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Effect of Structural Transformation of Nanoparticulate Zero-Valent Iron on Generation of Reactive Oxygen Species Di He,† Jinxing Ma,‡ Richard N. Collins,† and T. David Waite*,† †
School of Civil and Environmental Engineering, University of New South Wales, Sydney, New South Wales 2052, Australia State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, People’s Republic of China
‡
S Supporting Information *
ABSTRACT: While it has been recognized for some time that addition of nanoparticlate zerovalent iron (nZVI) to oxygencontaining water results in both corrosion of Fe0 and oxidation of contaminants, there is limited understanding of either the relationship between transformation of nZVI and oxidant formation or the factors controlling the lifetime and extent of oxidant production. Using Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, we show that while nZVI particles are transformed to ferrihydrite then lepidocrocite in less than 2 h, oxidant generation continues for up to 10 h. The major products (Fe(II) and H2O2) of the reaction of nZVI with oxygenated water are associated, for the most part, with the surface of particles present with these surface-associated Fenton reagents inducing oxidation of a target compound (in this study, 14C-labeled formate). Effective oxidation of formate only occurred after formation of iron oxides on the nZVI surface with the initial formation of high surface area ferrihydrite facilitating rapid and extensive adsorption of formate with colocation of this target compound and surface-associated Fe(II) and H2O2 apparently critical to formate oxidation. Ongoing formate oxidation long after nZVI is consumed combined with the relatively slow consumption of Fe(II) and H2O2 suggest that these reactants are regenerated during the nZVI-initiated heterogeneous Fenton process.
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INTRODUCTION Zero-valent iron (ZVI) has been used to degrade a variety of contaminants. In particular, reductive reactions induced by ZVI have been employed in permeable reactive barriers to remediate groundwaters contaminated with chlorinated and nitrosubstituted organics.1,2 Use of nanoscale zerovalent iron (nZVI) has been suggested as an alternative means of exploiting the reactivity of ZVI without the need to construct passive barriers.3 The main advantages of nZVI are its high reactivity and the potential for introducing the particles directly into contaminated soil and groundwater.4 Generally, nZVI-mediated degradation of contaminants is implemented under anoxic conditions since the presence of oxygen would be expected to lower the reductive efficiency of the process as a result of competition with the contaminants being degraded. However, it has been reported that the reaction of nZVI with O2 can produce reactive oxygen species (ROS) either on the particle surface or in solution with these ROS capable of oxidizing both inorganic and organic compounds.5−10 The generation of hydrogen peroxide (H2O2) during the oxygenation of Fe(0) has been confirmed, with H2O2 subsequently reacting with Fe(II) to produce hydroxyl radicals (HO•) and/or Fe(IV)O2+ species via the Fenton reaction.5,6,8,9 Meanwhile, the oxidation of Fe(0) by O2 also results in the formation of Fe(II) and Fe(III) species with subsequent precipitation of particulate amorphous and © 2016 American Chemical Society
crystalline Fe oxyhydroxides on, or in the vicinity of, the nZVI surface.11 While the formation of a surface layer of Fe oxides might be expected to result in a decrease in the rate of generation of ROS and, thus, the ability of nZVI to induce the oxidation of target contaminants, there has been no detailed investigation of the correlation between the structural transformation of nZVI and the oxidizing capacity of nZVI and any associated iron oxide products. While the reaction of nZVI with target contaminants (and O2) has been emphasized above, consideration must also be given to reaction of Fe(0) with water, with this reaction expected to be of particular importance for nZVI where the high surface area and thus high reactivity could result in the rapid corrosion of the surface and, potentially, influence the ability of these reactive particles to degrade contaminants. With respect to the anaerobic reaction of nZVI with water, the identification of primary products (particularly Fe(OH)2 and magnetite) by 57Fe Mossbauer Spectroscopy and elucidation of the reaction mechanism have been well addressed.12 In terms of the aerobic reaction of nZVI with water, however, there is surprisingly limited information on both the Fe(0) oxidation Received: Revised: Accepted: Published: 3820
October 11, 2015 February 4, 2016 March 9, 2016 March 9, 2016 DOI: 10.1021/acs.est.5b04988 Environ. Sci. Technol. 2016, 50, 3820−3828
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Environmental Science & Technology
