RESEARCH ARTICLE

Phosphorus-Assisted Biomass Thermal

Conversion: Reducing Carbon Loss and
Improving Biochar Stability
Ling Zhao1,2, Xinde Cao1*, Wei Zheng2, Yue Kan1
1. School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240,
China, 2. Illinois Sustainable Technology Center, University of Illinois at Urbana-Champaign, Champaign,
Illinois 61820, United States of America

*xdcao@sjtu.edu.cn

Abstract
OPEN ACCESS There is often over 50% carbon loss during the thermal conversion of biomass into
Citation: Zhao L, Cao X, Zheng W, Kan biochar, leading to it controversy for the biochar formation as a carbon
Y (2014) Phosphorus-Assisted Biomass Thermal
Conversion: Reducing Carbon Loss and Improving sequestration strategy. Sometimes the biochar also seems not to be stable enough
Biochar Stability. PLoS ONE 9(12): e115373. due to physical, chemical, and biological reactions in soils. In this study, three
doi:10.1371/journal.pone.0115373
phosphorus-bearing materials, H3PO4, phosphate rock tailing (PRT), and triple
Editor: Zhi Zhou, Purdue University, United States
of America superphosphate (TSP), were used as additives to wheat straw with a ratio of 1: 0.4–
Received: June 15, 2014 0.8 for biochar production at 500˚C, aiming to alleviate carbon loss during pyrolysis
Accepted: November 21, 2014 and to increase biochar-C stabilization. All these additives remarkably increased
Published: December 22, 2014
the biochar yield from 31.7% (unmodified biochar) to 46.9%–56.9% (modified
biochars). Carbon loss during pyrolysis was reduced from 51.7% to 35.5%–47.7%.
Copyright: ß 2014 Zhao et al. This is an open-
access article distributed under the terms of the Thermogravimetric analysis curves showed that the additives had no effect on
Creative Commons Attribution License, which
permits unrestricted use, distribution, and repro- thermal stability of biochar but did enhance its oxidative stability. Microbial
duction in any medium, provided the original author mineralization was obviously reduced in the modified biochar, especially in the
and source are credited.
TSP-BC, in which the total CO2 emission during 60-d incubation was reduced by
Data Availability: The authors confirm that all data
underlying the findings are fully available without 67.8%, compared to the unmodified biochar. Enhancement of carbon retention and
restriction. All relevant data are within the paper biochar stability was probably due to the formation of meta-phosphate or C-O-PO3,
and its Supporting Information files.
which could either form a physical layer to hinder the contact of C with O2 and
Funding: This work was supported by the National
Natural Science Foundation of China bacteria, or occupy the active sites of the C band. Our results indicate that pre-
(No. 21107070, 21377081), Shanghai Science and treating biomass with phosphors-bearing materials is effective for reducing carbon
Technology Commission (13231202502), and
Shanghai Education Commission (14ZZ026). This loss during pyrolysis and for increasing biochar stabilization, which provides a novel
work was also partially supported by State Key
method by which biochar can be designed to improve the carbon sequestration
Laboratory of Pollution Control and Resource
Reuse Foundation (NO. PCRRF12009). The fun- capacity.
ders had no role in study design, data collection
and analysis, decision to publish, or preparation of
the manuscript.
Competing Interests: The authors have declared
that no competing interests exist.

