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Optimized preparation for large surface areaactivated carbon from date (Phoenix dactylifera L.)stone biomass
Mohammed Danish a,b,*, Rokiah Hashim a, M.N. Mohamad Ibrahim b,Othman Sulaiman a
aBioresource Research Laboratory, School of Industrial Technology, Universiti Sains Malaysia, Penang 11800,
Malaysiab Industrial Chemistry Division, School of Chemical Sciences, Universiti Sains Malaysia, Penang 11800, Malaysia
a r t i c l e i n f o
Article history:
Received 17 September 2012
Received in revised form
7 December 2013
Accepted 11 December 2013
Available online 31 December 2013
Keywords:
Activated carbon
Central composite design
Surface area
Chemical activating agent
Phosphoric acid
* Corresponding author. Bioresource ResearPenang 11800, Malaysia. Tel.: þ60 465 35217
E-mail addresses: [email protected], mdan
0961-9534/$ e see front matter ª 2013 Elsevhttp://dx.doi.org/10.1016/j.biombioe.2013.12.
a b s t r a c t
The preparation of activated carbon from date stone treated with phosphoric acid was
optimized using rotatable central composite design of response surface methodology
(RSM). The chemical activating agent concentration and temperature of activation plays a
crucial role in preparation of large surface area activated carbons. The optimized activated
carbon was characterized using thermogravimetric analysis, field emission scanning
electron microscopy, energy dispersive X-ray spectroscopy, powder X-ray diffraction, and
Fourier transform infrared spectroscopy. The results showed that the larger surface area of
activated carbon from date stone can be achieved under optimum activating agent
(phosphoric acid) concentration, 50.0% (8.674 mol L�1) and activation temperature, 900 �C.
The BrunauereEmmetteTeller (BET) surface area of optimized activated carbon was found
to be 1225 m2 g�1, and thermogravimetric analysis revealed that 55.2% mass of optimized
activated carbon was found thermally stable till 900 �C. The leading chemical functional
groups found in the date stone activated carbon were aliphatic carboxylic acid salt ʋ(C]O)
1561.22 cm�1 and 1384.52 cm�1, aliphatic hydrocarbons ʋ(CeH) 2922.99 cm�1 (CeH sym./
asym. stretch frequency), aliphatic phosphates ʋ(PeOeC) 1054.09 cm�1, and secondary
aliphatic alcohols ʋ(OeH) 3419.81 cm�1 and 1159.83 cm�1.
ª 2013 Elsevier Ltd. All rights reserved.
1. Introduction
The lignocellulosic material is most prevalently used in the
production of agro based activated carbons. In contemporary
research different agricultural biomass based raw material
like palm oil waste [1e3], date (Phoenix dactylifera L.) palm [4,5],
ch Lab., Room Number 3(office); fax: þ60 4 657 [email protected] (M.
ier Ltd. All rights reserved008
coconut shell [6e8], woods [9e12], rice husk [13], and bagasse
[14] etc., have been reported for production of activated car-
bons [15]. The date (P. dactylifera L.) stone is a waste product of
date fruit which is largely grown in theMiddle East Asia, North
African, and North American desert region. In these regions it
is a more economically viable material for the production of
activated carbon. Date (P. dactylifera L.) stone is on an average
20, School of Industrial Technology, Universiti Sains Malaysia,78.Danish).
.
b i om a s s a n d b i o e n e r g y 6 1 ( 2 0 1 4 ) 1 6 7e1 7 8168
about 10% of the total weight of date fruit. The chemical
composition of date stone consists of hemicelluloses (23%),
lignin (15%), cellulose (57%) and ash (5%) [5]. Date (P. dactylifera
L.) stone is not consumable by humans in any form; it has a
high content of crude fibre (around 19%) that may cause di-
gestibility problems in ruminant animals as well [16].
Activated carbon has been extensively used in wastewater
treatment, chemical recovery and catalytic support industries
primarily due to large surface area and presence of different
pore sizes [17]. Besides, being used as an adsorbent and cat-
alytic support, the activated carbon can also be used as an
electrochemical double layer capacitor material in the elec-
tronic industry. In most reported cases, getting better porous
and high yielding activated carbons chemical activation is
preferred [18]. The advantage of chemical activation is to
achieve higher yield with larger surface area, while requiring
lesser energy cost as lower temperatures are used. Like other
commodity production process, the production of high
yielding activated carbonswith large surface area and suitable
pore size involves the balancing of production conditions to
get the desired characteristics of the obtained activated car-
bon. In general, these balancing acts become cumbersome job
for producers as there are more than one operating factor and
resultant characteristic that need to be considered. The
desirability of the obtained product is not only large surface
area and high yield and pore size but also controlled produc-
tion cost. Higher yielding with large surface area at lower
operating cost is always desired by producers, but careful
balancing is needed to avoid overlooking other characteristics
[19]. According to literature reviews, no study has been con-
ducted on the optimization of activated carbon production
from date stone and H3PO4 activating agents using the RSM
approach. In contemporary research, RSM is a popular sta-
tistical tool for designing experiment, response surface
modelling through regression, and in process optimization.
RSM has been recently used for the optimization of activated
carbon production from Coconut shell [19], Bamboo waste
[20], Turkish lignite [21], Tamarind wood [22], Jatropha hull
[23] and Olive waste cake [24].
