Separation Science and Technology

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Removal of azo dyes from aqueous solutions by

adsorption and electrosorption as monitored with
in-situ UV-visible spectroscopy

Elif Merve Sahin, Timur Tongur & Erol Ayranci

To cite this article: Elif Merve Sahin, Timur Tongur & Erol Ayranci (2019): Removal of azo dyes
from aqueous solutions by adsorption and electrosorption as monitored with in-situ UV-visible
spectroscopy, Separation Science and Technology, DOI: 10.1080/01496395.2019.1676786

To link to this article: https://doi.org/10.1080/01496395.2019.1676786

Published online: 18 Oct 2019.

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SEPARATION SCIENCE AND TECHNOLOGY
https://doi.org/10.1080/01496395.2019.1676786

Removal of azo dyes from aqueous solutions by adsorption and electrosorption

as monitored with in-situ UV-visible spectroscopy
Elif Merve Sahin, Timur Tongur, and Erol Ayranci
Department of Chemistry, Faculty of Science, Akdeniz University, Antalya, Turkey

ABSTRACT ARTICLE HISTORY

Removal of three azo dyes, Acid Orange 8, Acid Yellow 14 and Acid Red 151 from aqueous Received 1 July 2019
solutions was investigated by adsorption and electrosorption techniques. The processes were Accepted 2 October 2019
monitored by in-situ UV-visible absorbance measurements. Activated carbon cloth (ACC) was used KEYWORDS
as an adsorbent in open circuit adsorption studies and as a working electrode in electrosorption Azo dye; adsorption;
studies. Electrosorption was seen to enhance the removal of dyes both in amount and in speed electrosorption; activated
compared to adsorption. Kinetic data fitted to pseudo-second-order model better than pseudo- carbon cloth; in-situ UV-
first-order model. Isotherm data fitted to the Freundlich model better than the Langmuir model. visible spectroscopy

Introduction transferred from aqueous media onto solid adsorbent

materials. It is one of the most effective, low cost, easily
Water contamination and access to clean water
applicable, simple design, and nontoxic techniques for
resources are among the biggest problems that
wastewater treatment. It is preferred that the adsorbent
adversely affect the living conditions of all living crea-
material used in this technique has a high reactive sur-
tures. The rapid development of industrialization and
face area and is environmentally friendly. Activated
rise of the human population contribute to this envir-
carbon, as an adsorbent material, has a high specific
onmental problem. Dyes, which are the biggest pollu-
surface area in the range of 500–2000 m2 g−1. Its struc-
tants of water resources, are used in the textile, plastic,
ture has carbons with different degrees of saturation
leather, and paper industries. The wastes of these
and oxidation states. Its three-dimensional structure
industries are carcinogenic, mutagenic, toxic, and ter-
involves micropores of different sizes which make it
atogenic. They contain dissolved acidic, basic, and toxic
suitable for dye adsorption.[7] Therefore, activated car-
components which are dangerous for human health
bon is one of the most preferred adsorbents to remove
and aquatic life.[1,2] Textile industry uses more than
dyes from wastewater.[4,8] Activated carbons can be
10,000 different commercial dyes and pigments in the
found in granule, powder, fiber, or cloth forms. They
production process. Annual worldwide waste produc-
also have a widespread use in the applied electrochem-
tion of the textile industry is about 7 × 105 tons. During
istry because of their quasi-three dimensional structure,
the production and processing operations of this indus-
high conductivity, and low electrical resistance. Cloth
try, 12% of the dyes are lost and 20% of this loss is
and fibers are more advantageous in terms of mechan-
involved in the wastewater.[3,4] Due to their complex
ical integrity and ease of use.[9]
molecular structures with stable aromatic rings, dyes
Electrosorption is another technique for the removal
have very low biodegradability.[5]
of pollutants from solutions. It can be described as the
Nowadays, many biological, physical, and chemical
adsorption taking place on the surface of an electrode
techniques are being used to remove dyes from waste-
which is charged by applying a potential or current.
water. They involve aerobic and anaerobic degradation
Studies on electrosorption were initiated by the early
via microorganisms, coagulation, flocculation, and
works of Soffer and Folman[10], Ayranci and
membrane separation. Adsorption is another com-
Conway[11] and became very attractive in the last two
monly applied technique in which various absorbents
decades. The electrosorption process is very effective in
such as perlite, carbon nanotubes, clay, magnetic salts,
removing especially charged or polar pollutants and is
activated carbon, and fly ash are used to adsorb dye in
highly harmless as it involves no oxidation/reduction
the wastewater.[1,2,6] In this technique, pollutants are
reaction using hazardous chemicals. After the

