Effects of Feedstock and Pyrolysis Temperature on Biochar Adsorption of Ammonium and Nitrate
Effects of Feedstock and Pyrolysis Temperature on Biochar Adsorption of Ammonium and Nitrate
Xiapu Gai 0 3 4
Hongyuan Wang 0 3 4
Jian Liu 0 1 3 4
Limei Zhai 0 3 4
Shen Liu 0 3 4
Tianzhi Ren 2 3 4
Hongbin Liu 0 3 4
0 Key Laboratory of Nonpoint Source Pollution Control, Ministry of Agriculture/Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences , Beijing , China,
1 United States Department of Agriculture-Agricultural Research Service (USDA-ARS), Pasture Systems and Watershed Management Research Unit, University Park, Pennsylvania, United States of America,
2 Institute of Agro-Environmental Protection, Ministry of Agriculture , Tianjin , China
3 Funding: This study was financially supported by the National Natural Science Foundation of China (41301311), the Special Fund for Agro-scientific Research in the Public Interest (201303095-10), and the National Natural Science Foundation of China (41203072). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
4 Editor: Jonathan A. Coles, Glasgow University , United Kingdom
Biochar produced by pyrolysis of biomass can be used to counter nitrogen (N) pollution. The present study investigated the effects of feedstock and temperature on characteristics of biochars and their adsorption ability for ammonium N (NH4+-N) and nitrate N (NO32-N). Twelve biochars were produced from wheat-straw (W-BC), corn-straw (C-BC) and peanut-shell (P-BC) at pyrolysis temperatures of 400, 500, 600 and 700C. Biochar physical and chemical properties were determined and the biochars were used for N sorption experiments. The results showed that biochar yield and contents of N, hydrogen and oxygen decreased as pyrolysis temperature increased from 400C to 700C, whereas contents of ash, pH and carbon increased with greater pyrolysis temperature. All biochars could sorb substantial amounts of NH4+-N, and the sorption characteristics were well fitted to the Freundlich isotherm model. The ability of biochars to adsorb NH4+-N followed: C-BC.P-BC.W-BC, and the adsorption amount decreased with higher pyrolysis temperature. The ability of C-BC to sorb NH4+-N was the highest because it had the largest cation exchange capacity (CEC) among all biochars (e.g., C-BC400 with a CEC of 38.3 cmol kg21 adsorbed 2.3 mg NH4+-N g21 in solutions with 50 mg NH4+ L21). Compared with NH4+-N, none of NO32-N was adsorbed to biochars at different NO32 concentrations. Instead, some NO32-N was even released from the biochar materials. We conclude that biochars can be used under conditions where NH4+-N (or NH3) pollution is a concern, but further research is needed in terms of applying biochars to reduce NO32-N pollution.
Today, biochar is receiving great research attention due to its potential
importance in agronomic and environmental applications. Biochar refers to a
carbon (C)-rich and porous substance, which is produced by thermal
decomposition of biomass under oxygen-limited conditions and at relatively low
temperatures (,700C) . It has a high specific surface area, a high density of
negative surface charges, and characteristic pores and surface functional groups
. Biochar has been reported to be able to improve soil fertility by sequestrating
C and enhancing retention of nutrients  and to suppress greenhouse gas
emissions to the air .
Leaching of nitrogen (N) from agricultural land caused by excessive application
of N fertilizers may pose a great threat to the quality of surface- and groundwater,
and results in eutrophication of water bodies . This is a particular concern in
China that is consuming about one third of the total N fertilizers in the world .
Biochar is considered as a potential applicable material to mitigate N leaching,
since a few studies have indicated that it can affect availability and cycling of N in
the soil . However, confounding results have been reported with regard to
the effect of biochar application on N leaching. For example, Ding et al. 
observed a reduction of NH4+-N leaching at 0.2-m soil depth by 15% and Laird et
al.  observed a reduction of total N leaching by 11% in typical US Midwestern
agricultural soils after addition of biochar to the surface soil layer. Based on these
findings, they concluded that biochar, mainly owing to a high N sorption
capacity, can be used as an effective soil amendment to reduce N losses from soils.
However, some studies oppositely showed a limited or no ability of biochar to
adsorb NO32-N. For instance, Hollister et al.  observed no adsorption of
NO32 to biochar derived from corn (Zea mays L.) or oak (Quercus spp.). Yao et
al.  found nine of thirteen biochars tested had little NO32 adsorption ability
and some even released NO32 into water solution. These contradictory results are
likely because of the differences in properties among the biochars, which poses an
urgent need to disclose the relationship between biochar characteristics and their
effects on adsorption of NH4+-N and NO32-N.
