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Influence of the Surface Characteristics of Activated Carbon on the Adsorption of Herbicides (A Review)

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The influence of activation methods on the surface characteristics of activated carbons (ACs) and the relationships of these characteristics and the parameters of a medium with the efficiency of adsorption of herbicide molecules on ACs are considered. The main factors affecting the adsorption efficiency of ACs with respect to these pollutants are discussed.
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ISSN 0361-5219, Solid Fuel Chemistry, 2022, Vol. 56, No. 3, pp. 181–198. © Allerton Press, Inc., 2022.
Russian Text © The Author(s), 2022, published in Khimiya Tverdogo Topliva, 2022, No. 3, pp. 30–47.
Influence of the Surface Characteristics of Activated Carbon
on the Adsorption of Herbicides (A Review)
S. A. Kulaishina,*, M. D. Vedenyapinaa,**, and A. Yu. Kurmyshevaa,***
a Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
Moscow, 119991 Russia
*e-mail: s.kulaishin@mail.ru
**e-mail: mvedenyapina@yandex.ru
***e-mail: aukurm@gmail.com
Received October 5, 2021; revised January 31, 2022; accepted February 2, 2022
Abstract—The influence of activation methods on the surface characteristics of activated carbons (ACs) and
the relationships of these characteristics and the parameters of a medium with the efficiency of adsorption of
herbicide molecules on ACs are considered. The main factors affecting the adsorption efficiency of ACs with
respect to these pollutants are discussed.
Keywords: activated carbon, porosimetric characteristics of sorbents, surface groups of activated carbons,
adsorption conditions, herbicides
DOI: 10.3103/S0361521922030041
INTRODUCTION
In recent decades, the production of biologically
active organic substances (BAOSs), in particular, her-
bicides, which are widely used in agricultural and for-
estry activities, has been growing [1]. As a result of
active use, BAOSs were found in natural water bodies,
where they enter together with wastewater from agro-
industrial and agricultural complexes [2, 3]. There is
no universal strategy for the elimination of BAOSs due
to the wide variety of herbicides, which pollute waste-
water, [4]. The methods currently used for the
extraction and degradation of BAOSs include photo-
catalytic degradation [5, 6], Fenton process [6], elec-
trooxidation [7, 8], biological degradation [9], and
adsorption [10–13]. The above methods are effective,
but they have a number of disadvantages, for example,
biodegradation is a long process, which largely
depends on environmental conditions [14]. Electroo-
xidation and photocatalytic purification are energy-
consuming processes, and they require an external
source of UV radiation [15, 16]. The Fenton process
requires the complete production of hydroxide radi-
cals [17]. Among all of the listed methods, adsorption
is one of the most promising processes because it is
simple in design and operation [18–21]. A high effi-
ciency of adsorption with respect to herbicides was
noted in many literature sources, [12, 13, 22–32].
A wide range of adsorbents can be used in the adsorp-
tion process [27]. The disadvantage of the adsorption
method is that it is effective mainly at low pollutant
concentrations in the aquatic environment; therefore,
it is successfully used for the advanced treatment of
wastewater [33].
Activated carbons (ACs), which are used as adsor-
bents, are obtained from nonrenewable (brown coal,
hard coal, and anthracite) [34–39] and renewable raw
materials (nut shells, wood, plant seeds, cake, etc.)
[12, 13, 22–24, 40–42]. In a number of literary
sources, it was noted that ACs can also be obtained
from waste polymer materials, for example, polyeth-
ylene terephthalate (PET) [43] and car tires [44]. The
possibility of reusing activated carbons as adsorbents
after their regeneration was studied [41, 42, 45, 46].
In view of a wide variety of activated carbons and
the possibility of their production with desired surface
properties [47], it is of interest to consider the effect of
their surface characteristics on the efficiency of herbi-
cide adsorption from aqueous media.
A great contribution to the efficiency of adsorption
is made by the adsorbent preparation conditions: the
carbonization and activation of the feedstock [48].
The efficiency of adsorption also depends on process
parameters such as the pH of a medium [10, 49–56],
the effluent temperature [38, 48, 52], the presence of
biological impurities [57, 58], and the specific surface
area of AC [11, 24, 59].
This review considers the adsorption of the follow-
ing classes of organic compounds used as herbicides
and pesticides: aryloxylcarboxylic acids, aryloxyalkan-
ecarboxylic acids, parabens, chlorotriazines, aryloxy-
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SOLID FUEL CHEMISTRY Vol. 56 No. 3 2022
KULAISHIN et al.
phenoxypropionates, oxazolidinones, chloroacetoni-
lides, urea, organophosphorus compounds, neonicot-
inoids, triazoles, benzenethiazole, benzathiadiazone,
viologen, carbamates, and heterocyclic aromatic com-
pounds.
1. EFFECT OF PREPARATION CONDITIONS
ON THE SURFACE CHARACTERISTICS
OF ADSORBENTS
An obligatory step in the preparation of AC as an
adsorbent is the activation of carbon, which improves
its surface characteristics. According to published data
[60, 61], the developed specific surface area of an
adsorbent and the predominance of micropores on its
surface promote the more efficient adsorption of her-
bicide molecules. The predominant presence of mes-
opores on the sorbent surface facilitates the adsorption
of larger molecules [59]. In this section, the influence
of temperature and activation method on the porosim-
etric characteristics of ACs is considered.
1.1. Dependence of the surface properties of AC on
temperature and activation method. In order to improve
the surface characteristics, the sorbent can be acti-
vated by two methods, physical and chemical, or by a
combination of these methods [62]. Physical activa-
tion methods include heat treatment and steam treat-
ment. According to Mandal et al. [63], the sample
activated by steam treatment at 700°С had larger spe-
cific surface area and pore volume than those of the
same material activated at 700°С without steam treat-
ment (576 and 421 m2/g and 109.1 and 57.6 cm3/kg,
respectively). According to Mandal et al. [63], a high
activation temperature contributes to the opening of
pore space due to the removal of volatile organic com-
pounds. In this case, the sorption capacity of the sam-
ple with the largest surface area and pore volume with
respect to 2,4-dichlorophenoxyacetic acid (2,4-D)
were maximal.
Bernal et al. [64] found that an increase in tem-
perature can contribute to a decrease in the specific
surface area and pore volume of a carbon sorbent. Two
AC samples (both based on a commercial micropo-
rous coconut shell sorbent) additionally thermally
treated at temperatures of 800 and 900°C, respectively,
were used for the adsorption of methyl paraben
(methyl 4-hydroxybenzoate). The initial commercial
AC had a specific surface area (SBET) of 864 m2/g and
a micropore volume of 0.34 cm3/g. Additional treat-
ment at 800°C resulted in a significant increase in SBET
and micropore volume (to 1127 m2/g and 0.42 cm3/g,
respectively). The values of SBET and pore volume in
the second sample (treated at 900°С) decreased to
814 m2/g and 0.29 cm3/g, respectively. According to
Bernal et al. [64], this dependence was due to the
blocking of micropores by condensed carbonization
products in the second AC sample. The adsorption
capacity of AC samples with respect to the herbicide
methyl paraben had the highest value for AC addition-
ally treated at a temperature of 800°C (1.53 mmol/g).
In the sample treated at 900°С, the adsorption capac-
ity was comparable with the adsorption capacity of the
initial sample (1.15 mmol/g).
Kulaishin et al. [65] studied the adsorption of 2,4-D
on sorbent samples synthesized from carbon black.
The first sample was activated with steam at a tem-
perature of 850–900°C for 8 h, and the second sam-
ple, for 9.5 h. Longer activation led to a decrease in the
number of mesopores larger than 10 nm and increased
the adsorption capacity of AC for 2,4-D (200 and
250 mg/g for the first and second samples, respec-
tively). According to Kulaishin et al. [65], the change
in the mesopore size distribution after additional
steam activation did not affect the specific surface area
(SBET, 560 m2/g). A sample activated for a longer time
was characterized by a smaller number of surface
groups, which were determined by the IR spectrum; it
is likely that this provided the ππ interaction between
the electron-deficient aromatic nucleus of 2,4-D mol-
ecules and the AC surface [65].
Chemical methods for the activation of carbon
materials to obtain AC consist in the introduction of
chemicals into the initial material followed by heat
treatment in an inert atmosphere. Zakaria et al. [66]
prepared AC from wood waste using phosphoric acid
(H3PO4) as an activating agent. They studied the effect
of a weight ratio between the activating agent and AC
(H3PO4/AC =3/1, 4/1, and 5/1) and activation tem-
perature (300, 400, and 500°C) on the specific surface
area of the AC samples obtained after activation. Acti-
vated carbon prepared at an impregnation ratio of 4/1
and an activation temperature of 300°C had the high-
est SBET value of 1012 m2/g. According to published
data [66, 67], in the course of chemical treatment of
AC with phosphoric acid, the activating agent mole-
cules penetrated deeply into the carbon structure to
facilitate the formation of new mesopores and micro-
pores and increase the surface area of the sorbent.
