Statistical optimization of activated carbon from Thapsia transtagana stems and dyes removal efficiency using central composite design

Journal of Science: Advanced Materials and Devices 4 (2019) 544e553

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Journal of Science: Advanced Materials and Devices
journal homepage: www.elsevier.com/locate/jsamd

Original Article

Statistical optimization of activated carbon from Thapsia transtagana
stems and dyes removal efficiency using central composite design
A. Machrouhi a, H. Alilou a, b, M. Farnane a, S. El Hamidi a, M. Sadiq a, M. Abdennouri a,
H. Tounsadi c, *, N. Barka a, **
a
Sultan Moulay Slimane University of Beni Mellal, Research Group in Environmental Sciences and Applied Materials (SEMA), FP Khouribga, B.P. 145, 25000
Khouribga, Morocco
b
Facult
e Polydisciplinaire de Taroudant, Universit
e Ibn Zohr, Agadir, Morocco
c
Laboratoire d'Ing
enierie, d'Electrochimie, de Mod
elisation et d'Environnement, Universit
e Sidi Mohamed Ben Abdellah, Facult
e des Sciences Dhar El
Mahraz, F
es, Morocco

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 29 March 2019
Received in revised form
21 August 2019
Accepted 7 September 2019
Available online 13 September 2019

This study focused on the preparation of activated carbons from Thapsia transtagana stems by boric acid
activation and their evaluation for dyes removal. The central composite design and response surface
methodology were used to optimize the preparation conditions. The effect of activation temperature,
impregnation ratio and activation time on iodine number (IN), methylene blue index (MB index) and
removal efficiencies of methyl violet (MV), methyl orange (MO) and indigo carmine (IC) were fully
evaluated. The activated carbon samples prepared in optimal conditions were characterized by FTIR,
XRD, SEM-EDX, Boehm's titration, and point of zero charge (pHPZC). The equilibrium data for dyes
sorption onto optimum activated carbons were best fitted with Langmuir isotherm.
© 2019 Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an
open access article under the CC BY license ( />
Keywords:
Thapsia transtagana stems
Dyes removal
Chemical activation
Central composite design

1. Introduction
Nowadays, the extensive uses of textile dyes are considered the
main sources of water pollution [1]. It has been estimated that
10%e15% of the dye used during the manufacturing of textile
products are released into the environment worldwide annually

[2]. Moreover, many of these organic compounds can cause allergies, skin irritation or even cancer and human mutations [3]. It is,
therefore, essential to remove dyes from wastewater and water
reuse to avoid contamination and destruction of natural resources.
Currently, there are numerous methods employed to remove
dye molecules from aqueous solutions including adsorption [4],
precipitation [5], ion-exchange [6], coagulation [7], membrane
filtration [8], photocatalytic degradation [9], etc. Among these
processes, the adsorption is more applicable because it is an efficient, simple and economic method for the removal of dyes from
aqueous solutions [10,11]. For that, various types of low-cost, easily
* Corresponding author.
** Corresponding author. Fax: þ212 523 49 03 54.
E-mail addresses: (H. Tounsadi), barkanoureddine@
yahoo.fr (N. Barka).
Peer review under responsibility of Vietnam National University, Hanoi.

available and highly effective adsorbents are reported such as
activated carbon, zeolite, clay, polymer, and nanomaterials [12e16].
From economic point of view, the process of adsorption onto
activated carbon is advantageous due to the plentiful accessibility
of low cost raw material. Also, activated carbon is basically
referred as carbonaceous materials, with a high physicochemical
stability, high porosity, high sorption capacity and with immense
surface area.
Recently, many studies have been carried out to investigate the
use of inexpensive biomasses to produce low-cost activated carbons using agricultural solid wastes including coffee ground [17],
Carob shell [18], Diplotaxis harra [19], Glebionis coronaria L. [20],
maize corncob [21], beetroot seeds [22], apricot stones [23],
hazelnut shells [24] and loofah sponge [25].
Activated carbon can be produced in a two-step process:
carbonization and activation. Carbonization is usually conducted

via pyrolysis at temperatures of 400e850  C in the absence of oxygen [26]. The activation process converts carbonized materials to
activated carbon via heating. Carbon dioxide, air or steam is used as
a physical activation method to develop the porosity of the carbonaceous materials. The increase of surface area and the pore volume is achieved through the elimination of internal carbon mass

/>2468-2179/© 2019 Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license (http://
creativecommons.org/licenses/by/4.0/).


