Preparation of nitrogen-doped cotton stalk microporous activated carbon fiber electrodes with different surface area from hexamethylenetetramine-modified cotton stalk for electrochemical degradation of methylene blue

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Preparation of nitrogen-doped cotton stalk microporous activated carbon fiber electrodes with different surface area from hexamethylenetetramine-modified cotton stalk for electrochemical degradation of methylene blue

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Kunquan Li a, Zhang Rong a, Ye Li a,⇑, Li Cheng a, Zheng Zheng b

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10 11 13 12 14 1 9 6 2 17 18 19 20 21 22 23 24 25 26 27 28

a b

College of Engineering, Nanjing Agricultural University, Nanjing 210031, China Environmental Science & Engineering Department, Fudan University, 200433 Shanghai, China

a r t i c l e

i n f o

Article history: Received 24 October 2016 Received in revised form 23 January 2017 Accepted 23 January 2017 Available online xxxx Keywords: Cotton stalk Nitrogen content Electrode Surface area Methylene blue

a b s t r a c t Cotton-stalk activated carbon fibers (CSCFs) with controllable micropore area and nitrogen content were prepared as an efficient electrode from hexamethylenetetramine-modified cotton stalk by steam/ammonia activation. The influence of microporous area, nitrogen content, voltage and initial concentration on the electrical degradation efficiency of methylene blue (MB) was evaluated by using CSCFs as anode. Results showed that the CSCF electrodes exhibited excellent MB electrochemical degradation ability including decolorization and COD removal. Increasing micropore surface area and nitrogen content of CSCF anode leaded to a corresponding increase in MB removal. The prepared CSCF-800-15-N, which has highest N content but lowest microporous area, attained the best degradation effect with 97% MB decolorization ratio for 5 mg/L MB at 12 V in 4 h, implying the doped nitrogen played a prominent role in improving the electrochemical degradation ability. The electrical degradation reaction was well described by first-order kinetics model. Overall, the aforesaid findings suggested that the nitrogendoped CSCFs were potential electrode materials, and their electrical degradation abilities could be effectively enhanced by controlling the nitrogen content and micropore surface area. Ó 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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46 47

Introduction

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Dye-synthesizing wastewater and textile wastewater are two types of difficultly degraded effluent contents of organic matter, suspended masses, and dissolved salts [1], which are not effectively treated by using traditional methods. So it is necessary to develop new technology or material to deal with these wastewaters. Electrocatalytic degradation, an environment-friendly advanced oxidation technology, has been found to be very effective for the disposal of various organic wastewaters [2–6]. One of the most key factors influencing the electrochemical efficiency is usually the performance of anode materials [7]. However, electrochemical degradation of organic wastewaters using conventional electrode materials such as metal, metal oxide, and nonmetal compound is often relatively complex and expensive for more energy expenditure, especially in dilute wastewater treatment processes [8]. Therefore, some materials have been proposed

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⇑ Corresponding author.

in recent years [9–11]. Porous carbon electrode, one of these materials, can effectively decrease the energy requirement of electrical degradation, increase the reaction area of electrode, and regulate electric current density for its high surface area. Hence it has attracted increasing interest and been tested as electrode for electrochemical oxidation of organic pollutes [9,10,12]. As one new carbon nanomaterial, activated carbon fiber has been regarded as an excellent porous carbon electrode material in a view of its advantages of ultrafine 3D network, high porosity, controllable surface chemistry, high electrical conductivity, and relatively highly accessible surface area for more active site formation [13]. Moreover, it has been widely applied to remove organic pollutants because of its greatly high adsorption capacity, unusual chemical stability, and easy preparation. Based on the above consideration, researchers had carried out some studies to explore the performance of activated carbon fiber electrode, and confirmed that activated carbon fiber was an effective electrode for electrochemical degradation of dying wastewater [1,3,5,14–16]. Also, nitrogen-containing basic groups on the surface of carbon materials were proved to be favorable for catalytic oxidation for its

E-mail address: [email protected] (Y. Li). http://dx.doi.org/10.1016/j.rinp.2017.01.030 2211-3797/Ó 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article in press as: Li K et al. Preparation of nitrogen-doped cotton stalk microporous activated carbon fiber electrodes with different surface area from hexamethylenetetramine-modified cotton stalk for electrochemical degradation of methylene blue. Results Phys (2017), http://dx.doi.org/ 10.1016/j.rinp.2017.01.030

