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Journal of Applied Research and Technology 14 (2016) 354–366 www.jart.ccadet.unam.mx

Review
A kinetic, equilibrium and thermodynamic study of l-phenylalanine
adsorption using activated carbon based on agricultural waste (date stones)
Badreddine Belhamdi a,∗ , Zoulikha Merzougui a , Mohamed Trari b , Abdelhamid Addoun a
a Laboratory of Physical and Chemical Study of Materials and Applications in the Environment, Faculty of Chemistry (USTHB), BP 32-16111 EL-Alia, Algeria
b Laboratory of Storage and Valorization of Renewable Energies, Faculty of Chemistry (USTHB), BP 32-16111 EL-Alia, Algeria

Abstract
The main purpose of this work is to produce low cost activated carbons from date stones wastes for the adsorption of l-phenylalanine.
The activated carbons were prepared by chemical activation with KOH (ACK) and ZnCl2 (ACZ) and characterized by scanning electron
microscopy, N2 adsorption–desorption isotherms and FT-IR spectroscopy. Both The activated carbons ACK and ACZ have high specific sur-
face areas and large pore volumes, favorable for the adsorption. Batch experiments were conducted to determine the adsorption capacities.
A Strong dependence of the adsorption capacity on pH was observed, the capacity decreases with increasing pH up to optimal value of 5.7.
The adsorption follows a pseudo-second order kinetic model. Additionally, the equilibrium adsorption data were well fitted to the Langmuir
isotherm, and the maximum adsorption capacities of l-phenylalanine onto ACK and ACZ were 188.3 and 133.3 mg g−1 at pH 5.7, respec-
tively. The thermodynamic study revealed that the adsorption of l-phenylalanine onto activated carbons was exothermic in nature. The proposed
adsorption mechanisms take into account the hydrophobic and electrostatic interactions which played the critical roles in the l-phenylalanine
adsorption.
© 2016 Universidad Nacional Autónoma de México, Centro de Ciencias Aplicadas y Desarrollo Tecnológico. This is an open access article under
the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Activated carbon; l-phenylalanine; Size distribution; Sorbent surface; Adsorption isotherm; Thermodynamic

1. Introduction they are interesting molecules as adsorbates because of their

molecular size and zwitterionic nature (O’Connor et al., 2006).
Adsorption of amino acids onto solid surfaces has received Similar to many amino acids, l-phenylalanine is essential for
much attention because of its scientific importance and appli- animals and the human body. It is extensively used as ingredi-
cations in the separation and purification processes (Han & ent in food or feed additive, in infusion fluids, neutraceutical
Yun, 2007; Hong & Bruening, 2006; Kostova & Bart, 2007; and pharmaceutical (Pimentel, Alves, Costa, Fernandes, et al.,
O’Connor et al., 2006; Sánchez-Hernández, Bernal, del Nozal, 2014; Pimentel, Alves, Costa, Torres, et al., 2014; Zhou, Liao,
& Toribio, 2016). Amino acids are biomolecules of great rel- Wang, Du, & Chen, 2010). Generally, the amino acids have been
evance that are widely used in many industries such as food, studied by adsorption on well-ordered surfaces of solids. On
cosmetic, medicine, biochemistry and others (Bourke & Kohn, the other hand, most of the current methods employed for the
2003; Hartmann, 2005; Infante et al., 2004; Oshima, Saisho, removal of l-phenylalanine from protein hydrolysates are based
Ohe, Baba, & Ohto, 2009; Palit & Moulik, 2001). They are on the adsorption on activated carbon, polymeric resins, zeo-
non-toxic and are used as building blocks for the production lites and ion exchangers (Lopes, Delvivo, & Silvestre, 2005;
of pharmaceutical and agrochemical compounds. In addition, Outinen et al., 1996; Shimamura et al., 2002). These studies
give information for practical researches on the purification and
∗
separation of amino acids. Over the last years, several stud-
Corresponding author.
ies have been reported for the adsorption of amino acids on
E-mail address: Badropg@gmail.com (B. Belhamdi).
Peer Review under the responsibility of Universidad Nacional Autónoma de porous solids (Casado et al., 2012; El Shafei, 2002; El Shafei
México. & Moussa, 2001; Ghosh, Badruddoza, Uddin, & Hidajat, 2011;
http://dx.doi.org/10.1016/j.jart.2016.08.004
1665-6423/© 2016 Universidad Nacional Autónoma de México, Centro de Ciencias Aplicadas y Desarrollo Tecnológico. This is an open access article under the
CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
 B. Belhamdi et al. / Journal of Applied Research and Technology 14 (2016) 354–366 355

