Reduction of Bromate by Cobalt-Impregnated Biochar Fabricated via

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Reduction of Bromate by Cobalt-Impregnated Biochar Fabricated via Pyrolysis of Lignin using CO2 as a Reaction Medium Dongwan Cho, Gihoon Kwon, Yong Sik Ok, Eilhann E. Kwon, and Hocheol Song ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00619 • Publication Date (Web): 31 Mar 2017 Downloaded from http://pubs.acs.org on April 1, 2017

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ACS Applied Materials & Interfaces

Reduction of Bromate by Cobalt-Impregnated Biochar Fabricated via Pyrolysis of Lignin using CO2 as a Reaction Medium Dong-Wan Cho a, Gihoon Kwon a, Yong Sik Ok b, Eilhann E. Kwon a, *, Hocheol Song a, * a

Department of Environment and Energy, Sejong University, Seoul 05006, South Korea

b

School of Natural Resources and Environmental Science & Korea Biochar Research Center, Kangwon National University, Chuncheon 24341, South Korea

ABSTRACT In this study, pyrolysis of lignin impregnated with cobalt (Co) was conducted to fabricate a Cobiochar (i.e., Co/lignin biochar) for use as a catalyst for bromate (BrO3-) reduction. Carbon dioxide (CO2) was employed as a reaction medium in the pyrolysis to induce desired effects associated with CO2; 1) the enhanced thermal cracking of volatile organic compounds (VOCs) evolved from the thermal degradation of biomass, and 2) the direct reaction between CO2 and VOCs, which resulted in the enhanced generation of syngas (i.e., H2 and CO). This study placed main emphases on three parts; 1) the role of impregnated Co in pyrolysis of lignin in the presence of CO2, 2) the characterization of Co/lignin biochar, and 3) evaluation of catalytic capability of Co-lignin biochar in BrO3- reduction . The findings from the pyrolysis experiments strongly evidenced that the desired CO2 effects were strengthened due to catalytic effect of impregnated Co in lignin. For example, the enhanced generation of syngas from pyrolysis of Coimpregnated lignin in CO2 was more significant than the case without Co impregnation. Moreover,

*

Corresponding author Name: Eilhann E. Kwon Address: 209 Neungdong-Ro, Gwangjin-Gu, Seoul 05006, South Korea Tel: 82 2 3408 4166 Fax: 82 2 3408 4320 e-mail: [email protected] * Corresponding author Name: Hocheol Song Address: 209 Neungdong-Ro, Gwangjin-Gu, Seoul 05006, South Korea Tel: 82 2 3408 3232 Fax: 82 2 3408 4320 e-mail: [email protected] 1

ACS Paragon Plus Environment


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pyrolysis of Co-impregnated lignin in CO2 led to production of biochar of which surface area (599 m2 g-1) is nearly 100 times greater than the biochar produced in N2 (6.6 m2 g-1). Co/lignin biochar produced in CO2 also showed a great performance in catalyzing BrO3- reduction as compared to the biochar produced in N2.

KEYWORDS: Lignin; Pyrolysis; Engineered Biochar; Bromate Reduction; Carbon Dioxide; Waste-toEnergy

1. INTRODUCTION Thermo-chemical conversion process has been recognized as one of the promising fuel processing techniques that can utilize virtually all biomass for energy recovery. The types of biomass applicable in the process range from raw organic materials to waste materials from agricultural, industrial and municipal activities. 1,2 Gasification can be defined as transferring heating value from carbonaceous solid materials to gaseous products. For instance, gasification is a thermo-chemical process that converts organic materials into synthetic gas (i.e., syngas: mixture of H2 and CO) at > 700 ˚C, which can be further purified. However, gasification aims to maximize the production of syngas, which strategically restricts (bio)char generation. Meanwhile, pyrolysis shares the common operational goal of biomass-to-energy conversion as an intermediate step for gasification, but it involves relatively low temperature and generates three phase products (i.e., syngas, pyrolytic oil, and char), with their composition dependent upon operational parameters. In this aspect, pyrolysis offers extended venues of opportunity to produce a variety of valuable materials. Among pyrolytic products, biochar is a carbon-rich product with porous structure and biological/chemical stability,

