interestingly, the catalytic activity of these heterogeneous Fenton catalysts were found to be positively correlated with surface Fe(II) content and/or the Fe(II)/Fe(III) cycle performance of iron-bearing materials.14, 20, 21 Pyrite (FeS2), as one of the most abundant sulfide minerals with high content ferrous ions,22 was widely used to reduce environmental contaminants including heavy metals,23 nitrate,24 and chlorinated organic pollutants.25 It was reported that the pyrite could be also used as a potential heterogeneous Fenton reagent for the decomposition of H2O2 to oxidize organic contaminants. For

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example, Matta et al. found that the pyrite could efficiently catalyze the H2O2 decomposition to degrade 2, 4, 6-trinitrotoluene (TNT). Its degradation kinetic constant was much higher than those of other ferric-bearing minerals (hematite, goethite, and lepidocrocite) Fenton systems, and the degradation enhancement was attribute to the oxidation state of iron on the pyrite surface and its dissolution rate of ferrous ions.26 In addition, Che et al. reported that the pyrite Fenton system exhibited higher trichloroethylene and carbon tetrachloride degradation efficiency than the homogeneous counterpart (Fe2+/H2O2).27 More importantly, the pH of the pyrite Fenton system was found to decrease and reach an acidic value during the oxidation process by releasing iron and hydrogen ions, which offered an optimal pH for the Fenton reaction without extra acid addition.27-30 Recently, Wang et al. reported that a natural pyrite could in situ generate H2O2 and hydroxyl radicals to oxidize lactate within 10 days.31 Unfortunately, this natural pyrite has impurities such as transition-metal ions. These impurities could also act as Fenton-like reagents and thus hamper the FeS2 Fenton reaction mechanism investigation.29 Moreover, the activity of the natural pyrite minerals highly depended on their places of origin, resulting in some contradictory experimental observations. In this study, we demonstrate hydrothermal synthesized FeS2 (syn-FeS2) is highly efficient to catalyze the H2O2 decomposition for the alachlor degradation in a wide range of initial pH (3.2-9.2). A series of experiments were designed to clarify the generation of reactive oxygen species and their effects on the alachlor degradation in the FeS2 Fenton system as well as the FeS2 Fenton reaction mechanism. Gas chromatography-mass spectrometry was used to determine the intermediates of alachlor degradation to understand the alachlor degradation mechanism.

2. EXPERIMENTAL SECTION

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2.1 Chemicals and Materials. Iron (II) sulfate heptahydrate, Sodium thiosulfate pentahydrate, sulfur power, carbon disulfide, 1, 10-phenanthroline, benzoic acid, ethanol and tert-butanol (TBA) were all of analytical grade and purchased from National Medicines Corporation Ltd. China. Alachlor (≥ 99.9 %). Superoxide dismutase (SOD) was purchased from Sigma-Aldrich (St. Louis, MO). Commercial pyrite (com-FeS2) was purchased from Alfa Aesar and ground to a fine powder with an agate mortar. High performance liquid chromatography grade methanol and acetonitrile were obtained from Fisher Scientific. All the chemicals were used as received without further purification. Deionized water was used throughout the experiments. 2.2 Sample Preparation. In a typical synthesis,32 FeSO4•7H2O (0.02 mol), Na2S2O3•5H2O (0.02 mol) and S (0.02 mol) were dissolved in 60 mL of deionized water in a 100 mL Teflon-lined stainless steel autoclave. The mixture was stirred for 0.5 h at room temperature and then heated at 200 oC for 24 h. When the reaction was finished, the autoclave was cooled down to room temperature naturally. Finally, the dark gray solid product was obtained by centrifuging and sequentially rinsing with distilled water, carbon disulfide, and ethanol several times and then dried in a vacuum oven at 60 oC for 6 h for further characterization. 2.3 Alachlor Degradation Experiments. The alachlor stock solution was prepared by dissolving alachlor in deionized water and stored in dark to avoid any photochemical degradation. The initial pH value of the alachlor solution was 6.2. All the degradation experiments were conducted in 25 mL conical flasks and covered with aluminum foil. The reaction suspension was prepared by adding 0.5 g/L of syn-FeS2 and a desired dosage of H2O2 into 20 mL of 0.074 mmol/L (20 mg/L) alachlor. And the conical flasks were shaked with a rotary shaker (HY-5, China) for a predetermined period at room temperature. HCl or NaOH (0.5 mol/L) was used to adjust the initial pH of reaction solutions

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to investigate the pH influence on the alachlor degradation efficiency. Control experiments were conducted by using the same procedures with certain amounts of com-FeS2 or H2O2, respectively. Samples were withdrawn at a regular time interval from the flask with a syringe and passed through a 0.22 µm polytetrafluoroethylene filter. Meanwhile, 100 µL of 17.5 mol/L ethanol stock solution was added into 900 µL sample to quench the Fenton reaction. All of the degradation experiments were replicated for three times. 2.4 Analytic Methods. The dissolved ferrous ions was quantified by the 1, 10-phenanthrolin method, and the total dissolved iron ions was quantified after adding hydroxylamine hydrochloride to the filtered solution. Samples were analyzed by a UV-vis spectrophotometer (UV-2550, Shimadzu, Japan) at maximum wavelength of 510 nm.33 The generation rate of •OH was measured with a modified molecule probe method.34 The concentration of alachlor was analyzed by high pressure liquid chromatography (HPLC, LC-20A, Shimadzu, Japan), and its degradation intermediates were detected by gas chromatography-mass spectrometry (GC-MS, TRACE 1300-ISQ, Thermo, USA). The low molecular weight organic acid and inorganic anions were quantified by a Dionex 900 ion chromatograph (IC, Thermo, USA). The total organic carbon (TOC) concentration was analyzed by a Shimadzu TOC-VCPH analyzer after filtration through 0.22 µm filter. Electron spin resonance (ESR) spectra were obtained with a Bruker EPR A300 spectrometer at room temperature with 5, 5-dimethyl-1-pyrroline- N-oxide (DMPO) as the radical spin-trapped reagent. Furthermore, the superoxide anion was examined in methanol, to avoid the facile disproportionation of the superoxide species in water.35

