materials
Article
The Effect of Pyrolysis Temperature and the Source Biomass on
the Properties of Biochar Produced for the Agronomical
Applications as the Soil Conditioner
Michal Kalina * , Sarka Sovova, Jiri Svec, Monika Trudicova, Jan Hajzler, Leona Kubikova and Vojtech Enev
Faculty of Chemistry, Brno University of Technology, Purkynova 464/118, 61200 Brno, Czech Republic
* Correspondence: kalina-m@fch.vut.cz; Tel.: +420-541-149-398
Citation: Kalina, M.; Sovova, S.; Svec,
J.; Trudicova, M.; Hajzler, J.;
Kubikova, L.; Enev, V. The Effect of
Abstract: Biochar is a versatile carbon-rich organic material originating from pyrolyzed biomass
residues that possess the potential to stabilize organic carbon in the soil, improve soil fertility and
water retention, and enhance plant growth. For the utilization of biochar as a soil conditioner, the
mutual interconnection of the physicochemical properties of biochar with the production conditions
used during the pyrolysis (temperature, heating rate, residence time) and the role of the origin of used
biomass seem to be crucial. The aim of the research was focused on a comparison of the properties of
biochar samples (originated from oat brans, mixed woodcut, corn residues and commercial compost)
produced at different temperatures (400–700 ◦ C) and different residence times (10 and 60 min). The
results indicated similar structural features of produced biochar samples; nevertheless, the original
biomass showed differences in physicochemical properties. The morphological and structural analysis
showed well-developed aromatic porous structures for biochar samples originated from oat brans,
mixed woodcut and corn residues. The higher pyrolysis temperature resulted in lower yields;
however, it provided products with higher content of organic carbon and a more developed surface
area. The lignocellulose biomass with higher contents of lignin is an attractive feedstock material for
the production of biochar with potential agricultural applications.
Pyrolysis Temperature and the
Source Biomass on the Properties of
Keywords: agriculture; biochar; biomass; pyrolysis; temperature; soil conditioner
Biochar Produced for the
Agronomical Applications as the Soil
Conditioner. Materials 2022, 15, 8855.
https://doi.org/10.3390/ma15248855
Academic Editors: Jianbin Zhou and
HuanHuan Ma
Received: 24 October 2022
Accepted: 9 December 2022
Published: 12 December 2022
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affiliations.
Copyright: © 2022 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
1. Introduction
Biochar is a porous, carbonaceous material that is produced by the thermal decomposition of carbon-rich biomass materials in the absence of air. This ubiquitous black organic
material attracts numerous scientific groups around the world due to its hyper-versatile
utilization in agriculture, environmental engineering and basic chemical technologies [1,2].
The European Biochar Certification [3] defines as highly attractive those utilizations where
biochar is applied in a way in which the contained carbon remains stored as a long-term
carbon sink (e.g., utilization in agriculture as the soil conditioner) or replaces fossil carbon
in industrial manufacturing (e.g., metallurgy [4]). The attractive chemical properties of
biochar produced for agronomical utilization as a soil supplement are associated with a
positive effect on a reduction in the total amount of greenhouse gasses released from the
soil decreased leaching of macro- (P, N, Mg, Ca, K) and micronutrients (Al, Fe, Cu, Cr, Mn,
Ni, Zn), plant health and growth enhancement, reduction in the mobility of various heavy
metals (e.g., As, Cu, Pb, Cr, Cd) and positive effects on the soil microorganism communities [1,5]. The possible agronomical potential of biochar is also closely linked with its
physicochemical properties, morphology and texture [6]. The idea of biochar utilization in
agriculture has the historical consequence as the direct proof of the positive effect of biochar
on soil properties is a Hortic anthrosol of the Amazon (Terra preta) [7]. This unusually fertile
black soil contains a large amount of pyrogenic material (biochar) originating from ancient
soil management practices, which persisted in the soil for centuries.
4.0/).
Materials 2022, 15, 8855. https://doi.org/10.3390/ma15248855
https://www.mdpi.com/journal/materials
Materials 2022, 15, 8855
2 of 13
The crucial biochar physicochemical properties and structural characteristics are
closely linked to the conditions used during the pyrolysis of biomass feedstocks (temperature of pyrolysis, heating rate, pressure, type of used carrier gas and its flow rate and
residence time), the origin of the used source biomass material (the content of cellulose,
hemicellulose or lignin), the residual biomass moisture and its particle (grain) size [8]. Generally, the temperatures used for the production of biochar samples vary between 200 and
700 ◦ C [1]; for the production of biochar samples with potential agricultural applications,
temperatures between 450 and 600 ◦ C are generally accepted as the optimal [9]. High
temperature applied to biomass materials transforms the organic matter in biomass into
three different components—gas (e.g., methane and other hydrocarbons), liquids (formed
by condensation of gases) and solids (charcoal or biochar). The high temperature induces
the polymerization of molecules, which leads to the formation of relatively large structures
(including both aromatic and aliphatic compounds), as well as the thermal decomposition
of some components of the feedstock material into smaller molecules [1]. The increase
in pyrolysis temperatures results in the production of biochar with increased persistence
due to resistance to microbial and chemical decomposition in soil [10]. The residence time,
according to the literature [8], also affects biochar yield and its internal porous structure due
to the repolymerization of vaporized molecules, which can take place at higher residence
times, resulting in a minor increase in biochar yield and a greater extent of the internal
porous structure development. Moreover, the characteristics of biochar and its yield can
also be affected by the selection of carrier gas (often used nitrogen, argon or water vapor),
its flow rate and a selection of pyrolysis type [8,11].
The second crucial aspect affecting biochar production is the origin of a biomass
feedstock material [2], which can vary from wood-based products (e.g., woodcut, woodchip,
sawdust, wooden pellets and tree bark) to plant-based materials (e.g., wheat straw, leaves,
husks, seeds and cobs) and organic wastes materials (e.g., manure and sludges) [12,13].
In total, the wide variety of potential biomass feedstock materials, together with the
opportunity to utilize a huge extent of possible pyrolysis production conditions, enable the
production of biochar samples with rather variable properties of produced material [14].
The wooden-based feedstock materials generate more resistant biochar with total carbon
contents of up to 80%. On the other side, biomass with high lignin content (e.g., olive husk,
woodcut, grass, vegetable) produces biochar with even higher carbon content. Therefore, at
the same temperature, lignin loss of material due to pyrolysis is less than half of cellulose
loss [13]. Besides organic carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur can
also be identified in the structure of biochar, and their presence can significantly contribute
to the heterogeneity of biochar surface and its reactivity [15]. During the pyrolysis process,
volatile compounds of biomass such as functional groups containing O, H and S are
volatilized, which leads to the accumulation of the non-volatile elements such as C and
partially also N, P, K, Ca, Mg, Fe, etc. The stability is closely linked to the O/C ratio [12],
which can vary in the range from 0.05 to 0.7. The content of other nutrients depends on the
origin of the used feedstock biomass (lignin-based vs. cellulose-based). Biomass moisture
content and feedstock particle (grain) size represent additional important parameters
of biochar production as the biomass feedstock can contain either internal water vapor,
chemically bound water in pores or free liquid water, which all in total can represent up
to 60 wt.% of total mass [8]. Pyrolysis of biomass containing various types of internal
moisture results in a decrease in heat transfer effectivity as the fraction of the heat energy
is connected with moisture evaporation. For these reasons, the addition of the biomass
drying step can contribute to a significant reduction in the necessary heat energy and also
lower the necessary production time and production costs.