CONTACT Erol AYRANCI eayranci@akdeniz.edu.tr Chemistry Department, Akdeniz University, Antalya, Turkey
© 2019 Taylor & Francis Group, LLC
2 E. M. SAHIN ET AL.

adsorption/electrosorption process is complete, deso- adsorption behavior. Adsorption of Acid Orange 8

rption of various ions and organic compounds from from aqueous solution onto modified/unmodified
the adsorbent surface is possible by applying opposite zeolites[19,20] investigation of adsorption of AR151
potential or current, leading to the renewal of the from aqueous solution onto modified bentonite and
adsorption capacity of the adsorbent.[9] hectorine[21,22]; adsorption of Acid Red 151 from aqu-
The majority of dyes in wastewater are azo dyes eous solution using magnetic nano-composite MnFe2
which show carcinogenic and toxic effects, just as O4, kaolin and MnFe2O4/kaolin particles[23]; adsorption
most other dyes. There are many studies carried out of Acid Red 151 onto activated carbon which was
in the literature for the removal of azo dyes from obtained from waste wood pellets[24] and investigation
wastewater. Hoda et al.[12] investigated kinetics and of kinetics and thermodynamics of adsorption of Acid
thermodynamics of adsorption of azo dyes Acid Blue Yellow 14 onto activated carbon[25] are among those
92 and Acid Blue 120 from aqueous solutions onto limited number of studies reported in literature about
activated carbon cloth. Metivier-Pignon et al.[13,14] stu- the adsorption of dyes which are under study in the
died adsorption kinetics and thermodynamics of some present work. In a series of work on capacitive
azo dyes onto activated carbon cloth from aqueous deionization[26–28], it was found that the process is
solutions. Li et al.[15] studied the production of cyclic effective for the removal of a few other azo dyes from
organics by the electrochemical oxidation of Orange II aqueous solutions. Adsorption/electrosorption pro-
azo dye and then deposition onto activated carbon cesses employed in the present work involve some
fiber. Wang et al.[16] succeeded the adsorption of Acid interesting features. One is the usage of very high
Red 14 with an electro-Fenton separator using an acti- specific surface area activated carbon cloth (ACC) as
vated carbon fiber cathode. Decomposition of an adsorbent in adsorption and as a working electrode
Amaranth azo dye in aqueous solution by electroche- in electrosorption. Another is the possibility of mon-
mical method under potentiostatic conditions using an itoring adsorption/electrosorption processes, while they
activated carbon fiber electrode was investigated by Fan are taking place, using in-situ UV-vis spectroscopic
et al.[17] Removal of Acid Orange 7 azo dye via adsorp- technique by which an enormous number of data
tion and electrosorption onto activated carbon fiber could be recorded during the processes compared to
and then desorption of the dye into the aqueous those obtained by classical batch type adsorption pro-
media were reported by Yahne et al.[18] All these studies cesses with ex-situ analysis.
have proven the adsorption and electrosorption pro-
cesses to be effective in removal of azo dyes from
wastewater. Materials and methods
The purpose of the present study is to investigate the
Chemicals and materials
adsorption/electrosorption behavior of azo dyes Acid
Orange 8 (AO8), Acid Red 151 (AR151), and Acid The dyes Acid Orange 8 (AO8), Acid Yellow 14
Yellow 14 (AY14). No study was found in the literature (AY14), and Acid Red 151 (AR151) were purchased
on electrosorption of these dyes while only a limited from Aldrich. The chemical structures of them are
number of studies could be found about their given in Fig. 1. Water used in the adsorption,

Acid orange 8 (AO8) Acid yellow 14 (AY14)

Acid red 151 (AR151)

Figure 1. Structures of the three azo dyes.

