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O. C. TAIWO et al.: Recycling of Waste Expanded Polystyrene as an Effective Adsorbent of ..., Kem. Ind. 70 (9-10) (2021) 519−534 519
1 Introduction
Polycyclic aromatic hydrocarbons (PAHs) are a class of
organic compounds characterised by two or more fused
benzene rings. They are carcinogenic, mutagenic, and tox-
ic.1 There are sixteen listed PAHs as priority pollutants that
have been linked to various health challenges in humans.2,3
PAHs are by-products of various anthropogenic and indus-
trial activities, such as incomplete combustion of coal, fuel,
garbage, oil, oil spillage, organic substances, polymers, re-
fuse, tobacco smoke, and wood, among others.4,5,6,7 The
negative effect of PAHs in the environment has been a
great concern to researchers8 to mitigate their serious ef-
fects on the human body.5,8,9
PAHs have a strong resistance to biological degradation,
and some conventional physicochemical processes have
not demonstrated the desired potency for their remov-
al.5 However, adsorption processes, involving the use of
activated carbon derived from synthetic, natural, and re-
newable sources have been deployed for the successful
removal of PAHs.5,8,10 This may not be unconnected to the
advantages of the ease in operation, cost-effectiveness,
and insensitivity to toxic substances compared to other
separation techniques.9 Activated carbon, commonly used
in the adsorption process, has high adsorption capacities
for a wide range of pollutants because of its porous micro-
structures and large surface areas.11 However, the purchase
cost and the cost of regeneration of AC are expensive,12
besides there is 10–15 % loss during regeneration.13 Ad-
sorbents such as carbon nanotube, zeolite, diatomite, and
organoclay have been used for the adsorption of PAHs
from aqueous solutions.14,15,16 Naphthalene is an important
PAH that has a molecule containing two benzene rings17
with molecular formula C10H8, obtainable from petroleum
rening and coal tar distillation.18 Its presence in the envi-
ronment is more pronounced, relative to the other types
of PAHs.13 Some authors have used various types of adsor-
bent originated from clay, coal, and agricultural biomass
for the removal of naphthalene from wastewater.19,20,21,22
Expanded polystyrene (EPS) are agglomerated small and
expandable plastic “beads” used to produce food packag-
es,23 because of their calendaring surface, which prevents
absorption of water, oil, beverages, and other processed
food products. Used EPS has relatively low scrap value
and is discarded after use.24,25 Its non-biodegradable na-
ture makes it persist in the environment and its build-up
reduces the holding capacity of landlls or dumpsites.26,27
Reuse of EPS for environmental remediation is reported in
the works of Gwenzi et al., Siyal et al., Alsewailem and Aljlil,
and Ruziwa et al.26,27,28,29,30 This study aimed at recycling
EPS waste into an effective adsorbent for the removal of
naphthalene from the aqueous solution.
2. Experimental
2.1 Materials and sample preparation
WEPs, sourced from dumpsites were washed to remove
oil and dirt from their surfaces. The collected WEPs were
soaked in detergent solution and stirred until the strength
Recycling of Waste Expanded Polystyrene as
an Effective Adsorbent of Naphthalene from
Aqueous Solution
https://doi.org/10.15255/KUI.2020.084
KUI-37/2021
Original scientific paper
Received December 25, 2020
Accepted June 26, 2021
O. C. Taiwo
,a,b
T. J. Afolabi
,a,b
F. N. Osuolale
,a
A. O. Ajani
,a
O. A. Aworanti
,a
O. R. Ogunleye
, a,b and
A. O. Alade
a,b,c*
This work is licensed under a
Creative Commons Attribution 4.0
International License
a Department of Chemical Engineering, Ladoke Akintola University of Technology,
Ogbomoso, Nigeria
b Bioenvironmental, Water and Engineering Research Group, (BWERG),
Ladoke Akintola University of Technology, Ogbomoso, Nigeria
c Science and Engineering Research Group, (SEARG), Ladoke Akintola University of Technology,
Ogbomoso, Nigeria
Abstract
Batch adsorption process factors [contact time (20–150 min), adsorbent dosage (0.5–1.5 g), adsorbate concentration
(5–30 mg l−1), and agitation rate (100–250 rpm)] were optimised based on D-optimal Design under the Response Surface
Methodology (RSM) of the Design-Expert Software (7.6.8) for the removal of naphthalene from aqueous solution using adsor-
bent developed from Acetylated Waste Expanded Polystyrene (AWEPs). The maximum adsorption capacity (5.6608 mg g−1)
achieved was well tted to Dubinin-Radushkevich Isotherm (R2 = 0.9949). The SSE (< 0.05) and ARE (< 4.0 %) indicated pseu-
do-second-order as the most suitable model. This research has demonstrated the effectiveness of the WEPs for the removal of
naphthalene from the aqueous solution.