3

and it has been utilized in various applications such as land restoration,

CO2 adsorption, and crop yield enhancer. 4,5 Recently, engineered biochar has attracted much attention for 2


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use as a soil carbon sequestrator and adsorbent in soil/water remediation. 6,7 Harvey et al. 6 evaluated the ability of engineered biochar to sequestrate carbon in soil matrix via newly developed recalcitrance index calculation. In addition, Qui et al. 7 fabricated hydrous zirconium oxide-loaded biochar and successfully removed phosphate from water. These examples of environmental applications of biochar signify the practical merits of pyrolysis as a desirable end-use of waste materials. Bromate (BrO3-) is a suspected human carcinogen, frequently produced in ozonation processes in water treatment plants. Due to its potential carcinogenicity, the maximal contaminant level of BrO3- is regulated to 10 µg L−1 in drinking water by the United States Environmental Protection Agency (US EPA) and the World Health Organization (WHO). 8 It is not only resistant to microbial degradation, but also not readily removed during water treatment processes. Several conventional methods have been investigated to address BrO3- contamination, including adsorption,

9

ion exchange

10

and filtration.

11

But, these

techniques showed a large variation in BrO3- removal efficiencies, and due to their non-destructive nature toward target contaminants, it still leaves a concern about secondary pollution by phase-transferred BrO3in the treatment medium. As an alternative approach, several researchers have recently investigated BrO3- removal by catalytic hydrogenation.

12,13

In the process, BrO3- is transformed into Br- by hydrogen via a redox reaction

mediated by metallic catalysts. Recently, effectiveness of some transition metals including Pd, Pt, Ru, Ro, Sn, Cu, Zn, Fe, and Ni incorporated in various supports has been evaluated in the catalytic reduction of BrO3-.

14,15

But, the number of researches on catalytic hydrogenation of BrO3- is indeed very limited

compared to those on other oxyanionic and organic contaminants (i.e., NO3-, ClO4-, and organic dyes), 1621

and there are still many unknowns about the effectiveness of catalysts composed of different

combinations of metals and supports. For example, Co has yet been used in BrO3- reduction although its catalytic capability has been demonstrated in the reduction of other contaminants.

22,23

It has practical

advantages over other precious metal-based catalysts for the cost and availability. For the support 3



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materials, carbon materials such as activated carbon, carbon nanotube, and carbon nanofibers have been frequently used due to their favorable structural matrix as catalyst supports.

24-26

But, up to date carbon-

based material generated from pyrolysis of bio-waste has not been reported as a support. Utilizing wastederived carbonaceous materials (i.e., biochar) instead of commercial carbons will be beneficial in terms of environmental impacts and economic feasibility. Therefore, fabricating metallic catalysts embedded on biochar could be a viable approach to develop a treatment medium for BrO3- in aqueous phase. In this study, lignin was used as an initial feedstock for pyrolysis since it is generated in massive amounts as a by-product from various industries, especially in lignocellulosic-based bioethanol

27,28

and

pulp industry. 29 The main objective was to investigate the feasibility of fabricating Co-containing biochar for use as a catalyst for BrO3- reduction, and the effect of Co pretreatment of lignin on syngas generation during pyrolysis. In brief, the effect of Co impregnation on the generation of syngas from pyrolysis of lignin was explored in two atmospheres of N2 and CO2. The biochar samples produced from pyrolysis were characterized using a series of spectroscopic instruments and a surface analyzer. Lastly, their catalytic performances in BrO3- reduction were evaluated in batch experiments under varying experimental parameters (mass percentage of Co, initial NaBH4 concentration, initial BrO3- concentration, and reusability).

