Catalytic Reduction of Bromate Using ZIF-Derived Nanoscale Cobalt

Research Article pubs.acs.org/journal/ascecg

Catalytic Reduction of Bromate Using ZIF-Derived Nanoscale Cobalt/ Carbon Cages in the Presence of Sodium Borohydride Kun-Yi Andrew Lin* and Shen-Yi Chen Department of Environmental Engineering, National Chung Hsing University, 250 Kuo-Kuang Road, Taichung, Taiwan S Supporting Information *

ABSTRACT: Conventional catalytic hydrogenation of bromate involves continuous feeding of H2 gas, which leads to excessive consumption of H2 because of limited solubility of H2 gas in water. Herein, we propose to employ borohydride as a solid-phase H2 source which releases H2 with controllability depending on the presence of catalysts. A catalyst, possessing immobilized cobalt, porosity, and magnetic properties, was prepared via a one-step carbonization of a cobalt-based zeolitic imidazole framework. The resultant nanoscale cobalt/carbon cage (NCC) was used to activate NaBH4 to release H2. The presence of H2 might also allow the variation of oxidative states of cobalt in NCC to act as a reducing agent to reduce bromate. Thus, NCC/NaBH4 was found to completely convert bromate to bromide in water. Parameters affecting the bromate reduction were also investigated including NCC loading, borohydride dosage, temperature, pH, and competing anions. Hydrogen production of NCC/NaBH4 was also evaluated, and a mechanism for bromate reduction by NCC/NaBH4 was proposed. The multiple-cycle usability of the NCC/ NaBH4 system was also demonstrated, and the catalytic activity of NCC remained almost the same even though no regeneration treatments were applied on the used NCC. These features enable NCC as an effective and promising catalyst to activate NaBH4 for bromate reduction. KEYWORDS: ZIF-67, Cobalt, Carbon, Bromate, Reduction, Sodium borohydride

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INTRODUCTION

bromide seems to be a practical and sustainable approach to control bromate in water. The reduction of bromate typically is achieved by hydrogenation of bromate to yield bromide and water.10,11 To facilitate hydrogenation of bromate, metallic catalysts are usually employed. For instance, Chen et al. prepared a composite of palladium/alumina together with hydrogen (H2) gas to reduce bromate.12 Marco et al. also used ruthenium and palladium nanocomposites accompanied by H2 gas to reduce bromate,13 while Restivo et al. employed various metals immobilized on carbon particles with H2 gas to reduce bromate.10 In order to conveniently recover heterogeneous catalysts, Sun et al. synthesized a magnetic nanoparticlesupported palladium and used it with H2 gas to reduce bromate.14 One can find that these studies of catalytic hydrogenation all involved continuous feeding of H2 gas. Besides, the above-mentioned metal catalysts were also reduced using H2 gas prior to the catalytic reaction. The constant feeding of H2 gas during the hydrogenation, however, unavoidably wastes a large amount of H2 considering the low solubility of H2 in water.15 Therefore, a more efficient and easy-

As ozone technology has been increasingly adopted for water purification and wastewater treatment,1 occurrence of bromate (BrO3−) during ozonation is expected to pose serious threats to public health considering the carcinogenicity of bromate.2 Thus, the maximal level of bromate in drinking water has been restricted to 10 μg L−1 by the World Health Organization (WHO)3 and the United States Environmental Protection Agency (EPA).4 To eliminate bromate in water, a number of methods have been attempted. First, the bromate precursor (i.e., bromide) has to be removed from water prior to the ozonation process. Although this appears to be an optimal strategy to control bromate, it is quite challenging to completely and effectively eliminate bromate precursors during typical wastewater treatments.5 The second method is to prevent the formation of bromate during ozonation. Nevertheless, this approach has been considered to be exceedingly difficult accompanied by several drawbacks such as the slow down and decrease in ozonation efficiency as well as the occurrence of ammonia.6 The third strategy is to eliminate bromate after it forms via adsorption,7,8 filtration,9 or reduction to bromide. While adsorption and filtration can remove bromate from water, the pollutant is just transferred from the aqueous phase to solid phase. Thus, reduction of bromate to © 2015 American Chemical Society

Received: June 24, 2015 Revised: November 7, 2015 Published: November 11, 2015 3096

DOI: 10.1021/acssuschemeng.5b00570 ACS Sustainable Chem. Eng. 2015, 3, 3096−3103

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ACS Sustainable Chemistry & Engineering

Figure 1. NCC derived from ZIF-67: (a) synthesis scheme, (b) morphology of ZIF-67, and (c) morphology of NCC. The scale bar is 500 nm.

