The influence of microporous area, nitrogen content, voltage and initial concentration on 33 the electrical degradation efficiency of methylene blue MB was evaluated by using CSCFs as an
Trang 17
8
9 Kunquan Lia, Zhang Ronga, Ye Lia,⇑, Li Chenga, Zheng Zhengb
10 a
College of Engineering, Nanjing Agricultural University, Nanjing 210031, China
11 b
Environmental Science & Engineering Department, Fudan University, 200433 Shanghai, China
12
1 6 a r t i c l e i n f o
17 Article history:
18 Received 24 October 2016
19 Received in revised form 23 January 2017
20 Accepted 23 January 2017
21 Available online xxxx
22 Keywords:
23 Cotton stalk
24 Nitrogen content
25 Electrode
26 Surface area
27 Methylene blue
28
2 9
a b s t r a c t
30 Cotton-stalk activated carbon fibers (CSCFs) with controllable micropore area and nitrogen content were
31 prepared as an efficient electrode from hexamethylenetetramine-modified cotton stalk by
steam/ammo-32 nia activation The influence of microporous area, nitrogen content, voltage and initial concentration on
33 the electrical degradation efficiency of methylene blue (MB) was evaluated by using CSCFs as anode
34 Results showed that the CSCF electrodes exhibited excellent MB electrochemical degradation ability
35 including decolorization and COD removal Increasing micropore surface area and nitrogen content of
36 CSCF anode leaded to a corresponding increase in MB removal The prepared CSCF-800-15-N, which
37 has highest N content but lowest microporous area, attained the best degradation effect with 97% MB
38 decolorization ratio for 5 mg/L MB at 12 V in 4 h, implying the doped nitrogen played a prominent role
39
in improving the electrochemical degradation ability The electrical degradation reaction was well
40 described by first-order kinetics model Overall, the aforesaid findings suggested that the
nitrogen-41 doped CSCFs were potential electrode materials, and their electrical degradation abilities could be
effec-42 tively enhanced by controlling the nitrogen content and micropore surface area
43
Ó 2017 The Authors Published by Elsevier B.V This is an open access article under the CC BY-NC-ND
44 license (http://creativecommons.org/licenses/by-nc-nd/4.0/)
45 46
47 Introduction
48 Dye-synthesizing wastewater and textile wastewater are two
49 types of difficultly degraded effluent contents of organic matter,
50 suspended masses, and dissolved salts [1], which are not
effec-51 tively treated by using traditional methods So it is necessary to
52 develop new technology or material to deal with these
wastewa-53 ters Electrocatalytic degradation, an environment-friendly
54 advanced oxidation technology, has been found to be very effective
55 for the disposal of various organic wastewaters[2–6]
56 One of the most key factors influencing the electrochemical
effi-57 ciency is usually the performance of anode materials[7] However,
58 electrochemical degradation of organic wastewaters using
conven-59 tional electrode materials such as metal, metal oxide, and
non-60 metal compound is often relatively complex and expensive for
61 more energy expenditure, especially in dilute wastewater
treat-62 ment processes[8] Therefore, some materials have been proposed
63
in recent years[9–11] Porous carbon electrode, one of these
mate-64
rials, can effectively decrease the energy requirement of electrical
65
degradation, increase the reaction area of electrode, and regulate
66
electric current density for its high surface area Hence it has
67
attracted increasing interest and been tested as electrode for
elec-68
trochemical oxidation of organic pollutes[9,10,12]
69
As one new carbon nanomaterial, activated carbon fiber has
70
been regarded as an excellent porous carbon electrode material
71
in a view of its advantages of ultrafine 3D network, high porosity,
72
controllable surface chemistry, high electrical conductivity, and
73
relatively highly accessible surface area for more active site
forma-74
tion[13] Moreover, it has been widely applied to remove organic
75
pollutants because of its greatly high adsorption capacity, unusual
76
chemical stability, and easy preparation Based on the above
con-77
sideration, researchers had carried out some studies to explore
78
the performance of activated carbon fiber electrode, and confirmed
79
that activated carbon fiber was an effective electrode for
electro-80
chemical degradation of dying wastewater [1,3,5,14–16] Also,
81
nitrogen-containing basic groups on the surface of carbon
materi-82
als were proved to be favorable for catalytic oxidation for its
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/ ).
⇑ Corresponding author.
E-mail address: yeli800521@sina.com (Y Li).