At an activation temperature of 500°C, the surface area
of the activated carbon decreased sharply to 729 m2/g as
the impregnation ratio was increased from 3/1 to 5/1.
This can be due to the shrinkage of a carbon structure
as a result of the combined action of a large amount of
H3PO4 and a high activation temperature (500°C)
[66], which was also observed in [68, 69].
A carbon sorbent was obtained by thermal activa-
tion of bituminous fossil coal (F400) [48, 70]. The
resulting sample (F400An) was subjected to additional
chemical activation in nitric acid (F400NH2 sample).
F400NH2 had a larger number of micropores than
those of F400 and F400An without changes in the
meso- and macropores (Fig. 1). According to Allwar et
al. [71], this was associated with the action of nitric
SOLID FUEL CHEMISTRY Vol. 56 No. 3 2022
INFLUENCE OF THE SURFACE CHARACTERISTICS 183
acid in the process of activation. This action led to the
development of the surface, which could be caused by
the splitting of the C–C bonds of graphene layers in
the sample. In the case of the calcination of the F400
(F400An) AC sample (for 3 h at 900°C and a heating
rate of 10 K/min) [48], the number of surface oxygen
groups and some other groups that are unstable to high
temperatures, for example, weak acidic groups (phe-
nolic, carboxyl, carbonyl, etc.) present in the original
F400 sample, decreased. The presence of acidic
groups was determined by the degree of affinity for
sodium ions [72]. Figure 2 shows the micrographs of
the original sample (Fig. 2a) and two samples sub-
jected to additional processing. Figure 2 indicates how
the surface of F400 AC changed in the course of ami-
nation and calcination. Chingombe et al. [48] found
that, on the calcination of the F400 sample, the for-
mation of a larger fraction of micropores on the sur-
face was observed, which can be explained by the
elimination of some oxygen-containing functional
groups that are unstable at high temperatures. Accord-
ing to Chingombe et al. [48], micropores are most effec-
tive for the adsorption of low-molecular-weight com-
pounds, such as the herbicide atrazine (6-chloro-4-N-
ethyl-2-N-propan-2-yl-1,3,5-triazine-2,4-diamine).
The advantage of micropores is due to the overlap of
potential energy forces in the pores because of the
close location of their walls [73]. This leads to a stron-
ger interaction of adsorbate molecules with the surface
of a microporous adsorbent. However, F400An had
the highest adsorption capacity. Therefore, pore dis-
tribution is not the only factor that determines the effi-
ciency of adsorption. Chemical activation led to the
formation of acidic functional groups, which have a
significant effect on the sorption mechanism, on the
surface of the F400NH2 sample. These acidic groups
interact with water molecules to create cluster groups
on the adsorbent surface and hinder the transport of
atrazine molecules to the adsorbent surface. Demiral
et al. [62] carried out a similar study and impregnated
a starting material for the preparation of AC with
H3PO4 at different H3PO4/AC weight ratios of 1/1,
2/1, and 3/1. Each of the obtained materials was then
subjected to heat treatment at temperatures of 400,
500, and 600°C. Additional chemical activation of the
resulting samples was carried out using HNO3 of vari-
ous concentrations (15, 30, 45, and 69% HNO3). The
largest surface area (1399 m2/g) and micropore vol-
ume were obtained for AC with the weight ratio
H3PO4/AC = 3/1 activated at a temperature of 400°C.
Upon the subsequent activation of the samples with
nitric acid, the BET surface area of ACs decreased
from 1399 to 15 m2/g with an increase in the concen-
tration of nitric acid from 15 to 69%. The porosimetric
analysis showed that the micropores were completely
closed, and the mesopore volume decreased from
0.127 to 0.042 cm3/g. According to published data [62,
74], the treatment with nitric acid led to the destruc-
tion of micropores and to an increase in the number of
surface oxygen-containing functional groups.
Gamiz et al. [75] found that the heat treatment of a
grapevine sample followed by chemical activation with 3%
H2O2 at room temperature made it possible to change the
composition of the surface groups of biochar. Activation-
induced changes increased the adsorption capacity of the
biochar for cichalofop ((2R)-2-[4-(4-cyano-2-f luoro-
phenoxy)phenoxy]propionic acid) rather than for clom-
azone (2-(2-chlorobenzyl)-4,4-dimethyl-3-isoxalidin-
3-one). This fact suggests that the activation with
H2O2 increases the adsorption capacity for organic
acids, but it does not increase the adsorption capacity
for polar nonionized compounds. The ability of bio-
char prepared under mild conditions to adsorb a
Fig. 1. Pore distribution for the samples F400, F400An, and F400NH2 [48].
10000100010010
0.050
Dierential pore volume, cm3/g
0.045 F400 F400An F400NH2
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0
Pore width, Å
1
184
SOLID FUEL CHEMISTRY Vol. 56 No. 3 2022
KULAISHIN et al.
weakly acidic herbicide is possible due to their anionic
nature.
Alkali metal hydroxides were used as an activating
agent at high activation temperatures to create a devel-
oped microporous structure and high values of SBET
[76–78]. KOH is an efficient and environmentally
friendly activating agent, which makes it possible to
obtain highly porous activated carbons from a wide
range of agricultural wastes [79]. When these samples
were activated with alkali metal hydroxides, carbonates
were formed, which led to a larger amount of oxygen-
containing surface groups in the final product [80].
Moura et al. [81] used ZnCl2 as an activating agent
to activate charcoal from wood waste by heat treat-
ment at 900°C. The SBET of the obtained sample was
907 m2/g. Sun et al. [82] obtained activated carbon
from lignin using K2CO3 as an activating agent.
According to Sun et al. [82], the specific surface area
(the areas of micropores and mesopores) and the vol-
ume of pores (micropores and mesopores) increased
as the activation temperature was increased to 500°C.
However, with a further increase in the temperature,
the increase in the surface area was insignificant. Sim-
ilar results were obtained by Vieira et al. [40], who did
not observe an increase in the value of SBET with an
increase in the temperature of biomass activation by
potassium carbonate above 500°C. The pyrolysis of
macauba fruit endocarp followed by chemical activa-
tion with K2CO3 at 400°C provided the formation of
an average pore width of 1.7 nm (micropores); the
sample upon pyrolysis at 600°C had an average pore
width of 2.7 nm (mesopores) [40]. Both of the samples
showed high atrazine recovery.
For the maximum values of the specific surface
area and pore volume of AC, the activation tempera-
ture should be selected based on the data on the pres-
ence of thermally unstable groups in the composition
of the feedstock. At the same time, it should be taken
into account that an increase in the temperature can
lead to the opposite effect due to the blocking of
micropores by condensed carbonization products.
To obtain activated carbons from biomass, activa-
tion is carried out in a temperature range of 300–
600°C using alkali and alkali metal salts as activating
agents, which are favorable for the activation of plant
materials under high-temperature treatment condi-
tions, as compared to the use of acids.
The formation of micropores is equally affected by
both chemical and physical methods of activation. The
volume of micropores is not always the key factor influ-
encing the efficiency of adsorption of herbicide mole-
cules. When choosing a sorbent activation method, it is
also necessary to take into account the presence of oxy-
gen-containing surface functional groups, which can
Fig. 2. Electronic images of the surfaces of the samples (a) F400, (b) F400An, and (c) F400NH2 [48].
200 nm
(a) (b)
(c)
200 nm
200 nm
SOLID FUEL CHEMISTRY Vol. 56 No. 3 2022
INFLUENCE OF THE SURFACE CHARACTERISTICS 185
be formed on AC as a result of activation and interfere
with the adsorption process.
2. INFLUENCE OF THE SPECIFIC SURFACE
AREA AND FUNCTIONAL GROUPS
OF ACTIVATED CARON ON THE EFFICIENCY
OF ADSORPTION
The main surface characteristics of activated car-
bons include specific surface area, pore size distribu-
tion, and the composition and number of surface
functional groups. It is of interest to consider the
effects of these parameters on the efficiency of herbi-
cide adsorption on ACs from aqueous media.
2.1. Specific surface area. SBET and pore size distri-
bution are key characteristics of a carbon adsorbent.
According to published data [61], microporous sor-
bents with a high specific surface area are effective for
extracting herbicide molecules from aqueous media.