3. RESULTS AND DISCUSSION

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3.1 Characterization. XRD pattern in Figure 1a revealed that the hydrothermal synthesized sample (syn-FeS2) mainly consisted of pyrite FeS2 (JCPDS No.6-710) and tiny amount of marcasite FeS2 (JCPDS No.3-779). The sharp diffraction XRD peaks implied their high crystallinity. Meanwhile, the XRD analysis confirmed the pyrite phase (JCPDS No.6-710) of commercial FeS2 (com-FeS2) (Figure 1b). The morphology of the samples was investigated (Figure 2). It was found that the hydrothermal synthesized sample was composed of many aggregated “pyrite framboids”. These framboids consisted of numerous polyhydrons of 1-1.5 µm in size. In contrast, only irregular larger particles were found in com-FeS2. 3.2 The alachlor degradation in different Fenton systems. Figure 3a shows the degradation curves of alachlor in different systems with initial pH of 6.2. It was found that the H2O2 could not induce the alachlor degradation because of the weak oxidation ability of H2O2. Only 8% of alachlor was removed with syn-FeS2, suggesting the slight direct alachlor adsorption and/or reduction with syn-FeS2. Interestingly, more than 99% of alachlor was degraded in the syn-FeS2 Fenton (syn-FeS2/H2O2) system. For comparison, the alachlor degradation with commercial pyrite Fenton (com-FeS2/H2O2) system was also investigated. 8% of alachlor degradation in com-FeS2/H2O2 within 60 min was much lower than that (> 99%) of syn-FeS2/H2O2. Both of the alachlor degradation curves in the two FeS2/H2O2 systems were found to fit pseudo-first-order kinetics equations. The apparent alachlor degradation constant (0.0764 min-1, R2 = 0.9929) of syn-FeS2/H2O2 was almost 55 times that (0.0014 min-1, R2 = 0.9839) of com-FeS2/H2O2 (Figure 3b). As surface area could affect the reactivity of catalyst, we therefore compared their rate constants normalized with specific surface area (Table S1). It was interestingly found that the normalized rate constant of syn-FeS2 was even more than 4 times that of com-FeS2, which suggested that the higher alachlor degradation efficiency of syn-FeS2

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was not only ascribed to its high surface area. Subsequently, a pure pyrite was also synthesized by a solvothermal method (denoted as syn-FeS2-ST; the XRD and BET were shown in Figure S1 and Table S1, respectively), it was found that the normalized rate constants of the solvothermally and hydrothermally synthesized FeS2 were almost the same, this result could indirectly rule out the effects of marcasite on the alachlor degradation. Furthermore, the H2O2 concentration changes during the heterogeneous Fenton process were measured in these two FeS2 systems (Figure S2). It was found that the H2O2 decomposition efficiency with syn-FeS2 was much higher than that of the com-FeS2, which matched very well with the alachlor degradation efficiency in these two systems, revealing that the syn-FeS2 exhibited higher heterogeneous Fenton reactivity than com-FeS2. 3.3 The pH effects on the alachlor degradation in the FeS2 Fenton systems. As the pH value strongly affects the speciation of iron in aqueous solution and the generation of hydroxyl radicals during Fenton or Fenton-like processes, we thus investigated the influence of initial pH values on the alachlor degradation in the syn-FeS2/H2O2 system and found that alachlor could be efficiently degraded with efficiencies of more than 95% in a wide initial pH range of 3.2-9.2, but the alachlor degradation efficiency considerably decreased to 29% and 7% after the initial pH increased to 10.7 and 12.2 (Figure 4a). We therefore monitored the pH variations during the alachlor Fenton oxidation process and found that the pH values decreased and reached an acid equilibrium point of 3.2 in the initial pH range of 3.2-9.2 (Figure 4b). This interesting pH buffer-like property of FeS2 was attributed to the oxidation of FeS2 and the subsequent releasing of iron and hydrogen ions,27, 29, 36, 37 and/or the hydrolysis of iron ions. Obviously, the strong pH buffer property of FeS2 is promising for the Fenton oxidation of organic compounds. For comparison, we also monitored the pH variation in

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the com-FeS2/H2O2 system with initial pH of 6.2, and found that the pH was decreased to 4.7 within 60 min (Figure 4c), suggesting the weaker acid effect of com-FeS2 than that of syn-FeS2. Subsequently, the alachlor degradation in the com-FeS2/H2O2 system was carried out at the initial pH of 3.0 by adding certain amount of extra acid. Although the solution pH could also be kept at 3.0, only 24% of alachlor was degraded within 60 min in the com-FeS2/H2O2 system (Figure 4d), which was much lower than that (> 95%) of syn-FeS2/H2O2 under the same conditions. Therefore, we conclude that the dramatically enhanced alachlor degradation in the syn-FeS2/H2O2system was not attributed to the strong pH buffer property of syn-FeS2. 3.4 The molecular oxygen effects on the alachlor degradation in the FeS2 Fenton system. Pyrite (FeS2), as an excellent electron donor via the conversion of low valent S22- to sulfate, is related to the cycles of iron, sulfur, oxygen, and carbon.22 When FeS2 is exposed to the air, molecular oxygen could be reduced to superoxide or hydrogen peroxide directly by FeS2 via single-electron or two-electron transfer routes,36, 38 and indirectly by ferrous ions via the Haber-Weiss reactions.31, 37, 39 We thus investigated the effects of molecular oxygen on the alachor degradation in the syn-FeS2/H2O2 system under air or argon atmospheres (Figure 5), respectively. It was found that the alachlor degradation efficiency under argon was slightly lower than that under air. The alachlor degradation rate constant under argon (0.0703) was 92 percent of that under air (0.0764) when the initial alachlor concentration was 0.074 mmol/L (20 mg/L). However, the alachlor degradation efficiency under air was significantly higher than that under argon if the initial alachlor concentration was increased to 0.333 mmol/L (90 mg/L), where the rate constant (0.0130) under argon was only 59 percent of that under air (0.0221). These results revealed that alachlor degradation in the syn-FeS2/H2O2 system involved a molecular oxygen activation process. The higher inhibition ratio of