Contents lists available atScienceDirect
Results in Physics
j o u r n a l h o m e p a g e : w w w j o u r n a l s e l s e v i e r c o m / r e s u l t s - i n - p h y s i c s
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
Trang 283 increasing the electron mobility[17–19] So it is very important to
84 illuminate the effects of physicochemical properties on the
electro-85 chemical oxidation for optimizing the design of activated carbon
86 fiber electrode To our knowledge, however, the researches,
espe-87 cially the effects of nitrogen content and micropore area of
acti-88 vated carbon fibers on electrochemical catalytic activity for dying
89 wastewater have not yet been reported
90 On the other hand, the traditional preparation raw materials of
91 activated carbon fiber mainly consist of asphalt, phenolic, and
styr-92 ene/olefin copolymer, which are non-renewable and high cost
93 compared with biomass resources such as dedicated energy crops,
94 residues from agriculture and forestry, and both wet and dry waste
95 materials[20] It is known that most biomass resources, which are
96 often rich in cellulose, hemicellulose and lignin, are suitable raw
97 materials for porous activated carbon fiber In 2015, the cotton
out-98 put of China reached 5,605,000 t Most of the cotton stalks were
99 burned as fuel in rural areas, and the rest became unserviceable
100 refuses[21] Therefore, it can decrease not only circumstance
pol-101 lution but the preparation cost of activated carbon fiber electrode
102 by application of cotton stalk instead of traditional raw materials
103 to prepare activated carbon fiber
104 In this study, a series of nitrogen-doped cotton-stalk-based
acti-105 vated carbon fibers (CSCFs) with different micropore area were
106 synthesized from hexamethylenetetramine-modified cotton stalk
107 as a cost-effective electrode by liquidation, spinning, and steam/
108 ammonia activation for electrical degradation of refractory organic
109 dye wastewater Methylene blue (MB), widely used in the dyeing
110 of cotton, hair colorants, color photographic paper, and other
111 industries, was selected as the typical target The electrical
degra-112 dation efficiency of the prepared CSCFs was investigated by using
113 the prepared CSCFs as anode And the effects of microporous
struc-114 ture and nitrogen content of CSCFs on the electrolytic efficiency of
115 MB and the possible mechanisms were discussed
116 Materials and methods
117 Production of CSCFs
118 Preparation of cotton-stalk fiber fabric
119 Chinese cotton stalk was first ground and screened to particle
120 sizes of 60–80 meshes and then mixed with phenols containing
121 10 wt% H3PO4 as the active catalyst (stalk/phenol ratio, 1/5 by
122 weight) The mixture was then liquefied at 160°C for 2.5 h After
123 liquefaction, a synthesis agent (5 wt% hexamethylenetetramine)
124 was added to the liquefied cotton-stalk solution The mixture
125 was then heated to 170°C at 5 °C/min and maintained for 10 min
126 to prepare the spinning solution The as-spun fiber fabrics were
127 prepared by fusion spinning at 120°C with a laboratory spinning
128 apparatus When the fusion spinning was completed, the as-spun
129 fiber fabrics were cured by soaking in an acid solution with HCHO
130 and HCl (1:1 by volume) as the main components at 85°C for 2 h
131 The fabric precursors were then washed with deionized water and
132 finally dried at 90°C for 4 h
133 Preparation of CSCFs
134 The cotton-stalk fiber fabric precursor was impregnated with a
135 4% NH4H2PO4solution with a mass ratio of 1:60 and stirred for
136 10 min Thereafter, the mixed precursors were filtered and dried
137 in an oven at 105°C The mixed precursors were then placed in a
138 10 cm stainless steel container positioned in the horizontal tubular
139 furnace Stabilization was conducted by heating to 250°C at the
140 rate 1°C/min under a constant high-purity nitrogen flow of
141 80 cm3/min and temperature maintained for 60 min
Carboniza-142 tion was conducted by raising the temperature to 600°C at a rate
143 of 1°C/min and maintaining the temperature for 30 min The
fur-144
nace was then heated to different target temperatures, and the
145
gas flow was switched to water vapor or ammonia water
146
(0.4 mL min1g1) Different reaction times were then applied
147
for the production of CSCF The CSCFs obtained at different
activa-148
tion temperatures and times were labeled as
CSCF-Temperature-149
Time-Gas For example, the CSCF activated with water vapor at
150
800°C for 15 min was designated as CSCF-800-15-W The resultant
151
CSCFs were then cooled in a stream of gaseous nitrogen To remove
152
all chemicals and mineral matters, the prepared CSCFs were
153
washed with deionized water and then dried in an oven at
154
105°C The CSCF samples were stored in a desiccator The
155
ammonia-activated sample CSCF-800-15-N was activated with
156
ammonia water (liquid ammonia/H2O ratio, 1/5 by weight) from
157
the NH4H2PO4impregnated precursors by the same carbonization
158
conditions as those of CSCF-800-15-W
159
Pretreatment of CSCF electrode
160
The CSCF was cut in desired dimensions (20 mm 20 mm) and
161
weighed accurately The CSCF was immersed three times in
162
200 mg/L MB solution to saturate the adsorption before electrical
163
degradation CSCF was then dipped in MB solution at the test
con-164
centration before the electrical degradation experiment was
165
conducted
166
Electrical degradation experiments
167
The experimental setup employed in the study is shown in
168
Fig 1 The experiments were conducted in an open, undivided glass
169
vessel In the collection tank, 200 mL MB solution was used as the
170
reactant The flasks were then partially immersed in a water bath
171
fitted with a hot-type magnetic heating stirrer (DF-101s) The
tem-172
perature was maintained at 25°C, and the solution was stirred
173
with the same speed of 250 rpm The dimension of the CSCF, which
174
was used as anode, was 20 mm 20 mm The same geometrical
175
working area of the stainless steel was used as the cathode The
176
two electrodes immersed in the solution were installed in parallel,
177
and the distance between the electrodes was 20 mm The initial
178
concentration of MB solution and voltage changed respectively
179
The experiments commenced when Na2SO4was added to the
solu-180
tion as the supporting electrolyte The current and amount of
181
charge passed through the solution were measured and displayed
182
continuously throughout the electrolysis by using a direct-current
183
power supply (LWDQGS, PS-1505D) Samples were withdrawn
184
from the reactor every 10 min The concentrations of MB solution
185
were then calculated by measuring the absorbance of the solution
186
at a wavelength of 665 nm with spectrophotometer The COD
187
removal rate was determined by using CODcr The MB decoloration
188
ratio and COD removal rate were calculated using the following
189
formula:
Magnetic heating stirrer
DC supply
Cathode(stainless steel)
Reactant Water bath Anode (CSCF, 20mm*20mm)
Rotor
-Fig 1 CSCF anode electrolytic experiment setup.
