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Vermiculite modification increases carbon retention and stability of rice straw biochar at different carbonization temperatures
Journal Pre-proof Vermiculite modification increases carbon retention and stability of rice straw biochar at different carbonization temperatures
Yuxue Liu, Chengxiang Gao, Yuying Wang, Lili He, Haohao Lu, Shengmao Yang PII:
S0959-6526(20)30158-X
DOI:
https://doi.org/10.1016/j.jclepro.2020.120111
Reference:
JCLP 120111
To appear in:
Journal of Cleaner Production
Received Date:
27 July 2019
Accepted Date:
09 January 2020
Please cite this article as: Yuxue Liu, Chengxiang Gao, Yuying Wang, Lili He, Haohao Lu, Shengmao Yang, Vermiculite modification increases carbon retention and stability of rice straw biochar at different carbonization temperatures, Journal of Cleaner Production (2020), https://doi.org /10.1016/j.jclepro.2020.120111
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Journal Pre-proof Vermiculite modification increases carbon retention and stability of rice straw biochar at different carbonization temperatures
Yuxue Liu
a,b,
Chengxiang Gao
a,c,
Yuying Wang
a,b,
Lili He
a,b,
Haohao Lu
a,b,
Shengmao Yang a,b,*
aInstitute
of Environment, Resource, Soil and Fertilizer, Zhejiang Academy of
Agricultural Sciences, 298 Desheng Middle Road, Hangzhou 310021, China b
Engineering Research Center of Biochar of Zhejiang Province, Hangzhou 310021,
China c
Institute of Resource and Environment, Northwest Agricultural & Forestry
University, Yangling 712100, Shanxi, China
*Corresponding
author: Shengmao Yang
Tel.: +86 571 8641 9218 Fax: +86 571 8641 9218 E-mail: [email protected]
Word count for the manuscript: 5392 Declarations of interest: none.
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Abstract: Biochar is considered a promising material for sequestering CO2 from the
2
atmosphere, thus helping to alleviate climate change when returned to the soil.
3
Biochar stability is the most decisive factor determining its C sequestration potential.
4
Mineral modification may improve biochar characteristics, but systematic research on
5
the effect of mineral modification on the C retention and stability of biochar and the
6
associated mechanisms is limited. Therefore, in this study, rice straw was used to
7
produce biochar at various temperatures (300, 400, 500, 600, and 700 °C), with
8
vermiculite
9
thermogravimetric analysis, Fourier transform infrared (FTIR) spectroscopy, X-ray
10
photoelectron spectroscopy (XPS), and nuclear magnetic resonance (NMR)
11
spectroscopy were used to evaluate the effect of vermiculite modification and
12
carbonization temperature on biochar stability. Biochar yield and C retention ratio
13
decreased with increasing temperatures but increased by 13.5–38.8% and 5.2–22.1%,
14
respectively, after vermiculite modification. The ratios of C thermal weight loss,
15
atomic H/C, and C oxidation loss in the biochar were reduced with increasing
16
carbonization temperature, indicating improved thermal, aromatization, and chemical
17
oxidation stability. A trade-off that did not compromise C sequestration potential was
18
optimized at 700 and 600 °C for the unmodified and modified biochar, respectively.
19
Furthermore, the total mineral content of the biochar, particularly Fe, Al, Mg, and Si,
20
were increased by vermiculite modification. FTIR results showed that chemical bonds,
21
such as Si–O–C and Fe–O, were formed or enhanced on the biochar surface after
22
vermiculite modification. This was further certified by the XPS survey spectra. NMR
as
a
modified
mineral
material.
2
Several
methods
including
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results indicated that biochar stability was enhanced by increasing the aromatization
24
rate during carbonization, that is, by the conversion of C from alkyl and carbonyl C to
25
aromatic C. This study provides a basis for research into and the development of
26
functional biochar and its application in C sequestration.
27
Keywords: biochar; stability; mineral modification; vermiculite; carbon sequestration
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1 Introduction
29
By 2017, human-induced global warming had caused an average global temperature
30
increase of approximately 1 °C compared to pre-industrial levels (IPCC, 2018).
31
Ongoing climate change poses an increasing threat to humanity that is likely to be
32
aggravated if substantial and timely reductions of greenhouse gas emissions are not
33
achieved (Mora et al., 2018). Biochar, as a C-rich byproduct of biomass pyrolysis
34
(Chen et al., 2008; Lehmann, 2007a), is considered a promising material for locking
35
CO2 from the atmosphere and thus alleviating climate change when returned to the
36
soil (Lehmann, 2007b). Furthermore, biochar has multiple functions in soil
37
improvement (Major et al., 2010; Liu et al., 2016), contaminant removal (Ahmad et
38
al., 2014; Zhang et al., 2013) and reducing greenhouse gas emissions (Wang et al.,
39
2019). This is mainly based on two typical characteristics of biochar: one is its high
40
biochemical stability due to its highly aromatic C structure, and the other is its
41
excellent adsorption properties resulting from its developed pore structures and large
42
surface area (Wang et al., 2016). Biochar’s C stability is the key to its long-term
43
environmental functions, such as C fixation and emission reduction (Chandra &
44
Bhattacharya, 2019; Crombie et al., 2013; Ullah et al., 2019). The development of
45
biochar with high C retention and stability is of great theoretical and practical
46
significance for its C sequestration ability.
47
Recent studies have shown that mineral modification has an important effect on
48
improving the functional properties of biochar, such as its adsorption ability (Han et
49
al., 2016; Wang et al., 2015; Wang & Wang, 2019). FeCl3-modified biochar showed a
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high adsorption capacity for Cr(VI) from aqueous solution, which was 1–2 orders of
51
magnitude higher than that of unmodified biochar from the same feedstock (Han et al.,
52
2016). The As(V) and Pb(II) sorption capacities of MnCl2•4H2O-modified biochar
53
(0.59 and 4.91 g kg−1, respectively) and birnessite-modified biochar (0.91 and 47.05 g
54
kg−1, respectively) were significantly higher than that of unmodified biochar (0.20 and
55
2.35 g kg−1, respectively) (Wang et al., 2015). Therefore, the modification of biochar
56
with mineral materials has great potential to produce functional biochar with
57
improved properties.
58
However, in terms of C fixation, studies on the influence of mineral modification on
59
the C retention and stability of biochar remain limited. Li et al. (2014) found that
60
kaolin and CaCO3 had little effect on the C retention of biochar derived from rice
61
straw, whereas Ca(H2PO4)2 increased the C retention of modified biochar by up to 29%
62
compared to that of unmodified biochar. A recent study showed increased C retention
63
and stability in biochar when Ca(OH)2 was added to the sludge feed; this was
64
attributed to the formation of CaCO3 and increased C-containing functional groups on
65
the biochar surface (Ren et al., 2018). Therefore, different minerals may influence the
66
C stability of biochar, the mechanism of which needs to be further explored.
67
Moreover, the carbonization temperature during biochar production is the most
68
significant factor influencing aromatization and biochar stability (Leng & Huang,
69
2018; Mašek et al., 2013; Zhao et al., 2013). Many studies have found that as the
70
carbonization temperature increases, the content of stable aromatic ring structures
71
indicated by the H/C ratio of biochar increases (Liu et al., 2017; McBeath et al., 2015).
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Even so, it is not clear whether the presence of minerals affects the biomass
73
carbonization process and, thus, the stability of biochar. Furthermore, it is noteworthy
74
that biochar with a high stability does not always have a high C retention ratio.
75
Therefore, it is necessary to make a trade-off between the C retention ratio and the
76
stable C content for the research on C sequestration by biochar.
77
In China, large quantities of crop residues including rice/wheat straw are produced
78
annually. Most of the straw is discarded or burned, leading to resource losses and
79
serious air pollution caused by emissions of carbon monoxide, non-methane
80
hydrocarbons, nitrogen oxides, and sulfur dioxide (Wang et al., 2015; Wang et al.,
81
2018). Therefore, the development of a proper straw treatment method is required
82
urgently.