The literature provides a wide variety of published research dealing with the optimization of biochar application dosage and the corresponding agronomical impacts on soil
and plant growth and yields [1,2,5,15,16]. Kocsis et al. [16] defined the application dose of
biochar between 1 and 2.5 wt.% of dry soil (corresponding dosage between 20 and 50 t/ha
of soil) as the optimal for the support of yields (22% yield increase in a model plant—
Materials 2022, 15, 8855
3 of 13
Zea Mays). Moreover, these application doses have a positive effect on soil pH, cation and
nutrition retention (mainly K; P; and partially Mg, Na and N) and soil microbial activity.
These findings are supported by other published research [17–19], where the authors concluded regardless of the well-described positive impact of biochar on the horticultural and
agricultural systems, the application doses above 20 t/ha are too expensive (considering
the production costs). The production of biochar and its utilization as a soil conditioner
is connected with the carbon neutrality assessment. Verheijen et al. [2] pointed out that
the role of biochar is also connected with its effects on C cycling and mobility in soil. The
global flux of C from soil to atmosphere in the form of CO2 is approximately 60 Gt of C per
year, mainly as the result of microbial respiration (microbial decomposition of soil organic
matter). Components of biochar introduced into the soil provide a considerably more stable
form of organic structures compared to soil organic matter. This results in so-called carbon
negativity of the process, as carbon inputs into the soil through the application of biochar
as the stable C source are increased greatly compared to its output through soil microbial
respiration. Lehman et al. [1,7] described the potential of a global C-sequestration of 0.16 Gt
per year through the use of current biomass waste materials from forestry and agriculture
and urban wastes for biochar production.
Despite the positive effects of biochar on soil properties and soil fertility, the biggest
issue obstructing its broader agrochemical utilization is nowadays still hidden in the lack of
description of possible routes how soil and soil organic matter properties can be affected by
different types of biochar. Additionally, the mutual interconnection of the effect of biomass
feedstock selection and the setting of optimal pyrolysis conditions on the properties of
produced biochar is not fully described. Both these agronomically important aspects
guiding the properties of produced biochar are addressed in the present manuscript. The
main aim of our research is to focus on a description of the effect of source biomass origin
(woody, non-woody, commercially composted biowaste) as well as the corresponding
effect of pyrolysis conditions (temperature, residence time) on resulting biochar properties.
The attention is paid mainly to the properties of biochar, which are significant for the
agronomical utilization of this ubiquitous black carbon-rich material.
2. Materials and Methods
2.1. Materials
The biomass feedstocks for biochar production originated from oat brans (Mlyny
Vozenilek, spol. s.r.o., Predmerice, Czech Republic), mixed woodcut (Central composting
plant of Brno, SUEZ CZ, a.s., Brno, Czech Republic), corn residues (residual biomass
from other research at Faculty of Chemistry, Brno University of Technology, Brno, Czech
Republic) and commercial compost (Black Dragon, Central composting plant of Brno, SUEZ
CZ, a.s., Czech Republic). All these biomasses belong to waste materials, which fulfill the
recent action plans of the European Commission for the maximal use of natural resources
without undesirable wastes.
2.2. Methods
2.2.1. Pyrolysis of Biomass Feedstocks
The biomass samples (oat brans, mixed woodcut, corn residues and commercial compost) were air-dried at 45 ◦ C (to reduce the residual biomass moisture), homogenized
using an electric chopper (R-5115, Rohnson, Vassilias, Greece) and dry-sieved (size fraction
below 2 mm). The pre-treated biomass samples were subjected to pyrolysis under a driven
oxygen-free atmosphere using a laboratory furnace with vacuum and variable controlled
atmosphere of pure gases CLASIC 501 (Classic CZ s.r.o., Revnice, Czech Republic) with
integrated advanced PLC controller Vision 280 (Unitronics GmbH, Esslingen, Germany).
Samples were evacuated in the furnace to clear off the oxygen, and pyrolysis was conducted
under a nitrogen atmosphere (flow of 5 dm3 /min) with a temperature gradient of 5 ◦ C/min
up to final temperatures of 400, 500, 600 and 700 ◦ C. The furnace temperature is maintained
according to the program by molybdenum disilicide heating elements controlled by cali-
Materials 2022, 15, 8855
4 of 13
brated thermocouple and the furnace computer. The inert nitrogen atmosphere flow assists
with the heat distribution in the furnace. The target pyrolysis temperature was kept in the
operating furnace for 10 and 60 min (residence time). After cooling still in nitrogen below
200 ◦ C, produced biochar samples removed from the furnace were cooled in a desiccator,
weighted and stored in airtight glass vessels.
2.2.2. Thermogravimetry
The content of moisture, organic matter and inorganic ash in analyzed samples was
determined by thermogravimetric analyzer Q5000 (TA Instruments, New Castel, DE, USA).
A total of 5 mg of each biochar sample was weighed into a platinum pan, which was
subsequently heated with the used heating rate 10 ◦ C/min from ambient temperature to a
temperature of 800 ◦ C (air atmosphere). The content of sample moisture (weight difference
at 110 ◦ C), the content of organic matter (the weight difference between 110 ◦ C and 800 ◦ C)
and the content of ash (the weight at 800 ◦ C) were determined from the temperature
dependence of residual weight.
2.2.3. Elemental Analysis
CHNS/O analyzer EA 3000 (Euro Vector, Pavia, Italy) was used for the analysis of
basic organic elements content (C, O, H, N) in the individual analyzed samples. An amount
of 0.5–1.0 mg of the sample was weighed into a tin capsule packed and combusted in the
analyzer at 980 ◦ C (oxygen—a combustion gas; helium—a carrier gas). The calibration of
the precise determination of carbon (C), hydrogen (H), nitrogen (N) and sulfur (S) content
was controlled using a sulphanilamide standard. The content of oxygen was calculated
using the total determined content of organic matter obtained by TGA analysis.
2.2.4. Conductivity and pH of Aqueous Extract
A total of 1 g of each dried and milled biochar sample was dispersed in 10 mL of
demineralized water. After 1 h of shaking, pH was measured directly in the suspension.
The samples from the pH measurements were filtered through 0.45 µm syringe filters
(nylon membrane), and conductivity was measured in obtained filtered samples.
