Heterogeneous Reduction of 2-Chloronitrobenzene by Co-substituted

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Heterogeneous Reduction of 2-Chloronitrobenzene by Co-substituted Magnetite Coupled with Aqueous Fe2+: Performance, Factors, and Mechanism Xiaoliang Liang, Ying Li, Gaoling Wei, Hongping He, Joseph W Stucki, Lingya Ma, Linda Pentrakova, Martin Pentrák, and Jianxi Zhu ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00204 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on April 1, 2019

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ACS Earth and Space Chemistry

Heterogeneous Reduction of 2-Chloronitrobenzene by Co-substituted Magnetite Coupled with Aqueous Fe2+: Performance, Factors, and Mechanism

Xiaoliang Liang,*, †, ┴, # Ying Li,*, †, §, ┴, # Gaoling Wei,‡ Hongping He,†, ┴, # Joseph W. Stucki,§ Lingya Ma,†, ┴, # Linda Pentrakova,§ Martin Pentrak,‖ and Jianxi Zhu†, ┴, # †

CAS Key Laboratory of Mineralogy and Metallogeny/Guangdong Provincial Key

Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, PR China; ‡

Guangdong Key Laboratory of Integrated Agro-environmental Pollution Control and

Management, Guangdong Institute of Eco-environmental Science & Technology, Guangzhou 510650, PR China; §

Department of Natural Resources and Environmental Sciences, University of Illinois

at Urbana-Champaign, Urbana, IL 61801, USA; ‖

Illinois State Geological Survey, Prairie Research Institute, University of Illinois at

Urbana-Champaign, Champaign, IL 61820, USA; ┴

University of Chinese Academy of Sciences, Beijing 100049, PR China.

#

Institutions of Earth Science, Chinese Academy of Sciences, Beijing 100029, PR

China.

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ACS Paragon Plus Environment

ACS Earth and Space Chemistry 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

ABTRACT Reductive transformation is the predominant degradation pathway for nitrobenzene (NB) derivatives in natural matrices. Magnetite coupled with aqueous Fe2+ (Fe2+aq) displays reducing capability towards NB derivatives, but it is still unclear whether the substitution of redox-active metals in magnetite has significant influence on the reducing capability of the coupled system. This study investigated the potential of the heterogeneous reduction of 2-chloronitrobenzene (2-Cl-NB) by Co-substituted magnetite (Fe3-xCoxO4, 0.00 ≤ x ≤ 1.00) coupled with Fe2+aq. Both reaction kinetics and extent of electron transfer illustrated that appropriate Co substitution (x < 0.85) significantly promoted the reduction activity of Fe3-xCoxO4/Fe2+aq systems, while excess Co (x ≥ 0.85) retarded the process. A good linear correlation (R2 ≥ 0.94) was established between the electrical conductivity of Fe3-xCoxO4 and the rate constant (kobs), calculated from a three-parameter single exponential decay model. The improvement of reduction activity was ascribed to the redox pairs Co(II)/Co(III) and Fe(II)/Fe(III) on the octahedral sites, which accelerated the electron transfer in magnetite. As Co substitution increased up to x = 0.85, however, structural Fe(II) occupying the octahedral sites of magnetite was too low, resulting in a decrease of reducing capability of the coupled system. During the redox reaction, the adsorbed Fe2+aq and structural Fe(II) were oxidized gradually while the spinel structure of Fe3xCoxO4 was

maintained. These results shed light on the role of magnetite-group

minerals and their impact on the fate of contaminants in anoxic environments.

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KEYWORDS: Co-substituted magnetite; Aqueous Fe2+; Heterogeneous reduction; Electron transfer; Reduction stability

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1. INTRODUCTION Chlorinated nitrobenzenes (CNBs) are used primarily as important intermediates in the manufacture of explosives, pesticides, drugs, rubber, etc.1 With high toxicity, bioaccumulation and strong resistance to biodegradation, CNBs are easily accumulated in sediments and soils, which brings great threat to humans and wildlife.2, 3 The nitro- and chlorine- groups of CNBs with pronounced electronwithdrawing capacity decrease the density of electron cloud around aromatic rings, which makes the benzene ring difficult for oxygenase to attack.4, 5 Under anaerobic conditions, however, nitro functional groups readily succumb to electrophilic attack and subsequently induce the reduction of CNBs into their corresponding anilines, the less harmful and more biodegradable compounds.6-8 This makes reductive transformation the predominant degradation pathway for CNBs in natural matrices. Under subsurface anoxic environments, biogeochemical processes provide various potential reductants for the abiotic transformation of contaminants. As the fourth most abundant element on Earth, iron is redox active and commonly occurs in sedimentary environments as iron (oxyhydr)oxide minerals. Aqueous Fe2+ (Fe2+aq) is continuously generated through the reduction of iron oxides by dissimilatory iron reducing bacteria (DIRB).9, 10 Ferrous (Fe2+) is significantly more soluble and mobile than Fe3+ and is readily absorbed on the iron (oxyhydr)oxide surfaces. As verified in previous studies, Fe2+aq associated with iron oxides, including ferrihydrite, goethite, hematite, and magnetite, efficiently reduced various environmental contaminants.11-13 Compared