PLOS ONE | DOI:10.1371/journal.pone.0115373 December 22, 2014 1 / 15

 Phosphorus-Assisted Reducing Carbon Loss of Biochar

Introduction
Turning biomass into biochar through pyrolysis is being actively explored as a
tool for long-term carbon sequestration in soil and as a promising strategy to
mitigate global warming [1, 2]. The thermal conversion of biomass into biochar is
a carbonization process which generally involves an initial carbon loss followed by
aromatization. Thus, the carbon sequestration efficiency of biochar depends on
both the carbon loss during pyrolysis and the stability of the final product leading
to carbon emission over time [3, 4].
In general, over 50% of the carbon present in biomass is lost during pyrolysis
due to thermal decomposition and volatilization. This results in low yields of
biochar production, especially during the conversion of some plant-based
biomasses [5]. Our recent study shows that yields of biochar produced at 500 ˚C
from twelve different biomass feedstocks ranged from 27.8% to 58.4%, but the
yield of plant-based biomasses such as sawdust or crop wastes was generally low,
between 27.8–32.0% [5]. In another study, Hossain et al. (2011) attained biochar
yields from sludge ranging from 72.3% to 52.4% with temperatures increasing
from 300 ˚C to 700 ˚C [6].
On the other hand, though biochar is regarded as being stable for 1000 years in
soil, it still undergoes a slow cumulative degradation, which determines the
dispute of biochar as a tool for long-term carbon sequestration. The potential
biotic or abiotic mineralization of biochars has been reported by many researchers
[6, 7]. Carbon release from abiotic incubations of biochar was 50–90% that of
microbially inoculated incubations, and carbon release from both incubations
generally decreased with increasing charring temperature [6]. An accelerated aging
method which models the long-term stability of biochar indicated that carbon loss
ranged between 41.6% and 76.1% [8].
It has been reported that rapid oxidation on the surface of biochar may have
important implications for its environmental stability, because aromatic ring
structures in biochar may be more available to further microbial decomposition
following surface oxidation [9]. Certain chemical substances may be able to
strengthen the oxidative resistance of lignocellulosic materials. For example,
H3PO4 is often used during the production of activated carbon because it
facilitates the generation of thermally stable phosphorus complexes on the surface
of activated carbons (C–O–PO3/(CO)2PO2) [10]. These phosphorus complexes
could reduce the reactivity of active sites on carbon and act as a physical barrier
for oxygen diffusion in micropores [11, 12]. To the best of our knowledge, no
studies have previously been conducted to study this effect on biochar yield and
carbon stability of chemical or mineral additives to biomass feedstock prior to
pyrolysis.
The overall objective of this study is develop a chemical pre-modification
method to improve the potential carbon sequestration capacity of biochar. Thus,
three P-bearing substances, including H3PO4, phosphate rock tailings (PRT), and
triple superphosphate (TSP) were used as additives during biochar production.
H3PO4 was chosen because it has been shown to enhance the oxidative resistance

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 Phosphorus-Assisted Reducing Carbon Loss of Biochar

of lignin in woodchips [9]. PRT and TSP may coexist with biochar in the soil since
these two phosphorus materials are widely used in soil remediation for
inactivation of heavy metals [13, 14].
The specific objectives of this study were (i) to determine carbon loss during
pyrolysis of the pretreated biomass, (ii) evaluate the effects of these additives on
biochar chemical and biological stability, and (iii) to explore the formation
mechanisms of these designed biochars.

Materials and Methods

Biomass and chemical materials
Wheat straw, a typical plant-based biomass was chosen for this study. Samples
were collected from a farm located in Baoshan district in Shanghai, China. No
specific field permits were required for this study. The land accessed is not
privately owned or protected. No protected species were sampled. All locations
used in our study did not involve endangered or protected species. The samples
was air-dried to a moisture content of ,2% and ground to ,1 mm prior to
pyrolysis. PRT and TSP are rich in P (14.1% and 20.2%, respectively) and their
main constituents are Ca5(PO4)3F and Ca(H2PO4)2?2H2O, respectively [15]. PRT
is alkaline and slightly soluble in water, while TSP is acidic and highly water
soluble [15]. The ground wheat straw was immersed in the water slurry of these
materials and mixed homogenously. After 24 h, the pretreated wheat straw was
air-dried and put into pyrolysis system to produce biochars [11, 12]. The biomass/
additive ratios were within a range of 1: 0.4–0.8: wheat straw/H3PO4 (g/g) 1: 0.86;
wheat straw/PRT (g/g) 1: 0.5; and wheat straw/TSP (g/g) 1: 0.4. The ratios were
chosen to ensure that the P composition is enough for mixing with the biomass
completely for their full contact in the process of biochar formation.