83
As a consequence, rice straw was used as raw material for biochar preparation in this
84
study. Vermiculite was selected as the mineral material to produce modified biochar
85
at serial temperature conditions of 300–700 °C. Several methods, namely
86
thermogravimetric analysis (TGA), scanning electron microscopy (SEM), Fourier
87
transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS),
88
nuclear magnetic resonance (NMR), and X-ray diffraction (XRD) were used to
89
evaluate the effect of vermiculite modification and carbonization temperature on the C
90
retention and stability of biochar. This research provides a science-based
91
underpinning for the production of mineral-modified functional biochar and its
92
application in C sequestration and climate change mitigation.
93
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2 Materials and methods
95
2.1 Mineral material and feedstock for biochar
96
Vermiculite ((Mg,Fe,Al)3[(Si,Al)4O10(OH)2]•4H2O; analytical reagents), obtained
97
from a chemical reagent company, was chosen as modifying material in this study.
98
Vermiculite is a natural and non-toxic 2:1 silicate clay mineral with strong cationic
99
exchange capacity and adsorption properties. Therefore, it can be used as a soil
100
amendment to loosen soil and promote crop growth. In addition, vermiculite has the
101
advantages of low cost and environmental friendliness. The biomass raw material for
102
biochar production was rice straw, which was collected from an experimental plot at
103
the Scientific Research Base of Zhejiang Academy of Agricultural Sciences, Jiaxing
104
City, Zhejiang, China (120°24′23′′E, 30°26′07′′N). The rice straw was air-dried to a
105
moisture content of <5% and ground to a particle size <2 mm for later use.
106
2.2 Preparation of modified and unmodified biochar
107
The vermiculite was mixed with rice straw at a ratio of 1:4 (w/w). The mixtures were
108
then placed into a laboratory-scale programmable tubular carbonization reactor
109
(Hangzhou Lantian Instrument Co., Ltd., China) and heated under a vacuum at a
110
heating rate of 15 °C min−1 to reach five settled temperatures (300, 400, 500, 600, and
111
700 °C). At each settled temperature, the heating residence time was 1.5 h. The solid
112
residue in the reactor was vermiculite-modified biochar, which was designated
113
VBC300, VBC400, VBC500, VBC600, and VBC700 according to the respective
114
temperatures. As a control, the rice straw not mixed with vermiculite was used to
115
produce unmodified biochar under the same conditions, designated BC300, BC400,
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BC500, BC600, and BC700, respectively.
117
2.3 Characterization of biochar
118
The characteristics of the vermiculite-modified and unmodified biochar were
119
determined as follows: The biochar yield was calculated by weighing the sample and
120
solid residue before and after pyrolysis and subtracting the contribution of minerals.
121
Y=(Ms–Mm)/Mbm×100
122
where Y is the biochar yield (%), Ms and Mm are the weights of solid and mineral
123
residues after carbonization (g), respectively, and Mbm is the weight of biomass before
124
carbonization (g).
125
R=(Y×Cbc/Cbm)×100%
126
where R is the C retention ratio of biochar (%), and Cbc and Cbm are the C contents of
127
biochar and biomass (%), respectively.
128
Biochar pH was measured at a 1:20 (w/v) solid-to-water ratio by a pH meter
129
(Mettler-Toledo, Switzerland). The C, H, N, and S contents of the biochar were
130
measured by an elemental analyzer (Vario EL/micro cube, Elementar, Germany). The
131
K, Ca, Mg, Al, Si, Fe, and P contents of the biochar were measured by inductively
132
coupled plasma-atomic emission spectrometry. The O content was calculated by
133
subtraction. SEM (instrument: JSM-6700F, Jeol, Japan) was used to compare the
134
surface morphological characteristics of the unmodified and modified biochar. The
135
surface functional groups of biochar were analyzed by FTIR spectroscopy (Varian
136
640-IR, USA) using KBr pellets at 25±1 °C. XPS spectra of the unmodified and
137
modified biochar were recorded with a spectrometer (ESCALAB 250Xi,
(1)
(2)
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138
ThermoFisher, USA) using Al Kα as the X-ray source. The
139
magic angle spinning (CP-MAS) NMR spectra were recorded at a frequency of
140
100.62 MHz using a Varian Unity Inova 400 NMR spectrometer (AVANCEIII 400,
141
Bruker, Switzerland) under the same detecting conditions as described in our previous
142
research work (Liu et al., 2018). The crystalline phases of biochar were identified by
143
their XRD patterns obtained using the same X-ray diffractometer and employing the
144
same conditions as described in Liu et al. (2017).
145
2.4 Measurement of biochar stability
146
The aromatization stability of biochar is represented by its H/C atomic ratio. Biochar
147
heat stability is represented by the thermal weight loss ratio of C in biochar, which
148
was measured by a thermogravimetric analyzer (Q50, TA, USA). Specifically, 5–10
149
mg biochar sample (through a 100-mesh sieve) was weighed in an alumina crucible
150
and heated from an initial temperature of 30 °C to a terminal temperature of 800°C at
151
a heating rate of 10 °C min−1 in a high-purity nitrogen atmosphere. The thermal
152
weight loss ratio of C was calculated according to the mass loss of C in the biochar
153
samples.
154
The chemical stability of biochar was determined by chemical oxidation treatment, in
155
which experimental potassium dichromate (K2Cr2O7) was used to assess the labile
156
fraction of C in the biochar samples. For the K2Cr2O7 treatment, 0.1 g biochar was
157
treated in a glass test tube with 40 mL 0.1 mol L−1 K2Cr2O7 and 2 mol L−1 H2SO4
158
solutions at 55 °C for 60 h. The C loss was calculated according to the difference
159
between the C content of biochar samples before and after K2Cr2O7 oxidation, and the
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cross-polarization
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C oxidation loss ratio was thus expressed as the percentage of C loss to the initial C
161
content.
162
A trade-off between C retention ratio (R) and stable C content of biochar was
163
optimized according to the following formula:
164
Trade-off value = R×(100–C oxidation loss ratio)/100
165
2.5 Data processing
166
Data were processed using Microsoft Excel 2007, and SigmaPlot 10.0 software was
167
used for drawing the figures.
(3)
168 169
3 Results and discussion
170
3.1 Basic physicochemical properties of biochar
171
The elemental compositions and total mineral contents of both vermiculite-modified
172
and unmodified biochar are shown in Fig. 1. The C and total mineral contents of all
173
the biochar increased with increasing temperature, whereas the H and O contents
174
decreased. After vermiculite modification, the C content of biochar decreased from
175
46.8–56.7% to 36.5–39.6%, whereas the total mineral content increased from
176
3.72–5.48% to 13.4–18.9%.
177
(Insert Fig. 1 here)
178 179
The contents of different mineral components of modified and unmodified biochar are
180
shown in Fig. 2. The unmodified rice-straw biochar is rich in mineral components,
181
such as Al, Ca, Fe, K, Mg, P, and Si, with the K content being the highest
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(2.35–3.41%). After vermiculite modification, the Al, Fe, Mg, and Si contents of
183
biochar increased from 0.08–0.25%, 0.11–0.34%, 0.26–0.48%, and 0.21–0.29% to
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2.66–3.59%, 4.16–5.53%, 2.77–4.14%, and 0.31–0.50%, respectively, and increased
185
with increasing carbonization temperature. This is because vermiculite contains a
186
large number of Al, Fe, Mg, and Si components, which combined with rice straw
187
during carbonization and ultimately retained in biochar. However, the P content of
188
biochar decreased from 0.22–0.64% to 0.14–0.39% after vermiculite modification.
189
(Insert Fig. 2 here)
190 191
The yield, C retention ratio, and pH of modified and unmodified biochar are shown in
192
Table 1. The biochar yield decreased with increasing carbonization temperature. In
193
addition, the biochar yield increased by 13.5–38.8% after vermiculite modification,
194
with the minimum increase occurring at 300–400 °C and the maximum at 500–600 °C.
195
Previous studies have reported similar results: Hossain et al. (2011) found that the
196
yield of sludge biochar decreased from 78 to 73% as the carbonization temperature
197
increased from 400 to 600 °C. Chen et al. (2014) showed that biochar yield decreased
198
from 63.1 to 53.3% with temperature increasing from 500 to 900 °C. The
199
decomposition of organic components in biomass materials with increasing
200
carbonization temperature is the main reason for decreased yields (Yuan et al., 2015).
201
(Insert Table 1 here)
202 203
The C retention ratio of biochar decreased with increasing carbonization temperature.