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with other iron oxides, the interaction between Fe2+aq and magnetite is more interesting, due to the unique structural and electrical properties of magnetite and its universality in surface environment. Magnetite is ubiquitous in soils and sediments as both a primary and secondary mineral phase.14 In supergene environments, magnetite can form via the reduction of Fe3+ oxides or the oxidation of ferrous minerals and iron metal by biological or abiotic processes.15, 16 Magnetite contains both Fe(II) and Fe(III) in inverse spinel structure with the cations Fe(II)[B]:Fe(III)[B]:Fe(III)[A] in a ratio of 1:1:1, where A and B represent tetrahedral and octahedral sites, respectively.17 Structural Fe(II) and Fe(III) can be oxidized and reduced reversibly, promoting electron transfer from the solid phase to contaminants, while leaving the structure unchanged.18 Surface Fe(II) in magnetite is an important electron donor for environmental contaminants.19-23 As the reduction reaction proceeds, magnetite is gradually oxidized and losses its reducing capability.11, 20-22 Interestingly, however, exposing magnetite to Fe2+aq can effectively recover its reducing capability via improving structural Fe(II) content.22 The adsorbed Fe2+aq can reduce octahedral Fe(III) in the underlying magnetite to octahedral Fe(II), accompanied by the oxidation of adsorbed Fe2+aq.22 Within natural environments, magnetite rarely exists as a pure iron phase but, instead, contains several substituting ions, e.g., Co(II), Zn(II), Cu(II), V(III), Cr(III), and Ti(IV), which usually modifies the structural features and reactivity of magnetite.24-26 According to previous study,27 Zn(II) substitution generally improved the reduction

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ability of magnetite coupled with Fe2+aq towards nitrobenzene. The increase in electrical conductivity of magnetite by Zn substitution that promoted electron transfer from magnetite to nitrobenzene accounted for the improved reduction activity. But the fate of Fe2+aq after reaction and variation in the magnetite structure were not studied systematically, which is vital for understanding the reduction mechanism and reduction stability. Meanwhile, compared to Zn(II) cation with unique valence, it is unclear whether the substitution of redox-active metals, e.g., Mn(II)/Mn(III), V(III)/V(IV), and Co(II)/Co(III), play a more significant role on the reducing capability of coupled systems. On one hand, these metals not only promote the electron transfer within the spinel structure, but also act as electron donors for reactants. This was verified in the heterogeneous reaction toward H2O2 decomposition and •OH generation catalyzed by substituted magnetite.18, 28 The incorporation of Co and Mn produced a remarkable increase in the activity of magnetite, whereas Ni with unique valence inhibited the H2O2 reaction.18 On the other hand, they also dramatically changed the surface properties and microstructure of magnetite, and accordingly varied the adsorption properties and redox reactivity.29, 30 Based on the above findings, the substitution of redox-active metals probably affects the reducing capability of magnetite coupled with Fe2+aq, but to date little information about this phenomenon has been reported. The purpose of the present study was to fill these gaps through the investigation of the reductive degradation of 2-chloronitrobenzene (2-Cl-NB) by a series of Co-

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substituted magnetites (Fe3-xCoxO4, x = 0, 0.23, 0.49, 0.77, 0.85, and 1.00) in the presence of Fe2+aq. Among the substituted magnetites, Fe3-xCoxO4 is environmentally friendly and catalytically active, because the strong Fe-Co interaction suppresses Co leaching and improves electron transfer in spinel structure.31 The specific aims of this study were to (i) investigate the effect of Co substitution on the reducing capability of magnetite coupled with Fe2+aq; (ii) analyze the variation of surface and structural properties of magnetite by Co substitution and its evolution during the reaction, and (iii) establish the relationship between structure and reactivity of Fe3-xCoxO4 and discuss the reaction mechanism.

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2. MATERIALS AND METHODS 2.1. Synthesis of Fe3-xCoxO4. Chemicals used in this study were HPLC or analytical grade (Text S1). Fe3-xCoxO4 (x = 0.23, 0.49, 0.77, 0.85, and 1.00) samples were prepared using a coprecipitation method.32 Briefly, a chloride salt solution was prepared by dissolving FeCl2·4H2O, CoCl2·6H2O, and FeCl3·6H2O in a molar ratio of (1-x): x: 2, where the total cation concentration was kept at 0.3 mol L-1. Several drops of hydrazine and 4.0 mL of HCl (12 mol L-1) were added to prevent hydroxide precipitation and iron oxidation. Then, the mixed solution was dropwise titrated with a NaOH solution (4.0 mmol L-1) under stirring. After mixing, the black aggregates were maintained at 90°C for 5 h to improve the recrystallization of particles. The solution was purged by N2 during the whole process to prevent iron oxidation. The particles were separated by centrifugation and washed five times with deionized water. After washing, the samples were collected, freeze dried, sieved through a 200mesh screen, and reduced under H2 at 350°C for 30 min. The prepared samples were reground and stored in an anaerobic glovebox. The Fe(II), Fe(III), and Co(II) contents in synthesized Fe3-xCoxO4 were close to the theoretical values (Tables S1 and S2). As the Co(II) content increased, the Fe(II) content decreased, whereas the Fe(III) content remained unchanged. This indicates that Co(II) replaced only Fe(II). Fe3-xCoxO4 had a well crystallized spinel structure, while no cobalt oxide phases were observed in XRD patterns (Figure S1). As depicted in TEM images, the particles grew well in an octahedral shape and aggregated with a

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diameter of 10 to 20 nm (Figure S2).