2. EXPERIMENTAL SECTION 2.1. Chemical Reagents & Materials Lignin (containing 1.22% S) and cobalt chloride hexahydrate (CoCl2·6H2O, 98%) were obtained from Sigma Aldrich (St. Louis, USA) and Alfa Aesar (UK), respectively. Sodium borohydride (NaBH4, extra pure) was purchased from Daejung Chemical (South Korea). The pretreatment of lignin with CoCl2 was performed to impregnate Co onto lignin. A cobalt solution was prepared by dissolving known amount of CoCl2·6H2O in 50 mL of deionized distilled water (DDW), and then varying amount of lignin was added 4


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into the cobalt solution. The mass percentage of Co to lignin in the mixture ranged in 0.5-2.0 wt. %. The mixture was magnetically stirred at 70-80 ˚C until water was fully vaporized, followed by drying at 60 ˚C for 24 h in an oven. The dried sample is denoted as Co/lignin.

2.2. Thermo-Gravimetric Analysis (TGA) of lignin and Co/lignin The TGA tests were conducted in N2 and CO2 with a Mettler Toledo TGA/DSC Star System (Mettler, Switzerland). The reactive and protective gases were controlled using the imbedded mass flow controllers (MFCs) in the TGA unit and the total flow rate was set at 150 mL min-1. Each TGA test was performed with 10 ± 0.1 mg of sample. All TGA tests were conducted at a heating rate of 10 ˚C min−1 from 25 to 800 ˚C in triplicates.

2.3. Tubular Reactor (TR) Setup for Pyrolysis A batch type tubular reactor (TR) was used in the pyrolysis under ambient pressure. A 25.4 mm stainless Ultra Torr Vacuum Fitting (Swagelok SS-4-UT-6-400) was used to connect quartz tubing (25.4 mm outer diameter and 610 mm length, Chemglass CGQ-0900T-13, USA). The TR was housed in a tube furnace equipped with temperature program module (Wisetherm, South Korea), and the temperature was constantly monitored using a S-type thermocouple. All the gases used in the experiments were of ultrahigh purity (i.e., 99.999%) and the gas flow rates were controlled with mass flow controller (Brooks, 6850E series, USA). Gas flow rate was set at 500 mL min-1. The amount of untreated lignin loaded in the TR was 3 g. For Co/lignin samples, 3.015, 3.03 and 3.06 g was loaded for the samples prepared in Co:lignin mass ratio of 0.5, 1.0 and 2.0 wt. %, respectively, so that the amount of biomass (i.e., lignin) was equivalent to each other. Pyrolysis process consisted of two stages as shown in Figure S1; initial nonisothermal pyrolysis (200-700 ˚C) at heating rate of 10 ˚C min-1 for 50 min, followed by isothermal pyrolysis at 700 ˚C for 120 min. The effluent from the TR was sent to a micro-GC (Agilent 3000A, USA) 5



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with a valve system to identify and quantify the chemical species evolved from pyrolysis of samples. The lag time of the sample transferring from the TR unit to the injection block, located on the GC/TOF-MS, was calculated to be less than 1 s, taking into account the volume of transfer line (3 mL).

2.4. Characterization of Pyrogenic Products After second stage of pyrolysis, pyrogenic products (i.e. biochar) were obtained. The pyrogenic products of Co/lignin produced from the pyrolysis in N2 and CO2 were denoted as Co/lignin/N2 and Co/lignin/CO2, respectively. Field-emission scanning electron microscopy/energy-dispersive X-ray spectroscopy (FE-SEM/EDX, JEOLJSM7401F) was used to investigate the morphology and elemental composition of the samples. X-ray diffractometer (XRD, D8 Advance, Bruker-AXS) analysis was performed to identify mineral phase changes in the biochar occurred during pyrolysis using Cu Kα radiation and a LynxEye position sensitive detector. Brunauer-Emmett-Teller (BET) surface area, mean pore volume, total pore volume, and the Barrett-Joyner-Halenda (BJH) pore size distribution of the biochar were also measured using a surface analyzer (Belsorp-mini II, USA). The magnetic moments of the samples were analyzed using an alternating gradient magnetometer (MicroMag 2900 Series).