The adsorption of 2,4-DNA on ACs from coconut
shells was studied [50, 83]. In these studies, a high
adsorption capacity of sorbent samples with respect to
2,4-D was noted, which was due to the high specific
surface areas (946 [50] and 1070 m2/g [83]) and the
predominance of micropores. Comparable results
were obtained by Kulaishin et al. [60], who performed
the adsorption of 2,4-D on a microporous GAC sample
(SBET, 1513 m 2/g); the adsorption capacity was 469 mg/g.
Cansado and Mourao [84] adsorbed 4-chloro-2-
methylphenoxyacetic acid (MCPA) on activated car-
bon samples obtained from PET and cork material
using various activation methods. The sorbent samples
with the highest values of SBET and pore volume had
the highest sorption capacity. According to Cansado
and Mourao [84], not only the large specific surface
area but also the pore diameter (which should exceed
the diameter of an adsorbate molecule by a factor of
about 1.5) are important parameters for the effective
adsorption of the herbicide MCPA. The MCPA mol-
ecule has a diameter of 0.8 nm, which corresponds to
the pore diameters of the most efficient adsorbent
samples (1.5 nm). A similar assumption was made by
Salomon et al. [41], who found that an adsorbent with
an average pore size of 2.3 nm, which is larger than the
size of a 2,4-D molecule (1.1 nm), was most effective
for the adsorption of 2,4-D. According to Putun and
Putun [85], this ratio facilitated the diffusion of the
2,4-D molecule in pores.
According to Li et al. [86], six samples of biochar
from a mixture of alkaline lignin of shavings of soft
(pine) and hard (poplar) tree species subjected to heat
treatment at a temperature of 500–550°C in an inert
atmosphere of nitrogen had BET specific surface areas
higher than 300 m2/g. A higher heat treatment tem-
perature (800°C) provided a larger specific surface
area (416–418 m2/g) due to a higher fraction of micro-
pores. According to Li et al. [86], the adsorption
capacity of these samples for atrazine, metolachlor,
and isoproturon molecules did not correlate with their
specific surface areas. A large adsorption capacity was
characteristic of a sample with a large mesopore vol-
ume. The reason for the dominant role of mesopores
in the adsorption of atrazine, metolachlor, and isopro-
turon was associated with the fact that the herbicides
have molecular weights of >200 g/mol, which corre-
sponds to a size of >1.0 nm. According to Li et al.
[86], the minimum pore diameter for the adsorption
of organic pollutants with a molecular mass of
>200 g/mol should be 1.7 nm; therefore, mesopores
are the most effective for the adsorption of such mole-
cules.
ACs can also be obtained from woodworking
wastes, as described by Cansado et al. [24], who pro-
posed a material in the form of a composite of wastes
from the production of chipboard and fiberboard.
Carbonization was carried out at different heating
rates and with the subsequent activation in a flow of
CO2. The resulting ACs were characterized by a
microporous structure and basic properties. The sam-
ples subjected to longer activation (240 min) had a
larger specific surface area (1200 m2/g) and micropore
volume in comparison with the samples activated for
shorter times (800 and 900 m2/g). In this case, all sam-
ples had sufficient adsorption activity with respect to
phenoxyacetic acids; however, the highest value of
adsorption was established for the sample with the
highest value of SBET, which was also confirmed by
Vukcevic et al. [87] for dimethoate (О,О-dimethyl-S-
(N-methylcarbamidomethyl)dithiophosphate), acetami-
prid (N1-methyl-N1-[(6-chloro-3-pyridyl) methyl]-N2-
cyanoacetamidine) and atrazine.
The adsorption capacity can be affected by the adsor-
bent particle size. Fontecha-Camara et al. [88] used gran-
ular ACs with different granule cross-sectional diameters
from 1.50 to 0.03 mm (1200 m2/g; Vmic, 0.480 cm3/g). The
adsorption values of diuron (3-(3,4-dichlorophenyl)-1,1-
dimethylurea) and amitrol (3-amino-1,2,4-triazole)
increased with a decrease in the size of the granules. This
dependence was explained by an increase in the fre-
quency of collisions between the adsorptive and the
adsorbent.
2.2. Surface functional groups of AC. The composi-
tion of functional groups on the AC surface can affect
the adsorption capacity of the sorbent with respect to
herbicide molecules. According to Chingombe et al.
[89], amphoteric groups, which change their proper-
ties with changes in the pH of solution and the nature
of the adsorbed substance, the dipole–dipole interac-
tion of the adsorbent and the adsorbed substance, the
formation of hydrogen bonds and covalent bonds, and
ion exchange, are the most important types of interac-
tion for adsorption.
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SOLID FUEL CHEMISTRY Vol. 56 No. 3 2022
KULAISHIN et al.
Hu et al. [11] found that not only a decrease in the
surface area but also an increase in the number of
acidic functional groups (carboxylic and phenolic) on
the AC surface can adversely affect the adsorption of
some herbicides (in particular, atrazine). These sur-
face functional groups promote the adsorption of
water on the surface of a carbon sorbent, thereby
reducing the number of available adsorption sites for
the target adsorbate, as was also confirmed by Li et al.
[90].
Chingombe et al. [48], who studied atrazine adsorp-
tion on three AC samples (initial, heat-treated by anneal-
ing, and chemically activated by amination), came to sim-
ilar conclusions. The heat-treated AC showed the lowest
concentration of acidic surface groups. A decrease in the
concentration of acidic groups on the surface of this
sample led to an increase in the volume of micropores
and a higher adsorption capacity for atrazine mole-
cules, as compared to the other samples. The ami-
nated sample with a large number of oxygen-contain-
ing groups was characterized by the lowest adsorption
capacity for atrazine, despite the tendency of this
compound to form hydrogen bonds [91]. It was also
noted [64] that an increase in the number of acidic
groups on the adsorbent surface led to a decrease in the
adsorption of the molecules of phenol derivatives (2,4-D
and methylparaben) because of the formation of water
clusters due to hydrogen bonds. An increase in the
number of basic surface groups, on the contrary, led to
an increase in the value of adsorption due to acid–base
and ππ interactions. However, at low concentrations
of herbicides in the aquatic environment (to 500 ppb),
the composition and amount of AC surface functional
groups rather than the SBET of the sorbent play an
important role in the adsorption efficiency [92].
Goscianska and Olejnik [93] studied the adsorp-
tion of 2,4-D on mesoporous AC modified with ami-
nosilane and noted the high adsorption capacity of the
AC due to a large number of basic groups, in particu-
lar, amino groups, on the surface of this sorbent.
Rambabu et al. [94] used AC prepared from date
palm coir by thermochemical activation with KOH as
an activating agent for the adsorption of 2,4-D. They
found that KOH activation promoted the formation of
many oxygen-containing groups on the sorbent sur-
face, which, in turn, served as active centers for 2,4-D
adsorption. In particular, Rambabu et al. [94] noted
the С=О group as the most active in the process
adsorption. The results of Rambabu et al. [94] were
largely confirmed by Suo et al. [95], who obtained AC
from starch by thermochemical activation with KOH
and used it as a sorbent for the herbicide pyraclos-
trobin (methyl N-{2-[1-(4-chlorophenyl)-1H-pyra-
zol-3-yloxymethyl]phenyl}(N-methoxy)carbamate).
They explained the high efficiency of the sorbent with
respect to pyraclostrobin by the presence of oxygen-
containing and nitrogen-containing surface func-
tional groups, namely, C=O and C–N in the sorbent.
To increase the adsorption capacity of AC for some
benzene derivatives of molecules [96], it is necessary
to increase the hydrophobicity of sorbents by decreas-
ing the concentration of oxygen groups on the AC sur-
face. For this purpose, the outer layers were removed
from the granules by abrasion because the concentra-
tion of oxygen groups noticeably decreased from the
outer surface to the center of the granule in both for
the initial samples and the samples activated with
nitric acid. In both cases, the decrease in the concen-
tration of oxygen groups on the sorbent surface was
about 20%.
The presence of hydrophilic groups on the surface
is desirable for the adsorption of hydrophilic sub-
stances; otherwise, hydrophobic ones are desirable.
3. INFLUENCE OF THE PARAMETERS
OF THE MEDIUM IN THE ADSORPTION
PROCESS
The structure of adsorbed molecules, the pH and
temperature of the medium, and the presence of
chemical and biological impurities in an aqueous
medium are important for the adsorption process.
Adsorbates and adsorbents are characterized by the
presence of functional groups, and the pH of the solu-
tion affects the charge of functional groups both on
adsorbents and in adsorbate molecules due to their
deprotonation or protonation [97].