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the alachlor degradation under argon with higher initial alachlor concentration may be caused by the more alachlor adsorption on the FeS2 surface and subsequently the effects on the surface reactions between reactive oxygen species and iron species or organic molecules. 3.5 The reactive oxygen species generated in the FeS2 Fenton systems. To check the effects of molecular oxygen on the FeS2 Fenton system, the electron spin resonance spectra (ESR) was then used to detect the radicals generated in the FeS2 heterogeneous Fenton systems. Four characteristic peaks with 1:2:2:1 quarter pattern and the hyperfine splitting parameters of aN = aH = 14.9 G were obtained after the mixing of com-FeS2, H2O2 and DMPO at room temperature (Figure 6a), suggesting the generation of •OH in the system. Meanwhile, a six-line spectrum with relative intensities of 1:1:1:1:1:1 and hyperfine splitting parameters of aN = 16.4 G, aHβ = 23.3 G was observed in the methanol, which could be attributed to the signal of DMPO-•CH3. Interestingly, a four-line spectrum with relative intensities of 1:1:1:1 and hyperfine splitting parameters of aN = 14.25 G and aH = 12.45 G was observed in the methanol for the syn-FeS2/H2O2 system (Figure 6b), indicating the generation of •O2-. It was found that the DMPO-•OH signal in the syn-FeS2/H2O2 system was weaker than that in the com-FeS2/H2O2 system, which may be caused by the hydroxyl radicals consumption via the parasitic reactions, as shown in Eqs. 1-3.40 These results suggested that both of two FeS2 samples could catalyze the decomposition of H2O2 to generate hydroxyl radicals, but syn-FeS2 could more efficiently activate molecular oxygen to generate more superoxide radicals. Fe2+ + •OH → Fe3+ + OH-

k = 3.2×108 M-1•S-1

(1)

•OH + •OH → H2O2

k = 6.0×109 M-1•S-1

(2)

•O2- + •OH→ OH- + O2

k = 1.0×1010 M-1•S-1

(3)

Subsequently, the hydroxyl radicals generation rate constant (k•OH) in the two FeS2 Fenton systems

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were measured by employing the benzoic acid oxidation as the model reaction.34, 41 It was found that the k•OH (2.56 × 10-4 s-1) of the syn-FeS2/H2O2 system under air was 2.8 times that (0.91 × 10-4 s-1) under argon, confirming that the molecular oxygen activation of syn-FeS2 did promote the generation of hydroxyl radicals in the syn-FeS2/H2O2 system (Figure 7). As expected, k•OH (3.62×10-6 s-1) of the com-FeS2/H2O2 system under air was 2 orders of magnitude less than that of syn-FeS2/H2O2 system, which was consistent with their alachlor degradation difference (Figure 3a and Figure 5). We also compared the •OH generation rate of syn-FeS2/H2O2 with those of previously reported iron-bearing materials for the H2O2 decomposition,34 and found that the k•OH of syn-FeS2/H2O2 was 1-3 orders of magnitude larger than those of goethite (α-FeOOH), hematite (α-Fe2O3), and ferrihyrite (Fe5HO8•4H2O) (Table S2). Furthermore, their hydroxyl radicals generation rates normalized with specific surface area were also calculated and the trend was the same as that of initial ones. We therefore conclude that the hydrothermal synthesized FeS2 is a more promising heterogeneous Fenton reagent than commercial pyrite and other iron-bearing materials. To further understand the reasons for different reactive oxygen species generated in these two FeS2 Fenton systems, the X-ray photoelectron spectroscopy (XPS) was then used to analyze the surface chemical composition of the catalysts (Figure S3). In the high-resolution spectra of Fe 2p, we found that the strong peak at 707.0 eV assigned to the lattice ferrous ions of FeS2. The high binding energy tail was fitted with four peaks at 708.4, 709.1, 710.1, and 711.4 eV, respectively. The peaks at 708.4 and 710.1 eV were attributed to the Fe(II)-S on the surface sites, and other two peaks were ascribed to the Fe(III)-S.42 Meanwhile, the S 2p spectra were also investigated. Both the S 2p spectra of these two samples could be fitted with three peaks located at 163.5, 162.4, and 161.4 eV, corresponding to sulfur atoms of intermediate oxidation state, lattice S22-, and surface S2-, respectively. Meanwhile, the