Biochar production
Biochar production was conducted in a laboratory-scale pyrolysis system with a
stainless steel column of about 3.5 L in a muffle furnace [16]. The production was
performed under N2 gas with the highest treatment temperature at 500 ˚C. Briefly,
about 100 g unmodified or chemically modified wheat straw was weighed into the
pyrolyzer, and the heating temperature was then raised at 15 ˚C?min21 to reach
four settled gradient temperatures (200 ˚C, 300 ˚C, 400 ˚C and 500 ˚C). At each
gradient temperature, the heat temperature was held for 1 h, respectively, to allow
enough time for carbonization. Biochar production is an aromatization process,
in which volatile compounds are separated from biomass and the rest of carbon is
converted to chemically and biological recalcitrant forms. Upon heating, the
organic compounds are initially cracked at different temperatures to smaller and
unstable fragments. These highly reactive fragments, mainly free radicals with a
very short average lifetime, can polymerize into a recalcitrant aromatic structure.
Thus it is assumed the gradual temperature enabled biomass components to

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 Phosphorus-Assisted Reducing Carbon Loss of Biochar

thoroughly carry out the decomposition and aromatization process. After the
pyrolysis treatment was completed, the carbon-rich solid left in the pyrolyzer was
the biochar product [5]. For simplicity, the biochars without pre-modification
and pre-modified with H3PO4, PRT, and TSP were referred to as BC, H3PO4-BC,
PRT-BC, and TSP-BC, respectively. The experiments were conducted in three
replicates.

Characterization
Biochar pH was measured in de-ionized water with a solid/liquid ratio of 1:20 (w/
v) after 48 h equilibrium (EUTECH pH510, USA). The elements concentration
was measured using an element analyzer (Vario EL III, Elementar, Germany). The
bonding environments of P and C atoms (P 2p, C 1s) on the surface of biochar
particles were determined using XPS (AXIS UltraDLD, Shimadzu, Japan) with a
beam diameter of 200.0 mm and a pass energy of 26 eV. The solid phases of
biochar were characterized by X-ray diffraction (D/max-2200/PC, Japan Rigaku
Corporation) operated at 35 kV and 20 mA. Data was collected over 2h range
from 10 to 50 using Cu Ka radiation with a scan speed of 2o per minute.

Carbon loss and residue during biochar formation in pyrolysis

Biochar yield (%) was calculated based on the weight of biomass and additives lost
during pyrolysis (eq. 1).

Wbiochar
Yield (%)~( )| 100 % ð1Þ
Wbiomass zWadditive

Carbon loss after pyrolysis was calculated based on yield and total carbon
content (eq. 2).
2 3
TCbiochar :Yield:
6 (W 7
6 biomass zWadditive ) 7
C loss in pyrolysis (%) ~61- 7|100% ð2Þ
4 TCbiomass :Wbiomass z 5
TCadditive :Wadditive

TCbiomass, TCadditive and TCbiochar refer to the total carbon (TC) content in
biomass, additive, and biochar, respectively. Wbiomass and Wadditive refer to the
weight of biomass and additive, respectively; whereas, additives were inorganic
substances and contained negligible amounts of TC.
Fixed carbon (FC) represents a relatively stable fraction of carbon in char as
calculated by proximate analysis [17]. The FC calculation is shown in eq 3.

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 Phosphorus-Assisted Reducing Carbon Loss of Biochar

FC%~100%-VS%-ASH% ð3Þ

Volatile solid (VS) is calculated as the weight loss of material under N2

atmosphere at 900 ˚C, and ash is evaluated as the weight loss of material under air
atmosphere at 900 ˚C.