3.1. Structure of adsorbed molecules. The positions
of functional group in the adsorptive molecule and the
number of substituents in its benzene nucleus can
affect the value of adsorption [98]. The effect of the
positions of halogen substituents in a benzene ring was
described by Derylo-Marczewska et al. [98]. Compar-
ing the adsorption isotherms of 4-bromophenoxypro-
pionic acid and 3-bromophenoxypropionic acid with
those of 2-chlorophenoxyacetic acid and 4-chloro-
phenoxypropionic acid, they determined that mole-
cules with substituents in the para position were
adsorbed better than those with substituents in the
ortho and meta positions. The para position of a sub-
stituent makes the molecule more linear and promotes
better penetration of the adsorbent into micropores.
The meta and ortho positions disturb the symmetry of
molecules to increase the width of a molecule in a
plane parallel to the benzene nucleus. Thus, the access
of the adsorptive with substituents in the ortho and
meta positions to micropores is limited [98].
Jain and Jayaram [54] described the donor–accep-
tor mechanism of adsorption of the molecules of phe-
nol derivatives on AC. In this case, the carbonyl group
oxygen on the AC surface acts as an electron donor,
while the aromatic nucleus of phenol acts as an accep-
tor. This mechanism is most typical for the adsorption
of 2,4-dichlorophenol because the presence of chlo-
rine (–Cl) promotes the formation of a bond between
the surface carbonyl groups and the electron-deficient
SOLID FUEL CHEMISTRY Vol. 56 No. 3 2022
INFLUENCE OF THE SURFACE CHARACTERISTICS 187
aromatic nucleus. This hypothesis was confirmed by
comparing the IR spectra recorded from pure activated
AC from the peel of jackfruit (Artocarpus heterophyllus)
and AC with phenol adsorbed on it. A decrease in the
intensity of the values of wavenumbers of tensile vibra-
tions (C=O) in the adduct was observed in compari-
son with that of a pure AC sample. The assumption
that the carbonyl group acts as an electron donor to the
aromatic nucleus of the adsorbate was also reported
[99, 100].
Figure 3 shows models of the most stable position
of phenoxyacetic acid molecules on the AC surface
[10]. Molecules of 2,4-D and 4-chloro-2-methylphe-
noxyacetic acid occupy a quasi-planar position on the
AC surface (Fig. 3). The energy of interaction between
the adsorbent and the adsorbate is weak in this case;
therefore, the physical nature of adsorption takes
place. According to Spaltro et al. [10], there were no
changes in the geometry of the test herbicide mole-
cules.
Based on the studies carried out by Chingombe et
al. [89], the steric factor of adsorbed molecules can
complicate the process of adsorption. Figure 4 shows
the structural formulas and chemical structure models
of 2,4-D and benazoline (2-(4-chloro-2-oxo-1,3-
benzothiazol-3-yl)acetic acid).
For 2,4-D and benazoline molecules, the func-
tional group that increases the molecular size in the
course of adsorption is a carboxyl group located out-
side the same plane as the plane of the aromatic
nucleus. In the case of benazoline, the molecule con-
tains a heterocyclic ring adjacent to the aromatic one,
consisting of nitrogen and sulfur atoms. The nitrogen
atom, which forms a bond with neighboring atoms,
forms four pairs of σ electrons, which take a tetrahe-
dral position to minimize the repulsion of electron
pairs. One of the four positions is occupied by a pair of
electrons, and the remaining three form σ bonds; thus,
the adsorbate molecule is structured in the form of a
trigonal pyramid. In this case, the two ring systems are
Fig. 3. Modeling of the positions of (a) 2,4-dichlorophenoxyacetic acid (2,4-D) and (b) 4-chloro-2-methylphenoxyacetic acid
molecules adsorbed on the AC surface [10].
'E = 0.779 eV 'E = 0.894 eV
(b)(а)
3.64Å 3.21Å 3.43Å 3.34Å 3.44Å 3.01Å 3.14Å
Fig. 4. Chemical structures of (a) 2,4-D and (b) benazoline [89].
3.113 Å
2.074 Å
(a)
O
O
O
O
OH
N
Cl
Cl
Cl
S
OH
(b)
188
SOLID FUEL CHEMISTRY Vol. 56 No. 3 2022
KULAISHIN et al.
not in the same plane; because of this, the molecule of
benazoline is less linear and more bulky in comparison
with that of 2,4-D [89]. Similar conclusions were drawn
by Salman et al. [22], who compared the adsorption val-
ues of bentazone (3-isopropyl-(1H)-2,1,3-benzothiadi-
azine-4(3H)-one-2,2-dioxide) (101 mg/g) and 2,4-D
(168 mg/g) on AC.
Derylo-Marczewska et al. [98] studied the effect of
various halogen substituents on the adsorption value.
In a comparison of the adsorption isotherms of 4-halo-
phenoxypropionic acids on AC, the adsorption values
decreased in the following order: 4-bromophenoxypro-
pionic acid > 4-chlorophenoxypropionic acid > 4-fluo-
rophenoxypropionic acid.
Such differences in the adsorption values correlate
well with the Hammett constant σ. The constant
determines the electron-withdrawing properties of
substituents. For the test molecules with the para posi-
tion of substituents, it was σBr = 0.23, σCl = 0.23, or
σF= 0.06. An increase in the electron-withdrawing
capacity leads to a weakening of the interaction
between the aromatic nucleus and the π electrons of
carbon atoms of the adsorbent, which reduces the
adsorption value.
Carboxyl and carbonyl groups of substrate mole-
cules can interact with oxygen-containing surface
groups of AC. As a result, hydrogen bonds are formed
both with surface groups and with neighboring sub-
strate and solution molecules [98, 99].
The effect of the structure of adsorbed molecules
on the efficiency of adsorption was manifested in the
presence and position of functional groups. The para
position of a substituent in the adsorptive molecule is
most effective for adsorption; it facilitates the forma-
tion of a linear structure of the molecule. An increase
in the electronegativity of a substituent in the benzene
nucleus leads to a shift in electron density in the aro-
matic system of the adsorptive molecule and, as a con-
sequence, to an increase in the adsorption capacity.
3.2. pH of solution. The pH of solution affects the
polarity of an adsorbent to make it positive or negative
at pH values below or above the pH point of zero
charge (pHPZC), respectively [55, 101]. The value of
pHPZC corresponds to a charge value at which the total
charge of the entire adsorbent surface is zero [102,
103]. The pH of a solution strongly affects the state of
organic molecules in nonionic, cationic, and anionic
forms and leads to a change in their adsorption char-
acteristics [104]. The pH of a solution determines not
only the charge of the adsorbent surface [105] but also
the degree of dissociation of an organic acid [106].
Jafari et al. [107] adsorbed the herbicide paraquat
(N,N'-dimethyl-4,4'-dipyridylium dichloride) on AC
from solutions at pH 2, 4, 6, 8, and 10. As the pH was
increased from 2 to 6, the adsorption efficiency for
paraquat increased from 35 to 95%. With a further
increase in pH to 10, the adsorption efficiency slightly
increased from 95 to 97%. According to Jafari et al.
[107], a lower concentration of H+ facilitated the
deprotonation of adsorption centers on the adsorbent
surface. In addition, the competition between H+ and
the paraquat molecule decreased. As a result, the max-
imum adsorption capacity of the adsorbent exceeded
95% at pH 7–10. The value of pHPZC of the AC was 5.4;
therefore, the surface charge was positive at pH < 5.4
and negative at pH > 5.4, and this fact indicated that
the sorbent surface was charged positively at pH < 5.4
and negatively at pH > 5.4. When the pH of solution is
greater than pHPZC, electrostatic attraction can be the
dominant mechanism of the adsorption of paraquat
on AC.
According to published data [108], the efficiency of
adsorption of 4-chlorophenol (4-CP) on activated
carbon from pomegranate peel increased insignifi-
cantly as the pH of the solution was increased from 3
to 6. With a further increase in pH to 9, the adsorption
efficiency decreased by 10%. At pH 6.0, 4-CP was
present in the solution mainly in an unprotonated
form, and the pKa constant of 4-CP was 8.96. At the
values of pH in adsorbate solutions higher than the
adsorbent pHPZC of 5.96–6.4, the adsorbent surface
became negatively charged. At pH 9.0, the ratio
between undissociated and dissociated forms of the
4-CP molecule was approximately the same. There-
fore, the dissociated form, which is an anion, is
repelled from the surface of the adsorbent (at pH 9) to
significantly decrease the efficiency of adsorption.
Wang et al. [109] studied the influence of the pH of
adsorbate solutions on the adsorption of the carba-
mate pesticides (carbamic acid derivatives) pirimi-
carb, metolcarb, isoprocarb, sevin, methiocarb, and
bendiocarb on ACs. The adsorption efficiency of pes-
ticides increased with the pH of solution. The pH of
point of zero charge (pHPZC) was 3.8, which indicated
that the AC surface was negatively charged above
pH 3.8 and positively charged below pH 3.8. There-
fore, the AC surface was negatively charged at pH 7,
which contributed to the adsorption of pesticide mol-
ecules on the AC surface due to electrostatic interac-
tions.
