equilibrium, kinetics and thermodynamics study of phenols
TRANSCRIPT
Equilibrium, kinetics and thermodynamics study of phenolsadsorption onto activated carbon obtained from lignocellulosicmaterial (Eucalyptus Globulus labill seed)
Nelson Giovanny Rincon-Silva1 • Juan C. Moreno-Pirajan1 • Liliana Giraldo2
Received: 30 August 2015 / Revised: 11 November 2015 / Accepted: 20 November 2015
� Springer Science+Business Media New York 2015
Abstract Activated carbon was prepared from lignocel-
lulosic material (Eucalyptus Globulus labill seed) by
chemical activation with ZnCl2 at two different concen-
trations (10 and 25 % m/v) named ACS25 and ACS10. The
textural characteristics of the activated carbons (ACs) were
determined by N2 adsorption isotherms; these exhibit
B.E.T. surface areas of 250 and 300 m2 g-1 for ACS25 and
ACS10, respectively, with micropore volume contents of
0.140 and 0.125 cm3 g-1 in the same order. In addition, the
FTIR and Boehm methods were conducted for the chemi-
cal characterisation of ACs, where many groups with basic
character were found, which favours the adsorption of
phenols. The prepared carbonaceous adsorbents were used
in the adsorption of wide pollutants monosubstituted phe-
nol derivatives: phenol, 4-nitrophenol and 4-chlorophenol.
The effect of temperature on the thermodynamics, kinetic
and equilibrium of phenols adsorption on ACs was thor-
oughly examined. The adsorption kinetics adjusted prop-
erly for a pseudo-second-order kinetic model. However, the
Elovich model (chemisorption) confirms that phenols
adsorption did not occur via the sharing of electrons
between the phenolic ring and basal plane of ACs because
is not properly adjusted, so the process is given by
physisorption. The thermodynamic parameters [i.e. Gibbs
free energy change (DG�), enthalpy change (DH�) and
entropy change (DS�)] were also evaluated. The overall
adsorption process was exothermic and spontaneous in
nature. The values found in the thermodynamic study,
confirm that the adsorption process corresponds to a clearly
physical process.
Keywords Activated carbon � Kinetic �Thermodynamics � 4-Chlorophneol � Elovich equation
Abbreviations
Qt Amount of adsorbate adsorbed at time t (mg g-1)
Kf The pseudo-first-order rate constant (h-1),
t Time
Ks The pseudo-first-order rate constant
(g gm-1 min-1)
a Desorption constant in Elovich Equation
b Initial adsorption rate in Elovich Equation
kid The intraparticle diffusion rate constant
(mg g-1 h-1/2)
I Thickness of the boundary layer in intraparticle
model of Weber and Morris
k Rate constant of adsorption of Dumwald–Warner
model (min-1)
F Fractional attainment of equilibrium in the
Dumwald-Warner model
kfd Film diffusion rate coefficient in the Dumwald–
Wagner mode
Qmax Maximum phenol uptake in Langmuir model
kL Constant in Langmuir model that denoted the
energy of adsorption and affinity of the binding
sites (L mg-1)
kf Constant in Freundlich model giving an
indication of adsorption capacity [mg g-1 (L
mg-1) n]
& Liliana Giraldo
1 Departamento de Quımica, Facultad de Ciencias, Grupo de
Investigacion de Solidos Porosos y Calorimetrıa, Universidad
de los Andes, Bogota, Colombia
2 Departamento de Quımica, Facultad de Ciencias, Universidad
Nacional de Colombia, Bogota, Colombia
123
Adsorption
DOI 10.1007/s10450-015-9724-2
n Constant in Freundlich model giving an
indication of adsorption intensity
QmDRK Amount adsorbed of solute on the monolayer in
Dubinin–Radusckevisch–Kanager model
Cs Concentrations of equilibrium saturation in
Dubinin–Radusckevisch–Kanager model
ES Is related to the energy characteristic of the
process in Dubinin–Radusckevisch–Kanager
model
DG0 Gibbs free energy change of adsorption process
R R is the gas constant (8.314 J mol-1 K-1)
Ko K0 is the apparent equilibrium constant, in this
study the Langmuir constant was used
DS0 Entropy change of adsorption process
DH0 Enthalpy change of adsorption process
T Temperature in Kelvin
A Arrhenius factor
Ea Arrhenius activation energy of adsorption
1 Introduction
Water pollution is one of the most undesirable environ-
mental problems in the world and requires solutions (Rain-
bown 2002). This is caused by the introduction of substances
which may be non-toxic but affect biological cycles at high
concentrations, in the case of substances such as nitrates,
phosphates and some organic compounds, and secondly the
introduction of toxic substances, such as heavy metals,
hydrocarbons, pesticides, phenols, etc. (Blanchard et al.
1984; Dabrowski et al. 2005; Qing-Song et al. 2010). The
latter are considered priority pollutants due to their toxicity
in living organisms, even at low concentrations (Kumar et al.
2007). Phenolic wastewater is generated from chemicals,
pharmaceuticals, papermaking, rubber, wood, dye, and
pesticide industries and are highly toxic and harmful. In
addition, phenols are considered priority pollutants by the
EPA since they are not only carcinogenic but also cause an
unpleasant taste and odour, even at low concentrations
(Ahmaruzzaman and Sharma 2005).
Various technologies have been developed to treat
phenolic wastewater. Among them, adsorption is an
effective technology for the removal of phenols from
wastewater (Qing-Song et al. 2010; Rincon-Silva et al.
2015). Activated carbon (AC) is an extremely versatile
carbonaceous material that is widely used as an adsorbent
and catalyst support in industries (Rodrıguez-Reinoso and
Linares-Solano 1989, Rodriguez-Reinoso 2007). More-
over, its large surface area and micropore volume content,
favourable pore size distribution, surface chemistry
including the oxygen functional groups, the degree of
polarity and the active surface area lend AC as an appro-
priate adsorbent for a variety of environmental applica-
tions, i.e. the removal of organic materials, and the
purification and storage of gases and organic compounds
from aqueous solution. The adsorption efficiency of AC
relies strongly on its special surface and structural char-
acteristics (Rodrıguez-Reinoso and Linares-Solano 1989;
Dabrowski et al. 2005; Rodriguez-Reinoso 2007).
Activated carbon is mainly produced by thermal and
chemical activation (Rodrıguez-Reinoso and Linares-
Solano 1989; Sun and Jian 2010). Thermal or physical
activation involves the primary carbonisation of a car-
bonaceous precursor (below 700 �C) followed by activa-
tion of the obtained char with oxidising gases such as air,
CO2 or steam at high temperature in the range of
700–1000 �C. Chemical activation consists of the
impregnation of raw material with chemical agents such as
ZnCl2, H3PO4 or KOH followed by carbonisation at tem-
peratures between 400 and 800 �C under a N2 atmosphere
(Rodrıguez-Reinoso and Linares-Solano 1989). An enor-
mous range of lignocellulosic materials including rice
husk, corn cobs, fruit stone, almond shell, coconut shell,
sugar cane bagasse, palm shell, pistachio-nut shell and
cotton stalk have been used as activated carbon precursors
(Li et al. 2008; Li et al. 2010; Chandra et al. 2009; Nor
et al. 2013). Eucalyptus Globulus labil is other kind of
lignocellulosic material which has a reasonably high con-
tent of carbon, utilized also as raw material for AC (Tan-
credi et al. 2004; Mojica-Sanchez et al. 2012; Rincon Silva
et al. 2014).