Feedstock and temperature during pyrolysis can influence molecular structure
and pores size distribution of biochar, and thus affect biochar sorption
characteristics . Sohi et al.  reported that different feedstocks resulted
in different magnitudes of surface area, pores and functional groups in biochars,
and all these variables affect sorption characteristics of biochars. Sun et al. 
reported that poultry-litter biochar had a larger specific surface area and porosity
than wheat-straw biochar, despite the two biochars were produced under the same
temperature (400C). In general, high pyrolysis temperature leads to greater
specific surface area and aromaticity of biochar . For example, charcoal made
from wheat residue at 500700 C is well carbonized and its specific surface area is
relatively high (.300 m2 g21), whereas chars formed at 300400C are partially
carbonized and have a lower specific surface area (,200 m2 g21) . Moreover,
low-temperature biochars (250400C) will probably be more suitable for
improving soil fertility than high-temperature ones due to the relatively stable
aromatic backbone from pyrolysis and more C5O and C-H functional groups
which may serve as nutrient exchange sites after oxidation . Addition of
lowtemperature biochars to soils was reported to improve soil fertility by raising soil
cation exchange capacity (CEC) . On the other hand, Gell et al.  reported
that low temperature biochars were more phytotoxic due to accumulation of tars
and other organic compounds. So temperature of pyrolysis plays a great role in
biochar properties. Moreover, decrease of atomic ratios H/C and O/C resulted
from removing H- and O-containing functional groups with increasing
temperature will produce high aromaticity and low polarity biochars .
In recent years, development of new techniques has provided a great
opportunity to better understand biochar components and structures. These new
techniques include Fourier transform-infrared spectroscopy (FT-IR) and field
emission-scanning electron microscopy (FE-SEM), which can be used to
characterize the surface functional groups and micro-morphology of biochars.
The main objectives of this study were to (i) investigate the effects of feedstock
types and pyrolysis temperature on biochar characteristics related to N adsorption
ability; and (ii) determine the main factors affecting the adsorption of NH4+-N
and NO32-N to biochars. These will help to gain insights in use of biochar to
mitigate nonpoint source pollution from agricultural soils.
Materials and Methods
Preparation of biochars
Biochar samples were produced from three common agricultural by-products:
wheat-straw, corn-straw and peanut-shell. Raw materials were cut into small
pieces (2 cm) and oven-dried (70C) for 2 days after washing with deionized (DI)
water for five times. The materials were then ground and sieved to yield a uniform
1 mm size fraction, and converted to biochar under oxygen-limited conditions
using a muffle furnace (SXZ-12-10). To minimize oxygen content at reaction, the
container was filled with the feedstock materials and tightly sealed. The pyrolysis
temperature was raised to the aimed values of 400C, 500C, 600C and 700C and
held constant for 1.5 h . Biochar yields were recorded and the resulting twelve
biochar samples were hereafter referred as W-BC400, W-BC500, W-BC600,
WBC700, C-BC400, C-BC500, C-BC600, C-BC700, P-BC400, P-BC500, P-BC600
and P-BC700. The biochar production rate at each temperature was calculated as:
Production rate (%) 5 (MBiochar/MFeedstock) 6100, where MBiochar is the mass of
biochar and MFeedstock is the mass of feedstock, both on a basis of dry weight.
Detailed information about the chemical and physical characteristics of biochars is
listed in Table 1 and Table 2.
The crude product of biomass pyrolysis includes biochar and ash. To
investigate the possible effects of ash on the sorption properties, samples of
ashfree biochar were prepared. Crude biochar produced at 500C was suspended in
either 1 mol L21 H2SO4 or DI water at 1.5 g in 30 mL and agitated for 2 h in an
Table 1. The yields, chemical compositions and atomic ratios of biochars produced from different feedstocks at different pyrolytic temperatures.
Biochars Temp. ( C) Yield (%)
The biochars include wheat-straw biochar (W-BC), corn-straw biochar (C-BC) and peanut-shell biochar (P-BC) as well as biochars pyrolyzed at 500C and
washed with acid (A-W-BC, A-C-BC and A-P-BC) and deionized water (W-W-BC, W-C-BC and W-P-BC).
Note: Different letters indicate significant difference for the results in the same column, excluding the biochars washed with acid and water.
Table 2. pH values, electrical conductivity (EC), ash content, cation exchange capacity (CEC), BET surface area, pore volume and pore size of W-BC, C-BC
and P-BC at different pyrolytic temperatures.
Temp. ( C)
Note: Different letters indicate significant difference for the results in the same column.
ultrasound bath . Then the suspension was pumping filtrated until the pH of
the filtrate stabilized between two consecutive extractions (pH 0.02), and the
filter cake was oven-dried (70C) to obtain the treated biochars for further
adsorption experiments. The acid-washed biochar is denoted as A-W-BC500,
AC-BC500, A-P-BC500, while the DI water-washed biochar as W-W-BC500,
W-CBC500, W-P-BC500. The biochar production rate and chemical compositions
after washing are listed in Table 1.