Similar results were obtained by Pastrana-Marti-
nez et al. [110], who studied the adsorption of the her-
bicide fluroxypyr on carbon fiber. The highest adsorp-
tion capacity was found at pH 2, at which fluroxypyr
molecules were weakly dissociated. According to Pas-
trana-Martinez et al. [110], the high adsorption capac-
ity at this pH was due to the predominance of disper-
sion interactions between the graphene layers of the
adsorbent and undissociated fluroxypyr molecules.
The solubility of fluroxypyr molecules increases with
pH; therefore, the fraction of negatively charged
adsorbate ions in solution increases, and the sorbent
surface acquires a negative charge. Thus, electrostatic
SOLID FUEL CHEMISTRY Vol. 56 No. 3 2022
INFLUENCE OF THE SURFACE CHARACTERISTICS 189
repulsion between adsorbate and adsorbent molecules
and a decrease in dispersion interactions between
undissociated molecules and the AC surface lead to a
decrease in the adsorption value.
Njoku et al. [12] used AC obtained from pumpkin
peel as a sorbent for extracting the herbicide 2,4-D
from an aqueous medium. It was found that the pHPZC
of the sorbent surface was 6.0; therefore, the AC sur-
face was negatively charged in an alkaline medium.
At a solution pH greater than 4.5, 2,4-D mainly
occurred in the ionized form as the 2,4-dichlorophe-
noxyacetate ion, which led to electrostatic repulsion
from the AC surface. In addition, in an alkaline
medium, the high mobility of OH ions can compete
with the 2,4-dichlorophenoxyacetate anion for the sur-
face adsorption sites of AC [35, 111, 112]. Similarly to the
data of Njoku et al. [12], the adsorption of 2,4-D on AC
from coconut shells largely depended on the pH of the
solution [50]. According to the IR spectrum data, the
dominance of hydroxyl groups was observed on the
AC surface. These groups became deprotonated as the
pH value of the solution was increased; in turn, this led to
the repulsion of the negatively charged 2,4-D anion.
According to published data [10, 49–56, 113], the
adsorption value of the herbicide 2,4-D decreased with
an increase in the pH value of the solution from 3 to 9.
Ania and Beguin [114], who studied the effect of
pH on the adsorption of bentazone on AC, obtained
the results on the pH dependence of the adsorption,
which were similar to those for 2,4-D. Similarly, accord-
ing to Bernal et al. [64], the adsorption of methylparaben
on AC decreased at pH values that determine the nega-
tive charge of the adsorbent (pH > pHPZC) and a high
degree of adsorbate ionization.
Blachnio et al. [115] studied the adsorption of the
herbicides 2,4-D and MCPA on AC produced from
agricultural wastes. The adsorption capacity for 2,4-D
relative to MCPA was higher due to the lower solubil-
ity of the 2,4-D molecule and its higher hydrophobic-
ity. However, for binary systems of 2,4-D and MCPA
in a molar ratio of 1 : 1, competition between the mol-
ecules for the adsorption centers of AC was observed.
The low solubility of 2,4-D provided a high affinity for
the hydrophobic AC surface. The small size and good
solubility of the MCPA molecule allow diffusion into
micropores, but the presence of adsorbed 2,4-D facil-
itates the desorption of some MCPA molecules into
the bulk of the solution.
Moreno-Castilla et al. [116] found that an increase
in the pH value led to a lower dissociation of amitrol
molecules and, consequently, to a higher adsorption
on AC. Amytrol molecules in an acidic medium pre-
dominated in the protonated form, and the AC surface
was positively charged (pHPZC 8); this led to a decrease
in the adsorption of amitrol on the sorbent surface.
3.3. Temperature of the medium. The temperature of
the medium can have a significant effect on the pro-
cess of adsorption. A decrease in the adsorption effi-
ciency with temperature can be explained by the exo-
thermic nature of the process. An increase in tempera-
ture leads to an increase in the vibrations of molecules
on the adsorbent surface, which leads to the desorp-
tion of molecules into the bulk of the solution, as
found previously [23, 100, 109, 117, 118]. For example,
according to Georgin et al. [117], the adsorption
capacity of AC with respect to 2,4-D decreased with
an increase in the temperature from 25 to 55°C, and
the solubility of 2,4-D increased with the temperature;
therefore, the affinity of adsorbate molecules and the
hydrophobic surface of the adsorbent decreased to
cause a decrease in the adsorption of 2,4-D at higher
temperatures. Herrera-Garcia et al. [119] explained
the decrease in the adsorption capacity of AC for
2,4-D by an increase in the vibrational energy of her-
bicide molecules, which led to desorption from the
sorbent surface into the bulk of the solution. Similar
results were shown by Marczewski et al. [120], who
studied the adsorption of 4-chlorophenoxyacetic acid
(4-CPA) on AC and observed a decrease in the
adsorption capacity of the sorbent for 4-CPA with
increasing temperature. According to Marczewski et
al. [120], this temperature effect was associated with
an increase in the solubility of 4-CFCs in water with
increasing temperature. As a result, this decreased the
hydrophobicity of the herbicide molecule and its
affinity for the carbon surface. In addition, an increase
in the vibrational energy of adsorbed molecules pro-
moted their desorption from the AC surface into the
solution.
The inverse dependence of the effect of tempera-
ture increase on the increase in adsorption efficiency
can be explained by the endothermic nature of the
process. For example, an increase in the sorption
capacity of activated carbon at a higher temperature
can be associated with an increase in the pore size or
with an additional temperature activation of the adsor-
bent surface and with the creation of some new active
centers on its surface. The mobility of adsorbate mol-
ecules can also increase with temperature, which
increases the rate of diffusion inside the adsorbent and
leads to an increase in the efficiency of adsorption
[121–123].
According to Rambabu et al. [94], the adsorption
capacity of AC with respect to 2,4-D slightly increased
with an increase in temperature from 30 to 60°C,
which indicated the endothermic nature of the process
[52]. A similar result was obtained by Lam et al. [124]
for the adsorption of 2,4-D on AC. The efficiency of
2,4-D adsorption slightly increased with an increase in
the temperature from 30 to 50°C. This was probably
due to the fact that a higher temperature increased the
190
SOLID FUEL CHEMISTRY Vol. 56 No. 3 2022
KULAISHIN et al.
mobility and penetration of 2,4-D ions from the her-
bicide solution into the AC pore structure.
3.4. Influence of impurities in an aquatic environ-
ment on the efficiency of adsorption. The presence of
chemical or biological impurities in the aquatic envi-
ronment can affect the adsorption capacity of the
adsorbent and the solubility of the adsorptive.
It is well known that the solubility of organic com-
pounds depends on the concentration of an electrolyte
present in the aqueous medium [10]. The electrostatic
interaction between the AC surface and the adsorbed
molecule manifests itself as electrostatic repulsion,
whereas an increase in the ionic strength by adding an
electrolyte to the solution leads to an increase in the
adsorption capacity. In the case of the attraction
between the adsorptive molecule and the adsorbent
surface and if the surface concentration is small, an
increase in the ionic strength decreases the adsorption
capacity. Thus, when a salt is added to a solution, a
salting out effect occurs, when the solubility of organic
molecules decreases. Ions in the solution form hydra-
tion spheres; therefore, water molecules become inac-
cessible to organic molecules, thereby decreasing the
solubility and increasing the diffusion of organic mol-
ecules to the AC surface. If the salt is in excess, then a
screening effect decreases the interaction of organic
molecules and the AC surface.
Pastrana-Martinez et al. [125] considered the possi-
bility of adsorption depending on the hardness of water
controlled by the addition of CaCO3. With an increase
in water hardness, the adsorption capacity of AC
increased, and this was also confirmed by Moreno-
Castilla et al. [126]. The decrease in the solubility of the
adsorptive in the presence of CaCO3 was due to the for-
mation of insoluble complexes between CaCO3 and the
adsorptive because, according to Moreno-Castilla et al.
[126], the nonelectrostatic adsorbent–adsorptive inter-
action was the predominant mechanism of adsorption.
In the aquatic environment, biological pollutants,
which decrease the efficiency of adsorption of the tar-
get substrate, can also occur in addition to toxic chem-
ical substances. Olenin et al. [127] defined biological
pollution of the aquatic environment as a decrease in
the quality of the environment as a result of changes in
the biological, chemical, and physical properties of an
aquatic ecosystem.
