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Value of Functionalized Charcoal for Increasing the Efficiency of Urea N Uptake: Insights into the Functionalization Process and the Physicochemical Characteristics of Charcoal

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Functionalized charcoal (CHox) incorporated into urea is known for its ability to reduce NH3 volatilization and increase agronomic efficiency. However, it is important to optimize the functionalization process and to elucidate its relationship with the physicochemical properties of CHox for N supply. Thus, charcoal obtained from eucalyptus wood was functionalized with different HNO3 concentrations and reaction times. Ammonia adsorption by CHox was evaluated in chambers with high NH3 concentrations. Dry matter yield, N uptake, and apparent N recovery efficiency of corn plants were evaluated after the application of the urea-CHox mixture to soil in a greenhouse experiment. The properties of CHox, such as pH, isoelectric point, and total acidity (carboxylic and phenolics groups) depended on the HNO3 concentration but were not influenced by the reaction time. The NH3 adsorption by the functionalized charcoal showed a positive correlation with the quantity of carboxylic and phenolic groups and a negative correlation with the pH value and the isoelectric point. The small differences observed in dry matter yield, N accumulation, and apparent N recovery efficiency among the corn plants from urea mixed with CHox or humic acids derived from charcoal (AHCH) are not sufficient to determine the higher efficiency of these sources.
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Rev Bras Cienc Solo 2019;43:e0180200
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https://doi.org/10.1590/18069657rbcs20180200
* Corresponding author:
E-mail: paivadmendes@gmail.com
Received: September 23, 2018
Approved: May 06, 2019
How to cite: Paiva DM,
Guimarães GGF, Teixeira
BC, Cantarutti RB. Value of
functionalized charcoal for
increasing the eciency of
urea N uptake: insights into the
functionalization process and the
physicochemical characteristics
of charcoal. Rev Bras Cienc Solo.
2019;43:e0180200.
https://doi.org/10.1590/18069657rbcs20180200
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and source are credited.
Value of Functionalized Charcoal
for Increasing the Eciency of
Urea N Uptake: Insights into the
Functionalization Process and the
Physicochemical Characteristics
of Charcoal
Diogo Mendes de Paiva(1)* , Gelton Geraldo Fernandes Guimarães(2) , Breno
Cardoso Teixeira(1) and Reinaldo Bertola Cantarutti(3)
(1) Universidade Federal de Viçosa, Departamento de Solos, Programa de Pós-Graduação em Solos e
Nutrição de Plantas, Viçosa, Minas Gerais, Brasil.
(2) Empresa de Pesquisa Agropecuária e Extensão Rural de Santa Catariana, Itajaí, Santa Catarina, Brasil.
(3) Universidade Federal de Viçosa, Departamento de Solos, Viçosa, Minas Gerais, Brasil.
ABSTRACT: Functionalized charcoal (CHox) incorporated into urea is known for its
ability to reduce NH3 volatilization and increase agronomic eciency. However, it is
important to optimize the functionalization process and to elucidate its relationship
with the physicochemical properties of CHox for N supply. Thus, charcoal obtained
from eucalyptus wood was functionalized with dierent HNO3 concentrations and
reaction times. Ammonia adsorption by CHox was evaluated in chambers with high
NH3 concentrations. Dry matter yield, N uptake, and apparent N recovery eciency of
corn plants were evaluated after the application of the urea-CHox mixture to soil in a
greenhouse experiment. The properties of CHox, such as pH, isoelectric point, and total
acidity (carboxylic and phenolics groups) depended on the HNO3 concentration but were
not inuenced by the reaction time. The NH3 adsorption by the functionalized charcoal
showed a positive correlation with the quantity of carboxylic and phenolic groups and a
negative correlation with the pH value and the isoelectric point. The small dierences
observed in dry matter yield, N accumulation, and apparent N recovery eciency among
the corn plants from urea mixed with CHox or humic acids derived from charcoal (AHCH)
are not sucient to determine the higher eciency of these sources.
Keywords: biochar, NH3 adsorption, NH3 volatilization, ecient fertilization.
Division – Soil Use and Management | Commission – Soil Fertility and Plant Nutrition
Paiva et al. Value of functionalized charcoal for increasing the eciency of urea N...
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Rev Bras Cienc Solo 2019;43:e0180200
INTRODUCTION
Urea is the main nitrogen (N) fertilizer used in crop fertilization in Brazil. However, the
agronomic eciency of urea is low due to N losses via NH
3
volatilization. Currently,
there are major research eorts to develop new technologies for the production of more
sustainable fertilizers (Lahr et al., 2016; He et al., 2017).
In this context, urea mixed with substances with high acid buer capacity, such as
functionalized charcoal, has shown potential for decreasing ammonia volatilization
(Paiva et al., 2012; Guimarães et al., 2015). Functionalized charcoal by HNO3 is
predominantly hydrophilic, and a large part of its functional groups contains oxygen.
This facilitates the production of a material that behaves as a base or acid and has ion
exchange properties, increasing its potential as adsorbent of heavy metals, organic
molecules, and gases (Liu et al., 2007; Shafeeyan et al., 2010; Troca-Torrado et al., 2011).