Keywords
Adsorption, D-optimal, naphthalene, waste expanded polystyrene
* Corresponding author: Abass Olanrewaju Alade, PhD
Email: aoalade@lautech.edu.ng
O. C. TAIWO et al.: Recycling of Waste Expanded Polystyrene as an Effective Adsorbent of ..., Kem. Ind. 70 (9-10) (2021) 519−534
520
of the detergent became weak. They were then rinsed
thoroughly with a copious amount of distilled water be-
fore being sun-dried and oven-dried at 105 °C to constant
weight. They were reduced to relatively uniform sizes to
reduce the impact of calendaring and expose the pores.30
All the reagents used in this research were of analytical
grade and used without further purication.31
[Acetic acid (γ: 60.052 g mol−1, Tb: 118 °C, ρ: 1.05 g cm−3,
Tm: 16.6 °C, CAS 64-19-7), NaOH (γ: 39.997 g mol−1,
ρ: 2.13 g cm−3, Tb: 1,388 °C, CAS 1310-73-2), Naph-
thalene (Tm: 80.26 °C, γ: 128.1705 g mol−1, Tb: 218 °C,
ρ: 1.14 g cm−3, CAS 91-20-3), Ethanol (Tb: 78.37 °C,
γ: 46.07 g mol−1, Tm: −114.1 °C, Density: 789 kg m−3, CAS
64-17-5)].
2.2 Activation of WEPs
The WEPs were mixed in 100 ml of acetic acid
(c = 4.15 mol dm−3) at a ratio of 1.5 : 1 (mass of activant/
mass of precursor) and microwaved in the oven at 600 Hz
for 90 min, the excess acid was boiled off. The pH of the
acetylated WEPs (AWEPs) developed was neutralized with
NaOH and oven-dried to constant weight.28,30
2.3 Characterisation of AWEPs
The ash and moisture contents were determined by the
ASTM analytical method32 and the method adopted by Ek-
pete and Horsfall.33 The ash and moisture contents were
calculated according to Eqs. (1) and (2).31 Fourier Trans-
form Infrared Spectroscopy (FTIR) was used to determine
the functional group on the surface of AWEPs before and
after adsorption. The samples were prepared with potas-
sium bromide at a 1 : 10 ratio and pressed into the pel-
letized disc.34 The FTIR spectrum of WEPs, AWEPs, and
spent WEPs (SWEPs) adsorbents were recorded within the
range of 400–4000 cm−1.
( )
= ⋅
1
2
ashcontent % 100
m
m
(1)
% moisture content =
−⋅
−
43
53
100
mm
mm
(2)
where m1 is the mass of ash, m2 is the mass of the dried
sample, m3 is the mass of crucible, m4 is the mass of cruci-
ble with the wet sample, and m5 is the mass of the crucible
with the dry sample.
2.4 Batch adsorption
2.4.1 Adsorption studies
A stock solution of naphthalene of 200 mg l−1 was pre-
pared by dissolving 200 mg of naphthalene in 100 ml of
ethanol. Distilled water was added to make 1 l. The stock
solution was further diluted with distilled water according-
ly to produce the desired concentration.35
The study type used for this experimental design for the
adsorption study was the Response Surface Methodology
(RSM).36 The initial design suggested by the Design-Expert
software (7.6.8) was D-optimal. Zero (0) centre point was
chosen for the design with no blocks selected, and a build
time of 875 min was used for the design model. The factors
are activant concentration (A), impregnation ratio (B), mi-
crowave time (C), and microwave frequency (D) while the
response is adsorption capacity.26,28 Determined amount
(0.5–1.5 g) of AWEPs was mixed with 100 ml naphthalene
solution of specic concentration (5–30 mg l−1) and shaken
on a rotary shaker at a specic agitation rate (100–250 rpm),
and at room temperature (28±2 °C),26,28 according to the
D-optimal Design (Table 1). The mixture was centrifuged
and the supernatant was analysed using UV-Spectrometer
(UV-6100A, manufacturer: METASH A-MATRIX) at a wave-
length of 275 nm.37 Different concentrations (5–50 mg l−1)
of naphthalene were prepared earlier, and UV-Spectrom-
eter (UV-6100A) at a wavelength of 275 nm was used to
their corresponding absorbances, which were used to de-
velop the calibration curve from which the equation for
evaluating the naphthalene concentration from absorb-
ance was determined. The adsorption capacity and remov-
al efciency were evaluated by Eqs. (3) and (4).
(3)
(4)
where γo (mg l−1) is the initial concentration of naphthalene
solution in contact with adsorbent, γt (mg l−1) is the nal
concentration of naphthalene solution after the batch ad-
sorption procedure at any time t, m (g) is the mass of adsor-
bent, and V (l) is the volume of the naphthalene in solution.