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K. Li et al. / Results in Physics xxx (2017) xxx–xxx

increasing the electron mobility [17–19]. So it is very important to illuminate the effects of physicochemical properties on the electrochemical oxidation for optimizing the design of activated carbon fiber electrode. To our knowledge, however, the researches, especially the effects of nitrogen content and micropore area of activated carbon fibers on electrochemical catalytic activity for dying wastewater have not yet been reported. On the other hand, the traditional preparation raw materials of activated carbon fiber mainly consist of asphalt, phenolic, and styrene/olefin copolymer, which are non-renewable and high cost compared with biomass resources such as dedicated energy crops, residues from agriculture and forestry, and both wet and dry waste materials [20]. It is known that most biomass resources, which are often rich in cellulose, hemicellulose and lignin, are suitable raw materials for porous activated carbon fiber. In 2015, the cotton output of China reached 5,605,000 t. Most of the cotton stalks were burned as fuel in rural areas, and the rest became unserviceable refuses [21]. Therefore, it can decrease not only circumstance pollution but the preparation cost of activated carbon fiber electrode by application of cotton stalk instead of traditional raw materials to prepare activated carbon fiber. In this study, a series of nitrogen-doped cotton-stalk-based activated carbon fibers (CSCFs) with different micropore area were synthesized from hexamethylenetetramine-modified cotton stalk as a cost-effective electrode by liquidation, spinning, and steam/ ammonia activation for electrical degradation of refractory organic dye wastewater. Methylene blue (MB), widely used in the dyeing of cotton, hair colorants, color photographic paper, and other industries, was selected as the typical target. The electrical degradation efficiency of the prepared CSCFs was investigated by using the prepared CSCFs as anode. And the effects of microporous structure and nitrogen content of CSCFs on the electrolytic efficiency of MB and the possible mechanisms were discussed.

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Materials and methods

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Production of CSCFs

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Preparation of cotton-stalk fiber fabric Chinese cotton stalk was first ground and screened to particle sizes of 60–80 meshes and then mixed with phenols containing 10 wt% H3PO4 as the active catalyst (stalk/phenol ratio, 1/5 by weight). The mixture was then liquefied at 160 °C for 2.5 h. After liquefaction, a synthesis agent (5 wt% hexamethylenetetramine) was added to the liquefied cotton-stalk solution. The mixture was then heated to 170 °C at 5 °C/min and maintained for 10 min to prepare the spinning solution. The as-spun fiber fabrics were prepared by fusion spinning at 120 °C with a laboratory spinning apparatus. When the fusion spinning was completed, the as-spun fiber fabrics were cured by soaking in an acid solution with HCHO and HCl (1:1 by volume) as the main components at 85 °C for 2 h. The fabric precursors were then washed with deionized water and finally dried at 90 °C for 4 h.

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nace was then heated to different target temperatures, and the gas flow was switched to water vapor or ammonia water (0.4 mL min
1 g
1). Different reaction times were then applied for the production of CSCF. The CSCFs obtained at different activation temperatures and times were labeled as CSCF-TemperatureTime-Gas. For example, the CSCF activated with water vapor at 800 °C for 15 min was designated as CSCF-800-15-W. The resultant CSCFs were then cooled in a stream of gaseous nitrogen. To remove all chemicals and mineral matters, the prepared CSCFs were washed with deionized water and then dried in an oven at 105 °C. The CSCF samples were stored in a desiccator. The ammonia-activated sample CSCF-800-15-N was activated with ammonia water (liquid ammonia/H2O ratio, 1/5 by weight) from the NH4H2PO4 impregnated precursors by the same carbonization conditions as those of CSCF-800-15-W.

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Pretreatment of CSCF electrode

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The CSCF was cut in desired dimensions (20 mm  20 mm) and weighed accurately. The CSCF was immersed three times in 200 mg/L MB solution to saturate the adsorption before electrical degradation. CSCF was then dipped in MB solution at the test concentration before the electrical degradation experiment was conducted.

