Stepwise Deprotonation of Magnetite-Supported Gallic Acid

Key Laboratory of Leather Chemistry and Engineering, (Sichuan University), Ministry of ...... This work is financially supported by Science and Techno...
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Stepwise Deprotonation of Magnetite-Supported Gallic Acid Modulates Oxidation State and Adsorption-Assisted Translocation of Hexavalent Chromium Xiaoyu Guan,† Yi Chen,*,†,‡ and Haojun Fan† †

Key Laboratory of Leather Chemistry and Engineering, (Sichuan University), Ministry of Education, Chengdu 610065, P.R. China Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States

‡

S Supporting Information *

ABSTRACT: Recently, a synergistic strategy involving reduction of carcinogenic Cr(VI) into less toxic Cr(III) followed by Cr(III) adsorption and subsequent separation by surface-engineered magnetite nanoparticles has emerged as a promising alternative to address the environmental hazards associated with Cr(VI)-contaminated water. Despite several previous attempts exploiting this synergy, modulating the oxidation state and translocation of Cr(VI) with high spatiotemporal precision remains a major challenge. Here, we report how Cr(VI) responds accordingly in a well-defined manner to deprotonation of gallic acid covalently immobilized on magnetite nanoparticles, which proceeds through a fixed spatial sequence of distinct stages. To the best of our knowledge, this proof-of-principle study, for the first time, demonstrates that accurate spatiotemporal control over the cascading reduction−adsorption process of Cr(VI) by magnetic adsorbents is feasible, which provides guidance for rational design of more exquisite, magnetite-supported surfaces, where a predictable, and hence controllable, synergy can manifest for Cr(VI) detoxification. KEYWORDS: chromium, gallic acid, magnetite, redox, adsorption

1. INTRODUCTION

detoxification of Cr(VI)-contaminated water has been and will continue to be a global research priority. Recently, a synergistic strategy that involves reducing Cr(VI) into less toxic Cr(III) followed by Cr(III) adsorption by surface-engineered magnetite nanoparticles emerged and has attracted much interest.10,11 This strategy is promising because it offers a possibility for thorough detoxification of Cr(VI)contaminated water by fundamentally changing the oxidation state of the metal contaminants, combined with adsorptionmediated removal of the resultant Cr(III) along with the exhausted adsorbent under magnetic field. One compelling example of material synergies leading to reduction and concurrent removal of the carcinogenic metal is that of Cr(VI) remediation by humic acid-coated magnetite nanoparticles.10 In this study, magnetite nanoparticles were coated with humic acids, a naturally occurring mixture derived from microbiological degradation of vegetation and animal decay. Multiple substitutes abundant in humic acids were allegedly capable of initiating the reduction of Cr(VI) to Cr(III). Carboxylate, phenoxide, etc. otherwise captured Cr(III) cations by coordination, which enabled final removal of chromium

Over the past decades, escalating worldwide use of chromium in anthropogenic activities, as exemplified by leather tanning, electroplating, stainless steel production, and wood preservation, has heightened concerns over consequent contamination of aquatic ecosystems. Speciation of chromium in aqueous media occurs predominately between trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)). Compared with Cr(III), an essential micronutrient responsible for carbohydrate and lipid metabolism, Cr(VI) is a well-established carcinogen and one of the most frequently encountered occupational hazards.1−6 Particularly in 2016, Environmental Working Group, a nonprofit environmental research organization, examined water systems throughout United States and raised alarm that the tap water of 218 million Americans contained levels of carcinogenic Cr(VI) exceeding the public health goal (0.02 parts per billion) proposed by the Environmental Health Hazard Assessment Office of California.7 In China, the government has also set a maximum Cr(VI) discharge limit of 0.5 mg/L for industrial wastewater, whereas a maximum allowable Cr(VI) concentration of 0.05 mg/L in drinking water is legislatively required.8,9 To eliminate or substantially reduce potential environmental hazards associated with Cr(VI), rational development of strategies that aim at © XXXX American Chemical Society

Received: March 2, 2017 Accepted: April 24, 2017

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DOI: 10.1021/acsami.7b03061 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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spatiotemporal precision. By investigating how Cr(VI) responds accordingly around a specific magnetic adsorbent with finely tunable surface chemistry, this proof-of-principle study may shed light on strategies to devise more exquisite, magnetite-supported surfaces, where a predictable, and hence controllable, synergy can manifest for Cr(VI) detoxification.