Table 1 – Range of selected factors for naphthalene adsorption
Activation factors Unit Ranges
Low High
naphthalene concentration mg l−1 5.0 30.0
dosage g 0.5 1.5
time min 30 60
agitation rate rpm 100 250
2.4.2 Effect of adsorption factors
The optimum conditions for the adsorption of naphtha-
lene using AWEPs was subjected to OFAT procedures dur-
ing which the effects of contact time and concentration
were investigated when one of the factors was varied at a
time.38,39,40
2.5 Investigation of suitable adsorption isotherms
The adsorption data obtained were tted to selected iso-
therm models. Their constants were evaluated, and the
O. C. TAIWO et al.: Recycling of Waste Expanded Polystyrene as an Effective Adsorbent of ..., Kem. Ind. 70 (9-10) (2021) 519−534 521
correlation coefcient (R2) was used to express the extent
of correlation between the experimental data and the
model predicted values.41
2.5.1 Langmuir isotherm model
The Langmuir isotherm model is expressed by Eq. (5) and
the linear form Eq. (6) was used to generate the plot of
against γe, which gave a straight-line graph with a slope
m
1
q
and intercept of
mL
1
qK
.
(5)
(6)
where qe is the adsorption capacity at equilibrium (mg g−1),
γe is the equilibrium concentration of the adsorbate solu-
tion (mg l−1), KL is the constant related to the free energy
of adsorption (l mg−1), and qm is the maximum adsorption
capacity at monolayer coverage (mg g−1).
2.5.2 Freundlich isotherm model
The Freundlich isotherm model equation, Eq. (7), assumes
a heterogeneous adsorbent surface with its adsorption sites
at varying energy levels. Its linear form Eq. (8) was used to
generate the plot of lnqe against lnγe that is needed to de-
termine the Freundlich constants (kF
, and 1/n).42
(7)
(8)
where kF is the Freundlich constant, and qe is the adsorp-
tion capacity at equilibrium (mg g−1)
2.5.3 Temkin isotherm model
Temkin isotherm model explicates that the adsorbate-ad-
sorbent interactions and the related change in heat and/or
energy of adsorption are assumed to be linear ---+-char-
acterised by a uniform distribution of binding energy and
up to some maximum binding energy.42 Such an assump-
tion cannot hold for a logarithmic relationship. The Tem-
kin isotherm model is expressed by Eq. (9), while its linear
form is expressed by Eq. (10), and was further simplied
to Eq. (11).
(9)
(10)
(11)
where B = RT/b, B is the molecular interaction parameter
related to the heat of adsorption. A and B are the Temkin
isotherm constants, T (K) is the absolute temperature, and
R is the ideal gas constant (8314 J mol−1 K−1).
2.5.4 Dubinin-Radushkevich isotherm
Dubinin-Radushkevich (D-R) isotherm Eq. (12) assumes
that pore lling inuenced the adsorption mechanism in
micropores and not a layer-by-layer formation of a lm in
the walls of the adsorbent pores.43 The linear form of the
D-R isotherm equation is expressed in Eq. (13) and was
used to plot lnqe against ε2 needed to determine the qm and
β from the intercept and slope.
(12)
lnqe = lnqm – βε2(13)
where β (KJ2 mol2) is the free energy of sorption per mole of
the naphthalene as it migrates to the surface of WEPs from
an innite distance in the solution, qm is the maximum ad-
sorption capacity, and ε is the Polanyi potential (J mol−1),
which is expressed by Eq. (14):
(14)
where R is the universal gas constant (8.314 J mol−1 K−1), T
is the absolute temperature (K), and γe is the equilibrium
concentration of naphthalene.
2.6 Adsorption kinetics studies
The pseudo-rst-order model, pseudo-second-order mod-
el, and intraparticle diffusion model were employed to
evaluate the experimental data generated in this study.
2.6.1 Pseudo-first-order model
This model (Eq. (15)) is based on a solid capacity and its
plot of ln(qe − qt) vs t that gives a straight line from which
K1 and qt were evaluated based on the slope and intercept.
ln(qe − qt) = lnqt – K1t(15)
where qe is the equilibrium adsorption capacity (mg g−1), qt
is the adsorption capacity at time (mg g−1), K1 is the pseu-
do-rst-order rate constant (l min−1), and t is the time tak-
en.
2.6.2 Pseudo-second-order model
This model (Eq. (16)) was used to plot t/qe vs t, which gave
a straight line from which qe and K2 were evaluated.