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Electrical degradation experiments

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The experimental setup employed in the study is shown in Fig. 1. The experiments were conducted in an open, undivided glass vessel. In the collection tank, 200 mL MB solution was used as the reactant. The flasks were then partially immersed in a water bath fitted with a hot-type magnetic heating stirrer (DF-101s). The temperature was maintained at 25 °C, and the solution was stirred with the same speed of 250 rpm. The dimension of the CSCF, which was used as anode, was 20 mm  20 mm. The same geometrical working area of the stainless steel was used as the cathode. The two electrodes immersed in the solution were installed in parallel, and the distance between the electrodes was 20 mm. The initial concentration of MB solution and voltage changed respectively. The experiments commenced when Na2SO4 was added to the solution as the supporting electrolyte. The current and amount of charge passed through the solution were measured and displayed continuously throughout the electrolysis by using a direct-current power supply (LWDQGS, PS-1505D). Samples were withdrawn from the reactor every 10 min. The concentrations of MB solution were then calculated by measuring the absorbance of the solution at a wavelength of 665 nm with spectrophotometer. The COD removal rate was determined by using CODcr. The MB decoloration ratio and COD removal rate were calculated using the following formula:

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Anode (CSCF, 20mm*20mm) Cathode(stainless steel)

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Preparation of CSCFs The cotton-stalk fiber fabric precursor was impregnated with a 4% NH4H2PO4 solution with a mass ratio of 1:60 and stirred for 10 min. Thereafter, the mixed precursors were filtered and dried in an oven at 105 °C. The mixed precursors were then placed in a 10 cm stainless steel container positioned in the horizontal tubular furnace. Stabilization was conducted by heating to 250 °C at the rate 1 °C/min under a constant high-purity nitrogen flow of 80 cm3/min and temperature maintained for 60 min. Carbonization was conducted by raising the temperature to 600 °C at a rate of 1 °C/min and maintaining the temperature for 30 min. The fur-

Reactant

+

Water bath

-

Rotor

DC supply Magnetic heating stirrer Fig. 1. CSCF anode electrolytic experiment setup.

Please cite this article in press as: Li K et al. Preparation of nitrogen-doped cotton stalk microporous activated carbon fiber electrodes with different surface area from hexamethylenetetramine-modified cotton stalk for electrochemical degradation of methylene blue. Results Phys (2017), http://dx.doi.org/ 10.1016/j.rinp.2017.01.030

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d ¼ ðC 0
C t Þ=C 0

ð1Þ

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where C0 is the initial concentration and Ct is the concentration at time t.

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Characterization

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Nitrogen adsorption/desorption isotherms were collected at 77 K by using ASAP-2020 adsorption analyzer (Micromeritics, USA). Prior to the measurement, samples were degassed at 300 °C for 3 h under vacuum. The Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface area (SBET) by using adsorption data in a relative pressure (P/P0) range of 0.02 to 0.2. The total pore volume was (Vtotal) estimated from the adsorbed amount at P/P0 of 0.995 by a single-point method. The micropore surface area (SMic) and the volume (VMic) were then calculated from the t-plot method. Mesopore volume (VMes) was calculated with the Barrett–Joyner–Halenda equation. The pore size distribution (PSD) was derived in accordance with the NLDFT regularization method. The elemental analysis of the CSCFs was obtained from a CHN-O-Rapid Elemental Analytical Instrument (Elementer, Germany). Scanning electron microscopic(SEM) images of surface and cross section of the activated carbon fiber CSCF-800-15-N was taken under vacuum with an accelerated voltage of 15 kV using Hitachi S4800 field emission scanning electron microscope. The X-ray photoelectron spectroscopy (XPS) spectra were obtained on an Axis ultra-spectrometer (Kratos, Manchester, UK) with a mono Al-K (1486.6 eV) X-ray source at a power of 225 W (15 kV, 15 mA).

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Results and discussion

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Preparation and characterization of CSCFs