NCC). ZIF-67 was also particularly selected as a NCC precursor because ZIF-67 can be easily prepared with high yield under ambient conditions in water.21 Despite the fact that ZIF-67 itself can also facilitate the hydrolysis of NaBH4 to reduce bromate,22 ZIF-67 nanocrystals are very fine powders. These nanocrystals have to be recovered from solutions to prevent secondary pollutions; it would be challenging to directly use ZIF-67 in realistic applications. However, after carbonization, ZIF-67 can be converted to magnetic NCC, which still exhibits a carbonaceous porous structure derived from the 3-D structure of ZIF-67. Besides, it has been revealed that the hydrolysis of NaBH4 is a highly exothermic reaction, and catalysts used in the hydrolysis of NaBH4 would be exposed to a hot and caustic environment when the reaction is operated in a continuous flow at large-scale.23 This indicates that active catalytic components have to be supported on substrates. Carbon has been recognized as an appropriate support for metal catalysts in the hydrolysis of NaBH4.24 Also the dispersion of metals on the carbon support appears to be a critical factor to improve the catalytic performance in terms of hydrolysis of NaBH4.24 Since cobalt is present in ZIF-67 as the node, cobalt ions are evenly distributed within ZIF-67 frameworks. This enables ZIF-67 to be an ideal and quite promising precursor to prepare a cobalt-supported carbon nanocomposite. Thus, NCC can exhibit immobilized cobalt, porosity, and magnetic controllability, enabling it as a promising catalyst for bromate reduction using metal-activated borohydrides. NCC was obtained via the one-step carbonization of ZIF-67 and characterized by scanning electron microscopy (SEM), Xray diffraction (XRD) analysis, infrared (IR) and Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), N2 sorption/desorption isotherms, magnetization, and thermogravimetric analysis. Parameters affecting the bromate reduction using the NCC/borohydride system were investigated including NCC loading, borohydride dosage, temperature, pH, and competing inorganic anions. Hydrogen production of such a NCC/borohydride system was also revealed, and a possible mechanism for bromate reduction by NCC/borohydride was

to-handle method to introduce hydrogen should be developed. To this end, solid-form borohydrides seem to be a suitable and promising source to provide highly pure hydrogen.16 Opposed to H2 gas, solid-form borohydrides are relatively safe and easy to use.17,18 Even though H2 production from self-hydrolysis of borohydrides is quite slow, the hydrolysis reaction can be considerably expedited using catalysts. This feature in fact renders the catalytic hydrolysis of borohydrides usefully controllable because hydrogen is released only in the presence of catalysts under certain conditions. Recently, transition metals have been proven to be excellent catalysts to activate borohydrides. These transition metals, being exposed to H2, can also change their oxidative states to act as reducing agents to reduce bromate. Therefore, the combination of transition metals and borohydrides seems a promising and efficient means to reduce bromate. Nonetheless, studies of combining a metal catalyst and solid-form borohydrides are still quite limited; it is necessary to explore the capability and effectiveness of such a system more thoroughly. Since most of the transition metals used to activate borohydrides are typically noble metals (e.g., Ru and Pd),10 non-noble metals (e.g., cobalt and iron) should be preferentially adopted especially in the view that cobalt has been recognized as one of the most effective catalysts to activate borohydrides.19,20 In addition, the hydrolysis of borohydrides has to proceed in water, where the hydrogenation of bromate is also carried out. Thus, cobalt must be immobilized on substrates and equipped with controllable features (e.g., magnetic properties) to be easily recovered to prevent secondary contamination from catalysts. Considering these prerequisites, a catalyst possessing substrate-supported cobalt, porosity, and magnetic properties should be an ideal heterogeneous catalyst for bromate reduction using borohydrides. In this study, we propose to prepare a cobalt/carbon nanocomposite derived from a cobalt-based zeolitic imidazole framework (ZIF) (i.e., ZIF-67). ZIF represents a unique and intriguing class of metal organic frameworks (MOFs). By onestep carbonization, ZIF-67, consisting of a hierarchical structure and cobalt-imidazole coordination, can be converted to a nanoscale cobalt/carbon cage-like structure (abbreviated as 3097