O. C. TAIWO et al.: Recycling of Waste Expanded Polystyrene as an Effective Adsorbent of ..., Kem. Ind. 70 (9-10) (2021) 519−534
522
e2 e
1
t
tt
q qK q
= + (16)
where K2 is the rate constant of pseudo-second-order ad-
sorption (g mg−1 min−1).43
6.3 Intraparticle diffusion model
This model indicates that the rate-limiting step is the trans-
port of the solute from the bulk of the solution to the ad-
sorbent pores through the intraparticle process. It was ex-
pressed according to Eq. (17):44
1
2
diff
t
q kt C= +
(17)
where kdiff is the intraparticle diffusion rate constant
(mg g−1 min−0.5), t is time, and C is constant.
2.7 Test of the kinetics model
The impact of various error functions on the predicted
isotherm parameters was analysed to determine the order
of suitability of the selected isotherm models. Error func-
tions such as Average Relative Error (ARE) and Sum of Er-
ror Square (SSE) were calculated according to the Eqs. (16)
and (17).45,46
e,exp e,cal
1
e,exp
100
ARE n
i
qq
nq
=
−
=∑(16)
( )
2
e,cal e,exp
1
SSE= n
i
qq
n
=
−
∑(17)
where qe,exp is the adsorption capacity at equilibrium ex-
perimental (mg g−1), qe,cal is the adsorption capacity at equi-
librium calculated (mg g−1), and n is the number of data
points.
3 Results and discussion
3.1 Physicochemical analysis of the adsorbent
There was a signicant difference in the ash content be-
tween the WEPs (0.10 %) and AWEPs (0.39 %) (Table 2).
This may be due to the impact of the activant on the com-
position of the untreated WEPs, and this further suggested
that the activation process was evident.47,48 The moisture
content of AWEPs (14.82 %) was higher than that of WEPs
(0.90 %), and this may be due to the soaking step during
the activation process.
3.2 FTIR characterisation of WEPs and AWEPs
The IR peaks observed in the WEPs ranged from
623.1 cm−1 to 3933.4 cm−1 and the peak height ranged
from 21.8 cm−1 to 36.9 cm−1 (Fig. 1a). AWEPs had IR peaks
that ranged from 613.7 cm−1 to 3892.3 cm−1 and peak
height ranged from 3.5 cm−1 to 21.8 cm−1. The IR peak
for the spent SAWEPs ranged from 618.5 cm−1 to 3930.4,
while the peak height ranged from 10 cm−1 to 27.4 cm−1.
The IR peaks observed in the WEPs shifted from (623.1–
3933.4 cm−1) to (613.7–3892.3 cm−1) on AWEPs. All these
changes suggested the impact of acetic acid activation on
the surface improvement of the WEPs. The aromatic C−H
band present in both WEPs and AWEPs is attributed to the
presence of an aromatic benzene ring that is not split by the
acidic modication on the WEPs. C−C−O at 1332 cm−1
was found in WEPs but disappeared in AWEPs due to the
modication by acetic acid. These bands indicate the pos-
sible involvement of these functional groups on the surface
of AWEPs in the naphthalene adsorption process.47 WEPs
and AWEPs surface chemistry were found to be different as
some of the functional groups such as O−H and C−C−O
disappeared due to the activation process.48
transmittance ⁄ %
wavelength ⁄ nm
(a)
(b)
(c)
Fig. 1 –FTIR spectra for (a) Waste Expanded Polystyrene, (b)
Acetylated Waste Expanded Polystyrene, and (c) Spent
Acetylated Waste Expanded Polystyrene
3.3 Design summary for the adsorption capacity
of AWEPs for naphthalene
Run 4 (60 min, 100 rpm, 5 mg l−1, and 1.5 g) gave the
lowest adsorption capacity (0.1623 mg g−1), while Run 3
(30 min, 100 rpm, 30 mg l−1, and 0.5 g) gave the high-
est adsorption capacity (5.6608 mg g−1) (Table 3), which
Table 2 – Proximate analysis of WEPS and AWEPS
Sample Ash content ⁄ % Moisture content ⁄ %
WEPs 0.10 0.90
AWEPs 0.39 14.82
WEPs – Waste Expanded Polystyrene, AWEPs –
Acetylated Waste Expanded Polystyrene
O. C. TAIWO et al.: Recycling of Waste Expanded Polystyrene as an Effective Adsorbent of ..., Kem. Ind. 70 (9-10) (2021) 519−534 523
is higher than adsorption capacity of 1.44, 1.595, 1.527,
and 4.39 mg g−1, reported by Ania et al.,49 Murilo et al.,10
and Alade et al.,8 for activated carbons, activated clay, and
amboyant pod activated carbon studied for the removal
of naphthalene. A quadratic model was selected for this
study because of its least standard deviation (10.33) and
high R2 (0.9835). The R2 (0.9548) was very close to the ad-
justed R2 of 0.9548, with less than 0.2 differences, as nor-
mally expected.44 This indicated no large block effect nor
any possible problem with the model and data obtained.50
Adequate Precision, which measures the signal-to-noise
ratio of the data, was 28.793, which was greater than the
desired value (4.0), thus making the developed model very
suitable to navigate the design space.51
3.4 Analysis of variance (ANOVA)
for adsorption capacity of AWEPs
Prob> F of any term or model, less than 0.05, at a 95 %
condence interval is taken as signicant.31 The model
F-value of 34.16 implies the model is signicant and it has
about 0.01 % chance of occurrence due to noise. Thus, C,
AB, AC, AD, BC, BD, and CD are signicant model terms
(Table 4). The “Lack of t F-value” of 0.63 implies the Lack
of t is not signicant relative to the pure error, which
makes the model t31 and there is a 62.61 % chance that
a “Lack of t F-value” this large could occur due to noise.