2.2.5. Morphological Characterization
Scanning electron microscopy (SEM) was used for the visualization of the internal
structure of biochar samples. The selected parts of the biochar specimen were gold-coated
in a sputtering device and analyzed by a scanning electron microscope ZEISS EVO LS 10
(secondary electrons mode, accelerating voltage of 5 kV). The specific surface area (SSA) of
biochar samples was determined by a specific surface analyzer Nova 2200e (Quantachrome
Instruments, Florida, CA, USA), using the Brunauer–Emmett–Teller (BET) analysis. The
samples were degassed for 24 h at 200 ◦ C, and the measurement of the SSA was performed
in an inert atmosphere (nitrogen) at a temperature of liquid nitrogen (77.3 K).
2.2.6. Water Holding Capacity
The water holding capacity (WHC) of produced biochar samples was determined by
the standard EBC method [3]. The determination of WHC was based on drop-by-drop
addition of water on the defined weight of milled and dried biochar sample (placed on
the filtering pater into the Petri dish) until it reached the point of water-saturation (the
surplus water is dried with filtering paper). The saturated biochar sample was weighed
and subsequently air-dried at 105 ◦ C in the oven for 24 h. The dried biochar samples were
weighed, and WHC capacity was calculated from the weight difference between wet and
dried biochar.
2.2.7. FTIR Spectrometry
The structural features of the analyzed sample were characterized by the Fouriertransform infrared (FTIR) spectrometry using the attenuated total reflectance (ATR) tech-
Materials 2022, 15, 8855
5 of 13
nique with the single reflection germanium ATR crystal. The individual FTIR spectra were
recorded by Nicolet iS50 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). The
used spectral range was 4000–600 cm−1 , and the resolution was 8 cm−1 . A background
spectrum was collected from the clean, dry surface of the ATR crystal in an ambient atmosphere. The absorption spectra shown in the manuscript represent the average of 128 scans
and are presented with no further processing (baseline or ATR corrections).
2.2.8. Statistical Analysis
The experimental data were statistically processed using a Dean Dixon Q-test for the
identification and rejection of outliers (significance level α ≤ 0.05) in Statistica® 13.3 software
(TIBCO, Palo Alto, CA, USA). The individual data shown in the manuscript are in the form
of averaged values ± SD.
3. Results
The experimental work was focused on the assessment of the mutual interconnection
of the origin of used biomass feedstock (oat brans, mixed woodcut, corn residues and
commercial compost) and the pyrolysis conditions (temperature, residence time) on yield
and physicochemical properties of produced biochar samples. All these aspects, according
to the literature [20], strongly influence the distinctive characteristics of biochar. The proper
selection of the optimal production feedstock materials and pyrolysis conditions can access
the possibility to tune the final properties of biochar according to the need of the application.
In our research, the attention was focused on the agronomically important properties of
biochar, such as its physicochemical properties, the main structural motives, specific surface
area and water-holding capacity. The possible utilization in agriculture as a soil supplement
is subsequently also discussed with respect to obtained experimental characteristics.
3.1. Biomass Characterization
The biomass feedstocks were characterized by their content of organic matter and
organic elements—C, H and N. The results indicate the highest content of organic matter
for cellulose-based biomass materials—oat brans (93.7 wt.%) and corn residual biomass
(87.4 wt.%), followed by lignocellulose-based woodcut biomass (85.4 wt.%) to the lowest
content for commercial compost sample (34.4 wt.%). Similarly, the content of organic
carbon was highest for oat brans (43.7 wt.%), followed by woodcut (40.7 wt.%) and corn
residual biomass (40.3 wt.%), with the lowest value for compost biomass (13 wt.%).
The ATR- FTIR spectrometry was used for the structural characterization of used
biomass feedstocks. The results shown in Figure 1 indicate several common spectral
features [10]. The broad absorption band between 3600 and 3200 cm−1 corresponds to the
O–H stretching of moisture (water molecules) connected to the intramolecular hydrogen
bond. It can be found in spectra of all biomass feedstock, and the most pronounced is in the
case of commercial compost. The bands between 3000 and 2700 cm−1 can be assigned to
asymmetric and symmetric C–H stretching in methylene groups. The spectra of woodcut,
oat brans and corn biomass indicate a more significant content of aliphatic C–H structure
compared to the sample of compost. The spectral band between 1800 and 1700 cm–1 can be
assigned to the vibration of C = O; the vibration between 1700 and 1600 cm–1 originates
from the symmetric C = C stretching in aromatic moieties and a region 1600–1500 cm–1
is related to the C–C vibration in aromatics. The vibration bands at 1300–1200 cm−1 and
1150–1050 cm−1 correspond to the mixture structures of alkyl/aromatic ethers. The bands
at 1080 ± 10 cm−1 and 1050 ± 10 cm−1 can be associated with the asymmetric C–O–C and
C–C–O stretching in mixture ethers, and the spectral region at 1020 cm−1 can be assigned
to the vibration of the aliphatic planar hydrocarbon chains (e.g., cellulose).
Materials 2022, 15, x FOR PEER REVIEW
Materials 2022, 15, 8855
6 of 14
1050 cm−1 correspond to the mixture structures of alkyl/aromatic ethers. The bands at 1080
± 10 cm−1 and 1050 ± 10 cm−1 can be associated with the asymmetric C–O–C and C–C–O
stretching in mixture ethers, and the spectral region at 1020 cm−1 can be assigned to the
vibration of the aliphatic planar hydrocarbon chains (e.g., cellulose).
Figure
1. FTIR
spectra
of the individual
biomass feedstocks.
Figure
1. FTIR
spectra
of the individual
biomass
6 of 13
feedstocks.
3.2.
Biochar
YieldYield
3.2.
Biochar
The observed yields of produced biochar samples were in the range of 25–37 wt.%
The observed
yields offeedstock
produced
biochar
samples
were
inresidual
the range of 25–37 wt.%
for individual
lignocellulose-based
materials
(woodcut,
oat brans,
corn
for
individual
lignocellulose-based
feedstock
materials
(woodcut,
oat
brans, corn residual
biomass) depending on the used pyrolysis temperature and residence time (Table 1).
The
obtained
biochar
yields, ason
well
as used
the calculated
matter
loss during pyrolysis
of lignocelbiomass)
depending
the
pyrolysis
temperature
and residence
time (Table 1). The oblulose-based
non-woody
feedstock
are comparable
with
published
[21]. of lignocellulosetained biochar
yields,
as wellmaterials,
as the calculated
matter
loss
duringdata
pyrolysis
The biochar samples originating from lignocellulose biomass feedstocks contained high
based non-woody feedstock materials, are comparable with published data [21]. The
ratios of organic matter (>77.2 wt.%) and organic carbon (>62.5 wt.%). These results are in
biochar
samples
frominlignocellulose
biomass
contained high ratios
good
agreement
with originating
the data published
the literature [10,14].
Beuschfeedstocks
[22] defined the
most
crucial biochar
effects
on soil
in itsand
potential
to increase
the(>62.5
soil organic
matter
and results are in good
of organic
matter
(>77.2
wt.%)
organic
carbon
wt.%).
These
soil
organic carbon
content.