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Effect of activation temperature and time Activation process such as activation temperature and dwell time plays a key role in controlling activation degree, yield and developing the pore structure of activated carbon. In this study, the steam activation of CSCFs was performed from 750 to 900 °C for 15 min to investigate the effect of activation temperature. As shown in Fig. 2(a), the BET surface area of CSCF increased with the activation temperature while the yield showed a reverse trend. The experimental data confirmed that 900 °C provided with a highest BET surface area (1578 m2/g) for preparing CSCF, but a lowest yield (8.3%). This phenomenon is due to the promotion effect of temperature on C–H2O reaction since the reaction is an endothermic reaction [22]. The promotion effect of C–H2O reaction becomes more effective at higher temperature, which enhances both the exterior and internal hydrogen gasification from C–H2O reaction and constantly increases pores, BET surface area, and decreases the production yield. Product yield greatly decreased from 16.1% to 8.3% when temperature increased from 750 °C to 900 °C. In order to conform to practical reality, a compromise should be made between the product yield and BET surface area of the product. Thus 850 °C was selected as the optimum activation temperature, with a comparative high BET surface area of 1400 m2 g
1 and modest yield of 12.6%. The effect of activation time on BET surface area and yield were also investigated with different dwell time from 10 to 20 min at 850 °C. As shown in Fig. 2(b), the BET surface area of prepared CSCF showed an uptrend (from 873 to 1400 m2/g) before 15 min and then decreased to 1326 m2/g at 20 min. This phenomenon should be due to following reasons. Firstly, increasing dwell time may cause more reaction between the carbon and steam, which brings more pores and increases BET surface. However, over carbonsteam reaction could also bring about the collapse of established

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3

Fig. 2. Effect of activation temperature (a) and time (b) on BET surface area and yield of activated CSCFs.

pores and thus decrease the BET surface area. On the other hand, prolonged dwell time might increase the degree of burn off and consequent decreases in CSCF production yields. The observed trends are consistent with previous studies [22–25]. Thus, the yield value decreased from 18.5% to 11.6% when pyrolysis time raised from 5 to 20 min. Above all, a pyrolysis time of 15 min was adopted to prepare CSCFs.

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Morphology and pore structure of CSCFs SEM and nitrogen adsorption/desorption isotherms of selected CSCFs were performed to illustrate the surface morphology and pore structure. As shown in Fig. 3a (CSCF-850-15-W) and Fig. 3b (CSCF-850-20-W), the morphology of CSCF fiber is smooth, and the microcrystals are clear. Fig. 3c shows the nitrogen adsorption isotherms for four selected samples prepared at different activation temperature and dwell times. It can be seen from Fig. 3c that all the isotherms of four selected samples show a steep increase at a low relative pressure (P/P0 < 0.05), indicating the generation of numerous micropores in the carbon framework [26]. The following part of the isotherm of CSCF-800-15-W at P/P0 > 0.1 is nearly linear, and the branches of adsorption and desorption are basically coincident without hysteresis loops, indicating the nitrogen adsorption of CSCF-800-15-W is of a type I isotherm by IUPAC guidelines, characteristic of a microporous solid [15]. However, the isotherms of CSCF-800-15-N, CSCF-850-15-W, and CSCF-85020-W display a hysteresis loop in the high-relative-pressure range (P/P0 > 0.4), confirming the existence of some mesopores.

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Element analysis The carbon, nitrogen and hydrogen contents obtained by elemental analysis are summarized in Table 1. The nitrogen contents of five steam-activated samples are nearly identical despite the small decreases with the rise in activation temperature and dwell time. Obviously, the ammonia-activated CSCF has higher nitrogen amount. Compared with the water-vapor-activated sample CSCF800-15-W, the nitrogen content of the ammonia-activated CSCF800-15-N prepared at the same conditions except the activator increases by 36%, indicating that ammonia activation is an effective means for introducing nitrogen to CSCF. The possible scheme for nitrogen-doping procedure on activated carbon by ammonia activation is followed as Eq. (2).

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Fig. 3. SEM and nitrogen adsorption-desorption isotherms (a) of CSCFs.

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Pore size distribution Fig. 4 shows the pore size distribution (PSD) plots of the selected prepared CSCFs (CSCF-800-15-W, CSCF-800-15-N, CSCF850-15-W and CSCF-800-20-W) by using the DFT method. Evidently, the PSD plots further confirm that the prepared CSCF samples belong to microporous carbon. Multiple peaks are displayed within 2 nm, and the aperture is centered within 2 nm. A certain amount of mesopores ranging from 2 nm to 3 nm are also notable. It can be seen that the number of mesopores increase with activation temperature and dwell time, which is consistent with the analysis of the isotherms.