Goscianska, Olejnik, & Pietrzak, 2013b, 2013c; Jiao, Fu, Shuai, France), NaHCO3 (>99%, Sigma Aldrich, USA), Na2 CO3
& Chen, 2012; Long et al., 2009; Mei, Min, & Lü, 2009; Palit (>99%, Sigma Aldrich, USA), HCl (37%, Sigma Aldrich, USA),
& Moulik, 2001; Silvério, Dos Reis, Tronto, & Valim, 2008; KCl (>98%, Fluka, France), NaCl (>99%, Sigma Aldrich, USA),
Titus, Kalkar, & Gaikar, 2003; Wu, Zhao, Nie, & Jiang, 2009). NaOH (>99%, Sigma Aldrich, USA). Ultrapure water was
However, their adsorption capacity is still low because of the obtained from milli-Q system (Millipore, France).
small pore volume or wide pores of these adsorbents, which are
consequently inappropriate to the molecular size of amino acids. 2.2. Preparation of the activated carbons
Moreover, the major drawback of the adsorption process is the
high cost for the production and regeneration of adsorbents. Such The activated carbons were prepared from date stones. At
inconvenient resulted in growing research on inexpensive adsor- first, the stones were thoroughly washed with distilled water
bents (Alves, Franca, & Oliveira, 2013a, 2013b; Clark, Alves, and dried in an air oven at 120 ◦ C; such protocol was effective
Franca, & Oliveira, 2012; Goscianska, Nowicki, & Pietrzak, to facilitate crushing and grinding. A fraction particle size of
2014; He, Lin, Long, Liang, & Chen, 2015; Sebben & Pendleton, between 0.5 and 1 mm was used for the preparation of activated
2015). carbons by impregnation with ZnCl2 and KOH. The precursor
In this study, porous activated carbon-based materials was impregnated with a chemical activating agent in a solid form.
obtained from date stones (seeds) are potential adsorbents for The impregnated precursor was carbonized in a horizontal tubu-
l-phenylalanine amino acid. This is mainly due to their physi- lar furnace under nitrogen flow with a heating rate of 5 ◦ C min−1 ,
cal and chemical characteristics such as highly developed porous to allow free evolution of volatiles, up to the hold temperature for
structure, good thermal stability, low cost and more accessibility. 1 h. The resulting activated carbon was immersed in HCl solu-
Date stones are among the most common agricultural by prod- tion (0.1 mol L−1 ) under reflux ebullition (3 h) in order to extract
ucts available in palms growing in the Mediterranean countries the compound formed and reagent excess. Then, the solution
like Algeria, which is one of the largest producers in the world. was filtered and the black solid was washed with hot distilled
Algeria produces more than 400 different varieties of dates with water until the test with AgNO3 became negative. The adsor-
an annual production of about 400,000 tons (Chandrasekaran bent was dried at 120 ◦ C, and kept in tightly closed bottles until
& Bahkali, 2013). Date stones constitute roughly 10% of the use. The activated carbons were named ACZ (1 g ZnCl2 : 1 g
date weight and this lignocellulosic-based agricultural waste is date stones, activated at 600 ◦ C), ACK (9 mmol KOH: 1 g date
a good precursor for preparing activated carbon because of its stones, activated at 800 ◦ C).
excellent natural structure and low ash content (Bouchenafa-
Saib, Grange, Verhasselt, Addoun, & Dubois, 2005; Merzougui 2.3. Characterization
& Addoun, 2008). As it is well known, two methods are com-
monly used for the preparation of activated carbon: physical and The specific surface area and pore structure of the activated
chemical activations. Compared with the physical process, the carbons were characterized by nitrogen adsorption-desorption
chemical activation presents some advantages like low activa- isotherms at −196 ◦ C using the ASAP 2010 Micromeritics
tion temperature, short activation time, high surface area, well equipment. All the activated carbons were outgassed at 150 ◦ C
developed microporosity of activated carbon, simple operation overnight. The specific surface area was calculated by the
and low energy consumption (Deng, Yang, Tao, & Dai, 2009; Brunauer–Emmett–Teller (BET) equation (Brunauer, Emmett,
Pereira et al., 2014). Therefore, the date stones can be activated & Teller, 1938). The external surface area, micropore area and
with chemical agents such as KOH, ZnCl2 , H3 PO4 , K2 CO3 and micropore volume were calculated by the t-plot method. The
NaOH, to obtain activated carbons with well-developed textural total pore volume was evaluated from the liquid volume of N2
characteristics. To the best of our knowledge, the use of acti- at a high relative pressure near unity 0.99 (Guo & Lua, 2000).
vated carbons for the l-phenylalanine recovery from aqueous The mesopore volume was calculated by subtracting the micro-
solutions by adsorbents based on date stones are not available pore volume from the total volume. The pore size distribution
in the open literature. Thus, the principal objective of this work (PSD) was determined using the density functional theory (DFT)
was to prepare porous activated carbons with high surface areas model. The morphology of activated carbons was visualized by
from date stones by chemical activation with KOH and ZnCl2 . scanning electron microscopy (SEM) using a Philips XL 30
The activated carbons proved to be good candidates for the equipped with an energy dispersive spectrometer (EDS). The
adsorption of l-phenylalanine in an aqueous medium. Fourier transform infrared (FT-IR) spectroscopy was used to
determine the functional groups of the activated carbons; the
2. Materials and methods spectra were recorded over the range (400–4000 cm−1 ) on a
Perkin-Elmer spectrum two spectrometer using KBr pellets.
2.1. Materials
2.4. Determination of zero point charge pHPZC
The date stones used in this study were from Algerian
origin. The following reagents were used: l-phenylalanine The determination of the point of zero charge (pHPZC ) was
standard (>98%, Fluka, France), potassium hydroxide (>98%, conducted to investigate how the surface charge of ACK and
Sigma Aldrich, USA), zinc chloride (>98%, Sigma Aldrich, ACZ adsorbents depends on pH. pHPZC of the activated car-
USA), KH2 PO4 (>99%, Fluka, France), K2 HPO4 (>99%, Fluka, bons was determined using the procedure described elsewhere
356 B. Belhamdi et al. / Journal of Applied Research and Technology 14 (2016) 354–366

Fig. 1. SEM images of ACZ (A) and ACK (B) before adsorption.