The objective of this work is study the capacity of
adsorption of derivatives phenolics monosubstituted: phe-
nol, 4-clorophenol and 4-nithrophenol (phenol, 4-CP and
4-NP, respectively) on activated carbons prepared from
Eucalyptus shell by chemical activation. Additionally, an
extensive kinetic study was conducted to evaluate the
efficiency of the adsorption process. The pseudo-first-order
and pseudo-second-order models were used to correlate the
adsorption kinetics data of phenols onto AC, the kinetic as
well as the diffusion parameters were evaluated. Thermo-
dynamics studies have also been performed to understand
the process of removal of the selected phenols on ACs.
2 Materials and methods
2.1 Preparation of activated carbon
In this research, the lignocellulosic materials were treated
by impregnation with zinc chloride at low concentrations in
order to obtain microporous activated carbons at low costs.
This type of activation leads to ACs with high porosity, and
although the distribution of pore size is largely determined
Adsorption
123
by the precursor, the amount of zinc chloride used also
influences the porosity of the final product (Khalilia et al.
2000; Azevedo et al. 2007).
The suggested mechanism for activation using zinc
chloride can be summarised as follows: during impregna-
tion, the chemical reagent is introduced into the interior of
the precursor particles and causes some hydrolysis reactions
which are seen in a weight loss, in the output volatile mate-
rial, in the weakening of the structure and the increase in
elasticity, the chemical also causes the swelling of particles.
The two processes are more evident with increasing con-
centrations of zinc chloride. During heat treatment, zinc
chloride prevents the formation of volatiles thus increasing
process yield. During the impregnation and carbonisation at
low ratios impregnation occurs minimal weight loss, since
the amount of zinc chloride may be distributed uniformly
throughout the precursor with a large dispersion in the
interior of the particles resulting of activated carbons after
extensive washing with uniform microporosity and low
macroporosity. At higher impregnation ratios, hydrolysis
and swelling are accentuated, the zinc chloride may not be
distributed uniformly within the particles, and although the
total pore volume increases, the pore size distribution is more
heterogeneous, with meso- and macroporosity becoming
more important (Khalilia et al. 2000; Azevedo et al. 2007).
Thus, activated carbon was prepared by mixing of the
eucalyptus shell (ranging in size from 2.5 to 3.5 mm) at
two different concentrations of ZnCl2 (10 and 25 m/v),
which will be called ACS10 and ACS25. The impregnation
ratio of ZnCl2 and eucalyptus shell was 2.0/1.0; this pro-
cess was performed for 12 h. After mixing both together,
the salt impregnated material was dried overnight at 90 �C,
then a weighed sample was carbonised in a quartz tube
furnace at temperature of 550 �C, holding time of 2 h and
heating rate of 5 �C min-1. After carbonisation, the sample
was cooled down to room temperature in a flow of nitrogen
and then removed from the reactor. In order to remove
impurities in the synthesised ACs, it was dispersed in
distilled water at 80 �C. After that, the sample was washed
sequentially with hot and cold distilled water until the wash
water reached a pH of 6-7 (Qing-Song et al. 2010; Rincon-
Silva et al. 2015).
2.2 Characterisation of the ACs samples
The textural characterisation of the prepared ACs included
the surface area, the extent of micro- and mesoporosity was
conducted using N2 adsorption/desorption at 77 K using a
computer system Autosorb 3B, Quantachrome Co. The
specific surface areas were calculated from the N2
adsorption isotherm with Brunauer, Emmet and Teller
equation (B.E.T.) at the relative pressure in the range of
0.001–0.3 bar. The total pore volume was determined from
the amount of N2 adsorbed at P/P0 0.99. The volume of
micropores was estimated using the Dubinin–Radushke-
vich (D.R.) method (Qing-Song et al. 2010; Mojica-San-
chez et al. 2012; Anisuzzaman et al. 2014)
The selective method for determining of the total acid
and basic sites on the carbon surface was employed
(Boehm 1994, 2002; Lopez-Ramon et al. 1999). The pH at
point of zero charge (pHPZC) was determined using the
titration method of mass using a CG840B Schott pH meter
(Babic et al. 1999; Mojica-Sanchez et al. 2012).
The surface functional groups of the ACs samples were
also detected by Fourier Transform Infrared (FTIR) spec-
troscope (FTIR—Nicolet Impact 410) using a potassium
bromide (KBr) pellet prepared by mixing 0.033 % of dried
AC sample in KBr. The spectra were recorded between
4000 and 400 cm-1 (Pakulaa et al. 2005; Saka 2012).
2.3 Adsorption kinetics
Adsorption kinetic experiments were carried out using a
shaker water bath. Kinetic experiments were carried out by
agitating 50 mL of solution of phenols 100 mg of ACs at a
constant agitation speed, 20 �C and natural pH, because
measuring the pH of the solution showed that this was
below the pKa of phenols. Therefore, the molecular form of
each phenol of the ionic form predominates, favouring the
adsorption process due to electrostatic interactions (Mor-
eno-Castilla et al. 1995; Moreno-Castilla 2004; Qing-Song
et al. 2010; Rincon-Silva et al. 2015). Agitation was per-
formed for 120 min, which is more than sufficient time to
reach equilibrium at a constant stirring speed of 120 rpm.
Preliminary experiments had shown that the effect of
separation time on the adsorbed amount of phenol was
negligible. Two millilitres of samples was drawn at suit-
able time intervals. The samples were then centrifuged at
500 rpm and the omitted concentration in the supernatant
solution was analysed using UV–VIS spectrophotometry
with Milton Roy Co, Spectronic Genesys, equipment by
monitoring the absorbance changes at a wavelength of
maximum absorbance: 269, 280 and 319 nm for phenol,
4-CP and 4-NP, respectively (Tseng et al. 2010; Qing-Song
et al. 2010). Each experiment continued until equilibrium
conditions were reached when no further decrease in the
phenol concentration was measured; then, we proceeded to
plot the adsorption capacity of phenols on activated car-
bons versus time (Kumar et al. 2007; Kunwar et al. 2008;
Tseng et al. 2010; Qing-Song et al. 2010).
2.4 Adsorption isotherms of phenol, 4-nitrophenol
and 4-chlorophenol
The equilibrium isotherm of phenols adsorption on ACs
was determined by performing an adsorption test in
Adsorption
123
100 mL flasks, where 50 mL of phenols solutions with
different initial concentrations (50–1500 mg L-1) were
placed in each flask. Then, 100 mg of each of the prepared
activated carbon was added to each flask and kept in shaker
of 120 rpm at 20 �C for 72 h to reach equilibrium. The
equilibrium concentration of phenolic compounds was
determined with respect to calibration curves, at wave-
lengths (kmax) (Dabrowski et al. 2005; Qing-Song et al.
2010; Rincon-Silva et al. 2015).
Data amount Qe adsorbed at equilibrium (mg g-1) were
calculated from the following equation:
Qe ¼ V � C0 � Ce
mð1Þ
where Co and Ce are the initial and equilibrium phenol
concentration in mg L-1, V is the volume of solution
(L) and m is the mass of ACs (g) (Qing-Song et al. 2010;
Rincon-Silva et al. 2015).
2.5 Thermodynamic analysis
A thermodynamic study of adsorption process of phenols
on ACs to estimate the feasibility of the adsorption process
was performed. Therefore, the enthalpy of adsorption was
determined by the isosteric method which is known as the
standard differential enthalpy of adsorption, where exper-
imental adsorption isotherms were used at different tem-
peratures (20, 30 and 40 �C) with the procedure described
in Sect. 2.4 (Humpola et al. 2013; Rincon-Silva et al. 2015;
Ashraf et al. 2014).