The aim of this study was to find the optimum operating
conditions for the preparation of large surface area activated
carbon from date stone waste by simultaneously considering
the activating agent (H3PO4) concentration and activation
temperature. The activation time was fixed at 120 min for all
experiments used in this study. Desirable product output
based on surface area and yield were considered as a
response.
2. Material and methods
Date (P. dactylifera L.) stoneswere procured from theArab shop
inside Universiti SainsMalaysia (USM)main campus and used
as precursor material in the present study (the dates were of
safawi cultivar packed by the national factory for dates
packing and supplied by the Yousef Mouhsen Al-Haidry
exporting agency from Madinah Munawarah, Saudi Arabia).
The date stones were ground into powder and sieved (model
number BS410/1986, ENDECOTTS LTD, London, England) to a
particle size range of 0.25e0.50 mm. Chemical activation of
the precursor was conducted with H3PO4. Fifteen grams of
dried precursor were well mixed with 50 mL of different
concentrations of the H3PO4 solutions. The mixing was per-
formed at room temperature and then the samplewas dried in
the oven at 70 �C. The activation was carried out in a steel
reactor fitted inside an automated muffle furnace
(Nabertherm�) equipped with a temperature and time
controller C7 panel in the presence of continuous flow of pu-
rified nitrogen gas with the flow rate of 200 cm3 min�1. The
steel reactor was perfectly closed with steel lids and nuts
equipped with inlet and outlet pipes to supply purified nitro-
gen and exhaust volatile matters with nitrogen into a water
filled glass trap. The automated sensor of the muffle furnace
was programmed to carbonize the date stone powder at 300 �Cfor 30 min then to adjust to a selected activation temperature.
Samples were held at the final activation temperature for
activation times of 120 min before cooling down under nitro-
gen flow.
All the experiments were conducted at a constant heating
rate and nitrogen flow rate. The experiments were conducted
for different H3PO4 concentrations (1.7% (0.298mol L�1)e58.3%
(10.113 mol L�1)) treatment, activation temperatures
(357e993 �C), and fixed activation time (120 min). Volatile
species formed during the pyrolysis stage was removed from
the furnace through an outlet tube connected with a sealed
iron reactor into a water filled glass trap. After cooling, the
agglomerated activatedwood carbonswere ground to uniform
size and sieved with laboratory test sieve to keep particle size
in the range of 0.25e0.50 mm. The obtained activated carbons
were washed repeatedly with hot and cold distilled water to
remove residual organic and inorganic by-products formed
during the activation process, and then dried at 110 �C for 24 h.
Later, the obtained activated carbons were kept in air tight
glass bottles for further characterization studies.
2.1. Model fitting and statistical analysis
The statistical software package Design-Expert 6.0.6, stat-
Ease, Inc., Minneapolis, USA, was used for model fitting and
statistical analysis such as regression analysis of experi-
mental data to fit the empirical mathematical equation,
analysis of variance (ANOVA) and 3D plots of response sur-
face. RSM was utilized to fit a surface to a set of data. It de-
termines optimum factor levels as a part of its methodology.
In this method, the shape of the surface was determined by
the model that was fit to the response values (i.e. the data
points) [25,26]. Details of the model approach can be accessed
via the Supplementary data file.
The experiments were conducted to obtain the optimized
value for the activating agent concentration and activation
temperature by keeping the activation time fixed at 120min to
get maximum surface area activated carbon with relatively
higher yield. The desired ranges of the variables were defined
and coded to be 1 for factorial points, 0 for centre points, and
�a for axial points. Table 1 described the levels and ranges of
the studied process parameters that affect the yield and sur-
face area formation during the activation process of the date
stone activated carbons.
Table 1 e Levels and ranges of independent variablesused during the experiments (H3PO4 concentrations1.72% w 0.298 mol LL1; 10.0% w 1.734 mol LL1;30.0% w 5.204 mol LL1; 50.0% w 8.674 mol LL1;58.3% w 10.113 mol LL1).
Parameters Factors Variable level
�a �1 0 1 a
Activating agent (H3PO4)
concentration (%)
X1 1.7 10.0 30.0 50.0 58.3
Temperature (�C) X2 357 450 675 900 993
b i om a s s a n d b i o e n e r g y 6 1 ( 2 0 1 4 ) 1 6 7e1 7 8 169
2.2. Thermogravimetric analysis (TGA)
Thermal degradation against temperature and time was
recorded using PYRIS-1, Perkin Elmer instrument. Approxi-
mately 5 mg samples were placed in a platinum crucible on
the pan of a microbalance and then heated between 22 �Cand 900 �C at a heating rate of 10 �C min�1, with nitrogen flow
rate of 20 mL min�1 and constantly weighed. Percent weight
loss versus temperature plots were taken for thermogravi-
metric analysis (TGA) and derivative weight loss against
temperature was taken for differential thermogravimetric
analysis (DTG).
Fig. 1 e Single operating factor effects on yield and surface
area of the H3PO4 activated date stone activated carbon
(H3PO4 concentrations 10.0% w 1.734 mol LL1;
20.0% w 3.469 mol LL1; 30.0% w 5.204 mol LL1;
40.0% w 6.939 mol LL1; 50.0% w 8.674 mol LL1).