Ova and Ovez [58] studied the adsorption of
2,4-dichlorophenoxyacetic acid in the presence of
biological pollutants. They found that the presence of
biological pollutants in the solution determines the
polarity of the sorbent surface and leads to narrowing
and clogging of pores as a result of the adsorption of
biological pollutants. As a result, preliminary treat-
ment for the removal of biological pollutants was pro-
posed.
Table 1 summarizes the main porosimetric charac-
teristics (SBET, pHPZC, and pore volume) of AC adsor-
bents and the adsorption capacity of ACs for the her-
bicides considered in this work.
CONCLUSIONS
In the considered material, a number of factors
were identified that have a significant effect on the
adsorption process of herbicide molecules from aque-
ous media on activated carbons. The parameters that
can be varied in the manufacture of adsorbents include
porosimetric characteristics and the composition and
number of surface functional groups in ACs. Charac-
teristics such as temperature, pH, and the presence of
impurities are the factors affecting the efficiency of an
adsorption process.
Activation methods make it possible to control the
porosity of the sample surface and change the compo-
sition and number of surface groups. The SBET and the
micropore volume of an AC sample correlate with the
adsorption capacity for herbicide molecules. A signif-
icant contribution to the adsorption process is made
by the interaction of the groups of atoms of adsorbate
molecules with surface functional groups of the AC.
Their location and quantity on the AC surface, the
positions of substituents in the adsorbate molecules,
and their electronegativity directly affect the adsorp-
tion capacity of the sorbent.
The electrostatic interaction between an adsorbent
and an adsorptive depends on the pH of the medium.
The pH value determines the surface charge of the sor-
bent, which affects the electrostatic adsorbent–
adsorptive interaction.
An increase in the adsorption temperature can
change the adsorption capacity of AC due to various
thermodynamic factors of the adsorption process. The
presence of molecules that increase water hardness and
are capable of forming complexes with the adsorbed sub-
stance can reduce the solubility of the adsorbed sub-
stance and increase the value of adsorption.
An increase in the adsorption temperature can
change the adsorption capacity of AC due to various
thermodynamic factors of the adsorption process. The
presence of molecules that increase water hardness and
are capable of forming complexes with the adsorbed
substance can reduce the solubility of the adsorbed sub-
stance and increase the value of adsorption.
The structure of adsorptive molecules and the
position of substituents in them affect the access of
organic molecules to micropores. The electronegativ-
ity and the number of substituents at the benzene
nucleus of the adsorbate determine the adsorption
capacity of AC.
The literature does not cover the efficiency of ACs
in adsorption in multicomponent systems and in real
wastewater. The applicability of the reviewed ACs to
real effluents containing pollutants other than herbi-
cides has yet to be assessed.
SOLID FUEL CHEMISTRY Vol. 56 No. 3 2022
INFLUENCE OF THE SURFACE CHARACTERISTICS 191
Table 1. Characteristics of ACs for the adsorption of herbicides
Compound, dissociation constant,
solubility, and structural formula
Adsorbent
sample
Maximum ad-
sorption capa-
city* (mg/g)
pHPZC of
the ad-
sorbent
SBET
(m2/g)
Vpore,
cm3 g–1
Aver age p ore
diameter,
nm
Refe-
rences
2,4-Dichlorophenoxyacetic acid
(2,4-D), 2.73, 0.31 g/L
GAB
400.000 7.5 1189
Vtotal 0.530
Vmicro 0.270
Vmeso 0.260
2.25
[10]
CPB
385.000 4.8 1288
Vtotal 1.100
Vmicro 0.040
Vmeso 1.0 60
4.39
PSHAC
253.560 6.0 738
Vtotal 0.370
Vmicro 0.301
Vmeso 0.069
2.26
[12]
MDF 5950 0.263 >10.0 1195 Vtotal 0.480 1.02 [24]
MDF 5933 0.157 >10.0 805 Vtotal 0.330 0.66
PB 5960 0.303 >10.0 1211 Vtotal 0.580 1.15
PB 5936 0.245 >10.0 926 Vtotal 0.380 0.80
AC_Ar 0.332 9.7 243 Vtotal 0.120 1.05 [115]
CD_3 h 0.332 11.4 556 Vtotal 0.280 0.55
ST_CD_3 h 0.332 11.1 669 Vtotal 0.520 1.50
CD_MV_1.5 h 0.332 11.0 685 Vtotal 0.350 0.59
AC 250.000 1016 Vtotal 0.560
Vmicro 0.290 н/д [27]
CSAC 282.100 986 Vtotal 0.540 2.20 [50]
CCAC 259.400 3.5 1274 Vtotal 0.900
Vmicro 0.101 3.00 [52]
F300
0.375 9.8 762
Vtotal 0.460
Vmicro 0.280
Vmeso 0.180
0.52
[98]
TW-BCS 55.000 12.0 576 Vtotal 0.109 2.00 [63]
OW-BC 22.000 10.0 270.7 Vtotal 0.120 1.1
BU-BC 20.000 11.0 2.3 Vtotal 0.109 2
B-BC 20.000 10.0 476 Vtotal 0.209 1.1
TW-BC 8.000 11.0 421 Vtotal 0.058 1.9
QPPAC 360.000 5.1 782 Vtotal 0.441 2.26 [41]
CKIT-6 66.000 834 Vtotal 1.090 5.50 [93]
CKIT-6-A1 118.000 228 Vtotal 0.360 5.60
CKIT-6-A2 126.000 169 Vtotal 0.270 5.80
CKIT-6-A3 152.000 71 Vtotal 0.150 7.90
MNUM-1
185.000
557
Vtotal 0.690
Vmicro 0.040
Vmeso 0.650
13.00
[65]
MNUM-2
232.000
569
Vtotal 0.640
Vmicro 0.030
Vmeso 0.610
9.00
Cl Cl
OOH
O
192
SOLID FUEL CHEMISTRY Vol. 56 No. 3 2022
KULAISHIN et al.
ACKO 383.000 1070 Vmicro 0.430 0.80 [83]
GAC 469.000 1514 Vmicro 0.640 0.80 [60]
F400 0.010 7.5 790 [89]
F400AN 0.010 8.0 960
SSAC 125.000 490 4.6 [85]
DPC-AC 50.250 5.5 947 Vtotal1.646 2.90 [94]
4-Chloro-2-methylphenoxyacetic acid
(MCPA), 3.73, 0.825 g/L
GAB
590.000 7.46 1189
Vtotal0.530
Vmicro 0.270
Vmeso 0.260
2.25
[10]
CPB
270.000 4.76 1288
Vtotal 1.100
Vmicro 0.040
Vmeso 1.0 60
4.39
MDF 5950 0.293 >10.0 1195 Vtotal 0.480 1.02 [24]
MDF 5933 0.147 >10.0 805 Vtotal 0.330 0.66
PB 5960 0.375 >10.0 1211 Vtotal 0.580 1.15
PB 5936 0.157 >10.0 926 Vtotal 0.380 0.80
AC_Ar 0.080 9.7 243 Vtotal 0.120 1.05 [115]
CD_3 h 0.302 11.4 556 Vtotal 0.280 0.55
ST_CD_3 h 0.281 11.1 669 Vtotal 0.520 1.50
CD_MV_1.5 h 0.211 11.0 685 Vtotal 0.350 0.59
P-K
0.300 7.21 1255
Vtotal 1.000
Vmicro 1.000
Vmeso 0.000
1.07
[84]
P-UD5
0.748 7.08 2420
Vtotal 1.920
Vmicro 1.770
Vmeso 0.150
1.54
P-CDT-D1
0.792 6.71 2222
Vtotal 1.910
Vmicro 1.530
Vmeso 0.380
1.71
P-HU-D1
0.784 7.09 2076
Vtotal 1.720
Vmicro 1.500
Vmeso 0.220
1.55
P-PEI-D1
0.360 7.16 1431
Vtotal 1.130
Vmicro 1.130
Vmeso 0.000
0.94
C-K
0.271 7.14 900
Vtotal 1.000
Vmicro 1.000
Vmeso 0.000
0.82
C-UD5
0.774 8.50 2339
Vtotal 2.760
Vmicro 2.140
Vmeso 0.620
1.7
Compound, dissociation constant,
solubility, and structural formula
Adsorbent
sample
Maximum ad-
sorption capa-
city* (mg/g)
pHPZC of
the ad-
sorbent
SBET
(m2/g)
Vpore,
cm3 g–1
Aver age p ore
diameter,
nm
Refe-
rences
Cl
OOH
O
Table 1. (Contd.)