2 K Li et al / Results in Physics xxx (2017) xxx–xxx
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
Trang 3192
193 where C0is the initial concentration and Ctis the concentration at
194 time t
195 Characterization
196 Nitrogen adsorption/desorption isotherms were collected at
197 77 K by using ASAP-2020 adsorption analyzer (Micromeritics,
198 USA) Prior to the measurement, samples were degassed at
199 300°C for 3 h under vacuum The Brunauer–Emmett–Teller (BET)
200 method was utilized to calculate the specific surface area (SBET)
201 by using adsorption data in a relative pressure (P/P0) range of
202 0.02 to 0.2 The total pore volume was (Vtotal) estimated from the
203 adsorbed amount at P/P0of 0.995 by a single-point method The
204 micropore surface area (SMic) and the volume (VMic) were then
cal-205 culated from the t-plot method Mesopore volume (VMes) was
cal-206 culated with the Barrett–Joyner–Halenda equation The pore size
207 distribution (PSD) was derived in accordance with the NLDFT
reg-208 ularization method The elemental analysis of the CSCFs was
209 obtained from a CHN-O-Rapid Elemental Analytical Instrument
210 (Elementer, Germany) Scanning electron microscopic(SEM)
211 images of surface and cross section of the activated carbon fiber
212 CSCF-800-15-N was taken under vacuum with an accelerated
volt-213 age of 15 kV using Hitachi S4800 field emission scanning electron
214 microscope The X-ray photoelectron spectroscopy (XPS) spectra
215 were obtained on an Axis ultra-spectrometer (Kratos, Manchester,
216 UK) with a mono Al-K (1486.6 eV) X-ray source at a power of
217 225 W (15 kV, 15 mA)
218 Results and discussion
219 Preparation and characterization of CSCFs
220 Effect of activation temperature and time
221 Activation process such as activation temperature and dwell
222 time plays a key role in controlling activation degree, yield and
223 developing the pore structure of activated carbon In this study,
224 the steam activation of CSCFs was performed from 750 to 900°C
225 for 15 min to investigate the effect of activation temperature As
226 shown inFig 2(a), the BET surface area of CSCF increased with
227 the activation temperature while the yield showed a reverse trend
228 The experimental data confirmed that 900°C provided with a
high-229 est BET surface area (1578 m2/g) for preparing CSCF, but a lowest
230 yield (8.3%) This phenomenon is due to the promotion effect of
231 temperature on C–H2O reaction since the reaction is an
endother-232 mic reaction[22] The promotion effect of C–H2O reaction becomes
233 more effective at higher temperature, which enhances both the
234 exterior and internal hydrogen gasification from C–H2O reaction
235 and constantly increases pores, BET surface area, and decreases
236 the production yield Product yield greatly decreased from 16.1%
237 to 8.3% when temperature increased from 750°C to 900 °C In
238 order to conform to practical reality, a compromise should be made
239 between the product yield and BET surface area of the product
240 Thus 850°C was selected as the optimum activation temperature,
241 with a comparative high BET surface area of 1400 m2g1and
mod-242 est yield of 12.6%
243 The effect of activation time on BET surface area and yield were
244 also investigated with different dwell time from 10 to 20 min at
245 850°C As shown inFig 2(b), the BET surface area of prepared CSCF
246 showed an uptrend (from 873 to 1400 m2/g) before 15 min and
247 then decreased to 1326 m2/g at 20 min This phenomenon should
248 be due to following reasons Firstly, increasing dwell time may
249 cause more reaction between the carbon and steam, which brings
250 more pores and increases BET surface However, over
carbon-251 steam reaction could also bring about the collapse of established
252
pores and thus decrease the BET surface area On the other hand,
253
prolonged dwell time might increase the degree of burn off and
254
consequent decreases in CSCF production yields The observed
255
trends are consistent with previous studies[22–25] Thus, the yield
256
value decreased from 18.5% to 11.6% when pyrolysis time raised
257
from 5 to 20 min Above all, a pyrolysis time of 15 min was adopted
258
to prepare CSCFs
259
Morphology and pore structure of CSCFs
260
SEM and nitrogen adsorption/desorption isotherms of selected
261
CSCFs were performed to illustrate the surface morphology and
262
pore structure As shown inFig 3a (CSCF-850-15-W) andFig 3b
263
(CSCF-850-20-W), the morphology of CSCF fiber is smooth, and
264
the microcrystals are clear.Fig 3c shows the nitrogen adsorption
265
isotherms for four selected samples prepared at different
activa-266
tion temperature and dwell times It can be seen fromFig 3c that
267