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changed from 250 to 1000 mg L−1 to evaluate the effect of reductant dosage. To study the effect of temperature, batch-type experiments were performed at 25, 50, and 75 °C using a temperature controllable water circulation system. Effects of pH and Competing Anions on Bromate Reduction. Bromate reduction was also implemented at various pH to examine the effect of initial pH ranging from 3 to 11. The initial pH of bromate solutions was adjusted using 0.1 M of hydrocholoric acid and sodium hydroxide aqueous solutions. Considering other inorganic anions may be present in bromate solutions, we also investigated the effect of other anions on removal capacity by preparing a solution containing equal-molar concentrations of bromate, sulfate, phosphate, and nitrate. Recyclability of NCC for Bromate Reduction. Recyclability appears to be one of the most important aspects of a heterogeneous catalyst; thus, the recyclability of NCC was demonstrated and evaluated. After bromate reduction, 0.25 g of the used NCC was collected from bromate solutions using a permanent neodymium magnet. The recovered NCC was added to 0.5 L of DI water containing 100 mg L−1 of bromate as well as 0.25 g of NaBH4 to start another batch-type experiment of the bromate reduction at 25 °C. The kinetics of the bromate reduction using the recovered NCC was measured, and the corresponding removal capacity was obtained for four cycles without any regeneration treatments on NCC.

proposed. Multiple cyclicity of reusability of NCC without regeneration treatments was also tested.

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EXPERIMENTAL SECTION

Materials. Chemicals involved in this study were purchased from fine chemicals suppliers and employed without additional purification: cobalt nitrate hexahydrate (Co(NO3)2·6H2O) (Choneye Pure Chemicals, Taiwan), 2-methylimidazole (H-MIM) (Acros Organics, U.S.A.), sodium bromate (NaBrO3) (Sigma-Aldrich, U.S.A.), sodium borohydride (NaBH4) (Sigma-Aldrich, U.S.A.), sodium phosphate dibasic (Na2HPO3) (Sigma-Aldrich, U.S.A.), sodium nitrate (NaNO3) (Acros Organics, U.S.A.), and sodium sulfate (Na2SO4) (Acros Organics, U.S.A.). Deionized (DI) water was prepared to exhibit less than 18 MOhm cm. Preparation of ZIF-Derived Nanoscale Cobalt/Carbon Cages (NCC). NCC was prepared according to the scheme shown in Figure 1a. ZIF-67 was first prepared by mixing cobalt nitrate with H-MIM in water at ambient temperature based on the reported protocol.21 The as-obtained ZIF-67 nanocrystals were then placed into the tubular furnace and carbonized in nitrogen at 600 °C for 6 h to obtain black fine powder, which was washed with ethanol and DI water repeatedly and dried in a conventional oven at 65 °C to yield the final product, NCC. Characterization of NCC. Morphology of NCC was first characterized using a field emission SEM (JEOL JSM-6700, Japan). X-ray diffraction (XRD) patterns of NCC and its precursor were obtained by an X-ray diffractometer (PANalytical, The Netherlands). Raman spectroscopic analysis was performed in a Raman spectrometer (Tokyo Instruments, Inc. Nanofinder, Japan). IR spectrum of NCC was measured using a Fourier-Transform IR spectrometer (Jasco 4100, Japan) with KBr sample holders. X-ray photoelectron spectroscopic (XPS) analysis was obtained by Versa Probe/Scanning ESCA microprobe (PHI 5000, ULVAC-PHI, Inc., Japan). N2 sorption/ desorption isotherms were measured at a relative pressure (P/P0) ranging from 0.0001−0.99 by a volumetric gas adsorption analyzer (Micromeritics ASAP 2020, U.S.A.). Pore size distribution was estimated using the micropore method for pores smaller than 1.7 nm and the BJH method for pores ranging from 1.7 to 300 nm, respectively. Saturation magnetization of NCC was measured using a Superconducting Quantum Interference Device (SQUID) Vibrating Sample Magnetometer (Quantum Design MPMS SQUID VSM, U.S.A.) at 27 °C. Thermal decomposition behaviors of ZIF-67 and NCC were determined using a thermogravimetric analyzer (Instrument Specialists, Inc., TGA i1000, U.S.A.) at a heating rate of 20 °C min−1 from 25 to 800 °C in nitrogen/air. Catalytic Reduction of Bromate Using NCC in the Presence of NaBH4. Reduction of bromate using NCC in the presence of NaBH4 was implemented via batch-type experiments. First, 0.25 g of NaBH4 powder was dissolved in 0.5 L of DI water containing an initial concentration (C0) (e.g., 100 mg L−1) of bromate. Next, a certain amount of NCC (e.g., 0.25 g) was introduced to the bromate solution. At preset times, sample aliquots were sequentially withdrawn from the batch reactor, and NCC powders were immediately separated from the solution using a permanent magnet. Concentrations of the remaining bromate and generated bromide were then determined by an ion chromatography system (Thermo Dionex Basic Integrated IC System, U.S.A.). Removal capacity for bromate and amount of generated bromide were calculated using eq 1: qt =