 SEPARATION SCIENCE AND TECHNOLOGY 3

electrosorption, and isotherm experiments was from

Millipore Mill-Q Direct Q-3 ultrapure water system.
The adsorbent ACC, having a code of Spectracarb
2225 was obtained from Spectra Corp. (MA, USA).
A deionization and cleaning procedure was employed
to ACC as described earlier.[12] Various properties of
ACC related to the use of it as an adsorbent in adsorption
studies and as an electrode in electrosorption studies
were determined in our earlier works.[29] These proper-
ties are given collectively in Table 1.

Adsorption and electrosorption of dyes on ACC and

in-situ optical absorbance measurements
The design of the cell in which adsorption/electrosorption
studies were carried out and the procedure followed in
these studies have been described in detail in earlier
works.[9,29] The specially designed V-shaped cell (Fig. 2)
was used to accomplish both the adsorption and the
electrosorption processes. Electrosorption process is
accomplished by connecting the electrodes to
a potentiostat (PalmSens Instruments Inc.). Furthermore,
the whole process of adsorption or electrosorption could
be monitored by in-situ UV-vis absorbance measurements
using Varian Carry 100 UV-vis spectrophotometer con-
taining the special adsorption/electrosorption cell in its Figure 2. Diagram of the adsorption/electrosorption cell. CE:
counter electrode, RE: reference electrode, WE: working elec-
sample compartment.[9] Such an adsorption/electrosorp- trode. N2 is bubbled for mixing and expulsion of dissolved CO2.
tion experimental design allowed the collection of many
data points, for example, once in every 2 min, throughout
the process without disturbing the system. This is consid- Determination of adsorption isotherms
ered to be the superiority of the design over those in Isotherms for the adsorption of three dyes onto ACC
classical batch adsorption experiments. ACC pieces of were derived by batch analysis.[30] ACC pieces with vary-
about 18 mg (approximately 0.5 × 1.5 cm) were used as ing masses from 4 to 22 mg were placed in dye solutions
an adsorbent or working electrode while a Pt plate acted as at an initial concentration of 5 × 10−5 M, temperature of
a counter electrode and a Ag/AgCl electrode (BAS, MF- 25°C, and for a duration of 48 h which was determined to
2030, analytical systems Inc., USA) acted as a reference be sufficient to ensure equilibration by preliminary tests.
electrode. One further important advantage of in-situ UV- The concentrations of the AO8, AY14, and AR151 dye
vis absorbance measurements is the possibility of scanning solutions at the end of the equilibrium period were
the whole wavelength range in the UV-vis region in each determined spectrophotometrically at 490 nm, 223 nm,
measurement using the scanning kinetic mode of the and 511 nm, respectively.
instrument. Any probable change in the structure of The amount of dye adsorbed per unit mass of ACC at
adsorbate during adsorption/electrosorption could easily equilibrium, qe, was determined from the following
be detected with such scans. equation;

VðCo  Ce Þ
qe ¼ (1)
Table 1. Properties of activated carbon cloth [29]. m
Specific surface area 1870 m2 g−1
Micropore volume 0.709 cm3 g−1 where V is the volume of dye solution in L, C0, and Ce
Carbon content 95.14%
Hydrogen content 0.37% are the initial and equilibrium concentrations, respec-
Oxygen content 4.49% tively, of dye solutions in mole per liter and m is the
pHPZC 7.4
weight of ACC.
4 E. M. SAHIN ET AL.