contaminants along with the magnetite core via a simple magnetic process. Despite this effectual synergy, molecular-level understanding of the cascading reduction−adsorption process was complicated by the fact that humic acids are not a single compound, but rather a collection of chemicals whose composition and functionalities vary greatly with sources. With this complexity, researchers found it challenging to distinguish the reduction-governing functionalities from those responsible for Cr(III) coordination such that spatiotemporal precision was undermined in delineating the mechanism underlying the measured phenomenology, which imposed fundamental limitation on our ability to tailor the surface chemistry of the composite adsorbent for improved synergy. Another example highlighting how such synergy occurs is a recently published study of poly(m-phenylenediamine)@Fe3O4 core−shell nanocomposite for Cr(VI) detoxification.11 In this study, it was proposed that potent Cr(VI) removal was attributed, in part, to efficient reduction of Cr(VI) by benzenoid amines in the poly(m-phenylenediamine) shell, followed by Cr(III) coordination with iminos. However, this two-substaged process was found intertwined with direct and spontaneous Cr(VI) adsorption on protonated iminos in the polymer shell via electrostatic interaction. Thus, spatiotemporal control over the oxidation state and translocation of Cr(VI) with high precision was compromised, again, giving rise to a further difficulty that the experimental phenomenology could not be captured by a simple yet generality-leading model, which might aid rational design of new magnetite-based adsorbents with improved efficiency for Cr(VI)-targeting detoxification. In our previous study,12 gallic acid (GA), a natural, lowmolecular-weight triphenolic compound with well-defined structure, was covalently immobilized on magnetite nanoparticles, in anticipation that this nanocomposite might offer an alternative to address Cr(III) contaminates in tannery effluents. The rationale behind this effort was grounded in the fact that the 3,4,5-trihydroxy functionalities in the immobilized gallate deprotonated consecutively with increasing pH, which yielded Cr(III)-coordinating phenoxides. Under magnetic field, the enhancement in Cr(III) removal potency was found a complex function of multiple variables, particularly the deprotonation degree of gallate moieties. Unlike phenoxides, it is noteworthy that phenolic hydroxyls are liable to oxidation via hydrogen atom transfer (HAT) mechanism in the presence of reducible metal ions.13,14 In principle, protonated GA can be oxidized by Cr(VI), especially under acidic condition where the GA/orthoquinone redox couple displays an standard oxidation potential of 0.799 V,15,16 while the Cr(VI)/Cr(III) redox couple possesses a high standard reduction potential of 1.33 V,17,18 stoichiometrically yielding a positive total reaction potential that favors spontaneous redox reaction. Therefore, as the magnetite-supported gallates experience stepwise deprotonation, leading to coexistence of phenolic hydroxyls and phenoxides, we hypothesize and demonstrate herein that a synergy may manifest that reduces the Cr(VI) species into Cr(III) first, which then can be removed from water via a combination of chemical adsorption and magnetite-mediated enrichment. More importantly, as a result of specific substitute arrangement in the aromatic ring, deprotonation of 3,4,5trihydroxy functionalities in GA has been known to follow a fixed spatial sequence of distinct stages.13,14 This feature offers us a unique opportunity to locate and quantify the reductiongoverning sites as well as those responsible for Cr(III) coordination, endowing the manifested synergy with high