2.3. Field emission electron microscopy (FESEM) andenergy dispersive X-ray (EDX) spectroscopy
The FESEM images were recorded using Leo Supra 50 VP Field
Emission Scanning Electron Microscope (Carl-Zeiss SMT,
Oberkochen, Germany) equipped with Oxford INCA 400 en-
ergy dispersive X-ray microanalysis system (Oxford In-
struments Analytical, Bucks, U.K.) that can give FESEM and
EDX with the same sample. A thin layer of gold was sputter-
coated on the samples for charge dissipation during imaging.
2.4. X-ray diffraction (XRD)
Powder X-ray diffraction patterns were recorded using Bruker
diffrac Plus (Model No. PW3050/65, made in The Netherlands)
Advance Powder X-ray diffractometer equipped with a Cu Ka
radiation (40 kV, 30mA)with thewavelength of 1.54�A at a step
size of 0.05� glancing angle q andwith the holding time of 1 s at
fixed q. The 1 mm thick activated carbon powder sample was
placed on a sample holder and the reflection spectra was
recorded at 298 K and obtained data was analyzed by the
BrukerDiffracPlus� computer software.
2.5. Fourier transform infrared (FTIR) spectroscopy
For surface functional group analysis Fourier transform
infrared spectroscope (Nicolet iS10, serial number: AKX
0901513) was used. The samples were prepared in the form of
KBr pellets with the sample to KBr ratio 1:100. The FTIR
spectrum was recorded by 64 scan per sample with 4 cm�1
resolution. The frequency range was selected between
4000 cm�1 to 400 cm�1.
3. Results and discussion
3.1. Effect of parameters of activation
The following section will explain the complexity of balancing
the production conditions for the chemical activations. The
production of activated carbons was mainly guided by the
process variables such as activating agent concentration and
activation temperature. In preliminary study [27] we had
found that the activation time beyond 45 min till 120 min did
not have any effect on physical and textural properties of the
activated carbon; so in this study we have fixed activation
time at 120 min. The independent variables such as activating
agent concentration and activation temperature have their
own effect on physical, textural characteristics and yield of
the obtained activated carbons.
3.1.1. Effect of concentration of activating agentsThe chemical activating agents commonly used in activation
of biomass based materials are dehydrating agents. These
b i om a s s a n d b i o e n e r g y 6 1 ( 2 0 1 4 ) 1 6 7e1 7 8170
dehydrating agents penetrate deep into the biomass structure
and cause the organic molecules to disintegrate into smaller
molecules. After the release of these smaller molecules, tiny
pores were created. Besides helping in the development of
new pores or expansion of the pore, it also affects in
enhancing the surface area. Normally, it was observed that
micropores and mesopores formation results in to a larger
surface area of the activated carbons.
In Fig. 1(a) and (b) the effect of H3PO4 concentration on the
preparation of the activated carbon in terms of yield and
surface area can be seen. The curve in Fig. 1(a) shows that
yield has the highest value in between the two extremes
values of H3PO4 concentration. Haimour and Emeish [5] also
reported for date stone activation with H3PO4 that with the
rise of impregnation ratio (rise of activating agent concen-
tration), the yield increases at a lower activation temperature
of 400 �C and decreases at a higher activation temperature of
800 �C.Whereas, in the case of the surface area plot in Fig. 1(b),
a linearly increasing trend with the rise of concentration at
fixed temperature, 675 �C can be observed. Though the rise of
surface area was not steep, it was rising gradually. Haimour
and Emeish [5] have not studied the surface area of the ob-
tained activated carbons through N2 adsorption isotherm
technique, which is the more reliable and authentic method
for evaluating surface area, but tried to calculate the iodine
number to evaluate the surface area. It was quite interesting
that they found the maximum iodine number for impregna-
tion ratio 0.4 (w13.3% (2.306 mol L�1) H3PO4 Solution) at an
activation temperature of 800 �C. We have found experimen-
tally themaximum surface area at 50.0% (8.674mol L�1) H3PO4
solution treatment and 900 �C activation temperature.
It was observed that the surface area of the activated car-
bon of date stone was increased from 216 m2 g�1 to
1214 m2 g�1 when activating agent (H3PO4) concentration was
increased from 1.7% (0.298 mol L�1) to 50.0% (8.674 mol L�1)
respectively. Vernersson et al. [28] also reported the increase
of surface area for untreated Arundo donax canes to H3PO4
treated A. donax canes from 38 to 1151 m2 g�1. The general
trend reported [19,28] that for all lignocellulosic precursors, as
the concentration of the activating agents rises, the surface
area also increases up to an optimum level. After optimum
concentration level of the activating agent saturation was
achieved, further rise in activating agent concentration causes
either no increment of surface area or sometimes decrease in
surface area. A larger pore formed in macropore range which
corresponds to smaller surface area was developed when
excess acid was used. When the pores size rises from meso-
pore to macropores, they do not contribute to the surface area
significantly [29]. It was reported that only a 35.0%
(6.071 mol L�1) concentration of H3PO4 solution was best for
maximum surface area and porosity development of sorghum
grain [30]. At this concentration, the weakening of the sor-
ghum grain intra-molecular structure was minimized. As a
result, the intra-molecular structure strength of the sorghum
grain caused the hardness and bulk density of the resulting
activated carbon to remain almost same. Gradual develop-
ment of porosity, particularly of large pores, occurs as more
acid was thrust until a bearable limit was reached. Further
increase in acid concentration leads to collapse of pore
structure due to structural weakness caused by the intensified
acid concentration. Above optimized H3PO4 concentration, a
reduction in pore volume marked a decrease in surface area
for A. donax canes as reported by Vernersson et al. [28]. Ac-
cording to our results, 50.0% (8.674 mol L�1) concentration of
activating agent (H3PO4) was suitable for producing large
surface area activated carbon from date stone. Further rise in
H3PO4 concentration from 50.0% (8.674 mol L�1) to 58.3%
(10.113 mol L�1) results in the decrease of surface area.