A. Machrouhi et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 544e553

and the removal of volatiles [27]. However, in chemical activation,
the carbonization temperature is done only between 400 and
600  C. This method highlights an impregnation of the precursor or
raw material with dehydrating agents such as alkali metal hydroxide or acid. This method produces an activated carbon with
higher yield and well developed microporosities.
The objective of this research was to investigate the feasibility of
activated carbon produced from Thapsia transtagana stems
biomass, by H3BO3 activation and their ability for cationic and
anionic dyes removal from aqueous solution. Central composite
design (CCD) combined with response surface methodology (RSM)
was used to optimize the process. The factors chosen are impregnation ratio, activation temperature and activation time. Five responses including iodine number (IN), methylene blue index (MB
index) and removal efficiencies for methyl violet (MV), methyl orange (MO) and indigo carmine (IC) are investigated.
2. Material and methods
2.1. Material

The independent variables were coded as þ1 and À1 which
represent the eight factorial points at their low and high levels,
respectively. The six axial points were located at (±a, 0, 0), (0, ±a,
0), (0, 0, ±a), and the six replicates were at the center (0, 0, 0) were
run to examine the experimental error and the reproducibility of

the data. Where a is the distance of axial point from center which
makes the design rotatable; its value was fixed at 1.682. This value
of rotatability a, which depends on the number of parameters in
the experiment, was obtained from the following equation [30]:

a ¼ Np1/4

(3)

In this study, the independent variables studied were activation
temperature (A), impregnation ratio (B) and activation time (C).
These three variables with their respective ranges were selected
based on the literature and preliminary studies as given in Table 1.
The responses were determined using the optimal quadratic
model predictor Eq. (4) given as:

Y ¼ b0 þ

All the chemicals/reagents used in this study were of analytical
grade. H3BO3 (100%), HCl (37%), I2 (99.8e100.5%), Na2S2O35H2O,
Na2CO3, NaHCO3 (99.5e100.5%), commercial activated carbon
(powder form) (100%), methyl violet, methyl orange and indigo
carmine (100%) were purchased from SigmaeAldrich (Germany)
(100%). Methylene blue was purchased from Panreac (Spain)
(100%). HNO3 (65%) was provided from Sharlau (Spain). NaOH
(!99%) from Merck (Germany), potassium iodide (KI) (100%) was
obtained from Pharmac (Morocco).

545

Xn

!2

Xn

bx þ
i¼1 i i

þ

b x
i¼1 ii i

Xn-1 Xn

b xx
j¼1þ1 ij i j

i¼1

(4)
0where Y is the predicted response, bo is the offset term, bi the
linear effect, bii the squared effect, bij the interaction effect and xi, xj
are the coded values of the variables considered.
The quality of the fit of the polynomial model was expressed by
the correlation coefficient (R2). The significance and adequacy of the
used model was further explained using F-value (Fisher variation
ratio), probability value (Prob > F), and adequate precision (AP) [31].

2.2. Preparation of activated carbons
2.4. Iodine number (IN)
The T. transtagana plant was collected from the region of Oued
zem, Morocco. Steams were cut into small pieces and were
powdered to a particles of size <125 mm using a domestic mixer.
15 g of the biomass were impregnated with H3BO3 as the activating
agent at the desired mass ratio. Later, the sample was loaded in a
stainless steel vertical tubular reactor placed into a furnace under
purified nitrogen atmosphere. The obtained activated carbons were
washed with distilled water and dried at 105  C for 24 h. The
powder was sieved in particles of size lower than 125 mm using a
normalized sieve and kept in a hermetic bottle for a further use.
The impregnation ratio of the activating agent with the precursor was computed using Eq (1):
Impregnation ratio ¼ (dried weight of H3BO3/precursor of TTS) (1)

Iodine number is a measure of micropore content (0e2 nm) by
adsorption of iodine from solution. The iodine number is defined as
the milligrams of iodine adsorbed by 1.0 g of carbon when the
iodine concentration of the filtrate is 0.02 N. It was determined
according to the ASTM D4607-94 method [32].
2.5. Methylene blue index (MB index)
The methylene blue index is a measure of mesoporosity
(2e50 nm) present in activated carbon. Sorption equilibrium
was established for different methylene blue initial concentrations between 20 and 500 mg/L for 12 h at room temperature.
Residual concentrations were determined by a spectrophotometric method at the wavelength of maximum absorbance of
665 nm [33].