Determination of physical and chemical properties of the biochars
The specific surface area and porous texture of biochar were determined from N2
adsorption isotherms at 77 K with a Surface Area and Porosity Analyzer (ASAP
2020 HD88, USA). Biochar samples were degassed under vacuum at 363 K for 1 h
and at 623 K for another 3 h, before being filled with N2 gas at different vapor
pressures. The N2 adsorbed per g biochar was plotted versus the relative vapor
pressure (P/Po) of N2 ranging from 0.02 to 0.2, and the data was fitted to the
Brunauer-Emmett-Teller equation (BET) by computer to calculate surface area
. Biochar shapes and surface physical morphology were examined using
FESEM (SU8000, Hitachi, Tokyo, Japan) at 15 KeV. The X-ray powder diffraction
(XRD) patterns were determined using a Macscience-M18XHF instrument (UK)
with Cu-Ka radiation at 40 mA and 40 kV. The data was collected over a 2h range
of 1090 using the Cu-Ka radiation at a scan rate of 2 min21 . The FT-IR
spectra were recorded on Bruker Vertex 70 Fourier transform infrared
spectrometer using the oven-dried KBr (at 105110 C) pellet technique (1:100). The total
number of scans was 32 with the spectral resolution of 4 cm21.
Elemental contents of C, N, hydrogen (H) and oxygen (O) were determined
using the Elemental Analyzer (vario PYRO cube). Biochar pH was measured using
a pH meter (Mettler Toledo Delta 320) and electrical conductivity (EC) was by an
electrical conductivity meter (DDS-307A), both with biochar to DI water ratio of
1:30 w/w, after stirring for 1.5 min and equilibration for 1 h. Ash was separated by
placing biochar sample in a nickel crucible and it was heated at 700C for 2 h
under air . The content of ash was calculated as: Ash content (%) 5 (MAsh/
MBiochar) 6100, where MAsh was the mass of ash and MBiochar was the mass of
biochar. The CEC of biochar was measured by a modified NH4+-acetate
compulsory displacement method . An amount of 0.2 g biochar was leached
with 20 mL DI water for five times, and the contents of K+, Na+, Ca2+ and Mg2+ in
the collective leachate were determined as the soluble base cations of the biochar.
After this, the biochar sample was leached with 20 mL of 1 M Na+-acetate (pH 7)
for five times to determine K+, Ca2+ and Mg2+ in the leachate as the exchangeable
base cations. The biochar samples were then washed with 20 mL of ethanol for
five times to remove the excessive Na+. Afterwards, the Na+ on the exchangeable
sites of the biochar was displaced by 20 mL of 1 mol NH4+-acetate (pH 7) for five
times, and CEC was calculated from the Na+ displaced by NH4+. The contents of
K+ and Na+ in the leachate were determined by flame photometry, and Ca2+ and
Mg2+ by atomic absorption spectrometry.
To investigate the ability of biochars to adsorb NH4+-N and NO32-N, adsorption
experiments were conducted by adding biochar samples to water solutions with
different concentrations of NH4+-N and NO32-N. The same experimental
procedure was used for each type of biochar (excluding those treated with acid
and DI water) and N solution. Specifically, 0.2 g biochar was added to 50 mL
NH4Cl (or KNO3) solutions with concentrations of 10, 30, 50, 70, 100, 150, 300
and 500 mg NH4+ (or NO32) L21, respectively. The mixture was then shaken in a
thermostatic shaker at 25C and 200 rpm for 24 h to achieve equilibrium. The
supernatant was filtered and analyzed for concentrations of NH4+-N (or NO32-N)
by a Flow Injector Auto analyzer (Auto Analyzer 3, High Resolution Digital
Colorimeter). For the biochar samples treated with acid and DI water, the
adsorption experiments were conducted only in NH4Cl (or KNO3) solution with
50 mg NH4+ (or NO32) L21, while the other experimental procedures were the
same as for the non-washed biochar samples. The experiment for each sample was
run in triplicate. The amount of NH4+-N (or NO32-N) adsorbed on biochar was
calculated as the difference between the original NH4+-N (or NO32-N)
concentration and the remaining aqueous concentration at equilibrium. The
amount of NH4+-N (or NO32-N) adsorbed per unit mass of biochar was
calculated as Eq.1 .
where, Qe is the amount of N adsorbed by biochars (mg g21) at equilibrium; C0
and Ce are the NH4+-N (or NO32-N) concentration in the initial and equilibrium
solution (mg L21), respectively; V is the volume of the aqueous solution (L) and
M is the mass of biochar (g).
where, Qm is the maximum sorption capacity of biochar (mg g21), and KL refers
to the Langmuir constants related to adsorption capacity and adsorption rate.