This is further illustrated by Fig. 2, showing the effects of
the model terms concerning Normal % probability. The
points are distributed on the normal line starting from ap-
proximately 2 to 97 % on normal percentage distribution,
Y-axis, and −1.5 to 2.5 on internally studentized residuals,
X-axis though there is a stacking of the points.31
Table 3 – Results of responses from adsorption experimental data
Run Factors Response
A: time ⁄ min B: agitation rate ⁄ rpm C: concentration ⁄ mg l−1 D: dosage ⁄ g adsorption capacity ⁄ mg g−1
1 30.00 100.00 17.50 1.50 0.6188
2 45.00 250.00 30.00 1.50 0.7565
3 30.00 100.00 30.00 0.50 5.6608
4 60.00 100.00 5.00 1.50 0.1623
5 30.00 100.00 5.00 0.50 0.4869
6 60.00 100.00 30.00 1.50 1.4231
7 30.00 250.00 5.00 1.50 0.3072
8 30.00 250.00 17.50 0.50 2.8130
9 60.00 100.00 17.50 0.50 2.9000
10 30.00 175.00 30.00 1.50 1.3072
11 60.00 250.00 5.00 0.50 0.9217
12 60.00 100.00 30.00 1.50 1.3072
13 30.00 100.00 30.00 0.50 3.0521
14 60.00 250.00 17.50 1.50 0.5898
15 60.00 250.00 30.00 0.50 4.4434
16 60.00 175.00 17.50 1.00 0.4065
17 30.00 250.00 5.00 1.50 1.9608
18 45.00 250.00 17.50 1.00 0.9717
19 60.00 250.00 30.00 0.50 4.7043
20 30.00 100.00 5.00 0.50 0.9838
21 30.00 175.00 17.50 1.00 0.4500
22 45.00 100.00 17.50 1.00 1.3630
23 45.00 175.00 23.75 1.00 2.2489
24 45.00 175.00 5.00 1.00 0.3304
25 30.00 250.00 30.00 1.00 2.2217
O. C. TAIWO et al.: Recycling of Waste Expanded Polystyrene as an Effective Adsorbent of ..., Kem. Ind. 70 (9-10) (2021) 519−534
524
Fig. 2 –Effects of the model terms with respect to normal %
probability
3.5 Model equation
The model equations, (Eqs. (18) and (19)), show the rela-
tionship between the adsorption capacity and the selected
factors that can be used to predict the naphthalene ad-
sorption. Model terms B, D, AD, B2, A2, C2, and D2 have
positive coefcients, which indicate an increase in the ad-
sorption capacity of AWEPs in the factors. The negative
coefcients as observed in A, C, AB, AC, BD, CD, and B2
indicate an antagonistic inuence of these factors on the
adsorption capacity of naphthalene. The empirical model
equations in terms of coded factors are given in Eq. (18),
for the signicant and non-signicant terms.
(adsorption capacity)−3 = +3.01 − 1.83A +
+ 2.46B − 25.23C + 6.27D − 30.38AB −
− 31.98AC + 24.36AD + 33.65BC − 22.59BD −
− 24.79CD + 8.26A2 − 0.6B2 + 7.35C2 + 19.45D2
(18)
A B, C, and D are the coded variables for activant concen-
tration, IMR, microwave time, and frequency, respectively.