2.2. Characterization of Fe3-xCoxO4. The Fe and Co contents of Fe3-xCoxO4 were measured using an Agilent 7700X ICP-MS after the complete dissolution of samples in an HCl solution (6.0 mol L-1) in an anaerobic glovebox. The total Fe and Fe(II) contents were measured with the 1,10-phenanthroline method and potassium dichromate titration, respectively, while the Fe(III) content was calculated by subtracting Fe(II) content from total Fe content.33 Powder X-ray diffraction patterns (PXRD) were recorded between 10 and 80° 2θ at a step of 1° 2θ min-1 using a Bruker D8 advance diffractometer with Cu Kα radiation (40 kV and 40 mA). Transmission electron microscopy (TEM) was conducted on a JEOL JEM-2100F instrument at an acceleration voltage of 200 kV. The Brunauer-Emmett-Teller (BET) specific surface area was measured by N2 adsorption using an ASAP2020 instrument. The point of zero charge (PZC) of Fe3-xCoxO4 was measured with an acid/base titration method.34 The electrical conductivity was measured on a physical property measurement system -9 (PPMS-9) instrument. The powder samples were pressed into a round slice. Four electronic probes were used to electrify, while electrical resistance was tracked by the instrument. The electrical conductivity was the reciprocal of electrical resistance. Xray photoelectron spectroscopy (XPS) analysis was recorded on a Thermo Scientific K-Alpha instrument equipped with an Al Kα source (10 mA and 14 kV). The room temperature 57Fe Mössbauer spectra were collected using a Web Research, Inc.,

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spectrometer equipped with a Janis Model SHE-850-5 closed-cycle cryostat.

2.3. Batch reduction experiments. The reduction of 2-Cl-NB in different systems was conducted in an anaerobic glovebox (O2(g) < 0.1 ppm, 25°C). The Fe3-xCoxO4 suspension (1.0 g L-1) was buffered by 3-(N-morpholino)propanesulfonic acid (MOPS, 50 mmol L-1) at a constant pH of 7.2.22 A predetermined volume of Fe2+ and/or Co2+ solution (0.2 mol L-1, pH 1.0) was injected into the suspension, followed by shaking for 30 min to achieve adsorption equilibrium. Then, an aliquot of 2-Cl-NB stock solution (1.0 g L-1 in methanol) was introduced into the system. At given intervals, 1-2 mL of solution was withdrawn and immediately filtered through a hydrophilic PTFE syringe filter (0.22 µm, Anpel) for further analysis. Recycle experiments were carried out to test the reduction stability of Fe32+ xCoxO4/Fe aq

systems. After 360 min of reaction, the Fe2.23Co0.77O4 particles were

magnetically separated, and the suspension was removed. The particles were left in the reactor and dried overnight in an anaerobic chamber, followed by grinding and sieving through a 200-mesh screen. Then another reduction cycle was initiated by repeating the same reaction procedure as described above. Aqueous Fe2+ with the same concentration was repeatedly added. The concentrations of 2-Cl-NB, 2-chlorohydroxylamine (2-Cl-HNOH), and 2chloroaniline (2-Cl-An) were determined by HPLC on an LC-20A Shimadzu instrument (Text S2). The leaching of Fe and Co during the reaction was measured on

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an Agilent 7700X ICP-MS. The Fe2+aq concentration was tracked with the 1,10phenanthroline method. The reduction kinetics was analyzed by a three-parameter single exponential decay model (Text S3).

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3. RESULTS 3.1. 2-Cl-NB reduction in contrasting systems. In the presence of Co2+aq, Fe2+aq, or Fe3-xCoxO4 alone, 2-Cl-NB was not readily reduced or adsorbed (Figures 1). Fe3O4 coupled with Fe2+aq was effective for 2-Cl-NB reduction, where about 71% of 2-ClNB was transformed in 360 min, with the generation of several reduction intermediates (Figures 1 and S3). This suggests that the interaction between Fe3O4 and Fe2+aq was responsible for the efficient reduction. Compared to Fe3O4/Fe2+aq, the reduction activity of Fe2.77Co0.23O4/Fe2+aq system was significantly enhanced, with more than 91% of 2-Cl-NB degraded in 360 min (Figure 1). However, in the presence of Co2+aq alone, whose amount was equal to that of the structural Co(II) in the Fe2.77Co0.23O4/Fe2+aq system, little 2-Cl-NB was reduced. Even when equivalent Co2+aq was introduced to the Fe3O4/Fe2+aq system, the removal efficiency (74%) was slightly enhanced (Figure 1). The effective reduction processes were further described by a three-parameter single exponential decay model, where the kobs value decreased in the order Fe2.77Co0.23O4/Fe2+aq > Fe3O4/Fe2+aq/Co2+aq ≈ Fe3O4/Fe2+aq (Table S3). This illustrates that Co(II) substitution improved the reactivity of Fe3O4/Fe2+aq coupled system for 2-Cl-NB reduction. To verify the favorable impact of structural Co(II) on the reducing capability of the coupled system, the effect of Co substitution level and aqueous Co2+aq concentration were further investigated for comparison.