The NH4+-N and NO32-N sorption data were fitted to linear Freundlich and
Langmuir models, which are the most frequently used models for describing
sorption isotherms. The Freundlich adsorption model is as Eq. 2 :
where, Qe is mass of NH4+-N or NO32-N adsorbed per mass of biochar (mg g21)
at equilibrium; Ce is equilibrium concentration (mg L21) of NH4+-N or NO32-N
in solution; KF and 1/n are experimentally derived constants.
The Langmuir isotherm model, which assumes homogeneous monolayer
surface sorption, can be written as Eq.3 :
When Ce/Qe is plotted against Ce, a straight line with a slope of 1/Qm and an
intercept of 1/(Qm KL) is obtained.
The results were expressed as means and standard deviations. Figures were
plotted with the Origin 8.1 software. Statistical analysis was performed using
Statistical Analysis System (SAS, version 9.1). Significant differences were tested
using Duncans multiple range test (P50.05) and the correlation was analyzed
with the Pearson test (two-tailed) at P50.05. Any differences between the mean
values at P,0.05 were considered statistically significant.
Results and Discussion
Yields and element contents of different biochars
The yields and element contents of biochars from wheat-straw, corn-straw and
peanut-shell at four different pyrolysis temperatures of 400, 500, 600 and 700C
are given in Table 1. The yields of W-BC, C-BC and P-BC samples were reduced
from 32.4%36.8% to 22.8%25.8% as pyrolysis temperature increased from 400
to 700C. This is due to greater losses of volatile components at the higher
pyrolysis temperatures .
Content of C, which is the major constituent of the biochars, increased with
higher pyrolysis temperature for W-BC, C-BC and P-BC (Table 1). This was due
to highly carbonization at high temperature (600C and 700C), with a high
degree of C in aromatic structures . However, contents of H and O decreased
by approximately 60% and 30%, respectively, as pyrolysis temperature increased
from 400C to 700C (Table 1). This was attributed to the removal of water,
hydrocarbons, tarry vapors, H2, CO and CO2 during the carbonization process
. Some of these H and O contents are likely presented in organic functional
groups on biochar surface . Decrease of their contents is likely to result in a
reduction in N sorption capacity. The biochar samples contained small amount of
N (W-BC, 1.21.5%; C-BC, 1.62.4%; P-BC, 1.41.8%) and the N content
remained relatively stable, which was consistent with the findings by Zheng et al.
. However, content of N in C-BC was always higher than that in W-BC and
PBC at a given temperature. This is most likely because corn straws had a much
higher content of total N (17.2 g kg21) than wheat straws (10.5 g kg21) and
peanut shells (12.4 g kg21). Atomic ratios of elements, which estimates the
aromaticity (H/C) and polarity (O/C, (O+N)/C) of the biochars, were
significantly affected by pyrolysis temperature (Table 1). A higher H/C ratio
shows a lower degree of carbonization and aromaticity of the biochar . The
atomic O/C ratios were also lower in W-BC700 (0.30), C-BC700 (0.42) and
PBC700 (0.29) than those in W-BC400 (0.56), C-BC400 (0.59) and P-BC400 (0.54),
indicating the less hydrophilic surface of biochars at higher temperature .
Acid washing effectively removed most of the inorganic fractions from the three
biochars (63.389.6%). Both acid and DI water washing affected the relative
contents of the remaining elements in the biochars. Specifically, washing caused
the proportion of C to increase, but not the proportions of H, N and O.
Characteristics of different biochars
1. pH and EC values of biochars
All biochars produced in this study were alkaline, with a pH between 8.2 and 10.4
(Table 2). This range of pH is common for thermally produced biochars [1, 27].
In terms of different feedstocks, the pH values of W-BC (8.29.2) and P-BC (9.3
9.9) were lower than C-BC (10.210.4). The biochar pH significantly increased
with higher pyrolysis temperature (P,0.05) (Table 2). For example, C-BC had a
pH of 10.2 at 400C and 10.4 at 700C, which was consistent with the finding by
Hossain et al. .
The biochars from the three feedstocks had a similar trend of EC values, that is,
the values increased significantly with the higher pyrolysis temperature (Table 2).
This apparent effect of pyrolysis temperature on EC values was consistent with the
results of Cantrell et al.  and Quilliam et al. . The EC estimates the
amount of total dissolved salts or the total amount of dissolved ions in samples
. Its increase with pyrolysis temperature was likely due to loss of volatile
materials at high temperatures, which promoted the relative concentrations of
salts in the ash fraction.