(Prahas, Kartika, Indraswati, & Ismadji, 2008): 0.01 M of NaCl where C0 and Ce are the initial and equilibrium l-phenylalanine
was prepared and the initial pH was adjusted between 2 and 12 amino acid concentrations in the liquid phase (mg L−1 ), W (g)
using HCl or NaOH solution (0.1 M). 50 mL of NaCl solution the weight of adsorbent and V (L) the volume of solution.
was placed in Erlenmeyer flakes with 0.1 g of adsorbent. The
flasks were kept under agitation (150 rpm, 48 h), and the final
2.6. Desorption study
pH of the solution was measured. The intersection point of the
curves pHfinal vs. pHinitial and the bisector was taken as pHPZC .
Desorption of l-phenylalanine was investigated in order to
explore the regeneration and recycling ability of the two acti-
vated carbons. For this, 50 mg of ACK and ACZ were mixed
2.5. Adsorption experiments with 50 mL of l-phenylalanine solution at a saturated concentra-
tion, and stirred at 150 rpm at optimum adsorption temperature
The batch adsorption experiments were performed in 100 mL (20 ◦ C) for 300 min. The amount of adsorbed amino acid was
Erlenmeyer flasks containing a mass of adsorbent mixed with a determined by the same equation used in the adsorption exper-
known volume of l-phenylalanine solution (200 mg L−1 ) under iments (see Section 2.5). Thereafter, ACK and ACZ were
agitation (150 rpm). The effect of the contact time was stud- washed with ultrapure water until the residual concentration
ied to determine the time required for equilibrium at natural of l-phenylalanine becomes negligible. The loaded activated
pH at 20 ◦ C. For the temperature effect, 50 mg of activated carbons were then allowed to be in contact with 50 mL of two
carbon was added to 50 mL of l-phenylalanine solutions with eluent solutions (M NaOH and M HCl, 10−2 M) for 300 min.
concentrations ranging from 50 to 1000 mg L−1 , prepared by The desorbed carbons were again subjected to the next batch in
dissolving appropriate amounts of l-phenylalanine in ultrapure order to check desorption and reusability of ACK and ACZ. The
water (18.2 M cm), the flasks were maintained under constant amount of desorbed amino acid was calculated from the con-
agitation at various temperatures (20, 25, 35 and 40 ◦ C) for centration of desorbed l-phenylalanine in liquid phase using
300 min. The effect of pH on the adsorption was performed by UV–vis spectrophotometer at 257 nm. The percentage of des-
mixing 50 mg of activated carbon into 50 mL of l-phenylalanine orbed l-phenylalanine from the activated carbons was calculated
solutions in the pH range (2.0–9.4), under constant agitation according to the following equation:
at 20 ◦ C for 300 min. The pH value of the l-phenylalanine
 
solutions was changed by using different buffer solutions (pH Mass desorbed
5.7–7.2 potassium phosphate buffer: pH 2.0 HCl–KCl buffer: pH Desorption (%) = × 100 (3)
Mass adsorbed
9.4 bicarbonate buffer). The concentrations of l-phenylalanine
remaining in the supernatant solutions were filtered using a
hydrophilic syringe filter with a pore size of 0.45 um. The 3. Results and discussion
adsorbed amount was determined using a UV–vis spectropho-
tometer at 257 nm. (The adsorption capacity) The equilibrium 3.1. Characterization of activated carbons
adsorption capacity per unit mass of activated carbons qe
(mg g−1 ) and the removal percentage of the l-phenylalanine η 3.1.1. Scanning electron microscopy (SEM)
(%) were calculated from the following equation: The SEM micrographs show the effects of ZnCl2 and KOH
on the surface pore structures of the activated carbons (Fig. 1).
The external morphology shows more or less homogeneous cav-
(C0 − Ce )V
qe = (1) ities on the surfaces of ACK and ACZ. These cavities resulted
W from the evaporation of chemical agents during the activation
process, leaving space previously occupied by KOH and ZnCl2 .
(C0 − Ce ) Figure 1 suggests that the large pores on the surface are con-
η= × 100 (2)
C0 nected to a whole network of smaller pores inside the activated
 B. Belhamdi et al. / Journal of Applied Research and Technology 14 (2016) 354–366 357

Table 1
Textural characteristics of ACK and ACZ.
Adsorbent Surface area (m2 g−1 ) Pore volume (cm3 g−1 ) DFT pore size (nm)

SBET Sext Smic VT Vmes Vmic

ACK 1209 276 933 0.550 0.180 0.370 1.48

ACZ 1235 525 710 0.630 0.341 0.289 1.59

500 0.020
ACZ
ACK

Incremental pore volume (cm3 g–1)