3.2. Effect of Co substitution level on 2-Cl-NB reduction by magnetite/Fe2+aq.

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After 360 min, approximately 71%, 91%, 94%, 100%, 83%, and 71% of 2-Cl-NB was degraded by Fe3O4/Fe2+aq, Fe2.77Co0.23O4/Fe2+aq, Fe2.51Co0.49O4/Fe2+aq, Fe2.23Co0.77O4/Fe2+aq, Fe2.15Co0.85O4/Fe2+aq, and Fe2.00Co1.00O4/Fe2+aq systems, respectively (Figure 2a), with the respective kobs values being 0.071, 0.095, 0.147, 0.956, 0.071, and 0.069 min-1. With the increase of Co substitution, both the efficiency and kobs of 2-Cl-NB reduction increased initially, but then decreased at higher substitution levels. The distribution of 2-Cl-NB and its reduction intermediates also reflected the variation in reducing capability of Fe3-xCoxO4/Fe2+aq systems. 2chlorohydroxylamine (2-Cl-HNOH), an intermediate product was quickly generated. Afterwards, it was too reactive to accumulate in the solution and quickly transformed to the final product 2-chloroaniline (2-Cl-An) (Figure S4). After 360 min, the production of 2-Cl-An gradually increased from 33% by Fe3O4/Fe2+aq to 86% by Fe2.23Co0.77O4/Fe2+aq, but decreased to 35% by Fe2.00Co1.00O4/Fe2+aq (Figure S4). Based on the product distribution after 360 min reaction, the electron transfer during the reaction was quantified using Eq. (1): [e-]ttransferred = 4[Ar-NHOH]t + 6[Ar-NH2]t

(1)

where Ar represents the chloro-substituted benzene ring. The quantity of transferred electrons also increased with Co substitution up to 0.77, but decreased at higher substitution levels (Figure S5). During the reduction by Fe3-xCoxO4/Fe2+aq, the added Fe2+aq was no longer detected after 30 min of reaction, revealing that Fe2+aq was completely adsorbed onto Fe3-

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xCoxO4

surface. Then no Fe species including Fe2+ and Fe3+ were detected in the

solution. Thus, during the reaction, Fe was not dissolved from magnetite, while the generated Fe(III) precipitated on magnetite surface. Moreover, only a small amount of Co leaching was observed, up to a maximum of 7.5 mg L-1 (3.0%) from the Fe2.00Co1.00O4/Fe2+aq system. This indicates that the heterogeneous mechanism dominates the reduction of 2-Cl-NB in the Fe3-xCoxO4/Fe2+aq system, where the interaction between Fe3-xCoxO4 and Fe2+aq played a vital role for the 2-Cl-NB reduction. The stability of Fe3-xCoxO4 for 2-Cl-NB reduction was evaluated by submitting the system to multiple reaction cycles (Figure S6). The removal efficiency of 2-Cl-NB by the Fe2.23Co0.77O4/Fe2+aq system decreased from 100% in the first cycle to 59% and 36% in the second and third cycles, respectively. This illustrates the deactivation of Fe3-xCoxO4/Fe2+aq during the reduction.

3.3. Effect of Co2+aq concentration on 2-Cl-NB reduction by magnetite/Fe2+aq. The effect of Co2+aq concentration on 2-Cl-NB reduction was studied in the range of 0 to 2.1 mmol L-1, where Co2+aq concentrations of 1.0 and 2.1 mmol L-1 were comparable to the amount of structural Co in 1.0 g L-1 Fe2.77Co0.23O4 and Fe2.51Co0.49O4, respectively. Compared to Fe3O4/Fe2+aq in the absence of Co2+aq, lower Co2+aq concentrations (≤ 1.0 mmol L-1) facilitated the reduction only slightly, while higher Co2+aq concentrations inhibited the reaction (Figure 2b and Table S3). The Co2+aq and

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Fe2+aq concentrations were also tracked during the reduction (Figure S7). As the Co2+aq concentration increased from 0.0 to 2.1 mmol L-1, Co2+aq could not be completely adsorbed onto the Fe3O4 surface and the amount of free Co2+aq increased (Figure S7). At Co2+aq concentrations below 1.0 mmol L-1, however, the adsorption of Fe2+aq and Co2+aq both increased and enhanced the reduction of 2-Cl-NB, which was attributed to interactions between adsorbed Fe2+ and adsorbed Co2+. As the Co2+aq concentration increased further, however, the amount of adsorbed Fe2+ decreased (Figure S7) due to the competitive adsorption by Co2+aq, and the reduction of 2-Cl-NB decreased.