all the isotherms of four selected samples show a steep increase at
268
a low relative pressure (P/P0< 0.05), indicating the generation of
269
numerous micropores in the carbon framework[26] The following
270
part of the isotherm of CSCF-800-15-W at P/P0 > 0.1 is nearly
lin-271
ear, and the branches of adsorption and desorption are basically
272
coincident without hysteresis loops, indicating the nitrogen
273
adsorption of CSCF-800-15-W is of a type I isotherm by IUPAC
274
guidelines, characteristic of a microporous solid [15] However,
275
the isotherms of CSCF-800-15-N, 15-W, and
CSCF-850-276
20-W display a hysteresis loop in the high-relative-pressure range
277
(P/P0> 0.4), confirming the existence of some mesopores
Fig 2 Effect of activation temperature (a) and time (b) on BET surface area and yield of activated CSCFs.
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
Trang 4278 Pore size distribution
279 Fig 4 shows the pore size distribution (PSD) plots of the
280 selected prepared CSCFs (800-15-W, 800-15-N,
CSCF-281 850-15-W and CSCF-800-20-W) by using the DFT method
Evi-282 dently, the PSD plots further confirm that the prepared CSCF
sam-283 ples belong to microporous carbon Multiple peaks are displayed
284 within 2 nm, and the aperture is centered within 2 nm A certain
285 amount of mesopores ranging from 2 nm to 3 nm are also notable
286 It can be seen that the number of mesopores increase with
activa-287 tion temperature and dwell time, which is consistent with the
288 analysis of the isotherms
289
Element analysis
290
The carbon, nitrogen and hydrogen contents obtained by
ele-291
mental analysis are summarized inTable 1 The nitrogen contents
292
of five steam-activated samples are nearly identical despite the
293
small decreases with the rise in activation temperature and dwell
294
time Obviously, the ammonia-activated CSCF has higher nitrogen
295
amount Compared with the water-vapor-activated sample
CSCF-296
800-15-W, the nitrogen content of the ammonia-activated
CSCF-297
800-15-N prepared at the same conditions except the activator
298
increases by 36%, indicating that ammonia activation is an effective
299
means for introducing nitrogen to CSCF The possible scheme for
300
nitrogen-doping procedure on activated carbon by ammonia
acti-301
vation is followed as Eq (2)
303
304
XPS
305
Fig 5a also shows the survey XPS spectra of the two selected
306
CSCF-800-15-W and CSCF-800-15-N Three main peaks were
307
observed at around 285, 400.2, and 533 eV corresponding to C1s,
308
N 1s and O 1s[27,28], implying that carbon, oxygen, and nitrogen
309
are the predominant elements for the CSCF samples Apparently,
310
the N1s peak intensity of CSCF-800-15-N is much stronger than
311
that of CSCF-800-15-W, implying the former has more nitrogen
312
content As seen inFig 5b, the N1s of CSCF-800-15-W could be
313
deconvoluted into three types of different N-containing species:
314
pyridine (398.7 eV), imine/amide/amine (399.7 eV), and
quater-315
nary N (401.6 eV) Compared to CSCF-800-15-N, CSCF-800-15-N
316
showed one more type of deconvoluted nitrogen spectrum at
317
400.8 eV (quaternary N incorporated in grapheme layers) (Fig 5c)
318
[29,30] These above changes made it confirmation that the
nitro-319
gen introduced by ammonia activation mainly existed in the form
320
of imine, amide, amine and quaternary N incorporated in
gra-321
pheme layers
322
Effect of different CSCF electrodes on the electrical degradation of MB
323
The effect of surface area and nitrogen content of CSCF on the
324
electrical catalytic degradation efficiency of MB was investigated
325
with the four different CSCF anodes at an initial MB concentration
326
of 25 mg/L, initial pH of 4.5, voltage of 12 V, cathode and anode gap
327
of 2 cm, and Na2SO4 dose of 10 g/L The results are shown in
328
Fig 6a and b
329
The MB electrical degradation efficiency, including the MB
330
decoloration rate and COD removal ability, varied with different
331
microporous CSCF electrodes (Fig 6a and b) For the three
water-332
activated CSCF electrodes, the MB electrical degradation efficiency
333
follows the order CSCF-850-15-W, CSCF-850-20-W, and
CSCF-800-334
15-W It is notable that the microporous surface area of
CSCF-850-335
15-W, CSCF-850-20-W and CSCF-800-15-W is 1400, 1326 and
336
1152 m2/g, respectively The above results show that the MB
elec-337
trical degradation efficiency is consistent with the order of the
338
microporous surface area For example, the discoloration rate of
339
MB on the CSCF-850-15-W electrode at 120 min reached 55%,
340
whereas that of CSCF-800-15-W was 45% The experimental data
341
proves that higher micro surface area can provide more active area
Fig 3 SEM and nitrogen adsorption-desorption isotherms (a) of CSCFs.