SOLID FUEL CHEMISTRY Vol. 56 No. 3 2022
INFLUENCE OF THE SURFACE CHARACTERISTICS 193
C-CDT-D1
0.353 8.05 1142
Vtotal 1.320
Vmicro 1.230
Vmeso 0.090
1.03
[84]
C-HU-D1
0.546 8.03 1331
Vtotal 1.610
Vmicro 1.320
Vmeso 0.290
1.38
C-PEI-D1
0.423 8.11 2170
Vtotal 2.470
Vmicro 2.320
Vmeso 0.150
1.0 9
3-(3,4-Dichlorophenyl)-1,1-dimethy-
lurea (diuron), not determined,
0.035 g/L
MDF 5950 0.205 >10.0 1195 Vtotal 0.480 1.02 [24]
PB 5960 0.226 >10.0 1211 Vtotal 0.580 1.15
PB 5936 0.082 >10.0 926 Vtotal 0.380 0.80
AC
60.000
1016
Vtotal 0.560
Vmicro 0.29
– [27]
6-Chloro-4-N-ethyl-2-N-
propan-2-yl-1,3,5-triazine-
2,4-diamine (atrazine),
1.7, 0.033 g/L
CTRTAC 104.900 NA 981 3.12 [44]
F400 0.005 7.5 790 [48]
SBET
data
[70]
F400An 0.006 8.0 960
F400NH2 0.003 6.5 836 ––
L800n 0.108 416 Vmicro 0.105
Vmeso 0.209
– [86]
S800n 0.022 418 Vmicro 0.162
Vmeso 0.037
S600n 0.008 301 Vmicro 0.111
Vmeso 0.013
S650 0.129 583 Vmicro 0.133
Vmeso 0.209
H550 0.020 302 Vmicro 0.098
Vmeso 0.040
S450 0.020 166 Vmicro 0.047
Vmeso 0.031
Isomer mixture of (aRS, 1S)-2-chloro-
6'-ethyl-N-(2-methoxy-methyl-
ethyl)aceto-o-toluidide and (aRS,
1R)-2-chloro-6'-ethyl-N-(2-methoxy-
methyl-ethyl)aceto-o-toluidide = 80–
100% : 20–0% (metolachlor), not
determined, 0.480 g/L
L800n
0.108
416
Vmicro 0.105
Vmeso 0.209
Compound, dissociation constant,
solubility, and structural formula
Adsorbent
sample
Maximum ad-
sorption capa-
city* (mg/g)
pHPZC of
the ad-
sorbent
SBET
(m2/g)
Vpore,
cm3 g–1
Aver age p ore
diameter,
nm
Refe-
rences
N
H
Cl
ClN
O
N
N
N
Cl
N
H
N
H
Table 1. (Contd.)
194
SOLID FUEL CHEMISTRY Vol. 56 No. 3 2022
KULAISHIN et al.
S800n 0.023 418 Vmicro 0.162
Vmeso 0.037
[86]
S600n 0.007 301 Vmicro 0.111
Vmicro 0.013
S650 0.113 583 Vmicro 0.133
Vmeso 0.209
H550 0.014 302 Vmicro 0.098
Vmeso 0.040
S450 0.017 166 Vmicro 0.0 47
Vmeso 0.031
3-(4-Isopropylphenyl)-1,1-dimethy-
lurea (isoproturon), does not
dissociate, 0.065 g/L
L800n 0.108 416 Vmicro 0.105
Vmeso 0.209
S800n 0.031 418 Vmicro 0.162
Vmeso 0.037
S600n 0.007 301 Vmicro 0.111
Vmeso 0.013
S650 0.124 583 Vmicro 0.133
Vmeso 0.209
H550 0.015 302 Vmicro 0.098
Vmeso 0.040
S450 0.015 166 Vmicro 0.047
Vmeso 0.031
2-(4,6-Dimethoxypyrimidin-2-ylcar-
bomoyl-sulfamoyl)-N,N-dimethylnic-
otinamide (nicosulfuron),
4.78, 0.044 g/L
Ach129
0.070
2192
Vmicro 1.059
1.79
[87]
Methyl ester of para-hydroxybenzoic
acid (methylparaben), 8.2, 2.5 g/L
CB 0.175 5.4 864 Vtotal 0.300
Vmicro 2.100
– [64]
CB1073 0.233 11.1 1127 Vtotal 0.480
Vmicro 2.100
CB1173 170.000 8.9 814 Vtotal 0.340
Vmicro 2.300
Compound, dissociation constant,
solubility, and structural formula
Adsorbent
sample
Maximum ad-
sorption capa-
city* (mg/g)
pHPZC of
the ad-
sorbent
SBET
(m2/g)
Vpore,
cm3 g–1
Aver age p ore
diameter,
nm
Refe-
rences
O
N
Cl
O
N
H
N
O
S
O
N
O
O
N
HN
H
O
N
N
O
O
HO
O
O
Table 1. (Contd.)
SOLID FUEL CHEMISTRY Vol. 56 No. 3 2022
INFLUENCE OF THE SURFACE CHARACTERISTICS 195
CONFLICT OF INTEREST
The authors declare that they have no conflicts of interest.
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4-Chlorophenoxyacetic acid
(4-CPA, paraphen), 8.5,
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G33 0.003 770 Vtotal 0.400
G66 0.003
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(Клофибровая кислота), 3.37, 0.582
г/л 2-(4-Chlorophenoxy)-2-methyl-
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rences
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OO
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Table 1. (Contd.)
196
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Translated by V. Makhlyarchuk
... Among the most used methods for the removal of pesticides, from the aqueous phase, adsorption on a diversity of adsorbents, particularly on activated carbon (ACs) was highlighted, in reviews made by Kulaishin et. al in 2022 [1] and Baskar et. al, 2022 [2]. ...
... al, 2022 [2]. Some disadvantages concerning other methods currently used were identified, such as time consumption and dependency on environmental conditions for the biodegradation process, energy consumption for electrooxidation, and photocatalytic process [1]. ...
... Adsorption is a process with simple implementation and operation, being therefore widely used, for the removal of inorganic and organic contaminants from gaseous [3,4] or liquid effluents, such as drinking and wastewater [2]. However, adsorption present also has some limitations, such as adsorbent production cost, regeneration, and sustainable management of spent adsorbents [1][2][3][4][5]. ...
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This paper describes the influence of the regeneration treatments on 4-chloro-2-methyl-phenoxyacetic acid (MCPA)-loaded activated carbons (ACs) on adsorption-desorption cycles with two commercial ACs, Merck and Norit 1240 X ( Norit 1240 X was used in a granular and powder form). The ACs were saturated with MCPA and then submitted to four regeneration cycles, by washing with ethanol, NaOH solutions and washed with NaOH solutions followed by a thermal treatment. The ACs regenerated with ethanol showed a better performance concerning the successive adsorption-desorption cycles. On the first cycle, the MCPA desorbed, from washing with ethanol, from all ACs was higher than 99%. After a fourth adsorption-desorption cycle, washing regeneration combined with thermal treatment allowed excellent regeneration results. The amount of MCPA adsorbed on Norit 1240 X AC was higher than the amount adsorbed in the first cycle. MCPA adsorption kinetic data were analyzed by applying pseudo-first-order, pseudo-second-order, and Weber-Morris models. The pseudo-second-order fit better fit to the data and the Weber-Morris representation allows confirming that on Norit 1240 X, in a granular form, the pore diffusion was the limiting factor concerning the MCPA adsorption.
... In reviews by Kulaishin et al. 2022 [2] and Baskar et al. 2022 [3], adsorption on a variety of adsorbents, particularly on activated carbon (ACs), was emphasized as one approach for removing pesticides from the aqueous phase. There are certain drawbacks to other methods now in use, including the time needed for the process and reliance on environmental conditions for the biodegradation process, the need for energy for electrooxidation, and the photocatalytic process [2]. ...
... In reviews by Kulaishin et al. 2022 [2] and Baskar et al. 2022 [3], adsorption on a variety of adsorbents, particularly on activated carbon (ACs), was emphasized as one approach for removing pesticides from the aqueous phase. There are certain drawbacks to other methods now in use, including the time needed for the process and reliance on environmental conditions for the biodegradation process, the need for energy for electrooxidation, and the photocatalytic process [2]. ...