2.3. Design of experiments using central composite design
2.6. Dyes removal
Central composite design (CCD) was used to study the individual

and synergetic effect of the three factors towards defined responses. This method can reduce the number of experimental trials
required to evaluate the main effect of each parameter and their
interactions [28]. It is characterized by three operations namely: 2n
factorial runs, 2n axial runs and six center runs [29]. For this case,
it's translated into eight factorial points, six axial points and six
replicates at the center which gives a total of 20 experiments as
calculated from Eq (2):
n

Total number of experiments (N) ¼ 2 þ 2n þ nc

Stock solutions of methyl orange, methyl violet and indigo
carmine at 500 mg/L were prepared by dissolving 0.5 g of each dye
in 1 L of distilled water. Sorption experiments were investigated in
a series of beakers containing 50 mL of dyes solutions at 500 mg/L
Table 1
Process factors and their levels.
Variables

Code

Unit

A
B
C



(2)

where n is the number of factors, nc is the number of center points
(six replicates).

Activation temperature
Impregnation ratio
Activation time

C
g/g
min

Coded variable levels
Àa

À1

0

1

þa

366
0.66
105

400
1
115

450
1.5
130

500
2
145

534
2.34
155


546

A. Machrouhi et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 544e553

and 50 mg of each activated carbon. The mixtures were stirred for
2 h without any pH adjustment. After each sorption experiment,
samples were centrifuged at 3400 rpm for 10 min and the dyes
concentration was determined using a UVevis spectrophotometer.
The adsorption capacities of the dyes at equilibrium were
defined as the amount of adsorbate per gram of adsorbent (in mg/g)
and were calculated using the following equation:

q¼

ðCo À CÞ
R

(5)

where q is the adsorbed quantity (mg/g), Co is the initial dye concentration (mg/L), C is the residual dye concentration (mg/L), and R
is the mass of activated carbon per liter of aqueous solution (g/L).
2.7. Surface and chemical characterization
Textural properties of optimized activated carbon were
observed by scanning electron microscopy (SEM) using TESCAN
VEGA3-EDAX equipped with an Energy-Dispersive X-Ray detector
(EDX). The functional groups present on the surface of the starting
material and AC was determined by the Fourier Transform Infrared
(FTIR) spectroscope (FTIR-2000, PerkinElmer) in a range of
4000e400 cmÀ1. Crystallographic characterization was examined
by XRD measurements in the 2q range from 10 to 70 using a
Bruker-axs D2-phaser advance diffractometer operating at 30 kV
and 10 mA with CuKa. The acidic and basic functional groups on the
surface of ACs were determined quantitatively by the Boehm's
titration method [34]. The pH of the point of zero charge (pHpzc)
was determined according to the method described by Noh and
Schwarz [35].
3. Results and discussion

Table 2
Values of model coefficients of the five responses.
Main coefficients

Y1

Y2

Y3

Y4

Y5

b0
b1
b2
b3
b12
b13
b23
b11
b22
b33

710.80
46.15
55.08
1.30
1.75
5.24
5.24
À27.52
À12.71
À20.11

143.79
13.17

29.11
4.97
10.04
À1.84
2.93
À7.78
À11.09
À5.40

100.28
9.69
16.57
2.19
0.63
À1.81
À0.16
À3.93
À6.15
À4.54

120.79
6.89
14.66
4.46
4.71
0.61
À2.27
À5.15
À6.01
À5.04

29.97
4.01
10.46
2.08
3.40
0.03
0.30
À3.29
À2.98
À0.32

relationship between each response and the significant variables.
The F value implies that the models are significant and the values of
“Prob > F” less than 0.05 indicate that model terms are significant.
Especially larger F-value with the associated P value (smaller than
0.05, confidence interval) means that the experimental systems can
be modeled effectively with less error. Therefore, interaction effects
as adequate model terms can be used for modeling the experimental system.
3.2.1. Iodine number
According to the ANOVA analysis for the iodine number, the
significant terms are the activation temperature (A), impregnation
ratio (B), activation time (C), the interaction between activation
temperature and impregnation ratio (AB), the interaction between
impregnation ratio and activation time (BC), the quadratic term of
activation temperature (A2) and the quadratic term of activation
time (C2) Eq. (6).
Y1 ¼ 703.26 þ 44.10 A þ 56.46 B À 0.74 C À 1.75 AB þ 1.75
BC À 28.35 A2 À 20.94 C2