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calculation of the peak area in Fe 2p spectra gave ratios of lattice ferrous iron to total iron (Fe2+lattice/Fetotal), surface ferrous iron to total iron (Fe2+surf/Fetotal) and surface ferric iron to total iron (Fe3+surf/Fetotal) (Table 1). It was interestingly found that the values of Fe2+lattice/Fetotal in the two FeS2 were almost the same, but the Fe2+surf/Fetotal ratio (0.270) of syn-FeS2 was significantly higher than that (0.200) of com-FeS2, indicative of more surface ferrous ions on syn-FeS2. As the surface ferrous ions of the Fe-bearing materials could activate molecular oxygen via a single electron reduction pathway to generate superoxide anion radicals,43-47 syn-FeS2 of more surface ferrous ions is more efficient to enhance the single-electron reduction of molecular oxygen to generate superoxide anions, and subsequently promote the hydroxyl radicals generation and the Fenton degradation of alachlor. 3.6 The roles of reactive oxygen species in the syn-FeS2 Fenton system. To clarify the effects of the reactive oxygen species on the alachlor degradation, a series of trapping experiments were carried out in the syn-FeS2/H2O2 system with adding different kinds of excess scavengers (TBA for •OH, SOD for •O2-) (Figure 8).48 The degradation of alachlor were significantly inhibited with adding TBA and SOD, revealing the contribution of •OH and •O2- to the alachlor degradation in the syn-FeS2/H2O2 system. It is well known that hydroxyl radicals of strong oxidation ability could directly oxidize the organic pollutants.7 However, the reaction between •O2- and alachlor is seldom investigated. Although •O2- was generated via the single-electron reduction of molecular oxygen by the surface ferrous ions of FeS2, alachlor could not be obviously degraded by syn-FeS2 in air (Figure 3a), ruling out the direct alachlor reduction induced by •O2-. Thus, •O2- generated in the syn-FeS2/H2O2 system might contribute to the alachlor degradation via other pathways instead of •O2induced direct alachlor reduction. As stated previously, the molecular oxygen activation could generate both superoxide anions and

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hydrogen peroxide, which might enhance the generation of hydroxyl radicals, and also accelerate the Fe(II)/Fe(III) cycle via Eqs. 4-5. We thus monitored the concentration changes of dissolved ferrous, ferric and total iron ions during the syn-FeS2/H2O2 alachlor degradation in the absence and presence of SOD (Figure 9a). In the absence of SOD, the ferrous ions concentration increased to the maximum value of 0.043 mmol/L in 15 min, and then decreased slowly to 0.031 mmol/L in 60 min. Ferrous ions could be generated by the slowly oxidation of FeS2 via Eq. 6 and subsequently react with H2O2 to produce hydroxyl radicals, resulting in a final equilibrium of ferrous ions in the solution. Interestingly, the maximum concentration (0.0036 mmol/L) of ferrous ions in presence of SOD was just about a tenth of that (0.031 mmol/L) in the absence of SOD (Figure 9b). This difference revealed that superoxide anions generated in the syn-FeS2/H2O2 system participated in the Fe(II)/Fe(III) cycle. Although H2O2 and FeS2 are abundant in the syn-FeS2/H2O2 system, it is still possible to rule out the ferric ions reduction by H2O2 and FeS2 via Eq. 4 and Eq. 7 because the rate constants of •O2- with ferric ions is ~10 orders of magnitude larger than that of H2O2 and ferric ions.40 In contrast, the concentrations of dissolved iron ions were lower than 0.004 mmol/L in the com-FeS2/H2O2 system either in the presence or absence of SOD (Figure 9c and 9d). Therefore, the major role of •O2generated in the syn-FeS2/H2O2 system was to accelerate the Fe(II)/Fe(III) cycle, which could subsequently favor the H2O2 decomposition to produce more hydroxyl radicals for the alachlor degradation. Fe3+ + H2O2 → 2Fe2+ + •HO2 + H+

k = 2.2×10-2 M-1•S-1

(4)

Fe3+ + •O2- → Fe2+ + O2

k = 1.5×108 M-1•S-1

(5)

2FeS2 + 7O2 + 2H2O → 2Fe2+ + 4SO42- + 4H+

(6)

FeS2 + 14Fe3+ + 8H2O → 15Fe2+ + 2SO42- + 16H+

(7)

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As the stoichiometric ratio of total iron ions to sulfate ions generated by the oxidation of FeS2 should be 1 to 2 (Eq. 6), the concentration of sulfate ions in the syn-FeS2/H2O2 system was then monitored (Figure S4). It was found that the SO42- increased to 0.513 mmol/L in 30 min and then kept unchanged in the last 30 min, suggesting that the concentration of total iron ions in the solution should be 0.256 mmol/L, much higher than the real concentration (0.056 mmol/L) of total iron ions generated in the syn-FeS2/H2O2 system, indicating that most of iron ions (Fe(II)/Fe(III)) may exist on the surface of FeS2, not in the solution. As the species distribution and reactivity of iron ions are strongly dependent on the solution pH, MEDUSA software was subsequently used to simulate the Fe(II)/Fe(III) species distribution curves in the FeS2/H2O2 system at pH 1.0-12.0 (Figures S5). It was found that the Fe(II) could exist as dissociative ferrous ions at pH < 8.0, and then ferrous hydroxylate when pH increased from 8.0 to 12.0. In contrast, Fe(III) was intended to form hydroxylation complexes at pH > 1.0. Actually, the pH of the syn-FeS2/H2O2 system quickly decreased and finally kept at 3.2 within 60 min in case of initial pH 6.2 (Figure 4b), suggesting the existence of dissociative ferrous ions and hydroxylated ferric ions (FeOH2+ and Fe(OH)2+) in the solution, which might adsorb on the surface of FeS2. XPS analysis revealed that Fe3+surf/Fetotal ratios of syn-FeS2 and com-FeS2 increased after the alacholar degradation, reflecting the adsorption of Fe3+ on the FeS2 surface. Furthermore, the Fe3+surf/Fetotal of syn-FeS2 increased from 0.276 to 0.358, much lower than that (from 0.339 to 0.486) of com-FeS2 (Table 1), confirming more efficient Fe(II)/Fe(III) cycle on the surface of syn-FeS2. Subsequently, the reusability of the syn-FeS2 for the alachlor degradation was investigated (Figure S6). It was found that the alachlor degradation efficiency was only 10% after 1 hour at the same conditions, however, which could obtain about 90% after 7 hours reaction. And the same trend was