2.5. BrO3- Reduction Experiments Catalytic reduction of BrO3- by Co/lignin biochar in the presence of NaBH4 was conducted in batch modes with 10 mL glass bottles. For kinetics experiments, 1 mL of the freshly prepared NaBH4 solution (510 mM) was dissolved in 4 mL DDW containing BrO3- (9.7 mg L-1). Then, 2 mg biochar (0.5 g L-1) was added to the BrO3- solution. At predetermined intervals, 1 mL of supernatant solution was collected with a micro-pipette to measure BrO3- concentration. To examine the effect of varying Co mass in the biochar on the reduction of BrO3-, separate kinetics experiments were performed with Co/lignin/N2 biochar (1.0 wt. %) and Co/lignin/CO2 biochar (0.5, 1.0, 2.0 wt. %). The effect of initial BrO3- concentration was 6


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investigated in the BrO3-concentration ranges of 4.2-78.7 mg L-1 under the same biochar dose and NaBH4 concentration as the kinetics experiments. The effect of NaBH4 concentration was also investigated by varying initial NaBH4 concentration from 32.5 to 130 mM. The reusability of the biochar was evaluated by running consecutive 10 reduction cycles. The concentration of BrO3- was determined using an ion chromatograph (Dionex CX-120, USA) equipped with a Dionex As-14 analytical column. After the reduction, the reacted solution was analyzed for Co with inductively coupled plasma- Optical Emission Spectrometry (ICP-OES, Ultima 2C, Horiba-Yuvon, France) to find out if there is any leaching of Co from the biochar.

3. RESULTS AND DISCUSSION 3.1. Thermo-Gravimetric Analysis (TGA) of Co/lignin The TGA test was conducted at a heating rate of 10 ˚C min-1 from 25 to 800 ˚C to characterize the thermal degradation of Co/lignin in N2 and CO2. To chase any catalytic effects attributed by Co in lignin, the TGA tests with pure lignin were also conducted in N2 and CO2. The representative mass decay of each sample and its thermal degradation rate (i.e., differential thermogram: DTG) were illustrated in Figure 1. One interesting observation is that the overall thermal degradation behavior of Co/lignin in N2 and CO2 is substantially different from that of lignin.

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Figure 1. Mass decay and DTG curves of (a) lignin and (b) 1.0 wt. % Co/lignin in N2 and CO2 For example, the thermal degradation of ligin in N2 and CO2 is nearly identical at temperatures lower than 720 ˚C. Accordingly, onset and end temperature associated with the thermally induced depolymerization of lignin is identical. This phenomennon indicates that the influence attributed by CO2 is negiligible because any physical and chemical effects triggered by CO2 should be reflected as the different thermal degradation rate (i.e., DTG) in the TGA test. Therefore, Figure 1a clearly suggests that the influence of CO2 during the thermal degradation of lignin is limited to the gas phase reactions. This observation is in good agreement with the previous work.

31

Figure 1a also evidences the Boudouard

reaction ( → 2 at temperatures higher than 720 ˚C, which can be seen from the different DTG curves in N2 and CO2 in Figure 1a. For example, the Boudouard reaction is thermodynamically favorable at temperatures higher than 720 ˚C.

32-34

However, substantial mass decay did not occur in

Figure 1a due to the slow reaction kinetics arising from mass transfer limitation between solid phase carbon and CO2 (i.e., heterogeneous reaction). Unlike the thermal degradation of lignin in CO2, the overall thermal degradation of Co/lignin in CO2 is different from that in N2. The thermal degradation rate depicted as DTG curve in Figure 1b is significantly delayed in CO2, which is indicative of any catalytic interactions between CO2 and sample 8


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surface. In light of catalytic mechanisms, surface reactions between Co/lignin and CO2 via mass transfer may delay the thermal degradation rate. Mechanistic understanding of any catalytic effects from the TGA test is very limited since the TGA test only provides mass decay as a function of pyrolytic temperature. However, as compared to experimential findings in Figure 1a, mass decay attributed to the Boudouard reaction at temperatures higher than 720 ˚C is very substantial due to catalytic effect of Co.

3.2. Pyrolysis of Lignin and Co/Lignin The scaled-up experimental work was conducted using a batch-type TR to investigate the chemical influence of CO2 in pyrolysis of Co/lignin. For example, total gas flow rate and sample loading were set at 500 mL min-1 and 10 ± 0.1 mg, respectively. Other experimental parameters were the same as the TGA test in Figure 1. To identify any catalytic effects attributed by Co in lignin, the concentration profiles of syngas evolved from pyrolysis of pure lignin in N2 and CO2 were monitored and depicted in Figure 2.