50 °C and finally ground into a fine powder in the anaerobic chamber prior to use. Lepidocrocite (Lpd) was synthesized by oxygenation of nZVI in aqueous phase for 24 h with the Fe oxide product harvested by centrifugation three times at 3000 rpm for 3 min (with the product washed with Milli-Q water between centrifugations) and subsequently freeze-dried at −45 °C. Ferrihydrite (Fhy) was synthesized by the addition of 1 M NaOH to 5 mM Fe(III) solution to a final pH of 7.5.21 The freshly made slurry was rapidly mixed at 150 rpm for 30 s followed by slow mixing at 30 rpm for 30 min, then centrifuged three times at 3000 rpm for 3 min (washed with Milli-Q water between centrifugations) and freeze-dried at −45 °C. Experimental Setup. Simultaneous measurements of adsorption capacity and oxidant production of nZVI were conducted in a gastight suite, as illustrated in SI Figure S1, using H14COO− as a probe compound. The system consisted of a reaction vessel fitted with a sparger input port and an outflow line to a trapping vessel containing 1.0 M CO2-free NaOH solution. Each assay was initiated by adding a preweighed amount of nZVI (or, in some studies, lepidocrocite or ferrihydrite) to 100 mL air-saturated Milli-Q water containing 1 μM H14COO−. Continuous air-sparging drove oxidized H14COO− (i.e., 14CO2) to the downstream trapping vessel followed by trapping in the form of 14CO32− in 1.0 M NaOH solution with an increase in the concentration of 14 CO32− in the trapping vessel representing “oxidized” H14COO−. Following 24 h reaction, 20 mL of an acidic quench solution (∼12 M HCl and 1 mM formic acid) was added to the reaction vessel with continuous air-sparging of the system. After the solids completely dissolved, the release of H14COOH into the solution within the reaction vessel represents “adsorbed” H14COO− while an increase in the concentration of 14CO32− in the trapping vessel represents “precipitated” carbonate (in the form of 14C-labeled siderite (FeCO3(s)) arising from the oxidation of H14COO−. Blank experiments have confirmed that, under acidic conditions (pH ∼ 1), ∼ 100% H14COOH remained in the reaction vessel with negligible 14C-labeled material found in the trapping vessel during 5 h air sparging (SI Figure S2). Samples were collected from both reaction and trapping vessels at predetermined time intervals for subsequent analysis. Experiments were undertaken in unbuffered systems with the pH changes following nZVI addition to these oxygenated solutions (i.e., 6.2−8.3) typical of those expected on addition of nZVI to natural waters. . Analyses of Fe(II), H2O2 and H14COO−. Fe(II) concentrations were determined spectrophotometrically at 510 nm using the phenanthroline method,22 with the concentration of Fe(II) in the aqueous phase quantified by addition of phenanthroline following filtration through 0.22 μm PVDF filters (Millipore) while the concentration of total Fe(II), including free and surface-associated Fe(II), was measured by addition of phenanthroline prior to filtration. It should be noted that the concentration of total Fe(II) is only available when nZVI particles are completely consumed (after 2 h reaction) as the relatively low pH (∼4) of phenanthroline stock solutions can cause rapid dissolution of nZVI particles, leading to dramatic discrepancies between the actual and measured concentrations of total Fe(II). H2O2 concentrations were measured using the Amplex Red method.23 Specifically, the samples were diluted with 10.0 mM MOPS buffer (pH 7.0) and mixed in a 1 cm quartz cuvette with AR/HRP stock solution at final AR and HRP concentrations of
kinetics and the nature of the final reaction products.13 While we have implied above that formation of oxide coatings will limit the extent of nZVI-mediated production of ROS, this need not be the case as it is recognized that some iron oxide surfaces (including magnetite, ferrihydrite, and lepidocrocite) have been identified as strong sorbents for contaminants14−16 and effective substrates for mediation of oxidation of these sorbed contaminants with highly reactive HO• and/or Fe(IV)O2+ species generated via heterogeneous iron oxide-mediated Fenton reactions.15−19 Of particular importance is the recognition in these studies that the presence of Fe(II), either at the surface or within the structure of the iron oxides, is critical to the oxidative transformation of the sorbed contaminants.15,16 As such, quantitative understanding of both the structural transformation of nZVI and surface corrosion-mediated release of Fe(II) is of great importance for evaluation of the oxidizing capacity of nZVI. We elucidate these effects in this study by examination of the nature of the Fe oxides formed on oxygenation of nZVI particles and, in parallel, monitor both the formation/release of Fe(II) and H2O2 and the oxidizing ability of the transforming nZVI assemblage over time. The key questions addressed in this work include: (i) What iron oxides are formed (and how fast they are formed) on exposure of nZVI to an oxygenated solution? (ii) How does the concentration of Fe(II) and H2O2 change with time of exposure of nZVI to an oxygenated solution? (iii) How does the oxidizing capacity of the transforming nZVI assemblage change with time (and how is this capacity related to the measured concentrations of Fe(II) and H2O2)? and (iv) What is the relationship between the changing nature of the nZVI assemblage with time and the oxidizing capacity of this assemblage?