3.6 Model graph for the selected factors
on adsorption capacity for naphthalene
The plot of agitation rate with time (Fig. 3) shows a grad-
ual decrease in adsorption capacity before a steep slope
moving upward, and this indicates that the two factors
cause a decrease, and then nally increase the adsorption
rate. There is a slight decrease in the slope of concentra-
tion against time before (Fig. 4) and thus, means that the
Table 4 – ANOVA for the Model and selected factors
Source Sum of squares DfMean square F value P-value Prob >F
Model 51038.03 14 3645.57 34.16 < 0.0001*
A – time 14.27 1 14.27 0.13 0.7240
B – agitation rate 25.36 1 25.36 0.24 0.6390
C – concentration 5903.74 1 5903.74 55.32 < 0.0001*
D – dosage 113.17 1 113.17 1.06 0.3332
AB 2923.37 1 2923.37 27.39 0.0008*
AC 7680.27 1 7680.27 71.97 < 0.0001*
AD 1786.01 1 1786.01 16.74 0.0035*
BC 8533.81 1 8533.81 79.97 < 0.0001*
BD 1517.79 1 1517.79 14.22 0.0055*
CD 5305.82 1 5305.82 49.72 0.0001*
A239.15 1 39.15 0.37 0.5615
B20.21 1 0.21 1.939 ∙ 10−3 0.9660
C237.79 1 37.79 0.35 0.5683
D269.13 1 69.13 0.65 0.4442
Residual 853.71 8 106.71
Lack of t 234.40 3 78.13 0.63 0.6261
Pure error 619.32 5 123.86
Cor total 51891.75 22
* Signicant at p < 0.05
O. C. TAIWO et al.: Recycling of Waste Expanded Polystyrene as an Effective Adsorbent of ..., Kem. Ind. 70 (9-10) (2021) 519−534 525
Fig. 3 – (a) Contour plot, and (b) 3D plot showing the relationship between time and agitation rate
Fig. 4 – (a) Contour plot, and (b) 3D plot showing the relationship between time and concentration
Fig. 5 – (a) Contour plot, and (b) 3D plot showing the relationship between dosage and time
(a)
(a)
(a)
(b)
(b)
(b)
O. C. TAIWO et al.: Recycling of Waste Expanded Polystyrene as an Effective Adsorbent of ..., Kem. Ind. 70 (9-10) (2021) 519−534
526
Fig. 6 – (a) Contour plot, and (b) 3D plot showing the relationship between concentration and agitation rate
Fig. 7 – (a) Contour plot, and (b) 3D plot showing the relationship between dosage and agitation rate
Fig. 8 – (a) Contour plot, and (b) 3D plot showing the relationship between dosage and concentration
(a) (b)
(a) (b)
(a) (b)
O. C. TAIWO et al.: Recycling of Waste Expanded Polystyrene as an Effective Adsorbent of ..., Kem. Ind. 70 (9-10) (2021) 519−534 527
lower value of these factors decreases the adsorption rate.
Contrary to Figs. 3 and 4, there is a parabolic curve, an
increase in adsorption capacity in the plot of dosage and
time (Fig. 5). Further increase in dosage value with time
caused a decrease in adsorption capacity (Fig. 6). Concen-
tration against the agitation rate plot shows an increase in
adsorption capacity (Fig. 4). Other factors that increased
the adsorption rate were the dosage value and agitation
rate (Fig. 7), but the contrary result was experienced in
Fig. 8, in which the dosage and concentration decreased
with the adsorption rate.
3.7 Numerical optimisation studies
on adsorption capacity
Numerical optimisation was obtained from the software.
The four factors (time, agitation rate, concentration, and
dosage) were all set to “is in range” (Table 4), while the
adsorption capacity was set to “maximise” with its upper
and lower limit, respectively. The desirability values were
0.994, and the optimum values suggested by the software
were 60 min, 100 rpm, 5 mg l−1, and 1.5 g for time, agita-
tion rate, concentration, and dosage, respectively (Fig. 9).
Table 5 –Selected factors used for optimisation showing their
respective ranges
Name Goal Lower Limit Upper
Limit
time is in range 30 60
agitation rate is in range 100 250
concentration is in range 5 30
dosage is in range 0.5 1.5
(adsorption capacity)^−3 maximise 0.00551244 233.908
3.8 Effect of concentration on adsorption of naphthalene
An increase in the adsorbate initial concentration
(10–30 mg l−1) leads to an increase in the adsorption ca-
pacity (1.921–5.9217 mg g−1) of the adsorbent due to the
increase in the driving force of the concentration gradient
(Fig. 10a). The adsorption of naphthalene was rapid at the
initial stage of the contact time (30 min) for all the con-
centrations. This was because, in the beginning, all active
sites on the adsorbent were vacant, hence, adsorption pro-
ceeded at a faster rate. After this, the rates of adsorption
and desorption tended to be equal, and the extent of ad-
sorption reduced and eventually became almost constant
at equilibrium.51 The percentage of removal also increased
with increasing time for all the concentrations (Fig. 10b).