3.1.2. Activation temperatureIt was evident that with the rise of temperature from 450 �C to
900 �C the yield was almost linearly decreasing from 64.7% to
24.9%; whereas, the surface area was linearly increasing from
240 m2 g�1 to 1210 m2 g�1 (shown in Fig. S1 supplementary
data file). Haimour and Emeish [5] also reported the effect of
temperature on H3PO4 treated date stone activated carbon in
line with our research. They studied the temperature range
from 200 �C to 800 �C and found the yield to have decreased
from 58% to 9.5%. This effect was due to the loss of volatile
materials with increasing temperature. It was observed that at
a higher activation temperature above 800 �C, a lower
decrease rate of yield was noticed due to the formation of a
stable backbone structure. Girgis and El-Hendawy [31] re-
ported the rate of yield decreased beyond 700 �C through TGA
study of date stone activated with H3PO4.
The surface area was found linearly increasing with an
increase of temperature due to the opening of pores at a
higher temperature; but the temperature above 900 �C gave a
lower surface area due to the conversion of mesopores into
macropores. Haimour and Emeish [5] also reported the linear
rise of iodine number for H3PO4 treated date stone activated
carbons with the rise of temperature up to 800 �C. The in-
crease in activation temperature till 900 �C enhances the
micropore formation in activated carbonswhich increases the
adsorptive capacity. This result was in line with our findings.
Similar results were also reported by the other researchers
[5,31,32]. Overall, the activation temperature has a negative
effect on yield. The yield reduces with the rise in temperature.
Whereas, for surface area, temperature has a positive impact
till 900 �C; above this temperature, we found that the surface
area of date stone carbon has decreased at 993 �C.In chemically impregnated date stone biomass, application
of heat accelerates the thermal degradation and volatilization
of organic molecules. This gives subsequent rise in pores and
surface area but adversely affects the mass of the activated
carbons. The loss in mass was obvious. The selection of suit-
able activation temperature depends upon factors including
the type of precursor and chemical activating agents. Activa-
tion temperature for different biomass based precursor ma-
terials is generally used in the range of 400e900 �C, while for
coal based precursors it is higher than 900 �C [19]. The appli-
cation of heat in the activation process plays a major role in
pore formation and surface area expansion.
3.1.3. Combined effect of activating agent concentration andactivation temperatureThe RSM plot of the regression model analysis for the graph-
ical interpretation of the interaction has been recommended
[33]. Three dimensional (3D) response surface plots can be
highly informative about the behaviour of system variables
b i om a s s a n d b i o e n e r g y 6 1 ( 2 0 1 4 ) 1 6 7e1 7 8 171
within the experimental design. It can facilitate an examina-
tion of experimental factors such as activating agent con-
centration and activation temperature on selected responses
such as yield and surface area. Therefore, 3D response surface
curves were plotted for a statistically significant model to
understand the interaction of applied operating factors. The
graphical representation of the obtained models (Eqs. (1) and
(2)) facilitated an examination for the effects of experimental
factors on the yield and surface area.
The plots for yield (Fig. 2) are having a curvature in
response surface plot as well as a contour plot; whereas, the
surface areas in the response surface plot as well as the con-
tour plot is having linear lines (Fig. 3). It is clear from the
response surface 3D figure that response of yield has
quadratic interactive variables; whereas, the response of
surface area has linearly guided by temperature and concen-
tration. From Fig. 2(a), it can be observed that the yield of
Fig. 2 e Three dimensional response surfaces and contour
plot for yield (%) after activation: effect of activating agent
concentration (H3PO4 concentrations
10.0% w 1.734 mol LL1; 20.0% w 3.469 mol LL1;
30.0% w 5.204 mol LL1; 40.0% w 6.939 mol LL1;
50.0% w 8.674 mol LL1) and activation temperature
(activation time fixed at 120 min).
activated carbon was increasing at a low activation tempera-
ture and increasing H3PO4 concentration; at high activation
temperature and high H3PO4 concentration the yield was
lower than the average condition. The maximum yield of
activated carbon was at high H3PO4 concentration and low
activation temperature. From Fig. 3(a) it can be seen that at
50.0% (8.674 mol L�1) H3PO4 concentration and 900 �C tem-
perature, the surface area have highest value. In general, the
circular nature of the contour expressed that the interaction
between the operating variables are insignificant. Whereas
elliptical nature of the contour expressed the interaction be-
tween the variables are significant [33]. The contour plot for
yield and surface area are represented in Figs. 2(b) and 3(b). It
can be observed from the contour plots that the yields plots
have elliptical curve lines; whereas, the surface area plots
have linear lines.