0.016
Volume adsorbed (cm3 g-1)

400

0.012

300
0.008

200 Adsorption isotherm ACZ 0.004

Desorption isotherm ACZ
Adsorption isotherm ACK
Desorption isotherm ACK 0.000
100 0 10 20 30 40 50 60 70 80 90 100 110
0.0 0.2 0.4 0.6 0.8 1.0
Pore diameter (nm)
Relative presure (p/p0)
Fig. 3. DFT pore size distribution for ACK and ACZ samples.
Fig. 2. Nitrogen adsorption–desorption isotherms of ACK and ACZ samples at
−196 ◦ C.
Demiral, 2009; Zhu, Wang, Peng, Yang, & Yan, 2014). The pore
size distribution is an important property in the adsorption mech-
carbon. Therefore, the ACK and ACZ have great potential as anism because the adsorption of molecules of different sizes and
good adsorbents for l-phenylalanine. shapes is directly related to the pore size of adsorbents. Accord-
ing to the classification adopted by IUPAC, adsorbent pores
3.1.2. Nitrogen sorption are classified as micropores (<2 nm), mesopores (2–50 nm) and
The N2 adsorption–desorption isotherms of activated car- macropores (>50 nm). Figure 3 shows the pore size distribution
bons ACK and ACZ are given in Figure 2. The isotherms for the activated carbons, which clearly indicates that the pore
are type I, according to the IUPAC classification, assigned to diameter is in the micropore range. It is important to mention
microporous materials, and they present at a very low rela- that a l-phenylalanine molecule is relatively small with a size
tive pressure (P/P0 < 0.2) a significant increase of N2 adsorption of 0.7 nm × 0.5 nm × 0.5 nm (Alves et al., 2013b; Long et al.,
corresponding to the micropores filling (Fig. 2). However, the 2009). Therefore, such micropores with a size of (<2 nm) are
amount of adsorbed nitrogen is reduced at higher pressures, sug- accessible for l-phenylalanine molecules.
gesting the development of both micro and meso-porosity in
ACK and ACZ. The presence of hysteresis loops indicates that 3.1.3. Infrared spectroscopy (FT-IR)
some mesoporosity starts to be developed by capillary conden- The FT-IR is an important technique to qualitatively deter-
sation. The textural parameters of activated carbons determined minate the characteristic functional groups of the adsorbents.
from nitrogen adsorption-desorption are gathered in Table 1. It The spectra of porous activated carbons (Fig. 4) show multiple
can be concluded that the activation of date stones by ZnCl2 functions which can also be observed in other carbons activated
and KOH leads to active coals of a well-developed surface by KOH and ZnCl2 (Huang, Ma, & Zhao, 2015; Lua & Yang,
area and a high pore volume (Table 1). Among the activated 2005; Saka, 2012). The FT-IR analysis indicates that ACK and
carbons, ACZ exhibits the highest surface area (1235 m2 g−1 ) ACZ exhibit a similar shape and the same functional groups.
and pore volume (0.63 cm3 g−1 ) while ACK has a smaller sur- The broad band in the range (3000–3500 cm−1 ) is ascribed
face area (1209 m2 g−1 ) and pore volume (0.55 cm3 g−1 ). As to the O–H stretching mode of hydroxyl groups with hydro-
shown in Table 1, the activated carbon prepared by KOH is gen bending of adsorbed water. Bands (2900–2950 cm−1 ) are
essentially microporous, with 77% of its surface area. Simi- assigned to asymmetric and symmetric stretching vibrations of
lar trends have been found for the influence of the chemical aliphatic bond –CH, –CH2 and –CH3 while the bands around
activating agent on the development of the surface area and 1580 cm−1 may be due to the presence of aromatic C C ring
pore volume of activated carbons obtained through ZnCl2 and stretching vibration. The band at 1400 cm−1 is associated with
KOH activation of other lignocellulosic materials (Angin, 2014; –COO– asymmetric vibration of carboxylic groups while that at
Bagheri & Abedi, 2009; Foo & Hameed, 2011; Sreńscek-Nazzal, 1384 cm−1 is due to stretching vibration of –CH3 group. Finally,
Kamińska, Michalkiewicz, & Koren, 2013; Yorgun, Vural, & the vibration band centered at 1115 cm−1 is attributed to C–O
358 B. Belhamdi et al. / Journal of Applied Research and Technology 14 (2016) 354–366

120
ACK

100
Transmittance, %

80

qe (mg g–1)
ACZ 60

40
ACZ 20 °C
ACZ 25 °C
20
ACZ 35 °C
4000 3500 3000 2500 2000 1500 1000 500 ACZ 40 °C
Wavenumber (cm–1) 0
0 100 200 300 400 500 600 700 800

Fig. 4. The FTIR spectra of ACK and ACZ samples before adsorption in the Ce (mg L–1)
range 4000–500 cm−1 .
200

140

160
120

qe (mg g–1)
100 120
qt (mg g–1)

80
80

60
ACK 20 °C
40 ACK 25 °C
40
ACK 35 °C
ACK 40 °C
20
ACK 0
ACZ 0 100 200 300 400 500 600 700
0
Ce (mg L–1)
0 60 120 180 240 300 360

t (min) Fig. 6. Effect of temperature on l-phenylalanine adsorption onto ACK and ACZ
(adsorbent dose = 1 g L−1 , agitation speed = 150 rpm, contact time = 300 min,
Fig. 5. Effect of contact time on the l-phenylalanine adsorption onto ACK and
natural pH).
ACZ (l-phenylalanine concentration = 200 mg L−1 , adsorbent dose = 1 g L−1 ,
agitation speed = 150 rpm, contact time = 300 min, temperature = 20 ◦ C, natural
pH). (68 mg g−1 ) (Fig. 5). Comparing the results obtained in this
work (porous activated carbons) with mesoporous materials,
stretching vibrations, as in alcohols, phenols, acids, ethers or such as the SBA-3 mesoporous silica tested elsewhere
esters. The presence of the functional groups such as carboxyl (Goscianska, Olejnik, & Pietrzak, 2013a), it can be concluded
and hydroxyl are potential adsorption sites for l-phenylalanine that the mesoporous materials need a much longer period to
amino acid. reach the equilibrium; in this case the pore size distribution of
ACK and ACZ, centered at 1.48 and 1.59 nm, respectively, are
3.2. Adsorption studies beneficial for the adsorption because l-phenylalanine molecules
have easy access to the pores.
3.2.1. Effect of contact time
The contact time is a fundamental parameter in any trans- 3.2.2. Temperature effect
fer phenomena such as adsorption. The equilibrium adsorption The temperature has a direct influence on the adsorption
capacity of l-phenylalanine on activated carbon was investigated of amino acids. Figure 6 shows the temperature effect on the
to determine the time required to reach the equilibrium between adsorption of l-phenylalanine by activated carbons. The same
adsorbents (50 mg) and l-phenylalanine solution, (200 mg L−1 ) behavior is observed for ACK and ACZ and the isotherms plot-
(Fig. 5); it can be observed that the adsorption capacities of ted at various temperatures show that the equilibrium adsorption
activated carbons gradually increase with the contact time and capacity decreases with increasing temperature from 20 to 40 ◦ C,
does not stop until an equilibrium state is reached (180 min). No indicating that the adsorption of l-phenylalanine is of exother-
obvious variation in the amount of adsorbed l-phenylalanine mic nature. These results show that the decrease of adsorption
was observed; the adsorbed mass at equilibrium reflects the at a high temperature can be ascribed to the greater tendency
maximum adsorption capacity of the activated carbons under of l-phenylalanine molecules to form hydrophobic bonds in
the operating conditions; the equilibrium adsorption capacity an aqueous medium, thus hindering their hydrophobic interac-
of l-phenylalanine on ACK (114 mg g−1 ) is higher than ACZ tions with the adsorbent surface (El Shafei & Moussa, 2001).
 B. Belhamdi et al. / Journal of Applied Research and Technology 14 (2016) 354–366 359