(6)

3.1. Experimental results
The experimental results obtained at the designed experimental
conditions according to the central composite design are presented
in Table S1. From this table, it could be seen that the activated
carbon sample activated at 500  C for 145 min with an impregnation ratio of 2 g/g gives the optimum of MB index (188.75 mg/g),
MO adsorption (116.84 mg/g) and MV adsorption (140.76 mg/g. The
greater iodine number of 794.58 mg/g is obtained for the activated
carbon prepared at 450  C for 130 min with an impregnation ratio
of 2.34 g/g. Under these same conditions, the optimum for IC
adsorption (44.87 mg/g) is also acquired.
On the other hand, the regression analysis was performed to fit
the response functions with the experimental data. Table 2 shows
the values of the regression coefficients obtained. According to this
table, the three studied factors present a positive effect on the five
responses. The table also indicates that the targeted responses are
more influenced by activation temperature and impregnation ratio
than by activation time.
3.2. Analysis of variance (ANOVA)
The analysis of variance (ANOVA) was used to determine the
significance of the curvature in the responses at a confidence level
of 95%. After discarding the insignificant terms, the ANOVA data of
the coded quadratic models for the five responses are presented in
supplements (Tables S2eS6). The effect of a factor is defined as the
change in response produced by a change in the level of the factor.
This is frequently called a main effect because it refers to the primary factors of interest in the experiment. The ANOVA results
showed that the equations adequately represent the actual

The activation temperature, the impregnation ratio and the

interaction between impregnation ratio and activation time
showed a positive effect on the iodine number. Although, the
activation time, the interaction between activation temperature
and impregnation ratio, the quadratic term of activation temperature and the quadratic term of activation time showed a negative
effect on the iodine number. Besides, the impregnation ratio has the
largest significant effect on the iodine number due to the high Fvalue (99.05) followed by the activation temperature, the quadratic
term of activation temperature and the quadratic term of activation
with an F-value of 60.43, 26.61, and 14.52, respectively (Table S2).
Hence, it could be seen that the number of micropores are higher
with the impregnation ratio of 2.34 g/g in the studied domain. In
fact, at the high level of the significant model terms, the activation
reaction may take place rapidly producing a development of
porosity of the obtained activated carbons and an increase in the
microporosity.
3.2.2. Methylene blue index
The most significant effects for the methylene blue index are
activation temperature (A), impregnation ratio (B), activation time
(C), interaction between activation temperature and impregnation
ratio (AB) and the quadratic term of impregnation ratio (B2) Eq. (7).
Y2 ¼ 133.98 þ 13.17 A þ 29.11 B þ 4.97 C þ 10.04 AB À 9.90 B2 (7)
The activation temperature, impregnation ratio, activation time
and interaction between activation temperature and impregnation
ratio showed a positive effect on the methylene blue index
response. Although, the quadratic term of the impregnation ratio


A. Machrouhi et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 544e553

presented a negative effect on the development of mesopores.
According to Table S3, the impregnation ratio has the most significant effect on the methylene blue index due to the higher F-value

(43.55). After that, follow the activation temperature, the quadratic
term of impregnation ratio and the interaction between activation
temperature and impregnation ratio with F-values of 8.92, 5.41 and
3.03, respectively.
3.2.3. Methyl orange and methyl violet removal
Based on the ANOVA data for methyl orange and methyl violet
removal responses, the most significant factors are activation
temperature (A), impregnation ratio (B) activation time (C), interaction between activation temperature and impregnation ratio
(AB), interaction between impregnation ratio and activation time
(BC) and quadratic term of impregnation ratio (B2) Eqs. (8) and (9).
Y3 ¼ 93.97 þ 9.69 A þ 16.57 B þ 2.18 C þ 0.63 AB À 0.16 BC À 5.38
B2
(8)
Y4 ¼ 113.21 þ 9.89 A þ 14.66 B þ 4.46 C þ 4.71 AB À 2.27 BC À 5.09
B2
(9)
The activation temperature, impregnation ratio, activation time
and interaction between activation temperature and impregnation
ratio showed a positive effect on the methyl orange and methyl
violet removal response. Although, the interaction between
impregnation ratio and activation time and the quadratic term of
impregnation ratio presented a negative effect. From Tables S4 and
S5, it could be seen that the impregnation ratio has the largest effect on the methyl orange and methyl violet removal with an Fvalue of 53.15 and 26.10, respectively, followed by the activation
temperature and quadratic term of impregnation ratio for the MO
and MV removal.
3.2.4. Indigo carmine removal
The significant model terms for indigo carmine removal are the
activation temperature (A), impregnation ratio (B), activation time
(C), the interaction between activation temperature and impregnation ratio (AB), the interaction between activation temperature
and activation time (AC) and the quadratic term of activation

