Reinventing Fenton Chemistry: Iron Oxychloride ... - ACS Publications

Letter Cite This: Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX

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Reinventing Fenton Chemistry: Iron Oxychloride Nanosheet for pHInsensitive H2O2 Activation Meng Sun,†,⊥ Chiheng Chu,†,⊥ Fanglan Geng,‡ Xinglin Lu,† Jiuhui Qu,§ John Crittenden,*,∥ Menachem Elimelech,† and Jae-Hong Kim*,† †

Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286, United States Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China § School of Environment, Tsinghua University, Beijing 100084, China ∥ School of Civil and Environmental Engineering, Georgia Institute of Technology, 828 West Peachtree Street, Atlanta, Georgia 30332, United States ‡

S Supporting Information *

ABSTRACT: This study intends to reinvent classical Fenton chemistry by enabling the Fe(II)/Fe(III) redox cycle to occur on a newly developed FeOCl nanosheet catalyst for facile hydroxyl radical (•OH) generation from H2O2 activation. This approach overcomes challenges such as low operating pH and large sludge production that have prevented a wider use of otherwise attractive Fenton chemistry for practical water treatment, in particular, for the destruction of recalcitrant pollutants through nonselective oxidation by •OH. We demonstrate that FeOCl catalysts exhibit the highest performance reported in the literature for •OH production and organic pollutant destruction over a wide pH range. We further elucidate the mechanism of rapid conversion between Fe(III) and Fe(II) in FeOCl crystals based on extensive characterizations. Given the low-cost raw material and simple synthesis and regeneration, FeOCl catalysts represent a critical advance toward application of iron-based advanced oxidation in real practice.

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INTRODUCTION The strong oxidation potential of the hydroxyl radical (•OH) has been widely exploited for nonselective destruction of a range of refractory organic pollutants in water treatment processes.1,2 In typical advanced oxidation processes (AOPs; oxidation processes employing •OH as a main oxidant), various activation schemes have been used for in situ generation of highly reactive •OH from more stable oxidants such as H2O2. The Fenton reaction that relies on the Fe(II)/Fe(III) redox cycle for H2O2 activationdespite its rich history, engineering simplicity, and low costhas not been as widely adopted in practice as other activation processes that employ UV and O3; this lack of adoption is due to the requirement for an impractically low pH (< 4) and production of a large mass of iron precipitates after neutralization.3−11 Heterogeneous catalysts such as FeOCl have been explored to avoid iron precipitates but have been found to activate H2O2 effectively only at acidic pH.12 Here, we present FeOCl nanosheet catalysts that efficiently produce •OH over a wide pH range without generating iron precipitates. We demonstrate effective oxidation of select organic pollutants of significant public health concern including a plastic additive (bisphenol A), pharmaceuticals (17αethinylestradiol, cimetidine, and carbamazepine), antibiotic (ampicillin), pesticide (4-chlorophenol), and fungicide (2,4,6© XXXX American Chemical Society

trichlorophenol) as well as inactivation of a model microbe Escherichia coli (E. coli) at neutral pH. We further discuss the unique mechanism of H2O2 activation by FeOCl nanosheet catalysts based on extensive analysis on the Fe coordination environment and redox cycle occurring on the catalyst surface.

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MATERIALS AND METHODS The FeOCl nanosheet catalyst was synthesized by heating FeCl3·6H2O powder at a heating rate of 10 °C·min−1 to 220 °C and annealing for 2 h (Scheme S1). The structure of the catalyst was characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). The electronic state and coordination environment of Fe in the catalyst were characterized by X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure spectroscopy (XANES), extended X-ray adsorption fine structure spectroscopy (EXAFS), and Mössbauer spectroscopy (Text S1). The generation of •OH was qualitatively and Received: February 6, 2018 Revised: February 14, 2018 Accepted: February 15, 2018

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DOI: 10.1021/acs.estlett.8b00065 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX

Letter

Environmental Science & Technology Letters

Figure 1. (a) TGA-MS and corresponding derivative thermogravimetric (DTG) curve obtained from heating the FeCl3·6H2O precursor from 50 to 300 °C at a ramp rate of 10 °C/min. Escaped gases (H2O vapor, HCl, HClO, and Cl2) were detected using a mass spectra (MS) detector as mass-tocharge ratios (m/z) of 18, 37, 53, and 71, respectively. (b) HAADF-STEM images of as-synthesized FeOCl nanosheets with EDS (elemental line scanning and mappings) indicate a uniform distribution of Fe (red), O (blue), and Cl (green) within the nanosheet. (c) SEM image of the FeOCl nanosheets. (d) TEM image of FeOCl nanosheets. The top-left inset shows the SAED pattern acquired from the nanosheet area marked by a black rectangle, illustrating its crystalline structure. The bottom-right inset illustrates the high resolution TEM image of the same area with lattice fringe spacing of 3.44, 2.40, and 1.95 Å in different orientations, corresponding to the {110}, {021}, and {131} facets of FeOCl, respectively. The top-right inset shows the essential atom matrix of FeOCl from the (1−12) facets; green represents Cl atoms, red O atoms, and brown Fe atoms. (e) XRD pattern of as-prepared FeOCl nanosheets and standard PDF card of FeOCl particle crystal. (f) High resolution XPS spectra of Fe 2p, O 1s, and Cl 2p for FeOCl nanosheets. (g) Mössbauer spectrum of the FeOCl nanosheets with IS of 0.390 mm·s−1, QS of 0.887 mm·s−1, and LW of 0.380 mm· s−1. (h) First-order derivatives of the normalized XANES of Fe K-edge of the FeOCl nanosheet and Fe reference samples. (i) Fourier transforms of the χ(k)·k3 into R space in the range from 0 to 6 Å for FeOCl nanosheet and Fe reference samples.

pH 6.6) prior to further experiments. E. coli inactivation was assessed using the plate count method.15−17 Briefly, Fenton catalyst was added into a diluted bacteria suspension (107 colony forming unit (CFU)/mL) to achieve a final concentration of 0.2 g/L, followed by an immediate addition of H2O2 (1 mM) to initiate the reaction. After 5 min of reaction, aliquot (100 μL) was collected, spread on nutrient agar plates, and incubated overnight at 37 °C for enumeration. Control experiments in the absence of H2O2 were performed to evaluate the intrinsic toxicity of FeOCl.

quantitatively assessed using electron paramagnetic resonance (EPR) spectroscopy and a chemical probe (terephthalic acid),13,14 respectively (Text S2). Batch experiments were performed to evaluate the capability of Fenton catalysts to degrade select organic pollutants using an aqueous suspension of 0.2 g/L of catalyst containing 2 mM phosphate buffer and 1 μM organic pollutant. The reaction was initiated by adding 15 mM H2O2. Aliquots of the catalyst suspension were passed through a syringe filter (0.22 μm poly(ether sulfone) membrane; Sterlitech) to remove catalysts. Each aliquot (50 μL) was injected into a C18 column at 20 °C, and the concentration of pollutant before and after 1 min reaction was determined using an Agilent high performance liquid chromatography (HPLC) coupled to a photodiode array detector (Agilent 1100). The composition of the mobile phase, UV absorption maximum, and retention time for analysis of each pollutant species are summarized in Table S1. E. coli (ATCC BW26437; Yale E. coli Genetic Stock Center) was used as the model microorganism to test the disinfection capability of Fenton catalysts. E. coli was grown in Luria− Bertani broth at 37 °C overnight, diluted in fresh medium, and washed three times with a sterile saline solution (0.9% NaCl at

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RESULTS AND DISCUSSION The one-pot, readily scalable method newly developed in this study for FeOCl catalyst synthesis drastically simplifies the previously reported procedure.12 During calcination, a gradual weight loss down to 40% of the original mass at ∼200 °C was observed by a thermogravimetric analysis (TGA; Figure 1a). Crystalline water evaporation contributed to the major weight loss from 100 to 140 °C, while the loss from HCl and HClO evolution became dominant beyond 160 °C. The absence of Cl2 evolution suggested a synchronous stripping of Cl and O B


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Figure 2. (a) •OH generation in FeOCl suspension confirmed by DMPO trapped EPR spectra. (b) 1 min accumulation of •OH in FeOCl suspension at varying pH. Control experiment (black circle) shows that •OH was fully quenched in the presence of isopropanol (1% v/v, 125 mM). (c) Cumulative •OH formation up to 1 min by various Fenton reagents at neutral pH. Error bars represent the standard deviation of three replicates. Reaction conditions: Fenton catalyst (0.2 g/L), H2O2 (15 mM), terephthalate probe (1 mM), pH buffer (2 mM), and room temperature.