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also found in the third cycle. The decrease of alachlor degradation efficiency may ascribe to the surface changes of the syn-FeS2, as revealed with XPS. The high degradation efficiency after prolonging the reaction time suggested that the syn-FeS2 could reused for several cycles for the alachlor degradation. 3.7 The proposed enhanced Fenton oxidation mechanism in the syn-FeS2 Fenton system. On the basis of the above results and analyses, we propose a possible Fe(II)/Fe(III) cycle mechanism during the aerobic degradation of alachlor in the syn-FeS2 Fenton system (Scheme 1). First, ferrous ions could be released via the oxidation of FeS2, and then reacted with hydrogen peroxide to generate hydroxyl radicals via the Fenton reaction, accompanying with the formation of ferric ions. These generated •OH may directly oxidize and mineralize alachlor to generate low molecular byproducts and a series of organic acids. Simultaneously, the generated ferric ions could form Fe-hydroxo complexes and adsorb on the surface of FeS2, which disfavored the further reaction. Fortunately, the surface bound ferrous ions on the syn-FeS2 could activate molecular oxygen via the single-electron transfer pathway to generate superoxide anions, which would reduce the surface bound ferric ions into ferrous ions. Subsequently, the efficient Fe(II)/Fe(III) cycle in the syn-FeS2/H2O2 system could supply enough surface ferrous ions for the Fenton reaction to generate more hydroxyl radicals for the enhanced alachlor degradation in this study. 3.8 The alachlor degradation pathways in the FeS2 Fenton system. Total organic carbon (TOC) analysis was then employed to evaluate the mineralization efficiency of the syn-FeS2/H2O2 system (Figure 10). Although the alachlor could be completely degraded in 1 h, the TOC decrease was merely 7.5%. When the reaction time was extended to 3 h, about 26% of TOC could be removed. It is known that the pollutant mineralization efficiency of Fenton system depends on the ratio of the

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H2O2/pollutant. Therefore, the TOC removal was carried out by increasing the H2O2 concentration from 0.8 to 80 mmol/L. As expected, the TOC removal percentages improved to 42% and 53% after 1 and 3 hours' reaction, respectively. Therefore, it is possible to efficiently mineralize alachlor if adding enough H2O2 into the syn-FeS2 Fenton system. GC/MS analysis was then used to investigate the intermediates generated during this heterogeneous Fenton degradation of alachlor. Eight aromatic and cyclic molecules were identified. They

were

2-chloro-2’-acetyl-6’-ethyl-N-(methoxymethyl)

acetanilide

(2),

2-chloro-2’,6’-ethyl-acetanilide (3), 2-chloro-2’,6’-diacetyl-N- (methoxymethyl) acetanilide (4), 2,6-diethylphenyl

isocyanate

(5),

8-ethyl-quinoline

(6),

pyroquilon

(7),

1-chloroacetyl-2,3-dihydro-7-ethyl-indole (8), 2-ethylphenyl isocyanate (9). On the basis of these intermediates, we therefore proposed the alachlor degradation mechanism as follows (Scheme 2), which involved alkylic oxidation, cleavage of C-N and/or C-C bond, cyclization, and dealkylation. First, the compound 2 was generated by the hydroxylation of lateral chain upon the attack of hydroxyl radicals, and then further oxidized by hydroxyl radicals to generate compound 4. Subsequently, the cleavage of C-N bond via hydrogen abstraction followed by α/β scission of the bond to generate compound 3, which could be oxidized via the further cleavage of the C-N bond or cyclization to form compounds 5 and 6, 7, 8, respectively. Meanwhile, the direct dealkylation of compound 4 on the lateral chain would also take place to generate compound 9. Ions chromatogram (IC) was then used to identify and quantify the low-molecular weight organic acids and inorganic anions generated during the alachlor degradation process. Chloride, formate, acetate, and oxalate were detected and their concentrations increased along with extending the syn-FeS2 heterogeneous Fenton degradation time at the initial H2O2 concentration of 0.8 mmol/L

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(Figure 11a). Nitrite and nitrate formation during the alachlor oxidation was negligible, indicative of the N-containing group remaining. However, these N-containing groups could be converted into nitrite and nitrate ions in the syn-FeS2/H2O2 system by increasing the initial H2O2 concentration to 80 mmol/L (Figure 11b). Therefore, syn-FeS2 offers a superior heterogeneous Fenton catalyst to decompose H2O2 for producing hydroxyl radicals to mineralize alachlor.

4. CONCLUSIONS In summary, we have demonstrated that the hydrothermal synthesized FeS2 is highly efficient to catalyze the H2O2 decomposition for the alachlor degradation. The alachlor oxidation enhancement in the syn-FeS2 Fenton system was attributed to the molecular oxygen activation induced by its more surface bound ferrous ions. The molecular oxygen activation process could generate superoxide anions to accelerate the Fe(II)/Fe(III) cycle on the syn-FeS2 surface, which favored the H2O2 decomposition to generate more hydroxyl radicals for the alachlor oxidation. The alachlor could be mineralized with enough H2O2 in the syn-FeS2 Fenton system. These interesting findings could provide some new insights on the molecular oxygen activation induced by FeS2 minerals and the subsequent heterogeneous Fenton degradation of organic pollutants in the environment.

ASSOCIATED CONTENT Supporting Information Solvothermal synthesis of FeS2; XRD patterns of the syn-FeS2-ST; Concentrations of H2O2 as function of time in FeS2/H2O2 systems; High-resolution XPS of Fe 2p and S 2p; Concentration of sulfate ions generated in the syn-FeS2/H2O2 system; Ferrous ions and ferric ions species distribution in the syn-FeS2/H2O2; Reusability of syn-FeS2 in alachlor removal; The k (min-1) over FeS2, BET 17



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surface area (m2•g-1) of the FeS2, and the k′ (g•min-1•m-2); Comparison of •OH generation rates by the H2O2 decomposition over different catalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements. This work was supported by National Natural Science Funds for Distinguished Young Scholars (Grant 21425728), National Science Foundation of China (Grants 21177048 and 21477044), and Self-Determined Research Funds of CCNU from the Colleges’ Basic Research and Operation of MOE (Grants CCNU14Z01001 and CCNU14Z01010).