The observed
trends
between
our produced
biochar
samples
agreement
with
the data
published
in the
literature
[10,14].
Beusch
[22] defined the most
are straightforward, the increase in pyrolysis temperatures resulted in a decrease in biocrucial biochar effects on soil in its potential to increase the soil organic matter and soil
char yield and content of organic matter in the product but significantly increased the
organic
carbon
content.
Theinobserved
trends
between
our produced
biochar samples are
residual
content
of organic
carbon
a product, which
is highly
desirable.
The most sigstraightforward,
thetemperature
increase in
pyrolysis
temperatures
resulted
a decrease in biochar
nificant
effect of pyrolysis
was
observed for
oat brans and corn
biomass.inBoth
these
materials
represent
on in
cellulose
and hemicellulose.
A less signifiyield
and content
offeedstock
organic based
matter
the product
but significantly
increased the residual
cant
effect was
observed carbon
in the case
woodcut
These findings
content
of organic
in ofa lignin-based
product, which
is feedstock.
highly desirable.
The most significant
can be explained by the different temperature stability of cellulose, hemicellulose and ligeffect
of
pyrolysis
temperature
was
observed
for
oat
brans
and
corn
biomass. Both these
nin as the main constituents of organic matter of biomass. The less stable hemicellulose
materials
represent
feedstock
based
on
cellulose
and
hemicellulose.
degradation occurs at temperatures 220–315 °C, cellulose degradation temperature lies at A less significant
315–400
lignin isin
more
has a more heterogeneous
structure. This
effect °C,
waswhile
observed
thecomplex
case ofand
lignin-based
woodcut feedstock.
These findings can be
results
in its broad
degradation
range
(160–900
[21]. A decrease
in uti- and lignin as the
explained
by the
different temperature
temperature
stability
of°C)
cellulose,
hemicellulose
lized pyrolysis residence time showed a positive effect on the yield of biochar as well as
main constituents of organic matter of biomass. The less stable hemicellulose degradation
on the content of organic matter and organic
carbon in the product in the case of oat brans
◦
occurs
at temperatures
220–315 ◦feedstock).
C, cellulose
degradation
temperature
and
corn biomass
(mostly cellulose-based
The determined
content
of C, H and lies at 315–400 C,
lignin iswith
more
and has[23].
a more
heterogeneous
structure.
Nwhile
are comparable
the complex
published literature
The biochar
samples produced
from This results in its
oat
bransdegradation
reflected highertemperature
content of N, which
is attractive
agronomical
applications,
broad
range
(160–900for◦ C)
[21]. A decrease
in utilized pyrolysis
asresidence
N is one oftime
the most
important
microelements.
Our
results
agreement
showed
a positive
effect on
the
yieldareofinbiochar
as with
well as on the content
Beusch [22], who defines agronomically highly attractive biochar samples produced from
of organic matter and organic carbon in the product in the case of oat brans and corn
lignin-rich feedstocks, which often tend to have higher carbon content and more aromatic
biomass (mostly cellulose-based feedstock). The determined content of C, H and N are
comparable with the published literature [23]. The biochar samples produced from oat
brans reflected higher content of N, which is attractive for agronomical applications, as N
is one of the most important microelements. Our results are in agreement with Beusch [22],
who defines agronomically highly attractive biochar samples produced from lignin-rich
feedstocks, which often tend to have higher carbon content and more aromatic structure.
These materials applied to the soil reflect significantly higher long-term stability, higher
porosity and SSA, lower pH and cation-exchange capacity.
The value of the H/C atomic ratio (calculated from the molar concentration of both
elements) is the indication of the extent of biomass-to-biochar conversion. The EBC certification [3] defines the atomic ratio between H and C (must be below 0.7) as the crucial
parameter for the utilization of biochar in agriculture as soil conditioners. This condition
was achieved for all the biochar samples produced above 500 ◦ C from lignocellulose-based
feedstocks (oat barns, mixed woodcut, corn residual biomass), which, together with the
Materials 2022, 15, 8855
7 of 13
physicochemical properties and observed SSA (further discussed in Section 3.3), is the
indication of the high agronomical potential of these biochar samples.
Table 1. Summary of yields of individual prepared biochar samples, their content of organic matter
(determined by TGA) and organic elements (determined by EA).
Pyrolysis
Biomass Feedstock
Yield
(wt. %)
Organic
Matter
(wt. %)
C
(wt.%)
H
(wt.%)
N
(wt.%)
H/C
(–) *
Temp.
(◦ C)
Time
(min)
oat brans
700
600
600
500
400
60
10
60
60
60
26.6
29.7
24.7
31.3
33.0
77.2
79.6
77.2
76.5
81.3
72.7 ± 0.8
82.4 ± 0.9
72.8 ± 0.5
70.6 ± 0.4
67.8 ± 0.7
0.4 ± 0.1
2.8 ± 0.2
1.6 ± 0.2
3.0 ± 0.1
5.2 ± 0.2
3.7 ± 0.2
4.4 ± 0.3
3.6 ± 0.3
4.5 ± 0.1
4.7 ± 0.1
0.057
0.398
0.257
0.508
0.908
mixed woodcut
700
600
600
500
400
60
10
60
60
60
25.8
31.4
30.1
28.8
30.8
94.2
56.4
89.0
95.5
95.2
88.6 ± 0.3
92.5 ± 0.7
87.7 ± 0.1
87.9 ± 0.6
77.3 ± 0.8
0.6 ± 0.1
4.1 ± 0.3
2.6 ± 0.3
4.5 ± 0.3
5.5 ± 0.1
1.2 ± 0.1
1.4 ± 0.2
1.1 ± 0.1
1.3 ± 0.1
1.5 ± 0.1
0.085
0.533
0.347
0.611
0.851
corn residues
700
600
600
500
400
60
10
60
60
60
33.3
34.5
32.9
35.6
37.3
81.0
83.9
83.8
82.9
79.6
78.8 ± 0.4
75.7 ± 0.4
79.1 ± 0.6
75.2 ± 0.7
62.5 ± 0.8
0.5 ± 0.1
2.6 ± 0.2
0.6 ± 0.2
3.3 ± 0.2
4.7 ± 0.2
1.1 ± 0.1
0.8 ± 0.1
0.4 ± 0.1
1.4 ± 0.1
1.5 ± 0.2
0.076
0.405
0.096
0.520
0.903
commercial
compost
700
600
600
500
400
60
10
60
60
60
72.2
76.7
63.1
79.0
81.8
22.4
16.3
20.6
24.7
26.7
18.7 ± 0.3
14.0 ± 0.4
14.9 ± 0.6
19.9 ± 2.2
17.8 ± 0.7
1.0 ± 0.3
0.8 ± 0.1
0.6 ± 0.3
0.2 ± 0.1
0.3 ± 0.1
0.9 ± 0.1
0.4 ± 0.1
0.4 ± 0.1
1.5 ± 0.1
1.5 ± 0.1
0.658
0.698
0.504
0.108
0.195
* H/C ratio was determined using a molar concentration of C and H in samples.