temperature (A2) Eq. (10).
Y5 ¼ 27.67 þ 3.76 A þ 10.63 B þ 1.83 C þ 2.95 AB À 0.39 AC À 3.22
A2
(10)
The activation temperature, impregnation ratio, activation time
and interaction between activation temperature and impregnation
ratio showed a positive effect on the indigo carmine removal
response. However, the interaction between activation temperature and activation time and the quadratic term of activation
temperature presented a negative effect on the indigo carmine
removal. According to Table S6, it could be seen that the impregnation ratio has the most pronounced effect on the indigo carmine
removal based on the highest F-value of 60.03. In contrast, the
activation temperature, quadratic term of activation temperature
and interaction between activation temperature and impregnation
ratio have an F-value of 7.52, 5.94, and 2.72, respectively.
3.3. Response surface analysis
The mathematical models for the iodine number, MB index and
dyes removal were used to build response surfaces as well as to
determine the optimal conditions of the process. Fig. 1 present the
3D response surfaces plots for the significant interactions.

547

For the iodine number, the most significant interactions were
the impregnation ratio/activation temperature and the activation
time/impregnation ratio. The Fig. 1(a) indicates that the iodine
number increased with the increase of activation temperature and
impregnation ratio. Fig. 1(b) shows that the iodine number
increased with increase of the impregnation ratio and decrease of
activation time when the activation temperature is fixed at 500  C.
For the MB index, the most significant interaction was the

impregnation ratio/activation temperature. From Fig. 1(c), it can be
observed that the MB index increased with the increase of the
activation temperature and the impregnation ratio. The maximal
MB index response was obtained at an activation time of 145 min.
In the removal of MV and MO dyes, the same significant interactions are found, including the impregnation ratio/activation
temperature and the activation time/impregnation ratio. From
Fig. 1(d)e(f), it can be observed that the MV and MO removal
increased with increase of the activation temperature and
impregnation ratio. The maximal MV and MO removal responses
were obtained at an activation time of 145 min. Fig. 1(e)e(g), shows
that the MO and MV removal increased with increasing impregnation ratio and decreasing activation time in case the activation
temperature is fixed at 500  C.
For the removal of IC, the most significant interactions were the
impregnation ratio/activation temperature and the activation time/
activation temperature. From the 3D response surface plot as
shown in Fig. 1(h), it was observed that the indigo carmine removal
increases with increase of the impregnation ratio and activation
temperature in case the activation time is fixed at 130 min. Fig. 1(i)
shows that for a decrease of the activation time and an increase of
the activation temperature, the IC removal response increased.
In general, the impregnation of the precursor allows the
development of the internal structure of the activated carbon by
the creation of new pores and the enlargement of existing pores. In
this context, several parameters including the activation time, the
activation temperature and the impregnation ratio play an important role in the development of the porosity of activated carbons
and, consequently, the evolution of the adsorption performance. In
fact, during activation, the boric acid catalyzes the dehydration and
promotes the formation of aromatic structures during pyrolysis
[36]. In addition, the formation of an impenetrable glassy coating
on the solid surface from boric acid decomposition products inhibits the release of volatile substances, which also promotes the

formation of carbon [37,38]. Then, this vitreous coating prevents
the diffusion of oxygen and prevents the propagation of exothermic
combustion reactions [39].
3.4. Diagnostic model
Table S7 summarizes the information of the proposed models of
statistic the actual and predict values for testing the significant effects
of the regression coefficients. Predicted values obtained were
compared with experimental values. These values for the models
nearly coincide, which indicates a correspondence between the
mathematical model and the experimental data. The correlations
between the theoretical and experimental responses, calculated by
the model, are satisfactory. Therefore, the R2 values are in reasonable
agreement with those of the Radj2. In addition, the model F-value of
the iodine number, methylene blue index, methyl orange, methyl
violet and indigo carmine removal read as 28.21, 12.44, 13.06, 6.60 and
13.00 respectively. These values implicate that models are significant.
3.5. Normal probability plot of residuals
The normal probability plot of the residuals is presented in
Fig. 2. The normality of the data can be checked by plotting a