3.4. Surface properties and microstructure of Fe3-xCoxO4. The physicochemical properties (e.g., the specific surface area, pHpzc, and electrical conductivity), as well as the microstructure (e.g., the valence and occupancy of Co dopant and Fe) of fresh and reacted Fe3-xCoxO4 were characterized to understand the effect of Co substitution on the reducing capability of coupled system. Across the Fe3-xCoxO4 series, only a modest change in specific surface area was observed, from about 25 to 43 m2 g-1 (Table 1). The pHpzc (where the total net surface charge is zero) was in the range of 6.8 to 7.2. The electrical conductivity of Fe3xCoxO4

increased rapidly with Co substitution up to 0.77, but then decreased at higher

Co contents (Table 1). The maximum conductivity was 3.86×106 Ω-1cm-1 of Fe2.23Co0.77O4 and then decreased to 0.46×106 Ω-1cm-1 during the three-cycle

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experiments (Figure S6). XPS Fe 2p3/2 peaks at binding energies of 709.3 and 711.0 eV were assigned to Fe(II) and Fe(III), respectively (Figure 3). Other strong signals at higher binding energies of 715.0 and 719.5 eV were attributed to the satellites of magnetite.35 The percentage of Fe(II) in fresh Fe2.23Co0.77O4 calculated from the XPS spectrum was 11.2%, and it increased slightly to 13.7% after 30 min of Fe2+aq adsorption onto the magnetite surface. After 360 min of 2-Cl-NB reduction, however, the Fe(II) percentage on the Fe2.23Co0.77O4 surface decreased dramatically to 8.5%, 7.0%, and 5.6% after the first, second, and third cycle, respectively, thus giving clear evidence of Fe(II) oxidation on the magnetite surface with 2-Cl-NB reduction. In the Co 2p3/2 region, binding energies at 780.3 and 782.5 eV corresponded to Co(II) and Co(III), respectively. As the percentage of Co(II) in fresh Fe2.23Co0.77O4 was 90.2%, Co in Fe2.23Co0.77O4 was predominantly in the divalent state. After reaction, the ratio Co(II)/Co(III) did not change obviously (Figure S8). The parameters obtained from the room-temperature 57Fe Mössbauer spectra (Figure 4, Table 2) revealed that Co(II) replaces Fe(II) in the magnetite structure. The Mössbauer spectrum of magnetite consists of two overlapping magnetically ordered sextets, corresponding to 57Fe in tetrahedral (A) and octahedral (B) sites of the spinel lattice (Figure 4a). The isomer shift (IS, which is a reflection of the oxidation state of Fe) of the sextet derived from the B sites in fresh magnetite was 0.620 mm s-1, which is about mid-way between the IS for structural Fe(III) (~0.31 mm s-1) and structural

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Fe(II) (~1.1 mm s-1). This suggests that it is a composite of Fe(II)-Fe(III) pairs, giving the appearance of a valence of +2.5 because of the fast electron hopping between Fe(II) and Fe(III).16, 22 Upon substitution of Co(II) in the spinel lattice, the IS for B sites decreased, dropping to 0.342 mm s-1 in sample Fe2.00Co1.00O4. This value is close to that of structural Fe(III), indicating the complete replacement of Fe(II)oct by Co(II). With the increase in Co(II) substitution, a remarkable decrease in the intensity of the signal of B sites was also observed (Table 2). The relative area of B sites decreased continually from 62.2% to 40.0% as Co content increased from x = 0 to x = 1.00. Co(II), therefore, preferentially substitutes for octahedral Fe(II) in magnetite.36 For the Mössbauer spectra of reacted Fe2.23Co0.77O4, two overlapping sextet hyperfine patterns were also observed, corresponding to 57Fe in A and B sites of spinel lattice (Figure 4b), revealing that no other mineral phase was formed after 2-Cl-NB reduction. The relative area of B sites decreased from 50.6% to 41.0% as Fe2.23Co0.77O4 was reused for three reductions (Table 2). Growth of the tetFe(III) sextet accompanied with reduction of octFe(IIS) sextet resulted from the formation of unpaired octFe(III) sites, which was indistinguishable from the tetFe(III) sites above the Verwey temperature.37, 38 This indicates that structural Fe(II) was oxidized to Fe(III) on the octahedral sites in magnetite.20, 21

4. DISCUSSION 4.1. 2-Cl-NB reduction in different systems. Considering the inert adsorption and

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reduction of NB by Co2+aq, Fe2+aq, or Fe3-xCoxO4 alone, the effective reduction of 2Cl-NB by Fe3O4/Fe2+aq coupled system relied on the interaction between Fe3O4 and Fe2+aq. Through Mössbauer spectra and isotopic (56Fe and 57Fe) tracing experiments, Gorski et al. found that the added aqueous Fe2+ was adsorbed on the nonstoichiometric magnetite surface, accompanied with the reduction of octahedral Fe(III) to octahedral Fe(II) in the underlying magnetite.20, 22 This increased the structural Fe(II) content and, thus, greatly improved the reducing capability of magnetite. Therefore, 2-Cl-NB was reduced by structural Fe(II) of magnetite, while the generated structural Fe(III) was reduced by adsorbed Fe2+. Based on the comparative study on the effects of Co2+aq concentration and Co substitution level on 2-Cl-NB reduction by Fe3O4/Fe2+aq, the reducing capability of Fe3-xCoxO4/Fe2+aq depended on the distribution of Fe and Co on the surface and structure of magnetite. Adsorbed Fe2+ facilitates reduction much more than adsorbed Co2+ does; so, at high concentration, the surface absorbed Co2+ not only failed to significantly enhance, but even restrained the reduction performance of the Fe3O4/Fe2+aq system. The standard potential of E0 (Fe3+/Fe2+) is 0.77 V, which is much lower than that of redox pairs Co3+/Co2+ (1.81 V).18 This explains that Fe2+ species rather than Co2+ was responsible for the reduction of 2-Cl-NB. As Co2+aq concentrations was below 1.0 mmol L-1, the slight enhanced reduction of 2-Cl-NB was ascribed to interactions between adsorbed Fe2+ and adsorbed Co2+, which promoted the electron transfer among adsorbed Fe2+ – adsorbed Co2+ – Fe3O4 – 2-Cl-