Figure 2. Concentration profiles of (a) H2, (b) CO from the thermal degradation of lignin in N2 and CO2, (c) ratio of CO to H2 9



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As evidenced in Figure 2, the overall syngas evolution patterns in N2 follow typical pyrolysis of biomass in a way that the concentration of H2 is proportional to pyrolytic temperatures via dehydrogenation. This provides a favorable condition for carbonization, thus, the concentration of CO tends to be maintained at lower levels than that of H2. This can be attributed to the large amounts of carbon deposits in biochar. However, the syngas evolution patterns in CO2 are different from those in N2. For example, the generation of CO is substantially enhanced at temperatures higher than 550 ˚C. This observation is well consistent with our previous studies.

34-37

This can be explained by the enhanced

thermal cracking of VOCs attributed by CO2 and direct reaction between CO2 and VOCs.

30

These

genuine effects of CO2 lead to the subsequent tar reduction because condensable hydrocarbons (i.e., tar and VOCs) are consumed as substrates for syngas. In addition, H2 concentrations from the thermal degradation of lignin in N2 were higher than that in CO2. This phenomenon is also explained by the dilution effect from the enhanced generation of CO in pyrolysis of lignin in CO2. Thus, the concentration of H2 begins to decrease as the concentration of CO begins to increase, which can also be validated from the ratios of CO to H2 in Figure 2c.

10


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Figure 3. Concentration profiles of (a) H2, (b) CO from the thermal degradation of 1.0 wt. % Co/lignin in N2 and CO2, (c) ratio of CO to H2

The syngas evolution patterns from pyrolysis of Co/lignin in N2 follow the very typical pyrolytic patterns of biomass (Figure 3), which is similar to the experimental observation in Figure 2. However, the concentration profiles of syngas from the thermal degradation of Co/lignin in CO2 is very genuine. Compared to the experimental results in Figure 2, the enhanced generation of CO occurs at the relatively lower temperature regime. This experimental result can be explained in terms of catalytic effects attributed by Co in lignin, which is consistent with the previous discussion in Figure 1. This claim can be supported by Figure 3c since the ratios of CO to H2 from the thermal degradation of Co/lignin in CO2 exhibit the higher values than those of lignin in CO2 at temperature from 520 to 660 ˚C. However, this identified catalytic behavior does not occurs at temperatures higher than 660 ˚C. This suggests that the reaction kinetics of CO2 is fast enough not to be affected by Co.

3.3. Characterization of Biochar FE-SEM image of Co/lignin/CO2 biochar is shown in Figure S2. The image reveals that the biochar 11



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has aggregate morphology of needle-like particles and plate-shaped particles. EDX analysis with elemental mapping showed inclusion of C, O, Co, and S. The presence of Co (15.2 wt. %) indicates incorporation of Co into the biochar surface, which is evenly distributed over the surface. BET surface area and total pore volume of 1.0 wt. % Co/lignin/N2 were found to be 6.6 m2 g-1 and 0.0103 cm3 g-1, respectively (Figure 4a). When using CO2 as a feeding gas, the surface area of 1.0 wt. % Co/lignin/CO2 increased nearly 100 times (599 m2 g-1) as compared to that of N2 condition. In addition, the BJH pore size distribution of 1.0 wt. % Co/lignin/CO2 biochar showed the predominant contribution of the pores < 50 nm diameter to the total pore volume (Figure 4a). A few recent studies have observed a similar porosity-enhancing effect of CO2 on biochar. 32, 34, 37 The underlying mechanism of the pore enhancement has not been fully elucidated, but it is speculated that CO2 reacts with condensable hydrocarbon (i.e., tar) trapped into the pores

38

and turns them into gaseous phases, leaving open pores behind. As indirect

evidence, several researchers have observed a decrease of tar mass generated during pyrolysis in CO2 as compared to N2 condition. 34-36 N2 adsorption-desorption isotherm for 1.0 wt. % Co/lignin biochar in N2 and CO2 are presented in Figure 4b. The sharp increase in adsorbate volume in the low range of p/p0 indicates the presence of a large amount of micropores in the biochar matrix.