548

A. Machrouhi et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 544e553

Fig. 1. Surface response plots for the iodine number (aeb), MB index (c), MV removal (dee), MO removal (feg) and IC removal (hei).

normal probability plot of the residuals. If the data points on the
plot fall fairly close to the straight line, the data are normally
distributed [40]. It appears that for the iodine number, methylene

blue index and dyes responses, the data points were fairly close to
the straight line, indicating that the experiments come from a
normally distributed population.
3.6. Optimization analysis
The optimum conditions for the three variables, activation
temperature, impregnation ratio and activation time were obtained
using numerical optimization features of Design-Expert 10.0.0. The
software searches for a combination of factors that simultaneously
satisfy the requirements placed on each of the response factors. The
goal was to find the optimum process parameters that will produce
activated carbons with high iodine number, high dyes removal, as
well as high methylene blue index. From the experimental results,
the optimized activated carbon sample activated at 500  C for
145 min with an impregnation ratio of 2 g/g, under which a
maximum MB index of 188.75 mg/g, a MO and MV removal of
116.84 mg/g and 140.76 mg/g could be achieved, respectively. For

the maximum IN and removal of IC the optimal preparation conditions were determined as: activation temperature of 450  C,
impregnation ratio of 2.34 g/g and activation time of 130 min.
Under this condition the maximum values of IN and the adsorption
capacity for IC were 794.58 mg/g and 44.87 mg/g, respectively. In
addition, it was observed that experimental values obtained were
in good agreement with the values predicted from the models, with
relatively small errors between the predicted and the experimental
values, which were only 0.01% for iodine number and MO removal
responses, 0.04% for IC removal, 0.15% for the MB index and 0.22%
for the MV removal responses.
3.7. Structural and textural properties of activated carbons
3.7.1. Morphology of ACs
SEM images of raw and activated TTS surfaces are illustrated in

Fig. 3. Only a very limited number of pores were found on the
smooth and irregular surface of TTS (Fig. 3(a)). The activated
carbon prepared with an impregnation ratio of 2 g/g at an activation temperature of 500  C for 145 min and which exhibits the
high adsorption performance of dyes has an heterogeneous


A. Machrouhi et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 544e553

549

Fig. 2. Normal probability plots of residuals for the five responses: (a) Iodine number, (b) MB, index, (c) MV, (d) MO and (e) IC removal capacities.

texture with irregular cavities distributed on the surface Fig. 3(b).
Fig. 3(c) presents a smooth and featureless surface with very little
pores available on activated carbon prepared at 450  C for 130 min
with an impregnation ratio of 2.34 g/g. The activated carbon
prepared at 500  C for 115 min with an impregnation ratio of 2 g/g

shown in Fig. 3(d) indicates a rough and heterogeneous surface
and contains a limited number of pores. These observations
indicate that the activation with boric acid at different conditions
produces an increase in surface area and pore volume in the inner
surface of the ACs.


550

A. Machrouhi et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 544e553

Fig. 3. SEM micrographs of: (a) precursor (TTS), (b) 500  C/145 min/2 g/g, (c) 450  C/130 min/2.34 g/g and (d) 500  C/115 min/2 g/g.

3.7.2. Energy dispersive X-ray (EDX) analysis
The proximate analysis indicates that T. transtagana stems is a
good alternative for producing activated carbon due to its high
content of fixed carbon (27.85%) and volatile matter (39.45%) and
low ash (7.25%). Moreover, the energy dispersive X-ray analysis
(Table 3) showed that TTS contains 63.27% of carbon and 31.45% of
oxygen associated with some minerals such as potassium, nitrogen,
and calcium. After the impregnation of TTS with H3BO3, an increase
in the carbon content by 13.26% and a decrease in the oxygen
content by 13.72 for activated carbon prepared at 500  C for
145 min with an impregnation ratio of 2 g/g can be seen. For activated carbon prepared at 450  C in 130 min with an impregnation
ratio of 2.34 g/g, there is an increase in the carbon content by 11.92%
and a decrease of the oxygen content by 12.63%. For activated
carbon prepared at 500  C in 115 min with an impregnation ratio of
2 g/g, there is an increase in the carbon content by 8.42% and a
decrease in the oxygen content by 6.89%. This may be ascribed to
the oxygen removal and carbon enrichment, resulting from loosened oxygen attached to the carbonaceous material during chemical activation. The presence of carbon in significant quantity
provided the active surface for the attachments of the organic
pollutants to the surface of the activated carbons.
3.7.3. X ray diffraction
In order to determine the crystal structure of optimized activated carbons, X-ray diffraction analyzes were performed. Fig. 4
presents the XRD patterns of the studied activated carbons. This
Table 3
Percent atomic of: (a) precursor (TTS), (b) 500  C/145 min/2 g/g, (c) 450  C/130 min/
2.34 g/g and (d) 500  C/115 min/2 g/g.
Element