The results in Table 1 indicate that all the biochar samples produced above 500 ◦ C
(for the residence time of 60 min) showed comparable contents of organic matter and the
organic elements (C, H, N), and the effect of pyrolysis temperature was minimal. A compost
sample provided biochar with a significantly lower content of organic matter and organic
carbon, even at high pyrolysis temperatures. The yields of biochar produced by pyrolysis
of compost feedstock were significantly lower due to the lower initial content of organic
matter in the original feedstock biomass, which caused minor losses by the degradation of
volatile organic fractions [21]. On the other hand, the produced biochar samples originating
from compost feedstock also contained the lowest ratios of organic matter and organic
carbon. The residual matter after the pyrolysis consisted of inorganic ash (73.3–83.7 wt.%
in dry matter). The H/C ratio of biochar produced from commercial compost biomass was
below 0.7, which, together with the observed low content of organic carbon in the sample,
are not optimal characteristics for agronomical applications as the soil supplement.
3.3. Biochar Characterization
The conductivity and pH of biochar represent parameters characterizing its agronomical potential as they both describe the content and character of available macro- and
micro-elements available in biochar. These results correlate with the pyrolysis temperature and also with the type of lignocellulose feedstock [10,14]. All the prepared biochar
samples had a pH above 7 (Table 2). The alkaline nature has increased with pyrolysis
temperature [24,25]; most alkaline samples originated from corn and compost. The biochar
originating from corn feedstock represents extremely interesting material for agricultural
applications, as it possesses the highest conductivity of extract, which, together with the
higher pH response, is an indication of the presence of inorganic ash impurities with
Materials 2022, 15, 8855
8 of 13
alkaline pH response (salts of alkaline metals). These salts of alkaline cations such as K, Na,
Ca and Mg could represent important macronutrients introduced into the soil after biochar
application. These macronutrients could provide important nutrition for the various soil
components (microorganisms, plants) at the initial stages after biochar application as the
soil conditioner. The high pH of biochar originating from commercial compost is connected
with the presence of inorganic impurities even in original feedstock biomass. The high conductivity of biochar, together with its neutral or slightly alkaline pH response (pH between
7 and 10), is important for the agronomical application as these parameters guarantee
the higher content of nutrients, which are accessible for plants after biochar application
into the soil [10,26]. On the other hand, the low conductivity of biochar originating from
woodcuts is not optimal for agricultural applications. Veksha et al. [27] defined co-pyrolysis
of wooden biomass in a mixture with additional biomass feedstocks (e.g., corn wastes)
as a possible way to overcome undesirable properties originating from wooden biomass,
increase biochar yield and maintain the well-developed internal microporous structures.
Table 2. pH and conductivity of aqueous extracts, specific surface area (SSA) and water holding
capacity (WHC) of biochar samples produced from the individual biomass feedstocks at various
pyrolysis conditions (temperature, residence time).
Pyrolysis
pH
(–)
Conductivity
(mS/m)
SSA
(m2 /g)
WHC
(wt.%)
60
10
60
60
60
8.62 ± 0.11
8.08 ± 0.18
8.34 ± 0.09
8.48 ± 0.21
8.33 ± 0.12
98 ± 8
95 ± 5
80 ± 8
102 ± 11
89 ± 6
–
0.58 ± 0.07
2.05 ± 0.24
–
–
–
32.1 ± 1.3
35.2 ± 0.8
–
–
700
600
600
500
400
60
10
60
60
60
8.01 ± 0.08
8.20 ± 0.16
7.88 ± 0.04
7.69 ± 0.06
7.59 ± 0.05
26 ± 2
38 ± 4
39 ± 7
38 ± 7
31 ± 6
220.41 ± 9.97
7.19 ± 2.49
58.30 ± 5.41
14.54 ± 3.00
3.54 ± 0.56
56.2 ± 1.6
42.1 ± 2.1
49.8 ± 0.9
45.6 ± 1.8
38.9 ± 3.2
corn residues
700
600
600
500
400
60
10
60
60
60
9.51 ± 0.14
9.49 ± 0.12
9.66 ± 0.09
9.89 ± 0.11
9.72 ± 0.09
596 ± 18
702 ± 11
852 ± 21
553 ± 13
615 ± 22
–
8.95 ± 3.73
27.90 ± 3.11
–
–
–
36.4 ± 1.8
42.6 ± 1.7
–
–
compost
700
600
600
500
400
60
10
60
60
60
10.64 ± 0.11
10.52 ± 0.13
10.11 ± 0.14
10.42 ± 0.16
10.22 ± 0.08
196 ± 8
198 ± 9
314 ± 7
182 ± 13
222 ± 11
–
3.11 ± 1.13
14.30 ± 2.18
–
–
–
19.0 ± 2.1
22.3 ± 1.5
–
–
Feedstock Material
Temp.
(◦ C)
Time
(min)
oat brans
700
600
600
500
400
mixed
woodcut
The effect of the biomass feedstock origin and the conditions of pyrolysis on the
corresponding morphological characteristics of produced biochar was studied using SEM
visualization of the internal structure together with the measurement of specific surface
area (SSA) by BET analysis (Table 2). The development of internal surface and porosity
is crucial for the potential agronomical application of biochar as these characteristics can
directly influence possible interaction with soil nutrients, pollutants and/or water holding
and ion-change capacity [6,28]. Amalina et al. [20] described the direct connection between
the extent of SSA development of biochar with its agronomical potential. The increase in
SSA results in a decrease in the mobility of heavy metals and organic contaminants. The
corresponding neutralization of soil pH, together with a boost of cation exchange capacity,
can result in increased soil fertility. The importance of SSA of biochar was also confirmed
by other published research [1,2,22], where the wood-based biochar sample reflected the
greatest SSA, while crop and grass-based exhibited the greatest ion-exchange capacity.
The comparison of SEM images (Figure 2) of biochar samples originating from the
different feedstocks indicated the most developed porous internal structure for biochar
samples prepared from woodcut and corn biomass. These results correlate well with the
observed highest SSA as well as with data published in the literature [12,29,30], where the
most developed internal structure was identified for biochar samples from lignocellulose
Materials 2022, 15, 8855
SSA results in a decrease in the mobility of heavy metals and organic contaminants. The
corresponding neutralization of soil pH, together with a boost of cation exchange capacity,
can result in increased soil fertility. The importance of SSA of biochar was also confirmed
by other published research [1,2,22], where the wood-based biochar sample reflected the
greatest SSA, while crop and grass-based exhibited the greatest ion-exchange capacity.