120 14

ACK pH PZC =6.80

100 12 ACK pH PZC =6.18
Bisector
10
80
qe (mg g–1)

Final pH
60

6
40
ACZ pH 2.0 4
ACZ pH 5.7
20 ACZ pH 7.2
2
ACZ pH 9.4
0
0
0 100 200 300 400 500 600 700 800
0 2 4 6 8 10 12 14
Ce (mg L–1) Initial pH

200 Fig. 8. Determination of the pH of zero point charge (PZC) of ACZ and ACK.
180

160

140 of the activated carbon is positively charged below pHpzc and

120
negatively charged above pHpzc. The maximum adsorption
qe (mg g–1)

capacity of both activated carbons was obtained at pH 5.7, which

100
is close to the isoelectric point (PI = 5.48) of l-phenylalanine.
80
The latter is known as a Zwitterion containing both amine
60
ACK pH 2.0 and carboxylic groups, near to the isoelectric point and
40 ACK pH 5.7 presents both negative and positive charges (Jiao et al., 2012).
20
ACK pH 7.2 Therefore, the Coulomb repulsive interaction between the l-
ACK pH 9.4
phenylalanine molecules is almost negligible. Furthermore, the
0
0 100 200 300 400 500 600 700 800 strong hydrophobic interactions amino acid/adsorbents, and
Ce (mg L–1) intra-molecular interaction between amino acid molecules are
responsible of close packing of l-phenylalanine in the micro-
Fig. 7. Effect of pH on l-phenylalanine adsorption onto ACK and ACZ pores of adsorbents, leading to the highest adsorption capacity
(adsorbent dose = 1 g L−1 , agitation speed = 150 rpm, contact time = 300 min, at this pH. At low pH (<2), the surface charge of the activated
temperature = 20 ◦ C).
carbon is negative and the cationic form of the amino acid is
On the other hand, the decreased adsorption at equilibrium is not favorable for the adsorption because of the electrostatic
due to decreased surface activity at higher temperatures. The repulsion; this explains the decrease in the adsorption efficiency.
best uptake of l-phenylalanine was obtained at 20 ◦ C which is Figure 7 also shows that the adsorbed amount of l-phenylalanine
selected as an optimal adsorption temperature and will be used decreases with increasing pH from 5.7 to 9.4 and this can be
for further experiments. explained by the strong electrostatic repulsion adsorbent/amino
acid molecules, negatively charged. This effect is similar to
3.2.3. Effect of pH that reported previously (Alves et al., 2013b; Goscianska et al.,
pH is known to be a crucial parameter that affects the adsorp- 2013c); as consequence, the adsorption of l-phenylalanine is
tion behavior at water–solid interfaces. Its effect on the amino inhibited above and below pH 5.7. Based on the experimental
acid adsorption on ACK and ACZ at different buffer solutions results, pH 5.7 was selected as an optimum value.
ranging from 2.0 to 9.4 at 20 ◦ C is illustrated in Figure 7. All
equilibrium isotherms are of the L-type (Langmuir isotherms)
and the amount of adsorbed l-phenylalanine increases with rais- 3.2.4. Adsorption kinetic study
ing the initial concentration. At a low concentration, the amino Several kinetic models were proposed to understand the
acid is randomly deposited on the adsorbent, and the fast uptake behavior of adsorbents and to study the mechanisms control-
can be attributed to a large number of empty sites on the surface ling the adsorption. In this study, the experimental data of
(Goscianska et al., 2014). In contrast, at high concentrations, the l-phenylalanine adsorption are examined using a pseudo-first
nonpolar groups of amino acids are close to each other until they and pseudo-second order kinetic model. The pseudo first-order
touch inside their van der Waals radii, leading to a dense packing kinetic model is expressed in its linear form by the following
of molecules on the active sites of the adsorbent. The removal of equation (He et al., 2015):
l-phenylalanine from an aqueous solution is strongly depend-
ent on pH (Fig. 7) and this can be explained by pHpzc. pHPZC
(Fig. 8) is found to be 6.18 (ACZ) and 6.80 (ACK). The surface ln(qe − qt ) = ln qe − k1 t (4)
360 B. Belhamdi et al. / Journal of Applied Research and Technology 14 (2016) 354–366