2. Ash contents and CEC of biochars
Ash contents in different biochars ranged from 11% to 18%, which were low
compared with those in their feedstocks (wheat-straw 28%, corn-straw 31% and
peanut-shell 27%). Apparently, ash content increased with rise in temperature due
to increased concentrations of minerals and organic combustion residues .
Change of ash content in the biochars with temperature had a trend similar to that
of biochars originated from other organic wastes such as pine needle and animal
Biochar CEC values significantly differed with both feedstock and pyrolysis
temperature. The CEC of C-BC (19.068.6 cmol kg21) was much higher than that
of P-BC (0.38.5 cmol kg21) and W-BC (0.55.1 cmol kg21), despite the fact that
they had similar CEC in feedstocks (9.814 cmol kg21). The trend of CEC
changing with pyrolysis temperature was similar for the biochars from all
feedstocks. All biochars pyrolysed at 400C and 500C had higher CEC than that at
600C and 700C. Whereas in the findings of Yuan et al. , CEC of biochar
prepared from corn at 500C was higher than that at 300C and 700C; and CEC of
biochar prepared from peanut at 700C was higher than that at 300C and 500C.
3. Specific surface area and morphology structures of biochars
Specific surface area, pore volume and pore size of the biochars obtained from
different feedstocks are summarized in Table 2. Biochar SBET ranged from 3 to
185 m2 g21, which was significantly affected by biochar feedstock and pyrolysis
temperature . In general, the SBET of C-BC was much lower than that of
W-BC and P-BC. Ahmad et al.  attributed this difference to the compositional
compounds (lignin, cellulose and hemicellulose) in the original feedstocks, but the
mechanisms behind were not clear. The SBET of C-BC, W-BC and P-BC showed
the same trend as affected by temperature, that is, SBET increased as the
temperature increased from 400 to 600C, but substantially decreased at 700C
(Table 2). This is likely because of the removal of H- and O-carrying functional
groups, including aliphatic alkyl-CH2, ester C5O, aromatic -CO and phenolic
OH groups, in biochars produced at 600C, greatly enlarged their surface areas
Pore structures of biochars as described by FE-SEM provide information about
the structural change in biochar particles after thermal treatment. After pyrolysis,
the biochars obtained rough surface and multiple sizes of pores, which resulted in
a large specific surface area, a very important property for being sorbent materials
. FE-SEM micrographs of the morphological changes in the pore structure of
the biochars at different temperatures and with different washing treatments
implied that the clear and well-developed pore structure of the biochar consisted
of cylinder-like tubes. The FE-SEM micrographs of C-BC as an example are
shown in Figure S1af. The biochars contained microparticles and micropores,
and the unregular fold structure changed into regular layer with the increasing
temperature. But at 700C, the biochars showed laminated texture. FE-SEM
micrographs also demonstrated a homogeneous pore size distribution with a pore
arrangement, and the pores in the inner portion of the biochars were obvious and
well arranged in an array of cylinder-like structures. The above features of the
FESEM micrographs, such as well-developed pore structure and pore size
distribution, implied an excellent possibility for NH4+-N to be adsorbed by the
biochars according to Sun et al. . Compared with the non-washed C-BC500,
biochar treated with diluted H2SO4 and DI water had some convexity structures
(Figure S1ef), and the pores increased after washing with acid.
4. Crystal structure of biochars
Spectra for biochar crystal structure determined by XRD are shown in Figure S2a
c. Sharp peaks in all samples indicated presence of miscellaneous inorganic
components, which suggested that there were quartz and sylvite in the biochar
. The XRD patterns for C-BC revealed sharp peaks, which showed a high
degree of crystallinity with characteristic peaks at 26.6 (Figure S2b). The values
matched the characteristic peaks of silicate carbonaceous (SiCO322) material,
according to the database of the Joint Committee on Powder Diffraction
Standards . The XRD spectra analysis revealed that W-BCs (Figure S2a) and
P-BCs (Figure S2c) had similar crystal substances as in C-BCs. Moreover, peak
intensities decreased with higher temperature, indicating that inorganic
components were well crystallized during low-temperature pyrolysis process .
However, the XRD spectra of different feedstocks at the same pyrolysis
temperature showed no significant difference among the three types of biochars.