Paiva et al. (2012) have promoted the functionalization of charcoal by oxidizing it with
HNO
3
, followed by the separation of fractions according to the methodology of the
International Society of Humic Substances (Swift, 1996). With this amendment, coating
urea reduces the volatilization of NH
3
by approximately 50 % compared to uncoated urea.
In sequence, Guimarães et al. (2015) have observed a 40 % reduction in volatilization by
using functionalized charcoal obtained according to Paiva et al. (2012), but without the
separation of fractions. Both authors concluded that the volatilization of N-NH3 from urea
can be controlled with substances that have a high CEC (Charge Exchange Capacity),
which do not only enhance NH4
+ retention formed by urea hydrolysis but also contribute
to the buering capacity, thereby moderating the pH increase caused by hydrolysis.
Given the above, it is important to know the best combination of HNO3 concentration
and reaction time for obtaining the functionalized charcoal. According to Boehm (2002),
this binomial (HNO3 concentration vs. reaction time) determines the oxidizing power of
HNO3, which is highest with the acid heated to boiling. However, the concentration of
HNO3 and the reaction time to optimize the process of obtaining functionalized charcoal
as well as its physicochemical properties are still unknown.
Thus, the objective of this work was to study the inuences of HNO3 concentration and
reaction time on the physicochemical properties of charcoal and to evaluate the ability
of urea-functionalized charcoal to retain NH3 and to provide N to corn plants.
MATERIALS AND METHODS
Production of functionalized charcoal
Charcoal (CH) was obtained from Eucalyptus grandis wood blocks carbonized in a
laboratory oven at 350 °C in the presence of O2. This temperature was attained in 4 h
(following a constant heating rate until obtaining the nal temperature of 350 °C) and was
maintained for 4 h, resulting in 8 h of carbonization. The temperature was monitored by
a thermostat wrapped around the wood. The charcoal was crushed in an ultracentrifugal
mill until its granulometry was less than 149 μm and was then dried at 105 °C for 12 h.
Nine CHox were obtained using the HNO
3
concentrations of 0.5, 1.7, 4.5, 7.3, and 8.5 mol L
-1
and reaction times of 0.5, 1.5, 4.0, 6.5, and 7.5 h, according to the experimental central
compound matrix (2k + 2k + 1) and describe CHox1, CHox2, CHox3, CHox4, CHox5,
CHox6, CHox7, CHox8, CHox9 in according the table 1. The central point of the matrix
was dened by the concentration of 4.5 mol L-1 and a reaction time of 4 h, which were
used by Paiva et al. (2012) with two replicates.
For the production of the each CHox, 50 g of CH and 1 L of HNO3, with the respective
concentrations, were mixed, heated to boiling, and kept under reux at the respective
Paiva et al. Value of functionalized charcoal for increasing the eciency of urea N...
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Rev Bras Cienc Solo 2019;43:e0180200
reaction times. At the end of each period, the mixtures were kept at rest for 12 h at room
temperature and subsequently slowly ltered through a lter paper under vacuum; the
retained material was transferred to cellophane bags where it underwent dialysis in
distilled water. The water was changed twice daily until the increase in conductivity 1 h
after the replacement was lower than 1 μS cm-1 (Benites et al., 2005). After dialysis, the
CHox samples were dried at 60 °C, weighed, and ground to obtain a granulometry less
than 149 μm; subsequently, they were stored in a desiccator.
We also produced a compound according to the method described in Paiva et al.
(2012) to obtain humic acids from CH (AH
CH
), following the methodology described to
obtain CHox of the central point of the matrix previously related. This compound was
applied to the extraction of fulvic acids, humic acids, and humins, using the extraction
procedure of the International Society of Humic Substances, according to Swift (1996).
For this, the material retained in the lter was added to a solution of NaOH 1 mol L-1
until a pH of 12.0 was obtained and left to stand for 12 h. Subsequently, the pH
of the mixture was adjusted to about 2.0 with H2SO4, and the mixture was kept at
room temperature for 12 h and subsequently centrifuged at 3,345 g for 30 min; the
supernatant was discarded. The decanted material was resolubilized in 500 mL of
NaOH 1.0 mol L-1, held for 4 h, re-acidied, and centrifuged as previously described.
The supernatant was discarded, and the decanted material (AHCH) was transferred to
cellophane paper bags where it was dialyzed with distilled water as described above.
Subsequently, the AHCH was dried at 60 °C, crushed to a granulometry of less than
149 μm, and stored in a desiccator.
Characterization of functionalized charcoal
The contents of C, H, and N were determined by combustion in an elemental analyzer
(Perkin Elmer 2400 Series II CHNS/O), and oxygen was determined by the dierence
between the initial mass and the C, H, N, and ash contents. The atomic ratio of C/N
was determined.