2. EXPERIMENTAL SECTION 2.1. Materials. Iron(III) acetylacetonate (Fe(acac)3, ≥ 98.0%), (3aminopropyl) triethoxysilane (AMEO, ≥ 97.0%), and tetraethyl orthosilicate (TEOS, ≥ 99.0%) were purchased from Chengdu Kelong Chemical Reagent Company (China). Gallic acid (GA, ≥ 99.0%) in fine powder, N-hydroxysuccinimide (NHS, ≥ 98.0%), and 1-ethyl-3(3-dimethylaminepropyl) carbodiimide hydrochloride (EDC·HCl, ≥ 98.5%) were obtained from Alfa Aesar (Ward Hill, MA). Potassium dichromate (K2Cr2O7, ≥ 99.8%), benzyl ether (≥95%), oleylamine (≥70%), and polyoxyethylene (5) nonylphenylether (Igepal CO-520) were supplied by Sigma-Aldrich (St. Louis, MO). Ultrapure water with a resistivity equal to 18.2 MΩ cm at 25 °C was obtained from a Millipore Synergy water purification system, which was deoxygenated three times using a freeze−pump−thaw method before use. 2.2. Synthesis of GA-Modified Magnetite Nanoparticles. Monodisperse Fe3O4 nanoparticles were prepared by thermal decomposition of iron precursor under argon atmosphere using standard Schlenk-line techniques according to a previously disclosed procedure.19,20 In brief, 0.796 g of Fe(acac)3 was dissolved in a mixture containing 10 mL of benzyl ether and 10 mL of oleylamine. After dehydrated at 110 °C for 1 h, the solution was heated with a heating rate of 10 °C/min, followed by refluxing at 300 °C for another 2 h. Upon cooling, the mixture was poured into 50 mL of anhydrous ethanol and centrifuged (25 °C, 6000 rpm) to discard the supernatant. The pellet was then redispersed in hexane and precipitated with anhydrous ethanol five times before vacuum-dried at 50 °C. A reverse microemulsion method21 was further employed to encapsulate the as-synthesized Fe3O4 nanoparticles with a layer of hydroxyl-rich silica, thus amenable to secondary modification. Typically, 1.65 mg of Fe3O4 was dispersed into 11 mL of cyclohexane by ultrasonic oscillation (120 W, 40 kHz) for 10 min. Then 0.5 g of Igepal CO-520 and 0.2 mL of ammonium hydroxide (25−28% NH3 basis) were then added into the mixture under continuous stirring, followed by addition of 0.05 mL of TEOS via the equivalently fractionated drop method at ambient temperature within 24 h. The product was collected by centrifugation, washed with ethanol five times, and vacuum-dried until a constant weight. To covalently immobilize GA onto the silica-coated Fe3O4 surface, the composites prepared above were further modified by AMEO to introduce primary amines. Typically, 0.1 g of silica-coated Fe3O4 nanoparticles and 100 mL of toluene were mixed at 30 °C by ultrasonic oscillation (120 W, 40 kHz) for 20 min. Afterward, 2 mL of AMEO was added, and the mixture was stirred continuously at 80 °C for another 20 h under N2 atmosphere. The product was isolated using a magnet, washed with ethanol five times, and dried at 50 °C until a constant weight. Following this, EDC/NHS chemistry was used to activate the 1carboxyl in GA, yielding an intermediate that subsequently coupled with those surface primary amines offered by AMEO via chemically stable amide bonds. To this end, 0.5 g of GA, 0.305 g of NHS, and 0.57 g of EDC·HCl were mixed in 100 mL of phosphate buffered saline by stirring for 30 min. Subsequently, 0.5 g of AMEO-engineered Fe3O4 was added, with amide coupling carried out at 40 °C under N2 atmosphere for 6 h. The resultant magnetic nanocomposite, referred to henceforth as GA@Fe3O4, was collected by a magnet, washed with deoxygenated ultrapure water, and stored in a sealed vacuum desiccator at 4 °C prior to use. The synthesis procedure and structure of GA@Fe3O4 were schematically illustrated in Scheme 1. 2.3. Structural Characterization of GA@Fe3O4 Nanocomposite. The morphology of the nanocomposite was visualized by a transmission electron microscope (TEM, JEM-2100F, JEOL, Japan), which operated at a voltage of 200 kV, and atomic force microscope B

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where C0 (mg/L) and V0 (L) stand for the initial concentration and volume of the Cr(VI) solution, respectively, Ct (mg/L) and Vt (L) represent the residual concentration of total chromium and residual volume of the solution, respectively, at time t, and m (g) denotes the initial GA@Fe3O4 dose. All analyses were performed in triplicate, and means ± standard deviation were reported.

Scheme 1. Synthesis Procedure and Structure of GA@Fe3O4

3. RESULTS AND DISCUSSION Herein, GA molecules with well-defined structure were covalently immobilized on the surface of monodisperse magnetite nanoparticles, which exhibited an average diameter of 10.5 nm (Supporting Information, Figure S1a) and a typical cubic inverse spinel structure (Supporting Information, Figure S1b−d). The resultant nanocomposite possessed an experimentally measured BET specific surface as large as 40.8 ± 0.5 m2/g and displayed a shell-multicore morphology, visible in the TEM image present in Figure 1a. In particular, the presence of (AFM, SHIMADZU, SPM-9600, Japan) at ambient temperature using a tapping mode. Brunauer−Emmett−Teller (BET) specific surface area analysis was conducted at 77 K on an ASAP 2020 Accelerated Surface Area and Porosimetry System (Micromeritics, U.S.). Wideangle X-ray diffraction (XRD) patterns were recorded on a PANalytical X’Pert Pro MPD, powered by a Philips PW3040/60 Xray generator and fitted with an X’Celerator detector. Diffraction data were acquired by exposing the sample to Cu−Kα X-ray radiation, which had a characteristic wavelength (λ) of 1.5406 Å. X-rays were generated from a Cu anode supplied with 40 kV and a current of 100 mA. Magnetic measurements were performed at 300 K on a vibrating sample magnetometer (735 Model, Nanjing, China) with a maximum magnetic field of 10 kOe. X-ray photoelectron spectroscopy (XPS) measurements were performed on an AXIS Ultra DLD instrument (Kratos, U.K.), equipped with a standard and monochromatic Al Kα X-ray excitation source (1486.6 eV). The binding energies obtained were corrected by referencing the C 1s peak at 284.6 eV. Fourier transform infrared (FT-IR) spectra were collected using a Nicolet iS10 FTIR spectrometer (Thermo Scientific, U.S.) over a wavenumber range from 400−4000 cm−1 after 64 scans at a resolution of 2 cm−1. Total organic carbon (TOC) content was measured using an Apollo 9000 TOC combustion analyzer (Tekmar-Dohrmann, U.S.). Excitation−emission fluorescence spectra were acquired on a Cary Eclipse fluorimeter (Varian, Palo Alto, CA) using a 10 mm quartz cell. 2.4. Cr(VI) Reduction−Adsorption Experiment. Cr(VI) aqueous solution was prepared by dissolving a stoichiometric amount of K2Cr2O7 in deoxygenated ultrapure water. The initial pH of the solution was controlled by using 0.1 M NaOH and HCl and confirmed with a pH meter (Fisher Scientific). The reduction−adsorption experiments were carried out in dark, using a standard Schlenk line, pre-evacuated, and then purged with dry oxygen-free nitrogen. Under vigorous stirring, 60 mg of GA@Fe3O4 was added into 100 mL of Cr(VI) aqueous solution with an initial concentration of 5 mg/L, which was held in a thermostatic shaker at 25 °C agitating at a rate of 200 rpm. At regular time intervals, the solid and the liquid were momentarily separated by using a magnet, followed by measuring the residual Cr(VI) concentration in the liquid phase by 1,5diphenylcarbohydrazide spectrophotometric method (λmax = 540 nm) on a UV−vis spectrophotometer (UV-3600, SHIMADZU, Japan). In addition, the concentration of residual total chromium species was also measured by using an Optima 2100 DV inductively coupled plasma-optical emission (ICP-OES) instrument (PerkinElmer, Inc., U.S.) as a function of time. Before ICP-OES analysis, the solution was digested by H2SO4−HNO3 method and filtered through a 0.2 μm nylon syringe filter. Then Cr(III) concentration in the liquid phase could be calculated from the concentration difference between total chromium and Cr(VI). The amount of chromium adsorbed per unit GA@Fe3O4 mass at time t (qt) was determined as per the following equation: qt =