XPS Fig. 5a also shows the survey XPS spectra of the two selected CSCF-800-15-W and CSCF-800-15-N. Three main peaks were observed at around 285, 400.2, and 533 eV corresponding to C1s, N 1s and O 1s [27,28], implying that carbon, oxygen, and nitrogen are the predominant elements for the CSCF samples. Apparently, the N1s peak intensity of CSCF-800-15-N is much stronger than that of CSCF-800-15-W, implying the former has more nitrogen content. As seen in Fig. 5b, the N1s of CSCF-800-15-W could be deconvoluted into three types of different N-containing species: pyridine (398.7 eV), imine/amide/amine (399.7 eV), and quaternary N (401.6 eV). Compared to CSCF-800-15-N, CSCF-800-15-N showed one more type of deconvoluted nitrogen spectrum at 400.8 eV (quaternary N incorporated in grapheme layers) (Fig. 5c) [29,30]. These above changes made it confirmation that the nitrogen introduced by ammonia activation mainly existed in the form of imine, amide, amine and quaternary N incorporated in grapheme layers.

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Effect of different CSCF electrodes on the electrical degradation of MB

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The effect of surface area and nitrogen content of CSCF on the electrical catalytic degradation efficiency of MB was investigated with the four different CSCF anodes at an initial MB concentration of 25 mg/L, initial pH of 4.5, voltage of 12 V, cathode and anode gap of 2 cm, and Na2SO4 dose of 10 g/L. The results are shown in Fig. 6a and b. The MB electrical degradation efficiency, including the MB decoloration rate and COD removal ability, varied with different microporous CSCF electrodes (Fig. 6a and b). For the three wateractivated CSCF electrodes, the MB electrical degradation efficiency follows the order CSCF-850-15-W, CSCF-850-20-W, and CSCF-80015-W. It is notable that the microporous surface area of CSCF-85015-W, CSCF-850-20-W and CSCF-800-15-W is 1400, 1326 and 1152 m2/g, respectively. The above results show that the MB electrical degradation efficiency is consistent with the order of the microporous surface area. For example, the discoloration rate of MB on the CSCF-850-15-W electrode at 120 min reached 55%, whereas that of CSCF-800-15-W was 45%. The experimental data proves that higher micro surface area can provide more active area

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Fig. 4. Pore size distribution plots of CSCFs by using NL-DFT method.

Table 1 Element analysis of the prepared CSCFs.

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CSCFs

SBET (m2/g)

SMic (m2/g)

Vtotal (cm3/g)

Vmic (cm3/g)

CSCF-800-15-N CSCF-800-15-W CSCF-850-15-W CSCF-850-20-W CSCF-850-10-W CSCF-850-5-W

1162 1152 1400 1326 1281 873

1103 1116 1334 1255 1207 858

0.594 0.540 0.671 0.650 0.621 0.352

0.540 0.506 0.603 0.568 0.591 0.346

and active sites, leading to the acceleration of hydroxyl radical generation, and thus maximizing the decay of MB. Furthermore, the MB electrical degradation efficiency of CSCF800-15-N is the highest among those of the four prepared CSCF electrodes. As seen in Table 1, the micropore area of the ammonia-activated CSCF-800-15-N (1162 m2/g) is far lower than those of CSCF-850-15-W (1400 m2/g) and CSCF-850-20-W (1326 m2/g). These findings indicate that nitrogen introduction favored the electrical degradation of MB because CSCF-800-15-N has the highest nitrogen content and lowest surface area among the four CSCFs. The above-mentioned results suggest that augmenting the number of nitrogen functional groups considerably affected the electrical catalytic degradation performance of the CSCF electrodes. As the XPS results, the introduced nitrogen functional groups show different electron-rich nitrogen species such as imine, amide, amine and quaternary N. Previous studies [31] reported that the nitrogen species doped on the carbon surface were more radical and electronegative than the carbon atom, hence the carbon atoms around these nitrogen species showed higher positive charge density, which could alter the chemisorption mode of O2 on the electrode catalyst and consequently weaken the O-O bonding. Therefore, nitrogen atoms doped on the CSCF electrodes could efficiently cause more active sites for the electrochemical reduction of O2. Additionally, N-doped CSCF also exhibited the highest MB adsorption ability, suggesting that the

Elementary Content (wt.%) C

H

N

86.34 88.07 88.32 82.67 88.15 87.51

0.45 0.97 1.03 1.21 2.15 2.17

2.98 2.19 2.13 2.11 1.86 2.10

nitrogen-doping structural groups likely acted as the active sites for both adsorption and oxidation catalysis. Combining the above results, it might be concluded that high defects concentration and distortion can be effectively improved by nitrogen introduction, leading to the enhanced MB electrical degradation activity and ability. Since the CSCF-800-15-N electrode exhibited the highest MB electrical degradation efficiency, the electrode was selected to further investigate the influence of parameters such as voltage, initial concentration, and reaction time on the decolorization ratio and COD removal rate in subsequent experiments.