The isoelectric point of CHox and AH
CH
was determined by mass titration, according
to Noh and Schwarz (1989). For the quantication of total acidity, which characterizes
the potential cation exchange capacity (CEC), acid-base potentiometric titration was
Table 1. Carbon, H, N, and O contents of the nine functionalized charcoals (CHox) obtained according to the concentration of nitric
acid and the reaction time of the humic acid obtained from charcoal (AHCH) and eucalyptus charcoal (CH) as raw materials
CHox HNO3Time C H N O C/N(1)
mol L-1 h %
1 1.7 1.5 61.87 3.13 2.60 32.40 27.75
2 1.7 6.5 59.79 3.00 2.75 34.46 25.35
3 7.3 1.5 55.81 3.01 3.23 37.95 20.15
4 7.3 6.5 55.90 3.05 3.18 37.87 20.50
5 0.5 4.0 65.88 3.15 2.84 28.13 27.05
6 8.5 4.0 54.70 2.96 3.39 38.95 18.82
7 4.5 0.5 58.60 3.08 3.69 34.63 18.52
8 4.5 7.5 56.17 3.03 3.28 37.52 19.97
9 4.5 4.0 56.39 3.01 3.79 36.81 17.35
Average 58.35 3.05 3.19 35.41 21.72
Condence level (95 %) ±1.76 ±0.03 ±0.20 ±1.67 ±1.90
AHCH 4.5 4.0 52.80 3.19 2.91 41.62 21.16
CH 78.42 3.71 0.05 17.82 1,829.04
(1)
Molar ratio. C, H, and N were determined by combustion in an elemental analyzer (Perkin Elmer 2400 Series II CHNS/O), and oxygen was determined
by the dierence between the initial mass and the C, H, N, and ash contents.
Paiva et al. Value of functionalized charcoal for increasing the eciency of urea N...
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Rev Bras Cienc Solo 2019;43:e0180200
performed to quantify carboxylic and phenolic functional groups, according to Inbar et al.
(1990) and Bowles et al. (1989).
Photoelectric X-ray spectroscopy (XPS) was performed in a VSW HA-100 spherical
analyzer using an aluminum anode (Al Kα, hv = 1486.6 eV). The constant passage of
incident energy was 44 eV, in a xed transmission mode providing a line width of 1.6 eV
of Au 4f 7/2. The pressure employed during the analysis was less than 2 × 10-8 mbar.
To correct the binding energies, the C 1s line with the binding energy of 284.6 eV was
used as reference. Qualitative interpretation of the spectra was performed according
to Abe et al. (2000).
Adsorption of NH3 by functionalized charcoal
The adsorption capacity of NH3 by CHox and AHCH was evaluated in chambers with high
concentrations of NH
3
. The chambers were made of 300 cm
3
glass vials with internal
support to hold the Petri dishes containing 1 g of each of the nine CHox and AHCH samples.
Inside the chambers, 10 mL of 0.3 mol L
-1
(NH
4
)
2
SO
4
were placed, and the chambers
were hermetically sealed. Subsequently, 10 mL of 0.67 mol L-1 NaOH were injected into
the chambers through a rubber-sealed hole. For alkalinization, a high concentration of
NH3 was provided.
The samples remained in the chamber for 7 days in a temperature-controlled environment
at 25 °C (± 2 °C). After this period, 10 mL of H2SO4 0.3 mol L-1 were added to each
chamber, and after 2 h, the chambers were opened. The Petri dishes with the samples
were transferred to an oven at 30 °C and kept there for 24 h. Subsequently, the N content
was determined with an elemental analyzer (Perkin Elmer 2400 Series II CHNS/O Analyzer).
Adsorbed N-NH
3
was estimated by the dierence between the N contents after and before
incubation. Evaluations were performed based on three replicates of each sample.
Supply of N from Urea-CHox
A greenhouse experiment was carried out using urea mixtures with CHox6 (U-CHox6),
CHox9 (U-CHox9), and AHCH (U-AHCH) in a proportion of 500 g kg-1, as well as urea and
NH4NO3, applied at four N doses (0, 90, 180, and 270 mg dm-3) to evaluate N supply
to corn plants (Zea mays L.). According to table 1, CHox6, CHox9, and AHCH represent
the functionalized charcoal; NH4NO3 was used as a reference N fertilizer without N-NH3
volatilization. The experiment was arranged in randomized blocks with four replicates.
Samples of the 0.00-0.20 m layer of an air-dried Oxisol (Typic Hapludox/Latossolo) were
used, with particle size less than 2 mm and the following characteristics: pH(H2O) 4.6,
measured with a glass electrode; organic carbon of 12.5 g dm
-3
, measured by the
Walkley-Black method according to Nelson and Sommers (1996); 2.3 mg dm
-3
of available
P (Mehlich-1) according to Oliveira et al. (1979); 21 mg dm-3 of available K, according to
Embrapa (1979); Ca, Mg, and Al contents of 0.17, 0.05, and 0.92 cmol
c
dm
-3
, respectively,
according to Embrapa (1979); residual remaining phosphorus (P-rem) of 9 mg L
-1
according
to Alvarez et al. (2000); and potential acidity (H+Al) of 6.4 cmol
c
dm
-3
, according to
Oliveira et al. (1979).
The soil was limed to neutralize Al, and the levels of Ca+Mg were raised to 2 cmolc dm-3
with the application of 2.0 g dm
-3
of a mixture of CaCO
3
+MgCO
3
in the molar ratio
of 3:1, reaching a pH of 6.0 after soil incubation. The experimental units consisted
of plastic pots containing 2.5 dm3 of soil and ve early maturing hybrid corn plants.