This method is useful for estimating the enthalpy dif-
ferential adsorption, under the constraint of constant frac-
tion of coating of the surface adsorption. Initially, for a
system in thermodynamic equilibrium, the following fun-
damental equation is satisfied (Stoeckli et al. 1995;
Douillard 1996; Arias et al. 2009):
DG�ads ¼ �RTLnkeq ¼ DH�
ads � TDS�ads ð2Þ
where Keq is the equilibrium constant dynamic, chemical
equilibrium type during the adsorption process. Thus, the
constant kL of the Langmuir isotherm (see Eq. (11) in
Sect. 3.4), can be directly related to the equilibrium con-
stant of Eq. 2 in the vicinity of the boundary of Henry law
(Douillard 1996; Arias et al. 2009). Incorporating the
Langmuir isotherm in this equation, we obtain:
DG�ads ¼ �RTIn
hð1 � hÞ
1
p
� �¼ DH�
ads � TDS�ads ð3Þ
As a result, it is possible to obtain the equation of Vant
Hoff, which can be used to calculate the values of entropy
and enthalpy. Additionally, the equilibrium conditions
could be used to obtain the free energy of the adsorption
process (Stoeckli et al. 1995; Douillard 1996; Arias et al.
2009; Ashraf et al. 2014).
The Gibbs free energy change (DG�) values can discern
whether a process is spontaneous or not; negative values of
DG� imply a spontaneous process. The enthalpy change
(DH�) provides information about the exothermic or
endothermic nature of the process and differentiates
between physical and chemical adsorption process. The
entropy change (DS�) predicts the magnitude of the chan-
ges on the adsorbent surface, allowing the randomness of
the adsorbate-adsorbent interface to be evaluated (Hum-
pola et al. 2013; Rincon-Silva et al. 2014; Ashraf et al.
2014).
3 Results and discussion
3.1 Characterisation of activated carbons
3.1.1 Textural and chemical properties
The surface area of samples was calculated by B.E.T.
equation. Figure 1 shows the nitrogen adsorption isotherms
for the samples ACS25 and ACS10, demonstrating that
activated carbons obtained fit to Langmuir isotherm or
Type I related with the IUPAC classification (Martinez
1988; Rodrıguez-Reinoso and Linares-Solano 1989;
Lovera 2003). A greater volume was adsorbed at low rel-
ative pressures, characteristic of microporous adsorbents,
and the next part of the isotherm is not completely linear,
indicating the presence of a larger pore size generated by
the activation type (Martinez 1988; Lovera 2003). In the
adsorption isotherms for the carbon ACS10, it can be
observed that the curve displayed a small hysteresis loop,
indicating the presence of mesopore volume (Rodrıguez-
Reinoso and Linares-Solano 1989; Lovera 2003). Finally, it
0.0 0.2 0.4 0.6 0.8 1.00
20
40
60
80
100
120
Relative Pressure (P/P0)
V a
dsor
bed
(cm
3 g-1
STP
)
ACS10ACS25
Fig. 1 Nitrogen adsorption isotherms at 77 K for activated carbons
Adsorption
123
is evident that the ACS25 sample adsorbs greater volumes
of nitrogen, exceeding 100 cm3 g-1; this happens because
the increase in concentration of ZnCl2 favours the devel-
opment of porosity and apparent surface area, because zinc
chloride is a dehydrating agent, which produces the greater
removal of water molecules from the lignocellulosic
matrix, which increases porosity development. Also, due to
the increase in zinc chloride atoms on ACS25 sample,
more pores will be developed when the temperature of
carbonisation and washing the material remove particles of
this activating agent.
Table 1 shows the apparent surface area calculated by
the B.E.T. method, the micropore volume content, meso-
pore and total pore volume which were calculated by the
D.R. method, where the characteristic energy also was
determined. The results show changes in textural charac-
teristics of carbonaceous materials. It was observed that for
sample ACS25, the apparent surface area was 300 m2 g-1
and for sample ACS10, it was 250 m2 g-1, demonstrating
similar values. Results of micropore volume obtained by
the equation D.R. showed a similar tendency to the car-
bonaceous samples, in which the micropore volume was
0.140 and 0.125 cm3 g-1 for ACS25 and ACS10, respec-
tively. On the other hand, the volume of mesopores pre-
sented low values at both samples. However, an increased
content of mesopores for ACS10 carbon is observed.
Additionally, the average pore diameter was determined
and reported in Table 1, which shows the larger diameter
pore (1,820 nm) for the sample with the higher surface area
and greater micropore volume content, i.e. the activated
carbon ACS25.
In general, the textural properties presented in Table 1
are best developed for ACS25 sample, which happens
because with increasing concentration of zinc chloride
the high development of porosity is evidenced and a
higher surface area is apparent. As reported in other
studies of lignocellulosic materials activated by zinc
chloride (Khalilia et al. 2000; Azevedo et al. 2007).
Likewise, as shown in Fig. 1 and Table 1, the microp-
orosity can be attributed to concentrations of activated
agent in samples, which favours the development of
microporosity (Rodrıguez-Reinoso and Linares-Solano
1989; Lovera 2003).
The results of total surface groups are also shown in
Table 2. The carbons immersed in HCl, determine the total
amount of basic sites, indicating that sample ACS25 had
the largest concentration with 0.238 meq g-1, and sample
ACS10 presented the lowest concentration of basic sites
with a value of 0.178 meq g-1. Moreover, immersion in
NaOH carbons can determine the concentration of total
acid sites, finding that ACS25 sample contained the highest
concentration of total acid sites with 0.092 meq g-1, while
the other sample had a value of 0.057 meq g-1. The
obtained carbons had a higher content of basic groups and
pH values in the relatively neutral zero point of charge,
with values between 6.98 and 6.91, which favours
adsorption of phenols derivatives. This is because if the
samples have a greater amount of acidic groups, they are
located on the edges of graphene layers, withdrawing the
electron density of electrons p, leading to a weaker inter-
action between the p electrons of the aromatic ring of the
phenol and graphene layers, which reduces the adsorptive
capacity (Moreno-Castilla et al. 1995; Moreno-Castilla
2004).
In order to detect the functionality present in ACs,
adsorption in the infrared (IR) region takes place
(4000–400 cm-1) due to the rotational and vibrational
movement of the molecular groups and chemical bond of a
molecule. The FT-IR spectra were obtained to evaluate
qualitatively the chemical structures of ACs.
Figure 2 Shows the FT-IR spectrum of ACs, which
indicated various surface functional groups. The broad
band at around 3500 cm-1 is typically attributed to the
hydroxyl group of phenol, alcohol, and carboxylic acid.
The relatively intense band at about 1200 cm-1 observed
in the samples is attributed to C–O–C stretching in ethers.
In the FT-IR spectra, the peaks observed at 1577 and
1586 cm-1 can be attributed to C = O stretching in
ketones. Moreover, the region of the spectrum of
2220 cm-1 is attributed to alkyne group (C:C) (Saka
2012). The bands observed from 700 to 750 correspond to
stretching by the presence of the C–Cl group, due to the
activating agent used. Finally, to appreciate the bands
corresponding to specific surface chemical groups, no
specific differences were evidenced by variation in the
concentration of the activating agent (Pakułaa et al. 2005).
Table 1 Textural and chemical properties of the activated carbons
Sample AB.E.T.