3.9 Effect of dosage on the adsorption of naphthalene
An increase in adsorbent dosage (0.5–2.5 g) led to a re-
duction (5.5739–1.1930 mg g−1) in adsorption capacity
(Fig. 11a), because of the effect of partial aggregation of
naphthalene on the adsorbent surface, resulting in a de-
crease in total surface area available for naphthalene mol-
ecules. An increase in the adsorbent dosage increased the
removal efciency of naphthalene from 92.9 % to 99.4 %
Fig. 10 –Effect of concentration on (a) adsorption capacity, and (b) removal efciency
Fig. 9 – Graphical representation of the numerical optimisation
O. C. TAIWO et al.: Recycling of Waste Expanded Polystyrene as an Effective Adsorbent of ..., Kem. Ind. 70 (9-10) (2021) 519−534
528
at 180 min, due to the availability of more adsorption sites
on the adsorbent (Fig. 11b).52,53
3.10 Adsorption isotherm study
3.10.1 Langmuir isotherm model
The values of qm and KL for AWEPs (Fig. 12) were
−8.7108 mg g−1 and −0.1106, respectively. The low value
of R2 (0.4238) indicated that the experimental equilibri-
um data were not well described by the Langmuir mod-
el. The maximum adsorption capacity (qm) obtained from
this research (−8.7108 mg g−1) (Table 5), was lower for the
adsorption of naphthalene onto mesoporous molecular
sieves, zeolite, Mesoporous organosilica, and banana peel
activated carbon, respectively.10,54
3.10.2 Freundlich isotherm model
The Freundlich model to estimate Kf and 1/n are
−0.0511 l mg−1 and 0.8059 from its intercepts and the
slope, respectively (Fig. 13). The values of 1/n ranging from
0 to 1, indicates the model’s favourability for the adsorp-
tion process,55 and this value is lower than the previous re-
search, except 0.7986 derived by Gupta and Gupta.57 The
negative Kf value obtained from this study was lower than
Fig. 11 –Effect of adsorbent dosage on (a) adsorption capacity, and (b) removal efciency
Fig. 12 – Plot of ce/qe against ce for the effect of concentration Fig. 13 – Plot of lnqe against lnce for the effect of concentration
Fig. 14 – Plot of qe against lnce for the effect of concentration Fig. 15 – Plot of lnqe against ԑ2 for the effect of concentration
O. C. TAIWO et al.: Recycling of Waste Expanded Polystyrene as an Effective Adsorbent of ..., Kem. Ind. 70 (9-10) (2021) 519−534 529
the Kf value derived by Carla et al.,54 Chang et al.,56 Gupta
and Gupta,57 respectively. The R2 (0.9777) was relatively
high, thus making the Freundlich isotherm a better model
compared to the Langmuir (0.4238) (Table 5).
3.10.3 Temkin isotherm model
Estimated Temkin isotherm parameters A and B were
6.0670 l g−1 and 1.2408 J mol−1, respectively, (Fig. 14),
with an R2 value of 0.9883, which is higher than the R2
values of 0.4238 and 0.9777 obtained for Freundlich and
Langmuir, suggesting that the data better tted Temkin iso-
therm than the other two.
3.10.4 Dubinin-Radushkevich isotherm
for the effect of concentration
The Dubinin-Radushkevich isotherm model is described
by the plot of lnqe against E2 (Fig. 15) and the estimated pa-
rameters, qm and β are 25.1851 mg g−1 and 1 ∙ 106 KJ2 mol2,
respectively.
The mean free energy of biosorption β determines the bi-
osorption mechanism as either a physical or chemical pro-
cess. The biosorption process is chemically driven if the
value of β is greater than 8 KJ2 mol2, but involves physical
mechanism if less. The value obtained in this study indi-
cated that the adsorption of naphthalene onto AWEPs was
driven by a chemical process. The R2 value was 0.9949,
which was the highest of all the isotherm models investi-
Table 6 – Comparison of Langmuir and Freundlich Isotherm Parameter to other studies
Adsorbent Langmuir parameter Freundlich parameters Author
qm ⁄ mg g−1 Kf1/n
Mesoporous Molecular Sieves 0.10624 NR NR Murilo et al. (2004)10
Zeolite 0.769 4.215 1.074 Chang et al. (2004)56
Mesoporous Organosilica 0.0466 0.227 0.97 Carla et al. (2011)54
Banana Peel Activated Carbon NR*21.54 0.7986 Gupta and Gupta (2015)57
AWEPS −8.7108 −0.0511 0.8059 This study
*NR – not reported
Fig. 16 –Plot of ln(qm-qe) against time for the effect of concen-
tration
Fig. 17 – Plot of t/qe against time for the effect of concentration
Fig. 18 – Plot of qe against lnt for the effect of concentration Fig. 19 – Plot of qe against T0.5 for the effect of concentration
O. C. TAIWO et al.: Recycling of Waste Expanded Polystyrene as an Effective Adsorbent of ..., Kem. Ind. 70 (9-10) (2021) 519−534
530
gated, thereby giving the order of suitability as Langmuir <
Freundlich < Temkin < Dubinin-Radushkevich isotherm.
