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

The effect of activation temperature, impregnation ratio and activation time on iodine number (IN), methylene blue index (MB index) and removal ef ﬁ ciencies of methyl violet (MV), methy[r]

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Original Article

Statistical optimization of activated carbon from Thapsia transtagana

A Machrouhia, H Aliloua,b, M Farnanea, S El Hamidia, M Sadiqa, M Abdennouria,

H Tounsadic,*, N Barkaa,**

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 Faculte Polydisciplinaire de Taroudant, Universite Ibn Zohr, Agadir, Morocco

c Laboratoire d'Ingenierie, d'Electrochimie, de Modelisation et d'Environnement, Universite Sidi Mohamed Ben Abdellah, Faculte des Sciences Dhar El

Mahraz, Fes, Morocco

a r t i c l e i n f o

Article history:

Received 29 March 2019

Received in revised form

21 August 2019

Accepted 7 September 2019

Available online 13 September 2019

Keywords:

Thapsia transtagana stems

Dyes removal

Chemical activation

Central composite design

a b s t r a c t

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 bestﬁtted with Langmuir isotherm

open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

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

al-lergies, 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 ef

ﬁ-cient, simple and economic method for the removal of dyes from

aqueous solutions[10,11] For that, various types of low-cost, easily

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 car-bons 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 400e850C in the absence of ox-ygen[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 carbo-naceous materials The increase of surface area and the pore vol-ume is achieved through the elimination of internal carbon mass

* Corresponding author.

** Corresponding author Fax: þ212 523 49 03 54.

E-mail addresses: hananetounsadi@gmail.com (H Tounsadi), barkanoureddine@

yahoo.fr (N Barka).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2019.09.002

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

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and the removal of volatiles[27] However, in chemical activation,

the carbonization temperature is done only between 400 and

600C This method highlights an impregnation of the precursor or

raw material with dehydrating agents such as alkali metal

hy-droxide 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

impreg-nation ratio, activation temperature and activation time Five

re-sponses including iodine number (IN), methylene blue index (MB

index) and removal efﬁciencies for methyl violet (MV), methyl

or-ange (MO) and indigo carmine (IC) are investigated

2 Material and methods

2.1 Material

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)

2.2 Preparation of activated carbons

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<125mm using a domestic mixer

15 g of the biomass were impregnated with H3BO3as 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 125mm using a

normalized sieve and kept in a hermetic bottle for a further use

The impregnation ratio of the activating agent with the

pre-cursor was computed using Eq(1):

Impregnation ratio¼ (dried weight of H3BO3/precursor of TTS) (1)

2.3 Design of experiments using central composite design

Central composite design (CCD) was used to study the individual

and synergetic effect of the three factors towards deﬁned

re-sponses 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):

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

where n is the number of factors, nc is the number of center points

(six replicates)

The independent variables were coded as þ1 and 1 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 Whereais the distance of axial point from center which makes the design rotatable; its value wasﬁxed at 1.682 This value

of rotatabilitya, which depends on the number of parameters in the experiment, was obtained from the following equation[30]:

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 inTable 1 The responses were determined using the optimal quadratic model predictor Eq.(4)given as:

Y¼ b0þXni¼1bixiþ Xni¼1biixi

!2

þ Xni¼1-1Xnj¼1þ1bijxixj

(4)

0where Y is the predicted response, bois the offset term, bithe linear effect, biithe squared effect, bijthe interaction effect and xi, xj are the coded values of the variables considered

The quality of thefit 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] 2.4 Iodine number (IN)

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 theﬁltrate 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 concentra-tions between 20 and 500 mg/L for 12 h at room temperature Residual concentrations were determined by a spectrophoto-metric method at the wavelength of maximum absorbance of

665 nm[33] 2.6 Dyes removal 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 Coded variable levels

Activation temperature A C 366 400 450 500 534

A Machrouhi et al / Journal of Science: Advanced Materials and Devices 4 (2019) 544e553 545

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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:

q¼ðCo CÞ

where q is the adsorbed quantity (mg/g), Cois the initial dye

con-centration (mg/L), C is the residual dye concon-centration (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 cm1 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

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 500C for 145 min with an

impregna-tion 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 450C 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 toﬁt

the response functions with the experimental data.Table 2shows

the values of the regression coefﬁcients obtained According to this

table, the three studied factors present a positive effect on theﬁve

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 theﬁve 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

pri-mary factors of interest in the experiment The ANOVA results

showed that the equations adequately represent the actual

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 experi-mental 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)

Y1¼ 703.26 þ 44.10 A þ 56.46 B 0.74 C 1.75 AB þ 1.75

BC 28.35 A2 20.94 C2 (6) 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 tempera-ture 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 F-value (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)

Y2¼ 133.98 þ 13.17 A þ 29.11 B þ 4.97 C þ 10.04 AB 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

Table 2 Values of model coefﬁcients of the ﬁve responses.