Table 2 6
Kinetic parameters for l-phenylalanine adsorption on ACK and ACZ.
ACK pH 2.0
Model Parameters Adsorbent 5 ACK pH 5.7
ACK pH 7.2
ACK ACZ ACK pH 9.4
4
(mg g−1 )
qe,exp 114.36 68.03
η (%) 56.89 33.85
qe,cal (mg g−1 ) 3

Ce/qe
Pseudo-first order 62.49 45.01
k1 (min−1 ) 0.0194 0.0205
R2 0.982 0.995 2
q (%) 30.63 23.00
Pseudo-second order qe,cal (mg g−1 ) 119.05 72.46
k2 (g mg−1 min−1 )
1
0.0007 0.0009
R2 0.999 0.999
q (%) 2.65 4.00 0
0 100 200 300 400 500 600 700 800
Ce (mg L–1)
While the pseudo second-order kinetic model is based on the
12
assumption of a chemisorption of the adsorbate on the adsorbent
(He et al., 2015): ACZ pH 2.0
10 ACZ pH 5.7
t 1 t ACZ pH 7.2
= + (5)
qt k2 qe2 qe 8
ACZ pH 9.4

where qt and qe (mg g−1 ) are the amount of amino acid adsorbed
Ce/qe

at time t (min) and at equilibrium, respectively, k1 (min−1 ) and 6

k2 (g mg−1 min−1 ) are the rate constant of the pseudo first order
6
and pseudo second order adsorption models.
The best fit was validated on the base of the correlation
coefficient (R2 ), the difference between the experimental and 2

theoretical adsorption capacities and the normalized standard

deviation q (%) (Sen Gupta & Bhattacharyya, 2011): 0
0 100 200 300 400 500 600 700 800

 Ce (mg L–1)
[(qe,exp − qe,cal )/qe,exp ]2
q(%) = 100 (6)
n−1 Fig. 9. Langmuir isotherm for adsorption of l-phenylalanine on ACK and ACZ
at different values of pH (adsorbent dose = 1 g L−1 , agitation speed = 150 rpm,
where n is the number of data points, qe,exp and qe,cal are the contact time = 300 min, temperature = 20 ◦ C).
experimental and calculated equilibrium adsorption capacity
values (mg g−1 ), respectively.
The aim of this kinetic study was to find the appropriate model the pH range (2–9.4) at the optimal temperature of 20 ◦ C. The
that better describes the experimental data and that determines curves were fitted by the most used models namely the Lang-
the kinetic parameters of the mass transfer of l-phenylalanine. muir and Freundlich ones. The Langmuir model is based on the
The data were examined by using Eqs. (4)–(6). The calculated assumption that the maximum adsorption corresponds to a satu-
kinetic parameters for l-phenylalanine adsorption on activated rated monolayer of adsorbate molecules on homogeneous sites
carbons are gathered in Table 2. The adsorption capacities cal- of adsorbent with a constant energy, and no interaction between
culated from the pseudo second-order model are very close adsorbed species. The linear form is expressed as follows (Yang,
to the experimental ones as evidenced from the low values Yu, & Chen, 2015):
q (%) and high correlation coefficient (R2 > 0.99). Although
the R2 values for the pseudo first-order model are all above Ce 1 Ce
= + (7)
0.98 (Table 2), the eminent variances (the relative error of l- qe qmax KL qmax
phenylalanine onto ACK and ACZ are 45.35% and 33.84%,
respectively) between the experimental and calculated adsorp- where qmax and qe are the equilibrium and maximum adsorption
tion capacities reflect the poor fitting of the pseudo first-order capacities (mg g−1 ), respectively; KL , the Langmuir constant
model. Hence, l-phenylalanine adsorption kinetics on both ACZ related to the affinity of the binding sites (L mg−1 ); and Ce , the
and ACK is well described by the pseudo second-order kinetics. equilibrium concentration of adsorbate in an aqueous phases
(mg L−1 ). The values of qmax and KL are calculated from the
3.2.5. Adsorption isotherms slopes (1/qmax ) and intercept (1/qmax KL ) of the linear plots of
The adsorption isotherm is generally applied to analyze the (Ce /qe ) vs. Ce (Fig. 9).
experimental data at equilibrium. The isotherms were further The main characteristics of the Langmuir isotherm can be
investigated by performing batch adsorption experiments over expressed by a dimensionless separation factor, RL , which is
 B. Belhamdi et al. / Journal of Applied Research and Technology 14 (2016) 354–366 361

defined by the following formula (Liu, Zheng, Wang, Jiang, & 5.5

Li, 2010): ACK pH 2.0

ACK pH 5.7
1
RL = (8) 5.0
ACK pH 7.2
1 + KL C 0 ACK pH 9.4

The RL value indicates the possibility of the adsorption pro-

cess being favorable (0 < RL < 1), unfavorable (R1 > 1), linear 4.5

Inqe
(RL = 1), or irreversible (RL = 0).
The Freundlich isotherm model is based on heterogeneous
adsorbent surface (Yang et al., 2015): 4.0