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At the same temperatures, vermiculite modification increased the C retention ratio of
205
biochar by 5.2–22.1%, indicating that the presence of vermiculite during biomass
206
carbonization can effectively improve the C retention of biochar and reduce C loss
207
during carbonization. Li et al. (2014) found that the modification of kaolin and CaCO3
208
had no effect on the C retention ratio of biochar.
209
With carbonization temperature increasing from 300 to 700 °C, the pH gradually
210
increased from 5.56 to 10.5 for unmodified biochar and from 6.06 to 9.86 for
211
vermiculite-modified biochar (Table 1). The pH of modified biochar was lower than
212
that of unmodified biochar at carbonization temperatures of 400–700 °C but higher at
213
300 °C, indicating that vermiculite modification generally decreased biochar pH,
214
except at 300 °C.
215
3.2 Effects of mineral modification and carbonization temperature on biochar
216
stability
217
3.2.1 TGA
218
The stability of modified and unmodified biochar is shown in Table 1. The thermal
219
weight loss ratio can be used to characterize the thermal stability of biochar. With
220
increasing carbonization temperature, the thermal weight loss ratio of biochar
221
decreased gradually, indicating increased thermal stability. The modified vermiculite
222
reduced the thermal weight loss ratio of biochar by 14.9–45.6%, indicating that
223
vermiculite modification enhanced the thermal stability of biochar. This may be
224
because vermiculite has a high heat-absorption ability and thus protects biochar from
225
thermal decomposition by heat blocking (Kariya et al., 2016).
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3.2.2 H/C atomic ratio
227
The H/C atomic ratio can be used to characterize the degree of C aromatization of
228
biochar, with lower H/C atomic ratios indicating higher aromatization stability. With
229
increasing carbonization temperature, the H/C atomic ratio of biochar gradually
230
decreased, indicating that its aromatic structure gradually increased. Compared with
231
more amorphous C structures in low-temperature biochar, the aromatization stability
232
of high-temperature biochar gradually increased. The H/C atomic ratio of biochar was
233
reduced by 1.4–15.1% after vermiculite modification. This indicated that vermiculite
234
modification enhanced the aromatization stability of biochar. Guo and Chen (2014)
235
found that Si plays an important role in the C arrangement and structure composition
236
of rice-straw biochar. Si and C may combine to form a dense protection structure, thus
237
improving biochar stability (Han et al., 2018).
238
3.2.3 Chemical oxidation
239
The C oxidation loss ratio (K2Cr2O7 or H2O2 method) can be used to characterize the
240
chemical oxidation stability of biochar. Our results showed that with increasing
241
carbonization temperature, the C oxidation loss ratio of biochar (K2Cr2O7 method)
242
gradually decreased, indicating that its chemical oxidation stability improved.
243
Vermiculite modification reduced the C oxidation loss ratio of biochar by 6.80–45.8%,
244
except for biochar produced at 300 °C. This shows that vermiculite can generally
245
improve the chemical oxidation stability of biochar.
246
Li et al. (2014) used CaCO3 and hydroxyapatite to modify biochar and found that the
247
C oxidation loss ratio (H2O2 method) of the modified biochar decreased by 18.6 and
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58.5%, respectively, which may be due to increased aromatic C content and the
249
formation of C–O–P and C–P bonds. Ren et al. (2018) studied the co-pyrolysis of
250
CaOH2 and sludge to produce biochar and found that the C oxidation loss ratio (H2O2
251
method) of the biochar decreased from 31.3 to 9.71% at 300 °C and from 2.15 to 1.32%
252
at 700 °C. The reason may be that mineral modification prompted the formation of
253
CaCO3, which may have prevented oxidants from entering the biochar by means of a
254
physical barrier, thus improving the chemical oxidation stability of biochar; Zhao et al.
255
(2016) found similar results. Yang et al. (2016) modified biochar with soil minerals
256
(such as FeCl3, AlCl3, CaCl2, and kaolin) and found that the C oxidation loss ratio
257
(H2O2 method) of biochar was reduced by 13.4–79.6% compared with that of
258
unmodified biochar. This may be because the minerals act as a physical barrier
259
preventing oxidants from entering the biochar. Moreover, the formation of organic
260
mineral complexes (e.g., Fe–O–C) during the interaction between minerals and
261
biochar may play a protective role.
262
It is worth mentioning that although biochar produced at higher carbonization
263
temperatures achieved higher thermal and oxidative stability, the decreased C
264
retention ratio with increasing temperature (Table 1) indicated lower C sequestration
265
at higher carbonization temperature. In this study, a trade-off between reduced C
266
retention ratio and more stable C content (K2Cr2O7 method) with increasing
267
temperature was optimized at 700°C for the unmodified biochar and at 600 °C for the
268
modified biochar (Trade-off value in Table 1). This was consistent with the results of
269
McBeath et al. (2015), who found that the optimal carbonization temperature was
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500–700 °C for most feedstock in a temperature range of 300–900 °C. Our results
271
showed that vermiculite modification reduced the carbonization temperature by
272
100 °C (from 700 to 600 °C) without sacrificing the C sequestration potential of
273
biochar. This indicated that vermiculite can be used as a catalyst in the carbonization
274
process. Therefore, carbonization of rice straw with vermiculite modification could be
275
considered as a cleaner production technology.
276
3.3 SEM
277
SEM was used to determine the morphological characteristics of the modified and
278
unmodified biochar. As shown in Fig. 3, many small particles existed on the surface
279
or in the pores of the modified biochar (Fig. 3B, D, F, H, and J), whereas no or few
280
such particles were found on the unmodified biochar (Fig. 3A, C, E, G, and I). This
281
could be explained as the loading of metals (such as Fe, Mg, and Si) on the surface of
282
the modified biochar due to vermiculite modification.
283
(Insert Fig. 3 here)
284 285
3.4 FTIR
286
The surface functional groups in biochar can be qualitatively analyzed by infrared
287
spectroscopy. The FTIR spectra of the modified and unmodified biochar are presented
288
in Fig. 4. With increasing carbonization temperature, the peak at 3400 cm−1 indicates
289
that the stretching vibration of hydroxyl bonds (–OH) gradually decreased. When the
290
temperature increased to 400–500 °C, the stretching vibration of aliphatic C–H bonds
291
(2950 cm−1) decreased and disappeared at around 600 °C. The stretching vibration of
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C=O bonds (1710 cm−1) disappeared entirely when the temperature exceeded 700 °C.
293
This indicates that the O-containing functional groups of biochar gradually decreased
294
with increasing carbonization temperature (Novak et al., 2009). Intense bands of
295
aromatic C=C bonds (1600 and 1450 cm−1) and aromatization C–H surface bending
296
vibration (810 cm−1) were increasingly observed, indicating the intensification of the
297
dehydrogenation reaction and the enhancement of the biochar aromatization structure
298
with increasing temperature. This is consistent with the decreasing trend in H/C
299
atomic ratio (Table 1). In addition, after vermiculite modification, the vibration of
300
Fe–O bonds on the biochar surface (450 cm−1) was significantly enhanced. The
301
stretching vibration of C–O–C functional groups (1100 cm−1) was weakened and
302
replaced by Si–O–C or Si–O–Si groups (1000 cm−1), indicating that more stable
303
mineral organic complexes were formed on the surface of modified biochar (Guo &
304
Chen, 2014). (Insert Fig. 4 here)
305 306 307
3.5 XPS
308
To verify the FTIR results obtained and described in section 3.4, the XPS spectra of
309
the modified and unmodified biochar at a series of carbonization temperatures were
310
provided. As shown in Fig. 5(A-E), remarkable O 1s and C 1s peaks were observed in
311
the XPS survey spectra of both the modified and unmodified biochar. Furthermore,
312
Mg 1s, Fe 2p, Si 2s, and Si 2p signals were clearly observed for all modified biochar
313
(VBC300, VBC400, VBC500, VBC600, and VBC700), but not obvious for the
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unmodified biochar (BC300, BC400, BC500, BC600, and BC700); this indicated that
315
the Mg, Fe, and Si metals were loaded on the vermiculite modified biochar. The XPS
316
survey spectra indicated that the modifications can remarkably alter the properties of
317
the functional groups of biochar, which agreed well with the FTIR spectra presented
318
in Fig. 4. (Insert Fig. 5 here)
319 320 321
3.6 NMR
322
The NMR spectra of modified and unmodified biochar are shown in Fig. 6. The
323
C-containing functional groups of rice-straw biochar mainly comprise aromatic C
324
(165–95 ppm), which has a highly aromatic structure. Compared with
325
high-temperature biochar (600 and 700 °C), low-temperature biochar (300–500 °C)
326
also contains alkyl C (0–90 ppm) and carbonyl C (220–165 ppm) groups. These
327
results show that the aromatization degree of biochar increased with increasing
328
carbonization temperature. Previous studies have indicated that the macromolecule
329
content of solid biomass decreased while aromatic rings formed during the initial
330
heating period of carbonization (200–300 °C); small and defective sheets of
331
condensed aromatic rings subsequently stacked up (from >300 to 600 °C) and finally
332
formed turbostratic crystallites (>700 °C) with further temperature increases (Aller,
333
2016; Keiluweit et al., 2010).