References (1) Tiedje, J. M.; Hagedorn, M. L. Degradation of Alachlor by a Soil Fungus, Chaetomium Globosum. J. Agric. Food Chem. 1975, 23, 77-81. (2) Charizopoulos, E.; Papadopoulou-Mourkidou, E. Occurrence of Pesticides in Rain of the Axios River Basin, Greece. Environ. Sci. Technol. 1999, 33, 2363-2368. (3) Kolpin, D. W.; Thurman, E. M.; Goolsby, D. A. Occurrence of Selected Pesticides and Their Metabolites in Near-Surface Aquifers of the Midwestern United States. Environ. Sci. Technol. 1995, 30, 335-340. (4) Battaglin, W.; Furlong, E.; Burkhardt, M.; Peter, C. Occurrence of Sulfonylurea, Sulfonamide, Imidazolinone, and Other Herbicides in Rivers, Reservoirs and Ground Water in the Midwestern United States, 1998. Sci. Total Environ. 2000, 248, 123-133. (5) Wong, C. C.; Chu, W. The Direct Photolysis and Photocatalytic Degradation of Alachlor at Different TiO2 and UV Sources. Chemosphere 2003, 50, 981-987. (6) Pipi, A. R. F.; De Andrade, A. R.; Brillas, E.; Sires, I. Total Removal of Alachlor from Water by 18


Page 18 of 30

Page 19 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Electrochemical Processes. Sep. Purif. Technol. 2014, 132, 674-683. (7) Qiang, Z. M.; Liu, C.; Dong, B. Z.; Zhang, Y. L. Degradation Mechanism of Alachlor During Direct Ozonation and O3/H2O2 Advanced Oxidation Process. Chemosphere 2010, 78, 517-26. (8) Levanon, D. Roles of Fungi and Bacteria in the Mineralization of the Pesticides Atrazine, Alachlor, Malathion and Carbofuran in Soil. Soil Biol. Biochem. 1993, 25, 1097-1105. (9) Walling, C. Fenton's Reagent Revisited. Acc. Chem. Res. 1975, 8, 125-131. (10) Bautista, P.; Mohedano, A. F.; Gilarranz, M. A.; Casas, J. A.; Rodriguez, J. J. Application of Fenton Oxidation to Cosmetic Wastewaters Treatment. J. Hazard. Mater. 2007, 143, 128-34. (11) Zazo, J.; Casas, J.; Mohedano, A.; Gilarranz, M.; Rodriguez, J. Chemical Pathway and Kinetics of Phenol Oxidation by Fenton's Reagent. Environ. Sci. Technol. 2005, 39, 9295-9302. (12) Pignatello, J. J.; Oliveros, E.; MacKay, A. Advanced Oxidation Processes for Organic Contaminant Destruction Based on the Fenton Reaction and Related Chemistry. Crit. Rev. Environ. Sci. Technol. 2006, 36, 1-84. (13) Nie, Y. L.; Hu, C.; Qu, J. H.; Zhou, L.; Hu, X. X. Photoassisted Degradation of Azodyes over FeOxH2x-3/Fe0 in the Presence of H2O2 at Neutral pH Values. Environ. Sci. Technol. 2007, 41, 4715-4719. (14) He, J.; Yang, X. F.; Men, B.; Bi, Z.; Pu, Y. B.; Wang, D. S. Heterogeneous Fenton Oxidation of Catechol and 4-chlorocatechol Catalyzed by Nano-Fe3O4: Role of the Interface. Chem. Eng. J. 2014, 258, 433-441. (15) Huling, S. G.; Jones, P. K.; Lee, T. R. Iron Optimization for Fenton-Driven Oxidation of MTBE-Spent Granular Activated Carbon. Environ. Sci. Technol. 2007, 41, 4090-4096. (16) Pham, A. L. T.; Lee, C.; Doyle, F. M.; Sedlak, D. L. A Silica-Supported Iron Oxide Catalyst

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Capable of Activating Hydrogen Peroxide at Neutral pH Values. Environ. Sci. Technol. 2009, 43, 8930-8935. (17) Ramirez, J. H.; Maldonado-Hódar, F. J.; Pérez-Cadenas, A. F.; Moreno-Castilla, C.; Costa, C. A.; Madeira, L. M. Azo-dye Orange II Degradation by Heterogeneous Fenton-Like Reaction Using Carbon-Fe Catalysts. Appl. Catal., B 2007, 75, 312-323. (18) Kasiri, M. B.; Aleboyeh, H.; Aleboyeh, A. Modeling and Optimization of Heterogeneous Photo-Fenton Process with Response Surface Methodology and Artificial Neural Networks. Environ. Sci. Technol. 2008, 42, 7970-7975. (19) Feng, J.; Hu, X.; Yue, P. L. Discoloration and Mineralization of Orange II Using Different Heterogeneous Catalysts Containing Fe: A Comparative Study. Environ. Sci. Technol. 2004, 38, 5773-5778. (20) Matta, R.; Hanna, K.; Kone, T.; Chiron, S. Oxidation of 2,4,6-trinitrotoluene in the Presence of Different Iron-Bearing Minerals at Neutral pH. Chem. Eng. J. 2008, 144, 453-458. (21) Xue, X. F.; Hanna, K.; Deng, N. S. Fenton-Like Oxidation of Rhodamine B in the Presence of Two Types of Iron (II, III) Oxide. J. Hazard. Mater. 2009, 166, 407-414. (22) Rickard, D.; Luther, G. W. Chemistry of Iron Sulfides. Chem. Rev. 2007, 107, 514-562. (23) Kang, M.; Chen, F.; Wu, S.; Yang, Y.; Bruggeman, C.; Charlet, L. Effect of pH on Aqueous Se(IV) Reduction by Pyrite. Environ. Sci. Technol. 2011, 45, 2704-10. (24) Engesgaard, P.; Kipp, K. L. A Geochemical Transport Model for Redox ‐ Controlled Movement of Mineral Fronts in Groundwater Flow Systems: A Case of Nitrate Removal by Oxidation of Pyrite. Water Resour. Res. 1992, 28, 2829-2843. (25) Lee, W.; Batchelor, B. Abiotic Reductive Dechlorination of Chlorinated Ethylenes by