1
ln qe = ln KF + ln Ce (9)
n
3.5
The linear plot of lnqe vs. lnCe (Fig. 10) enables the determi- 2 3 4 5 6 7
nation of the Freundlich constants KF and n from the intercept lnce
and slope, respectively.
5.0
The isotherm parameters of l-phenylalanine are calculated
ACZ pH 2.0
from the Langmuir and Freundlich models (Table 3), all the ACZ pH 5.7
R2 values of the Langmuir model are greater than 0.99. These ACZ pH 7.2
4.5
values are much higher than those of the Freundlich model, ACZ pH 9.4
whatever the pH, suggesting the applicability of the Langmuir
model which reveals a monolayer coverage of l-phenylalanine
4.0
on homogeneous sites for both adsorbents. The RL values are Inqe

between 0 and 1, indicating that the l-phenylalanine adsorption

on ACK and ACZ is favorable under the operating conditions.
3.5
As the pH increases from 2.0 to 9.4, qmax shows a significant
decrease, reflecting that the adsorption is more favorable at pH
5.7, which is close to the isoelectric point (PI = 5.48). ACK
3.0
exhibits a maximum monolayer adsorption of l-phenylalanine 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0
(188.3 mg g−1 ) compared to ACZ (133.3 mg g−1 ) due to its lnce
largest microporous surface area. As the pore size of ACK is
1.48 nm, most pores including micropores are easily accessible Fig. 10. Freundlich isotherm for adsorption of l-phenylalanine on ACK
to l-phenylalanine with a size of 0.7 × 0.5 × 0.5 nm3 . Conse- and ACZ at different values of pH (adsorbent dose = 1 g L−1 , agitation
speed = 150 rpm, contact time = 300 min, temperature = 20 ◦ C).
quently, the microporous surface area plays an important role for
determining the adsorption capacity of the small biomolecule l-
phenylalanine. The maximum adsorption capacities of ACK and
ACZ for l-phenylalanine are compared to those values reported

Table 3
Isotherm parameters for l-phenylalanine adsorption on ACK and ACZ.
Model Adsorbent Parameters pH

2.0 5.7 7.2 9.4

Langmuir ACK qe,exp (mg g−1 ) 144.07 176.01 154.68 122.66
qmax (mg g−1 ) 151.97 188.32 170.06 137.74
kL (L mg−1 ) 0.0246 0.0262 0.0181 0.0146
RL 0.0483 0.0455 0.0646 0.0789
R2 0.999 0.998 0.998 0.993
ACZ qe,exp (mg g−1 ) 84.01 107.45 96.94 75.65
qmax (mg g−1 ) 99.40 133.33 119.33 91.99
kL (L mg−1 ) 0.0076 0.0063 0.0065 0.0073
RL 0.1412 0.1655 0.1613 0.1462
R2 0.988 0.993 0.993 0.997
Freundlich ACK kF 37.738 43.613 27.222 26.016
n 4.667 4.364 3.520 3.986
R2 0.963 0.952 0.911 0.921
ACZ kF 8.874 7.927 7.667 6.191
n 2.860 2.443 2.516 2.529
R2 0.978 0.985 0.980 0.944
362 B. Belhamdi et al. / Journal of Applied Research and Technology 14 (2016) 354–366

Table 4 6.0
Comparison of the maximum adsorption capacities (Qm ) of various adsorbents ACK
for l-phenylalanine. 5.8
ACZ

Adsorbent Qm (mg g−1 ) Reference

5.6
Polymeric adsorbent 115.6 (Grzegorczyk and Carta, 1996)
Polymeric resins 65.9–100.8 (Díez et al., 1998) 5.4

ln(ρKC)
Carbonated calcium 44.0 (Bihi et al., 2002)
phosphates 5.2
NAZSM-5 zeolite 41.3 (Titus et al., 2003)
Commercial activated 100.0 (Garnier et al., 2007) 5.0
carbon
Organic-inorganic 1.2 (Wu et al., 2009) 4.8
hybrid membranes
Spherical carbon 66.1 (Long et al., 2009) 4.6
aerogels 3.15 3.20 3.25 3.30 3.35 3.40 3.45
Macroporous resins 12.8–84.0 (Mei et al., 2009) 1000/T(K–1)
Activated defective 69.5 (Clark et al., 2012)
coffee beans Fig. 11. Regressions of Van’t Hoff for thermodynamic parameters of l-
Calcined 46.4 (Jiao et al., 2012) phenylalanine adsorption on ACK and ACZ.
CuZnAl-CO3
layered double
hydroxides
1/T, respectively (Fig. 11 and Table 5). The negative enthalpies
Activated corn cobs 109.2 (Alves et al., 2013a, 2013b)
Mesoporous materials 0.27–0.30 (Goscianska et al., 2013b) (H◦ ) indicate the exothermic nature of the adsorption, in agree-
CSBA-15 , CSBA-16 ment with the temperature effect study (see Section 3.2.2). It
and CKIT-6 has been reported that H◦ is in the range (2.1–20.9 kJ mol−1 ),
Mesoporous silica 36.0–69.0 (Goscianska et al., 2013a) indicating a physisorption (Liu, 2009). H◦ (−6.947 and
Mesoporous carbon 273.0 (Goscianska et al., 2014)
−7.153 kJ mol−1 for ACK and ACZ, respectively) showed a
CMK-3
Multi-walled carbon 233.0 (Goscianska et al., 2014) physisorption of l-phenylalanine, with weak interactions while
nanotubes CNTs the negative free enthalpy (G◦ ) indicates the spontaneous
Activated carbon 188.3 This study nature of l-phenylalanine uptake over the studied temperature
ACK range. The variation of G◦ for physisorption is in the range
Activated carbon ACZ 133.3 This study
(0–20.9 kJ mol−1 ), whereas this energy ranges from 80 to
200 kJ mol−1 for a chemisorption (Liu, 2009). In our case,
in the literature for other adsorbents (Table 4). In comparison G◦ (Table 5) is characteristic of a physical adsorption.
with various adsorbents, our activated carbons ACK and ACZ The entropy (S◦ ) is used to describe the randomness at
have high adsorption capacities and can be considered as effec- the solid–solution interface during the recovery process. The
tive adsorbents for the recovery of l-phenylalanine from aqueous positive values of S◦ demonstrate an increase in random-
solutions. ness during the adsorption of l-phenylalanine on ACK and
ACZ.
3.2.6. Adsorption thermodynamics
A thermodynamic study was performed for the determi- 3.3. Proposed mechanism of adsorption
nation of the free energy change (G◦ ), entropy (S◦ ) and
enthalpy (H◦ ). A previous study of the temperature effect To further understand the adsorption behavior and select
on l-phenylalanine adsorption over ACZ and ACK enabled us a desorption approach, the adsorption mechanism of l-
to determine the thermodynamic parameters at 20–40 ◦ C and phenylalanine amino acid was discussed. The adsorption
800 mg L−1 . They were calculated from the following equations mechanisms occurred mainly because of the hydrogen bonding
(Bouguettoucha, Reffas, Chebli, Mekhalif, & Amrane, 2016; formation, hydrophobic and electrostatic interactions of amino
Milonjic, 2007): acid molecules with the activated carbons surface. The main
qe active sites for binding of l-phenylalanine by the activated car-
KC = (10) bons are the hydroxyl and carboxyl groups on the surface of ACK
Ce
and ACZ, which react with polar molecules and various func-
ΔG0 = −RT ln(ρKC ) (11) tional groups. The surface of porous activated carbon can include
electrically charged groups (ACZ/ACK surface –OH2 + : below
ΔS 0 ΔH 0
ln(ρKC ) = − (12) pHpzc) and (ACZ/ACK surface –O− : above pHpzc), electrically
R RT neutral groups (ACZ/ACK surface –OH: near pHpzc). l-
where KC is the equilibrium constant (L g−1 ), T the absolute tem- phenylalanine amino acid has dissociation constants (pK1 = 1.83
perature (K), R the universal gas constant (8.314 J mol−1 K−1 ) and pK2 = 9.13) and isoelectric point (PI = 5.48) (Jiao et al.,
and ρ the density of water (g L−1 ). H◦ and S◦ are calcu- 2012). The molecule is positively charged (+ NH3 –R–COOH) for
lated from the slope and intercept of the plots of ln (ρKC ) vs. pH < PI and negatively charged (NH2 –R–COO− ) for pH > PI,
 B. Belhamdi et al. / Journal of Applied Research and Technology 14 (2016) 354–366 363