(m2 g-1)
Model D.R. Chemistry properties
Vlpore
(cm3 g-1)
Vmesopore
(cm3 g-1)
Vtotal
(cm3 g-1)
E0
(KJ mol-1)
Average pore
diameter (nm)
Acidity
(meq g-1)
Basicity
(meq g-1)
pHPCC
ACS25 300 0.140 0.007 0.146 10.946 1.510 0.092 0.238 6.98
ACS10 250 0.125 0.015 0.140 8.354 1.820 0.057 0.178 6.91
Adsorption
123
Table
2P
seu
do
-firs
to
rder
,p
seu
do
-sec
on
do
rder
and
chem
iso
rpti
on
mo
del
con
stan
tsan
dco
rrel
atio
nco
effi
cien
tsfo
rp
hen
ols
adso
rpti
on
on
toA
Cs
Ph
eno
lsC
arb
on
C0
(mg
L-
1)
Qeexp
(mg
g-
1)
Pse
ud
o-fi
rst
ord
erP
seu
do
-sec
on
do
rder
Elo
vic
hk
inet
icm
od
el
Qe
(mg
g-
1)
Kf*
10-
2(h
-1)
R2
DQ
(%)
Qe
(mg
g-
1)
b(g
-1
h-
1)
R2
DQ
(%)
a(m
gg-
1
h-
1)
b(g
h-
1)
R2
DQ
(%)
Ph
eno
lA
CS
25
10
4.6
74
3.6
00
2.3
03
0.9
15
0.0
95
4.7
50
1.9
61
0.9
54
0.0
54
0.1
09
6.5
79
0.8
37
1.4
52
40
19
.02
51
7.8
59
1.6
12
0.8
56
0.3
42
20
.85
90
.09
70
.96
80
.09
50
.64
72
.94
10
.80
00
.89
5
70
36
.52
63
5.4
73
1.3
82
0.9
27
0.1
62
40
.47
30
.60
90
.97
90
.00
90
.79
40
.92
30
.86
60
.96
3
10
04
5.0
55
39
.46
64
.60
60
.96
80
.09
34
6.4
66
0.0
72
0.9
48
0.0
28
1.4
08
0.5
51
0.8
81
2.6
24
AC
S1
01
03
.60
14
.41
74
.93
00
.90
80
.25
62
.41
71
4.0
13
0.9
88
0.0
31
0.1
16
5.6
50
0.9
38
2.6
58
40
20
.20
81
5.5
97
1.3
82
0.9
07
0.4
27
16
.59
71
3.1
53
0.9
79
0.2
56
0.3
95
1.7
57
0.8
83
0.8
96
70
31
.40
03
0.8
97
1.1
52
0.8
56
0.2
27
32
.89
70
.06
80
.98
80
.05
60
.51
21
.50
60
.83
42
.36
5
10
04
1.9
58
36
.93
21
.15
20
.92
80
.12
84
6.9
32
2.9
86
0.9
79
0.0
25
1.0
56
0.8
91
0.8
70
3.3
65
4-N
PA
CS
25
10
6.3
44
3.0
76
3.6
85
0.9
95
0.1
74
6.0
76
1.9
55
0.9
88
0.0
43
1.5
26
3.3
67
0.9
88
3.5
41
40
22
.93
52
0.5
40
8.9
82
0.9
94
0.3
51
24
.54
02
.70
60
.99
80
.17
41
.25
01
.12
20
.98
12
.65
8
70
37
.56
33
9.2
35
9.9
82
0.9
27
0.1
30
38
.65
23
.60
90
.92
70
.35
11
.23
52
.94
10
.88
00
.98
5
10
05
6.6
24
51
.26
31
1.9
82
0.9
38
0.0
86
68
.21
44
.60
90
.98
90
.00
31
.21
00
.92
30
.86
01
.02
3
AC
S1
01
05
.53
72
.05
43
.22
40
.99
50
.78
14
.05
44
.03
40
.98
80
.00
40
.78
84
.44
40
.93
32
.36
5
40
29
.64
71
8.2
96
2.7
64
0.9
96
0.3
88
22
.29
66
.23
00
.96
90
.78
11
.12
81
.15
30
.97
71
.36
5
70
38
.52
63
6.1
23
3.7
36
0.9
25
0.1
44
33
.24
19
.23
00
.93
90
.38
81
.15
92
.05
40
.87
92
.98
4
10
06
8.2
35
47
.14
24
.76
49
.40
40
.08
26
0.6
35
14
.03
40
.98
80
.00
52
.14
82
.65
40
.87
84
.36
5
4-C
PA
CS
25
10
7.7
90
3.1
68
2.7
64
0.9
87
0.4
21
7.1
68
2.3
40
0.9
89
0.0
05
0.1
00
5.2
08
0.8
45
3.6
54
40
23
.09
41
9.8
77
2.3
03
0.9
75
0.1
82
28
.87
71
.61
20
.96
90
.42
10
.51
70
.94
20
.91
71
.36
5
70
43
.30
23
5.6
70
1.3
82
0.9
26
0.0
55
48
.67
00
.84
20
.98
90
.00
90
.74
50
.65
40
.90
40
.89
5
10
07
1.9
58
57
.25
71
.15
20
.96
50
.04
56
9.2
57
0.6
36
0.9
78
0.0
05
1.2
29
0.4
00
0.9
15
1.3
54
AC
S1
01
06
.46
54
.53
82
.53
30
.90
40
.12
26
.53
81
0.5
69
0.9
89
0.0
20
0.1
07
6.3
29
0.9
21
3.5
64
40
21
.70
92
2.3
30
1.3
82
0.9
24
0.1
88
25
.33
01
.77
80
.95
80
.12
20
.32
41
.55
50
.92
22
.69
8
70
49
.61
73
9.1
77
1.3
82
0.9
95
0.0
66
41
.17
70
.65
20
.97
80
.10
91
6.7
90
0.0
60
0.9
11
4.6
58
10
07
8.2
68
59
.03
01
.15
20
.96
50
.04
76
9.0
30
0.4
93
0.9
89
0.0
14
0.9
90
0.4
69
0.9
02
5.6
54
Adsorption
123
3.2 Adsorption kinetics
The study of adsorption kinetics is important because the
rate of adsorption (which is one of the criteria for efficiency
of adsorbent) and also the mechanism of adsorption can be
concluded from kinetic studies (Ho and McKay 2000; Ho
2006b; Wu et al. 2009; Tseng et al. 2010; Boparai et al.
2011; Aljeboree et al. 2014).
The relationship between contact time and phenol
adsorption onto ACs at different initial phenol concentra-
tions (10.0 to 100 mg L-1) is shown in Fig. 3. This graph is
obtained by plotting the absorption capacity versus time t
in the time required to reach equilibrium system in hours,
proving the variation of the amount of adsorbed (Q) as a
function of time. The rate of adsorption for phenols is high
at initial times of adsorption (Azizian 2008; Plazinski and
Plazinska 2012). For phenols, most of the adsorption takes
place within 300 min which indicate that the rate of phe-
nols adsorption by ACs is high (Figure to 4-CP and 4-NP
not was shown here, but had a similar behaviour). Figure 3
indicates that while the adsorption of phenol was quite
rapid initially, the rate of adsorption became slower with
the time and reached a constant value (equilibrium time).
Additionally, it showed that the speed increased at lower
concentrations. The initial faster rate may be due to the
availability of the uncovered surface area of the adsorbents
(Ho and McKay 1999; Ho 2006a; Manasi and Rajesh
2014).
In order to analyse the adsorption kinetics of mono-
substituted phenols by ACs the pseudo-first-order, pseudo-
second order, Elovich equation, Dumwald–Wagner equa-
tion and intra-particle diffusion model were tested (Leyva-
Ramos and Geankoplis 1994; Qiu et al. 2009; Feng-Chin
et al. 2009; Siminiceanu et al. 2010; Gihan and El-Khaiary
2010; Plazinski et al. 2013) The result of fitting is listed in
Tables 2 and 3.