v |C 0 − C t | M

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RESULTS AND DISCUSSION Characterization of NCC. NCC’s precursor, ZIF-67, prior to carbonization is shown in Figure 1b. The morphology of ZIF-67, as other reported ZIF-67 nanocrystals, was found to be sodalite with distinct edges, and the size range of ZIF-67 nanocrystals was in the range of 10−100 nm. The XRD pattern of ZIF-67 can be found in Figure 2a, and it can be readily indexed to the typical pattern of ZIF-67, suggesting that ZIF-67 nanocrystals were well-developed.25 Once ZIF-67 was carbonized in nitrogen, the sodalite shape was shrunken and concaved as shown in Figure 1c. The edges of the ZIF-67 nanocrystals, however, can be still preserved, and thus, the whole sodalite

(1)

where M (g) is the amount of NCC used in the reduction experiment, and v (L) is the total volume of solution. To directly compare the removal capacity for bromate and the amount of generated bromide, Ct, denoting the concentration of bromate (or bromide) at a given reaction time t, was expressed in mmol L−1 and so was C0 (mmol L−1), the initial concentration of bromate (or bromide). To examine the effect of catalyst loading, the concentration of NCC was varied from 250 to 1000 mg L−1, while the concentration of NaBH4 was also

Figure 2. Characteristics of NCC: (a) XRD pattern of NCC precursor, ZIF-67, (b) XRD pattern of NCC, and (c) Raman spectrum of NCC. 3098


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which after the deconvolution reveals various cobalt species within NCC. The peaks at 780.2 and 781.8 eV are attributed to Co3+ and Co2+, respectively,27,28 while the peaks at 786.3 and 804.8 eV are the shakeup satellite peaks of Co2+.28 This indicates that the cobalt content in NCC consists of both Co2+ and Co3+. Furthermore, the surface area and porosity of NCC were also determined by the N2 sorption/desorption isotherms as displayed in Figure 3c, which appears to be a combination of the IUPAC type I and II sorption isotherms. The surface area was found to be 220 m2 g−1 estimated by the BET model, whereas the porosity was 0.34 cm3 g−1 calculated by the singlepoint adsorption model. Figure 3d also displays the pore size distribution of NCC, which consisted of a small fraction of micropores and a large amount of mesopores, revealing the porous structure of NCC. Moreover, the magnetic property of NCC was measured and is shown in Figure 3e. It can be seen that the saturation magnetization could reach 45 emu g−1, revealing a high magnetic controllability of NCC, which enables NCC to be easily separated from water after bromate reduction. Considering that NCC was derived from ZIF-67, the thermal decomposition behavior of ZIF-67 in N2 was also examined as shown in Figure 3f. It can be seen that ZIF-67 started losing weight from 200 °C, where the carbonization of the ligand began and continued to 500 °C.29 The weight subsequently remained almost unchanged until 800 °C. The residue from the carbonization of ZIF-67 was considered to be NCC, which accounts for 20 wt % of ZIF-67, indicating that the yield of NCC was around 20 wt % of ZIF-67. Figure 3f also reveals the thermogravimetric curve of the resultant NCC in N2, and the weight change was negligible, showing the highly stable thermal stability of NCC in N2. However, we additionally examined the thermal decomposition behavior of NCC in air in order to determine fractions of metal and carbonaceous contents. The curve exhibits a significant weight loss starting at 200 °C, possibly owing to the conversion of carbon content to CO2, and remained unchanged beyond 250 °C. The remaining weight was around 50 wt %, suggesting that the carbonaceous content in a single NCC was around 50% and the rest of it was cobalt oxide. Catalytic Bromate Reduction Using NCC in the Presence of NaBH4. The present study focuses on bromate removal by catalytic reduction using NCC in the presence of NaBH4. To distinguish bromate removal caused by adsorption to NCC from the reduction, we first measured the bromate adsorption to NCC as depicted in Figure 4a. After a 2 h adsorption, only a very slight amount of bromate was removed