Sowing was performed after soil fertilization with 300 mg dm-3 of P, 150 mg dm-3 of K,
40 mg dm
-3
of S, 0.81 mg dm
-3
of B, 1.33 mg dm
-3
of Cu, 1.55 mg dm
-3
of Fe (as Fe-EDTA),
3.66 mg dm
-3
of Mn, 0.15 mg dm
-3
of Mo, and 4 mg dm
-3
of Zn. The nutrient doses,
including N, were dened according to Novais et al. (1991). The N doses, according
to the treatment, were applied at 30 days after emergence (stage of high nutrient
demand) by locating the fertilizer on the soil in the central part of the pot surface. The
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Rev Bras Cienc Solo 2019;43:e0180200
moisture content of the soil was maintained close to 70 % by applying distilled water
periodically on the soil surface.
After 60 days of growth (stage of owering), the plants were cut 1 cm above the ground.
The plant material was dried (65 °C for 72 h), ground, and the N content was determined
by Kjeldahl digestion and distillation, according to Bremner and Mulvaney (1982). Based
on the accumulated dry mass and N content, the accumulated N content was estimated,
and thus, the apparent N recovery eciency was estimated using equation 1:
× 100
Apparent N recovery index (%) = N1 – N0
total N applied
Eq. 1
in which N1 refers to the N content in the corn plants in each dose of N applied and N0
refers to the N content in the corn plants at an N dose of zero.
Statistical analysis
The potentiometric characterization and NH
3
adsorption data of the functionalized charcoal
were submitted to variance analysis. Regression equations were adjusted according to
the complete model: ŷ = β0 + β1 t + β2 t2 + β3 H + β4 H2 + β5 tH + Ɛ, where “H” is
the concentration of HNO3 in mol L-1 and “t” is the reaction time in hours. The eects of
these factors were evaluated by the signicance of the regression coecients at p<0.10.
The average values of N adsorbed by each treatment were submitted to contrast analyses
among treatments. Also, the values of N adsorbed were submitted to the linear correlation
of variable responses.
Dry matter yield, N uptake, and apparent N recovery eciency values were submitted to
variance analysis, and the eects of the treatments were compared through regression,
with the coecients of the equations being based on the mean square of the residue
of the analysis of variance at p<0.10. Also, averages of dry matter yield and N uptake
were compared for each N dose by Tukey’s test (p = 0.05).
The package Minitab Statistical Software 14 (Minitab Inc., State College, Pennsylvania, US)
was used to carry out all analyses.
RESULTS
Characterization functionalized charcoal
Functionalization with HNO3 promoted an increase up to 75 times in the N and up to
2.2 times in the O content of CHox and AHCH, while the amount of C decreased with
respect to CH (Table 1). The AH
CH
showed a higher O content than CHox, while the
contents of C and N were higher for CHox. The pH of CH in water was 5.86, while that
of AHCH was 3.56 and that of CHox ranged from 2.73 to 3.25 (Table 2). The isoelectric
points indicate that the positive and negative charges in the compounds are balanced
at a pH between 1.57 and 2.08, while for charcoal, the PI was 6.06, being higher than
its pH in water (Table 2).
The pH of CHox was inuenced only by the concentration of HNO
3
, as shown by the adjusted
regression equation (Table 3). The isoelectric point, however, was neither signicantly
inuenced by the concentration of HNO3 nor by the reaction time, corresponding to a
mean value of 1.68 (Table 2).
The total acidity of AHCH was 11 times higher than that of CH, while the total acidity of
CHox was between 5.7 and 12.9 times higher than that of CH (Table 2). For CHox, the
carboxyl groups presented a total acidity of 59 to 74 %, while for AHCH, total acidity was
76 %, based on the data presented in table 2. The carboxyl group content of the CHox
Paiva et al. Value of functionalized charcoal for increasing the eciency of urea N...
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Rev Bras Cienc Solo 2019;43:e0180200
increased linearly with the concentration of HNO3, while the contents of phenolic groups
increased in a quadratic manner (Table 3). Considering the prevalence of carboxylic
groups, total acidity also increased linearly, rearming the irrelevance of the reaction
time (Table 3).
Characteristic peaks of carboxylic, phenolic, and ether groups were identied in the
surface structure of the compounds evaluated (Table 4). For CHox, the C identied
between 9 and 14 % is attributed to carboxylic groups, whereas for AH
CH
, this ratio
was 15 % and for CH, it was only 5 %. The relative proportion of C, based on C 1s
spectra, associated with phenolic groups or ether for CHox, varied between 13 and
24 %; for AHCH, it was 16, whereas it was 15 % for CH. The structures described based
on C 1s spectra are conrmed with the spectra obtained for O 1s (Table 4), with the
identication of peaks related to C=O bonds, which may represent carboxylic acids
and aromatic bonds of O and C (CAr) of phenols or ether. There was a lower amount
of O-CAr in both CHox and AHCH than in CH, agreeing with the previous assertion that
there is a reduction in the amount of ether groups with the formation of carboxylic
groups after functionalization with HNO3.