The comparison of SEM images (Figure 2) of biochar samples originating from the
9 of 13
different feedstocks indicated the most developed porous internal structure for biochar
samples prepared from woodcut and corn biomass. These results correlate well with the
observed highest SSA as well as with data published in the literature [12,29,30], where the
most developed internal structure was identified for biochar samples from lignocellulose
biomass
withwith
a more
significant
oflignin
lignin(woody
(woody
biomass,
woodcut,
plant residues,
biomass
a more
significantcontent
content of
biomass,
woodcut,
plant resisoft-plant
stems and
straws,
etc.).etc.).
dues, soft-plant
stems
and straws,
Figure 2. SEM images (magnification 1000×) of biochar produced at 600 °C (residence time 60 min)
◦C (residence time 60 min) from
Figure
2. individual
SEM images
(magnification
1000×
) of
biochar
produced
600 residual
from
biomass
feedstocks—(A)
oat
brans;
(B) woodcut;
(C)atcorn
biomass; (D)
commercial
compost.
individual biomass feedstocks—(A) oat brans; (B) woodcut; (C) corn residual biomass; (D) commercial compost.
Biochar originating from oat brans showed rod-like structures preserved from the
original cellulose microfibrils. The extent of porous internal structures of oat brands
is significantly less developed compared to biochar samples from woodcut and corn
biomass. The biochar samples originating from compost showed particle-like structures
with limited development of internal structure, which can be explained by a high ratio of
inorganic structures present in the original feedstock material. The effect of the pyrolysis
temperature increase was limited in the case of oat brans and compost but more significant
for biochar samples originating from woodcut (Figure 3) and corn biomass feedstocks.
These conclusions are in agreement with the published literature [30,31]. The great extent
of internal porous structure development was defined by Ma et al. [30] as a sign visible in
biochar samples produced from woody biomass (lignin-based) at pyrolysis temperatures
above 600 ◦ C. Production of biochar samples at such conditions results in materials with
maintained original microcellular morphology of their feedstock with a large extent of pores
development formed as the result of hemicellulose and cellulose moieties degradation.
The cellulose-based biomass (non-woody plant residues) are on the other side, tending to
provide biochar products with spherical microparticles with significantly less or almost
no apparent pores on the surface as the structure is not maintained due to the presence of
relatively temperature-stable lignin structures [14,29,30].
The differences in the extent of development of the internal structure of biochar
samples originating from different biomass feedstocks observed by SEM (Figure 2) correlate
well with the determined values of SSA (shown for selected biochar samples in Table 2).
The lignin-based woodcut biochar showed the highest SSA due to the presence of more
temperature-resistant lignin structures, followed by corn residues-based and commercial
Materials 2022, 15, x FOR PEER REVIEW
Materials 2022, 15, 8855
10 of 14
Biochar originating from oat brans showed rod-like structures preserved from the
10 of 13
original cellulose microfibrils. The extent of porous internal structures of oat brands is
significantly less developed compared to biochar samples from woodcut and corn biomass. The biochar samples originating from compost showed particle-like structures with
limited development of internal structure, which can be explained by a high ratio of inorcompost-based
biochar
The
lowest
extentThe
ofeffect
SSAofwas
observed
ganic structures
present samples.
in the original
feedstock
material.
the pyrolysis
tem-for biochar
perature
increase
was limited in the
of oatSimilar
brans and
compost regarding
but more significant
produced
from
cellulose-based
oatcase
brans.
findings
the effect of used
for biochar
samples
originating
from woodcut
(Figure 3) and
corn biomass feedstocks.
biomass
feedstock
(woody
lignin-based
vs. non-woody
cellulose-based)
on the determined
These conclusions are in agreement with the published literature [30,31]. The great extent
values
of
SSA
can
be
found
in
the
literature
[30,31].
For
all
the
studied
samples,
the increase
of internal porous structure development was defined by Ma et al. [30] as a sign visible in
in pyrolysis
residence
time
increased
SSA,
as
the
organic
matter
degradation
biochar samples produced from woody biomass (lignin-based) at pyrolysis temperatures processes
wereabove
more600
pronounced.
The
resultssamples
of BETatanalysis
also indicated
a strongwith
effect of used
°C. Production
of biochar
such conditions
results in materials
maintained
original
microcellular
morphology
of
their
feedstock
with
a
large
extent
of
pyrolysis temperature on determined SSA, mainly for the woodcut-based biochar
samples.
pores
development
formed
as
the
result
of
hemicellulose
and
cellulose
moieties
degradaThese findings are in good agreement with results published by Chen et al. [32], who
tion. The cellulose-based biomass (non-woody plant residues) are on the other side, tenddescribed the strong Gaussian dependence of SSA on the pyrolysis temperature for the
ing to provide biochar products with spherical microparticles with significantly less or
lignocellulose
materials,
with
thesurface
maximal
SSA
formed
at maintained
pyrolysis due
temperatures
in the
almost no apparent
pores
on the
as the
structure
is not
to the
rangepresence
between
600 andtemperature-stable
800 ◦ C.
of relatively
lignin structures [14,29,30].
Figure 3. SEM visualization (magnification 1000×) of the pyrolysis temperature effect (residence
Figure
3. 60
SEM
visualization
(magnification
×) originating
of the pyrolysis
temperature
effect (residence time
time
min)
on the morphology
of produced1000
biochar
from corn
residual biomass—(A)
400on
°C;the
(B) 500
°C; (C) 600 °C;
(D) 700 °C. biochar originating from corn residual biomass—(A) 400 ◦ C;
60 min)
morphology
of produced
(B) 500 ◦ C; (C) 600 ◦ C; (D) 700 ◦ C.
The determined values of WHC (Table 2) indicate a strong correlation with the result
obtained in the morphological analysis of produced biochar samples. As far as the development of the internal porous structure of biochar is guided by the pyrolysis conditions
(mainly pyrolysis temperature), it is not surprising that we also found a direct connection
to determined WHC [29]. The highest WTC was determined for biochar samples produced
from mixed woodcut, followed by corn residues and commercial compost. Our results are
comparable with data published by Werdin et al. [33] for biochar samples produced from
woody biomass at different pyrolysis temperatures. The authors also found a strong correlation with the density of used wooden biomass. Suo et al. [34] compared the WHC of biochar
samples originating from a wide variety of biomass feedstock, and they summarized that
besides woody biomass, corn residues-based biochar also showed high WHC. The observed
values of WHC, together with well-developed SSA and porosity of both woodcut and corn
biomass feedstock-based biochar samples created from these materials suitable candidates
Materials 2022, 15, 8855
11 of 13
for agrochemical applications as soil supplements as this specific biochar property can
significantly improve soil water holding capacity mainly in the case of soil samples with
lower content of soil organic matter [22,26].
The results of ATR-FTIR spectrometry shown in Figure 4 indicate several common
spectral features, which were interpreted according to the data published in the literature [9,12]. The first characteristic absorption, observed between wavenumber 3600 and
3200 cm−1 , corresponds to the O–H stretching of moisture (water molecules) connected
to the intramolecular hydrogen bond. This broadband (originally present in all biomass
feedstocks—Figure 1) was significantly decreasing in its intensity as the pyrolysis temperature increased, which is the indication of O–containing functional groups degradation.