Atomic %
a

b

c

d

C
O
Al
Na
Cl
K
Ca

63.27
31.45
4.62
0.2
0.14
0.12
0.12

76.53
17.73
e
e
e
e
e

75.19
18.82
e
e
e
e
0.08

71.69
22.56
e
0.03
e
0.02
0.02

figure shows generally an amorphous structure of all activated
carbons with similar profiles and broad band at 23 . The simple
band at 23 may be due to the disordered stacks of graphite layers
[41]. Activated carbons have interplanar distances of d002, higher
than those of graphite. These activated carbons are considered to be
in disorder and out of graphitization.
3.7.4. Boehm titration and pH of zero charge
Table 4 presents the estimated chemical groups on the surface of
ACs and pHPZC. The pHPZC values were 6.62, 5.86 and 6.28, which
assign an acid character to the samples. While, acid groups are
greater than basic groups (1.4179, 1.4481 and 1.4432 meq/g
compared to 0.3665, 0.3549 and 0.3575 meq/g, respectively). This
acidity gave them greater exchange properties with the cationic

dyes than with the anionic dyes. Although, studied activated carbons have an important quantity of phenolic and lactonic groups in
comparison of the amount of carboxylic groups. Hence, the greater
adsorption performance of cationic and anionic dyes of these three
activated carbons optimized can be related to the availability of this
type of functional groups.
3.7.5. Infrared spectroscopy
The functional groups of activated carbons and precursor material are presented in Fig. 5. According to this figure, the surface
groups of the activated carbons were different from those of the
biomass. Some peaks have a low intensity or even disappeared in
the prepared ACs relative to the raw T. transtagana stems, as many
of the functional groups disappeared after the activation processes.
This result is due to the thermal degradation effect during the
activation processes which resulted in the destruction of some
intermolecular bondings. Concerning the FT-IR spectra of the precursor material (TTS), there are large band at 3700 and 3200 cmÀ1
attributed to the stretching vibration of hydrogen bonds of the
hydroxyl group linked in cellulose, lignin, adsorbed water and to
NeH Stretching, respectively [42]. The bands at 3000e2800 cmÀ1
are attributed to aliphatic CeH stretching vibrations of an aromatic
methoxyl group in methyl and methylene groups of side chains.
The small band at 1676 cmÀ1 is assigned to OeH bending. The
spectra also indicate a band at 1615.95 cmÀ1 which is characteristic
of C]O stretching vibrations of ketones, aldehydes, lactones or


A. Machrouhi et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 544e553

551

Fig. 4. XRD patterns of ACs-: (a) 500  C/145 min/2 g/g, (b) 450  C/130 min/2.34 g/g and (c) 500  C/115 min/2 g/g.

Table 4
Chemical groups on the surface of the ACs and pHpzc.
Activated carbon

AC-500  C/145 min/2 g/g
AC-450  C/130 min/2.34 g/g
AC-500  C/115 min/2 g/g

Surface group (meq/g)

pHPZC

Carboxylic

Lactonic

Phenolic

Total acid

Total basic

0.4324
0.4310
0.4231

0.4962
0.5049
0.5013

0.4893
0.5122
0.5188

1.4179
1.4481
1.4432

0.3665
0.3549
0.3575

6.62
5.86
6.28

Fig. 5. FT-IR spectra of: (a) precursor (TTS), (b) AC-500  C/145 min/2 g/g, (c) AC-450  C/130 min/2.34 g/g and (d) AC-500  C/115 min/2 g/g.