from FeCl3·6H2O, attributed to HCl and HClO formation. A drastic decline of HCl and HClO and no further gas formation above 220 °C indicated redeposition of Cl and O atoms on the substrate and completion of FeOCl crystal formation, respectively. Consequently, O and Cl were found to be uniformly distributed along with Fe, according to HAADFSTEM and energy-dispersive X-ray spectrum (EDS) images (Figure 1b). As-prepared FeOCl consisted of nanoscale sheets with a thickness of 70−80 nm that were randomly stacked in a corallike architecture according to the SEM image (Figure 1c). These layered structures are known to be tightly bound together via strong attractive van der Waals interactions between adjacent Cl atoms (Figure S1).18 The Brunauer− Emmett−Teller (BET) isotherm (Figure S2) exhibited strong absorption at high N2 pressure and Type-IV hysteresis during N2 desorption due to capillary condensation.19 Accordingly, FeOCl appeared to have pores with an average size of ∼80 nm, which likely represented void spaces between nanosheets, as well as much smaller pores at around 3 nm, presumably formed by the evaporation of pyrolytic gases during calcination (Figure S2, inset). FeOCl nanosheets appeared highly crystalline, as evidenced by distinctive lattice fringes that were spaced 3.44, 2.40, and 1.95 Å apart (Figure 1d, bottom-right inset) in HRTEM image. These d-spacings matched well with {110}, {021}, and {131} facets in a previously identified FeOCl crystal (Figure 1d, topright inset) when viewed from its (1−12) facets. The selected area electron diffraction (SAED) pattern of the nanosheet (Figure 1d, left inset) also demonstrated a highly uniform and ordered diffraction pattern. These diffraction spots can be assigned to orthogonal planes of {−100} and {010} as well as their vector sum, {−110} plane, along the {001} zone in orthorhombic FeOCl.20 XRD peaks were located at 10.8°, 26.0°, 35.5°, and 38.1° (Figure 1e), corresponding to the {010}, {110}, {021}, and {111} crystal facets, respectively, consistent with pure sheet-like FeOCl crystals (JCPDS No. 72-

0619). The crystallite size was estimated to be around 50 nm based on the Scherrer equation and was similar to the SEM observation. Electronic State and Coordination Environment of Fe. The majority of Fe in the FeOCl nanosheets was found to be in the trivalent state, according to XPS (Figure 1f). A deconvoluted Fe 2p spectrum exhibited major peaks at 711.9 and 725.5 eV, which correspond to the binding energies of 2p3/2 and 2p1/2 in Fe(III), respectively, along with satellite peaks at 718.6 and 730.1 eV.21 A separate Fe 2p3/2 peak observed at 714.1 eV implies the minor existence of Fe(II).22 In addition, the prominent O 1s peak (532.4 eV) and Cl 2p peak (199.2 eV) indicate the presence of Fe−O and Fe−Cl bonds. The dominance of Fe(III) over Fe(II) in FeOCl nanosheets was further confirmed by the hyperfine interactions among elements using Mössbauer spectroscopy (Figure 1g). The Mössbauer fingerprint doublets were characterized by an isomer shift (IS) of 0.39 mm·s−1, quadrupole splitting (QS) of 0.89 mm·s−1, and line width (LW) of 0.38 mm·s−1. Such a small IS value is very similar to that of ferrihydrite (5(Fe3+)2O3· 9H2O, IS = 0.38 mm·s−1; QS = 0.9 mm·s−1),23 suggesting that Fe in FeOCl mainly exists in the Fe(III) state (IS ranging from 0.3 to 0.6 mm·s−1, whereas that of Fe(II) would reach 0.7 mm· s−1).23,24 The molar fraction of Fe(II) was estimated to be less than 33% of the total Fe in FeOCl, based on XANES analysis (Figure 1h). The highest K-edge absorption energy of Fe in asprepared FeOCl appeared at 7121.8 eV, which was very close to Fe3O4 (7121.3 eV) but far from Fe2O3 (7125.3 eV) and iron foil (7111.4 eV).25 EXAFS analysis elucidated Fe coordination with O and Cl. Fe K-edge oscillations (Figure S3) and their Fourier transformed curves (Figure 1i) revealed a unique coordination structure of Fe in FeOCl. In the distal shell, a peak at 2.79 Å in FeOCl was assigned to Fe(III)−Fe(III) coordination, similar to the peak observed at 2.73 Å in Fe2O3 and Fe3O4. The peak at 2.0 Å in FeOCl indicates the second shell of Fe−Cl coordination.26,27 Peaks at 1.07 and 1.56 Å C