nanocrystal was turned into a cage-like cobalt/carbon nanocomposite. The XRD pattern of NCC is shown in Figure 2b, which is quite different from the XRD pattern of ZIF-67, showing that ZIF-67 had been converted to another material. Nevertheless, the XRD pattern can be also indexed to cobalt oxide according to JCPDS file No. 42-1467, validating its cobalt content, which was derived from cobalt ions of ZIF-67. On the other hand, the carbonaceous content of NCC can be revealed by Raman spectroscopy as shown in Figure 2c. The D and G bands are found at 1350 and 1590 Raman shift (cm−1), representing disordered structure carbon and graphitic carbon, respectively. In addition, we also found a few noticeable bands at 192, 470, 510, 608, and 682 Raman shift (cm−1), which can be attributed to the cobalt content of NCC.26 In fact, the existence of cobalt oxide can be also validated by IR spectroscopic analysis (Figure 3a). The peaks at 570 and 661

Figure 3. Chemical and physical properties of NCC: (a) FT-IR spectrum, (b) Co 2p core-level XPS spectrum, (c) N2 sorption/ desorption isotherm, (d) pore size distribution, (e) saturation magnetization, and (f) thermogravimetric analyses of ZIF-67 and NCC.

cm−1 can be associated with the stretching vibrations of Co−O bond. The peak at 661 cm−1 has been reported for A−B−O3, where A denotes Co cations in the tetrahedral position,26 whereas B represents Co cations in an octahedral position. The peak at 570 cm−1 can be correlated to the B−O−B3 vibrations in the spinel lattice. Besides, the peak at 1585 cm−1 can be attributed to the C−N bonding originated from H-MIM. Figure 3b also shows the Co 2p core-level XPS spectrum of NCC,

Figure 4. Comparison of adsorption to NCC, NaBH4 alone, and NCC with NaBH4 for (a) reduction of bromate and (b) the subsequent generation of bromide at 25 °C. 3099


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Figure 5. Proposed mechanism for bromate reduction using NCC/NaBH4: (a) hydrogen production and (b) illustration scheme.

bromate ions might still reside on the surface of NCC and then interact with hydrogen produced from NCC/NaBH4 as illustrated by route (I) in Figure 5b. Moreover, the used NCC was also recovered and analyzed using XPS. The result (Figure S1) reveals that the fractions of cobalt(II) and cobalt(III) changed accompanied by the presence of metallic cobalt. This suggests that during the reduction of bromate by the hydrolysis of NaBH4, the valence state of cobalt was changed, and some cobalt sites were reduced to the lower valence state. These reduced cobalt sites provided electrons and converted bromate to bromide as depicted by route (III) in Figure 5b. Effects of NCC Loading and NaBH4 Dosage on Bromate Reduction. To further investigate the roles of the heterogeneous catalyst, NCC, and the reductant, NaBH4, the effects of NCC loading and NaBH4 dosages were examined by varying each factor from 250 to 1000 mg L−1. Figure 6a shows