39

The hysteresis loop observed in the p/p0 range of

0.45-0.7 is generally associated with the mesoporous structure. In addition, the high N2 uptake attained at

high p/p0 (> 0.8) indicates macropores were also created to some extent. Therefore, these overall observations suggest that 1.0 wt. % Co/lignin/CO2 biochar possesses pores all size ranges from micro- to macropores although micropores constitute the majority of the pores. The magnetic properties of the samples are shown in Figure 4c. It exhibited hysteresis loop in both 1.0 wt. % Co/lignin/N2 and 1.0 wt. % Co/lignin/CO2 biochar samples, representing a common property of ferromagnetic material. 40

(a)

(b)

(c)

12


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Figure 4. (a) Barrett-Joyner-Halenda (BJH) pore size distribution of 1.0 wt. % Co/lignin biochar fabricated in N2 and CO2 [inset table: BET surface area (m2 g-1), total pore volume (cm3 g-1), and mean pore diameter (nm)] (b) Catalytic reduction of bromate (BrO3-) reduction in the presence of NaBH4 by different Co/lignin biochar (initial BrO3- concentration = 9.7 mg L-1, biochar dose = 0.5 g L-1, NaBH4 concentration = 130 mM) (c) Magnetic hysteresis of 1.0 wt. % Co/lignin biochar fabricated in N2 and CO2 at room temperature.

XPS analysis result of 1.0 wt.% Co/lignin/CO2 biochar is shown in Figure 5a, and it revealed the presence of C, S, O, and Co. The peak centered at 284.2 eV in C 1s spectra indicates the formation of sp2 graphitic structure (Figure 5b).

41

In S 2p spectrum (Figure 5c), two peaks of S 2p3/2 and S 2p1/2 are of

interest as they indicate spin-orbit coupling in cobalt sulfide.42 In the spectrum, S 2p3/2 appeared at 161.0 eV 43, but S 2p1/2, typically observed at ~162.4 eV, was not detected as a peak, probably because it was masked by the broad peak at 164.1 eV. In fact, the presence of cobalt sulfide was confirmed in XRD analysis as discussed below. The large peak at 164.1 eV is attributable to the sulfur species in the form of aromatic C-S-C. 43 The peak at 168.1 eV is assumed to be a satellite peak, which is similar to the reported value.

44

The Co 2p spectra exhibited two predominant peaks at 797.2 and 781.2 eV (Figure 5d), which

correspond to the Co 2p3/2 and Co 2p1/2 spin-orbit peaks of CoO, respectively. 40 In addition, two satellite peaks observed at 786.4 eV and 803.2 eV are the characteristic peaks of CoO phase.

13



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Figure 5. XPS analysis of 1.0 wt. % Co/lignin/CO2 biochar (a) full spectra (b) C 1s spectra (c) S 2p spectra (d) Co 2s spectra

Figure S3 shows XRD patterns of 1.0 wt. % Co/lignin biochar generated at 700 ˚C in N2 and CO2. Zero-valent cobalt (Co0)

40

was the major mineral phase in 1.0 wt. % Co/lignin/ N2 whereas mixture of

Co0 and cobalt pentlandite (Co9S8) (JCPDS card No. 65−6801) was found to be the major phase in 1.0 wt. % Co/lignin/ CO2. The interesting observation is the appearance of Co9S8 despite the lignin feedstock contains very low S content (approximately 1.0 wt. %). 30 In general, most carbon composites containing Co9S8 have been synthesized with precursors having high S content, N2/Ar atmosphere, and through procedures involving multiple steps. 45-49 The time-dependent phase transformation of Co minerals in the isothermal region (i.e., later phase of pyrolysis) is presented in Figure 6a. Prior to the isothermal region, the magnitude of crystallinity for Co 14