5. Surface functional groups of biochars
The FT-IR spectra of the twelve biochars are illustrated in Figure 1ac. Different
spectra reflected changes in the surface functional groups of biochars produced at
different temperatures. The peak assignments in the spectra represented methyl
CH stretching compounds (,2930 cm21), methylene C-H stretching
(,2860 cm21), aromatic carbonyl/carboxyl C5O (,1700 cm21), aromatic C5C
and C5O (,1600 cm21), aliphatic C-O-C and alcohol-OH (11601030 cm21),
and aromatic C-H (,815 cm21) . All these bands experienced different
changes with increasing pyrolytic temperature, which is consistent with the study
of Chen et al. . At low pyrolysis temperatures (400500C) for W-BC, the
band intensities were at 3438 cm21 (-OH), which dramatically decreased and
almost diminished at 600700C, whereas other bands (e.g., -CH2-, C5C and
ester C5O) were preserved. The polar groups (-OH and C-O) exhibited the lower
magnitude of peaks upon heating at high temperature (600C and 700C),
suggesting a decrease in the polar functional groups with an increase in pyrolysis
temperature. The maximum loss occurred in -OH, CH2- and C-O functional
groups in biochars produced at 700C, which was also apparent from their
elemental compositions and element atomic ratio (Table 1). Thermal destruction
of cellulose and lignin in the feedstocks might result in the exposure of aliphatic
alkyl CH2-, hydroxyl -OH, ester C5O and aromatic C5O functional groups in
biochars . The changes in the peaks and their intensities and consequently
functional groups of C-BC and P-BC were similar to those of W-BC. This is a
result of strong dependence of the extent of carbonization on production
The comparisons of the functional groups between washed and non-washed
biochars, as determined by FT-IR spectra, are presented in Figure 1df. After
different treatments, the bands of W-BC, C-BC and P-BC changed considerably.
FT-IR spectra confirmed that acid and DI water washing effectively removed most
of the inorganic fractions of biochars. As seen from the FT-IR spectra in W-BC
(Figure 1d), surface functional groups did not change by washing, which was
supported by other studies [19, 32]. Compared with C-BC500 (Figure 1e), there
were some differences in the FT-IR spectrum of A-C-BC500 and W-C-BC500. The
strong peak at 1446 cm21 (aromatic C5C) and 1600 cm21 (-OH) decreased due
to C condensation for A-C-BC500 and W-C-BC500 . There was no peak at
1109 cm21 (C-O-C) in C-BC500 and the new aromatic structure formed in
A-CBC500. But for P-BC (Figure 1f), the FT-IR spectra showed great difference.
Compared with the non-washed samples, acid-washing decreased the intensities
of surface functional groups at 3438 cm21 (-OH) and 1600 cm21 (aromatic C5C
and C5O), but they were increased by DI water-washing. In addition,
CH2(1438 cm21) diminished with acid- and DI water-washing.
Ammonium nitrogen sorption on different types of biochars
The equilibrium adsorption isotherms of NH4+-N, which are essential to
understand the mechanism controlling biochar adsorption process, are presented
in Figure 2ac. The twelve tested biochars had considerable NH4+-N sorption
capacity, e.g. 0.52.4 mg NH4+-N g21 at an initial NH4+ concentration of 50 mg
L21. Biochars usually carry negative surface charges, which enhances the ability of
soil to adsorb and retain cations (e.g. NH4+) and thus inhibit cation loss by
leaching from acid soils [17, 20]. In general, C-BC had a greater NH4+-N sorption
ability than W-BC and P-BC at a given pyrolysis temperature. For example,
CBC500 had a much higher Qe value than W-BC500 and P-BC500, when the initial
NH4+ concentration was 50 mg L21 (Table 3). More NH4+-N was adsorbed by the
low-temperature biochars (400500C) than by the high-temperature biochars
(600700C) for each feedstock at a given NH4+-N concentration. Taking an
initial concentration of 100 mg NH4+ L21 as an example, C-BC400 (Qe 3.6 mg
g21) and C-BC500 (Qe 3.0 mg g21) had relatively higher Qe values than C-BC600
Figure 2. Sorption isotherms of NH4+-N on wheat-straw biochar (W-BC), corn-straw biochar (C-BC) and peanut-shell biochar (P-BC) at different
pyrolytic temperatures (Qe: the amount of NH4+-N sorbed by per unit mass of biochar at equilibrium; Ce: concentration of NH4+-N in the solution at
equilibrium). Bars indicate standard deviation of three replicates.
(2.8 mg g21) and C-BC700 (2.4 mg g21). Table 4 shows the Freundlich and
Langmuir isotherm constants and NH4+-N adsorption correlation coefficients for
different biochars. Sorption of NH4+-N to different biochars was better fitted to
Freundlich isotherm model, with higher r values than that of Langmuir model.