A simple kinetic analysis of adsorption (pseudo-first-
order equation or Lagergren equation) is in the form:
Qt ¼ Qe 1 � expðkf tÞ� �
ð4Þ
Where Qt is the amount of adsorbate adsorbed at time t
(mg g-1), Qe is the adsorption capacity in the equilibrium
(mg g-1), kf is the pseudo-first-order rate constant (h-1),
and t is the contact time (Rudzinski and Plazinski 2006;
Qiu et al. 2009).
A pseudo-second-order equation based on adsorption
equilibrium capacity may be expressed in the equation:
Qt ¼ksQ
2e t
1 þ ksQetð5Þ
where ks is the pseudo-first-order rate constant (g gm-1 -
min-1) and t is the contact time (Qiu et al. 2009).
The applicability of the kinetic model to describe the
adsorption process was further validated by the normalised
standard deviation DQ (%), which is defined as (Qiu et al.
2009; Valderrama et al. 2010):
DQ ¼ 100
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPðQexp � Qcal=QexpÞ� �2
N � 1
s8<:
9=; ð6Þ
The Elovich equation has been applied satisfactorily to
some chemisorption processes and has been found to cover
a wide range of slow adsorption rates. Moreover, this
describes the adsorption of adsorbate by solid adsorbents in
aqueous medium. The same equation is often valid for
3600 3000 2400 1800 1200 600
60
70
80
90
100
C=CC=O
C-O-COH-
1586
700
1200
1577
2220
3500Tran
smitt
ance
% T
Wavenumber cm-1
ACS10ACS25
Fig. 2 FT-IR spectra for the ACs derivate from Eucalyptus shell
activated with ZnCl2
0 1 2 3 4 5 6 7 8 9 100
10
20
30
40
50
ACS25 10 mg L-1 ACS10 10 mg L-1
ACS25 40 mg L-1 ACS10 40 mg L-1
ACS25 70 mg L-1 ACS10 70 mg L-1
ACS25 100 mg L-1 ACS10 100 mg L-1
Qt (
mg
g-1)
Time (h)
Fig. 3 Kinetics of phenol adsorption at different concentrations onto
ACs
Adsorption
123
systems in which the adsorbing surface is heterogeneous,
and is formulated as (Qiu et al. 2009; Feng-Chin et al.
2009):
Qt ¼1
b
� �LnðabÞ þ 1
b
� �LnðtÞ ð7Þ
The intra-particle diffusion model based on the theory
proposed by Weber and Morris was used to identify the
diffusion mechanism (Gihan and El-Khaiary 2010).
According to this theory, the adsorbate uptake Qt varies
almost proportionally with the square root of the contact
time, t� rather than t, Eq. (7) (Qiu et al. 2009; Gihan and
El-Khaiary 2010):
Qt ¼ kidffiffit
pþ I ð8Þ
where I is the intercept and kid (mg g-1 h-1/2) is the
intraparticle diffusion rate constant.
The Dumwald–Warner model is another intraparticle
diffusion model, which is written as (Acharya et al. 2009;
Qiu et al. 2009; Siminiceanu et al. 2010; Gihan and El-
Khaiary 2010; Qing-Song et al. 2010; Theydana and
Ahmed 2012):
F ¼ Qt
Qe
¼ 1 � 6
p2
X1n¼1
1
n2expð�n2ktÞ ð9Þ
where k (min-1) is the rate constant of adsorption. Equa-
tion (9) can be simplified as:
Inð1 � FÞ ¼ �kfdt ð10Þ
Where F is the fractional attainment of equilibrium
(F = Qt/Qe) and kfd (min-1) is the film diffusion rate
coefficient. The Dumwald–Wagner model proved to be a
reasonable model for different kinds of adsorption systems
(Qiu et al. 2009; Siminiceanu et al. 2010).
The results of the kinetic data of adsorptions of phenol,
4-NP and 4-CP at different initial concentrations are given
in Tables 2 and 3. In the pseudo-first order model kf and
Qe, were calculated using the slope and intercept of plots of
Log (Qe - Qt) versus t (Fig. 4; Table 2). It shows that the
Table 3 Intraparticle diffusion and liquid film diffusion model constants and correlation coefficients for phenols adsorption onto ACs
Phenols Carbon C0 (mg L-1) Intraparticle diffusion Liquid film diffusion
kid (mg g-1 h0.5) I R2 DQ (%) Qe
(mg g-1)
kfd (h-1) R2 DQ (%)
Phenol ACS25 10 0.094 0.042 0.979 0.854 3.546 0.009 0.993 1.526
40 0.487 0.365 0.978 1.562 18.5621 0.009 0.998 2.651
70 0.733 0.552 0.989 3.541 40.621 0.010 0.990 2.865
100 1.221 0.806 0.998 2.654 40.896 0.404 0.982 3.365
ACS10 10 0.099 0.045 0.998 2.456 3.456 0.010 0.978 2.365
40 0.380 0.304 0.998 1.365 12.365 0.010 0.985 0.954
70 0.456 0.332 0.989 2.365 25.658 0.008 0.993 1.984
100 0.768 0.338 0.998 2.321 40.998 0.010 0.971 2.654
4-NP ACS25 10 0.196 0.428 0.899 2.654 5.654 0.019 0.863 3.256
40 0.262 1.419 0.959 2.365 20.652 0.066 0.988 3.651
70 0.362 2.019 0.979 2.123 36.654 0.009 0.928 2.365
100 0.982 2.119 0.979 2.415 62.365 0.046 0.938 2.654
ACS10 10 0.140 0.34 0.819 1.025 3.658 0.023 0.801 1.254
40 0.502 0.514 0.899 2.125 23.654 0.043 0.883 2.365
70 0.620 0.564 0.979 2.451 35.568 0.078 0.873 2.854
100 0.730 0.614 0.979 3.562 65.651 0.033 0.889 4.654
4-CP ACS25 10 0.216 0.101 0.989 3.214 6.854 0.011 0.998 3.254
40 0.700 0.362 0.999 3.245 25.658 0.014 0.985 3.652
70 1.034 0.584 0.998 3.354 46.365 0.012 0.990 4.562
100 2.122 0.988 0.988 3.654 60.652 0.016 0.965 4.658
ACS10 10 0.295 0.042 0.997 2.124 4.658 0.011 0.984 2.654
40 0.425 0.197 0.997 2.365 20.654 0.011 0.974 2.854
70 0.629 0.497 0.918 2.854 40.654 0.011 0.989 3.654
100 1.303 0.969 0.978 2.654 62.687 0.013 0.972 3.854
Adsorption
123
correlation coefficients (R2) for the pseudo-first order
kinetic model fit are far from 1.00 and the standard devi-
ation values are high compared with the pseudo-second-
order. Moreover, a low correlation was also observed
between Qe exp and Qe calculated of model. Thereby, the
pseudo-first-order model isn’t a suitable equation to
describe the adsorption kinetics of phenols on the ACs (Qiu
et al. 2009; Siminiceanu et al. 2010).
In the pseudo-second order adsorption parameters Qe
and ks in Eq. (3) were determined by plotting t/Qt and
t (Fig. 5; Table 2). The values of Qe calculated of model
are close from the experimental value, the R2 values
derived from the second-order kinetic model were rela-
tively high in comparison with the Pseudo-first order
model. Therefore this model fit the adsorption process of
phenol onto ACs.
As shown in Table 2, phenol and 4-CP demonstrate
similar adsorption kinetics, while other phenols exhibit
slower initial adsorption rates. For phenol, it should be
noted that its adsorption driving force is weaker due to a
relatively lower ultimate uptake; in fact, its adsorption
kinetics is remarkable, as suggested by the second-order
rate index. For 4-NP slower adsorption rates should be
ascribed to the steric effects, i.e., the adsorbate molecules
have difficulties in moving within pores with size not large
enough. Adsorption kinetics is more sensitive to the steric
effects, which demonstrates their influences during the
adsorption process. The similar adsorption kinetics of
phenol and 4-CP indicate that their adsorption process is
not hindered to an appreciable extent, suggesting that steric
effects are negligible if the molecular dimensions are
below some limits (Qiu et al. 2009; Siminiceanu et al.