The O 1s spectra results indicate greater similarity between CHox and AHCH. However, the
XPS analysis of O was less conclusive than for C, since the identication of these types of
bonds may relate to more than one functional group, for example, C=O for carboxylic or
aldehydes (Abe et al., 2000). The N 1s spectra revealed only two absorption peaks, with
Table 2. Isoelectronic points (PI), pH values in water, quantity of carboxylic groups, phenolics, and total acidity values of the
functionalized charcoals (CHox) obtained according to nitric acid concentration and reaction time of humic acid obtained from charcoal
(AHCH) and eucalyptus charcoal (CH)
CHox HNO3Time pH(H2O)(1) PI(2) Carboxylic(3) Phenolic(3) Total(4)
mol L-1 h mmolc kg-1
1 1.7 1.5 2.97 1.68 1,671 1,157 2,828
2 1.7 6.5 2.91 1.66 2,216 1,266 3,482
3 7.3 1.5 2.81 1.57 2,970 1,516 4,486
4 7.3 6.5 2.73 1.59 3,381 1,148 4,529
5 0.5 4.0 3.25 2.08 1,263 884 2,147
6 8.5 4.0 2.84 1.62 3,616 1,269 4,885
7 4.5 0.5 2.88 1.67 2,081 1,387 3,468
8 4.5 7.5 2.82 1.62 3,435 1,400 4,835
9 4.5 4.0 2.79 1.64 3,134 1,407 4,541
Average 2.89 1.68 2,641 1,271 3,911
Condence level (95 %) ±0.12 ±0.10 ±586 ±131 ±671
AHCH 3.56 1.64 3,214 1,008 4,222
CH 5.86 6.06 251 126 377
(1) Solid:solution ratio of 1:10. (2) PI obtained by mass titration. (3) Estimated by potentiometric titration, from the titrant volumes to raise the pH from
3 to 8 (carboxylic) and from 8 to 10 (phenolics). (4) Sum of the contents of carboxylic and phenolic groups.
Table 3. Regression equations adjusted to pH in water and functional groups contents according
to the concentration of HNO3 (H) and reaction time (t) expressed in hours
pH and content of
functional groups Regression equation R2
pH(H2O) ŷ = 3.29 - 0.158***H - 0.011 t + 0.013***H20.88
Carboxylic ŷ = 1531.4 + 257.4***H 0.70
Phenolics ŷ = 901.3 + 212.4**H - 12.2 t - 19.7**H20.74
Carboxylic + Phenolics ŷ = 2658.2 + 292.4***H 0.68
*** Signicant at p<0.01; ** signicant at p<0.05.
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Rev Bras Cienc Solo 2019;43:e0180200
one of them being related to C-N-C type bonds, which could be attributed to N insertion
into an aromatic C structure (pyrrole or pyridone), with the formation of amines (Table 4).
Adsorption of NH3 by functionalized charcoal
Table 5 shows the adsorption of NH3 on the surface of the charcoals. The AHCH adsorbed
5.9 times more N-NH
3
than CH, whilst CHox, with the exception of CHox5, presented
a 3 to 9 times higher adsorption than CH. The highest correlation observed was with
the carboxylic groups (Table 6), indicating that its greater presence favors adsorption
compared to the presence of phenolic groups. These results were conrmed through the
lower correlation and the signicance observed for NH3 adsorption on phenolic groups.
The pH and PI also showed a correlation with N-NH3 adsorption, indicating that lower pH
and PI values favor NH3 adsorption on CHox.
Due to the better result showed by CHox6 in terms of NH3 adsorption, this functionalized
charcoal was selected from the matrix (Table 1) to evaluate its interaction with urea in a
plant fertilization experiment. In addition to CHox6, the centre point of the matrix “CHox9”
was evaluated and represented the functionalized charcoal under the same conditions
as described in Paiva et al. (2012) and Guimarães et al. (2015).
Supply of N from Urea-CHox
The corn fertilized with urea had a lower rate of dry matter accumulation as a function of
the N dose than corn fertilized with other fertilizers, as shown by the linear coecients
of the regression equations (Figure 1). The linear coecients of the equations for
U-CHox6, U-CHox9, and U-AH
CH
were greater or close to those adjusted for the response
to NH4NO3, indicating that the responses of corn plants fertilized with urea combined
with CHox or AH
CH
could be equivalent to the response to NH
4
NO
3
. The levels of N
accumulated in the corn shoots of the corn plants increased linearly with the N rates
for all fertilizers (Figure 2). The highest increase in accumulated N was veried for
NH4NO3, while urea and U-CHox6 had the lowest rates. However, U-CHox9 and U-AHCH
presented lower increase rates than NH
4
NO
3
, but 8 and 13 % higher rates, respectively,
than those estimated for urea.