Results and discussion conditions alone can easily be seen from this figure. It
can also be seen that the order of affinity of the three dyes
Absorption characteristics of azo dyes
is AR151< AY14< AO8. The concentration of dyes in
The wavelengths of absorption maxima (λmax) for the solution decreased to 9.86 × 10−7 M, 7.07 × 10−6 M, and
azo dyes were determined from their UV-vis spectra 8.48 × 10−6 M for AO8, AY14, and AR151, respectively, at
and given in Table 2. Calibration data were obtained by the end of total adsorption period of 300 min.
measuring absorbances, A, of standard azo dye solu- In order to quantify kinetic findings, the collected
tions at different concentrations, C, at their λmax. Then, data were fitted to pseudo-first-order and pseudo-
these data were regressed linearly according to second-order kinetic models which can be formulated
Lambert–Beer law. The calibration equations and in linear forms as in Eqs. (2) and (3), respectively;
squared regression coefficients are included in Table
lnðqe  qt Þ ¼ lnðqe Þ  k1 t (2)
2. These calibration equations were used to convert
the absorbance data obtained in kinetic and isotherm 1 1 1
¼ 2
þ t (3)
studies into concentration. qt k2 qe qe
where qt and qe are the amounts of dye adsorbed per
Kinetics of adsorption/electrosorption unit mass of ACC at any time and at equilibrium,
respectively. qt is obtained using Eq. (1) after replacing
Open-circuit adsorption qe by qt and Ce by Ct which is the concentration of dye
In-situ absorbance measurements of dye solutions during in solution at any time, t. k1 and k2 are pseudo-first-
the course of open-circuit (OC) adsorption lead to the order and pseudo-second-order rate constants,
collection of kinetic data involving the change in dye respectively.
concentration with time after making the necessary absor- The plots of ln(qe-qt) vs t and 1/qt vs t are presented in
bance-concentration conversions. The initial dye concen- Fig. 4a–b, respectively. Linear regression of ln(qe-qt) vs
tration, the amount of ACC, and the duration of OC t data provides the parameter k1 as the slope according to
adsorption were 5 × 10−5 M, 18.0 ± 0.1 mg and 300 Eq. (2). Linear regression of 1/qt vs t data provides the
min, respectively, for all three dyes in order to make the parameter qe from the slope and then the parameter
comparison of adsorption kinetics easier. Concentration k2from the intercept according to Eq. (3). qe values can
vs time plots produced under these conditions are given also be estimated from the plots (as experimental value,
in Fig. 3. The affinity of dyes toward ACC in OC qe,exp) in Fig. 3 using concentration reached at equili-
brium (the plateau value). All these parameters are collec-
Table 2. Wavelengths of absorption maxima and calibration tively given in Table 3. Two criteria can be used in
data for azo dyes. deciding which model represents the kinetics of adsorp-
λmax Squared regression Calibration equation (A: tion better. One is the squared regression coefficient (r2)
Dye (nm) coefficient (r2) absorbance, C: concentration)
given in Table 3. It is well known that the closer the r2 to 1,
AO8 489 0.9986 A = 26968C + 0.0172
AY14 225 0.9979 A = 15128C–0.0248 the better is the fit. Although all the r2 values are very close
AR151 511 0.9956 A = 33937C–0.0067 to 1, the ones for the pseudo-second-order model are

4
C / (10-5 M)

0
0.00 100.00 200.00 300.00
t / min

Figure 3. Concentration vs time plots for the open-circuit adsorption of AO8 (•), AY14 (■), and AR151 (▲).
 SEPARATION SCIENCE AND TECHNOLOGY 5

-5 -5 -5
(a1) (a2) (a3)
-10

ln (qe-qt)
-10 -10

ln (qe-qt)
ln (qe-qt)
-15
-15 -15
-20

-25 -20 -20

0 200 400 0 200 400 0 200 400
time / min time / min time/ min

8.0E+06 6.0E+06 8.0E+06

(b1) (b2) (b3)
6.0E+06 6.0E+06
4.0E+06

1/qt
1/qt

4.0E+06

1/qt
4.0E+06
2.0E+06
2.0E+06 2.0E+06

0.0E+00 0.0E+00 0.0E+00

0 200 400 0 200 400 0 200 400
time / min time / min time / min

Figure 4. Plots of ln(qe-qt) vs t for pseudo-first-order treatment of adsorption data for AO8 (a1), AY14 (a2), AR151 (a3), and 1/qt vs
t for pseudo-second-order treatment of adsorption data for AO8 (b1), AY14 (b2), AR151 (b3). Straight lines result from the regression
analysis of experimental data points shown by thicker lines.