C0 × V0 − Ct × Vt m

Figure 1. (a) TEM image, (b1) N 1s, (b2) C 1s, and (b3) O 1s highresolution XPS spectral envelopes, (c) contour plot of the excitation− emission photoluminescence matrix, and (d) FT-IR spectrum of GA@ Fe3O4 nanocomposite. The inset in panel a demonstrates the appearance of 2.0 mg/mL GA@Fe3O4 nanocomposite dispersed in water (pH = 5.5) and their fast response upon application of an external magnetic field. To separate different species of the same element, CasaXPS processing software was employed to deconvolve the XPS signals with Gaussian−Lorentzian function (Gaussian = 80%, Lorentzian = 20%) after subtraction of a Shirley background. The deconvolved FT-IR spectra were fitted with Gaussian band shapes by an iterative curve fitting procedure.

gallate moieties on the outmost layer of the nanocomposite was systematically verified by (1) XPS analysis detecting an −NHinduced signal with BE centered at 399.1 eV (Figure 1b1), attributable to amide,22,23 as well as simultaneous occurrence of GA-derived CO, C−O−H, and C−O/C−N signals via deconvolution of the high resolution C 1s (Figure 1b2) and O 1s (Figure 1b3) spectral envelopes; (2) excitation−emission photoluminescence matrix (Figure 1c) that yielded two