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presented a negative effect on the development of mesopores.

According toTable S3, the impregnation ratio has the most

signif-icant 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),

inter-action 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)

Y3¼ 93.97 þ 9.69 A þ 16.57 B þ 2.18 C þ 0.63 AB 0.16 BC 5.38

Y4¼ 113.21 þ 9.89 A þ 14.66 B þ 4.46 C þ 4.71 AB 2.27 BC 5.09

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 FromTables S4 and

S5, it could be seen that the impregnation ratio has the largest

ef-fect on the methyl orange and methyl violet removal with an

F-value 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

impreg-nation ratio (AB), the interaction between activation temperature

and activation time (AC) and the quadratic term of activation

temperature (A2) Eq.(10)

Y5¼ 27.67 þ 3.76 A þ 10.63 B þ 1.83 C þ 2.95 AB 0.39 AC 3.22

The activation temperature, impregnation ratio, activation time

ratio showed a positive effect on the indigo carmine removal

response However, the interaction between activation

tempera-ture and activation time and the quadratic term of activation

temperature presented a negative effect on the indigo carmine

removal According toTable S6, it could be seen that the

impreg-nation 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

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 1present the

3D response surfaces plots for the signiﬁcant interactions

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 isfixed at 500C. For the MB index, the most significant interaction was the impregnation ratio/activation temperature FromFig 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 in-teractions 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 impreg-nation ratio and decreasing activation time in case the activation temperature isﬁxed at 500C.

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 inFig 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 isﬁxed 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 impor-tant 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 in-hibits 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 S7summarizes 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 R2values 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

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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 toﬁnd 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 con-ditions 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 acti-vation temperature of 500C for 145 min and which exhibits the high adsorption performance of dyes has an heterogeneous

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

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texture with irregular cavities distributed on the surfaceFig 3(b).

Fig 3(c) presents a smooth and featureless surface with very little

pores available on activated carbon prepared at 450C for 130 min

with an impregnation ratio of 2.34 g/g The activated carbon

prepared at 500C for 115 min with an impregnation ratio of 2 g/g

shown inFig 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

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.

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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 ofﬁxed 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

acti-vated carbon prepared at 450C 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 500C 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

loos-ened oxygen attached to the carbonaceous material during

chem-ical 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

acti-vated carbons, X-ray diffraction analyzes were performed Fig 4

presents the XRD patterns of the studied activated carbons This

ﬁgure shows generally an amorphous structure of all activated carbons with similar proﬁles and broad band at 23 The simple band at 23may 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 4presents the estimated chemical groups on the surface of ACs and pHPZC The pHPZCvalues 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 car-bons 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 ma-terial are presented inFig 5 According to thisﬁgure, 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 pre-cursor material (TTS), there are large band at 3700 and 3200 cm1 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 cm1 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 cm1 is assigned to OeH bending The spectra also indicate a band at 1615.95 cm1which is characteristic

of C]O stretching vibrations of ketones, aldehydes, lactones or

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

Table 3

Percent atomic of: (a) precursor (TTS), (b) 500C/145 min/2 g/g, (c) 450C/130 min/

2.34 g/g and (d) 500C/115 min/2 g/g.

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carboxyl groups The activated carbons present similar proﬁles with

different bands intensities The band between 3200 and 3700 cm1

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

re-action between H3BO3and precursor[43] The band appearing in

the spectrum between 1500 and 1700 cm1is attributed to

vibra-tions 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 cm1is 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 cm1could 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

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

Table 4

Chemical groups on the surface of the ACs and pHpzc.

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

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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 500C 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

or-ange and indigo carmine adsorption It was observed fromFig 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 bestﬁtted 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

Lang-muir isotherm model are 219.70, 118.10, 137.80 and 44.70 mg/g for

MB, MO, MV and IC adsorption, respectively These capacities

as-sume 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 com-posite design with response surface methodology Results indi-cate 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 How-ever, 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 ob-tained 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 500C 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 https://doi.org/10.1016/j.jsamd.2019.09.002

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.

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