3.2. Outcome of statistically designed experiments
The experiments were designed to obtain a second-order
model consisting of 22 trial plus a star configuration (a¼�1.41)
Fig. 3 e Three dimensional response surfaces and contour
plot for surface area after activation: effect of activating
agent concentration (H3PO4 concentrations
10.0% w 1.734 mol LL1; 20.0% w 3.469 mol LL1;
30.0% w 5.204 mol LL1; 40.0% w 6.939 mol LL1;
50.0% w 8.674 mol LL1) and activation temperature
(activation time fixed at 120 min).
b i om a s s a n d b i o e n e r g y 6 1 ( 2 0 1 4 ) 1 6 7e1 7 8172
and five replicates at the centre point. The effect of the
experimental variable on the production of activated carbon
from date stone with a large surface area and considerably
good yield has been visualized through a mathematical
modelling.
The surface area more than 900 m2 g�1 of the activated
carbonwas obtained in two caseswith variables in coded form
(1, 1) and (0, 1.41); for these cases the yield was found to be
24.9% and 17.8%, respectively.
3.2.1. Central composite design regression model analysisSecond-order model was used to develop the correlation
between the activating agent concentration and activation
temperature for yield. According to the sequential model
sum of squares, the models were considered based on the
highest order polynomials. For yield, the quadratic model
was prescribed and for surface area, the linear model was
suggested by Design-Expert 6.0.6 programming. The layout
of this proposed design can be seen in Table 2, along with
the response results. Regression analysis was conducted to
fit the response functions yield and surface area. The
empirical model expressed by equation (Eq. (2)), where
variables use coded values, represents percent activated
carbon yield (Y1) and surface area (Y2) as a function of
activating agent concentration (X1) and activation temper-
ature (X2).
The final empirical models in terms of coded factors after
excluding the insignificant terms for yield (Y1) and surface
area (Y2) are given in Eqs. (1) and (2), respectively.
Yield ðY1Þ¼58:0þ4:01X1�22:6X2�9:24X21�3:32X2
2þ2:03X1X2
(1)
Surf:Area ðY2Þ ¼ 595þ 109X1 þ 305X2 (2)
3.2.2. Interpretation of statistical resultsThe results of the experiments conducted and the model
predicted values; where, design flexibility for coming closure
to the optimum values of the parameters and responses star
points (�1.41) were analyzed statistically. The star pointswere
Table 2 e Coded level combinations for a five level, twovariable designs.
Run Variables Responses
X1
(H3PO4 conc.)X2
(Temperature)Y1
(%yield)Y2
(surface area,m2 g�1)
1 0.00 0.00 59.4 622
2 �1.00 1.00 21.3 785
3 �1.41 0.00 30.5 216
4 1.00 �1.0 60.2 392
5 0.00 0.00 55.2 688
6 1.00 1.00 24.9 1214
7 0.00 0.00 58.1 686
8 0.00 �1.41 90.2 187
9 0.00 0.00 57.0 663
10 0.00 1.41 17.8 946
11 0.00 0.00 60.2 670
12 �1.00 �1.00 64.7 240
13 1.41 0.00 53.8 421
equidistant from the centroid in either direction of the studied
parameters. For this experiment the star point �1.41 was
below minimum level �1, and star point þ1.41 was above
maximum level þ1 of the studied parameter variables. The
central run (0, 0) was repeated five times, as it contributed to
the estimation of deviation in each run.
We can get the general idea about the behaviour of the
parameters on the responses from the signs andmagnitude of
the coefficient variables in the equations. The signs of the
quadratic terms in Eq. (1) are negative for H3PO4 concentration
and temperature coefficients; each represents some form of
maximum or approaches to it. Therefore, if moved on either
concentration axis or temperature axis, the yield will
decrease. The yield will decreasewithmore a rapid rate on the
concentration axis due to a higher magnitude of the concen-
tration coefficient compared to the temperature one. The
larger the coefficients, the more thorough the control of the
variables; so, the results were in the acceptable region [26].
Actual values are experimentally measured for a particular
run, and the predicted values are estimated by using an
empirical mathematical model (shown in Fig. S2
supplementary data file). The values of R2 and R2adj for yield
were found to be 0.945 and 0.905 respectively. For surface area,
the values of R2 and R2adj were found to be 0.778 and 0.734,
respectively.
Table 3 presents the comparison between the actual and
model predicted values of the responses and their residuals.
It can be seen that for all the activated carbons prepared
with different combinations of process variables, the
experimentally calculated response and the theoretically
predicted response through mathematically developed
model have minor differences except for a few runs. For
runs 3, 6, and 13, the difference was slightly higher in the
surface area, which was due to a high standard deviation
between the replicate samples in surface area calculation.
For this experiment the standard deviation calculated for
yield and surface area was 6.39% and 155 m2 g�1, respec-
tively. The little variation in the input variables due to
human and instrumental error has also propagated its effect
on the responses. That an error has been estimated through
the propagation of error curves.
The fair correlation coefficients might be due to the insig-
nificant terms present in ANOVA table (shown in
Supplementary data file Table ST1). The nonlinear influence of
the investigated variables also gives poor correlation coeffi-
cient on process response outcomes. For yield, model F-value
was found to be 23.8 which implied that the model was sig-
nificant. Only 0.03% chance was there, that this large F-value
occurred due to noise. The “lack of fit” F-value of yieldwas 23.7
implying that the “lack of fit” was significant. The “Pred R2” of
yieldwas 0.622,whichwas not as close to the “Adj R2” of 0.9049
as expected. This was due to no blocks being selected for
running these experiments.