(1) C

DOI: 10.1021/acsami.7b03061 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces maximums in the presence of borax/sodium dodecyl sulfonate enhancer, corresponding to the π* → π* (λex/λem = 226/361 nm) and n → π* (λex/λem=(280/361 nm) electronic transition in gallate structural motif,24 which resembled that of pristine GA (Supporting Information, Figure S2); and (3) FT-IR spectrum, deconvolution of which demonstrated amide I (peaking at 1670 cm−1) and II (peaking at 1514 and 1545 cm−1, respectively) absorption bands, stemming from amide couplings between amine terminals from AMEO and the activated 1-carboxyl in GA. From the measured TOC values of AMEO-functionalized Fe3O4 (122 ± 2 mg/g) and GA@Fe3O4 (159 ± 3 mg/g), as well as the theoretical carbon content (49.4 wt %) in GA, it could be quantitatively calculated that the resultant nanocomposite contained ∼7.5 wt % GA on its surface, coinciding well with the result (∼7.9 wt %) estimated from GA concentration decrease in the reaction mixture after amide couplings. By virtue of abundant surface hydroxyls from gallate moieties, GA@Fe3O4 was found to be readily dispersible in water (insert in Figure 1a). Upon application of an external magnetic field, the nanocomposite, with a saturated magnetization value of 31.5 emu/g (Supporting Information, Figure S3), could be quickly enriched around the magnet within 4 min (insert in Figure 1a), allowing convenient separation of any contaminant adsorbed on its surface. To investigate how Cr(VI) responds to stepwise deprotonation of gallate moieties on GA@Fe3O4 surface, Bjerrum plot for the aqueous nanocomposite dispersion was constructed from acid−base titration curve experimentally obtained (Supporting Information, Figure S4a), revealing the pH regions where specific gallate species dominated. In GA, the 3,4,5-trihydroxy functionalities are readily deprotonated owing to extended delocalization and conjugation of the p-electrons on phenoxides enhanced by resonance.13,14 Moreover, such deprotonation occurs sequentially, with the 4-hydroxyl in para position with respect to the 1-carboxyl always deprotonating first, followed by successive deprotonation of the 3- and 5-hydroxyl.13,14 This sequence is dictated by special substitute arrangement in GA aromatic ring, which offers a geometry to better stabilize the negative charge of the 4-phenoxide via two intramolecular hydrogen bonds with bilateral hydroxyl substitutes.14,25 From the titration curve (Supporting Information, Figure S4a), the 4hydroxyl seemed deprotonated more easily in GA@Fe3O4 than that in pristine GA, displaying a remarkably lower pKa1 (∼5.5) compared with the counterpart before immobilization (∼8.8, Supporting Information, Figure S4b). This was also the case for the other two meta-substituted hydroxyls, the pKa values of which decreased to ∼6.3 and 8.1 (Supporting Information, Figure S4a), respectively, relative to the original values (∼10.8 and >13.0, Supporting Information, Figure S4b) in GA. Such shifts in pKa were normal and could be well rationalized given that amide displays stronger electron-withdrawing capability, and hence resonance stabilization for phenoxide than carboxyl. According to the Bjerrum plot in Figure 2, protonated gallates, abbreviated as H3A, dominated the GA@Fe3O4 surface once the pH decreased below 5.5; a pH greater than 8.1 favored complete deprotonation of gallates, yielding A3−; over the pH range in between, partially deprotonated gallates, H2A− or HA2−, were the prevailing species. As the immobilized gallates deprotonated consecutively as a function of pH, we assumed that Cr(VI) could be reduced into Cr(III) by phenolic hydroxyls, then remaining in the solution or being adsorbed onto the adsorbent surface depending on whether extra phenoxides were present. Only if the gallate moieties

Figure 2. Bjerrum plot illustrating species distribution of gallate moieties on GA@Fe3O4 nanocomposite at 25 °C as a function of pH. H3A represents protonated gallate moieties, H2A− denotes gallate species with the 4-hydroxyl deprotonated, HA2− stands for gallate species with two adjacent hydroxyls deprotonated, and those gallates completely deprotonated are abbreviated as A3−. Ka1 is the equilibrium constant for reversible deprotonation of H3A into H2A−, Ka2 is the equilibrium constant for the deprotonation reaction H2A−⇌HA2− + H+, and Ka3 is the equilibrium constant for reversible deprotonation of HA2− into A3−.

deprotonated completely at pH values significantly higher than pKa3, possessing neither reduction nor adsorption capability toward anionic metal species, should Cr(VI) remain inert, in terms of both oxidation state and physical distribution. More importantly, deprotonation of gallates proceeds through a fixed spatial sequence of distinct stages. This feature allowed us to distinguish the reduction-governing sites from those responsible for Cr(III) coordination, qualitatively and quantitatively, yielding high spatiotemporal precision in modulating the reduction and translocation of Cr(VI). To demonstrate the foregoing hypothesis, GA@Fe3O4 nanocomposite was incubated with Cr(VI) aqueous solution with an initial concentration of 5 mg/L at varying pH. At a predetermined time interval, the nanocomposite was separated temporarily using a magnet, with residual concentrations of Cr(VI) and total chromium measured by spectrophotometric method and ICP-OES, respectively. On the basis of the measured data and the initial Cr(VI) concentration, the amount of Cr(III), remaining in the solution or being adsorbed by GA@Fe3O4, could be quantitatively estimated, as described in the Experimental Section. Control experiments were specifically carried out wherein the Cr(VI) aqueous solution was incubated at corresponding pH without GA@Fe3O4. Spectrophotometric analysis indicated that the Cr(VI) species were chemically stable in the system, yielding a constant concentration of ∼5 mg/L throughout incubation. After extensive rinsing using deoxygenated ultrapure water, surface chemistry of the exhausted GA@Fe3O4 nanocomposite was analyzed, which provided experimental evidence for gallate oxidation and translocation of chromium species. When the pH dropped to 2.0, where H3A accounted for 100% of total gallate species on GA@Fe3O4, it was found that Cr(VI) concentration in the liquid phase decreased sharply D