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Figure 3. Degradation of select organic pollutants and inactivation of E. coli by various Fenton catalysts after 1 min (organic pollutants) or 5 min (E. coli) reactions. Reaction conditions: organic pollutant (1 μM), Fenton catalyst (0.2 g/L), H2O2 (15 mM), pH buffer (2 mM phosphate buffer at pH 7.0), and room temperature. Reaction conditions for E. coli: Fenton catalyst (0.2 g/L), H2O2 (1 mM), room temperature. Error bars represent the standard deviation of three replicates.

(Scheme 1). Note that a similar Fe2+/Fe3+ redox cycle is the core mechanism of homogeneous Fenton chemistry where the reduction of Fe3+ to Fe2+ is the rate-limiting step responsible for the overall slow kinetics. An increase in Fe(II) content in FeOCl upon exposure to H2O2 was observed by a peak shift of XANES (Figure 4a) and XPS (Figure S7).29 The shift of the XANES peak to Fe(II) was in stark contrast to previously reported FeOCl,12 for which the XANES peak shifted to Fe(III) upon reaction with H2O2 (Figure S8).

suggest the existence of Fe(II)−O and Fe(III)−O bonds, respectively. Compared to Fe3O4, the 1.07 Å peak in FeOCl was much more intense, assuring the presence of Fe(II) which was less obvious in the aforementioned XANES results. The curve fitting of EXAFS results (Table S2) suggests that one Fe atom was surrounded by four O atoms and two Cl atoms. Catalytic Performance. The occurrence of a distinctive EPR signal for the •OH adduct upon H2O2 addition to FeOCl suspension at pH 7.0 confirmed the generation of •OH (Figure 2a). This signal was fully quenched by excess tert-butyl alcohol (•OH quencher; k = 6.1 × 108 M−1·s−1).28 The cumulative • OH generation in 1 min decreased with increasing pH (Figure 2b). Despite this pH dependency, •OH generation was still significant at neutral pH (i.e., 11.0 μM). This was in stark contrast to previously explored heterogeneous Fenton catalysts, which do not form a detectable amount of •OH at neutral pH (Figure 2c). FeOCl was 3-fold more efficient than a homogeneous Fenton catalyst (Fe2+) across the entire pH range investigated (Figure S4). The higher •OH production by FeOCl nanosheets compared to other Fenton reagents translates to a much faster oxidative degradation of organic pollutants at neutral pH (Figure 3). FeOCl outperformed other Fenton catalysts by a significant margin for the degradation of all the organic pollutants tested. Effective degradation of recalcitrant compounds such as halogenated benzene and carbamazepine, whose removal by other Fenton catalysts is dismal, is particularly noteworthy. Further, up to 80% inactivation of a model microorganism, E. coli, was achieved within 5 min (see Figure S5 for control experimental results). The FeOCl catalyst was found to perform efficiently through repetitive use (Figure S6) with an insignificant amount of dissolved iron leaching (Table S3). Mechanistic Insight. The efficient activation of H2O2 and • OH generation by the FeOCl nanosheet catalyst is attributed to the uniquely advantageous mechanism. First, FeOCl facilitates rapid conversion of Fe(III) to Fe(II), with a concurrent production of O2−• from the reaction with H2O2. Resultant Fe(II) further reacts with H2O2 to generate •OH and to complete the redox cycle involving the Fe(III)/Fe(II) pair

Scheme 1. Turnover of Fe(II)−Fe(III) in FeOCl Nanosheets upon Reaction with H2O2

Second, FeOCl nanosheets promote •OH generation by obviating a detrimental oxidation of Fe(II) to Fe(IV). The slight XANES shift of FeOCl toward a lower absorption edge energy located at 7121.3 eV provides critical evidence for the absence of Fe(IV); the absorption edge energy of Fe(IV) is estimated to be higher than 7125.3 eV (Figure 4a). The formation of Fe(IV) from Fe(II) from the reaction with H2O2 (i.e., without generating •OH) is not desirable since Fe(IV) has a much lower oxidative reactivity than •OH.30 However, Fe(II) that is hydrolyzed at nonacidic pH tends to form Fe(IV) when reacting with H2O2. This futile reaction has been identified as the major reason why a homogeneous Fenton catalyst is D