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Effect of initial MB concentration on electrical degradation efficiency

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A series of concentration tests was conducted to study the effect of initial MB concentration on the electrical degradation efficiency at 12 V, pH 4.5, and cathode–anode gap 2 cm with 10 g Na2SO4 (Fig. 6c and d). As shown in Fig. 6c, the decoloration ratio decreased with increasing MB concentration. For example, as the initial MB concentration increased from 5 mg/L to 50 mg/L, the decolorization ratio decreased by 50% from 97% to 47% in 240 min. Fig. 6d shows that the COD removal rate also decreased by 42% from 78% to 36% with increasing initial MB concentration in 240 min. This might be due to the following reasons. First, the increase in initial concentration resulted in the increase in mass

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a

b

timely manner. The MB solution appeared in a certain concentration gradient along the electrode direction, referred to as concentration polarization. The rate of the target MB moving towards the electrode decreased with increasing initial MB concentration, which led to a decrease in electrical degradation efficiency with increasing initial concentration [32,33].

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Effect of voltage on MB electrical degradation efficiency

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To investigate the influence of voltage on the MB degradation process, a series of experiments was conducted at various voltage levels (6, 9, 12, and 15 V), 25 mg/L MB solution concentration, and 10 g Na2SO4 dose (Fig. 6e and f). MB decoloration ratio and COD removal rate greatly increased with increasing voltage. The electrical degradation efficiency of MB was the lowest at 6 V, in which the decolorization ratio and the COD removal rate were only 21% and 7%, respectively, after electrical catalytic degradation for 150 min. As the voltage increased from 6 V to 15 V, the COD removal rate of MB and the decolorization ratio reached 71% and 53%, which increased by approximately by 3.3 and 7.5-fold. These results indicated that the increasing voltage exerted a beneficial effect on the electrical degradation capacity. One possible explanation for these findings is the higher degradation rate at higher electrode potential, which resulted from the higher oxygen evolution at such conditions. This phenomenon increased the mass transfer of MB or enhanced the electrode surface ability against passivation. Other studies have arrived at similar conclusions [34].

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Reaction kinetics

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Kinetics investigates the non-equilibrium dynamics system of material properties with time variation [35–37]. The mathematical model of the first-order kinetic equation was established by researching the reaction rate. The first-order reaction rate equation is expressed as follows [38]:

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404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420

423 424 425 426

427

dC t n V ¼
¼ kC t dt

c

The above-mentioned differential equation can be integrated to the following first-kinetic equation at n = 1 [39]


ln ðCt =C0 Þ ¼ kt

Fig. 5. XPS survey spectrum (a) and deconvoluted-N1s spectrum (b, c) of CSCFs.

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transfer resistance, which further diminished the current and blocked the electron transfer rate [32]. Therefore, the removal efficiency decreased with increasing initial concentration. Second, concentration polarization reduced the removal efficiency. During electrical degradation, the target MB near the electrodes decayed, whereas the surrounding MB target was not replenished in a

ð2Þ

ð3Þ

where V is the reaction rate (mg/L min
1), Ct is the MB residual concentration or the remaining rate of COD at t min (mg/L), C0 is the initial concentration of MB (mg/L), t is the reaction time (min), k is the surface-area-normalized reactivity constant, and n is the reaction order. The kinetics was studied in the following conditions of the MB solution: pH 4.5, 200 mL solution volume, and 10 mg Na2SO4. With t as the X axis and –ln (Ct/C0) as the y axis, the scatter diagram was drawn and fitted. The reaction kinetics followed a pseudo-firstorder kinetics, and the parameters including rate constant k and coefficient R2 are listed in Table 2. The R2 values in different experimental conditions ranged from 0.80 to 0.99, showing that the electrical degradation of MB by CSCF electrodes was compliant with the mathematical model of the first-order kinetic equation (Table 2). Both the rate constant k values of MB decoloration and COD removal increased with voltage and decreased with MB initial concentration, which is consistent with the aforementioned results. More importantly, CSCF-800-15-N (highest nitrogen content) and CSCF-850-15-W (largest microporous area) attained higher reaction rate k values than those of the other 2 CSCF electrodes. This result further indicated that increasing micropore number and nitrogen content in the CSCF can greatly promote the electrical degradation of MB.