4 K Li et al / Results in Physics xxx (2017) xxx–xxx
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
Trang 5342 and active sites, leading to the acceleration of hydroxyl radical
gen-343 eration, and thus maximizing the decay of MB
344 Furthermore, the MB electrical degradation efficiency of
CSCF-345 800-15-N is the highest among those of the four prepared CSCF
346 electrodes As seen in Table 1, the micropore area of the
347 ammonia-activated CSCF-800-15-N (1162 m2/g) is far lower than
348 those of CSCF-850-15-W (1400 m2/g) and CSCF-850-20-W
349 (1326 m2/g) These findings indicate that nitrogen introduction
350 favored the electrical degradation of MB because CSCF-800-15-N
351 has the highest nitrogen content and lowest surface area among
352 the four CSCFs The above-mentioned results suggest that
aug-353 menting the number of nitrogen functional groups considerably
354 affected the electrical catalytic degradation performance of the
355 CSCF electrodes As the XPS results, the introduced nitrogen
func-356 tional groups show different electron-rich nitrogen species such
357 as imine, amide, amine and quaternary N Previous studies[31]
358 reported that the nitrogen species doped on the carbon surface
359 were more radical and electronegative than the carbon atom,
360 hence the carbon atoms around these nitrogen species showed
361 higher positive charge density, which could alter the
chemisorp-362 tion mode of O2on the electrode catalyst and consequently weaken
363 the O-O bonding Therefore, nitrogen atoms doped on the CSCF
364 electrodes could efficiently cause more active sites for the
electro-365 chemical reduction of O2 Additionally, N-doped CSCF also
exhib-366 ited the highest MB adsorption ability, suggesting that the
367
nitrogen-doping structural groups likely acted as the active sites
368
for both adsorption and oxidation catalysis Combining the above
369
results, it might be concluded that high defects concentration
370
and distortion can be effectively improved by nitrogen
introduc-371
tion, leading to the enhanced MB electrical degradation activity
372
and ability
373
Since the CSCF-800-15-N electrode exhibited the highest MB
374
electrical degradation efficiency, the electrode was selected to
fur-375
ther investigate the influence of parameters such as voltage, initial
376
concentration, and reaction time on the decolorization ratio and
377
COD removal rate in subsequent experiments
378
Effect of initial MB concentration on electrical degradation efficiency
379
A series of concentration tests was conducted to study the effect
380
of initial MB concentration on the electrical degradation efficiency
381
at 12 V, pH 4.5, and cathode–anode gap 2 cm with 10 g Na2SO4
382
(Fig 6c and d) As shown in Fig 6c, the decoloration ratio
383
decreased with increasing MB concentration For example, as the
384
initial MB concentration increased from 5 mg/L to 50 mg/L, the
385
decolorization ratio decreased by 50% from 97% to 47% in
386
240 min.Fig 6d shows that the COD removal rate also decreased
387
by 42% from 78% to 36% with increasing initial MB concentration
388
in 240 min This might be due to the following reasons First, the
389
increase in initial concentration resulted in the increase in mass
Fig 4 Pore size distribution plots of CSCFs by using NL-DFT method.
Table 1
Element analysis of the prepared CSCFs.