from water, and no bromate had been converted to bromide (Figure 4b). This indicates that the adsorption of bromate to NCC, even though a slight amount was detected, was insignificant, and NCC alone could not reduce bromate to bromide. In addition, we also investigated whether NaBH4 alone could reduce bromate. Nevertheless, when NaBH4 alone was present, bromate removal was insignificant, and the removal capacity for bromate was only 0.06 mmol g−1 after 2 h, which merely accounts for 3% of total bromate present in the solution. The conversion of bromate to bromide was also found to be negligible as displayed in Figure 4b. This indicates that NaBH4 alone was ineffective because of the extremely slow selfhydrolysis of NaBH4 without catalysts. Once NCC was introduced to the bromate solution containing NaBH4, the removal capacity increased rapidly along the reaction time and approached saturation within just 60 min. At equilibrium, the removal capacity reached 1.575 mmol g−1, which means that total bromate added to the solution had been completely eliminated. Figure 4b also reveals that the bromate eliminated from water had been almost completely turned into bromide. Especially, a comparison between amounts of bromate removed and generated bromide at a given reaction time reveals that at the moment bromate was eliminated it was converted to bromide. This suggests that the mechanism for bromate removal using the NCC/NaBH4 system was owing to the catalytic reduction of bromate to bromide. As discussed in the Introduction section, NaBH4 can be hydrolyzed to generate hydrogen with the assistance of metallic catalysts. Thus, the combination of NCC and NaBH4 was expected to produce hydrogen, which can react with bromate to form bromide and water as follows (eq 2): BrO3− + 3H 2 → Br − + 3H 2O

Figure 6. Effect of NCC loading on (a) reduction capacity (mmol g−1) and (b) removal efficiency (Ct/C0) at 25 °C.

(2)

the removal capacity in mmol g−1 for bromate using different concentrations of NCC while fixing the NaBH4 concentration. With various loadings, the bromate removal all increased rapidly initially and reached saturation afterward. The bromate eliminated was also almost completely converted to bromide as shown in Figure S2. However, a lower NCC loading in fact resulted in a higher qt, whereas a higher loading led to a relatively low qt. The same trend can be observed in the bromide generation in Figure S2. This is because regardless of the NCC loading bromate was completely eliminated as shown in Figure 6b (Ct/C0 → 0), even though the loading changed from 250 to 1000 mg L−1. Thus, the removal capacity, defined as bromate removed (mmol) over NCC loading (g), was higher at a lower NCC loading and vice versa. Although bromate present in the solution can be completely eliminated by the three NCC loadings, the reduction kinetics appeared to be different with various NCC loadings. When NCC loading

Figure 5a shows hydrogen production by the NCC/NaBH4 system at ambient temperature as tested for bromate reduction in Figure 4a. It can be seen that hydrogen production continued to increase along with the reaction time and approached saturation at around 100 min. The hydrogen production rate seems to agree with the kinetics of bromate reduction as displayed in Figure 4a. This validates that the bromate removal using the NCC/NaBH4 system primarily involved the reaction in eq 2, which in fact can be achieved by two routes. First, hydrogen generated from the NCC/NaBH4 system reacts with bromate ions in the bulk phase as depicted by route (II) in Figure 5b. Second, as reported in the study conducted by Restivo et al.,10 bromate can be adsorbed to the surface of the carbon support and then react with the resultant hydrogen. Even though the amount of bromate removed by NCC via the adsorption was insignificant, a small fraction of 3100


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ACS Sustainable Chemistry & Engineering increased from 250 to 500 mg L−1, the reaction rate was improved noticeably. With NCC loading of 1000 mg L−1, the reduction rate was remarkably accelerated, indicating that a higher loading of NCC could facilitate the reaction kinetics possibly owing to more available catalytic sites provide by NCC. To quantitatively analyze the kinetic change with different NCC loadings as observed in Figure 6b, we adopted a common kinetic model, the pseudo first-order rate law, to determine the rate constant as follows (eq 3): C t = C0 exp( −k1t )