334
Furthermore, vermiculite modification enhanced the aromatization rate of biochar
335
during carbonization by increasing the Al, Fe, Mg, and Si contents and other mineral
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336
components (Fig.2), that is, the transformation rate of C from alkyl and carbonyl C to
337
aromatic C enhanced biochar stability. The conversion of raw materials into
338
increasingly aromatized structures contributed greatly to the environmental
339
recalcitrance of biochar (Leng & Huang, 2018). (Insert Fig. 6 here)
340 341 342
3.7XRD
343
XRD was used to study the crystalline structures of the minerals present in the biochar
344
(Wang et al., 2016). Typical XRD patterns of the modified and unmodified biochar
345
are shown in Fig. 7. The broad peak located at about 24° in the XRD pattern of
346
biochar represents a typical amorphous C diffraction pattern (Wang et al., 2018). The
347
peaks at 28 and 41° confirmed the presence of sylvite (KCl) in the unmodified
348
rice-straw biochar. The peak at 26.7° was due to quartz (SiO2) in the modified biochar.
349
More peaks were observed in the XRD results of the modified biochar than in those of
350
the unmodified biochar, which revealed that the former contained more mineral
351
components than the latter did. (Insert Fig. 7 here)
352 353 354
4 Conclusions
355
With increasing carbonization temperature, the C content of rice-straw biochar
356
gradually increased, whereas the yield and C retention ratio gradually decreased.
357
Compared with that of the unmodified biochar, the C content of the
18
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358
vermiculite-modified biochar decreased significantly, but the yield and C retention
359
ratio increased significantly. The thermal weight loss, H/C atomic, and C oxidation
360
loss ratios (K2Cr2O7 method) of biochar gradually decreased with increasing
361
temperature, indicating that the thermal, aromatization, and chemical oxidation
362
stability of biochar were enhanced. Vermiculite modification enhanced biochar
363
stability by increasing the content of mineral components, promoting the formation of
364
chemical bonds, such as Si–O and Fe–O, on the biochar surface, and improving the
365
aromatization rate during carbonization. The CO2 released from modified biochar
366
would be less and/or slower than that from unmodified biochar. Furthermore, a
367
trade-off without sacrificing the C sequestration potential of biochar was optimized at
368
700 and 600 °C for the unmodified and modified biochar, respectively, indicating that
369
vermiculite can be used as a catalyst in the carbonization process. Therefore,
370
carbonization of rice straw with vermiculite modification could be considered as a
371
cleaner production technology. This research provides a scientific basis for research
372
on and development of functional biochar and its application in C sequestration and
373
climate change mitigation.
374 375
Acknowledgements
376
This research was financially supported by the National Natural Science Foundation
377
of China (41701334) and Natural Science Foundation of Zhejiang Province
378
(LY20D010005). There are no conflicts of interest to declare.
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379
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Journal Pre-proof Author Contributions
Yuxue Liu: Conceptualization, Methodology, Formal analysis, Writing - Original Draft, Writing - Review & Editing, Project administration, Funding acquisition
Chengxiang Gao: Investigation, Validation, Formal analysis
Yuying Wang: Resources, Methodology, Formal analysis
Lili He: Software, Formal analysis
Haohao Lu: Software, Formal analysis
Shengmao Yang: Resources, Supervision, Funding acquisition
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Declaration of interests ■ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Journal Pre-proof Figure captions Fig. 1 Elemental analysis and total mineral contents of modified and unmodified biochar (BC: unmodified biochar; VBC: vermiculite-modified biochar; 300, 400, 500, 600, and 700 indicate the carbonization temperatures (°C) of both modified and unmodified biochar) Fig. 2 Contents of mineral components of modified and unmodified biochar (BC: unmodified biochar; VBC: vermiculite-modified biochar; 300, 400, 500, 600, and 700 indicate the carbonization temperatures (°C) of both modified and unmodified biochar) Fig. 3 SEM images of modified and unmodified biochar (A, C, E, G, I: unmodified biochar at the carbonization temperatures of 300, 400, 500, 600, and 700, respectively; B, D, F, H, J: modified biochar at the carbonization temperatures of 300, 400, 500, 600, and 700, respectively) Fig. 4 FTIR spectra of modified and unmodified biochar (BC: unmodified biochar; VBC: vermiculite-modified biochar; 300, 400, 500, 600, and 700 indicate the carbonization temperatures (°C) of both biochars) Fig. 5 XPS spectra of modified and unmodified biochar (BC: unmodified biochar; VBC: vermiculite-modified biochar; 300, 400, 500, 600, and 700 indicate the carbonization temperatures (°C) of both biochars) Fig. 6 NMR spectra of modified and unmodified biochar (BC: unmodified biochar; VBC: vermiculite-modified biochar; 300, 400, 500, 600, and 700 indicate the carbonization temperatures (°C) of both biochars) Fig. 7 XRD spectra of modified and unmodified biochar (BC: unmodified biochar; VBC: vermiculite-modified biochar; 300, 400, 500, 600, and 700 indicate the carbonization temperatures (°C) of both biochars)
1
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1 2
Fig. 1
2
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3 4
Fig. 2
3
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5 6
Fig. 3 4
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7 8
Fig. 4
5
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9 10
Fig. 5
6
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11 12
Fig. 6
7
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BC300 BC400 BC500 BC600
Intensity
BC700
VBC300
VBC400
VBC500
VBC600 VBC700
10
13 14
20
30
40
50
60
2 (degree)
Fig. 7
8
70
80
Journal Pre-proof Highlights Biochar was derived from rice straw with vermiculite modification at 300–700 °C Higher C retention ratio and mineral content were obtained after modification Si–O–C and Fe–O bonds were formed/enhanced on biochar surface after modification Biochar stability was enhanced by converting alkyl and carbonyl C to aromatic C Trade-off without sacrificing C sequestration was optimized for modified biochar
1
Table 1 Yield, carbon retention ratio, pH, and stability of modified and unmodified biochars Biochar
Yield (%)
Carbon retention
pH
ratio (%)
Carbon thermal
H/C atomic ratio Carbon oxidation Trade-off value
weight loss rate (%)
loss rate (%)
BC300
55.5±0.1
75.6±0.1
5.56±0.03
75.4
1.36±0.06
51.8±0.4
36.4
BC400
53.9±0.1
71.7±0.2
7.04±0.01
68.6
1.16±0.04
48.1±0.6
37.2
BC500
42.7±2.0
62.5±2.7
7.96±0.17
58.7
0.92±0.05
30.4±1.0
43.5
BC600
35.3±0.2
54.0±0.3
10.2±0.05
34.8
0.66±0.03
10.2±0.5
48.5
BC700
34.5±1.8
51.9±1.7
10.5±0.04
32.8
0.60±0.04
2.43±0.1
50.6
VBC300
68.6±0.1
84.7±0.2
6.06±0.06
43.3
1.27±0.05
58.0±1.4
35.6
VBC400
61.2±2.0
75.3±2.7
6.88±0.02
37.3
1.14±0.06
44.9±0.9
41.5
VBC500
52.8±0.6
70.6±0.9
7.85±0.04
38.3
0.91±0.05
24.3±0.4
53.4
VBC600
49.0±0.8
66.0±0.9
9.74±0.04
29.6
0.56±0.02
5.52±0.2
62.4
VBC700
46.6±0.1
63.0±0.1
9.86±0.05
18.6
0.51±0.03
2.03±0.1
61.7
BC: unmodified biochar; VBC: vermiculite-modified biochar; 300, 400, 500, 600, and 700 indicate the carbonization temperatures (°C)
Yuxue Liu, Chengxiang Gao, Yuying Wang, Lili He, Haohao Lu, Shengmao Yang PII:
S0959-6526(20)30158-X
DOI:
https://doi.org/10.1016/j.jclepro.2020.120111
Reference:
JCLP 120111
To appear in:
Journal of Cleaner Production
Received Date:
27 July 2019
Accepted Date:
09 January 2020
Please cite this article as: Yuxue Liu, Chengxiang Gao, Yuying Wang, Lili He, Haohao Lu, Shengmao Yang, Vermiculite modification increases carbon retention and stability of rice straw biochar at different carbonization temperatures, Journal of Cleaner Production (2020), https://doi.org /10.1016/j.jclepro.2020.120111
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Journal Pre-proof Vermiculite modification increases carbon retention and stability of rice straw biochar at different carbonization temperatures
Yuxue Liu
a,b,
Chengxiang Gao
a,c,
Yuying Wang
a,b,
Lili He
a,b,
Haohao Lu
a,b,
Shengmao Yang a,b,*
aInstitute
of Environment, Resource, Soil and Fertilizer, Zhejiang Academy of
Agricultural Sciences, 298 Desheng Middle Road, Hangzhou 310021, China b
Engineering Research Center of Biochar of Zhejiang Province, Hangzhou 310021,
China c
Institute of Resource and Environment, Northwest Agricultural & Forestry
University, Yangling 712100, Shanxi, China
*Corresponding
author: Shengmao Yang
Tel.: +86 571 8641 9218 Fax: +86 571 8641 9218 E-mail: [email protected]
Word count for the manuscript: 5392 Declarations of interest: none.