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NB. As the Co2+aq concentration increased further, however, due to the competitive adsorption by Co2+aq, the amount of adsorbed Fe2+ (electron donor) decreased gradually and, thus, reduction of 2-Cl-NB decreased. Both the kobs and electron transfer amount suggest that substituting Co(II) in the spinel structure rather than Co2+aq greatly improved the reduction performance of the magnetite/Fe2+aq system. But when the optimum substitution level, i.e., x=0.77 was surpassed, the effect of Co substitution was less favorable.

4.2. Reactivity-structure relationship. A key factor of 2-Cl-NB reduction by Fe32+ xCoxO4/Fe aq

coupled system was the interaction between Fe3-xCoxO4 and Fe2+aq

adsorbed on the Fe3-xCoxO4 surface, which recharged Fe3-xCoxO4 during the reduction of 2-Cl-NB by structural Fe(II).39, 40 This is accompanied by the oxidation of adsorbed Fe2+aq and structural Fe(II) on the surface and/or in the bulk structure of reacted Fe3xCoxO4,

as verified by a decrease in the Fe(II)/Fe(III) ratio determined by XPS and

Mössbauer analyses.22 Adsorbed Co2+aq in place of Fe2+aq failed to regenerate structural Fe(II). Substitution of Co(II) in the structure decreased structural Fe(II) content (Table S1), however, obviously improved the reduction activity of the Fe3O4/Fe2+aq system, but an overdose of Co(II) substitution retarded the reduction, as demonstrated by a decrease in the kobs values and electron transfer quantity. The Co substitution did not greatly vary the specific surface area and pHpzc, nor did these properties affect the adsorption capacity of Fe2+aq on magnetite. Aqueous Fe2+ (Fe2+aq)

19



was completely adsorbed on the Fe3-xCoxO4 surface. However, a good linear correlation was found between the electrical conductivity of Fe3-xCoxO4 and the kobs values (R2 ≥ 0.94), and between the electrical conductivity and the amount of electrons transferred (R2 ≥ 0.91) (Figure 5). This verifies that the electrical conductivity was a significant indicator of the reducing capability of Fe3-xCoxO4/Fe2+ coupled systems. According to previous study, the variation in conductivity of Fe3xCoxO4

was related not only to the occupancy and valence of Co(II) in the magnetite

structure, but also to the structural Fe(II) content.40 As the octahedral sites are exclusively exposed at the surface of the spinel structure, the reactivity of magnetite is primarily dependent on the octahedral cations.41 Fe(II) and Fe(III) on the octahedral sites can be oxidized and reduced reversibly, while keeping the structure unchanged, resulting in the benign conductivity of magnetite as an electron shuttle.18 From the Mössbauer analysis, both Co(II) and Fe(II) occupied the octahedral sites, where the thermodynamically favorable redox pairs Co(II)/Co(III) and Fe(II)/Fe(III) formed large quantities of galvanic couples and accelerated electron transfer in magnetite. This improved the conductivity and reducing capability of the magnetite/Fe2+aq system.42, 43 However, with the further increase of Co substitution, the octahedral structural Fe(II) was too low; especially in sample Fe2.00Co1.00O4, nearly all of the octahedral Fe(II) was replaced by Co(II). This resulted in the great weakening of electron transfer and, thus, the low conductivity in the structure. The stable valency of Co species also reveals that Co(II) acts as medium for the transfer of electrons from

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absorbed Fe2+ and structural Fe(II) to the contaminant. As implied in the reduction of 2-Cl-NB by Fe3O4/Fe2+aq/Co2+aq, the reducing capability of Fe3-xCoxO4/Fe2+aq depended on the distribution of Fe and Co on the surface and in the structure of the magnetite, where the surface and structural Fe2+ could not be too low. The deactivation of Fe3-xCoxO4/Fe2+aq over three redox cycles was also ascribed to the decrease in the electrical conductivity of Fe2.23Co0.77O4. The poor conductivity may be related to the oxidation of adsorbed Fe2+aq/structural Fe(II). As found in the XPS and Mössbauer analyses, the overall Fe(II)/Fe(III) ratio decreased, leading to the decrease of conductivity of Fe2.23Co0.77O4 and accordingly the reducing capability of Fe2.23Co0.77O4/Fe2+aq coupled system (Figure S8).