2010).
In the Elovich model, a and b were calculated from the
slope and intercept of the plot of Qt vs In t and the results
are present in Table 2 (figure not shown here). The values
show that the correlation coefficients were not satisfactory
for most of the cases, which indicated that the Elovich
model is not appropriate for the description of phenol, 4-CP
and 4-NP adsorption by ACs. Therefore, phenols adsorp-
tion on the ACs does not follow chemisorption models.
Thus, suggested phenols adsorption does not occur via the
sharing of electrons between the phenolic ring and basal
plane of ACs (Qiu et al. 2009; Wu et al. 2009; Ahmaruz-
zaman and Laxmi 2010; Al-Khateeb et al. 2014).
3.3 Diffusion kinetic model
A detailed understanding of adsorption mechanisms facil-
itates the determination of the rate-limiting step. This
information can then be used to optimise the design of
adsorbents and adsorption conditions. The overall rate of
adsorption can be described by the following three steps:
(i) film or surface diffusion where the sorbate is transported
from the bulk solution to the external surface of sorbent (ii)
intraparticle or pore diffusion, where sorbate molecules
move into the interior of sorbent particles, and (iii)
adsorption on the interior sites of the sorbent (Theydana
and Ahmed 2012). Since the adsorption step is very rapid,
it is assumed that it does not influence the overall kinetics.
The overall rate of adsorption process, therefore, will be
0 1 2 3 4 5 6 7 8 9 100.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Log
( Qe-Q
t )
ACS25 10 mg L-1 ACS10 10 mg L-1 ACS25 40 mg L-1
ACS10 40 mg L-1 ACS25 70 mg L-1 ACS10 70 mg L-1
ACS25 100 mg L-1 ACS10 100 mg L-1
Time (h)
Fig. 4 Pseudo-first-order kinetic model fit for phenol adsorption at
different concentrations onto ACs
0 1 2 3 4 5 6 7 8 9 100
15
30
45
60
Time (h)
t/Qt (
h g /
mg)
ACS25 10 mg L-1 ACS10 10 mg L-1 ACS25 40 mg L-1
ACS10 40 mg L-1 ACS25 70 mg L-1 ACS10 70 mg L-1
ACS25 100 mg L-1 ACS10 100 mg L-1
Fig. 5 Pseudo-second-order kinetic model fit for phenol adsorption at
different concentrations onto ACs
Adsorption
123
controlled by either surface diffusion or intraparticle dif-
fusion (Gihan and El-Khaiary 2010; Theydana and Ahmed
2012).
The Weber–Morris intraparticle diffusion model has
often been used to determine whether intraparticle diffu-
sion is the rate-limiting step. According to this model, a
plot of Qt versus t0.5 should be linear if intraparticle dif-
fusion is involved in the adsorption process and if the plot
passes through the origin then intraparticle diffusion is the
sole rate-limiting step. It has also been suggested that in
instances when Qt versus t0.5 is multilinear two or more
steps govern the adsorption process, the multilinearity of
this plot for adsorption on activated carbon suggests that
adsorption occurred in three phases. The initial steeper
section represents surface or film diffusion, the second
linear section represents a gradual adsorption stage where
intraparticle or pore diffusion is rate-limiting and the third
section is final equilibrium stage. In Fig. 6, the adjustment
graph of the intraparticle diffusion model for phenol is
shown (Figures for 4-CP and 4-NP are not shown here), as
the plot passes through the origin, intraparticle diffusion
could be rate-limiting step for most phenol concentrations
studied; however, for some concentrations of phenol
(100 mg L-1), in both carbons, there were three processes
controlling the adsorption rate but only one was rate lim-
iting in any particular time range. The intraparticle diffu-
sion rate constant kid was calculated from the slope linear
section (Fig. 6; Table 3). The value of the intercept I in the
second section provides information related to the thick-
ness of the boundary layer. Larger intercepts suggest that
surface diffusion has a larger role as the rate-limiting step
(Gihan and El-Khaiary 2010; Theydana and Ahmed 2012;
Ocampo-Perez and Leyva-Ramos 2013).
In the liquid film diffusion models, a linear plot of
In(1 - F) versus t with zero intercept suggests that the
kinetics of the adsorption process is controlled by diffusion
through the liquid film (this Figure is not shown here).
Application of the liquid film diffusion model to the
adsorption of phenol, 4-CP and 4-NP by ACs did not
converge, and the regression coefficient values were very
low; however, the standard deviation was higher, as shown
in Table 3. This indicates that the liquid film diffusion was
not the rate-determining step (Acharya et Al 2009; Gihan
and El-Khaiary 2010; Qing-Song et al. 2010; Theydana and
Ahmed 2012; Ocampo-Perez and Leyva-Ramos 2013).
Based on the results presented in Table 3, it is clear that
the mechanism of interaction between the phenolic com-
pounds and the ACs is somewhat complex. The application
of the intra-particle diffusion model, and liquid film dif-
fusion model, on the experimental data yielded different
straight lines but only in intraparticle model the line
passing through the origin, which indicates some degree of
boundary layer control. This further show that the intra-
particle diffusion and liquid film diffusion are not the only
rate-controlling step, but other processes may also control
the rate and mechanism of adsorption (Acharya et Al 2009;
Gihan and El-Khaiary 2010; Theydana and Ahmed 2012;
Ocampo-Perez and Leyva-Ramos 2013).
3.4 Adsorption isotherms
The adsorption isotherm can describe the distribution of
phenol between solid phase and the solution at a certain
temperature when the equilibrium was reached.
Figure 7 shows the experimental adsorption isotherms
for phenol, 4-CP and 4-NP, for the two samples, which
depicts the phenol adsorption capacity (Qe expressed in mg
per gram of activated carbon retained). It shows that phenol
adsorption behaviour follows the Freundlich isotherm,
because the mass of adsorbed phenol in a wide range of
concentrations as considered in this work does not became
asymptotic at high concentrations; the same behaviour
applied for the adsorption of 4-NP by ACS10. Although in
some isotherms, e.g. phenol adsorption on ACS10, it
behaves according to the Langmuir model, since the mass
of phenol remains constant when the phenol concentration
at equilibrium is greater than 400 mg L-1. Compared with
phenol, substituted phenols showed greater intensity of
adsorption and adsorption capacity given in the following
order: 4-CP[ 4-NP[ phenol. As phenol has smaller
molecular size than substituted phenols, these results imply
that only a small part of the micropores is filled in phenol,
adsorption and the micropore filling phenomenon is more
evident for the substituted phenols (Moreno-Castilla et al.
1 2 3 40
10
20
30
40
Qt
(mg
g-1)
Time 0.5 (h0.5)
ACS25 10 mg L-1 ACS10 10 mg L-1 ACS25 40 mg L-1
ACS10 40 mg L-1 ACS25 70 mg L-1 ACS10 70 mg L-1
ACS25 100 mg L-1 ACS10 100 mg L-1
Fig. 6 Intraparticle diffusion plots for phenols adsorption at different
concentrations onto ACs
Adsorption
123
1995; Dabrowski et al. 2005; Derylo-Marczewska et al.
2010; Qing-Song et al. 2010; Rincon-Silva et al. 2015).