20


Page 20 of 30

Page 21 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Iron-Bearing Soil Minerals. 1. Pyrite and Magnetite. Environ. Sci. Technol. 2002, 36, 5147-5154. (26) Matta, R.; Hanna, K.; Chiron, S. Fenton-Like Oxidation of 2,4,6-trinitrotoluene Using Different Iron Minerals. Sci. Total Environ. 2007, 385, 242-51. (27) Che, H.; Bae, S.; Lee, W. Degradation of Trichloroethylene by Fenton Reaction in Pyrite Suspension. J. Hazard. Mater. 2011, 185, 1355-1361. (28) Choi, K.; Bae, S.; Lee, W. Degradation of off-gas Toluene in Continuous Pyrite Fenton System. J. Hazard. Mater. 2014, 280, 31-37. (29) Wu, D. L.; Feng, Y.; Ma, L. M. Oxidation of Azo Dyes by H2O2 in Presence of Natural Pyrite. Water, Air, Soil Pollut. 2013, 224. (30) Zhang, Y. L.; Zhang, K.; Dai, C. M.; Zhou, X. F.; Si, H. P. An Enhanced Fenton Reaction Catalyzed by Natural Heterogeneous Pyrite for Nitrobenzene Degradation in an Aqueous Solution. Chem. Eng. J. 2014, 244, 438-445. (31) Wang, W.; Qu, Y. P.; Yang, B.; Liu, X. Y.; Su, W. H. Lactate Oxidation in Pyrite Suspension: A Fenton-Like Process in Situ Generating H2O2. Chemosphere 2012, 86, 376-382. (32) Wu, R.; Zheng, Y. F.; Zhang, X. G.; Sun, Y. F.; Xu, J. B.; Jian, J. K. Hydrothermal Synthesis and Crystal Structure of Pyrite. J. Cryst. Growth 2004, 266, 523-527. (33) Tamura, H.; Goto, K.; Yotsuyanagi, T.; Nagayama, M. Spectrophotometric Determination of Iron (II) with 1, 10-phenanthroline in the Presence of Large Amounts of Iron (III). Talanta 1974, 21, 314-318. (34) Yang, X. J.; Xu, X. M.; Xu, J.; Han, Y. F. Iron Oxychloride (FeOCl): An Efficient Fenton-Like Catalyst for Producing Hydroxyl Radicals in Degradation of Organic Contaminants. J. Am. Chem. Soc. 2013, 135, 16058-16061.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(35) Sawyer, D. T.; Valentine, J. S. How Super Is Superoxide? Acc. Chem. Res. 1981, 14, 393-400. (36) Schoonen, M. A. A.; Harrington, A. D.; Laffers, R.; Strongin, D. R. Role of Hydrogen Peroxide and Hydroxyl Radical in Pyrite Oxidation by Molecular Oxygen. Geochim. Cosmochim. Acta 2010, 74, 4971-4987. (37) Harrington, A. D.; Hylton, S.; Schoonen, M. A. A. Pyrite-Driven Reactive Oxygen Species Formation in Simulated Lung Fluid: Implications for Coal Workers' Pneumoconiosis. Environ. Geochem. Health 2012, 34, 527-538. (38) Cohn, C. A.; Fisher, S. C.; Brownawell, B. J.; Schoonen, M. A. A. Adenine Oxidation by Pyrite-Generated Hydroxyl Radicals. Geochem. Trans. 2010, 11, 2. (39) Jones, G. C.; van Hille, R. P.; Harrison, S. T. L. Reactive Oxygen Species Generated in the Presence of Fine Pyrite Particles and Its Implication in Thermophilic Mineral Bioleaching. Appl. Microbiol. Biotechnol. 2013, 97, 2735-2742. (40) Brillas, E.; Sires, I.; Oturan, M. A. Electro-Fenton Process and Related Electrochemical Technologies Based on Fenton's Reaction Chemistry. Chem. Rev. 2009, 109, 6570-6631. (41) Shi, J. G.; Ai, Z. H.; Zhang, L. Z. Fe@Fe2O3 Core-Shell Nanowires Enhanced Fenton Oxidation by Accelerating the Fe(III)/Fe(II) Cycles. Water Res. 2014, 59, 145-153. (42) Schaufuß, A. G.; Nesbitt, H. W.; Kartio, I.; Laajalehto, K.; Bancroft, G. M.; Szargan, R. Incipient Oxidation of Fractured Pyrite Surfaces in Air. J. Electron Spectrosc. Relat. Phenom. 1998, 96, 69-82. (43) Ai, Z. H.; Gao, Z. T.; Zhang, L. Z.; He, W. W.; Yin, J. J. Core–Shell Structure Dependent Reactivity of Fe@Fe2O3 Nanowires on Aerobic Degradation of 4-chlorophenol. Environ. Sci. Technol. 2013, 47, 5344-5352.