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substituent of a phenolic compound, the more likely and faster it will participate in HAT reactions. In gallate, the 4-hydroxyl possesses a higher propensity to donate a proton-coupled electron, or hydrogen atom, forming a phenoxyl radical resonance-stabilized by the adjacent aromatic ring, which is further augmented via the electron-withdrawing inductive effect of the para-amide coupling. Of equal importance, the generated 4-phenoxyl radical can also be well stabilized via two intramolecular hydrogen bonds with 3- and 5-hydroxyl substitutes.14 Thus, the 4-hydroxyl is considered an active hydrogen atom donor, with a bond dissociation enthalpy (BDE) as low as −14.0 kcal/mol, compared to that of phenol falling in the range of ∼87.0−88.7 kcal/mo.13 Following radicalization of the 4-hydroxyl, either 3- or 5-hydroxyl, displaying a BDE of −7.6 kcal/mol, may also experience a concerted donation of a proton and an electron, completing its oxidation to form ortho-quinone.13 Thus, the active sites responsible for Cr(VI) reduction on GA@Fe3O4 at pH 2.0 were clear, which involved the 4-hydroxyl or two adjacent hydroxyls (Figure 3e). In the presence of highly reducible Cr(VI), HAT-mediated oxidation of gallate moieties occurred spontaneously, considering a positive total redox potential, stoichiometrically calculated as E⊖Cr(VI)/Cr(III) (1.33 V)17,18 − E⊖GA/ortho‑quinone (0.799 V)15,16 = E⊖redox (0.531 V) in acidic solution. Such redox reaction did occur, manifested by the presence of a FT-IR stretching vibration peaking at 1715 cm−1 (Figure 3d), derived from semiquinone/ortho-quinone-associated ketone on the exhausted nanocomposite surface. However, high resolution XPS spectrum in Figure 3c suggested that neither Cr(III) (binding energy usually peaking at 577.0−578.0 eV and 586.0−588.0 eV23,26) nor Cr(VI) species (binding energy usually peaking at 580.0−580.5 and 589.0−590.0 eV23,26) existed on the exhausted GA@Fe3O4 surface, which indicated that all chromium remained in the liquid phase. This result was in good agreement with the observation that the final Cr(III) concentration in the solution was almost equivalent to the original Cr(VI) concentration. Also, it suggested that any adsorbed chromium indicated in Figure 3b stem from physical adsorption, which was difficult to exclude without disrupting the experiment. Translocation of Cr(III) was spatially confined in this case because both phenolic hydroxyl and quinone displayed negligible capability to coordinate with chromium ions. Compared with the above scenario, Cr(VI) responded quite differently as the pH increased up to 12.0, where the 3,4,5trihydroxy in gallate deprotonated completely. As could be seen in Figure 4a, the concentration of residual Cr(VI) species fluctuated slightly about 5 mg/L regardless of incubation time; they were neither reduced into Cr(III) nor adsorbed by GA@ Fe3O4, which led to almost undetectable Cr(III) in the liquid phase (Figure 4a) as a function of time and indiscernible color change after GA@Fe3O4 treatment (inset in Figure 4b). Herein, inertness of Cr(VI), in terms of both oxidation state and physical distribution, was further verified by experimental result that no chromium-associated XPS signal was detectable on the final nanocomposite (Figure 4c), and FT-IR analysis revealing the absence of ketone stretching vibration associated with semiquinone or ortho-quinone (Figure 4d). Of course, electron transfer from phenoxides to aid reduction of potential target compounds via single electron transfer (SET) chemistry was possible. This mechanism, however, seemed not to work herein, probably because the p-electron on phenoxide was delocalized over the entire molecule by resonance, leading to

from 5 mg/L, approaching zero after incubation for only 10 min (Figure 3a). Accordingly, Cr(III) remaining in the solution

Figure 3. (a) Concentrations of residual Cr(VI) and Cr(III) in the liquid phase after incubation with GA@Fe3O4 nanocomposite at pH 2.0 as a function of time. (b) Amount of chromium adsorbed per unit GA@Fe3O4 mass with increasing incubation time t at pH 2.0. The inset in panel b demonstrates the original pale-yellow Cr(VI) solution turned transparent after GA@Fe3O4 treatment. (c) Cr2p-associated high-resolution XPS spectral envelope, and (d) FT-IR spectrum across the range of interests of GA@Fe3O4 after reduction−adsorption experiment at pH 2.0. Before XPS and FT-IR analysis, the exhausted nanocomposite was extensively rinsed with deoxygenated ultrapure water. The deconvolved FT-IR spectrum was fitted with Gaussian band shapes by an iterative curve fitting procedure. (e) Proposed molecular mechanism underlying the measured phenomenology at pH 2.0, where fully protonated gallates overwhelmed the GA@Fe3O4 surface. Error bars show standard deviations calculated from measurements in triplicate.