slightly higher than those for pseudo-first-order. model are all lower than 5. So, it can be concluded that
The second criterion, which is considered to be a better the kinetics of OC adsorption is better represented by
criterion[31], is normalized percent deviation given by: the pseudo-second-order model for all three dyes. This



conclusion is further supported by the closeness of
P ¼ ð100=N Þ Σ qt;exp  qt;pred = qt;exp (4) experimental qe values (2nd column for adsorption in
Table 3) and qe values calculated according to pseudo-
second-order treatment (9th column for adsorption in
where N is the number of data pairs, qt,exp and qt,pred Table 3). It is to be noted that many of the dye adsorp-
are the qt values determined experimentally using Eq. tion processes were also found to follow pseudo-second
(1) and predicted from the model using Eq. (2) or Eq. -order kinetics in literature.[33–35]
(3), respectively. The P values obtained for both models OC adsorption results in Fig. 3 and in Table 3 can be
are included in Table 3. The P values for the pseudo- explained in terms of interactions between dyes and ACC.
second-order model are much lower than those for The order of affinity of the three dyes toward ACC was
pseudo-first-order model for all three dyes. determined above (AR151< AY14< AO8) by visual ana-
A generally accepted criterion about P values is that lysis of the curves in Fig. 3. When the structures of the
when the P value is lower than 5, the fit between three dyes are compared (Fig. 1), it can be seen that they
experimental data and model is considered to be commonly possess a negatively charged sulfonate group.
excellent.[31,32] The P values for pseudo-second-order AO8 has three aromatic ring structures, AR151 has four

Table 3. Experimental and calculated qe values and rate constants of pseudo-first-order and pseudo-second-order models for
adsorption and electrosorption, at 1.0 V, of three dyes onto ACC.
pseudo-first-order pseudo-second-order
Dyes qe/(mol g−1) exp k1/(min−1) P qe/(mol g−1) calc. r2 k2/(g(mol min) −1
) P qe/(mol g−1) calc. r2
ADSORPTION
AO8 5.27 x 10−5 2.81 x 10−2 17.59 1.10 x 10−4 0.9789 17139 3.28 5.26 x 10−5 0.9970
AY14 4.85 x 10−5 2.60 x 10−2 28.27 3.64 x 10−5 0.9753 16784 4.94 4.87 x 10−5 0.9838
AR151 4.47 x 10−5 1.62 x 10−2 40.11 3.29 x 10−5 0.8577 15144 4.68 4.46 x 10−5 0.9707
ELECTROSORPTION
AO8 5.61 x 10−5 3.48 x 10−2 11.73 2.60 x 10−5 0.9642 16185 4.83 5.62 x 10−5 0.9925
AY14 5.52 x 10−5 2.26 x 10−2 33.61 3.19 x 10−3 0.9482 15024 3.96 5.48 x 10−5 0.9934
AR151 5.36 x 10−5 1.90 x 10−2 20.77 9.89 x 10−5 0.9024 14953 2.27 5.34 x 10−5 0.9978
6 E. M. SAHIN ET AL.

Figure 5. CVs taken at a sweep rate of 5 mVs−1 in 0.1 M Na2SO4 solution a) without any dye, b) with 1 × 10−3 M AO8, c) with 1 ×
10−3 M AY14, and d) with 1 × 10−3 M AR151.

4
C / (10-5 M)

0
0 100 200 300
t / min

Figure 6. Electrosorption behavior at 1.0 V potential for AO8 (•), AY14 (■), and AR151 (▲) (supporting electrolyte: 0.1 M Na2SO4,
mass of ACC: 18 ± 0.1 mg).

aromatic ring structures, and AY14 has three ring struc- ACC is 7.4 (Table 1), the surface of ACC during the
tures, two of which are aromatic. AO8 has an – OH group adsorption of AO8 is slightly positive (pH of
as a substituent in one of the aromatic rings. The pH of solution<pHpzc), attracting AO8 molecules both from
adsorbate solutions was followed during OC adsorption negatively charged sulfonate and polar – OH centers.
experiments. It was almost constant throughout the pro- The surface of ACC during the adsorption of AY14 is
cess having a range of 6.01 to 6.10 for AO8, 6.54 to 6.59 also slightly positive (pH of solution<pHpzc), but less than
for AY14, and 7.78 to 7.83 for AR151. Since the pHpzc of in the case of AO8 adsorption. Thus, affinity of ACC
 SEPARATION SCIENCE AND TECHNOLOGY 7

Figure 7. The UV-vis spectra of aqueous solutions of AO8 (•), AY14 (■), and AR151 (▲) at a concentration of 5 × 10−5 M. Typical
scanning kinetic outputs for b) OC adsorption and c) electrosorption at 1.0 V of AR151 where scans were taken at 10 min intervals,
the top scans being at t = 0.