/g) S Mic (m 2
/g) V total (cm 3
/g) V mic (cm 3
/g) Elementary Content (wt.%)
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
Trang 6390 transfer resistance, which further diminished the current and
391 blocked the electron transfer rate[32] Therefore, the removal
effi-392 ciency decreased with increasing initial concentration Second,
393 concentration polarization reduced the removal efficiency During
394 electrical degradation, the target MB near the electrodes decayed,
395 whereas the surrounding MB target was not replenished in a
396
timely manner The MB solution appeared in a certain
concentra-397
tion gradient along the electrode direction, referred to as
concen-398
tration polarization The rate of the target MB moving towards
399
the electrode decreased with increasing initial MB concentration,
400
which led to a decrease in electrical degradation efficiency with
401
increasing initial concentration[32,33]
402
Effect of voltage on MB electrical degradation efficiency
403
To investigate the influence of voltage on the MB degradation
404
process, a series of experiments was conducted at various voltage
405
levels (6, 9, 12, and 15 V), 25 mg/L MB solution concentration,
406
and 10 g Na2SO4 dose (Fig 6e and f) MB decoloration ratio and
407
COD removal rate greatly increased with increasing voltage The
408
electrical degradation efficiency of MB was the lowest at 6 V, in
409
which the decolorization ratio and the COD removal rate were only
410
21% and 7%, respectively, after electrical catalytic degradation for
411
150 min As the voltage increased from 6 V to 15 V, the COD
412
removal rate of MB and the decolorization ratio reached 71% and
413
53%, which increased by approximately by 3.3 and 7.5-fold These
414
results indicated that the increasing voltage exerted a beneficial
415
effect on the electrical degradation capacity One possible
explana-416
tion for these findings is the higher degradation rate at higher
elec-417
trode potential, which resulted from the higher oxygen evolution
418
at such conditions This phenomenon increased the mass transfer
419
of MB or enhanced the electrode surface ability against passivation
420
Other studies have arrived at similar conclusions[34]
421
Reaction kinetics
422
Kinetics investigates the non-equilibrium dynamics system of
423
material properties with time variation[35–37] The mathematical
424
model of the first-order kinetic equation was established by
425
researching the reaction rate The first-order reaction rate equation
426
is expressed as follows[38]:
427
V¼ dCt
dt ¼ kCn
430
The above-mentioned differential equation can be integrated to
431
the following first-kinetic equation at n = 1[39]
432
435
where V is the reaction rate (mg/L min1), Ctis the MB residual
con-436
centration or the remaining rate of COD at t min (mg/L), C0is the
437
initial concentration of MB (mg/L), t is the reaction time (min), k
438
is the surface-area-normalized reactivity constant, and n is the
reac-439
tion order
440
The kinetics was studied in the following conditions of the MB
441
solution: pH 4.5, 200 mL solution volume, and 10 mg Na2SO4 With
442
t as the X axis and –ln (Ct/C0) as the y axis, the scatter diagram was
443
drawn and fitted The reaction kinetics followed a
pseudo-first-444
order kinetics, and the parameters including rate constant k and
445
coefficient R2are listed inTable 2 The R2values in different
exper-446
imental conditions ranged from 0.80 to 0.99, showing that the
elec-447
trical degradation of MB by CSCF electrodes was compliant with
448
the mathematical model of the first-order kinetic equation
449
(Table 2) Both the rate constant k values of MB decoloration and
450
COD removal increased with voltage and decreased with MB initial
451
concentration, which is consistent with the aforementioned
452
results More importantly, CSCF-800-15-N (highest nitrogen
con-453
tent) and CSCF-850-15-W (largest microporous area) attained
454
higher reaction rate k values than those of the other 2 CSCF
elec-455
trodes This result further indicated that increasing micropore
456
number and nitrogen content in the CSCF can greatly promote
457
the electrical degradation of MB
a
b
c
Fig 5 XPS survey spectrum (a) and deconvoluted-N1s spectrum (b, c) of CSCFs.
6 K Li et al / Results in Physics xxx (2017) xxx–xxx
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
Trang 7Fig 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.
Rate constants (k,min) Coefficient (R 2
) Rate constants (k,min) Coefficient (R 2
) Different CSCFs
MB concentration
Voltage
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
Trang 8458 Fig 7shows the first-order kinetic fitting for MB degradation at
459 the initial MB concentration of 5 mg/L, initial pH of 4.5, voltage of
460 12 V, cathode–anode gap of 2 cm, and Na2SO4dose of 10 g/L The
461 results revealed the good fit of the first-order kinetic model with
462 the data The corresponding correlation coefficient R2was 0.9927
463 for the electrical decolorization of MB and 0.9888 for COD removal
464 (Table 2), confirming that the electrical decolorization of MB was
465 well compliant with the mathematical model of the first-order
466 kinetic equation Furthermore, the highest reaction rate constants
467 of MB degradation, including those of MB decoloration (0.1491),
468 COD removal (0.00635), and MB decoloration ratio (97%), also
469 demonstrated that the nitrogen-doped cotton-stalk microporous
470 CSCF with high surface area can be served as potential electrode
471 material for the treatment of low-concentration dye contains
472 Conclusions
473 A series of CSCFs with different nitrogen and surface area were
474 prepared to serve as efficient electrodes by controlling activation
475 parameters and activators The influences of pore structure and
476 nitrogen content of CSCF, as well as the electrolytic parameters
477 (voltage and initial concentration), on the electrolytic efficiency
478 of MB were evaluated Results showed that the CSCFs achieved
479 an excellent electrochemical processing efficiency toward MB,
480 and the electrical catalytic degradation efficiency varied in terms
481 of BET surface area and nitrogen content CSCF-800-15-N exhibited
482
the best degradation effect with 97% of MB decolorization ratio and
483
78% of COD removal rate at 12 V in 4 h The decolorization ratio
484
and COD removal rate both conformed well to the first-order
kinet-485
ics equation The above findings demonstrated that the prepared
486
cotton-stalk-based CSCF was a potential electrode material that
487
could be used for the treatment of low-concentration dye
wastew-488
ater, and that the electrical degradation performance of the
cotton-489
stalk-based electrode could be effectively improved by controlling
490
the micropore area and nitrogen content
491
References
492 [1] Fan L, Zhou Y, Yang W, Chen G, Yang F Electrochemical degradation of aqueous
493 solution of Amaranth azo dye on ACF under potentiostatic model Dyes Pigm
494 2008;76:440–6.