3.5 Equilibrium adsorption of phenols
The experimental data of the adsorption isotherms were
fitted to models Langmuir, Freundlich and Dubinin–
Raduskevich–Kaganer (D.R.K.), which are presented in
Table 3. Each isotherm model was expressed by relative
certain constants which characterised the surface properties
and indicated adsorption capacity of this material (Kumar
et al. 2007; Derylo-Marczewska et al. 2010; Qing-Song
et al. 2010; Rincon-Silva et al. 2015).
The Langmuir model proposes that monolayer sorption
occurs on the solid surface with identical homogeneous
sites. It also suggests that no further adsorption takes place
once the active sites are covered with phenols molecules.
The saturated monolayer isotherm is presented by the fol-
lowing equation:
Qe ¼QmaxkLCe
1 þ kLCe
ð11Þ
where Ce is the concentration of phenol at equilibrium in
solution (mg L-1); Qe is unit equilibrium adsorption
capacity; Qmax is the maximum phenol uptake, giving the
information about adsorption capacity for a complete
monolayer (mg g-1); and KL is a constant denoted the
energy of adsorption and affinity of the binding sites
(L mg-1) (Qing-Song et al. 2010; Okelo and Odebunmi
2010; Rincon-Silva et al. 2015).
Freundlich isotherm is an empirical model assuming that
the distribution of the heat on the adsorbent surface is non-
uniform, namely a heterogeneous adsorption. The equation
is stated as follows (Kumar et al. 2007; Qing-Song et al.
2010; Okelo and Odebunmi 2010; Rincon-Silva et al.
2014):
Qe ¼ kf ðCeÞ1=n ð12Þ
where n and Kf [mg g-1 (L mg-1)n] are both the Fre-
undlich constants giving an indication of adsorption
intensity and capacity, respectively. The degree of non-
linearity between solution concentration and adsorption is
n dependent as follows: if the value of n is equal to unity,
the adsorption is linear; if the value is below to unity, this
implies that adsorption process is chemical; if the value is
above unity adsorption is a favourable physical process
(Mourao et al. 2006; Kumar et al. 2007; Qing-Song et al.
2010; Rincon-Silva et al. 2014).
The Dubinin–Radusckevisch–Kanager model is repre-
sented in Eq. (13)
Qe ¼ QmDRK exp �RTIn Cs=Ce
� �ES
nð13Þ
where QmDRK represents the amount adsorbed of solute on
the monolayer, Ce and Cs are the concentrations of equi-
librium saturation and adsorbate, respectively. ES is related
to the energy characteristic of the process, and n relates to
the heterogeneity variations of the microporous adsorbents.
The parameters n and Es are in principle responsible of
surface heterogeneity for adsorbate-adsorbent system
(Mourao et al. 2006; Rincon-Silva et al. 2015).
The coefficients of determination (R2) and isotherm
parameters from nonlinear regressive method were listed in
Table 4.
When the Langmuir model is applied for experimental
adsorption isotherms, it is observed that the value of Qmax
was greater for ACS25 with values of 55.566, 137.005
and 200.004 mg g-1 for phenol 4-nitrophenol and
4-chlorophenol respectively, the sample ACS10 also had
the same order in the adsorption capacity. The adsorption
was greater to 4-CP followed by 4-NP and finally phenol.
In general, ACS25 carbon present the best able to
adsorption of phenols compounds and this succeed because
this is the activated carbon with the highest apparent sur-
face area (300 m2 g-1) and the largest volume of microp-
ores (0.140 cm3 g-1), favouring the ability to adsorption of
phenolic compounds on this adsorbent.
In Freundlich analysis, the kf was higher for the sample
ACS25, in adsorption of 4-chlorophenol with a magnitude
of 29.818 mg1-1/nL1/ng-1, which is associated with
monolayer adsorption capacity and textural properties of
this sample. The value of 1/n is a measure of heterogeneity
of the surface, in which value close to 0 indicates a
heterogeneous surface. When the value of 1/n is less than 1
it is said that the adsorption process is favourable, as
0 400 800 12000
40
80
120
160
200
ACS10 phenol CS25 phenol ACS10 4-NP ACS25 4-NPACS10 4-CP ACS25 4-CP
Qe (
mg
g-1)
Ce (mg L-1)
Fig. 7 Experimental adsorption isotherms of phenols compounds at
20 �C
Adsorption
123
happened in this study. However, there was a greater value
in adsorption favouring 4-chlorophenol.
In the D.R.K. model, the maximum quantity of QmDRK
was on ACS25 for all adsorbates in the order
4-chlorophenol[ 4-nitrophenol[ phenol with values
155.544, 122.836 and 48.525 mg g-1 respectively; it was
noted that there was a correlation of values of adsorption in
monolayer between the two models. Value Es is the char-
acteristic energy of adsorption process, and it can be
observed that value ES was higher for the sample ACS25,
an aspect that is also related with the amount adsorbed on
the monolayer. Additionally, its increase was directly
proportional to values of QmDRK and Qmax. On the other
hand, it was observed that ES values were higher for the
phenol compared with 4-nitrophenol, because this value is
directly related to the solubility of phenol, and this com-
pound had the highest solubility with a value of 93 g L-1,
compared with the value 1.7 g L-1 of 4-nitrophenol. The
values of Es in the adsorption process of phenol were
similar of the adsorption process of 4-chlorophenol, this is
also explained by solubility, since 4-chlorophenol is also
significantly soluble in water, and finally, this is the one
with greater adsorption. Therefore, high values are also
related to this parameter. Again, the activated carbon
ACS25 is the solid that has greater adsorption capacity in
D.R.K model, which happens for the same reason as the
Langmuir model, that is, the textural properties favoured in
this solid compared with the other carbon ACS10, in
relation to the highest amount of used concentration of zinc
chloride.
According to the results presented in Table 4, the acti-
vated carbon with zinc chloride to 25 % m/V is more
efficient in the adsorption of phenolic compounds, relative
to the sample activated to 10 % m/V, since it is the higher
adsorption capacity presented in adjusting models of
Langmuir and D.R.K. This happens because, as explained
above, this sample is the one with higher development of
textural properties. Likewise, ACS25 is the one with higher
content of basic groups which favours adsorption, because
these do not withdraw electron density, which does not
weaken the interaction between the electrons of the aro-
matic ring and of the graphene layers of coal (Moreno-
Castilla et al. 1995). Similarly it is clear in Table 4 that the
best fit of the data is for the Freundlich model because its
correlation coefficient is very close to unity, Furthermore,
this is confirmed because it has the lowest percentage of
deviation (0.025–0.365 %), so it is assumed that the
adsorption occur on adsorbents with the energetically
heterogeneous surfaces (Kumar et al. 2007; Okelo and
Odebunmi 2010; Qing-Song et al. 2010).
Finally, another important aspect to consider in the
adsorption process is the pH of the solution, because this
also interacts with the solid charge, and indicates the
concentration of phenolic compound in solution. From the
diagram of phenol speciation it is known that a high pro-
portion of protonated species at low pH values and pres-
ence of species deprotonated at high pH values. The pH
experimental values of phenols isotherms in this study
were between 6.56 and 7.86. With these data and the dis-
tribution of species, it can be demonstrated that the process
was conducted for phenol protonated species, i.e. they are
not dissociated, and the process favours the dispersion
forces, because if solutions were in basic medium, they
would have decreased the adsorption due to electrostatic
repulsion between the negatively charged surface and the
anions phenolates and between each other (Kumar et al.
2007; Qing-Song et al. 2010; Rincon-Silva et al. 2015).
This behaviour is evidenced by the pKa value of phenol
(9.89) which is greater than the data obtained from
experimental solution pH, which proves that the adsorbed
species were in their protonated form (Kumar et al. 2007;
Qing-Song et al. 2010; Rincon-Silva et al. 2015).