4.3. Mechanism of the improvement effect of Co substitution on reduction activity. In view of the results given above, an improved reaction mechanism is proposed for the increasing reduction levels observed in the presence of increasing values for x in the Co(II)-substituted spinel structure of magnetite, Fe3-xCoxO4 (Figure 6). First, reduction occurs by way of electron transfer from structural Fe(II) in the octahedral sites to 2-Cl-NB. Second, electron transfer to 2-Cl-NB is directly related to the electrical conductivity in the structure and at the surface of the octahedral sites in the magnetite, which is enhanced and accelerated due to the large number of galvanic redox couples involving Fe(II)/Fe(III) and Co(II)/Co(III). Third, the optimum level of Co(II) substitution for structural Fe(II) is x = 0.77, above which reactivity with 2-Cl-

21



NB declines due to declining Fe(II) content and below which reactivity decreases because of lower electrical conductivity. Fourth, Fe(II) is regenerated in the structure through re-reduction by surface-adsorbed Fe2+aq. Fifth, the adsorbed Fe2+aq and structural Fe(II) are oxidized irreversibly, thus removing it as a source for regeneration of structural Fe(II). And sixth, regeneration of structural Fe(II) can continue until all of the adsorbed Fe2+aq is consumed; in the present study the limit was reached after three cycles.

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5. CONCLUSIONS This study revealed that 2-Cl-NB was rapidly reduced by Fe3-xCoxO4 coupled with Fe2+aq. Co substitution in magnetite up to the optimum level (x = 0.77) significantly improved the reduction activity of coupled systems, but surpassing that level retarded the process. This phenomenon was explained as being due to the variation of electrical conductivity through Fe3-xCoxO4, which was primarily determined by the occupancy, valence, and content of Co and Fe. The study not only suggests that the Fe3-xCoxO4/Fe2+ systems could be used for the remediation of anoxic environments containing CNBs, but also is of great importance in reaching a deeper understanding of the fate of CNBs in anoxic matrices and the application of natural magnetite in environmental remediation. During the redox reaction, the oxidation of adsorbed Fe2+aq and structural Fe(II) resulted in the decrease in the reductive performance of Fe3-xCoxO4/Fe2+aq systems, which may be inadequate for practical application in a long term. Therefore, in the future, the removal of the surface passivation and the regeneration of structural Fe(II) will be further studied to improve the reductive stability of Fe3-xCoxO4/Fe2+aq coupled systems.

23



ASSOCIATE CONTENT Supporting Information HPLC analysis, the measured and theoretical contents of Co, total Fe, Fe(II), and Fe(III) of Fe3-xCoxO4, XRD and TEM of synthetic Fe3-xCoxO4, the distribution of 2-Cl-NB and its products during the reduction by Fe3-xCoxO4/Fe2+aq, kinetics and electrical analysis, cycle experiments, different species in Fe3-xCoxO4/Fe2+aq/Co2+aq, and XPS Co 2p3/2 of fresh and reacted Fe2.23Co0.77O4. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX.

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AUTHOR INFORMATION Corresponding Author. E-mail: [email protected] (X.L. LIANG); [email protected] (Y. LI)

ORCID Xiaoliang Liang: 0000-0001-6674-9354 Jianxi Zhu: 0000-0002-9002-4457

Notes The authors declare no competing financial interest.

25



ACKNOWLEDGMENTS This work was financially supported by the Key Research Program of Frontier Sciences, CAS (Grant No. QYZDJ-SSW-DQC023), the National Natural Science Foundation of China (Grant No. 41572032), the Natural Science Foundation of Guangdong Province, China (Grant No. 2016A030313778), the Youth Innovation Promotion Association CAS, the Guangdong Special Branch Plans (Grant No. 201629015), and the Science and Technology Planning Project of Guangdong Province, China (Grant No. 2017B030314175).

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Tables Table 1. The surface physico-chemistry properties for Fe3-xCoxO4 series. Samples

Specific surface area (m2 g-1)

pHpzc

Conductivity (Ω-1·cm-1)

Fe3O4

25.2

6.8

3.36×104

Fe2.77Co0.23O4

37.8

6.9

1.74×105

Fe2.51Co0.49O4

43.3

7.0

1.30×106

Fe2.23Co0.77O4

35.7

7.2

3.86×106

Fe2.15Co0.85O4

41.2

7.1

2.08×105

Fe2.00Co1.00O4

32.7

7.1

3.49×104

Across the Fe3-xCoxO4 series, no obvious variations in specific surface area and pHpzc (where the total net surface charge is zero) were observed. The electrical conductivity of Fe3-xCoxO4 increased rapidly with Co substitution up to 0.77, but then decreased at higher Co contents.