22


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(44) Liu, W.; Ai, Z. H.; Cao, M. H.; Zhang, L. Z. Ferrous Ions Promoted Aerobic Simazine Degradation with Fe@Fe2O3 Core–Shell Nanowires. Appl. Catal., B 2014, 150–151, 1-11. (45) Schoonen, M. A. A.; Cohn, C. A.; Roemer, E.; Laffers, R.; Simon, S. R.; O'Riordan, T. Mineral-Induced Formation of Reactive Oxygen Species. Rev. Mineral. Geochem. 2006, 64, 179-221. (46) Fang, G. D.; Zhou, D. M.; Dionysiou, D. D. Superoxide Mediated Production of Hydroxyl Radicals by Magnetite Nanoparticles: Demonstration in the Degradation of 2-chlorobiphenyl. J. Hazard. Mater. 2013, 250-251, 68-75. (47) Fang, G. D.; Dionysiou, D. D.; Al-Abed, S. R.; Zhou, D. M. Superoxide Radical Driving the Activation of Persulfate by Mmagnetite Nanoparticles: Implications for the Degradation of PCBs. Appl. Catal., B 2013, 129, 325-332. (48) Atalla, S. L.; Toledo-Pereyra, L. H.; Mackenzie, G. H.; Cederna, J. P. Influence of Oxygen-Derived Free Radical Scavengers on Ischemic Livers. Transplantation 1985, 40, 584-589.

Figure Captions

Figure 1. XRD patterns of (a) syn-FeS2, (b) com-FeS2.

23



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Figure 2. SEM images of (a) syn-FeS2, (b) com-FeS2.

Figure 3. (a) The degradation curves of alachlor in different systems; (b) Plots of ln(C/C0) versus time for the degradation of alachlor in syn-FeS2/H2O2 and com-FeS2/H2O2 systems. The concentrations of syn-FeS2 and com-FeS2 were 0.5 g/L, the initial concentrations of H2O2 and alachlor were 0.8 and 0.074 mmol/L, respectively.

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Figure 4. (a) Effect of initial pH on the oxidative degradation of alachlor in syn-FeS2/H2O2 system; (b) Variation of syn-FeS2 suspension pH with respect to time; (c) The change of the pH value during the alachlor removal in the com-FeS2/H2O2 system with initial pH of 6.2; (d) The alachlor degradation and pH change curves in com-FeS2/H2O2 system with initial pH of 3.0. The concentration of FeS2 was 0.5 g/L, the initial concentrations of H2O2 and alachlor were 0.8 and 0.074 mmol/L, respectively.

Figure 5. The degradation of alachlor in the syn-FeS2/H2O2 system under different atmospheric 25



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conditions (air or argon): (a) 0.074 mmol/L (20 mg/L) of alachlor; (b) 0.333 mmol/L (90 mg/L) of alachlor. The concentration of FeS2 was 0.5 g/L, and the initial concentrations of H2O2 and alachlor were 0.8 and 0.074 mmol/L, respectively.

Figure 6. ESR spectra of DMPO spin-trapping adducts after 2 minutes reaction in different systems: (a) the com-FeS2/H2O2 system; (b) the syn-FeS2/H2O2 system. ( represents DMPO-•OH adduct, represents DMPO-•CH3 adduct and represents DMPO-•O2- adduct.)

Figure 7. Formation rates of •OH as a function of H2O2 concentration in different systems, the inset is the magnifying curve of k•OH in the com-FeS2/Air system.

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Figure 8. The degradation efficiency of alachlor in syn-FeS2/H2O2 system with scavengers (TBA for •OH, SOD for •O2-). The concentrations of syn-FeS2 was 0.5 g/L, the initial concentrations of H2O2, alachlor, SOD and TBA were 0.8 mmol/L, 0.074 mmol/L, 250-700 U/mL, and 200 mmol/L, respectively.

Figure 9. The concentration of dissolved ferrous, ferric and total iron ions determined by 1, 10-phenanthrolin method: (a) and (b) without and with SOD in the syn-FeS2/H2O2 system; (c) and (d)

27



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without and with SOD in the com-FeS2/H2O2 system. The concentration of FeS2 was 0.5 g/L, and the initial concentrations of H2O2, alachlor, and SOD were 0.8 mmol/L, 0.074 mmol/L, and 250-700 U/mL, respectively.

Figure 10. The comparison of alachlor degradation and TOC decay in the syn-FeS2/H2O2 system. The concentration of FeS2 was 0.5 g/L, and the initial concentration of alachlor was 0.074 mmol/L.

Figure 11. The inorganic ions and low molecular organic acids concentrations detect in the syn-FeS2/H2O2 system. (a) chloride, formate, acetate, and oxalate ions detected at the initial H2O2 concentration of 0.8 mmol/L; (b) nitrite and nitrate ions detected at the initial H2O2 concentration of 80 mmol/L. The concentration of FeS2 was 0.5 g/L, and the initial alachlor concentration was 0.074 mmol/L, respectively.

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Scheme 1. The proposed heterogeneous Fenton mechanism in the syn-FeS2/H2O2 system.

Scheme 2. The proposed alachlor degradation mechanism in the syn-FeS2/H2O2 system.

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Page 30 of 30

Table 1. The ratio of lattice ferrous iron (Fe2+ lattice /Fetotal), surface ferrous ions (Fe2+surf/Fetotal) and

ferric ions (Fe3+surf/Fetotal) on the surface of FeS2 to total iron in Fe 2p spectra of the different samples. sample

Fe2+ lattice/Fetotal

Fe2+surf/Fetotal

Fe3+surf/Fetotal

syn-FeS2

0.454

0.270

0.276

syn-FeS2-1a

0.386

0.256

0.358

com-FeS2

0.461

0.200

0.339

com-FeS2-1b

0.252

0.262

0.486

a, b

The samples obtained after the Fenton reaction with syn-FeS2 and com-FeS2, respectively.

TOC Art Figure

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Hydrothermal Synthesis of FeS2 as a High-Efficiency Fenton Reagent

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