The increase in pyrolysis temperature influenced the intensity of the bands between 3000
and 2700 cm−1 (asymmetric and symmetric C–H stretching in methylene groups), as the
aliphatic structural motives of the original biomass feedstocks (mainly as a part of cellulose
and hemicellulose structures) decomposed at higher pyrolysis temperature. The differences
in FTIR spectra are also visible at wavenumbers 1800–1700 cm–1 (vibration of C = O in
carboxylates), 1700–1600 cm–1 (symmetric C = C stretching in aromatic moieties) and region
1600–1500 cm–1 (C–C vibration in aromatic structures), where again a decrease in the intensity of bands was observed for biochar samples produced at higher pyrolysis temperatures.
The presence of aromatic rings and/or inorganic carbonates was confirmed by the intensive
band at 1415 ± 10 cm−1 in all samples. The intensive vibration bands at 1300–1200 cm−1
and 1150–1050 cm−1 correspond to the mixture structures of alkyl/aromatic ethers. The
bands at 1080 ± 10 cm−1 and 1050 ± 10 cm−1 , significant mainly in spectra of biochar
originating from woodcut and compost, belong to the asymmetric
C–O–C and C–C–O
Materials 2022, 15, x FOR PEER REVIEW
12 of 14
stretching in mixture ethers.
Figure 4. FTIR spectra of biochar samples: (A) originated from different feedstocks prepared at 500
Figure
4. time
FTIR
spectra
of biochar
samples:
(A)
originated
different feedstocks prepared at
°C
(residence
60 min);
(B) at different
pyrolysis
temperatures
and
residence timefrom
originating
◦ Cbrans.
from
500 oat
(residence time 60 min); (B) at different pyrolysis temperatures and residence time originating
from
oat brans.
4.
Conclusions
The results of our research have confirmed the necessity to select the optimal pyrolysis conditions (temperature, residence time) and the appropriate type of biomass feedstock used as the crucial parameters affecting the physicochemical properties of produced
biochar samples. The highest yields of produced biochar (in the range between 25 and 37
wt.% of original biomass) were achieved for lignocellulose-based feedstock materials
(woodcut, oat brans and corn residual biomass). The lignocellulose biomass feedstock materials containing higher ratios of lignin (woodcut and corn residual biomass) provided
Materials 2022, 15, 8855
12 of 13
4. Conclusions
The results of our research have confirmed the necessity to select the optimal pyrolysis
conditions (temperature, residence time) and the appropriate type of biomass feedstock
used as the crucial parameters affecting the physicochemical properties of produced biochar
samples. The highest yields of produced biochar (in the range between 25 and 37 wt.% of
original biomass) were achieved for lignocellulose-based feedstock materials (woodcut,
oat brans and corn residual biomass). The lignocellulose biomass feedstock materials
containing higher ratios of lignin (woodcut and corn residual biomass) provided biochar
samples with well-developed aromatic porous structures and optimal content of organic
carbon. A sample of commercial compost with a major content of inorganic structural
moieties was proved to be inappropriate due to the low yields of produced material and its
inappropriate properties (low level of internal porous structure development, low content
of organic carbon). The increase in the pyrolysis temperature resulted in slightly lower
yields of produced biochar; however, it provided products with higher content of organic
carbon, a more developed internal structure, and also more resistance to chemical and
microbial degradation. The feedstock materials with a higher ratio of lignin content in
the original biomass (woodcut) and feedstocks originating from agricultural corn residues
were identified as suitable source materials for the production of biochar with potential
agricultural applications. Biochar samples produced from these feedstocks at pyrolysis
temperatures above 500 ◦ C showed neutral or slightly alkaline pH response and welldeveloped internal porous structures, which, as a consequence, also reflected in high water
holding capacity. These conditions, together with the high content of pyrolyzed organic
carbon, represent crucial parameters considering the possible agronomical utilization of
biochar as the soil supplement. On the other hand, biochar produced from woody biomass
showed low conductivity of aqueous extract, which could be overcome by co-pyrolysis
of wooden-based biomass with additional feedstock having a higher content of inorganic
mineral moieties in the structure (e.g., corn waste biomass, oat brans).
Author Contributions: Conceptualization, M.K.; methodology, M.K., J.S. and V.E.; investigation, S.S.,
J.S., L.K., M.T., J.H. and V.E.; writing—original draft preparation, M.K., S.S., J.S. and V.E.; writing—
review and editing, M.K., S.S., J.S. and V.E.; supervision, M.K.; project administration, M.K.; funding
acquisition, M.K. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the Czech Science Foundation of the Czech Republic by grant
number GJ20-28208Y.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
Lehmann, J.; Rillig, M.C.; Thies, J.; Masiello, C.A.; Hockaday, W.C.; Crowley, D. Biochar effects on soil biota—A review. Soil Biol.
Biochem. 2011, 43, 1812–1836. [CrossRef]
Verheijen, F.; Jeffery, S.; Bastos, A.; Velde, M.; Diafas, I. Biochar Application to Soils—A Critical Scientific Review of Effects on Soil
Properties; Processes and Functions; European Commission: Luxembourg, 2010.
EBC. European Biochar Certificate-Guidelines for a Sustainable Production of Biochar (2012–2022); Version 10.0; European Biochar
Foundation (EBC): Arbaz, Switzerland. Available online: http://european-biochar.org (accessed on 1 January 2022).
Supriya, A.; Samantray, R.; Mishra, S.C. Biochar, a substitute for fossil fuels. IOP Conf. Ser. Mater. Sci. Eng. 2017, 178, 012022.
[CrossRef]
Kumar, A.; Joseph, S.; Tsechansky, L.; Schreiter, I.J.; Schüth, C.; Taherysoosavi, S.; Mitchell, D.R.G.; Graber, E.R. Mechanistic
evaluation of biochar potential for plant growth promotion and alleviation of chromium-induced phytotoxicity in Ficus elastica.
Chemosphere 2020, 243, 125332. [CrossRef] [PubMed]
Kalina, M.; Sovova, S.; Hajzler, J.; Kubikova, L.; Trudicova, M.; Smilek, J.; Enev, V. Biochar Texture—A Parameter Influencing
Physicochemical Properties, Morphology, and Agronomical Potential. Agronomy 2022, 12, 1768. [CrossRef]
Lehmann, J. A handful of carbon. Nature 2007, 447, 143–144. [CrossRef]
Tripathi, M.; Sahu, J.N.; Ganesan, P. Effect of process parameters on production of biochar from biomass waste through pyrolysis:
A review. Renew. Sust. Energ. Rev. 2016, 55, 467–481. [CrossRef]
Ding, Y.; Liu, Y.; Liu, S.; Li, Z.; Tan, X.; Huang, X.; Zeng, G.; Zhou, L.; Zheng, B. Biochar to improve soil fertility. A review. Agron.
Sustain. Dev. 2016, 36, 36. [CrossRef]
Materials 2022, 15, 8855
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
13 of 13
Ali, L.; Palamanit, A.; Techato, K.; Ullah, A.; Chowdhury, M.S.; Phoungthong, K. Characteristics of Biochars Derived from the
Pyrolysis and Co-Pyrolysis of Rubberwood Sawdust and Sewage Sludge for Further Applications. Sustainability 2022, 14, 3829.