Table 4. Relative proportions of the surface chemical species of C, O, and N, identied by the XPS spectra of C 1s, O 1s, and N 1s,
respectively, in CHox according to the concentration of HNO3 and the reaction time, for humic acids obtained from charcoal (AHcH)
and eucalyptus charcoal (CH)
CHox HNO3Time Aromatic or
aliphatic C
Carboxylic
groups
Phenolic or
ether groups
O bound to
aromatic C C=O
N pyrrol
pyridine or
amines
NO2 groups/
oxidized
nitrogen
functions
mol L-1 h %
1 1.7 1.5 76 9 15 55 43 65 35
2 1.7 6.5 54 10 24 39 49 59 41
3 7.3 1.5 68 13 16 49 51 49 51
4 7.3 6.5 66 10 13 48 49 51 49
5 0.5 4.0 68 10 19 37 60 69 31
6 8.5 4.0 70 14 13 48 43 51 49
7 4.5 0.5 73 10 17 48 52 79 21
8 4.5 7.5 63 13 24 50 50 59 41
9 4.5 4.0 71 13 16 50 50 68 32
Average 71.5 16.5 12.0 69.5 30.5 71.5 28.5
Condence level (95 %) ±0.9 ±0.9 ±1.9 ±38.2 ±35.7 ±6.8 ±5.9
AHCH 69 15 16 49 51 52 38
CH 80 5 15 74 26 - -
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Rev Bras Cienc Solo 2019;43:e0180200
For equivalent N doses required to reach 90 % of the estimated maximum yield
for NH
4
NO
3
, a 34 % higher dose for urea was required, whereas for U-CHox6 and
U-CHox9, the doses would be 1 and 11 % lower, respectively, and for U-AHCH, the
dose would be 7 % higher (Table 7). The lower eciency of urea is also evidenced in
the equivalent N dose to reach 80 % of the accumulated N content in relation to the
corn plants fertilized with NH4NO3, in that there is a need for a 30 % higher N dose
(Table 7). For U-CHox6, the required dose should be 40 % higher, and for U-CHox9
and U-AH
CH
,
these doses would be 15 and 20 %, respectively, higher in relation to
the dose of N applied as NH4NO3.
By evaluating the means of dry matter yield and N uptake within each dose (Table 8),
corn plants fertilized with U-CHox9 and U-AHCH showed statistically higher dry matter
yields at the N dose of 180 mg dm-3 and equivalence in the N content at 270 mg dm-3
of N to NH4NO3. Although the N content at this dose is still equivalent to that observed
Table 5. The N-NH3 adsorbed by functionalized charcoals (CHox) produced according to the
concentration of HNO3 and the reaction time, for the humic acid obtained from charcoal (AHCH)
and eucalyptus charcoal (CH), and the average contrasts between the amounts of N adsorbed in
treatments associated with statistical signicance
CHox HNO3Time N-NH3 adsorbed
mol L-1 h mg g-1
1 1.7 1.5 18.50
2 1.7 6.5 19.70
3 7.3 1.5 35.23
4 7.3 6.5 41.87
5 0.5 4.0 5.53
6 8.5 4.0 53.53
7 4.5 0.5 43.23
8 4.5 7.5 55.87
9 4.5 4.0 48.57
AHCH 36.80
CH 6.17
Average 33.18
Condence level (95 %) 6.18
Average contrast
C1 = - CH vs (CHox1+CHox2+ ... +CHox9) 26.65*
C2 = - CH vs AHCH 3.06*
C3 = - CHox vs AHCH -0.89ns
C4 = - CHox5 vs (CHox1+CHox2+ ... +CHox9) 27.69*
* Signicant at p<0.05. ns: not signicant.
Table 6. Coecients of linear correlation between N-NH3 adsorption (mg g-1) and the quantity
of carboxylic groups (mmolc kg-1), or quantity of phenolics groups (mmolc kg-1), or pH value, or
isoelectronic point (PI) values
Physical-chemical properties Coecient
Carboxylic groups 0.88***
Phenolics groups 0.68**
pH - 0.80***
PI - 0.70**
*** Signicant at p<0.01; ** signicant at p<0.05.
Paiva et al. Value of functionalized charcoal for increasing the eciency of urea N...
9
Rev Bras Cienc Solo 2019;43:e0180200
for urea, the fact that it is statistically equal to NH4NO3 indicates the greater potential
of these sources in providing N when compared to urea.
The apparent N recovery eciency of corn plants was between 75.22 and 84.30 % when
fertilized with NH4NO3 (Table 9), while for other fertilizers, it was between 50.21 and
83.48 %. However, among fertilizers, the apparent N recovery eciency was statistically
equal (Table 9), and in each fertilizer, statistical dierence was only found among doses
to U-AHCH and NH4NO3.
Dose of N (mg dm
-3
)
090180 270
Dry matter (g pot
-1
)
0
20
30
40
50
NH
4
NO
3
Ureia
U-CHox6
U-CHox9
U-AH
CH
Dose zero of N, commom for all sources
y = 21.67 + 0.252***N – 0.00059***N
2
R
2
= 0.96
y = 21.91 + 0.180***N – 0.00029**N
2
R
2
= 0.94
y = 21.20 + 0.273***N – 0.00072***N
2
R
2
= 0.99
y = 20.63 + 0.292***N – 0.00074***N
2
R
2
= 0.99
y = 20.15 + 0.252***N – 0.00055***N
2
R
2
= 0.98
Figure 1. Dry matter yield of corn plants as a function of N dosages applied as NH4NO3, urea, and
urea mixed with functionalized charcoals (U-CHox6 or U-CHox9) or humic acids derived from CHox
(U-AHCH). Equations representing the observed values are shown for p<0.01 (***) and p<0.05 (**).