dosages using the pseudo first-order rate law are also summarized in Table S1. k1 decreased from 0.071 to 0.008 min−1 as NaBH4 changed from 500 to 250 mg L−1, whereas it increased to 0.113 min−1 as NaBH4 increased 1000 mg L−1. This suggests that availability of NaBH4 was a critical parameter to both reduction kinetics and removal capacity. A certain dosage must be also provided in order to fully eliminate bromate using NCC/NaBH4 system. Considering that variation of NCC loading/NaBH4 dosage may change hydrogen production, the hydrogen production rates of NCC/NaBH4 with different NCC loadings and NaBH4 dosages were also measured to further probe into the bromate reduction mechanisms. A higher NCC loading or a higher NaBH4 dosage indeed increased the hydrogen production rate (Table S2). As the combination of NCC = 500 mg L−1 and NaBH4 = 500 mg L−1 was used as a reference, the variation in the hydrogen production rate was quite comparable to the change of the reduction rates (i.e., k1). This indicates that the bromate reduction was primarily achieved via the hydrogenation of bromate by hydrogen generated from the catalyzed hydrolysis of NaBH4. Nevertheless, much slower and much faster bromate reduction rates were observed in the cases of NaBH4 = 50 mg L−1 and NCC = 1000 mg L−1, respectively. This suggests that the bromate reduction might involve other mechanisms such as the adsorption of bromate to NCC and the hydrogenation of bromate residing on NCC. Other factors, including diffusion rate and hydrogen retention time, could also significantly affect the bromate reduction rate. Effect of Temperature on Bromate Reduction and Activation Energy. The effect of temperature was also examined as displayed in Figure 7b. As the temperature increased from 25 to 50 °C, the curve of removal capacity appears to reach saturation much faster, revealing the positive effect of temperature. The kinetics was further accelerated when temperature was raised to 75 °C. The kinetics of bromide generation was also found to accelerate at a higher temperature as displayed in Figure S4. The corresponding pseudo first-order rate constants are also summarized in Table S1, and the k1 value increased from 0.071 to 0.181 and 0.812 min−1 when temperature changed from 25 to 50 and 75 °C, respectively, validating the positive effect of elevated temperature. To correlate rate constants and the effect of temperature, the Arrhenius equation was employed as follows (eq 4):

(3)

where k1 represents the pseudo first-order rate constant of the bromate reduction. Table S1 lists k1 values with different NCC loadings and modeling correlation coefficients, which are all higher than 0.995, indicating that the bromate reduction using NCC/NaBH4 system can be well described by the pseudo firstorder rate law. The k1 value was found to change from 0.046 to 2.025 min−1 when NCC loading increased from 250 to 1000 mg L−1, validating that the higher loading significantly enhanced the reduction rate. Furthermore, the effect of NaBH4 was also investigated as shown in Figure 7a. While NaBH4 dosage was lowered from

Figure 7. Other effects on reduction capacity: (a) NaBH4 dosage at 25 °C and (b) temperature.

500 to 250 mg L−1, the removal capacity and reduction kinetics became lower and slower, respectively. On the other hand, when NaBH4 dosage increased to 1000 from 500 mg L−1, bromate was also completely eliminated, and the reduction kinetics was improved noticeably. Nevertheless, the effect caused by the higher NaBH4 dosage was less significant than that caused by a lower dosage. The same trends can be observed in the bromide generation as shown in Figure S3. The kinetic analyses of bromate reduction with different NaBH4

ln k1 = ln k − Ea /RT

(4)

Figure 8. Effects of (a) pH and (b) competing anions on removal capacity for bromate and (c) recyclability of NCC at 25 °C. 3101