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MATERIALS AND METHODS Reagents. All chemicals used in this work were analytical reagent grade and were purchased from Sigma-Aldrich unless otherwise stated. All solutions were prepared in ultrapure 18.2 MΩ cm Milli-Q water (Millipore). All glassware was soaked in 5% v/v hydrochloric acid (HCl) for at least 3 days before use. 5.0 mM Fe(II) and Fe(III) stock solutions were prepared by dissolving appropriate amounts of ferrous sulfate heptahydrate (FeSO4·7H2O) and ferric sulfate (Fe2(SO4)3) in 5.0 mM HCl solutions, respectively. A 3.0 mM 1,10-phenanthroline stock solution was prepared in a buffer solution containing 0.1 M acetic acid (pH ∼ 4) while 20 mM M H2O2 stock solutions were prepared weekly by dilution of a 30% w/v H2O2 solution and standardized by UV−vis spectrometry with a molar extinction coefficient of 22.7 M−1cm−1 at 250 nm.20 Stock solutions of 200 μM Amplex Red (AR; Invitrogen) mixed with 100 kU L −1 horseradish peroxidase (HRP) for H 2 O 2 determination were prepared and stored at −80 °C when not in use. Sodium hydroxide (NaOH) and hydrochloric acid (HCl) were used for the adjustment of pH. Experiments were performed at a controlled room temperature of 22 °C. Synthesis of nZVI, Lepidocrocite, and Ferrihydrite. nZVI was synthesized daily in an anaerobic chamber (855 Series; Plas-Lab Inc.) according to a protocol modified from literature.5 In brief, a 0.25 M ferrous chloride tetrahydrate (FeCl2·4H2O) solution was prepared in a 30% v/v methanol solution, and a 0.5 M sodium borohydride (NaBH4) solution was added dropwise to the Fe(II) solution with magnetic stirring. The freshly synthesized nZVI particles were then rinsed three times with deaerated methanol, dried overnight at 3821
DOI: 10.1021/acs.est.5b04988 Environ. Sci. Technol. 2016, 50, 3820−3828
Environmental Science & Technology
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2.0 μM and 1.0 kU L−1 respectively with the resorufin produced due to the oxidation of AR by H2O2 in the presence of HRP exhibiting a fluorescence emission maxima at 587 nm upon excitation at 563 nm. The concentration of aqueous H2O2 was determined by addition of AR/HRP into diluted samples after filtration while the concentration of total H2O2, which was attributed mainly to H2O2 on the surface of particles, was quantified by addition of AR/HRP into diluted samples before filtration. Calibration was performed daily by the addition of H2O2 into MOPS buffer at pH 7.0 over the concentration range of 0−800.0 nM. H2O2 calibration was unaffected by the addition of lepidocrocite (2g L−1 as Fe) indicating that the presence of Fe oxyhydroxide has negligible effect on the AR/ HRP fluorescence signal (SI Figure S3). H14COO− in the reaction vessel and 14CO32− in the trapping vessel were measured using a Packard Tri-Carb 2100TR scintillation counter following the addition of 1 mL of sample into 5 mL of liquid scintillation fluid (Ultima GOLD, PACKARD).24 Characterization of nZVI and Oxidation Products. Solid phase Fe speciation of nZVI was investigated using Fe Kedge extended X-ray absorption fine structure (EXAFS) spectroscopy. EXAFS measurements were conducted at the XAS beamline (ID 12) at the Australian Synchrotron (Clayton, Australia). The energy was selected with a Si(111) liquid nitrogen-cooled double crystal fixed-exit monochromator and the beam was focused (fully tuned) both vertically and horizontally with a rhodium-coated toroidal mirror. The beam energy was monitored with a Fe reference foil and found to vary by