1
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1
Abstract: Biochar is considered a promising material for sequestering CO2 from the
2
atmosphere, thus helping to alleviate climate change when returned to the soil.
3
Biochar stability is the most decisive factor determining its C sequestration potential.
4
Mineral modification may improve biochar characteristics, but systematic research on
5
the effect of mineral modification on the C retention and stability of biochar and the
6
associated mechanisms is limited. Therefore, in this study, rice straw was used to
7
produce biochar at various temperatures (300, 400, 500, 600, and 700 °C), with
8
vermiculite
9
thermogravimetric analysis, Fourier transform infrared (FTIR) spectroscopy, X-ray
10
photoelectron spectroscopy (XPS), and nuclear magnetic resonance (NMR)
11
spectroscopy were used to evaluate the effect of vermiculite modification and
12
carbonization temperature on biochar stability. Biochar yield and C retention ratio
13
decreased with increasing temperatures but increased by 13.5–38.8% and 5.2–22.1%,
14
respectively, after vermiculite modification. The ratios of C thermal weight loss,
15
atomic H/C, and C oxidation loss in the biochar were reduced with increasing
16
carbonization temperature, indicating improved thermal, aromatization, and chemical
17
oxidation stability. A trade-off that did not compromise C sequestration potential was
18
optimized at 700 and 600 °C for the unmodified and modified biochar, respectively.
19
Furthermore, the total mineral content of the biochar, particularly Fe, Al, Mg, and Si,
20
were increased by vermiculite modification. FTIR results showed that chemical bonds,
21
such as Si–O–C and Fe–O, were formed or enhanced on the biochar surface after
22
vermiculite modification. This was further certified by the XPS survey spectra. NMR
as
a
modified
mineral
material.
2
Several
methods
including
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23
results indicated that biochar stability was enhanced by increasing the aromatization
24
rate during carbonization, that is, by the conversion of C from alkyl and carbonyl C to
25
aromatic C. This study provides a basis for research into and the development of
26
functional biochar and its application in C sequestration.
27
Keywords: biochar; stability; mineral modification; vermiculite; carbon sequestration
3
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1 Introduction
29
By 2017, human-induced global warming had caused an average global temperature
30
increase of approximately 1 °C compared to pre-industrial levels (IPCC, 2018).
31
Ongoing climate change poses an increasing threat to humanity that is likely to be
32
aggravated if substantial and timely reductions of greenhouse gas emissions are not
33
achieved (Mora et al., 2018). Biochar, as a C-rich byproduct of biomass pyrolysis
34
(Chen et al., 2008; Lehmann, 2007a), is considered a promising material for locking
35
CO2 from the atmosphere and thus alleviating climate change when returned to the
36
soil (Lehmann, 2007b). Furthermore, biochar has multiple functions in soil
37
improvement (Major et al., 2010; Liu et al., 2016), contaminant removal (Ahmad et
38
al., 2014; Zhang et al., 2013) and reducing greenhouse gas emissions (Wang et al.,
39
2019). This is mainly based on two typical characteristics of biochar: one is its high
40
biochemical stability due to its highly aromatic C structure, and the other is its
41
excellent adsorption properties resulting from its developed pore structures and large
42
surface area (Wang et al., 2016). Biochar’s C stability is the key to its long-term
43
environmental functions, such as C fixation and emission reduction (Chandra &
44
Bhattacharya, 2019; Crombie et al., 2013; Ullah et al., 2019). The development of
45
biochar with high C retention and stability is of great theoretical and practical
46
significance for its C sequestration ability.
47
Recent studies have shown that mineral modification has an important effect on
48
improving the functional properties of biochar, such as its adsorption ability (Han et
49
al., 2016; Wang et al., 2015; Wang & Wang, 2019). FeCl3-modified biochar showed a
4
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50
high adsorption capacity for Cr(VI) from aqueous solution, which was 1–2 orders of
51
magnitude higher than that of unmodified biochar from the same feedstock (Han et al.,
52
2016). The As(V) and Pb(II) sorption capacities of MnCl2•4H2O-modified biochar
53
(0.59 and 4.91 g kg−1, respectively) and birnessite-modified biochar (0.91 and 47.05 g
54
kg−1, respectively) were significantly higher than that of unmodified biochar (0.20 and
55
2.35 g kg−1, respectively) (Wang et al., 2015). Therefore, the modification of biochar
56
with mineral materials has great potential to produce functional biochar with
57
improved properties.
58
However, in terms of C fixation, studies on the influence of mineral modification on
59
the C retention and stability of biochar remain limited. Li et al. (2014) found that
60
kaolin and CaCO3 had little effect on the C retention of biochar derived from rice
61
straw, whereas Ca(H2PO4)2 increased the C retention of modified biochar by up to 29%
62
compared to that of unmodified biochar. A recent study showed increased C retention
63
and stability in biochar when Ca(OH)2 was added to the sludge feed; this was
64
attributed to the formation of CaCO3 and increased C-containing functional groups on
65
the biochar surface (Ren et al., 2018). Therefore, different minerals may influence the
66
C stability of biochar, the mechanism of which needs to be further explored.
67
Moreover, the carbonization temperature during biochar production is the most
68
significant factor influencing aromatization and biochar stability (Leng & Huang,
69
2018; Mašek et al., 2013; Zhao et al., 2013). Many studies have found that as the
70
carbonization temperature increases, the content of stable aromatic ring structures
71
indicated by the H/C ratio of biochar increases (Liu et al., 2017; McBeath et al., 2015).
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72
Even so, it is not clear whether the presence of minerals affects the biomass
73
carbonization process and, thus, the stability of biochar. Furthermore, it is noteworthy
74
that biochar with a high stability does not always have a high C retention ratio.
75
Therefore, it is necessary to make a trade-off between the C retention ratio and the
76
stable C content for the research on C sequestration by biochar.
77
In China, large quantities of crop residues including rice/wheat straw are produced
78
annually. Most of the straw is discarded or burned, leading to resource losses and
79
serious air pollution caused by emissions of carbon monoxide, non-methane
80
hydrocarbons, nitrogen oxides, and sulfur dioxide (Wang et al., 2015; Wang et al.,
81
2018). Therefore, the development of a proper straw treatment method is required
82
urgently.