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ACS Sustainable Chemistry & Engineering where Ea denotes the activation energy (kJ mol−1) for the bromate reduction; k represents the temperature-independent factor (g mg−1 min−1); R is the universal gas constant; and T is the solution temperature in Kelvin (K). A plot of ln k1 versus 1/ T is shown in Figure S5. The data points are well fitted by the linear regression based on eq 4 with a correlation coefficient of 0.970, showing that the relationship between the reduction kinetics and temperature can properly be described by the Arrhenius equation. The Ea and k are also estimated to be 40.7 kJ mol−1 and 1282 g mg−1 min−1, respectively. Effects of pH and Competing Anions on Bromate Reduction. Considering that pH of wastewater spans a wide range, the effect of pH on the bromate reduction was also investigated and is shown in Figure 8a. One can find that the removal capacity of NCC/NaBH4 at equilibrium for bromate remained quite stable in the pH range of 3−10, revealing that NCC/NaBH4 system can be extensively applied under various conditions without significant activity loss. Nevertheless, a slight decrease in the removal capacity occurred at pH 11. This might be due to the excessive amount of hydroxyl anions accumulated on the surface of NCC at pH 11, leading to a highly negative surface charge on NCC as revealed in the zeta potential of NCC (Figure S6). As a result, the approach of bromate to NCC surface might be hindered owing to a stronger electrostatic repulsion between NCC and bromate, thereby slightly decreasing the removal capacity. Moreover, since bromate-containing solutions may also consist of other inorganic anions such as nitrate, phosphate, and sulfate, it is necessary to investigate whether these competing anions interfered with the bromate reduction using NCC/NaBH4 system. Figure 8b shows that slight amounts of phosphate and nitrate were removed after 2 h, while almost no sulfate was eliminated by the NCC/NaBH4 system. Although nitrate might be also converted to nitrite via hydrogenation, no significant amount of nitrite was observed after the 2 h reaction. This suggests that the low removal capacity for phosphate and nitrate might be mainly owing to adsorption to NCC. Nevertheless, one can find that the removal capacity for bromate in the presence of the competing anions was quite comparable to that in the absence of other anions. This suggests that these competing anions did not noticeably affect the capacity of NCC/NaBH4 system, which in fact exhibited a high selectivity toward the bromate reduction. Recyclability of NCC and a Proposed Scheme To Remove a byproduct. As a heterogeneous catalyst, it is essential to easily recover NCC from water and to reuse it multiple times. Thus, in this study, the used NCC was collected via a permanent magnet and introduced to a subsequent experiment without any regeneration treatments. Figure 8c shows the removal capacity of the used NCC over four cycles, and the kinetics and the removal capacity at equilibrium remained almost the same, indicating that the catalytic activity of NCC was highly stable even with being used for four cycles. This also enables NCC as a regeneration-free and durable heterogeneous catalyst to activate NaBH4 for bromate reduction. While NCC/NaBH4 appears to be an effective and durable process to reduce bromate, a byproduct (i.e., NaBO2) can be also produced during the hydrolysis of NaBH4. To deal with NaBO2, several approaches have been developed to convert NaBO2 back to NaBH4.30−32 Considering the context of the present study, the process of using NCC/NaBH4 to reduce bromate can be integrated with other processes to remove

NaBO2 from water and result in valuable products according to the modified Schlesinger and Brown process.30 Typically, CO2 gas can be introduced into a solution containing NaBO2 to generate NaHCO3 and B2O3 as follows (eq 5): 4NaBO2 + 4CO2 + 2H 2O → 4NaHCO3 + 2B2O3

(5)

where B2O3 precipitates and can be filtered out to remove boron. Next, the solution containing NaHCO3 can be utilized to reduce water hardness (e.g., Ca2+) based on the following eq (eq 6): Ca 2 + + 2HCO3− → CaCO3 + CO2 + H 2O

(6)

where CaCO3 precipitates and can be filtered out to reduce hardness of water. By integrating these water treatment processes, bromate and also hardness can be reduced with the generation of valuable boron oxide.

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CONCLUSION In this study, the nanoscale cobalt/carbon cage-like structure (NCC) was prepared via the one-step carbonization of a cobaltbased ZIF (i.e., ZIF-67). NCC exhibited immobilized cobalt, porosity, and magnetic properties, which allowed it to be a useful and easy-to-handle catalyst to activate NaBH4. The NCC/NaBH4 system was found to almost completely convert bromate to bromide in water. Parameters influencing bromate reduction were also examined. While NCC loading and NaBH4 dosage both affected the bromate reduction efficiency, NaBH4 dosage played a more important role as it acted as the source of hydrogen. Elevated temperature indeed accelerated the reduction kinetics, and the activation energy was also determined. NCC/NaBH4 was also found to be quite stable at a wide range of pH and exhibited a high selectivity toward bromate reduction even in the presence of other competing anions. Hydrogen generation of the NCC/NaBH4 system was also revealed to probe in the mechanism underlying this NCC/ NaBH4 system. Besides, the multiple-cycle usability of NCC/ NaBH4 was also demonstrated, and the catalytic activity of the NCC remained almost the same even though no regeneration treatments were applied on the used NCC. These abovementioned features enable NCC as an effective and promising catalyst to facilitate hydrolysis of NaBH4 for bromate reduction.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00570.

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Information as mentioned in the text. (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel: +886-4-22854709. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Ministry of Science and Technology, Taiwan (104-2221-E-005-007-MY2). 3102


Research Article


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Catalytic Reduction of Bromate Using ZIF-Derived Nanoscale Cobalt

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