495 [2] Wang Y, Qu J, Wu R, Lei P The electrocatalytic reduction of nitrate in water on
496 Pd/Sn-modified activated carbon fiber electrode Water Res 2006;40:1224–32.
497 [3] Jia J, Yang J, Liao J, Wang W, Wang Z Treatment of dyeing wastewater with ACF
498 electrodes1 Water Res 1999;33:881–4.
499 [4] Zhao H-Z, Sun Y, Xu L-N, Ni J-R Removal of Acid Orange 7 in simulated
500 wastewater using a three-dimensional electrode reactor: Removal
501 mechanisms and dye degradation pathway Chemosphere 2010;78:46–51.
502 [5] Fan L, Zhou Y, Yang W, Chen G, Yang F Electrochemical degradation of
503 Amaranth aqueous solution on ACF J Hazard Mater 2006;137:1182–8.
504 [6] Yi F, Chen S, Yuan CE Effect of activated carbon fiber anode structure and
505 electrolysis conditions on electrochemical degradation of dye wastewater J
506 Hazard Mater 2008;157:79–87.
507 [7] De Coster J, Vanherck W, Appels L, Dewil R Selective electrochemical
508 degradation of 4-chlorophenol at a Ti/RuO2-IrO2 anode in chloride rich
509 wastewater J Environ Manage 2017;190:61–71.
510 [8] Moreira FC, Boaventura RAR, Brillas E, Vilar VJP Electrochemical advanced
511 oxidation processes: a review on their application to synthetic and real
512 wastewaters Appl Catal B 2017;202:217–61.
513 [9] Yue L, Guo J, Yang J, Lian J, Luo X, Wang X, et al Studies on the electrochemical
514 degradation of Acid Orange II wastewater with cathodes modified by quinones.
515
J Ind Eng Chem 2014;20:752–8.
516 [10] Zhao W, Xing J, Chen D, Jin D, Shen J Electrochemical degradation of Musk
517 ketone in aqueous solutions using a novel porous Ti/SnO 2 -Sb 2 O 3 /PbO 2
518 electrodes J Electroanal Chem 2016;775:179–88.
519 [11] Wang L, Hu Y, Li P, Zhang Y, Yan Q, Zhao Y Electrochemical treatment of
520 industrial wastewater using a novel layer-upon-layer bipolar electrode system
521 (NLBPEs) Chem Eng J 2013;215–216:157–61.
522 [12] Xu M, Wang Z, Wang F, Hong P, Wang C, Ouyang X, et al Fabrication of cerium
523 doped Ti/nanoTiO 2 /PbO 2 electrode with improved electrocatalytic activity and
524 its application in organic degradation Electrochim Acta 2016;201:240–50.
525 [13] Wang K, Song Y, Yan R, Zhao N, Tian X, Li X, et al High capacitive performance
526
of hollow activated carbon fibers derived from willow catkins Appl Surf Sci
527 2017;394:569–77.
528 [14] Liu L, Chen F, Yang F Stable photocatalytic activity of immobilized Fe-0/TiO 2 /
529 ACF on composite membrane in degradation of 2,4-dichlorophenol Sep Purif
530 Technol 2009;70:173–8.
531 [15] Sun Y, Wang G, Dong Q, Qian B, Meng Y, Qiu J Electrolysis removal of methyl
532 orange dye from water by electrospun activated carbon fibers modified with
533 carbon nanotubes Chem Eng J 2014;253:73–7.
534 [16] Wang A, Qu J, Liu H, Ru J Mineralization of an azo dye Acid Red 14 by
535 photoelectro-Fenton process using an activated carbon fiber cathode Appl
536 Catal B Environ 2008;84:393–9.
537 [17] Chen J, Xu J, Zhou S, Zhao N, Wong C-P Nitrogen-doped hierarchically porous
538 carbon foam: a free-standing electrode and mechanical support for
high-539 performance supercapacitors Nano Energy 2016;25:193–202.
540 [18] Lei W, Han L, Xuan C, Lin R, Liu H, Xin HL, et al Nitrogen-doped carbon
541 nanofibers derived from polypyrrole coated bacterial cellulose as
high-542 performance electrode materials for supercapacitors and Li-ion batteries.
543 Electrochim Acta 2016;210:130–7.
544 [19] Lu W, Liu M, Miao L, Zhu D, Wang X, Duan H, et al Nitrogen-containing
545 ultramicroporous carbon nanospheres for high performance supercapacitor
546 electrodes Electrochim Acta 2016;205:132–41.