[CrossRef]
Zhang, H.; Xiao, R.; Huang, H.; Xiao, G. Comparison of non-catalytic and catalytic fast pyrolysis of corncob in a fluidized bed
reactor. Bioresour. Technol. 2009, 100, 1428–1434. [CrossRef]
Li, J.; Yu, G.; Pan, L.; Li, C.; You, F.; Xie, S.; Wang, Y.; Ma, J.; Shang, X. Study of ciprofloxacin removal by biochar obtained from
used tea leaves. J. Environ. Sci. 2018, 73, 20–30. [CrossRef]
Yin, Q.; Liu, M.; Ren, H. Biochar produced from the co-pyrolysis of sewage sludge and walnut shell for ammonium and phosphate
adsorption from water. J. Environ. Manage. 2019, 249, 109410. [CrossRef] [PubMed]
Tomczyk, A.; Sokołowska, Z.; Boguta, P. Biochar physicochemical properties: Pyrolysis temperature and feedstock kind effects.
Rev. Environ. Sci. Biotechnol. 2020, 19, 191–215. [CrossRef]
Qian, T.-T.; Wu, P.; Qin, Q.-Y.; Huang, Y.-N.; Wang, Y.-J.; Zhou, D.-M. Screening of wheat straw biochars for the remediation of
soils polluted with Zn (II) and Cd (II). J. Hazard. Mater. 2019, 362, 311–317. [CrossRef] [PubMed]
Kocsis, T.; Kotroczó, Z.; Kardos, L.; Biró, B. Optimization of increasing biochar doses with soil–plant–microbial functioning and
nutrient uptake of maize. Environ. Technol. Innov. 2020, 20, 101191. [CrossRef]
Robb, S.; Joseph, S.; Abdul Aziz, A.; Dargusch, P.; Tisdell, C. Biochar’s cost constraints are overcome in small-scale farming on
tropical soils in lower-income countries. Land. Degrad. Dev. 2020, 31, 1713–1726. [CrossRef]
Clare, A.; Shackley, S.; Joseph, S.; Hammond, J.; Pan, G.; Bloom, A. Competing uses for China’s straw: The economic and carbon
abatement potential of biochar. GCB Bioenergy 2015, 7, 1272–1282. [CrossRef]
Chew, J.; Zhu, L.; Nielsen, S.; Graber, E.; Mitchell, D.R.G.; Horvat, J.; Mohammed, M.; Liu, M.; van Zwieten, L.; Donne, S.; et al.
Biochar-based fertilizer: Supercharging root membrane potential and biomass yield of rice. Sci. Total Environ. 2020, 713, 136431.
[CrossRef]
Amalina, F.; Syukor Abd Razak, A.; Krishnan, S.; Sulaiman, H.; Zularisam, A.W.; Nasrullah, M. Advanced techniques in the
production of biochar from lignocellulosic biomass and environmental applications. Clean. Mater. 2022, 6, 100137. [CrossRef]
Yang, H.; Yan, R.; Chen, H.; Lee, D.H.; Zheng, C. Characteristics of hemicellulose, cellulose and lignin pyrolysis. Fuel 2007, 86,
1781–1788. [CrossRef]
Beusch, C. Biochar as a Soil Ameliorant: How Biochar Properties Benefit Soil Fertility—A Review. J. Geosci. Environ. Prot. 2021, 9,
28–46. [CrossRef]
Wijitkosum, S.; Jiwnok, P. Elemental Composition of Biochar Obtained from Agricultural Waste for Soil Amendment and Carbon
Sequestration. Appl. Sci. 2019, 9, 3980. [CrossRef]
Novak, J.M.; Lima, I.; Xing, B.; Gaskin, J.W.; Steiner, C.; Das, K.C.; Ahmedna, M.; Rehrah, D.; Watts, D.W.; Busscher, W.J.; et al.
Characterization of Designer Biochar Produced at Different Temperatures and Their Effects on a Loamy Sand. Ann. Civ. Environ.
Eng. 2009, 3, 195–206.
Gezahegn, S.; Sain, M.; Thomas, S. Variation in Feedstock Wood Chemistry Strongly Influences Biochar Liming Potential. Soil
Syst. 2019, 3, 26. [CrossRef]
Sovova, S.; Enev, V.; Smilek, J.; Kubikova, L.; Trudicova, M.; Hajzler, J.; Kalina, M. The effect of biochar application on soil
properties and growth of the model plant Zea mays. Ecocycles 2021, 7, 46–54. [CrossRef]
Veksha, A.; McLaughlin, H.; Layzell, D.B.; Hill, J.M. Pyrolysis of wood to biochar: Increasing yield while maintaining microporosity. Bioresour. Technol. 2014, 153, 173–179. [CrossRef] [PubMed]
Lu, S.; Zong, Y. Pore structure and environmental serves of biochars derived from different feedstocks and pyrolysis conditions.
Environ. Sci. Pollut. R. 2018, 25, 30401–30409. [CrossRef]
Méndez, A.; Terradillos, M.; Gascó, G. Physicochemical and agronomic properties of biochar from sewage sludge pyrolysed at
different temperatures. J. Anal. Appl. Pyrolysis 2013, 102, 124–130. [CrossRef]
Ma, X.; Zhou, B.; Budai, A.; Jeng, A.; Hao, X.; Wei, D.; Zhang, Y.; Rasse, D. Study of Biochar Properties by Scanning Electron
Microscope—Energy Dispersive X-Ray Spectroscopy (SEM-EDX). Commun. Soil Sci. Plant Anal. 2016, 47, 593–601. [CrossRef]
Xu, G.; Wei, L.L.; Sun, J.N.; Shao, H.B.; Chang, S.X. What is more important for enhancing nutrient bioavailability with biochar
application into a sandy soil: Direct or indirect mechanism? Ecol. Eng. 2013, 52, 119–124. [CrossRef]
Chen, Y.; Zhang, X.; Chen, W.; Yang, H.; Chen, H. The Structure Evolution of Biochar from Biomass Pyrolysis and Its Correlation
with Gas Pollutant Adsorption Performance. Bioresour. Technol. 2017, 246, 101–109. [CrossRef]
Werdin, J.; Fletcher, T.D.; Rayner, J.P.; Williams, N.S.G.; Farrell, C. Biochar made from low density wood has greater plant available
water than biochar made from high density wood. Sci. Total Environ. 2020, 705, 135856. [CrossRef] [PubMed]
Suo, F.; You, X.; Yin, S.; Wu, H.; Zhang, C.; Yu, X.; Sun, R.; Li, Y. Preparation and characterization of biochar derived from
co-pyrolysis of Enteromorpha prolifera and corn straw and its potential as a soil amendment. Sci. Total Environ. 2021, 798, 149167.
[CrossRef] [PubMed]