83
As a consequence, rice straw was used as raw material for biochar preparation in this
84
study. Vermiculite was selected as the mineral material to produce modified biochar
85
at serial temperature conditions of 300–700 °C. Several methods, namely
86
thermogravimetric analysis (TGA), scanning electron microscopy (SEM), Fourier
87
transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS),
88
nuclear magnetic resonance (NMR), and X-ray diffraction (XRD) were used to
89
evaluate the effect of vermiculite modification and carbonization temperature on the C
90
retention and stability of biochar. This research provides a science-based
91
underpinning for the production of mineral-modified functional biochar and its
92
application in C sequestration and climate change mitigation.
93
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94
2 Materials and methods
95
2.1 Mineral material and feedstock for biochar
96
Vermiculite ((Mg,Fe,Al)3[(Si,Al)4O10(OH)2]•4H2O; analytical reagents), obtained
97
from a chemical reagent company, was chosen as modifying material in this study.
98
Vermiculite is a natural and non-toxic 2:1 silicate clay mineral with strong cationic
99
exchange capacity and adsorption properties. Therefore, it can be used as a soil
100
amendment to loosen soil and promote crop growth. In addition, vermiculite has the
101
advantages of low cost and environmental friendliness. The biomass raw material for
102
biochar production was rice straw, which was collected from an experimental plot at
103
the Scientific Research Base of Zhejiang Academy of Agricultural Sciences, Jiaxing
104
City, Zhejiang, China (120°24′23′′E, 30°26′07′′N). The rice straw was air-dried to a
105
moisture content of <5% and ground to a particle size <2 mm for later use.
106
2.2 Preparation of modified and unmodified biochar
107
The vermiculite was mixed with rice straw at a ratio of 1:4 (w/w). The mixtures were
108
then placed into a laboratory-scale programmable tubular carbonization reactor
109
(Hangzhou Lantian Instrument Co., Ltd., China) and heated under a vacuum at a
110
heating rate of 15 °C min−1 to reach five settled temperatures (300, 400, 500, 600, and
111
700 °C). At each settled temperature, the heating residence time was 1.5 h. The solid
112
residue in the reactor was vermiculite-modified biochar, which was designated
113
VBC300, VBC400, VBC500, VBC600, and VBC700 according to the respective
114
temperatures. As a control, the rice straw not mixed with vermiculite was used to
115
produce unmodified biochar under the same conditions, designated BC300, BC400,
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BC500, BC600, and BC700, respectively.
117
2.3 Characterization of biochar
118
The characteristics of the vermiculite-modified and unmodified biochar were
119
determined as follows: The biochar yield was calculated by weighing the sample and
120
solid residue before and after pyrolysis and subtracting the contribution of minerals.
121
Y=(Ms–Mm)/Mbm×100
122
where Y is the biochar yield (%), Ms and Mm are the weights of solid and mineral
123
residues after carbonization (g), respectively, and Mbm is the weight of biomass before
124
carbonization (g).
125
R=(Y×Cbc/Cbm)×100%
126
where R is the C retention ratio of biochar (%), and Cbc and Cbm are the C contents of
127
biochar and biomass (%), respectively.
128
Biochar pH was measured at a 1:20 (w/v) solid-to-water ratio by a pH meter
129
(Mettler-Toledo, Switzerland). The C, H, N, and S contents of the biochar were
130
measured by an elemental analyzer (Vario EL/micro cube, Elementar, Germany). The
131
K, Ca, Mg, Al, Si, Fe, and P contents of the biochar were measured by inductively
132
coupled plasma-atomic emission spectrometry. The O content was calculated by
133
subtraction. SEM (instrument: JSM-6700F, Jeol, Japan) was used to compare the
134
surface morphological characteristics of the unmodified and modified biochar. The
135
surface functional groups of biochar were analyzed by FTIR spectroscopy (Varian
136
640-IR, USA) using KBr pellets at 25±1 °C. XPS spectra of the unmodified and
137
modified biochar were recorded with a spectrometer (ESCALAB 250Xi,
(1)
(2)
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138
ThermoFisher, USA) using Al Kα as the X-ray source. The
139
magic angle spinning (CP-MAS) NMR spectra were recorded at a frequency of
140
100.62 MHz using a Varian Unity Inova 400 NMR spectrometer (AVANCEIII 400,
141
Bruker, Switzerland) under the same detecting conditions as described in our previous
142
research work (Liu et al., 2018). The crystalline phases of biochar were identified by
143
their XRD patterns obtained using the same X-ray diffractometer and employing the
144
same conditions as described in Liu et al. (2017).
145
2.4 Measurement of biochar stability
146
The aromatization stability of biochar is represented by its H/C atomic ratio. Biochar
147
heat stability is represented by the thermal weight loss ratio of C in biochar, which
148
was measured by a thermogravimetric analyzer (Q50, TA, USA). Specifically, 5–10
149
mg biochar sample (through a 100-mesh sieve) was weighed in an alumina crucible
150
and heated from an initial temperature of 30 °C to a terminal temperature of 800°C at
151
a heating rate of 10 °C min−1 in a high-purity nitrogen atmosphere. The thermal
152
weight loss ratio of C was calculated according to the mass loss of C in the biochar
153
samples.
154
The chemical stability of biochar was determined by chemical oxidation treatment, in
155
which experimental potassium dichromate (K2Cr2O7) was used to assess the labile
156
fraction of C in the biochar samples. For the K2Cr2O7 treatment, 0.1 g biochar was
157
treated in a glass test tube with 40 mL 0.1 mol L−1 K2Cr2O7 and 2 mol L−1 H2SO4
158
solutions at 55 °C for 60 h. The C loss was calculated according to the difference
159
between the C content of biochar samples before and after K2Cr2O7 oxidation, and the
9
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C oxidation loss ratio was thus expressed as the percentage of C loss to the initial C
161
content.
162
A trade-off between C retention ratio (R) and stable C content of biochar was
163
optimized according to the following formula:
164
Trade-off value = R×(100–C oxidation loss ratio)/100
165
2.5 Data processing
166
Data were processed using Microsoft Excel 2007, and SigmaPlot 10.0 software was
167
used for drawing the figures.
(3)
168 169
3 Results and discussion
170
3.1 Basic physicochemical properties of biochar
171
The elemental compositions and total mineral contents of both vermiculite-modified
172
and unmodified biochar are shown in Fig. 1. The C and total mineral contents of all
173
the biochar increased with increasing temperature, whereas the H and O contents
174
decreased. After vermiculite modification, the C content of biochar decreased from
175
46.8–56.7% to 36.5–39.6%, whereas the total mineral content increased from
176
3.72–5.48% to 13.4–18.9%.
177
(Insert Fig. 1 here)
178 179
The contents of different mineral components of modified and unmodified biochar are
180
shown in Fig. 2. The unmodified rice-straw biochar is rich in mineral components,
181
such as Al, Ca, Fe, K, Mg, P, and Si, with the K content being the highest
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(2.35–3.41%). After vermiculite modification, the Al, Fe, Mg, and Si contents of
183
biochar increased from 0.08–0.25%, 0.11–0.34%, 0.26–0.48%, and 0.21–0.29% to
184
2.66–3.59%, 4.16–5.53%, 2.77–4.14%, and 0.31–0.50%, respectively, and increased
185
with increasing carbonization temperature. This is because vermiculite contains a
186
large number of Al, Fe, Mg, and Si components, which combined with rice straw
187
during carbonization and ultimately retained in biochar. However, the P content of
188
biochar decreased from 0.22–0.64% to 0.14–0.39% after vermiculite modification.
189
(Insert Fig. 2 here)
190 191
The yield, C retention ratio, and pH of modified and unmodified biochar are shown in
192
Table 1. The biochar yield decreased with increasing carbonization temperature. In
193
addition, the biochar yield increased by 13.5–38.8% after vermiculite modification,
194
with the minimum increase occurring at 300–400 °C and the maximum at 500–600 °C.
195
Previous studies have reported similar results: Hossain et al. (2011) found that the
196
yield of sludge biochar decreased from 78 to 73% as the carbonization temperature
197
increased from 400 to 600 °C. Chen et al. (2014) showed that biochar yield decreased
198
from 63.1 to 53.3% with temperature increasing from 500 to 900 °C. The
199
decomposition of organic components in biomass materials with increasing
200
carbonization temperature is the main reason for decreased yields (Yuan et al., 2015).
201
(Insert Table 1 here)
202 203
The C retention ratio of biochar decreased with increasing carbonization temperature.
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At the same temperatures, vermiculite modification increased the C retention ratio of
205
biochar by 5.2–22.1%, indicating that the presence of vermiculite during biomass
206
carbonization can effectively improve the C retention of biochar and reduce C loss
207
during carbonization. Li et al. (2014) found that the modification of kaolin and CaCO3
208
had no effect on the C retention ratio of biochar.
209
With carbonization temperature increasing from 300 to 700 °C, the pH gradually
210
increased from 5.56 to 10.5 for unmodified biochar and from 6.06 to 9.86 for
211
vermiculite-modified biochar (Table 1). The pH of modified biochar was lower than
212
that of unmodified biochar at carbonization temperatures of 400–700 °C but higher at
213
300 °C, indicating that vermiculite modification generally decreased biochar pH,
214
except at 300 °C.
215
3.2 Effects of mineral modification and carbonization temperature on biochar
216
stability
217
3.2.1 TGA
218
The stability of modified and unmodified biochar is shown in Table 1. The thermal
219
weight loss ratio can be used to characterize the thermal stability of biochar. With
220
increasing carbonization temperature, the thermal weight loss ratio of biochar
221
decreased gradually, indicating increased thermal stability. The modified vermiculite
222
reduced the thermal weight loss ratio of biochar by 14.9–45.6%, indicating that
223
vermiculite modification enhanced the thermal stability of biochar. This may be
224
because vermiculite has a high heat-absorption ability and thus protects biochar from
225
thermal decomposition by heat blocking (Kariya et al., 2016).
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3.2.2 H/C atomic ratio
227
The H/C atomic ratio can be used to characterize the degree of C aromatization of
228
biochar, with lower H/C atomic ratios indicating higher aromatization stability. With
229
increasing carbonization temperature, the H/C atomic ratio of biochar gradually
230
decreased, indicating that its aromatic structure gradually increased. Compared with
231
more amorphous C structures in low-temperature biochar, the aromatization stability
232
of high-temperature biochar gradually increased. The H/C atomic ratio of biochar was
233
reduced by 1.4–15.1% after vermiculite modification. This indicated that vermiculite
234
modification enhanced the aromatization stability of biochar. Guo and Chen (2014)
235
found that Si plays an important role in the C arrangement and structure composition
236
of rice-straw biochar. Si and C may combine to form a dense protection structure, thus
237
improving biochar stability (Han et al., 2018).
238
3.2.3 Chemical oxidation
239
The C oxidation loss ratio (K2Cr2O7 or H2O2 method) can be used to characterize the
240
chemical oxidation stability of biochar. Our results showed that with increasing
241
carbonization temperature, the C oxidation loss ratio of biochar (K2Cr2O7 method)
242
gradually decreased, indicating that its chemical oxidation stability improved.
243
Vermiculite modification reduced the C oxidation loss ratio of biochar by 6.80–45.8%,
244
except for biochar produced at 300 °C. This shows that vermiculite can generally
245
improve the chemical oxidation stability of biochar.
246
Li et al. (2014) used CaCO3 and hydroxyapatite to modify biochar and found that the
247
C oxidation loss ratio (H2O2 method) of the modified biochar decreased by 18.6 and
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58.5%, respectively, which may be due to increased aromatic C content and the
249
formation of C–O–P and C–P bonds. Ren et al. (2018) studied the co-pyrolysis of
250
CaOH2 and sludge to produce biochar and found that the C oxidation loss ratio (H2O2
251
method) of the biochar decreased from 31.3 to 9.71% at 300 °C and from 2.15 to 1.32%
252
at 700 °C. The reason may be that mineral modification prompted the formation of
253
CaCO3, which may have prevented oxidants from entering the biochar by means of a
254
physical barrier, thus improving the chemical oxidation stability of biochar; Zhao et al.
255
(2016) found similar results. Yang et al. (2016) modified biochar with soil minerals
256
(such as FeCl3, AlCl3, CaCl2, and kaolin) and found that the C oxidation loss ratio
257
(H2O2 method) of biochar was reduced by 13.4–79.6% compared with that of
258
unmodified biochar. This may be because the minerals act as a physical barrier
259
preventing oxidants from entering the biochar. Moreover, the formation of organic
260
mineral complexes (e.g., Fe–O–C) during the interaction between minerals and
261
biochar may play a protective role.
262
It is worth mentioning that although biochar produced at higher carbonization
263
temperatures achieved higher thermal and oxidative stability, the decreased C
264
retention ratio with increasing temperature (Table 1) indicated lower C sequestration
265
at higher carbonization temperature. In this study, a trade-off between reduced C
266
retention ratio and more stable C content (K2Cr2O7 method) with increasing
267
temperature was optimized at 700°C for the unmodified biochar and at 600 °C for the
268
modified biochar (Trade-off value in Table 1). This was consistent with the results of
269
McBeath et al. (2015), who found that the optimal carbonization temperature was
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270
500–700 °C for most feedstock in a temperature range of 300–900 °C. Our results
271
showed that vermiculite modification reduced the carbonization temperature by
272
100 °C (from 700 to 600 °C) without sacrificing the C sequestration potential of
273
biochar. This indicated that vermiculite can be used as a catalyst in the carbonization
274
process. Therefore, carbonization of rice straw with vermiculite modification could be
275
considered as a cleaner production technology.
276
3.3 SEM
277
SEM was used to determine the morphological characteristics of the modified and
278
unmodified biochar. As shown in Fig. 3, many small particles existed on the surface
279
or in the pores of the modified biochar (Fig. 3B, D, F, H, and J), whereas no or few
280
such particles were found on the unmodified biochar (Fig. 3A, C, E, G, and I). This
281
could be explained as the loading of metals (such as Fe, Mg, and Si) on the surface of
282
the modified biochar due to vermiculite modification.
283
(Insert Fig. 3 here)
284 285
3.4 FTIR
286
The surface functional groups in biochar can be qualitatively analyzed by infrared
287
spectroscopy. The FTIR spectra of the modified and unmodified biochar are presented
288
in Fig. 4. With increasing carbonization temperature, the peak at 3400 cm−1 indicates
289
that the stretching vibration of hydroxyl bonds (–OH) gradually decreased. When the
290
temperature increased to 400–500 °C, the stretching vibration of aliphatic C–H bonds
291
(2950 cm−1) decreased and disappeared at around 600 °C. The stretching vibration of
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C=O bonds (1710 cm−1) disappeared entirely when the temperature exceeded 700 °C.
293
This indicates that the O-containing functional groups of biochar gradually decreased
294
with increasing carbonization temperature (Novak et al., 2009). Intense bands of
295
aromatic C=C bonds (1600 and 1450 cm−1) and aromatization C–H surface bending
296
vibration (810 cm−1) were increasingly observed, indicating the intensification of the
297
dehydrogenation reaction and the enhancement of the biochar aromatization structure
298
with increasing temperature. This is consistent with the decreasing trend in H/C
299
atomic ratio (Table 1). In addition, after vermiculite modification, the vibration of
300
Fe–O bonds on the biochar surface (450 cm−1) was significantly enhanced. The
301
stretching vibration of C–O–C functional groups (1100 cm−1) was weakened and
302
replaced by Si–O–C or Si–O–Si groups (1000 cm−1), indicating that more stable
303
mineral organic complexes were formed on the surface of modified biochar (Guo &
304
Chen, 2014). (Insert Fig. 4 here)
305 306 307
3.5 XPS
308
To verify the FTIR results obtained and described in section 3.4, the XPS spectra of
309
the modified and unmodified biochar at a series of carbonization temperatures were
310
provided. As shown in Fig. 5(A-E), remarkable O 1s and C 1s peaks were observed in
311
the XPS survey spectra of both the modified and unmodified biochar. Furthermore,
312
Mg 1s, Fe 2p, Si 2s, and Si 2p signals were clearly observed for all modified biochar
313
(VBC300, VBC400, VBC500, VBC600, and VBC700), but not obvious for the
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314
unmodified biochar (BC300, BC400, BC500, BC600, and BC700); this indicated that
315
the Mg, Fe, and Si metals were loaded on the vermiculite modified biochar. The XPS
316
survey spectra indicated that the modifications can remarkably alter the properties of
317
the functional groups of biochar, which agreed well with the FTIR spectra presented
318
in Fig. 4. (Insert Fig. 5 here)
319 320 321
3.6 NMR
322
The NMR spectra of modified and unmodified biochar are shown in Fig. 6. The
323
C-containing functional groups of rice-straw biochar mainly comprise aromatic C
324
(165–95 ppm), which has a highly aromatic structure. Compared with
325
high-temperature biochar (600 and 700 °C), low-temperature biochar (300–500 °C)
326
also contains alkyl C (0–90 ppm) and carbonyl C (220–165 ppm) groups. These
327
results show that the aromatization degree of biochar increased with increasing
328
carbonization temperature. Previous studies have indicated that the macromolecule
329
content of solid biomass decreased while aromatic rings formed during the initial
330
heating period of carbonization (200–300 °C); small and defective sheets of
331
condensed aromatic rings subsequently stacked up (from >300 to 600 °C) and finally
332
formed turbostratic crystallites (>700 °C) with further temperature increases (Aller,
333
2016; Keiluweit et al., 2010).
334
Furthermore, vermiculite modification enhanced the aromatization rate of biochar
335
during carbonization by increasing the Al, Fe, Mg, and Si contents and other mineral
17
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336
components (Fig.2), that is, the transformation rate of C from alkyl and carbonyl C to
337
aromatic C enhanced biochar stability. The conversion of raw materials into
338
increasingly aromatized structures contributed greatly to the environmental
339
recalcitrance of biochar (Leng & Huang, 2018). (Insert Fig. 6 here)
340 341 342
3.7XRD
343
XRD was used to study the crystalline structures of the minerals present in the biochar
344
(Wang et al., 2016). Typical XRD patterns of the modified and unmodified biochar
345
are shown in Fig. 7. The broad peak located at about 24° in the XRD pattern of
346
biochar represents a typical amorphous C diffraction pattern (Wang et al., 2018). The
347
peaks at 28 and 41° confirmed the presence of sylvite (KCl) in the unmodified
348
rice-straw biochar. The peak at 26.7° was due to quartz (SiO2) in the modified biochar.
349
More peaks were observed in the XRD results of the modified biochar than in those of
350
the unmodified biochar, which revealed that the former contained more mineral
351
components than the latter did. (Insert Fig. 7 here)
352 353 354
4 Conclusions
355
With increasing carbonization temperature, the C content of rice-straw biochar
356
gradually increased, whereas the yield and C retention ratio gradually decreased.
357
Compared with that of the unmodified biochar, the C content of the
18
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358
vermiculite-modified biochar decreased significantly, but the yield and C retention
359
ratio increased significantly. The thermal weight loss, H/C atomic, and C oxidation
360
loss ratios (K2Cr2O7 method) of biochar gradually decreased with increasing
361
temperature, indicating that the thermal, aromatization, and chemical oxidation
362
stability of biochar were enhanced. Vermiculite modification enhanced biochar
363
stability by increasing the content of mineral components, promoting the formation of
364
chemical bonds, such as Si–O and Fe–O, on the biochar surface, and improving the
365
aromatization rate during carbonization. The CO2 released from modified biochar
366
would be less and/or slower than that from unmodified biochar. Furthermore, a
367
trade-off without sacrificing the C sequestration potential of biochar was optimized at
368
700 and 600 °C for the unmodified and modified biochar, respectively, indicating that
369
vermiculite can be used as a catalyst in the carbonization process. Therefore,
370
carbonization of rice straw with vermiculite modification could be considered as a
371
cleaner production technology. This research provides a scientific basis for research
372
on and development of functional biochar and its application in C sequestration and
373
climate change mitigation.
374 375
Acknowledgements
376
This research was financially supported by the National Natural Science Foundation
377
of China (41701334) and Natural Science Foundation of Zhejiang Province
378
(LY20D010005). There are no conflicts of interest to declare.
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Journal Pre-proof Author Contributions
Yuxue Liu: Conceptualization, Methodology, Formal analysis, Writing - Original Draft, Writing - Review & Editing, Project administration, Funding acquisition
Chengxiang Gao: Investigation, Validation, Formal analysis
Yuying Wang: Resources, Methodology, Formal analysis
Lili He: Software, Formal analysis
Haohao Lu: Software, Formal analysis
Shengmao Yang: Resources, Supervision, Funding acquisition
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Declaration of interests ■ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Journal Pre-proof Figure captions Fig. 1 Elemental analysis and total mineral contents of modified and unmodified biochar (BC: unmodified biochar; VBC: vermiculite-modified biochar; 300, 400, 500, 600, and 700 indicate the carbonization temperatures (°C) of both modified and unmodified biochar) Fig. 2 Contents of mineral components of modified and unmodified biochar (BC: unmodified biochar; VBC: vermiculite-modified biochar; 300, 400, 500, 600, and 700 indicate the carbonization temperatures (°C) of both modified and unmodified biochar) Fig. 3 SEM images of modified and unmodified biochar (A, C, E, G, I: unmodified biochar at the carbonization temperatures of 300, 400, 500, 600, and 700, respectively; B, D, F, H, J: modified biochar at the carbonization temperatures of 300, 400, 500, 600, and 700, respectively) Fig. 4 FTIR spectra of modified and unmodified biochar (BC: unmodified biochar; VBC: vermiculite-modified biochar; 300, 400, 500, 600, and 700 indicate the carbonization temperatures (°C) of both biochars) Fig. 5 XPS spectra of modified and unmodified biochar (BC: unmodified biochar; VBC: vermiculite-modified biochar; 300, 400, 500, 600, and 700 indicate the carbonization temperatures (°C) of both biochars) Fig. 6 NMR spectra of modified and unmodified biochar (BC: unmodified biochar; VBC: vermiculite-modified biochar; 300, 400, 500, 600, and 700 indicate the carbonization temperatures (°C) of both biochars) Fig. 7 XRD spectra of modified and unmodified biochar (BC: unmodified biochar; VBC: vermiculite-modified biochar; 300, 400, 500, 600, and 700 indicate the carbonization temperatures (°C) of both biochars)
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Fig. 1
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Fig. 2
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Fig. 3 4
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Fig. 4
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BC300 BC400 BC500 BC600
Intensity
BC700
VBC300
VBC400
VBC500
VBC600 VBC700
10
13 14
20
30
40
50
60
2 (degree)
Fig. 7
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Journal Pre-proof Highlights Biochar was derived from rice straw with vermiculite modification at 300–700 °C Higher C retention ratio and mineral content were obtained after modification Si–O–C and Fe–O bonds were formed/enhanced on biochar surface after modification Biochar stability was enhanced by converting alkyl and carbonyl C to aromatic C Trade-off without sacrificing C sequestration was optimized for modified biochar
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Table 1 Yield, carbon retention ratio, pH, and stability of modified and unmodified biochars Biochar
Yield (%)
Carbon retention
pH
ratio (%)
Carbon thermal
H/C atomic ratio Carbon oxidation Trade-off value
weight loss rate (%)
loss rate (%)
BC300
55.5±0.1
75.6±0.1
5.56±0.03
75.4
1.36±0.06
51.8±0.4
36.4
BC400
53.9±0.1
71.7±0.2
7.04±0.01
68.6
1.16±0.04
48.1±0.6
37.2
BC500
42.7±2.0
62.5±2.7
7.96±0.17
58.7
0.92±0.05
30.4±1.0
43.5
BC600
35.3±0.2
54.0±0.3
10.2±0.05
34.8
0.66±0.03
10.2±0.5
48.5
BC700
34.5±1.8
51.9±1.7
10.5±0.04
32.8
0.60±0.04
2.43±0.1
50.6
VBC300
68.6±0.1
84.7±0.2
6.06±0.06
43.3
1.27±0.05
58.0±1.4
35.6
VBC400
61.2±2.0
75.3±2.7
6.88±0.02
37.3
1.14±0.06
44.9±0.9
41.5
VBC500
52.8±0.6
70.6±0.9
7.85±0.04
38.3
0.91±0.05
24.3±0.4
53.4
VBC600
49.0±0.8
66.0±0.9
9.74±0.04
29.6
0.56±0.02
5.52±0.2
62.4
VBC700
46.6±0.1
63.0±0.1
9.86±0.05
18.6
0.51±0.03
2.03±0.1
61.7
BC: unmodified biochar; VBC: vermiculite-modified biochar; 300, 400, 500, 600, and 700 indicate the carbonization temperatures (°C)