Measurement of biochar carbon stability

Three methods were applied to test carbon stability of biochar including heat-
resistance stability, oxidation-resistance stability, and microbe-resistance stability.
Thermogravimetric analysis (TGA) of the materials under N2 and air were used as
quick and convenient methods to evaluate the heat-resistance stability and
oxidation-resistance stability of each material, respectively [18]. The TGA curves
were obtained by heating biochar from 25 ˚C to 900 ˚C at 20 ˚C?min-1 on the
machine (PerkinElmer Pyris 1 TGA) under both N2 flow and atmospheric air. The
TGA analysis can simultaneously provide data for analysis of material
constituents. For example, the weight loss before 200 ˚C is generally regarded as
moisture removal, and the subsequent weight loss can be largely attributed to the
decomposition of organic matter [19].
The biological carbon stability of biochar was measured by performing
microbial mineralization analysis in sterilized 20 mL borosilicate vials with rubber
septa. This method used a simulated microbial soil condition which was widely
used in previous reports [7, 9]. Before incubation, the biochars were washed
several times to eliminate the volatile smaller molecules from the decomposition
of organic matters attached on the surface of the biochars and equalize their initial
pH values. For each treatment, three replicate incubations of 0.05–0.30 g of
biochar and clean quartz sand with 0.3 mL aqueous nutrient solution [60 g?L21
of (NH4)2SO4 +6 g?L21 of KH2PO4] were conducted [7]. The addition of sand
served to increase permeability, thus increasing the water and oxygen accessibility
for the biochar. To culture the bacteria for inoculation, a soil containing bird
droppings was taken from a university forest and extracted with deionized water
at a 1:50 solid/liquid ratio (W/V). The supernatant was used for inoculation, and
0.3 mL of inoculation solution was added into the biochar-sand system. The
materials were at 30 ˚C. Headspace CO2 was measured every 6–10 days using gas
chromatography (GC-2010AF, Shimadzu, Japan). Before each measurement, the
vials were vacuumed evacuated and 20 mL simulated air (O2 and N2) with CO2
removed was injected into the vials. After 3 days of closed incubation, the released
CO2 due to decomposition was measured.

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 Phosphorus-Assisted Reducing Carbon Loss of Biochar

Results and Discussion

Selected properties of biomass and modified biochars
The compositions of all biochars were presented in Table 1. The C content of
H3PO4-BC and PRT-BC were relative low (28.3% and 29.8%) because of the
contribution of the additives residue to the total biochar weight. The N content
was a little higher in the PRT-BC and TSP-BC than in other biochars because
these two minerals contain a fraction of N. It indicates that the PRT- and TSP-
modified biochars may have a better fertility than the unmodified ones. The wheat
straw contained a high O content as 41.5%, and biochars contained less O (7.84–
29.6%). In H3PO4-BC, O content is relative high (29.6%), compared with the
unmodified biochar (11.6%), which was due to the formation of calcium
metaphosphate. P content was 12.5%, 5.98% and 6.78% in the H3PO4-BC, PRT-
BC and TSP-BC, while the value was low in BC. Biochar yields were elevated with
the addition of additives, increasing from 31.7% (unmodified biochar) to 40.3%–
56.9% (modified biochars) (Table 1). Both PRT-BC and TSP-BC contained
higher ash contents (69.3% and 51.9%, respectively) than H3PO4-BC (25.2%)
(Table 1).
The inclusion of different additives in the process of biochar production
resulted in changes in their properties (Table 1). The pH value of unmodified
biochar was 7.77, falling within the general alkaline pH range of biochars, between
7.5–10.5 [5, 20]. H3PO4 and TSP addition resulted in biochars with stronger
acidity (pH51.51 and 3.89, respectively), and PRT-BC was alkaline (pH58.04).
The pH of the modified biochars likely resulted largely from the acidity or
alkalinity of the additive itself.
The physicochemical properties of the modified biochars such as point of zero
net charge, cation exchange capacity, specific surface area, etc, were not
investigated because carbon residue and carbon stability of biochar are the main
focuses of this study. These properties will be investigated in detail in future
studies.