toward AY14 is expected to be less than AO8, being also the sizes of dye molecules may play a role in deter-
consistent with the experimental observations. During mining the order of affinity of dyes toward ACC. It is
the adsorption of AR151, the pH of the solution is slightly expected that the smaller molecules can penetrate into
higher than the pHpzc of ACC resulting in a slight negative smaller pores finding more sites available for adsorption.
charge on ACC. Thus, the attraction of AR151 by ACC is Visual analysis of structures in Fig. 1 suggests that AR151
expected to be only through hydrophobic and van der is the largest in size with its four aromatic rings.
Waals interactions which are weaker than electrostatic
interactions. This could be the reason for the least affinity Electrosorption
observed for AR151 among the three dyes toward ACC. Electrosorption can simply be defined as electrically
Not only the interactions between ACC and azo dyes but assisted adsorption. The adsorbent used as a working
8 E. M. SAHIN ET AL.

Figure 8. Scanning electron micrographs of ACC a) before adsorption/electrosorption, b) after open-circuit adsorption, and c) after
electrosorption by applying 1.0 V potential for the removal of AO8.

electrode is charged positively or negatively depending on achieved for AO8 already in OC adsorption (Fig. 3). In
the charge of the adsorbate by applying a potential in order electrosorption, this removal is speeded up (Fig. 6).
to facilitate the attraction of adsorbate species. It is prefer- While removal of AR151 and AY14 could not be com-
able that no faradaic reaction takes place at the electrode pleted even at the end of 300 min in OC adsorption,
during this charging process. For this reason, the safe their removals were again speeded up and reached
potential range in which no such reaction takes place has almost to completion at about 250 min period in elec-
to be determined for each system. This is usually achieved trosorption (Fig. 6).
by taking a cyclic voltammogram (CV) of each system. UV-vis spectra of all three dyes are shown in Fig. 7a.
CVs taken in 0.1 M Na2SO4supporting electrolyte In addition, sample scanning kinetic outputs from the
solutions with and without dye between −0.4 V and UV-vis spectrophotometer for AR151 are given in Fig.
+1.4 V are shown in Fig. 5. It is seen that no faradaic 7b for OC adsorption and in Fig. 7c for electrosorption
reaction is taking place from −0.4 V to about +1.0 V in at 1.0 V. No change is detected in the structure of dye,
any of these systems. Since all three azo dyes are anionic, as there is no change in the general appearance of
the electrosorption studies were carried out by applying spectra. Similar scans were obtained for the adsorption
a potential of +1.0 V, which is possibly the highest poten- and electrosorption of other dyes. The curves in Fig. 3
tial to be applied without any faradaic reaction. and in Fig. 7 are produced from the absorbance data at
Electrosorption behaviors of three azo dyes are given the λmax value of each dye.
in Fig. 6. When the curves in this figure are compared Kinetic data for electrosorption behavior given in
with the corresponding curves in OC adsorption given Fig. 6 were treated again according to pseudo-first-
in Fig. 3, the improvements in removal by electrosorp- order and pseudo-second-order models just in the
tion can clearly be seen. Almost complete removal was same way as in the case of OC adsorption. The
 SEPARATION SCIENCE AND TECHNOLOGY 9

30 a)

qe / (mol g-1 x 10-5)

15

0
0 0.5 1 1.5 2

Ce / (mol L-1 x 10-5)

80
b)
60
qe / (mol g-1 x 10-5)

40

20

0
0 1.5 3 4.5
Ce / (mol L-1 x 10-5)
30
c)

20
qe / (mol g-1 x 10-5)

10

0
0 1 2 3
Ce / (mol L-1 x 10-5)

Figure 9. Adsorption isotherms at 25 °C for a) AO8, b) AY14, and c) AR151. (▲): experimental data points, (―): fit of data according
to Langmuir equation, and (‒ ‒): fit of data according to Freundlich equation.