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Fig. 6. Effects of different CSCF electrodes on MB decoloration ratio (a) and COD removal rate (b). Effect of initial MB concentration on MB decoloration ratio (c) and COD removal rate (d). Effect of voltage on MB decoloration ratio (e) and COD removal rate (f).

Table 2 First-kinetic fitting parameters for MB degradation at different operating conditions. Parameters

MB decoloration

COD removal

Rate constants (k,min)

Coefficient (R2)

Different CSCFs CSCF-800-15-N CSCF-850-15-W CSCF-850-20-W CSCF-800-15-W

0.00665 0.00617 0.00506 0.00494

0.994 0.996 0.994 0.996

MB concentration 5 mg/L 25 mg/L 50 mg/L

0.01491 0.00552 0.00275

0.993 0.974 0.984

0.00635 0.00478 0.00183

0.989 0.999 0.925

Voltage 6 9 12 15

0.00142 0.00310 0.00661 0.00836

0.844 0.971 0.996 0.990

0.00495 0.00100 0.00050 0.00471

0.965 0.802 0.820 0.996

Rate constants (k,min) 0.00471 0.00374 0.00280 0.00248

Coefficient (R2) 0.996 0.961 0.877 0.838

Please cite this article in press as: Li K et al. Preparation of nitrogen-doped cotton stalk microporous activated carbon fiber electrodes with different surface area from hexamethylenetetramine-modified cotton stalk for electrochemical degradation of methylene blue. Results Phys (2017), http://dx.doi.org/ 10.1016/j.rinp.2017.01.030

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Fig. 7. First-order kinetic fitting for MB decoloration (a) and COD removal (b) at an initial MB concentration of 5 mg/L, initial pH of 4.5, voltage of 12 V, cathode–anode gap of 2 cm, and Na2SO4 dose of 10 g/L.

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Fig. 7 shows the first-order kinetic fitting for MB degradation at the initial MB concentration of 5 mg/L, initial pH of 4.5, voltage of 12 V, cathode–anode gap of 2 cm, and Na2SO4 dose of 10 g/L. The results revealed the good fit of the first-order kinetic model with the data. The corresponding correlation coefficient R2 was 0.9927 for the electrical decolorization of MB and 0.9888 for COD removal (Table 2), confirming that the electrical decolorization of MB was well compliant with the mathematical model of the first-order kinetic equation. Furthermore, the highest reaction rate constants of MB degradation, including those of MB decoloration (0.1491), COD removal (0.00635), and MB decoloration ratio (97%), also demonstrated that the nitrogen-doped cotton-stalk microporous CSCF with high surface area can be served as potential electrode material for the treatment of low-concentration dye contains.

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Conclusions

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A series of CSCFs with different nitrogen and surface area were prepared to serve as efficient electrodes by controlling activation parameters and activators. The influences of pore structure and nitrogen content of CSCF, as well as the electrolytic parameters (voltage and initial concentration), on the electrolytic efficiency of MB were evaluated. Results showed that the CSCFs achieved an excellent electrochemical processing efficiency toward MB, and the electrical catalytic degradation efficiency varied in terms of BET surface area and nitrogen content. CSCF-800-15-N exhibited

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the best degradation effect with 97% of MB decolorization ratio and 78% of COD removal rate at 12 V in 4 h. The decolorization ratio and COD removal rate both conformed well to the first-order kinetics equation. The above findings demonstrated that the prepared cotton-stalk-based CSCF was a potential electrode material that could be used for the treatment of low-concentration dye wastewater, and that the electrical degradation performance of the cottonstalk-based electrode could be effectively improved by controlling the micropore area and nitrogen content.

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Please cite this article in press as: Li K et al. Preparation of nitrogen-doped cotton stalk microporous activated carbon fiber electrodes with different surface area from hexamethylenetetramine-modified cotton stalk for electrochemical degradation of methylene blue. Results Phys (2017), http://dx.doi.org/ 10.1016/j.rinp.2017.01.030