Article
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Adsorbents presenting high adsorption capacity, fast adsorption rate, easy regeneration, and a good possibility for reusability are ideal for removing 4-chloro-2-methyl-phenoxyacetic acid (MCPA) or other pesticides from wastewater. Here, the effects of regeneration treatments on adsorption–desorption cycles are examined using two commercial activated carbons (ACs) (Merck and Norit 1240 X). MCPA adsorption was fast on Merck and Norit ACs in powder form (6 h) but on Norit AC, in granular form, adsorption was too slow, and the equilibrium time was reached only after 288 h. MCPA adsorption kinetic data were analyzed by applying pseudo-first-order, pseudo-second-order, and Weber–Morris models. The pseudo-second-order model fit better to all data, and the Weber–Morris representation allows confirming that on Norit 1240 X, in granular form, the pore diffusion was the limiting factor concerning the MCPA adsorption. Merck and Norit 1240 X (in powder and granular form) ACs loaded with MCPA were submitted to different regeneration process by washing with distilled water, ethanol, HNO3, and NaOH solutions and washed with NaOH solutions or ethanol followed by a thermal treatment. The ACs regenerated with ethanol performed well in the subsequent adsorption–desorption cycles. All ACs had more than 99% desorbed MCPA after the first cycle of washing with ethanol. The washing with NaOH solution was less efficient. The regeneration process, consisting of washing the sample with a solution of NaOH and subsequent heating at 573 K, was very effective. After this regeneration procedure, the amount of MCPA adsorbed on Norit 1240 X AC was even higher than the amount adsorbed in the first adsorption cycle. At present, washing methods for adsorbent regeneration are not used at an industrial level. However, research for environmentally friendly regeneration methods is necessary to achieve the objectives of the circular economy.
... The influence of carbon surface properties on adsorption effectiveness is widely investigated; however, despite much research, this area is not fully explained due to the many effects that determine the adsorption process. [33][34][35][36] However, the role of surface functional groups on other phenomena occurring in pores, like freezingmelting, is not a subject of many studies. 37,38 The role of solvents in adsorption systems is relatively rarely investigated. ...
Article
In this work, the adsorption and freezing/melting processes on activated carbons of different chemical nature (different concentrations of oxygen surface groups) were investigated. The role of solvent (cyclohexane and water)...
... Finally, the maximum amount of each pesticide adsorbed on both ACs was slightly higher than the values obtained previously on ACs prepared from Tectona Grandis sawdust, by physical activation with CO 2 , at 973 K (maximum adsorption capacity of MCPA on Teak-7480 was 1.56 mmol·g −1 ) [10]. Nevertheless, these data compare favorably with published results obtained on a diversity of commercial ACs used for this purpose [10,39,40]. ...
Article
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This work presents a first approach concerning the valorization of Tectona Grandis tree by-products, from East Timor through their transformation into high activated carbon (AC) by chemical activation with KOH and K2CO3. The better ACs, Teak-KOH-1-1-700 and Teak-K2CO3-1-2-700, presented a high ABET (995 and 1132 m²·g⁻¹) and micropore volume (0.43 and 0.5 cm³·g⁻¹), respectively. Both ACs were tested on the removal of four pesticides, from the liquid phase. Both ACs performed better than existing commercial types, presenting a maximum adsorption capacity of 1.88, 1.67, 1.10 and 0.89 mmol·g⁻¹, for 4-chloro-2-methylphenoxyacetic acid, 2,4-dichlorophenoxyacetic acid, diuron and atrazine, respectively. Pesticide adsorption from diluted and concentrated solutions confirms that diffusion is the limiting factor. The possibility of implementing a production unit for ACs in East Timor is very promising for that country. It presents an opportunity for job creation, biomass waste reduction and a contribution to environmental sustainability, thereby following the principles of a circular economy.
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This work presents some preliminary results on the direct use of untreated biomass from agricultural activities as adsorbents for water treatment. Waste was also used to produce activated carbons (ACs) by chemical activation with KOH. The efficacy of agricultural waste such as stubble, sawdust from Teak (Tectona Grandis), fibers from Imbondeiro (Adansonia digitata L.), bamboo flowers, and other regional plants were tested on methylene blue (MB) removal from the aqueous phase. Adsorption studies were conducted in a batch system and the influence of kinetics, pH, and temperature was evaluated. The adsorption performance of the natural adsorbents was significantly high concerning MB. In particular, Imbondeiro presented a maximum removal capacity of 188.3 mg per gram. This amount was similar to or even higher than the values obtained on ACs produced by their predecessors at 873 K. The studies were finished by constructing slow filters containing natural adsorbents or ACs. The maximum amounts of MB removed on a continuous flux were lower than those obtained for a diversity of untreated biomass types on a batch system. However, these amounts were comparable to the published results obtained on a diversity of untreated biomasses in batch mode.
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The present work discusses the influence of additives, designated as co-adjuvants agents of the chemical activation, containing nitrogen on their structure, on the activated carbons (ACs) produced from the waste of polyethylene terephthalate (PET) and cork by chemical activation, with KOH, at 973 K. The co-adjuvants agents used were urea, 2-chloro-4,6-diamino-1,3,5-triazine, (2-hydroxyethyl) urea and polyethylenimine. The ACs, produced from PET, only activated with KOH presented a porous volume of 0.53 cm ³ g⁻¹. The chemical activation of PET with KOH and urea, or 2-chloro-4,6-diamino-1,3,5-triazine or (2-hydroxyethyl) urea allows obtaining ACs with a porous volume upper than 0.91 cm ³g⁻¹. The same improvement (porous volume higher than 0.93 cm ³g⁻¹) was achieved with cork, with urea or polyethylenimine. On all ACs produced with the four co-adjuvants agents, an increment in the nitrogen content was very noticeable. The high thermal stability of these ACs allows inferring, that the nitrogen was directly connected to the ACs array, and not only retained on the surface. These ACs were successfully tested on 4-chloro-2-methyl-phenoxyacetic acid (MCPA) removals from the aqueous medium, with removal percentages ranging from 65% in solutions containing 2.50 mmol L⁻¹ to 100% in solutions with concentrations lower than 0.25 mmol L⁻¹. Five ACs had an MCPA adsorption capacity greater than 3.7 mmol g,−1 meanwhile, the highest value found in the literature was 2.99 mmol g⁻¹ on the commercial AC-GAB (Spaltro et al., 2018).
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The wastes formed from various industries' activities draw attention to their ability to be used as a natural adsorbent or as a source for activated carbon production. In this study, we aimed to produce activated carbon rich in surface functional groups from biomass. Surface functional groups of activated carbons (AC) were enriched by HNO3 oxidation. The activated carbons were synthesized with H3PO4 using peach stones (PSs), fruit juice industrial wastes, and can be found in abundance in the World and Turkey. The activated carbons' surface properties, such as BET surface area, micro-mesopore volume, and average pore diameter, were determined. The activated carbon with the highest surface area was produced with the impregnation ratio of 3/1 at 400°C (3AC400) as 1399 m²/g. The microporous structures of the 3AC400 synthesized were determined to be more than the mesopore structures. The effects of acid modification on the surface of 3AC400, whose surface characterization was completed, were investigated using HNO3 at different concentrations (15, 30, 45, and 69 % (v/v) HNO3). When the nitric acid concentration used in the modification was increased from 15% to 69%, the BET surface area of pure activated carbon (3AC400) decreased from 1399 m²/g to 15.44 m²/g. Besides, the micropores were wholly clogged, and the mesopore volume decreased from 0.127 cm³/g to 0.042 cm³/g. Vmic/Vtop of 3AC400 was reduced from 0.813 to 0.008. Modification using in this study was found to have a significant effect on the surface area of 3AC400. Nitric acid led micropores to collapse. So the structure of the surface of modified activated carbons has become more mesopores. The surface properties and surface functional group diversity of the synthesized modified activated carbons by various methods were investigated. With 69% nitric acid modification, the amount of carboxyl, lactone, and phenyl groups on the activated carbon surface was increased by 5.3, 9.75, and 9.00, respectively. The nitric acid modification affected the diversity of surface functional groups such as lactone> phenol> carboxyl. As the nitric acid concentration increased, the modified samples' total surface acidity increased, and IEP values shifted from 3.6 to the left. The total acidity value for the 69NAC/90/2 sample has increased approximately seven times that of the total acidity value of 3AC400.
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In this study, the chalice generated from the production of the Physalis peruviana fruit was subjected to a treatment with sulfuric acid and applied in the adsorption of 2,4-Dichlorophenoxyacetic acid (2,4-D). The precursor, and the treated material before and after the adsorption, were analyzed by different techniques. After the acid treatment, it was found that the surface was changed from a smooth to an irregular surface with the presence of cavities with irregular size. The adsorption was favored at pH = 2 and with a dosage of 0.8 g L⁻¹. The pseudo-second-order model was the best to represent kinetic data. The isothermal experiments were well represented by the Langmuir and Tóth models, reaching a high capacity of 244 and 320 mg g⁻¹, respectively. The computed thermodynamic values show that the 2,4-D adsorption was spontaneous and exothermic. Overall, this study indicates that the Physalis peruviana chalice treated with strong acid presents great potential as an alternative material for the adsorption/removal of 2,4-D herbicide from liquid effluents.