Letter


or additions from 1 to 4 Å) after the reaction with H2O2. Finally, FeOCl could be easily regenerated multiple times by simple incubation of spent FeOCl in the presence of HCl (37%) at 220 °C for 1 h, without loss of crystalline structure (Figure S10), •OH generation efficiency (Figure S11), or pollutant degradation capability (Figure S12). Given the simplicity of not only synthesis but also regeneration, and the most efficient •OH production at neutral pH ever reported, the FeOCl nanosheets provide a promising new opportunity to exploit heterogeneous Fenton chemistry for destruction of water pollutants.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.estlett.8b00065. Figures, tables, detailed experimental methods, and results of additional experiments described within the manuscript. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +1 (203) 432-4386. Fax: +1 (203) 432-0199. *E-mail: [email protected]. Tel: +1 (404) 8945676. Fax: +1 (404) 894-7896.

Figure 4. (a) First-order derivate of the normalized XANES of Fe Kedge of FeOCl nanosheet catalysts before and after reacting with 15 mM H2O2. (b) Fourier transforms of the χ(k)·k3 into R space for the as-prepared FeOCl nanosheet catalysts before and after reacting with 15 mM H2O2. The inset shows their corresponding K space curves in the range from 1 to 12 Å. The FeOCl samples used in these analyses were collected after FeOCl was reacted with H2O2 for 1 min, filtered, and dried at 50 °C for 12 h.

ORCID

Meng Sun: 0000-0002-8188-9264 Chiheng Chu: 0000-0001-9493-9120 Xinglin Lu: 0000-0002-0229-7712 John Crittenden: 0000-0002-9048-7208 Menachem Elimelech: 0000-0003-4186-1563 Jae-Hong Kim: 0000-0003-2224-3516

ineffective at neutral pH. Limiting this pathway results in steering the Fe(II)−H2O2 reaction more specifically toward • OH generation. The above-mentioned effects, i.e., more efficient conversion of Fe(III) to Fe(II) and improved selectivity of Fe (II) toward • OH generation rather than conversion to Fe(IV), are likely driven by the unique coordination environment of Fe in the FeOCl nanosheet. Exposed Fe on the surface of the nanosheet serves as a reactive center for catalytic H2O2 activation (Scheme 1). The coordination of electrophilic Cl and O is likely to increase the reduction potential of exposed Fe, resulting in more efficient single electron transfer from H2O2 (during the Fe(III) to Fe(II) reduction step) as well as homolytic cleavage of H2O2 (before the Fe(II) to Fe(III) oxidation step).31 Unlike homogeneous Fe2+ whose coordination is highly pH-dependent (i.e., more hydroxyl complexation at higher pH), the coordination of exposed Fe on the FeOCl surface would remain relatively unaffected by a change in solution pH. Similarly, the increased atom distances of Fe−O pairs (0.15 and 0.18 Å) and Fe−Cl pairs (0.15 Å) in the previously reported FeOCl material indicated looser interactions of Fe−O and Fe− Cl compared with that in FeOCl synthesized in this study (Figure S9). This condition could lead to a more pH-sensitive Fe coordination center when reacted with H2O2. In addition, the FeOCl nanosheet exhibited relatively robust stability; the FT (R space, Figure 4b) analysis of the Fe K-edge EXAFS (Figure 4b inset) suggests that the Fe coordination environment did not change (i.e., no obvious coordination deficiencies

Author Contributions ⊥

Meng Sun and Chiheng Chu contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the National Key Research and Development Program of China (No. 2016YFA0203101). This research was also supported by CrittendenTech (Atlanta, Georgia) and the Brook Byers Institute for Sustainable Systems, Hightower Chair, and the Georgia Research Alliance at Georgia Institute of Technology. We also are thankful for partial financial support from the National Science Foundation Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (EEC-1449500). C.H. Chu was financially supported by an Early Postdoc. Mobility Fellowship, Swiss National Science Foundation (P2EZP2_168796).

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