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mineral was low and no single Co mineral phase was identified. This indicates the phase transformation of Co minerals mainly occurred in the isothermal region of pyrolysis. Three major peaks corresponding to Co0 (44.3˚, 51.5˚, and 71.6˚) were exclusively observed for the 1.0 wt. % Co/lignin/CO2 biochar when the isothermal pyrolysis time was 30 min. The impregnated Co2+ ions onto lignin surface could act as a role in dissociating CO2 molecules to CO via electron transfer under CO2 environment (CoCl2 + C + CO2 = Co0 + 2CO + Cl2↑). This phenomenon may transform Co phase from ionic state to solid state, thereby forming metal Co. However, Co9S8 started to be observed from 60 min and its peak intensity gradually increased until 120 min, whereas the peak intensities of Co0 gradually decreased. The results indicate that Co0 minerals in the initial stage of isothermal pyrolysis reacted with S derived from thermal degradation of lignin in the presence of CO2 to form Co9S8. This phase transformation from Co0 to Co9S8 under CO2 atmosphere can be expressed as 9Co0 + 8S = Co9S8. To investigate the effect of Co loading on the formation of Co9S8, Co/lignin/CO2 biochar samples with different mass percentages of Co (0.5-4.0 wt. %) were also analyzed by XRD (Figure 6b). The results indicated the peak intensities of Co9S8 decreased with increasing Co content and Co0 mineral phases became more dominant in the biochar. Elucidating the Co9S8 formation in the presence of low S content and CO2 will be a future work.

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(a)

(b)

Figure 6. XRD spectra of (a) 1.0 wt. % Co/lignin biochar fabricated at final pyrolysis time (0-120 min) in the subsequent isothermal region of 700 ˚C, and (b) lignin biochar and Co/lignin biochar containing 0.54.0 wt. % cobalt

3.4. Catalytic Reduction of BrO3The Co/lignin biochar were used to catalytically reduce BrO3- in the presence of NaBH4. Borohydride (BH4−) is a strong reductant that can reduce a variety of redox-active contaminants. Direct reduction of BrO3- by BH4− is inhereently hindered due to the negative charges of both ions. Thus, the catalytic reduction is assumed to proceed via 1) adsorption of BH4− on Co surface with the simultaneous production of highly active nascent hydrogen, 51 and 2) the nascent hydrogen on the metal surface directly 16


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reduced BrO3- adsorbed on Co surface,

52

or else the nascent hydrogen reduced water molecules to

produce H2, which subsequently served as a reductant for BrO3-. 53 In preliminary experiments, both unmodified biochar in N2 and CO2 showed negligible BrO3removal capability (nearly 0 % removal) (data not shown). For 1.0 wt. % Co/lignin/N2 biochar, BrO3concentration was reduced from 9.7 to 7.2 mg L-1 within 15 min (25.9 % removal) (Figure 7). On the other hand, the catalytic performance of 1.0 wt. % Co/lignin/CO2 biochar was greatly improved. For example, 1.0 wt. % Co/lignin/CO2 biochar achieved 89.7% BrO3- removal under the same condition, which is quite comparable to other types of catalytic materials that have been utilized for BrO3- reduction, (Table S1). The high catalytic capability of Co/lignin/CO2 biochar could be attributed to the large surface area. Given that the presence of Co minerals plays the key role in the reduction of BrO3-, the Co minerals are likely to be more extensively distributed in the accessible pores of CO2 biochar. The observed pseudofirst-order rate constants (kobs) for BrO3- reduction increased in proportion to the Co loading (0.5-2.0 wt. %) on the biochar (Figure 7).

Figure 7. Effect of different Co mass percentages on the reduction of BrO3- in the presence of NaBH4 (initial BrO3- concentration = 9.7 mg L-1, biochar dose = 0.5 g L-1, NaBH4 concentration = 130 mM) 17



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The concentration of NaBH4 is an important factor governing overall redox reaction and thus optimizing NaBH4 concentration is necessary in the catalytic reactions. To investigate the effect of NaBH4 concentration on the reduction of BrO3-, a series of kinetics experiments were performed under the experimental condition that the amount of BrO3- (9.7 mg L-1) and biochar dose (1 g L-1) were kept constant while NaBH4 concentration changed from 32.5 to 130 mM. The time-dependent BrO3- reduction with varying NaBH4 concentration is presented in Figure 8a. The kobs values increased from 0.0191 to 0.132 min-1 with the increase of NaBH4 concentration. As a result, 130 mM NaBH4 was selected as an optimal concentration of NaBH4 for further experiments. The effect of initial BrO3- concentration on the reduction of BrO3- is shown in Figure 8b. The kobs values decreased slightly from 0.0455 to 0.0388 min-1 when BrO3- concentration was changed from 4.2 to 30 mg L-1, respectively. The reduction of organic compounds mediated by catalysts is considered as a surface-mediated process.