Table 5
Thermodynamic parameters for l-phenylalanine adsorption on ACZ and ACK.
Adsorbent H◦ (kJ mol−1 ) S◦ (kJ mol−1 K−1 ) G◦ (kJ mol−1 )

293 K 298 K 303 K 313 K

ACK −6.947 0.019 −13.820 −13.930 −14.184 −14.280

ACZ −7.153 0.017 −12.300 −12.418 −12.604 −12.646

Hydrogen bending Electrostatic

repulsion force
Electrostatic
repulsion force O
NH2
OH
O OH
O H3N+
O–
O– H2N OH H2O+
OH
Activated carbon
surface

π−π Interaction

pH above 5.7 and pHPZC pH=5.7 ≈PI pH below 5.7 and pHPZC

Fig. 12. Proposed adsorption mechanism of l-phenylalanine onto ACZ and ACK adsorbents.

and behaves in an aqueous medium as a dipolar zwitterion 3.4. Desorption behavior of l-phenylalanine from activated
(+ NH3 –R–COO− ) at pH–PI. In acid solution, the presence carbons
of H3 O+ ions in the surface of ACZ and ACK causes repul-
sion of protonated amino groups with the surface functional Regeneration and reuse of adsorbents for further cycles
groups, and thus lower the adsorption efficiency. In a basic is important from the economic perspective. Desorption of
solution, OH− ions present on the adsorbent surface compete l-phenylalanine from the activated carbons was evaluated
with anionic carboxylic groups for l-phenylalanine molecules using two different eluents: NaOH and HCl (0.01 M). The
(repulsion effect) and inhibit the adsorption. The highest uptake highest desorption was achieved in the NaOH solution with
of l-phenylalanine at pH 5.7 indicates a dominant hydropho- almost 95.7% for ACZ and 88.8% for CAK against 21 and
bic interaction with ␲–␲ type between the phenyl rings of 6.5% in the HCl solution. This may be due to the enhance-
amino acid molecules and graphene rings of the activated car- ment of the number of negatively charged sites at high pH
bons surface (Doulia, Rigas, & Gimouhopoulos, 2001; Rajesh, which increases the electrostatic repulsion, which liberates l-
Majumder, Mizuseki, & Kawazoe, 2009). Electrostatic attrac- phenylalanine from ACK and ACZ. To check the adsorption
tion between anionic carboxylic groups of l-phenylalanine efficiency, the desorbed ACK and ACZ were dried overnight and
molecules and OH− ions in the surface of activated car- subjected to a new adsorption/desorption cycle. During the sec-
bons also accounts for the increased adsorption. Additionally, ond cycle, the adsorption capacities obtained were 29.4 (ACK)
such strong bindings between l-phenylalanine and the adsor- and 23.5 mg g−1 (ACZ). A significant decay in the adsorp-
bent surface can be explained by the formation of hydrogen tion capacity of both activated carbons was observed and may
binding between oxygenated groups at the activated car- be attributed to the depletion of active sites of the adsorbents
bons surface and amino groups of l-phenylalanine. According being occupied by the amino acid. With the increase of the
to these results, the adsorption mechanism is proposed in repeated cycle, the rate of desorption was also greatly decreased
Figure 12. (Fig. 13).
364 B. Belhamdi et al. / Journal of Applied Research and Technology 14 (2016) 354–366

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