Dose of N (mg dm
-3
)
090180 27
0
N uptake (mg pot
-1
)
0
100
200
300
400
500
600
700
NH
4
NO
3
Ureia
U-CHox6
U-CHox9
U-AH
CH
Dose zero of N, commom for all sources
y = 133 + 1.89***N R
2
= 0.99
y = 131 + 1.46***N R
2
= 0.99
y = 147 + 1.31***N R
2
= 0.99
y = 149 + 1.58***N R
2
= 0.93
y = 115 + 1.65***N R
2
= 0.97
Figure 2. Nitrogen uptake by corn plants as a function of N dosages applied as NH4NO3, urea,
and urea mixed with functionalized charcoals (U-CHox6 or U-CHox9) or humic acids derived
from functionalized charcoals (U-AHCH). Equations representing the observed values are shown
for p<0.01 (***).
Paiva et al. Value of functionalized charcoal for increasing the eciency of urea N...
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Rev Bras Cienc Solo 2019;43:e0180200
DISCUSSION
The results of the elemental analyses (Table 1) indicate an increase in the amount of O with
the reduction of the amount of C in the CHox and AHCH, evidencing the functionalization
eect of HNO
3
on CH, providing the formation of functional groups. Also, we assume that
AHCH are more saturated than CHox, suggesting that the separation of the alkaline soluble
material favors AH
CH
. The increase in aliphatic and saturated structures can facilitate
the formation of functional groups (Figueiredo et al., 1999), consequently resulting in
Table 7. Nitrogen dose to reach 90 % of maximum dry matter (DM) yield and 80 % of the highest
amount of N absorbed. Values estimated from the adjusted regression equations for DM yield and
N absorbed of corn as a function of N dosages applied as NH4NO3, urea, urea mixed with oxidized
charcoals (U-CHox6, U-CHox9) or humic acids derived from CHox (U-AHCH)
Fertilizer N dose
90 % of maximum DM yield 80 % of maximum N absorbed
mg dm-3
NH4NO3122.85 201.93
Urea 165.10 262.77
U-CHox6(1) 121.30 280.64
U-CHox9(1) 109.40 231.42
U-AHCH 131.00 242.21
(1) Mixture of urea with oxidized carbon CHox in the proportion of 500 g kg-1.
Table 8. Dry matter yield and N uptake by corn plants for each N dosage applied as NH4NO3, urea, and urea mixed with functionalized
charcoals (U-CHox6 or U-CHox9) or humic acids derived from CHox (U-AHCH)
Fertilizer
N dose
90 mg dm-3 180 mg dm-3 270 mg dm-3 90 mg dm-3 180 mg dm-3 270 mg dm-3
Dry matter yield N uptake
g pot-1 mg pot-1
NH4NO342.34 a 45.20 ab 47.81 a 311.55 a 486.84 a 629.61 b
Urea 39.22 a 41.38 a 50.34 a 273.30 a 400.99 a 517.96 ab
U-CHox6(1) 41.30 a 45.72 ab 42.94 a 295.82 a 393.89 a 401.05 a
U-CHox9(1) 40.62 a 49.71 b 45.70 a 300.28 a 497.55 a 531.51 ab
U-AHCH 36.57 a 49.56 b 47.70 a 234.86 a 448.85 a 546.14 ab
(1)
Mixture of urea with oxidized carbon CHox in the proportion of 500 g kg
-1
. Means followed by the same letter in a column are not signicantly
dierent according to Tukey’s test (p=0.05).
Table 9. Apparent N recovery by corn plants according to the doses of NH4NO3, urea, and urea
mixed with the functionalized charcoals (U-CHox6 or U-CHox9) or humic acids derived from CHox
(U-AHCH)
Fertilizer
N dose
90 mg dm-3 180 mg dm-3 270 mg dm-3
Apparent N recovery
%
NH4NO384.30 A b 81.10 A a 75.22 A a
Urea 67.30 A ab 62.03 A a 58.68 A a
U-CHox6177.31 A ab 60.45 A a 53.21 A a
U-CHox9179.29 A ab 83.48 A a 60.69 A a
U-AHCH 50.21 A a 72.66 A a 62.85 A a
Means followed by the same small letters in a line and capital letters in a column are not signicantly dierent
according to Tukey’s test (p=0.05).
Paiva et al. Value of functionalized charcoal for increasing the eciency of urea N...
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Rev Bras Cienc Solo 2019;43:e0180200
compounds with a more hydrophilic character, favoring the interaction with the soil.
In addition, nitrogenous groups are formed due to the increase in the amount of N and
to the lower C/N ratio.
Both the pH and PI of AHCH were similar to those of the functionalized charcoals,
indicating that the separation of the alkaline soluble material did not contribute to a
specic modication of these potentiometric characteristics. The dierences observed
for the pH in water and the isoelectric point between CH and both CHox and AHCH
indicate modications the carbonaceous surface due to functionalization with HNO
3
. The
low pH values are due to the functional groups formed and their intense protonation.
While the PI of CH is greater than its pH, the PI of CHox and AHCH are lower than their
respective pH, which suggests the formation of functional groups with low dissociation
potential, acting as weak acids. The low isoelectric point is interesting because the
dissociation of functional groups will occur at low pH values, contributing to acidication
and cation retention.
The potential cation exchange capacity (CEC) values of AHCH and CHox were similar to
those observed by Paiva et al. (2012) and Guimarães et al. (2015), being 4,400 and
4,750 mmolc kg-1, respectively. This result can be attributed to the functionalization of
the charcoal.