3.6 Thermodynamic study
The thermodynamic behaviours for adsorption of phenol,
4-CP and 4-NP on ACs were investigated. The thermody-
namic parameters such as Gibbs free energy change (DG�),enthalpy change (DH�) and entropy change (DS�) were
calculated using the following equations:
Table 4 Parameters of the models, Freundlich, Langmuir and D.R.K. in the adsorption of phenols compounds on carbon samples obtained
Phenol Carbon Langmuir Freundlich Dubinin–Raduskevich–Kaganer
Qmax
(mg g-1)
KL (L
mg-1)
R2 %
Dev
kf (mg1-1/nL1/
n*g-1)
1/n R2 %
Dev
QmDRK
(mg g-1)
ES
(kJ mol-1)
R2 %
Dev
Phenol ACS10 27.035 0.010 0.964 1.524 3.735 0.285 0.997 0.025 28.504 18.804 0.994 2.365
ACS25 55.566 0.010 0.985 0.958 2.852 0.444 0.985 0.124 48.525 19.448 0.985 1.895
4-NP ACS10 54.954 0.010 0.978 0.745 5.596 0.315 0.997 0.365 45.384 9.376 0.986 3.541
ACS25 137.005 0.030 0.996 1.214 21.984 0.296 0.996 0.018 122.836 12.578 0.855 4.654
4-CP ACS10 125.008 0.030 0.975 1.523 28.447 0.221 0.997 0.087 101.606 22.364 0.935 2.365
ACS25 200.004 0.030 0.984 2.587 29.818 0.282 0.998 0.124 155.544 20.205 0.934 1.365
Adsorption
123
DG0 ¼ �RTInK0 ð14Þ
where K0 is the apparent equilibrium constant [In this case,
is the equilibrium constant of Langmuir model kL, as has
already been explained in Sect. 2.5), R is the gas constant
(8.314 J mol-1 K-1)], and T is absolute temperatures
(K) (Qing-Song et al. 2010; Al-Khateeb et al. 2014; Rin-
con-Silva et al. 2015).
The enthalpy change (DH�) of adsorption and entropy
change (DS�) of adsorption were calculated from adsorp-
tion data at different temperatures using the Van’t Hoff
Eq. (9) as follows (Kumar et al. 2007; Rodriguez et al.
2009; Qing-Song et al. 2010; Al-Khateeb et al. 2014):
InK0 ¼ DS0
R� DH0
RTð15Þ
Other thermodynamic quantity is Ea (Arrhenius activa-
tion energy), which can be calculated from the relationship
from the pseudo-second order rate constant of phenols
adsorption, which is expressed as a function of temperature
(Ashraf et al. 2014):
InkS ¼ LnAEa
RTð16Þ
where Ea is the Arrhenius activation energy of adsorption,
A is the Arrhenius factor, R is the gas constant is equal to
8.314 J mol-1 K-1 and T is the operating temperature. A
linear plot of Ln ks vs 1/T for the adsorption of phenols
onto ACs was constructed to generate the activation energy
from the slope (-Ea/R). The magnitude of activation
energy gives an idea about the type of adsorption, which is
mainly physical or chemical. Low activation energies
(5–40 kJ mol-1) are characteristics for physisorption,
while higher activation energies (40–800 kJ mol-1) sug-
gest chemisorptions (Stoeckli et al. 1995; Kumar et al.
2007; Qing-Song et al. 2010; Rincon-Silva et al. 2015).
The thermodynamic parameters of the system: Gibbs
free energy change (DG�), enthalpy change (DH�) and
entropy change (DS�) are presented in Table 5. The free
energy values at temperatures 20, 30 and 40 �C in the
adsorption of phenol; 4-NP and 4-CP were negative in all
cases, showing the spontaneous nature in the process. That
is to say, for three phenols, it was shown that the adsorption
process on ACs was a spontaneous process, and the
decrease of DG� values with the increase of temperature
indicated that the adsorption became less favourable at
higher temperatures. It is also shown that free energy
values increases with the addition of nitro or chlorine
groups to phenol. The maximum value of DG� for
adsorption at 20 �C was on ACS25 for the three phenols,
data directly related to the adsorption capacity of this
carbon. In this study the free energy values indicated that
the adsorption process occurs by physisorption (Stoeckli
and Centeno 1997; Kumar et al. 2007; Qing-Song et al.
2010; Boparai et al. 2011; Rincon-Silva et al. 2015).
DH� values are negative for all process on ACs, showing
the exothermic nature of the adsorption process. Enthalpy
change values are between -6.032 and -30.352 kJ mol-1;
therefore, it follows a behaviour of physisorption in the
process. DS� values were also reported; positive values
indicated the entropy of the system increased during the
adsorption, as in most cases. However, it should also be
noted that the entropy of the universe (including the system
and the surroundings) might increase because the adsorp-
tion reaction was not an isolated process. Additionally,
negative cases of DS� are also given, suggesting a
decreased randomness at the solid–liquid interface during
the adsorption process, as was observed in the adsorption
of 4-NP and 4-CP with values of -10.231 and
-5.583 J mol-1, respectively, on ACS25. Finally, the
values of Arrhenius activation energy are presented in
Table 4 also, the values indicated that the adsorption pro-
cess has a low potential barrier and also confirms the
physisorption process, since these are in the range
6.852–14.564 kJ mol-1 (Huang and Gao 2007; Kumar
et al. 2007; Qing-Song et al. 2010; Rincon-Silva et al.
2015).
4 Conclusions
Eucalyptus shell, a waste solid from trees, was successfully
utilised to synthesise activated carbon, a low cost alterna-
tive adsorbent for the removal of phenols (phenol,
4-chlorophenol and 4-nithrophenol). The samples obtained
present apparent surface area values of 250 and
300 m2 g-1 and micropore volume between 0.007 and
0.015 cm3. The micropore and mesopore distribution is
appropriate for carrying out efficient adsorption processes,
especially in aqueous phenols. Also, the adsorption iso-
therms of phenols on ACs were studied and modelled using
three isotherm models. Freundlich model gives the best
fitting for the adsorption isotherms in most cases, while
Langmuir model is reasonably applicable in all cases.
The values for the adsorption capacity were between
27.035 and 200.004 mg g-1. In kinetic studies, ACs show
high adsorption rate. The pseudo-second-order model gives
satisfactory fitting, and the intraparticle diffusion model
describes the adsorption process well. Steric effects on
adsorption kinetics were found for 4-NP, due to the limi-
tation of the pore structure and the retardation of the
adsorbed molecules. It is proposed that the steric effects are
notable on the adsorption kinetics. The classification of the
kinetic models according to the adsorption study is:
pseudo-first-order[ pseudo-second-order[ intra-particle-
diffuse[ chemisorption.
Adsorption
123
The thermodynamic study demonstrates the spontaneous
and exothermic nature of the adsorption process due to
negative values of both free energy change and enthalpy
change, the increased entropy change with the increased
substitution degree was observed; based on thermodynamic
parameters, the adsorption is primarily physical in nature.
Finally, the uptakes were observed at pH\ pKa which
indicates that the adsorbed phenols species were in their
protonated form which improves the adsorption process on
ACs.
Acknowledgments The authors wish to thank the Master Agree-
ment established between the ‘Universidad de los Andes’ and the
‘Universidad Nacional de Colombia’ and the Memorandum of
Understanding entered into by the Departments of Chemistry of both
Universities. Additionally, the authors are also thankful for financial
support to the Additionally, the authors are also thankful for financial
support to the convocation ‘Proyecto Semilla’ of the faculty of sci-
ences of Universidad de los Andes in the category Master Student
2015.
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