Despite both constants KF and n in Freundlich model affect NH4+-N adsorption
isotherms, it seems KF plays a main role in reflecting differences of NH4+-N
adsorption ability between biochars from different feedstocks. C-BC with a greater
KF value in the isotherm had a relatively high NH4+-N sorption ability compared
with W-BC and P-BC. Compared with the non-washed biochars, washing with
acid and DI water reduced adsorption of NH4+-N, especially for C-BC500
(Table 3). This decrease in NH4+-N sorption ability is probably because ash was
washed off from the biochar and some functional groups were removed from the
biochar surface, Zheng et al.  stated that ash could substantially improve
theNH4+-N adsorption capacity of biochars.
To investigate the main factors influencing biochar ability to adsorb NH4+-N,
correlations between Qe and contents of elements and CEC of biochars were
analyzed. The correlations for C-BCs as examples are shown in Figure 3af. The
Qe values were negatively correlated with C contents (r 520.9046) (Figure 3a).
Note: Different letters indicate significant difference for the results and the adsorbed amounts of NH4+-N and NO32-N were compared separately.
Figure 3. Correlations between mass of NH4+-N adsorbed per mass of biochar at equilibrium (Qe) and content of C (a), content of O (b), atomic
ratio O/C (c), atomic ratio H/C (d), atomic ratio (O+N)/C (e) and CEC of corn-straw biochar (f), respectively. The symbols &, $, m and . represented
pyrolysis temperatures at 400C, 500C, 600C and 700C, respectively.
C-BC700 that had the highest C content among the C-BCs as a result of a high
degree of carbonization at a high pyrolysis temperature , had the lowest
adsorption amount of NH4+-N. In contrast, Qe was positively correlated with
content of O in the biochar (r50.9104) (Figure 3b). This indicated that removal
of the O-carrying functional groups with increasing pyrolysis temperature
induced the increase in the hydrophobicity of the BC600 and the BC700. As
mentioned above, the biochars produced at high pyrolysis temperatures had low
polarity (i.e. low O/C ratio) and thus low ability to adsorb NH4+. Positive
correlations between Qe and O/C (r50.9264), H/C (r50.8633) and (O+N)/C
(r50.9275) were respectively observed (Figure 3ce). All these implied a decrease
in NH4+-N adsorption ability with decreasing polarity of biochars. In the present
study, C-BCs had distinctly higher NH4+-N adsorption than W-BCs and P-BCs
(Figure 2), despite the facts that C-BCs had obviously low specific surface area
compared with the other biochars (Table 2). In addition, there was no clear trend
of pore volume and pore size that could reflect the difference between biochars
from different feedstocks, which was consistent with the finding by Yao et al. 
in a test of thirteen biochars. These suggest that specific surface area and pore
structures were not dominant factors affecting NH4+-N adsorption to biochars.
CEC seems to be the dominating factor influencing NH4+-N adsorption ability
of biochars. The Qe values were positively correlated with CEC (r50.4633)
(Figure 3f). That is to say, the biochars with higher CEC values had larger
NH4+N sorption capacity. In the present study, C-BC had a higher adsorption capacity
compared with W-BC and P-BC (Figure 2), which is most likely to be a result of
the higher CEC values. For example, at 50 mg NH4+ L21 solutions, the C-BC with
CEC of 19.068.6 cmol kg21 presented obviously higher NH4+-N sorption
capacity (1.62.3 mg g21) than the W-BC (0.60.9 mg g21) and P-BC (0.6
1.2 mg g21) with CEC of 0.38.5 cmol kg21. In addition, biochars derived from
each feedstock at pyrolysis temperatures of 600700C had relatively low CEC,
compared with those at 400500C (Table 2). For instance, the CEC values of
CBCs decreased by 72% when the pyrolysis temperatures increased from 500 to
700C. Correspondingly, NH4+-N adsorption ability of the biochars decreased
with increasing pyrolysis temperatures. The decrease in CEC with increasing
temperature can be attributed to the loss of carboxyl functional groups during
Nitrate nitrogen sorption on different types of biochars
In contrast to NH4+, no NO32-N could be sorbed by W-BC400-700,
C-BC400700 and P-BC400-700 at series of NO32 concentrations (10300 mg NO32 L21).
On contrary, these biochars even released NO32-N into the solutions (Figure 4ac).
Disability of biochars to adsorb NO32-N was in agreement with the previous
sorption experiments with the biochar made from sugarcane (Saccharum
officinarum L.) bagasse (particle sizes 250500 mm) at a temperature range of 400
to 600C . In the present study, the six-biochars made at a lower temperature
(400500C) released 0.250.40 mg NO32-N g21 to the solution with an initial
Figure 4. Sorption isotherms of NO32-N on wheat-straw biochar (W-BC), corn-straw biochar (C-BC) and peanut-shell biochar (P-BC) at different
pyrolytic temperatures (Qe: the amount of NO32-N sorbed by per unit mass of biochar at equilibrium; Ce: concentration of NO32-N in the solution
at equilibrium). Bars indicate standard deviation of three replicates.