parameters of the two models are included in Table 3. However, this can be explained by an
It can be seen that pseudo-second-order model is better approximate percent coverage, Ɵ, calculation using
for the representation of electrosorption data based on Eq.(5)[36]:
both r2 and P values, just as in OC adsorption.
Closeness of experimental qe and qe calculated from ðCO  Cf ÞVNO
θ¼  100 (5)
pseudo-second-order model supports this conclusion 5:0  1017  1870  m
(7th and 9th columns for electrosorption in Table 3). where C0 and Cf are the initial concentration and final
Sample scanning electron micrographs (SEMs) of concentration at the end of adsorption/electrosorption
ACC before using in adsorption/electrosorption, after period, V is the volume of solution, N0 is Avogadro
OC adsorption, and after electrosorption by applying number and m is the mass of ACC. The constant 5.0 ×
1.0 V potential for the removal of AO8 are shown in 1017 is approximate saturation value in units of mole-
Fig. 8(a–c), respectively. At first glance, the amount of cules per m2 of the surface corresponding to 200 Å2
dye attached to the fibers may seem to be very small projected molecular area, e.g. AO8. One thousand eight
both after OC adsorption and electrosorption. hundred and seventy is the specific surface area of ACC
10 E. M. SAHIN ET AL.

0.08 0.08 0.15

(a1) (a2) (a3)
0.12
0.06 0.06

Ce/qe
0.09

Ce/qe

Ce/qe
0.04 0.04
0.06
0.02 0.02 0.03

0 0 0
0 1 2 0 2.5 5 0 2 4
Ce / 10-5 M Ce / 10-5 M Ce / 10-5 M

-7.6 -6.4
(b1) (b2)
-8 (b3)
-7
-8.2
-8.6
lnqe

-7.6

lnqe
lnqe
-8.8 -9.2
-8.2

-9.4 -8.8 -9.8

-15 -12.5 -10 -7.5 -14 -12 -10 -8 -16 -14 -12 -10 -8
lnCe lnCe lnCe

Figure 10. Isotherm plots according to Langmuir model for AO8 (a1), AY14 (a2), AR151 (a3), and Freundlich model for AO8 (b1),
AY14 (b2), AR151 (b3).

Table 4. The parameters of Langmuir and Freundlich isotherm equations for the adsorption of AO8, AY14, and AR151 azo dyes at 25
°C.
Langmuir Freundlich
Adsorbate qm/(mol g−1) b/(L mol−1) r2 P KF/(mol1-(1/n)L1/n g−1 1/n r2 P
AO8 3.25 x 10−4 3.1 x 105 0.8817 22.21 5.52 x 10−3 0.28 0.8756 9.75
AY14 7.35 x 10−4 1.5 x 105 0.9669 11.97 1.89 x 10−2 0.34 0.9853 3.02
AR151 2.43 x 10−4 4.6 x 105 0.9661 10.20 5.48 x 10−3 0.30 0.9592 5.89

(Table 1). Ɵ values calculated for ACC after OC where qm is the maximum adsorption at monolayer
adsorption were 3.57% and for ACC after electrosorp- coverage, b is a constant related to the energy of
tion was 3.65%. The overall specific surface area of adsorption, KF is Freundlich constant related to the
ACC is so large that the percentage of this surface adsorption capacity, and n is another constant related
covered by AO8 is only 3.57 after OC adsorption and to surface heterogeneity. Both equations are given in
3.65 after electrosorption. linearized forms so that Ce/qe vs Ce plots provide qm
and b from slope and intercept, respectively, and lnqe
vs lnCe plots provide 1/n and KF from the slope and
intercept, respectively. The corresponding plots are
Adsorption isotherms
shown in Fig. 10. The parameters obtained by linear
The amount of dye adsorbed onto ACC at equilibrium regression analysis of adsorption data are given in
is plotted as a function of dye concentration at equili- Table 4. The goodness of fit of data to the equations
brium, at 25°C, in Fig. 9(a–b) for AO8, AY14, and is tested using both r2 and P (Eq. (4)) values. Evaluation
AR151, respectively. The experimental data were fitted of these criteria parameters, especially the P values,
to two well-known isotherm models; Langmuir model suggests that the Freundlich model represents the
given by: adsorption isotherm data of all three dyes better than
the Langmuir model.
Ce Ce 1
¼ þ (6)
qe qm bqm
and Freundlich model given: Conclusion
1 AO8, AY14, and AR151 azo dyes could be removed to
ln qe ¼ ln KF þ Ce (7)
n a great extent from aqueous solutions onto ACC by OC
 SEPARATION SCIENCE AND TECHNOLOGY 11

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