experienced a surge in concentration to ∼5 mg/L over the first 10 min and afterward leveled off (Figure 3a). This synchronism underscored a complete conversion of Cr(VI) without Cr(III) adsorption onto the nanocomposite. As a consequence, the original pale-yellow Cr(VI) solution turned transparent after GA@Fe3O4 treatment (insert in Figure 3b). Herein, zero-order, first-order, and second-order kinetics models were employed to fit the experimental data in Figure 3a. The expressions for these models were given in Supporting Information, eqs 1−3, respectively, and the fitting results were tabulated in Supporting Information, Table S1. In terms of a high coefficient of determination (R2 = 0.99) and a low average relative error (ARE = 0.89), the first-order kinetics model provided fairly good fit to the time-dependent Cr(VI) concentration in Figure 3a, yielding a rate constant of 0.42 min−1, which temporally governed Cr(VI) reduction. In this scenario, HAT mechanism that involves proton-coupled electron transfer in a single kinetic step was applicable to interpret the measured phenomenology. HAT mechanism occurs when a phenolic reductant encounters highly reducible species, as exemplified by Cr(VI) herein. In general, the weaker the hydrogen atom is held to the hydroxyl E

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ACS Applied Materials & Interfaces

the nanocomposite surface at pH 5.5, the Cr(VI) species could be completely reduced into Cr(III) by GA@Fe3O4 after incubation for ∼40 min, while the incubation time favored complete conversion of Cr(VI) increased to ∼120 min at pH 7.5, where ∼75% HA2− and ∼20% A3− were present. Particularly at pH 9.0, where A3− accounted for 90% of the gallate moieties while the rest were mostly HA2−, not all Cr(VI) species could be reduced into Cr(III), with ∼0.7 mg/L of Cr(VI) still detectable in the liquid phase ultimately. This phenomenon demonstrated again that only phenolic hydroxyls, capable of proportionally providing more electrons via HAT mechanism, contributed to reduction of Cr(VI). As phenolic hydroxyls were the only sites on GA@Fe3O4 participating in HAT reaction, amount of which varied in a predictable manner as described by Figure 2, the redox component constituting the synergy for Cr(VI) detoxification could be well quantified, leading to a temporal control over the oxidation state of Cr(VI). Following the redox reaction, physical distribution of the resultant Cr(III) also varied in a well-defined manner. Compared with Cr(VI), Cr(III) concentration in the liquid phase displayed a complex time dependence, as illustrated in Figure 5a; it increased as incubation started, reaching a maximum after 20−30 min, and afterward decreased until reaching a plateau. This observation was a corroborating evidence that the reduction of Cr(VI) and Cr(III) adsorption proceeded as two distinct substages, and the resultant Cr(III) preferred to stay in the liquid phase rather than be adsorbed by phenoxides at the very beginning. With the 4-hydroxyl or two adjacent hydroxyls deprotonated, the 3- or 5-hydroxyl was oxidized into semiquinones, as confirmed by the presence of an FT-IR absorption band peaking at 1715 cm−1 associated with quinone-derived ketone (Figure 5d). Following the redox reaction, Cr(III) species were adsorbed by GA@Fe3O4, given the XPS results finding Cr(III) signals with BEs centered at 577.0 and 586.0 eV on the exhausted nanocomposite (Figure 5c). Considering that partially deprotonated gallates, HA2− and H2A−, were the prevailing species, the gallates might form a 1:1 complex with Cr(III) via the 4-phenoxide or chelate with the cationic metal by two adjacent phenoxides. By using nonlinear regression, pseudo-first order, pseudo-second order, and intraparticle diffusion models were adopted to fit the experimental data in Figure 5b. In terms of high R2 (≥0.93) and low ARE (≤2.67) (Supporting Information, Table S2), the kinetics data conformed well to pseudo-first order model. The pseudo-first order assumes that the uptake rate is limited by only one process or mechanism acting on a single class of adsorbing sites.28,29 Therefore, the fitting result suggested again that phenoxides were the only adsorption sites on GA@Fe3O4 for Cr(III) cations. Since the oxidation of phenolic hydroxyls in those immobilized gallates is irreversible, the exhausted GA@Fe3O4 nanocomposites prepared in this study are less likely to be reused. However, chromite resources are precious and considered nonrenewable. Theoretically, these adsorbed Cr(III) species can be conveniently desorbed from GA@Fe3O4 by chemical desorption (e.g., H2SO4 treatment), resulting in Cr(III) sulfate solution for potential reuse, for example, as chrome tanning agent. This advantage may compensate for the nonreusability of GA@Fe3O4 itself, which makes the synergistic strategy based on such nanocomposite economically feasible for Cr(VI) detoxification.