The model F-value of 17.6 implied that the model for sur-
face area was significant. There was only a 0.05% chance that
this result could be due to noise. The “lack of fit” F-value of
54.9 implies that there was only 0.08% chance that this much
of a high value had occurred due to noise. The “Pred R2” value
was 0.5378, which was in a reasonably close agreement with
the “Adj R2” of 0.7340.
Table 3 e Experimental design for the production of activated carbon comparison of actual and predicted responses.
Run Variables Responses
Y1 (%yield) Y2 (surface area, m2 g�1)
X1 (H3PO4 conc.) X2 (temp.) Actual Predicted Residual Actual Predicted Residual
1 0.00 0.00 59.4 58.0 1.40 622 595 91.9
2 �1.00 1.00 21.3 16.7 4.52 785 791 �6.02
3 �1.41 0.00 30.5 33.8 �3.30 216 441 �225
4 1.00 �1.0 60.2 70.0 �9.87 392 398 �6.80
5 0.00 0.00 55.2 58.0 �2.73 688 595 93.1
6 1.00 1.00 24.9 28.8 �3.94 1210 1010 206.0
7 0.00 0.00 58.1 58.0 0.13 686 595 91.9
8 0.00 �1.41 90.2 83.3 6.87 187 163 23.7
9 0.00 0.00 57.0 58.0 �0.99 663 595 68.7
10 0.00 1.41 17.8 19.3 �1.51 946 1030 �80.5
11 0.00 0.00 60.2 58.0 2.19 670 595 75.7
12 �1.00 �1.00 64.7 66.1 �1.41 240 181 59.4
13 1.41 0.00 53.8 45.2 8.66 421 749 �328.0
b i om a s s a n d b i o e n e r g y 6 1 ( 2 0 1 4 ) 1 6 7e1 7 8 173
3.2.3. Response surface methodology optimizationTo optimize the process with two or more output responses it
is recommended to use the desirability function. It is the
widely used method for optimizing multiple responses in the
process of optimization. This method can combine multiple
responses into one function known as a desirability function.
The range of a desirability function is between 0 and 1. A
desirability value of 0 indicates that one or more character-
istics of the desired responses are unacceptable; while,
maximum desirability value of 1 indicates that all process
characteristics responses are possible. The value of an indi-
vidual desirability increases as the desirability of the corre-
sponding response increases. Overall desirability is a
geometric mean of the individual desirability function [34].
The goal of optimizationwas to achieve activated carbon from
date stone with the largest surface area and yield within the
range. To achieve maximum desirability (0.803) for the largest
surface area activated carbon from date stone we fixed other
parameter such as temperature, H3PO4 concentration, and
yield were fixed within the range. With maximum desirability
(0.803), we found model predicted surface area, 1010 m2 g�1;
and on experimental verification under these conditions sur-
face area was found to be 1225 m2 g�1 (as shown in Table 4).
This difference in model predicted and experimentally found
surface area was due the deviation in input factors and sur-
face area measurement techniques. It was also represented
through histogram (shown in Fig. S3 supplementary data file).
The optimization for largest surface area and maximum yield
was also attempted and was found to have a maximum sur-
face area of 696 m2 g�1 with maximum yield being 54.5%. In
this case, the maximum desirability achieved was only 0.502.
The graphical optimization can also be done with the help
of the Design-Expert 6.0.0 software. The optimum range of
H3PO4 concentration was from 22.7% to 50% and the
Table 4 e Optimal processing conditions from numerical optim
Solution no. Parameter
Yield (%
X1 (mol L�1) X2 (�C) Model Exp
1 8.674 900 28.8
corresponding temperature of activation was from 675 �C to
800 �C for maximum surface area (>700 m2 g�1) and yield
(>42%). Since chemical activating agents are costlier than heat
generation; optimization for minimum concentration of
H3PO4, while varying the temperature of activation was con-
ducted. With the help of layout design, the output was
extrapolated by keeping the H3PO4 concentration around
25.0% (4.337 mol L�1) and activation temperature in the range
of 770e800 �C; where, the model predicted the surface area in
the range of 700e800 m2 g�1 and yield in the range of 42e46%.
The green coloured zone in Fig. 4 shows the region where
surface area greater than 700 and yield greater than 42% with
a maximum deviation in result of yield was 6.4%.
3.2.4. Propagation of errorThe deviation in input factor such as activating agent (H3PO4)
concentration and activation temperature on the responses
has been evaluated through Design-Expert software. During
the experimentation the deviation in H3PO4 concentration of
0.2% (0.0347 mol L�1) and deviation in activation temperature
of 1 �C was recorded. The propagation of error plot (shown in
Fig. S4 supplementary data file) showed how input error was
transmitted to the response (yield). The lower the propagation
of error the better, because less error will be transmitted to the
responses.
However, the propagation of error for the combination of
input factors can be obtained when the response surface fol-
lowed the nonlinear model such as quadratic or higher order
model. When the response surface followed the linear model
such as that for surface area followed here, the error was
transmitted equally throughout the region. For yield
maximum error obtained was 6.4%. Whereas, the deviation in
the surface area was found to be 155 m2 g�1, due to deviation
in all the input factors.
ization.