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Table 2. Mössbauer parameters for Fe3-xCoxO4 (0 ≤ x ≤ 1.0) samples before and after reaction. Samples

Site

IS (mm s-1) a

QS (mm s-1) b

Bhf (T) c

Relative area

Area ratio (B/A)

(%) Fresh Fe3O4

A

0.291

-0.006

48.7

37.8

B

0.620

-0.014

45.6

62.2

A

0.289

-0.015

49.1

40.7

B

0.598

0.018

46.2

59.3

A

0.295

-0.005

48.9

43.3

B

0.507

-0.010

46.2

56.7

A

0.310

-0.001

48.5

49.4

B

0.396

-0.036

45.4

50.6

A

0.315

-0.001

47.9

55.0

B

0.373

-0.032

44.5

45.0

A

0.316

-0.006

47.9

60.0

B

0.342

-0.030

44.4

40.0

Fe2.23Co0.77O4 after Fe2+aq

A

0.307

-0.012

48.5

47.8

adsorption

B

0.387

-0.012

45.6

52.2

Fe2.23Co0.77O4 after first

A

0.318

0.008

48.6

50.4

reduction reaction

B

0.390

-0.050

45.6

49.6

Fe2.23Co0.77O4 after second

A

0.311

-0.008

48.5

53.2

reduction reaction

B

0.405

0.005

45.2

46.8

Fe2.23Co0.77O4 after third

A

0.318

-0.024

48.6

59.0

reduction reaction

B

0.414

-0.022

44.9

41.0

Fresh Fe2.77Co0.23O4





a

Isomer shift (IS)

b

Quadrupole splitting (QS)

c

Magnetic hyperfine field (Bhf)

35


1.65

1.46

1.31

1.02

0.82

0.67

1.09

0.98

0.88

0.69


As Co content increased from x = 0 to x = 1.00, the IS value decreased gradually from 0.620 mm s-1 to 0.342 mm s-1, and the relative area of B sites decreased continually from 62.2% to 40.0%. Co(II), therefore, preferentially substitutes for octahedral Fe(II) in magnetite. The relative area of B sites decreased from 50.6% to 41.0% as Fe2.23Co0.77O4 was reused for three reductions.

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Figure captions Figure 1. Reduction of 2-Cl-NB (a) by Co2+aq, Fe2+aq, or Fe3-xCoxO4 alone, and (b) in different systems ([Fe3-xCoxO4]0 = 1.0 g L-1, [Fe2+aq]0 = 0.3 mmol L-1, [Co2+aq]0 = 1.0 mmol L-1, [2-Cl-NB]0 = 0.063 mmol L-1, pH = 7.2 ± 0.1). 2-Cl-NB was not readily reduced or adsorbed by Co2+aq, Fe2+aq, or Fe3-xCoxO4 alone, while Fe3O4 coupled with Fe2+aq was effective for 2-Cl-NB reduction. Figure 2. (a) Effect of Co substitution on the reduction of 2-Cl-NB by Fe32+ xCoxO4/Fe aq

([Fe3-xCoxO4]0 = 1.0 g L-1, [Fe2+aq]0 = 0.3 mmol L-1, [2-Cl-NB]0 = 0.063

mmol L-1, pH = 7.2 ± 0.1); (b) Effect of Co2+aq concentration on the reduction of 2-ClNB by Fe3O4/Fe2+aq/Co2+aq ([Fe3O4]0 = 1.0 g L-1, [Fe2+aq]0 = 0.3 mmol L-1, [2-Cl-NB]0 = 0.063 mmol L-1, pH = 7.2 ± 0.1). Substituting Co(II) rather than Co2+aq greatly improved the reduction performance of magnetite/Fe2+aq system. When the optimum substitution level, i.e., x=0.77 was surpassed, the effect of Co substitution was less favorable. Figure 3. Fe 2p3/2 spectra of (a) fresh Fe2.23Co0.77O4, (b) Fe2.23Co0.77O4 after Fe2+aq adsorption, (c) Fe2.23Co0.77O4 after first reduction reaction, (d) Fe2.23Co0.77O4 after second reduction reaction, and (e) Fe2.23Co0.77O4 after third reduction reaction. The Fe(II) percentage on the Fe2.23Co0.77O4 surface decreased dramatically after three cycle experiments. Figure 4. Room-temperature Mössbauer spectra of (a) fresh Fe3-xCoxO4 and (b) reacted Fe3-xCoxO4. The area of B sites decreased gradually as Co content increased

37



(a). After three cycle experiments, the area of B sites of Fe2.23Co0.77O4 decreased continually (b). Figure 5. Correlation among the electrical conductivity of Fe3-xCoxO4, reaction rate constant kobs of Fe3-xCoxO4/Fe2+aq, and amount of electrons transferred during the reaction. A good linear correlation was found between the electrical conductivity of Fe3-xCoxO4 and the kobs values (R2 ≥ 0.94), and between the electrical conductivity and the amount of electrons transferred (R2 ≥ 0.91). Figure 6. Schematic figure for the reduction of 2-Cl-NB by Fe3-xCoxO4/Fe2+aq coupled systems. 2-Cl-NB was reduced by structural Fe(II) of Fe3-xCoxO4, while the generated structural Fe(III) was reduced by adsorbed Fe2+aq, accompanied with the oxidation of Fe2+aq/bulk Fe(II). Appropriate octahedral Co substitution (x ≤ 0.77) significantly promoted the reduction activity of Fe3-xCoxO4/Fe2+aq systems, while excess Co (x > 0.77) retarded the promotion.

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Heterogeneous Reduction of 2-Chloronitrobenzene by Co-substituted

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