Carbon conversion during pyrolysis

Routine yield which is calculated only from the weight change during the biomass
conversion (i.e., eq 1) does not represent the real carbon-negativity of biomasses;
thus, the entire carbon budget was evaluated in this study. The total carbon loss
calculated from eq. 2 is presented in Fig. 1. There was 51.7% carbon loss during
wheat straw conversion into biochar. However, addition of all P-containing
chemicals greatly alleviated the carbon loss to 35.5–47.7%, with the carbon loss
reduced by 7.74–31.4% compared to the unmodified biochar. The underlying
mechanisms for carbon loss alleviation will be discussed in the next sections. The
extent of the alleviation may differ with different feedstock and production
conditions, which needs separate investigation.
The FC content of the biochars is presented in Table 1. FC is a fraction of
biochar and represents a relatively stable fraction in biochar which enables it to be

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 Phosphorus-Assisted Reducing Carbon Loss of Biochar

Table 1. Selected properties of biomass and biochars.

Wheat straw BC H3PO4-BC PRT-BC TSP-BC

pH 6.01 7.77 1.51 8.04 3.89
C (%) 46.8a 69.1 28.3 29.8 40.8
H (%) 0.151 0.285 0.023 0.655 0.541
N (%) 0.329 0.691 0.227 1.97 2.03
O (%) 41.5 11.6 29.6 7.84 14.3
P (%) 0.112 0.116 12.5 5.98 6.78
Yield (%) - 31.7 56.9 54.4 46.9
ASH (%) 4.35 27.1 25.2 69.3 51.9
VS (%) 78.6 13.0 57.6 12.8 12.0
FC (%) 17.1 59.9 17.2 17.9 36.1
a
PRT: phosphate rock tailing; TSP: triple superphosphate; TC: total carbon; VS: volatile solid; FC: fixed carbon. Mean value (n53)

doi:10.1371/journal.pone.0115373.t001

the ‘‘sequestrated C’’. FC partially originates from feedstock carbon, but it is also a
conversion product from pyrolysis processes [20]. FC was calculated as the non-
ash fraction that could be burned in air but not be lost in N2 at 900 ˚C. The FC
values for H3PO4-BC, PRT-BC and TSP-BC were 17.2%, 17.9% and 36.1%,
respectively, which were lower than that of the control BC (59.9%). Note that the
relative low FC values were due to the high ASH content in PRT-BC and the high
VS fraction in H3PO4-BC.

Fig. 1. Carbon loss during pyrolysis for biochar production from wheat straw at 500˚C with different
modifications.

doi:10.1371/journal.pone.0115373.g001

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 Phosphorus-Assisted Reducing Carbon Loss of Biochar

Fig. 2. TGA curves of the biochars in N2 (a) and air (b) atmosphere.

doi:10.1371/journal.pone.0115373.g002

Thermal and oxidation stability of modified biochars

TGA curves of biochars over the temperature ranging from 25 ˚C to 900 ˚C under
N2 conditions are shown in Fig. 2a, which reflects the thermal stability of biochar.
The modified biochars had similar TGA curves to the unmodified biochar. The
majority of weight loss occurred from 600 ˚C to 900 ˚C after a minor weight loss
from 200 ˚C to 500 ˚C (Fig. 2a). H3PO4-BC showed a larger initial weight loss than
the other biochars. The main weight loss of biochar was most likely due to the
decomposition of either organic fractions of the carbon skeleton or some
combination of carbon and additives (see section 3.5). Overall, there was not

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 Phosphorus-Assisted Reducing Carbon Loss of Biochar

much difference in the temperature at which the main weight loss occurred
between the unmodified and modified biochars, indicating that these additives
had no influence on the thermal stability of biochars.
The mass loss of biochars in air is generally related to their oxidative stability,
and it is believed that the more stable a substance is, the higher the temperature
needs to be for decomposition [21]. Fig. 2b shows TGA curves of biochars under
normal air conditions. All the modified biochars decomposed at higher
temperatures (490 ˚C–650 ˚C) than the unmodified ones (400 ˚C), especially the
H3PO4-BC (650 ˚C). The results indicate that these additives could increase the
oxidative stability of biochar. Significant oxidation inhibition by phosphorus has
also been observed in many other carbon materials production [22, 23, 24] and
possible mechanisms will be discussed in section 3.5.