Response Desirability
) Surface area (m2 g�1)
erimental Model Experimentally
24.9 1010 1225 0.803
Fig. 4 e Overlay plot for optimum region (green zone) of
H3PO4 concentrations (H3PO4 concentrations
10.0% w 1.734 mol LL1; 20.0% w 3.469 mol LL1;
30.0% w 5.204 mol LL1; 40.0% w 6.939 mol LL1;
50.0% w 8.674 mol LL1) and temperature combination for
producing activated carbon from date stone. (For
interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this
article.)
Fig. 5 e TGA and DTG plot of raw date stone and H3PO4
activated date stone carbon.
b i om a s s a n d b i o e n e r g y 6 1 ( 2 0 1 4 ) 1 6 7e1 7 8174
3.3. Thermal stability of date stone based activatedcarbons
Thermogravimetric analysis (TGA) and differential thermog-
ravimetric (DTG) measure the amount and rate of change in
the weight of a sample as a function of temperature and time
respectively, in a controlled atmosphere. This technique can
characterize the samples that exhibit weight loss due to
decomposition, oxidation, or dehydration on the application
of heat in the presence of air atmosphere. The weight loss
measurements are used primarily to determine the compo-
sition of materials and predict their thermal stability at a
temperature up to 900 �C. Fig. 5(a) and (b) depicts the
TGAeDTG curves for physically activated date stone carbon
and chemically (H3PO4) activated carbons. Although both are
activated carbons samples and show four steps to weight loss,
yet significance differences were observed in the peak in-
tensities and location of peaks. Chemically (H3PO4) activated
date stone carbon with H3PO4 brought about significant
modification to the course of pyrolysis and characteristics due
to the formation of phosphate complex.
The TGA and DTG curves of physically activated date stone
carbon in Fig. 5(a) consist of 90, 373, 473, and 570 �C. The first
peak at 90 �Cwasdue tomoisture loss from the surface. Till the
temperature reached 313.64 �C, the mass loss of the sample
was recorded as only 3.58%, this was attributed by water va-
pours and low molecular weight volatile matters. At temper-
ature 373.01 �C, maximum loss of sample mass 46.143% was
observed. This mass loss was due to CO2 release from the
sample and represents themain decomposition stage and can
be assigned to the decomposition of cellulose and hemi-
celluloses constituents. At temperatures 473.27 �C and
570.39 �C, the mass losses recorded were 18.94% and 7.37%,
respectively. Itwas likely due to lignindecomposition, because
lignin decomposes over a broad range of temperature [35].
The TGA and DTG curves for chemically (H3PO4) activated
date stone carbon (Fig. 5(b)) have decomposition peaks at
temperatures 163, 400, 835, and 900 �C. Between temperatures
30 and 163 �C, mass loss was recorded to be 10.45%. This mass
loss was attributed to moisture and volatile organic matters
associated with the activated carbon during preparation. At
temperature 400 �C, only 5.64%mass losswas observed. In this
temperature range, cellulose and hemicelluloses materials
were generally decomposed. The smallmass loss signifies that
the cellulose and hemicelluloses were converted into stable
phosphate complex which resists the decomposition [35]. At
temperatures 835 and 900 �C, themass losses were 22.11% and
5.41%, respectively; these values represent the largest mass
losses, probablydue to lossof ligninandphosphate complexes.
3.4. Surface area and surface morphology of date stoneactivated carbons
The H3PO4 activated date stone activated carbons have
comparable surface area and total pore volume as compared
b i om a s s a n d b i o e n e r g y 6 1 ( 2 0 1 4 ) 1 6 7e1 7 8 175
to commercially available activated carbons BDH (surface
area ¼ 1118 m2 g�1, pore volume ¼ 0.618 cm3 g�1), Merck F100
(surface area ¼ 957 m2 g�1, pore volume ¼ 0.526 cm3 g�1), and
BPL calgon corporation (surface area ¼ 972 m2 g�1, pore
volume ¼ 0.525 cm3 g�1) [36]. The larger surface area and total
pore volume with well developed pore diameter of the ob-
tained activated carbon was due to the use of suitable con-
centration of H3PO4 as an activating agent. The suitable
concentration of H3PO4 works as a strong dehydrating agent
that can penetrate the fibrous structure of date stone
biomass and enlarge it and removes organic molecule like
lignin and sugar from the pores and consequently, giving as
large surface area and mesoporous structure to activated
carbon.
Fig. 6 e FESEM images of raw date stone as well as
activated carbon samples.
Fig. 6(a)e(c) represents the SEM images of raw date stone
powder, low concentrated H3PO4 solution treated and average
concentration H3PO4 solution treated activated carbons,
respectively for the comparison of surface morphology due to
different treatments. It can be seen from the images (Fig. 6(a))
that raw date stone is very flat with no cracks or any kinds of
pores, looking very rigid and compact surface. At low con-
centration (10.0% (1.734mol L�1)), H3PO4 solution treatment of
date stone powder and activation at low temperature (450 �C)surrounds each pore by thick boundary walls as shown in
Fig. 6(b). When the date stone powder was treated with 30.0%
(5.204 mol L�1) solution of H3PO4 and activated at 993 �C, theobtained activated carbons have developed a large number of
pores with thin boundaries as shown in Fig. 6(c).
It seems from the figure that the surface was composed of
thin layered perforated sheets overlapping to each other.When
H3PO4 was an activating agent, the chemical activation process
was found effective in creating a large surface area with well
developed pores on the surface of date stone activated carbon.