carboxyl groups. The activated carbons present similar profiles with
different bands intensities. The band between 3200 and 3700 cmÀ1
that correspond to OeH stretching vibrations of the hydroxyl
functional groups including hydrogen bonding, was of low intensity
in ACs. This reduction in the peak intensity corresponds to a
reduction in the hydrogen bonding which may be due to the reaction between H3BO3 and precursor [43]. The band appearing in
the spectrum between 1500 and 1700 cmÀ1 is attributed to vibrations of the C]C bonds in the aromatic rings or of the groups C]O
of carboxylic acids, acetate groups (COOe), ketones, aldehydes or

lactones. The shoulder at 900e970 cmÀ1 is attributed to a chemical
ionized bonding PþeO or to symmetric vibrations in the PeOeP
chains (polyphosphate). This band is an indication of the presence

of phosphorus-oxygen compounds in the samples. It appears that
activation of the samples impregnated with H3BO3 leads to
decomposition of phosphoric compounds. Also, the shoulder at
600e640 cmÀ1 could correspond to vibration elongation of PeOeC
(aliphatic), asymmetric elongation of PeOeC (aromatic), PeO
stretching in >P]OOH, strain PeOH asymmetric stretching
PeOeP in polyphosphates in complex phosphate-carbon.


552

A. Machrouhi et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 544e553

Fig. 6. Experimentals points and nonlinear fitted curves isotherms of AC-500  C/145 min/2 g/g: removal of (a) MB, (b) MO, (c) MV and (d) IC.

3.7.6. Adsorption isotherm
The use of an experimental design allowed us to optimize the
preparation conditions and to evaluate the cationic and anionic
dyes removal onto prepared activated carbons. In fact, the sample
activated at 500  C during 145 min with an impregnation ratio of
2 g/g was considered as optimized activated carbon for studying
the isotherm models of methylene blue, methyl violet, methyl orange and indigo carmine adsorption. It was observed from Fig. 6
that the adsorption efficiency increases with the increase of the
initial concentration, indicating that the adsorption process is more
favorable at increasing concentration of dyes. The equilibrium
characteristics of this adsorption study were described using
Langmuir [44] and Freundlich [45] isotherm models.
Based on the result tabulated in supplements (Table S8), the
correlation coefficients of the Freundlich model are lower than the
values of the Langmuir model. While, the experimental equilibrium

data can be best fitted with the Langmuir isotherm model. In fact, r2
values of 0.996, 0.993, 0.990 and 0.997 are found for MB, MO, MV
and IC adsorption, respectively. The results from the Freundlich
isotherm model show that the value for n is greater than unity,
which further supports the favorable adsorption of MB, MV, MO
and IC onto the activated carbon. Moreover, the maximum
adsorption capacities obtained with the application of the Langmuir isotherm model are 219.70, 118.10, 137.80 and 44.70 mg/g for
MB, MO, MV and IC adsorption, respectively. These capacities assume monolayer adsorption processes for MB, MV, MO and IC that
are close to the observed adsorption capacities at equilibrium.
The maximum Langmuir adsorption capacities mentioned
above were compared to previous studies on various activated
carbons with different preparation conditions (Table S9). It could be
seen that the experimental data in the present study are higher
than those of the most often prepared activated carbons used in
cationic and anionic dyes adsorption in the literature.

4. Conclusion
This work has shown that T. transtagana stems is a new good
alternative precursor for the preparation of activated carbons for
the elimination of cationic and anionic dyes. The optimization of
preparation conditions was investigated using the central composite design with response surface methodology. Results indicate that the activation temperature and the impregnation ratio
are the most important factors in the activation process. The
iodine number increases as the activation temperature and the
impregnation ratio increase. It was also clear that the influence of
the activation time is more pronounced at higher temperature
and impregnation ratio. The adsorption of the dyes increases
when the impregnation ratio increases from 0.66 to 2 g/g. However, with an impregnation ratio of 2.34 there is a slight decrease
in the adsorption of MV and MO. In addition, with an activation
temperature of 366  C, the activated carbon indicates minor
adsorption capacities of the dyes also the iodine number and the

methylene blue index. The maximum adsorption capacities obtained with the application of the Langmuir isotherm model are
219.70, 118.10, 137.80 and 44.70 mg/g for MB, MO, MV and IC,
respectively, for AC activated at 500  C during 145 min with an
impregnation ratio of 2 g/g.
Conflict of interest
We have no conflict of interest to declare.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
/>

A. Machrouhi et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 544e553

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