Microbe-resistance stability of modified biochars

The emission rate of CO2 from biochar during aerobic incubation represents its
biological mineralization stability [25, 26]. The pH of these materials fluctuated in
the range of 5.5–6.5, which was appropriate for the microbial activity. All
additives were observed to reduce the CO2 emission rate from biochar to some
extent (Fig. 3). TSP-BC showed the lowest mineralization rate. The decrease in
mineralization rate by H3PO4 and PRT was also significant. The majority of
change in the CO2 emission rates occurred within the first 36 days. After the 48th
day, the CO2 emission of all samples decreased to a very low level, and the
differences among the unmodified and modified biochar seemed unremarkable.
However, because biochar can be oxidized in the soil for many years, small
differences in CO2 emissions may have a significant effect if summed over a long
period of time, thus, long-term stability of modified biochar needs further
investigation. Note that the evaluation of biological stability was made in this
work using a simulated microbial soil condition [7, 9]. However, the stability of
biochar may also be influenced by soil properties such as soil pH, soil carbon
content and the presence of other organic substances. Therefore, incubation tests
of modified biochar in a real soil system should be conducted in a future study
[27].
The cumulative CO2 emission amount during the 60-day aerobic incubation
was calculated according to the average emission rates every 12 days, which were
18.9, 15.8, 14.8 and 6.09 mg CO2? g C21 for BC, H3PO4-BC, PRT-BC, and TSP-
BC, respectively. H3PO4, PRT, and TSP treatments reduced microbial CO2
emission of biochar by 16.4%, 21.7%, and 67.8%, compared to unmodified
biochar. Many studies concluded that the main CO2 emission occurred in the first
60 days [7, 28]; thus, the obvious decrease in CO2 emission during the 60-day
incubation period in this study suggests that the three additives could enhance the
biological mineralization stability of biochar. The effectiveness followed the trend
TSP.PRT.H3PO4.

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 Phosphorus-Assisted Reducing Carbon Loss of Biochar

Fig. 3. Emission rates of CO2 from the biochars during aerobic incubation.

doi:10.1371/journal.pone.0115373.g003

Interference of phosphorus on biomass pyrolysis

It is not surprising that inorganic substances have influence on the charcoal yield,
product distribution, or product selectivity during pyrolysis [29–32]. For
example, iron improved the O-containing functional groups of biochar [33]. In
this study, the exact reactions between the additives and wheat straw cannot yet be
determined, but potential mechanisms of modification effects on biochar carbon
residue and stability could be proposed according to the results obtained from this
study. Fig. 4 shows the XPS spectra of P 2p and C 1s electron (spin-orbit) for the
unmodified and modified biochars. The additives made the peak of P 2p shift
from low binding energy (BC: 133.8 eV) to higher binding energy (H3PO4: 134.9
eV; PRT: 134.0 eV; TSP: 134.3 eV). The binding energy intensity of the main C 1s
peak at 284.8 eV was almost not changed (Fig. 4b), while the height of the peaks
was decreased by the introduction of the additive materials. This indicates that the
surface C content was reduced.
A P 2p peak around 135.0 eV is generally assigned to metaphosphate or C-O-
PO3 type groups [34, 35]. Addition of chemicals, especially H3PO4, increased the
P 2p peak from 133.8 eV in unmodified BC to about 135 eV, indicative of the P-C
compounds formation. This observation agrees with previous findings that
H3PO4 addition mainly resulted in the formation of oxygen-containing
phosphorus groups which may include metaphosphates, C–O–PO3 groups, or C–
PO3 groups [34, 35]. These groups are suggested to act as a physical barrier against
carbon decomposition, as well as to block the active carbon sites [22, 18], resulting
in reduced oxidation and mineralization of biochar. Formation of P-C
compounds was further evidenced by X-ray diffraction (XRD) analysis (Fig. 5).
Compared to the unmodified BC, a new peak at 2h0526.6, most likely
corresponding to the P-C compounds was observed in the H3PO4-BC and TSP-
BC. Although PRT is also rich in P, it is less soluble, allowing it behave differently

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 Phosphorus-Assisted Reducing Carbon Loss of Biochar

Fig. 4. XPS spectra of P 2p and C 1s electron for the unmodified and modified biochars.