From the XPS spectra, the highest intensity peaks were identied in relation to the
characteristic energies of the chemical bonds in which C, O, and N participate. However,
as the energy range for the identication of phenolic groups or ether is the same, it
is not possible to conclude with precision the formation of one or the other, especially
considering that some of the CHox and the AHCH presented a proportion close to that
of CH. As the ether functional group is common in the structure of CH, and considering
that the potentiometric evaluations indicated an increase in the amount of phenolic
groups, this supports the hypothesis that parts of the structure formed by ether have
been converted into phenolic groups. Moreover, the lower proportion of aromatic and
aliphatic C chains in CH, as opposed to the higher amount of carboxyl groups in CHox and
AHCH, suggests the predominance of the transformation of parts of the carbon structure
into carboxyl groups.
The XPS spectrum contributes to enhancing the evidence of functional groups in
the structure of the compounds. Certainly, these observations relate to the nitro
compounds characterized by the NO
2
-bond in both aliphatic and aromatic structures.
As no type of N-containing bond was identied in CH, the insertion of N in CHox and
AHCH is evident.
The adsorption of the NH
3
by functionalized charcoal strongly suggests that both obtaining
AH-like compounds from CH and functionalization with HNO3 lead to the improvement
of the adsorbent properties of charcoal. Considering that the evaluation was done in a
media with a high NH3 concentration, adsorption may have occurred by two processes:
the occlusion of NH3 in the pores of the compounds or protonation of NH3, forming an
NH
4
+
ion, followed by its adsorption by functional groups. The computed adsorption
must be predominantly attributed to this process since during the drying of the sample,
it is possible that the NH
3
in the pores has been removed. Kaneko et al. (1992) also
attributed the high NH3 adsorption by charcoal to the functional groups generated by
its functionalization. High adsorption capacity is important because this would be one
of the main factors involved in the reduction of N losses by NH3 volatilization from urea
mixed with functionalized charcoal. Thus, this characteristic is extremely important for
this purpose.
The results of dry matter yield indicated that the interaction of urea with the
functionalized charcoal potentiated corn growth. In addition, a lower N dose was
required to obtain the same corn dry matter yield when urea was applied as U-CHox
Paiva et al. Value of functionalized charcoal for increasing the eciency of urea N...
12
Rev Bras Cienc Solo 2019;43:e0180200
and U-AHCH in contrast to applying urea alone. According to Guimarães et al. (2016),
the high CEC of functionalized charcoal increases the availability of mineral N in the
soil due to reducing N losses.
The treatments U-CHox6 and U-CHox9 showed an improved N-supply eciency compared
to the use of pure urea, at lower doses. The reduction in apparent N recovery eciency
for urea at higher doses is more probable because, in addition to reducing the eciency
of dry matter yield, a more saturated NH3 environment is created, favoring its loss
into the atmosphere. These latter results have also been observed by Primavesi et al.
(2004), who found a smaller apparent N recovery by coastcross grass for higher doses
of urea applied to the soil. The higher linear coecient observed for U-CHox6 suggests
a lower eciency of this source compared to U-CHox9, indicating that the use of CHox,
produced from functionalized charcoal with HNO3, at a higher concentration does not
provide a higher eciency in the supply of N. The apparent N recovery eciency
of U-AHCH was lower than that of pure urea; in the same way, the dry matter and N
accumulation of corn plants fertilized with this source were lower than those observed
for U-CHox6 and U-CHox9. However, Guimarães et al. (2015) found that compounds
with physical-chemical properties similar to those of AHCH were eective in reducing
N losses of urea. As discussed above, the mode of incorporation of the compound into
urea may have contributed to this result.
CONCLUSIONS
In summary, the physical and chemical properties of CHox are not inuenced by the
reaction time, but by the HNO
3
concentration.
Moreover, the HNO
3
concentration also
aects the capacity of CHox to adsorb NH3. Adsorption of NH3 by functionalized charcoal
showed a positive correlation with the quantity of carboxylic and phenolic groups and a
negative correlation with the pH value and the isoelectric point. These results lead us to
infer that there is potential for the CHox or AHCH compounds to improve the agronomic
eciency of urea. However, further studies are necessary to determine the most suitable
dose to compose the urea-based fertilizer and the optimum N dose.
ACKNOWLEDGMENTS
The authors thank the Brazilian agencies CNPq (Conselho Nacional de Desenvolvimento
Cientíco e Tecnológico), and CAPES (Coordenação de Aperfeiçoamento de Pessoal de
Nível Superior) for funding this study.
AUTHOR CONTRIBUTIONS
Conceptualization: Diogo Mendes de Paiva and Reinaldo Bertola Cantarutti.
Investigation: Diogo Mendes de Paiva, Gelton Geraldo Fernandes Guimarães, and
Breno Cardoso Teixeira.
Formal analysis: Diogo Mendes de Paiva and Gelton Geraldo Fernandes Guimarães.
Methodology: Diogo Mendes de Paiva and Reinaldo Bertola Cantarutti.
Software: Diogo Mendes de Paiva.
Writing – original draft preparation: Diogo Mendes de Paiva and Gelton Geraldo
Fernandes Guimarães.
Writing – review and editing: Diogo Mendes de Paiva and Gelton Geraldo Fernandes
Guimarães.
Paiva et al. Value of functionalized charcoal for increasing the eciency of urea N...
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