The EDX surface analysis shows H3PO4 treated date stone acti-
vated carbon contains elemental composition of 80% C, 18% O
and 2% P (shown in Fig. S5 supplementary data file).
3.5. Interpretation of X-ray diffractogram
The optimized activated carbon produced from date stone
using H3PO4 activation was crystallographically characterized
by the reflection X-ray diffraction technique. To compare the
Fig. 7 e X-ray diffraction patterns of raw date stone as well
as H3PO4 activated date stone carbon samples.
Fig. 8 e FTIR spectra of raw date stone as well as activated carbon from date stone after H3PO4 activation.
b i om a s s a n d b i o e n e r g y 6 1 ( 2 0 1 4 ) 1 6 7e1 7 8176
change in the crystallographic surface after H3PO4 activation,
the raw date stone powder was also analyzed through XRD.
Fig. 7(a) represents the reflection diffractogram for raw date
stone powder. The diffractogram contains no peaks between
the applied scattering angles 20� and 80�. This characteristic ofthe diffractogram was due to amorphous nature of the ma-
terials [37]. Fig. 7(b) shows the diffractogram for H3PO4 treated
activated carbon. The activated carbon has one broad peak at
(002) with interlayer spacing (d002) 3.36 nm. Another peak
with a very weak signal was recorded at (100) with interlayer
spacing (d100) 2.14 nm. The diffraction pattern exhibit broad
peaks at around 2q angle 26� and 42�; this indicates increasingregularity of layered structure, in which consequently result-
ing in better layer alignment due to a short crystallographic
structure range of activated carbon network. Overall, H3PO4
treated activated carbon was having graphitic layered struc-
ture of amorphous nature.
3.6. Surface functional groups estimation
The FTIR spectrawere recorded for rawdate stone powder and
activated carbon obtained after optimized condition (H3PO4
concentration 50.0% (8.674 mol L�1) and temperature of acti-
vation 900 �C) of preparation (as shown in Fig. 8). On overall
comparison of spectra, it was found that raw date stone has
more numbers of transmittance peaks in the finger print re-
gion (1300e650 cm�1). This indicates the presence of more
chemical functional groups in the raw date stone compared to
its activated carbon. The major chemical functional groups
presented in raw date stone are primary aliphatic alcohol
ʋ(OeH) 3416.95 cm�1 and 1062.17 cm�1, ester ʋ(C]O)
1746.30 cm�1, and aliphatic hydrocarbons ʋ(CeH) for eCH3
2924.90 cm�1, OeCH3 2854.38 cm�1, eCH2e (CeH bend)
1457.82 cm�1, gem-dimethyl 1380.92 cm�1. After activation
withH3PO4, the surface functional groups changed to aliphatic
carboxylic acid salt ʋ(C]O) 1561.22 cm�1 and 1384.52 cm�1,
aliphatic hydrocarbons ʋ(CeH) 2922.99 cm�1 (CeH sym./asym.
Stretch frequency), aliphatic phosphates ʋ(PeOeC)
1054.09 cm�1 and secondary aliphatic alcohols ʋ(OeH)
3419.81 cm�1 and 1159.83 cm�1.
4. Conclusion
The experiments were carried out to determine optimum
condition for the production of large surface area activated
carbon from date stone. The optimized activated carbon
was characterized through BET surface area, micropore
surface area, mesopore surface area, pore diameter, surface
morphology, and molecular arrangement in carbon molec-
ular structure and chemical functional groups for its suit-
able use. The major findings of the design of experiments
are:
(1) The optimum points for maximum surface area
(>1000 m2 g�1) with compromising yield were 50.0%
(8.674 mol L�1), 900 �C, and 120 min for H3PO4 concen-
tration, activation temperature, and activation time
respectively. The optimumpoints formaximumsurface
area (w696 m2 g�1) and maximum yield (54.5%) were
43.1% (7.477 mol L�1) H3PO4 concentration, 697 �C acti-
vation temperature, and 120 min activation time
respectively.
(2) The optimization was done using quadratic program-
ming to maximize the yield (without compromising the
surface area) within the studied range. The surface area
optimization followed the linear model equation. The
maximum surface area experimentally achieved was
1225 m2 g�1 with yield 24.9%.
(3) The H3PO4 activated carbon from date stone has much
improved thermal stability compared to physically
activated carbon. Up to 900 �C, around 55% of the stable
backbone structure survived. The elemental composi-
tion of the chemically activated date stone carbon
comprises of 80% C, 18% O, and 2% P with some ordered
layered arrangement.
b i om a s s a n d b i o e n e r g y 6 1 ( 2 0 1 4 ) 1 6 7e1 7 8 177
(4) The H3PO4 activated carbon from date stone seems to be
graphitic layered structure and largely composed of
functional groups like aliphatic carboxylate salt, sec-
ondary alcohols, aliphatic phosphates, and aliphatic
hydrocarbons.
Acknowledgement
The authors acknowledged Universiti Sains Malaysia for Vice
Chancellor Award and Post-doctoral fellowship to Dr.
Mohammed Danish. The authors are grateful to Read-
WriteAnalyze team for English editing of this manuscript.
Appendix A. Supplementary data
Supplementary data related to this article can be found at
http://dx.doi.org/10.1016/j.biombioe.2013.12.008.
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