547 [20] Speirs J, McGlade C, Slade R Uncertainty in the availability of natural
548 resources: fossil fuels, critical metals and biomass Energy Policy
549 2015;87:654–64.
550 [21] Li J, Zhang S, Gao B, Yang A, Wang Z, Xia Y, et al Characteristics and
deoxy-551 liquefaction of cellulose extracted from cotton stalk Fuel 2016;166:196–202.
552 [22] Cagnon BT, Py X, Guillot A, Stoeckli F The effect of the carbonization/activation
553 procedure on the microporous texture of the subsequent chars and active
554 carbons Microporous Mesoporous Mater 2003;57:273–82.
555 [23] Dhawane SH, Kumar T, Halder G Central composite design approach towards
556 optimization of flamboyant pods derived steam activated carbon for its use as
557 heterogeneous catalyst in transesterification of Hevea brasiliensis oil Energy
558 Convers Manage 2015;100:277–87.
559 [24] Kan Y, Yue Q, Gao B, Li Q Preparation of epoxy resin-based activated carbons
560 from waste printed circuit boards by steam activation Mater Lett
561 2015;159:443–6.
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 Na 2 SO 4 dose of 10 g/L.
8 K Li et al / Results in Physics xxx (2017) xxx–xxx
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
Trang 9562 [25] Zhang Y-J, Xing Z-J, Duan Z-K, Meng L, Wang Y Effects of steam activation on
563 the pore structure and surface chemistry of activated carbon derived from
564 bamboo waste Appl Surf Sci 2014;315:279–86.
565 [26] Zemskova LA, Voit AV, Didenko NA Influence of modification on the
566 electrochemical properties and thermal oxidation stability of carbon fibers.
567 Fibre Chem 2014;46:178–83.
568 [27] Boudou JP, Parent P, Suárez-García F, Villar-Rodil S, Martínez-Alonso A, Tascón
569 JMD Nitrogen in aramid-based activated carbon fibers by TPD, XPS and
570 XANES Carbon 2006;44:2452–62.
571 [28] Jansen RJJ, van Bekkum H XPS of nitrogen-containing functional groups on
572 activated carbon Carbon 1995;33:1021–7.
573 [29] Burg P, Fydrych P, Cagniant D, Nanse G, Bimer J, Jankowska A The
574 characterization of nitrogen-enriched activated carbons by IR, XPS and LSER
575 methods Carbon 2002;40:1521–31.
576 [30] Pietrzak R XPS study and physico-chemical properties of nitrogen-enriched
577 microporous activated carbon from high volatile bituminous coal Fuel
578 2009;88:1871–7.
579 [31] Liu T, Wang K, Song S, Brouzgou A, Tsiakaras P, Wang Y New Electro-Fenton
580 gas diffusion cathode based on nitrogen-doped graphene@carbon nanotube
581 composite materials Electrochim Acta 2016;194:228–38.
582 [32] Hu X, Yu Y, Sun Z Preparation and characterization of cerium-doped
583 multiwalled carbon nanotubes electrode for the electrochemical degradation
584 of low-concentration ceftazidime in aqueous solutions Electrochim Acta
585 2016;199:80–91.
586 [33] Rocha JHB, Gomes MMS, Santos EVD, Moura ECMD, Silva DRD, Quiroz MA,
587
et al Electrochemical degradation of Novacron Yellow C-RG using
boron-588 doped diamond and platinum anodes: direct and Indirect oxidation.
589 Electrochim Acta 2014;140:419–26.
590 [34] Ajeel MA, Aroua MK, Daud WMAW Anodic degradation of 2-chlorophenol by
591 carbon black diamond and activated carbon composite electrodes Electrochim
592 Acta 2015;180:22–8.
593 [35] Dobrzeniecka A, Zeradjanin AR, Masa J, Blicharska M, Wintrich D, Kulesza PJ,
594
et al Evaluation of kinetic constants on porous, non-noble catalyst layers for
595 oxygen reduction—A comparative study between SECM and hydrodynamic
596 methods Catal Today 2016;262:74–81.
597 [36] Luque GC, de Chialvo MRG, Chialvo AC Influence of spontaneous
598 decomposition on the electrochemical formic acid oxidation on a
599 nanostructured palladium electrode Electrochem Commun 2016;70:69–72.
600 [37] Montero MA, Fernández JL, Gennero de Chialvo MR, Chialvo AC.
601 Characterization and kinetic study of a nanostructured rhodium electrode
602 for the hydrogen oxidation reaction J Power Sources 2014;254:218–23.
603 [38] Nolan JE, Plambeck JA The EC-catalytic mechanism at the rotating disk
604 electrode: Part I Approximate theories for the pseudo-first-order case and
605 applications to the Fenton reaction J Electroanal Chem Interfacial Electrochem
606 1990;286:1–21.
607 [39] Wang AM, Qu JH, Ru J, Liu HJ, Ge JT Mineralization of an azo dye Acid Red 14
608
by electro- Fenton’s reagent using an activated carbon fiber cathode Dyes
609 Pigm 2005;65:227–33.
610
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