54-56

At constant biochar

loading, the adsorption of BrO3- onto the catalyst sites should be a limiting factor that ultimately induces decrease of overall reaction at high BrO3- concentrations. 57 Another possibility is related to the diffusionlimited theory that the external carbonaceous matrix of biochar could inhibit vertical penetration or diffusion of BrO3- molecules to the catalyst sites, only allowing the molecules to diffuse laterally over the surface. 55

18


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Figure 8. Effects of (a) NaBH4 concentration (32,5, 65, and 130 mM) (initial BrO3- concentration = 9.7 mg L-1, biochar dose = 0.5 g L-1) and (b) initial BrO3- concentration (4.2, 17.2, and 30.0 mg L-1) on the reduction of BrO3- in the presence of 1.0 wt. % Co/lignin/CO2 biochar (NaBH4 concentration = 65 mM, biochar dose = 0.5 g L-1)

3.5. Reusability Reusability and stability of 1.0 wt. % Co/lignin/CO2 biochar is a very important material property of heterogeneous catalysts. The BrO3- solutions were repetitively added to the reactor after completion of each reduction cycle. The result of multiple reduction experiments by Co/lignin/CO2 biochar is shown in Figure 9. The complete removal (100 %) of 9.7 mg L-1 BrO3- was rapidly achieved within 2 min in every reduction cycle repeated for ten times with 2.5 g L-1 of 1.0 wt. % Co/lignin/CO2 biochar and 260 mM NaBH4. In addition, no leaching of Co from the biochar during the reaction was found as there was no Co ion in the reacted solution detected by ICP-OES analysis. Consequently, the biochar remained stable in the whole catalytic process while maintaining its original catalytic capability.

19



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Figure 9. Reusability of 1.0 wt. % Co/lignin/CO2 biochar for the reduction of BrO3- (initial BrO3concentration = 9.7 mg L-1, biochar dose = 2.5 g L-1, NaBH4 concentration = 260 mM)

4. CONCLUSIONS In summary, lignin was impregnated with Co and subsequently pyrolyzed in order to increase the generation of syngas and to produce biochar for environmental application as a catalyst. The enhanced generation of syngas was the most apparent in pyrolysis of Co/lignin in the presence of CO2 due to the synergistic effect of CO2 and metallic Co minerals formed during pyrolysis. The loading of 1.0 wt. % Co was suitable to enhance the generation of syngas. The Co-modified biochar, generated in CO2-assisted pyrolysis was more effective than the one generated in N2 to catalytically reduce BrO3- in the presence of NaBH4. The reduction rate of BrO3- was proportionally increased with the increase of Co mass embedded in the biochar. In addition, reusability and stability of the biochar generated in CO2 were demonstrated. Ultimately, the concept of the synergetic effect with Co impregnation in CO2-assisted pyrolysis will bring 20


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a good foundation to develop new approaches to increase syngas production and fabricate highperformance catalysts for environmental application.

ASSOCIATED CONTENT Supporting Information. Comparison of removal efficiencies of bromate by various materials (Table S1) A Scheme of pyrolysis process consisting of two stages (1st pyrolysis and 2nd pyrolysis) for syngas production and fabrication of modified porous biochar (Figure S1) FE-SEM image and EDX analysis with elemetal mapping of 1.0 wt. % Co/lignin/CO2 biochar (Figure S2) XRD spectra of 1.0 wt. % Co/lignin biochar generated in N2 and CO2 (Figure S3)

AUTHOR INFORMATION Corresponding authors *E-mail addresses: [email protected], [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1D1A1B03934605).

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Reduction of Bromate by Cobalt-Impregnated Biochar Fabricated via

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