Figure 4. (a) Concentrations of residual Cr(VI) and Cr(III) in the liquid phase after incubation with GA@Fe3O4 nanocomposite at pH 12.0 as a function of time. (b) Amount of chromium adsorbed per unit GA@Fe3O4 mass with increasing incubation time t at pH 12.0. The inset in panel b demonstrates the pale-yellow color of the original Cr(VI) solution remained constant after GA@Fe3O4 treatment. (c) Cr2p-associated high-resolution XPS spectral envelope, and (d) FT-IR spectrum across the range of interests of GA@Fe3O4 after reduction− adsorption experiment at pH 12.0. Before XPS and FT-IR analysis, the exhausted nanocomposite was extensively rinsed with deoxygenated ultrapure water. The deconvolved FT-IR spectrum was fitted with Gaussian band shapes by an iterative curve fitting procedure. (e) Proposed molecular mechanism underlying the measured phenomenology at pH 12.0, where completely deprotonated gallates overwhelmed the GA@Fe3O4 surface. Error bars show standard deviations calculated from measurements in triplicate.

limited propensity for donation. In addition to invariable oxidation state, Cr(VI) existed in aqueous solution in the form of chromate (CrO42−) anions at high pH value.27 Under such circumstance, it was impossible for negatively charged phenoxides to capture Cr(VI) anions due to electrostatic repulsion, accounting for restricted translocation of chromium species. The oxidation state and translocation of Cr(VI) became much more complicated, as the gallate moieties experienced partial deprotonation in the pH range from 5.5−9.0, leading to a predictable, and hence controllable, synergy for Cr(VI) detoxification. In this pH range, the concentration of residual Cr(VI) decreased from 5 mg/L as incubation time elapsed and then leveled off (Figure 5a), but the kinetics correlated with the deprotonation degree of gallate moieties. By fitting the experimental data using multiple kinetics models (Supporting Information, Table S1), the decrease of Cr(VI) concentration in Figure 5a was found to follow first-order kinetics model (R2 ≥ 0.92; ARE ≤ 1.28); the rate constant for Cr(VI) reduction increased from 0.02 min−1 at pH 9.0, through 0.07 min−1 at pH 7.5, to 0.19 min−1 at pH 5.5 (Supporting Information, Table S1). In addition to kinetics, the extent of Cr(VI) reduction also displayed a strong dependence on species distribution of gallate moieties. With equal amounts of H3A and H2A− dominating F

DOI: 10.1021/acsami.7b03061 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 5. (a) Concentrations of residual Cr(VI) and Cr(III) in the liquid phase after incubation with GA@Fe3O4 in a pH range from 5.5−9.0 as a function of time. (b) Amount of chromium adsorbed per unit GA@Fe3O4 mass at time t in a pH range from 5.5−9.0. (c) Cr2p-associated highresolution XPS spectral envelope, and (d) FT-IR spectrum across the range of interests of GA@Fe3O4 after reduction−adsorption experiment in a pH range from 5.5−9.0. Before XPS and FT-IR analysis, the exhausted nanocomposite was extensively rinsed with deoxygenated ultrapure water. The deconvolved FT-IR spectrum was fitted with Gaussian band shapes by an iterative curve fitting procedure. (e) Proposed molecular mechanism underlying the measured phenomenology in a pH range from 5.5−9.0, where partially deprotonated gallates dominated the GA@Fe3O4 surface. Error bars show standard deviations calculated from measurements in triplicate.

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4. CONCLUSIONS Herein, we demonstrated a proof-of-concept that the oxidation state and translocation of Cr(VI) could be modulated with high spatiotemporal precision by stepwise deprotonation of gallate moieties covalently immobilized on magnetite nanoparticles. Protonated phenolic hydroxyls from the immobilized gallates were responsible for reduction of Cr(VI) into Cr(III) via HAT mechanism, while phenoxide species captured cationic Cr(III) by coordination. As deprotonation of gallates proceeded through a fixed spatial sequence of distinct stages, the reduction-governing and adsorption-responsible sites could be well distinguished, located, and quantified in this simple nanocomposite model, which facilitated better understanding the measured synergy and thus supported rational design of more sophisticated, magnetite-based adsorbents with improved efficient for Cr(VI) detoxification.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiaoyu Guan: 0000-0001-5791-731X Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by Science and Technology Planning Project of Sichuan Province (2017JY0218), Fundamental Research Funds for the Central Universities, China, International Visiting Program for Excellent Young Scholars of SCU, and National Natural Science Foundation of China (21576172).

ASSOCIATED CONTENT

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b03061. AFM image, HRTEM image, SAED pattern, and XRD pattern of native Fe3O4 nanoparticles; contour plot of excitation−emission photoluminescence matrix of GA; room-temperature magnetic hysteresis loop of GA@ Fe3O4 nanocomposite; acid−base titration curves of aqueous GA@Fe3O4 dispersion and GA aqueous solution; analysis of Cr(VI) reduction kinetics; analysis of Cr(III) adsorption kinetics (PDF)

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DOI: 10.1021/acsami.7b03061 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX