Energies 2020 , 13, x doi: FOR PEER REVIEW www.mdpi.comjournal energies [616747]

Energies 2020 , 13, x; doi: FOR PEER REVIEW www.mdpi.com/journal/ energies
Article 1
Integrated leaching and thermochemical technologies 2
for producing high -value products from rice husk: 3
leaching of rice husk with the aqueous phases of 4
bioliquids 5
Wenran Gao1, Hui Li1, Karnowo1,2*, Bing Song3, Shu Zhang1,* 6
1 Joint International Research Laboratory of Biomass Energy and Materials, College of Materials Science and 7
Engineering, Nanjing Forestry University, Nanjing, 210037, PR China 8
2 Faculty of Engineering, Universitas Negeri Semarang (UNNES), Jawa Tengah, 50229, Indonesia 9
3 Scion, Te Papa Tipu Innovation Park, 49 Sala Street, Private Bag 3020, Rotorua 3046, New Zealand 10
* Correspondence: [anonimizat] (Karnowo); [anonimizat] (Shu Zhang) 11
Received: date; Accepted: date; Published: date 12
Abstract: It remains challenging to develop a techno -economically feasible metho d to remove alkali 13
and alkaline earth metal species (AAEMs) from rice husk (RH) , which is a widely available 14
bioresource across the world. In this study, the AAEMs leaching effect of aqueous phases of both 15
biocrude prepared by hydrothermal liquefaction (AP -HT) and bio -oil prepared by pyrolysis (AP – 16
Pyro) of RH were systematically investigated . The results indicated that although the acidity of AP – 17
HT and AP -Pyro are much lower than that of HCl, they performed a comparable removal efficiency 18
on AAEMs (Na: 56.2%, K: 96.7%, Mg: 91.0%, Ca: 46.1% for AP -HT, while Na: 58.9%, K: 96.9%, Mg: 19
94.0%, Ca: 86.3% for AP -Pyro) with HCl . The presence of phenolics in bio -oil could facilitate the 20
penetration of water and organic acids into the inner area of RH cells, thus enhancing the AAEMs 21
removal via chelate reactions. The thermal stability of leached RH during thermochemical 22
conversions were studi ed via TG and Py -GC-MS. The results showed that the heat conduction 23
efficiency in leached RH was enhanced with a high pyrolysis rate, resulting in a narrow carbon 24
chain distribution (C 5-C10) of derived chemical compounds. 25
Keywords: rice husk; AAEMs; leaching; hydrothermal; pyrolysis; a queous phase; biocrude; bio-oil 26
27
1. Introduction 28
The limited reserves of fossil fuel resources and their high environmental impacts leads to a 29
crucial demand for the development of the new energy sources [1, 2] . Solar, wind and biomass are 30
considered as the main renewable energy sources ，among which biomass is relatively affordable 31
and abundantly available with simple converting processes [3]. Gasification, pyrolysis and 32
hydrothermal liquefaction are the main thermal processes to obtain renewable biofuels, carbon 33
materials and biochemicals from various biomass resources [4-7]. To produce liquid biofuels, either 34
fast pyrolysis or hydrothermal liquefaction is required to liquefy biomass . Briefly, pyrolysis produces 35
bio-oil at 450 -550 ° C by thermal decompo sition of biomass [8], while hydrothermal liquefaction 36
considers converting biomass into bio -crude at 250 -374 ° C under high pressure (4 -22 MPa) [9]. These 37
techniques are capable to achieve liquefaction of feedstocks with a yield of bio -oil/biocrude at ~60%, 38
whereas the quality of these bioliquids are poor with high acidity, high moisture content and low 39
calorific values. To fully use the raw bioliquid more efficiently, fractionation (e.g., distillation, 40
extraction) has been recommended to separate raw products into different fractions (e.g., aqueous 41
phase, hydrophobic light bio -oil, and heavy bio -oil) before further refining [10, 11] . Both the light and 42
heavy fractions can then be upgraded into drop -in fuels via hydrotreatment. In contrast, the aqueous 43

Energies 2020, 13, x FOR PEER REVIEW 2 of 15
phase (rich in water, organic acids and various volatile compounds) is unsuitable to be used as a fuel, 44
and its application needs to b e further investigated. 45
Rice husk (RH) is widely distributed and has potential to be used for bio -oil production [12-16]. 46
According to the statistics in the United States Department of Agriculture, the global rice production 47
hit a record high of 657.9 million tons in 2019, which indicates the production of a large amount of 48
RH as a byproduct [17]. Hence, there is an increasing interest in utilizing RH as a fe edstock for the 49
production of renewable biofuels or other value -added products [18]. However, one of the major 50
challenges of using R H is its high ash content (11.4 –23.5 wt%) [19], which has inh ibited the production 51
of bioliquids from RH. The ash of RH is composed of various elements such as finely dispersed alkali 52
and alkaline earth metal species (AAEMs), which are bound to hydroxyl and/or phenolic groups in 53
the form of cations or exist as catio ns of inorganic salts [19]. These ash contents need to be removed 54
prior to the thermochemical liquefaction of biomass to avoid the inactivation of catalysts, the 55
corrosion of reactors, and the interferenc e of bioliquids upgrading. For instance, during the pyrolysis 56
of biomass, the ash content can cause fouling issues (such as corrosion, erosion and blocking etc.) 57
both inside the reactor and the downstream pipes [20]. Many ash contents exist in the form of chloride 58
salts, th e pyrolysis of which produces chloride acids that is both poisonous and corrosive. The 59
presence of inorganic species can also interfere the further refinery of produced bio -oil/bio -crude [21, 60
22]. Because of the side effects of ash content and the high ash content of RH, it is essential to conduct 61
pretreatment to remove these components. 62
Water leaching is a simple pretreatment that can be easily conducted for the removal of ash 63
content in biomass. The removal efficiency of leaching for AAEMs generally follows an order based 64
on the acidi ty of water: strong acid/water > weak acid/water > water [23-25]. Thus, the use of acids is 65
required to ach ieve a maximal ash removal. However, the side effects of using acids, especially strong 66
inorganic acids (e.g. HCl), are also obvious when considering the economic feasibility and the 67
environment impacts particularly in an industrial scale. The aqueous frac tion of bio -oil has been 68
recently recognized as an effective leaching agent for biomass pretreatment that can improve the 69
pyrolysis performance of pretreated biomass for the production of bio -oil [26]. The study conducted 70
leaching of biomass with the aqueous phase of fast pyrolysis bio -oils and investigated the pyrolysis 71
behavior of treated biomass using TGA/ DTG analysis and Py -GC-MS. However, details of the 72
leaching performances and mechanisms need to be further investigated. Moreover, considering that 73
hydrothermal liquefaction has also been widely used for biomass liquefaction, the aqueous phase of 74
bio-crude is also potential to be used for biomass leaching, whereas its performance on the ash 75
content removal has not been studied. As conceptualized in Figure 1, successful integration of 76
leaching and thermochemical liquefaction (i.e., pyrolysis or hydrothermal liquefaction) is believed to 77
not only achieve a well removal of ash content in the feedstock, but also produce a bio -oil or bio – 78
crude with higher qualities (i.e., lower moisture content, lower acidity, and higher calorific values) 79
by fractionating aqueous phases. In general, it is worthful to investigate the leaching performances 80
of aqueous phases of both biocrude and bio -oil in details prior to integrating leaching by aqueous 81
phases and liquefaction techniques for bioliquid production. 82
This study is conduc ted as a key step to investigate the feasibilities and fundamentals of utilizing 83
the aqueous phases of bio -oil and biocrudes for RH leaching pretreatment. Major contributions 84
include: 1. Preparing bio -oils and bio -crudes via thermochemical techniques and c onducting 85
fractionation of these bioliquids to collect aqueous phases; 2. Conducting leaching of RH using 86
different aqueous phases and investigate the details of ash removal by following the release of 87
AAEMs. 3. Conducting TG analysis and Py -GC/MS analysis to investigate the effects of different 88
leaching solutions on the pyrolytic decomposition of pretreated rice husk. 89
90
91
92
93
94

Energies 2020, 13, x FOR PEER REVIEW 3 of 15
Figure 1. Schematic diagram of the integrated leaching and thermochemical technologies for 95
producing high -value products from rice hus k. 96
2. Materials and Methods 97
2.1 Materials 98
RH samples as received were milled and sieved to obtain sample particle sizes of 300 -450 μm, 99
which w ere further dried in an oven at 105 ° C for 24 h before using. Table 1 shows the proximate and 100
ultimate analysis o f the RH samples. 101
Table 1. The proximate and ultimate analysis of RH sample s. 102
Sample Proximate analysis
(wt%, db) Ultimate analysis
(wt%, daf)
Volatile
matter Fixed
Carbon a Ash C H O a N S
Rice husk 71.95 12.08 15.97 41.28 5.12 53.11 0.42 0.07
db: dry basis; daf: dry and ash free basis; a: by difference (Fixed carbon=100 -Volatile
matter -Ash, O%=100% -C%-H%-N%-S%)
2.2 Preparation of leaching reagents and the leaching process 103
Preparation of aqueous phase of bio -crude by hydrothermal liquefaction: Hydrothermal 104
liquefaction on RH samples was carried out in a 100 mL stirred autoclave as is shown in Figure 2(a). 105
In each run, 3 g of RH samples and 50 mL deionized water were firstly loaded into the reactor. The 106
residual air in the reactor was then remov ed by flowing N 2 for 30 min. In the next step, the reactor 107
was pressurized up to 1 MPa with N 2, and then heated to 250 ° C in 40 min with the stirring speed 108
being 175 rpm. The reactor was then immersed into a water/ice bath for quick cooling. When the 109
reactor was cooled down to room temperature, the gas inside the reactor was released into a fume 110
hood. The remained slurry inside the reactor was separated into the liquid and solid phases by using 111
a filtration system where a filter paper (0.45 μm, PTFE) w as placed in the Buchner funnel connected 112
to the conical flask. The filtering speed of slurry was accelerated by a suction device. The solid on the 113
Buchner funnel was rinsed with 200 mL acetone (>99.5 wt%), till the falling filtrate became colorless. 114
The s olid residue was then collected and dried in an oven at 105 ° C for 12 h to obtain a constant 115
weight to determine the RH conversion rate. The filtrated liquid was precipitated at -5 ° C to further 116
separate the possible remained solid particles and were filte red again with a micro filter (0.25 μm). 117
Then the finally filtered liquid was weighed, which is the obtained aqueous phase of bio -crude by 118
hydrothermal treatment, hereafter referred to as AP -HT. It was stored in a refrigerator at -5 ° C before 119
use. 120

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Preparat ion of aqueous phase of bio -oil by pyrolysis: The slow pyrolysis of RH samples was 121
carried out in a tubular furnace system as is shown in Figure 2(b). During the experiment, N 2 with a 122
flow rate of 200 mL/min was supplied through the reactor. In each trial, 30 g RH samples were pre – 123
loaded into the reactor, followed by heating up from 25 to 110 ° C with a ramping rate of 10 ° C/min 124
(holding for 30 min at 110 ° C), then further heating up to 500 ° C with a same ramping rate (holding 125
for 60 min). The collected liqu id by an ice bath was filtered with a micro filter (0.25 μm), which is the 126
aqueous phase of bio -oil by pyrolysis, hereafter referred to as AP -Pyro. It was also stored in a 127
refrigerator at -5 ° C before usage. 128
129
Figure 2. Schematic diagram of (a) the hydroth ermal reactor system, and (b) the vertical drop -tube 130
reactor system. 131
Leaching process: It should be noted that except AP -HT and AP -Pyro, a typical strong 132
hydrochloric acid solution (1 mol/L, pH=0) was also prepared as leaching agent for comparison. RH 133
samp les were immersed in the leaching agent, and the mixture was stirred continuously for 4 h at 50 134
° C in a water bath. After the leaching process, the slurry was separated into the liquid and solid using 135
the same filtration system described above. Then the so lid was rinsed with deionized water 136
continuously until the pH of the effluent solution was higher than 6.5. It was then dried at 85 ° C for 137
12 h in the oven. The obtained leached RH samples were referred to as HT -RH, Pyro -RH and HCl – 138
RH, respectively. 139
2.3 An alytical methods 140
Chemical compounds in AP -HT and AP -Pyro . The AP -HT and AP -Pyro samples were analyzed 141
by gas chromatography/mass spectrometry (GC/MS, Agilent7890A, USA) with a HP -5MS column 142
(30m×0.25 mm, 0.25 μm film thickness ) after diluted in trichlorom ethane (> 99.8%) with a dilution 143
ratio of 1:1. The mass spectra operated in electron ionization mode at 70 eV and it was obtained in 144
the range of m/z=50 -300 amu. Helium (99.999%) was used as the carrier gas. The oven temperature 145
was kept at 100 ° C for 2 mi n, then raised to 250 ° C with a ramping rate of 15 ° C/min. It was finally 146
kept at 250 ° C for 15 min. The injector temperature was 250 ° C with a split ratio of 30:1. Identification 147
of each compound was performed based on retention time and matched with the mass spectrum in 148
the spectral library (NIST11 library). 149
pH values . The pH values of all leaching agents were measured with a pH electrode (pHS -25, 150
REX, Shanghai, China). Before each test, the instrument was always calibrated with the standard 151
buffer solut ion. The pH electrode was also rinsed with distilled water as well as the tested solution 152
before being immersed into the tested solution for measurement. The pH values were recorded when 153
the reading stabilizes under the constant stirring by a glass rod. 154
Water content . The water content of AP -HT and AP -Pyro samples were investigated with a Karl 155
Fisher titrator (KF -200, Mitsubishi, Japan). Before measurement, a calibration was done by titrating 156

Energies 2020, 13, x FOR PEER REVIEW 5 of 15
six times of pure water in triplicate. For every single test, abo ut 7 mg of the sample was dropped into 157
the titration vessel. Once the liquid was completely dissolved, the titration was started . The weight 158
of water was recorded when potential balance was reached , and the water content was calculated. 159
AAEMs contents . An ash-digestion -ICP-AES method was adopted to measure the AAEMs 160
content of raw and leached RH samples. Briefly, a known amount of the sample was subjected to an 161
ashing process following a previous method [27]. The obtained ash was weighed and digested in a 162
mixture solution of HF and HNO 3 (1 mol : 1 mol, 10 mL : 10 mL) and then solidified by evaporation 163
for 24 h in the fume hood. The solid was then digested by HCl (2 wt%, 20 mL) in a water bath at 90 ° C 164
before being dissolved in deionized water (40 mL). The l iquid was injected into ion chromatography 165
by inductively coupled plasma -atomic emission spectrophotometry (ICP -AES, ZX_2018 Avio200, 166
Perkin Elmer, USA). The removal rate of metallic species is defined by the following equation: 167
Removal efficiency (%)=(1−𝑀2
𝑀1)×100 (1) 168
where M 1 and M 2 present the amount of metallic species in RH before and after leaching (mg/kg 169
of corresponding samples), respectively. The standard deviation for the removal rate was below 2% 170
for each metallic species. 171
Surface functional groups . Functional groups in RH and leached RH were characterized by a 172
FTIR analyser (VERTES 80V, Brooke, Germany). The sample was evenly ground into fine particles in 173
an agate mortar and mixed with the KBr powder uniformly, from which a disc sa mple can be 174
prepared in an infrared tablet presser by holding for 10 s under a pressure of 10 MPa. 175
Pyrolysis behavior of various samples . Thermogravimetric analysis was carried out by 176
STA409PC, Netzsch, Germany. N 2 with high purity (99.99%) with a flow rat e of 100 mL/min was 177
applied as the gas environment in the oven. The temperature was scanned from room temperature 178
up to 900 ° C with a heating rate of 10 ° C/min. 179
Pyrolytic product distribution . The examination on the liquid products from the thermal 180
convers ion of the four samples were conducted in a pyroprobe (Py, CDS Analytical 5200) coupled 181
with GC/MS (Agilent 7890A/5975C) via a transferring line at 285 °C. Briefly, 500 ± 20 μg of sample 182
was loaded into the center of a quartz tube (i.d.=2 mm, length=20 mm) . The sample was then 183
pyrolyzed in Py at 550 ° C for 30 s, and the produced volatiles were rapidly transferred into GC/MS 184
for the analysis. The split ratio of GC/MS was 100:1, and a commercial capillary column (Agilent VF – 185
1701MS, 30 m × 0.25 mm i.d., 0.25 µ m film thickness) was employed for the separation of the 186
components. The temperature in the column increased from 40 ° C (stabilized for 2 min) to 280 ° C 187
(stabilized for 3 min) with a heating rate of 10 ° C/min. The MS was operating in an electron impact 188
mod e at an ionization energy of 70 eV with a scan range of m/z 25 to 500. The interpretation of the 189
mass spectra was mainly based on library search (NIST11, version 2.0) or other references. 190
3. Results 191
3.1 Acidity of AP-HT benchmarking against that of AP -Pyro and their compositions 192
Acidity of leaching reagents is an important parameter in determining its ability to eliminate 193
AAEMs. The pH values of AP -HT, AP -Pyro and HCl were measured as 3.2, 2.7 and 0, respectively. 194
The pH value of AP -HT is higher than AP -Pyro due to the lower content of total organic acid 195
compounds (peak areas of 4.22 ×108 versus 2.54 ×108) and higher water content (Table 2). In addition, 196

Energies 2020, 13, x FOR PEER REVIEW 6 of 15
the high abundance of organic compounds in the solution, such as phenols, could hinder the 197
ionization of acetic acid and consequently increased the pH value of the solution [28]. 198
Table 2. The water content and pH of prepared AP -HT and AP -Pyro . 199
Samples Water content (wt%) pH
AP-HT 98.10 3.24
AP-Pyro 58.66 2.70
GC-MS detectable chemical compounds in AP -HT and AP -Pyro are shown in Table 3. The data 200
shows that the dominant organic acid compounds in AP -HT are n -hexadecanoic acid (2.392%) and 201
octadecanoic acid (2.476%), whereas the amount of acetic acid (20.939%) is the highest in AP -Pyro. 202
Moreover, the relatively high content of ketones is observed in AP -Pyro (24.107% compared to 11.710% 203
in AP -HT). For instance, the relative amount of 2 -Propanone, 1 -hydroxy – is 10.199%, while its 204
abundance in AP -HT is only 1.825%. In addition, phenolics with different contents are observed in 205
both AP -HT and AP -Pyro. For instance, the phenol contents are 0.420 and 1.189% in AP -HT a nd AP – 206
Pyro, respectively. Compared with the total amount of phenolics in AP -Pyro, it is higher in AP -HT 207
(13.106% versus 10.982%). It should be noted that the content of the sugars inside AP -HT (18.983%) 208
is considerably high compared to that in AP -Pyro (9.6 32%). This is due to the existence of β -D- 209
Glucopyranose, 1,6 -anhydro – in AP -HT (17.818% versus 6.329% in AP -Pyro). Lastly, a slightly higher 210
amount of furanics is recorded in AP -Pyro (8.700 % compared to 7.034% in AP -HT). 211
Table 3. The identified chemical compounds in AP -HT and AP -Pyro by GC -MS. 212
Compounds Molecular
formula Relative content (%) in
AP-HT Pyro -HT
Acids
Formic acid CH 2O2 0.136 2.557
Acetic acid C2H4O2 0.399 20.939
Propanoic acid C3H6O2 0.000 1.850
n-Hexadecanoic acid C16H32O2 2.392 0.000
Octadecanoic acid C18H36O2 2.476 0.000
Total 5.402 25.346
Furans
Furfural C5H4O2 1.217 2.835
Benzofuran, 2,3 -dihydro – C8H8O 0.000 1.461
2(5H) -Furanone C4H4O2 1.285 1.143
5-Hydroxymethylfurfural C6H6O3 3.348 0.000
2-Furancarboxaldehyde, 5-methyl – C6H6O2 0.169 0.000
2,4(3H,5H) -Furandione, 3 -methyl – C5H6O3 0.361 0.000
2-Furanmethanol C5H6O2 0.282 3.261
4-Methyl -5H-furan -2-one C5H6O2 0.371 0.000
Total 7.034 8.700
Keton es
2,3-Butanedione C4H6O2 0.000 1.162

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1-Hydroxy -2-butanone C4H8O2 0.000 2.309
2-Propanone, 1 -hydroxy – C3H6O2 1.825 10.119
2-Butanone C4H8O 2.720 0.959
2-Propanone,1 -(acetyloxy) – C5H8O3 0.000 1.544
2-Cyclopenten -1-one C5H6O 0.000 1.048
1,2-Cyclopentanedione C8H12O2 1.375 0.000
Dihydroxyacetone C3H6O3 0.602 0.000
3-Pentanone C5H10O 0.437 0.532
Apocynin C9H10O3 0.579 0.000
2-Cyclopenten -1-one, 2 -hydroxy – C5H6O2 0.000 3.066
2-Cyclopenten -1-one, 2 -hydroxy -3-methyl – C6H8O2 0.567 2.364
2-Cyclopenten -1-one, 2 -hydroxy -3-methyl – C6H8O2 0.742 0.000
2-Propanone, 1-(4-hydroxy -3-methoxyphenyl) – C10H12O3 0.000 1.004
1-Hydroxy -2-pentanone C5H10O2 0.406 0.000
3-Pyrazolidinone, 1,4 -dimethyl C5H10N2O 1.071 0.000
3,6-Nonadecadione C19H36O2 1.101 0.000
3',5'-Dimethoxyacetophenone C10H12O3 0.283 0.000
Total 11.710 24.107
Phenol
Phenol, 2 -methoxy – C7H8O2 2.276 2.967
p-Cresol C7H8O 0.462 0.000
Creosol C8H10O2 2.860 1.919
Phenol C6H6O 0.420 1.189
Phenol, 4 -ethyl -2-methoxy – C9H12O2 0.556 0.671
2-Methoxy -4-vinylphenol C9H10O2 3.038 1.044
trans -Isoeugenol C10H12O2 0.644 0.000
Phenol, 2,6 -dimethoxy – C8H10O3 1.385 3.191
Phenol, 4 -methoxy -3-(methoxymethyl) – C9H12O3 0.589 0.000
Phenol, 2,6 -dimethoxy -4-(2-propenyl) – C11H14O3 0.258 0.000
Ethanone, 1 -(4-hydroxy -3,5-dimethoxyphenyl) – C10H12O4 0.528 0.000
Total 13.016 10.982
Sugars
d-Mannose C6H12O6 0.771 0.000
2,3-Anhydro -d-mannosan C6H8O4 0.000 0.807
1,43,6 -Dianhydro -α-d-glucopyranose C6H8O4 0.394 1.551
d-Glycero -d-galacto -heptose C7H14O7 0.000 0.945
β-D-Glucopyranose, 1,6 -anhydro – C6H10O5 17.818 6.329
Total 18.983 9.632
213
214
215

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3.2 Leaching effect of AP -HT and AP-Pyro benchmarking against that of HCl on AAEMs removal from RH 216
The AAEMs content in RH before and after leaching by the different leaching agents as well as 217
the AAEMs removal efficiency are shown in Figure 3. It can be seen that after leaching by the three 218
types of leaching agents, the AAEMs contents are dramatically decrease, especially for K and Mg . 219
This suggests both AP -HT and AP -Pyro are quite efficient to remove the AAEMs from RH. The 220
removal eff iciency of AP -HT is slightly lower than that AP -Pyro, while that of HCl is the best. This is 221
mainly attributed to their acidity difference. However, AP -HT exhibits almost equally high efficiency 222
on removing K and Mg (90 -95%), despite its lowest acidity. It should be noted that the high content 223
of K in RH could be related to the excess use of K -containing fertilizer in the agricultural sector. 224
Vacuoles in plant cells are the key hosts to accommodate K species [29]. Additionally, Mg2+ is an 225
important component of polyribosomes that can promote the synthesis of vitamins in the cell 226
structure of lignocellulose [30]. Therefore, the majorities of K and Mg may be leached easily through 227
the breakage of cell wall and then dissolved in the acidic aqueous phase. Besides the ion -exchanges 228
between metal ions and H+, previous studies have shown that the phenolics could penetrate into the 229
organic matrix of the RH material, forming hydro phobic interactions as well as hydrogen bonds with 230
macromolecules. This makes the matrix more accessible to acetic acid and water, promoting the 231
leaching of organically bound AAEMs [28]. Meanwhile, the synergistic effect could take place 232
between the acetic acid and typical organic components like furfural, guaiacol, phenol, 233
hydroxyacetone and ethylene glycol in AP -HT and AP -Pyro, which could also increase the removal 234
rate of metallic species [31]. 235
Figure 3. (a) AAEMs concentrations of RH, RH -AP, RH -Pyro and RH -HCl samples and (b) the 236
removal efficiency on AAEMs of AP -HT, AP -Pyro and HCl. RH -AP, RH -Pyro and RH -HCl stand for 237
RH leached by AP -HT, AP -Pyro and HCl; AP -HT and AP -Pyro stands for the aqueous phases of bio – 238
oil prepared by hydrothermal and pyrolysis; RH stands for rice husk; AAEMs stands for alkali and 239
alkaline earth me tal species. 240
On the other hand, the removal efficiencies of Na and Ca are relatively lower than those of K 241
and Mg. It is vital to note that the initial content of Na in RH is very low, so the contents of Na in 242
leached RH are much lower than those of other metals althou gh its removal efficiency is not high. 243
The removal efficiency of Ca by AP -HT is only about 46%, which is the lowest compared to those of 244
AP-Pyro and HCl. This may due to the presence of some non -acid organic components (e.g. 245
hydroxyacetone, guaiacol and ph enol) inside AP -HT, which hindered the removal of Ca [31]. The 246
majority of Ca2+ in the plants is bound to pectin polysaccharides and normally exists in the form of 247
CaCO 3 or Ca silicates, which is insoluble in water or other weak/organic acidic solutions [32]. The 248
strong acidity of HCl could effectively extract the pectin polysaccharides, and thus achieving the high 249
RH RH-HT RH-Pyro RH-HCl0150300450600750315032003250
AAEMs concentration (mg/kg) Na
K
Mg
Ca(a)
Na K Mg Ca020406080100120
Removal efficiency (%) RH-HT
RH-Pyro
RH-HCl(b)

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removal efficiency of Ca [28, 33]. What’s more, chelation bonding between cation and some organic 250
compounds like sugar acid may also promote the leaching of Ca and Mg [34, 35] . 251
To evaluate the possible effects of various leaching agents on the RH matrix and surface property, 252
the ratios of H/C and O/C have been calculated and presented in Table 4. As is shown, the atomic 253
ratios of O/C in the RH decrease after leached by the agents due to the partial extraction of 254
hemicellulose. This is confirmed by the decrease of -OH functionality after the leaching process. In 255
addition, the a cid-promoted dehydration of cellulose also increases the C content. Interestingly, H/C 256
ratio remained almost unchanged, indicating that the aromatic matrix of RH has been least affected. 257
Table 4. The O/C and H/C ratios of rice husk samples. 258
Item RH RH-HT RH-Pyro RH-HCl
O/C 1.57 1.33 1.30 1.17
H/C 0.11 0.12 0.12 0.11
The changes in O -containing functional groups was recorded by FTIR, as is shown in Figure 4. 259
It can be seen that all the spectra have similar patterns, suggesting the “core” of the sample structure 260
is not apparently alter after the acid leaching. Generally, the main bands show declining trends after 261
the acid leaching process, which is in a good agreement with the decreases of O/C ratios for the 262
processed samples. In Figure 4, the band at 34 30 cm-1 is ascribed to –OH functionality from alcohols. 263
This shows the disruption of hydrogen bonds in the chemical compositions of the samples [36]. The 264
peak centered at 1730 cm-1 is corresponding to the C=O stretching vibration and is mainly formed 265
from free carbonyl groups, which are abundantly available in hemicellulose and cellulose stru cture. 266
The next band appeared at 1650 -1510 cm-1 is ascribed to C=C stretching vibrations of aromatic 267
structures, which is produced from lignin [37]. Compared with raw RH, the intensity of the band 268
around 1730 cm-1 and 1650 -1510 cm-1 slightly decline after the acid leaching. This indicates the 269
breaking down of hemicellulose acetyl and uronic ester groups [38]. The band located at 1100 cm-1 is 270
related to C -O/Si -O-Si functional groups, which has almost similar intensities for all the analyzed 271
samples. 272
4000 3500 3000 2500 2000 1500 1000 500
Si-O
460Si-O
895
C-O
Si-O-Si
1096C=C
1517C=C
1630C=O
1730
C-H
2850C-H
2925 RH-HCl
RH-Pyro
RH-HTAbsorbance (a.u.)
Wavenumber ( cm-1)

RHO-H
3430
273
Figure 4. FTIR spectra of RH, RH -AP, RH -Pyro and RH -HCl samples. RH -AP, RH -Pyro and RH -HCl 274
stand for RH leached by AP -HT, AP -Pyro and HCl; AP -HT and AP -Pyro stands for the aqueous 275
phases of bio -oil prepared by hydrothermal and pyrolysis; RH stands for rice husk. 276
277

Energies 2020, 13, x FOR PEER REVIEW 10 of 15
3.3 Reaction pathway of the AAEMs removal from RH by the leaching agents 278
The reaction pathways of AAEMs removal by various acidic leaching agents are proposed and 279
shown in Figure 5. The main components of RH are the lemma and palea. Histochemical staining 280
shows that outer epidermis cells walls with high thickness, lignification and severe silicification, 281
while the walls of lower epidermis cells were not lignified [39]. Therefore, the sp ecial cell wall 282
structure of RH is a key factor affecting the removal efficiency of AAEMs. It is well known that RH 283
is a good adsorbent for the removal of heavy metals, phenols, pesticides, and dyes [40]. In the pickling 284
process, RH can also adsorb a certain amount of organic matters from the leaching agent. The 285
adsorbed organic species can disturb the original hydrogen bonds and leads to the swelling 286
phenomena in cells [28, 41, 42] . Therefore, the process of acid leaching in this study involves both 287
physical and chemical effects on the removal efficiency of AAEMs. 288
Figure 5. Mechanistic diagram of AAEMs removal by acids. AAEMs stands for alkali and alkaline 289
earth metal species. 290
The high acidity of the leaching agent could result in the sever changes in the physical properties 291
of biomass materials [25]. The H+ can promote the leaching of hemicellulose and the hydrolysis of 292
cellulose, which r educes the crystallinity as well as loosen the cell wall structure of RH and break 293
down the lemma. Consequently, the AAEMs can be released from the cell structure in RH. 294
For HCl solution, the removal of AAEMs is mainly depend on the action of H+ ions. The high 295
concentration of H+ can destroy the cell wall of RH and replace the cations in the polysaccharides -M 296
(M; K+, Na+, Ca2+ or Mg2+). This also promotes the decomposition of polysaccharides to 297
monosaccharides with disassociated AAEMs. At low pH values, th e functional groups on the surface 298
of the adsorbent are prone to protonation, which limits the adsorption of positive ions like AAEMs 299
on the surface of RH [26]. In the case of high pH values, the organic compounds in the solution have 300
important effects on the removal efficiency of AAEMs via the chelation reactions. Also the weak 301
organic acids with oxygen func tionalities present in AP -Pyro can remove AAEMs from the macro 302
organic complex inside the cells by ion exchanging process [31]. 303
3.4 Effects of AP -HT leaching on the pyrolysis characteristics and the liquid products 304
Thermal Gravimetric (TG) and derivative thermogravimetric (DTG) curves of raw and leached 305
RH samples are presented in Figure 6, while the corresponding characteristic parameters for each 306
curve are calcula ted and shown in Table 5. The pyrolysis characteristic parameters are defined as 307
follows: the mass loss at the pyrolysis temperature of 900 ° C was denoted as m l; the maximum mass 308
loss rate was called DTG max; the temperature of corresponding maximum loss ra te was named T max; 309
the initial decomposition temperature was denoted as T i. 310

Energies 2020, 13, x FOR PEER REVIEW 11 of 15
Figure 6. TGA (a) and DTG (b) of RH, RH -AP, RH -Pyro and RH -HCl samples. RH -AP, RH -Pyro and 311
RH-HCl stand for RH leached by AP -HT, AP -Pyro and HCl; AP -HT and AP -Pyro stands for the 312
aqueous phases of bio -oil prepared by hydrothermal and pyrolysis; RH stands for rice husk. 313
Table 5. Pyro lysis characteristic parameters of raw and pretreated rice husk samples. 314
Samples m1 (wt.%) DTGmax (wt.%/min) Tmax (℃) Ti (℃)
RH 61.8 6.1 339.5 270.2
HT-RH 67.2 7.6 355.8 294.9
Pyro -RH 64.9 7.8 358.5 279.6
HCl-RH 66.1 9.1 347.3 301.8
Clearly, the treatments by the leaching agents impact the pyrolysis behavior of RH sample. The 315
ml of RH, RH -HT, RH -Pyro and RH -HCl are 61.8%, 67.2%, 64.9% and 66.1%, respectively. 316
Additionally, the thermal stability of the samples improves after removing the AAEMs species, which 317
is conducive to the rapid pyrolysis of raw materials to produce bio -oil w ith a high purity and high 318
yield [43, 44] . A rapid weight loss is observed for RH between 270 -376 ℃, whereas it is between 319
279.6 -384 ° C for the leached RH samples. The presence of the AAEMs could significantly reduce the 320
initial temperature of rapid weight loss of cellulo se[32]. After leaching treatment, the "acromion" 321
peaks of DTG are decline due to the reduction in hemicellulose which is partially extracted by 322
leaching process [45]. The DTG max of RH -HT, RH -Pyro and RH -HCl are increase to 7.6%, 7.8% and 323
9.1% from 6.1% of RH, respectively. AAEMs play important roles in the cross -linked stru ctures 324
among lignin, cellulose/hemicelluloses structures [46] as well as in affecting the primary and 325
secondary r eactions of large organic fragments during pyrolysis. The partial removal of 326
hemicellulose, as indicated by the FTIR analysis and the TG curves, means that the cell wall of RH is 327
partially destroyed, and thus the heat conduction efficiency in leached RH is enhanced with a high 328
pyrolysis rate. In addition, the acid leaching also obviously changes the relative intensities of the 329
decomposition ranges, implying the variation in product distributions, which is further discussed 330
below. 331
The components in the bio-oil are divided into eleven groups including acids, ketones, phenols, 332
aldehydes, furans, esters, N -containing compounds, sugars, alcohol, alkanes and others. Their 333
relative abundances are shown in Figure 7(a). Compared to the relatively low content of sugar in the 334
bio-oil produced from RH (7.14 %), it is the dominant compound in all the other bio -oils produced 335
from leached RH samples. For instance, the sugar contents in the bio -oil obtained from RH -HT, RH – 336
Pyro and RH -HCl are 32.52%, 39.54% and 41.78%, r espectively. In all the bio -oil samples, acids (4.24 – 337
0 100 200 300 400 500 600 700 800 90030405060708090100110 TG (%)
Tempature ( C)

RH
RH-HT
RH-Pyro
RH-HCl(a)
0 100 200 300 400 500 600 700 800 9000246810
(b)DTG (%/ C)

Tempature ( C) RH
RH-HT
RH-Pyro
RH-HCl

Energies 2020, 13, x FOR PEER REVIEW 12 of 15
10.0%), ketones (5.96 -13.77%), phenols (8.02 -19.36%), furans (8.32 -13.3%), N -containing compounds 338
(14.52 -16.30%), esters (4.32 -7.28%) and aldehydes (7.03 -11.86%), alcohol and alkanes (both less than 339
4%) are present at various extents. In comparison to the bio -oil produced from RH, the bio -oil 340
obtained from all the leached samples has the lower contents of acids, ketones, phenols and furans. 341
Moreover, levoglucosan is the predominant anhydrosugar product, w hich is mainly derived from 342
the cellulose. The relative content of levoglucosan in the bio -oil from RH -HT, RH -Pyro and RH -HCl 343
are 9, 14 and 15 times higher in comparison with its content in the bio -oil from RH. This is due to the 344
high removal rate of K, wh ich has a distinguishable catalytic effect on the suppression of 345
levoglucosan formation [47]. 346
Figure 7. Distribution of (a) chemical compounds with different functional groups and (b) different 347
carbon atom number s in bio -oil. RH -AP, RH -Pyro and RH -HCl stand for RH leached by AP -HT, AP – 348
Pyro and HCl; AP -HT and AP -Pyro stands for the aqueous phases of bio -oil prepared by 349
hydrothermal and pyrolysis; RH stands for rice husk. 350
The distribution of the different groups of chemicals and the number of carbon atoms (C 2-C12+) 351
are shown in Figure 7(b). It should be noted that C 6 compounds are mainly related to levoglucosan 352
and levoglucosenone, while C 7–C10 compounds are ascribed to lignin derivatives. The C 11+ 353
compounds, which a re from the incomplete cracking of lignin, only account for a small proportion in 354
the bio -oils. The results indicate that the compounds in the range of C 6 and C 9 have the highest 355
concentration in the volatiles (57 -60%) obtained from the pyrolysis of the le ached RH. This shows 356
that acid leaching improves the quality of bio -oil. Correspondingly, both the relative contents of low 357
molecular weight compounds (C 3-C5) and high molecular weight compounds (C 10-C12+) in the bio -oil 358
decrease after acid leaching of RH. The removal of inorganic species from biomass sample has 359
weaken the catalytic secondary polymerization reactions [48]. 360
4 Conclusions 361
The performances of liqu id phases obtained from the fractionation of both bio -oil and biocrude 362
for the leaching of ash contents in RH have been investigated. Both AP -HT and AP -Pyro showed a 363
similar removal of AAEM species (especially for K and Mg) which is comparable to that of H Cl 364
solution, although their acidity is lower than that of HCl solution. Furthermore, the chelation 365
reactions and ion exchanges are primary responsible pathways for AAEMs eliminations by the 366
organics in the AP of bio -oils, which is favored by the physical s welling in cells induced from the 367
phenolic compounds. With the removal of AAEMs, the thermal stability of RH is obviously improved, 368
C2 C3 C4 C5 C6 C7 C8 C9C10 C11C12+051015202530354045
Relative content ( area% ) RH
RH-HT
RH-Pyro
RH-HCl(b)
Acids
KetonesPhenol
AldehydesN-containingSugars Furans EstersAlcohol AlkanesOthers051015202530354045
Relative content (area%) RH
RH-HT
RH-Pyro
RH-HCl(a)

Energies 2020, 13, x FOR PEER REVIEW 13 of 15
and the pyrolysis reactions are concentrated, which leads to the narrow distributions (C 5-C10) of 369
derived chemical compounds under pyrolytic conditions . 370
Author Contributions: Conceptualization, karnowo karnowo; Formal analysis, Wenran Gao and Bing Song ; 371
Funding acquisition, Shu Zhang; Investigation, Wenran Gao and Hui Li; Methodology, Hui Li, karnowo 372
karnowo and Shu Zhang ; Project administration, Shu Zhang; Resources, Shu Zhang; Supervision, karnowo 373
karnowo and Shu Zhang; Validation, Hui Li, Bing Song and Shu Zhang; Writing – original draft, Wenran Gao ; 374
Writing – review & editing, Bing Song and Shu Zhang . 375
Funding: This work was financially supported by the National Natural Science Foundation of China (Grants 376
51876093); An internationally collaborative project (BRICS2019 -040) under BRICS STI Framework Programme 377
with government funding organizations of Brazil CNPq (40 2849/2019 -1), Russia RFBR (19 -58-80016), India DST 378
(CRG/2018/004610, DST/TDT/TDP -011/2017), China MOST (2018YFE0183600), and South Africa NRF 379
(BRIC190321424123). 380
Conflicts of Interest: The authors declare no conflict of interest. 381
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