NO32 concentration of 50 mg L21 (Figure 4ac). The other six biochars pyrolysed
at higher temperatures released slightly less NO32-N, at 0.160.32 mg g21. Release
of N in proportion of total N in biochar ash was demonstrated with the following
example. At an initial concentration of 10 mg NO32 L21, the N content in the ash
of C-BC400 was 2.4% and an amount of 17 mg NO32 was added with biochars to
the 25 mL solution (Table 2). At equilibrium, the concentration of NO32 in the
solution was 18.95 mg L21. That is to say, the N released accounted for 2.1% of the
total N in the C-BC400. However, these results were opposite with the findings of
previous studies, which reported that NO32-N could be sorbed by biochars. For
example, Mizuta et al.  reported that bamboo biochar powder (280 mm) made
at 900C had NO32-N adsorption capacity of 20.2 mg g21 as estimated by the
Langmuir model. A recent study by Hollister et al.  demonstrated that
approximately 1.6 mg NO32-N g21 was adsorbed by the 800C treated
bagassebiochar in the solution with an initial concentration of 20 mg NO32-N L21.
Meanwhile, one study on NO32-N sorption to bamboo-biochar (300500 mm)
gave a maximum sorption capacity of 7.1 mg NO32-N g21 predicted with the
Langmuir adsorption model . The weak ability of biochar to adsorb NO32-N at
different NO32 concentrations in Mizuta et al.  and Hollister et al.  may be
because their biochars were produced at a higher pyrolysis temperature (.800C)
than in the present study. Moreover, there may be other mechanisms involved to
affect NO32-N leaching in soil than direct adsorption of NO32-N by biochars.
Knowles et al.  found that biochar application reduced nitrate leaching from
biosolid amended soils to levels at or below that in the control treatments in
lysimeter experiments. Since we did not investigate the pyrolysis temperature higher
than 700C or N behavior in soil, further studies are needed in this regard to fully
understand the mechanisms governing NO32-N retention to biochars.
Washing with acid and DI water had a significant effect on biochar adsorption
of NO32-N (Table 3). In contrast to releasing of NO32-N from the non-washed
C-BC to solutions, small quantities of NO32-N were adsorbed by A-C-BC
(0.14 mg g21) and W-C-BC (0.12 mg g21) from 50 mg NO32 L21 solutions. This
trend was similar for W-BC and for P-BC (Table 3). First of all, washing with acid
and DI water removed ash from biochars, and thus no NO32-N was added with
the biochar ash to the solution. In addition, removal of the ash from biochars
might have created additional sorption sites on biochar surface and facilitated
more sorption of NO32-N . Afkhami et al.  suggested that treatment of
biochars with acid tends to produce positive sites on the biochars, by protonation
of surface -OH groups that would cause an increase in electrostatic adsorption of
Feedstock types and pyrolysis temperature greatly influenced the biochar chemical
and physical characteristics, which further influenced N adsorption ability of the
biochars. Adsorption of NH4+-N was predominantly affected by the CEC of
biochars. The corn-straw biochar had the largest adsorption capacity for NH4+-N,
in particular at a pyrolysis temperature of 400C. In contrast, biochars released
NO32-N to the solutions rather than adsorb NO32-N. However, retention of
NO32-N by biochar may be enhanced by promoting pyrolysis temperature or
other mechanisms in soils. Therefore, we conclude that biochars, in particular
corn-straw biochar (400C), can be used under conditions where NH4+-N (or
NH3) pollution is a concern, but further research is needed in terms of applying
biochars to reduce NO32-N pollution.
Figure S1. Field emission-scanning electron microscopy (FE-SEM) images of
biochars derived from corn straw pyrolytic at different temperatures and with
different treatments (a: 400C, b: 500C, c: 600C, d: 700C, e: 500C and treated
with diluted H2SO4, f: 500C and treated with DI water).
Figure S2. The X-ray diffraction (XRD) spectrum of wheat-straw biochar
(WBC), corn-straw biochar (C-BC) and peanut-shell biochar (P-BC) at different
This work was conducted in the test station of Fluvo-aquic soil in Changping
County, Beijing, China. Thanks for the assistance of BoYang, Qian Zhang and
Conceived and designed the experiments: HBL HYW TZR. Performed the
experiments: XPG SL. Analyzed the data: XPG HBL HYW JL LMZ TZR.
Contributed reagents/materials/analysis tools: XPG HYW SL. Wrote the paper:
XPG HYW JL.
8. IFA/IFDC/IPI/PPI/FAO. Fertilizer Use by Crop. 2009.
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