doi:10.1371/journal.pone.0115373.g004

from soluble TSP and H3PO4. A weak peak of P-C compounds was observed at
2h0526.6 in the PRT-BC (Fig. 5). Qian et al. (2014) pointed out that the P-
containing radicals react with the aromatic rings produced by the pyrolysis of
lignin to form P-containing species, which is an important factor influencing the
distribution and stabilization of P in char [36]. Uchimiya and Hiradate (2014)
indicated that orthophosphate such as CH32O2PO322 and phenyl2O2PO322
formed in pyrolysis were stable [37]. Klupfel et al. (2014) also proposed that
biochar has redox properties and acts as electron-donating [38]. This indicated
that P has potential to react with the carbon in biochar. Overall, all three P-
bearing additives induced the formation of P-C compounds.
According to the results of this study, the P-bearing additives seemed to have
no influence on the thermal stability of biochar products, but they did improve
oxidative stability and biological degradation. Thermal stability is a function of

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 Phosphorus-Assisted Reducing Carbon Loss of Biochar

Fig. 5. X-ray diffraction (XRD) patterns of the unmodified and modified biochars.

doi:10.1371/journal.pone.0115373.g005

bond energy, while both the incineration of volatile matters and the carbon to
CO2 conversion require contact between C and O2 or bacteria. Thus it could be
speculated that the bond energy of the reaction products from additives is low,
and these additives could also form a physical layer to hinder the contact of C with
O2 or bacteria or occupy the active sites of the carbon band.

Implications in application of biochar as a carbon fixer

Although biochar has been considered as a multi-functional material for
improving the environment in many different ways, its primary environmental
significance has always been carbon sequestration [39, 40]. However, challenges
for the effectiveness of biochar as a carbon fixer still remain. About half of the
initial carbon from the biomass is not converted to biochar during pyrolysis, and
the final biochar product is still not as stable as necessary for long-term carbon
sequestration, which causes biochar to remain a controversial carbon fixer [41].
This study presents a novel idea that biochars with high carbon content residue
and stability can be designed using pre-modification during the feedstock
preparation. The P-containing materials chosen did show a tendency to act as
passivators to reduce carbon loss during pyrolysis and enhance the stability of
biochar, though the two effect extents were not consistent with one specific
additive. The potential capacity of carbon sequestration is expected to be
improved through the regulation of process conditions. Additionally, these
modifications might change biochar’s physicochemical properties, which may
affect the soil environment when biochar is applied into soil as a carbon
sequestration tool. Additional benefits to the soil may also be obtained; for
example, the modified PRT-BC and TSP-BC contain high P which may be

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 Phosphorus-Assisted Reducing Carbon Loss of Biochar

effective in immobilizing heavy metals in contaminated soils [42, 43]. Of course, it

must be noted that the low pH value of the H3PO4-BC biochar may limit its soil
application in acidic soils.
Overall, there is some promise that biochar can be designed to provide multi-
win effects, i.e., carbon sequestration, soil improvement, and contamination
remediation.

Conclusions
During the charring process of biomass to biochar, over 50% carbon is typically
lost, and the generated biochar may not be stable enough in soil due to physical,
chemical, and biological reactions. In this study, the biomass precursor was pre-
treated using three P-bearing chemical substances including H3PO4, PRT and
TSP, aiming to reduce carbon loss during charring and simultaneously increase
the carbon stability of the final biochar product. The results show that these
chemicals reduced the carbon loss, improved the oxidation stability and reduced
the microbial mineralization. This study put forward the novel idea that people
can design biochar to improve its carbon residue and stability through passivator
addition during feedstock preparation, which enables it to be a prospective tool
for carbon sequestration.

Author Contributions
Conceived and designed the experiments: LZ. Performed the experiments: LZ.
Analyzed the data: LZ XC WZ. Contributed reagents/materials/analysis tools: LZ
YK. Wrote the paper: LZ XC. Conducted the instrumental analysis: LZ YK.
Polished the manuscript language: XC WZ LZ.

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