Therefore, Dubinin-Radushkevich isotherm model best
ts the experimental data generated for the adsorption of
naphthalene on AWEPs.
3.11 Investigation of adsorption kinetics
3.11.1 Pseudo-first-order kinetics model
The estimated k1 values of 0.023, 0.020, and 0.026 had
no visible trend (Fig. 16, Table 7). There is a wide dispari-
ty between the calculated equilibrium adsorption capacity
qe,calc, and the experimental equilibrium adsorption capac-
ity (qe,exp) values, contrary to a good correlation expect-
ed.58 This suggests that the adsorption of naphthalene onto
AWEPs does not t the rst-order kinetics. The R2 for 10,
20, and 30 mg l−1 are 0.9609, 0.7838, and 0.8034, respec-
tively and are relatively high.31
Table 8 – Kinetic parameters obtained for the kinetics models
Kinetics model Parameters 10 mg l−1 20 mg l−1 30 mg l−1
Pseudo-rst
order qe,cal ⁄ mg g−1 1.648 1.389 1.933
qe,exp ⁄ mg g−1 1.922 3.878 5.922
K1 ⁄ min−1 0.023 0.020 0.026
R20.9609 0.7838 0.8034
Pseudo-second
order qe,cal ⁄ mg g−1 2.349 4.297 6.545
qe,exp ⁄ mg g−1 1.922 3.878 5.922
K2 ⁄ g mg−1 min−1 0.011 0.013 0.010
R20.9750 0.9804 0.9785
Elovich Α0.140 0.872 1.816
Β1.951 1.298 0.900
R20.8905 0.6349 0.5503
Intraparticle
diffusion Kid 0.120 0.172 0.246
C 0.434 1.821 3.060
R20.8194 0.5322 0.4504
3.11.2 Pseudo-second-order kinetics model
for the effect of concentration
The qe. cal obtained from the plot of t/qe vs t (Fig. 17) were
2.349, 4.297, and 6.545 mg l−1, showing a direct relation-
ship with the increase in concentration. The values of qe.cal
were relatively closer to the values of qe,exp. The values of
K2 (0.011, 0.013, and 0.010 g mg−1 min−1) had no particu-
lar pattern with increasing concentration. The R2, (0.9750,
0.9804, and 0.978) were higher than the R2, obtained for
the rst-order model (Table 7).
3.11.3 Elovich kinetic model
The plot of qe against lnt for Elovich kinetic model (Fig. 18)
gave the values of initial adsorption rate, α as 0.14, 0.872,
and 0.814, while the rate of surface coverage, β, were
1.951, 1.298, and 1.900 for 10, 20 and 30 mg l−1, respec-
tively. The R2 (0.8905, 0.6349, and 0.5503) displayed an
inverse relation with the initial concentration. The increas-
ing order of suitability of the kinetic models, based on R2
obtained, was Elovich < pseudo-rst-order < pseudo-sec-
ond-order kinetic model. Therefore, the adsorption exper-
iment of naphthalene onto AWEPs is best described by the
pseudo-second-order kinetic model.
3.11.4 Intraparticle diffusion model
The kinetic parameter (C) obtained from the plot of qe
against
1
2
t
(Fig. 19) was 0.434, 1.821, and 3.06 for 10, 20,
and 30 mg l−1, respectively, implying that values of C were
directly proportional to the surface adsorption of naphtha-
lene in the rate-controlling step. The intraparticle diffusion
rate constant, Kid, values were 0.12, 0.172, and 0.246,
showing that Kid, increased with concentration. Intraparti-
cle diffusion becomes the sole rate-limiting step if the plot
is linear and passes through the origin.59 This study deviat-
ed from this condition, thus, intraparticle diffusion is not
the sole rate-limiting step. The R2 value obtained reduced
with increasing concentration.
3.12 Error analysis for the kinetic models
It was found that the SSE(pseudo-rst-order) value for the effect
of concentration ranged between 0.017 and 0.89, while
the value obtained for the pseudo-second-order ranged
between 0.260 and 0.054. ARE(pseudo-rst-order) value for
the effect of concentration was greater than 9 %, while
ARE(pseudo-second-order) value for the same factor was less than
4 % (Table 8). Therefore, the pseudo-second order model
better predicts the adsorption of naphthalene on AWEPs
than the pseudo-rst order model.
Table 9 – SSE and ARE values for kinetic models
Kinetic model Concentration ⁄ mg l−1 SSE ARE ⁄ %
pseudo-rst order 10 0.0107 2.0366
15 0.8850 9.1689
20 0.5698 9.6227
pseudo-second order 10 0.0260 3.1737
15 0.0250 1.5435
20 0.0554 1.5028
4 Conclusion
This research successfully demonstrated the suitability of
expanded polystyrene waste (WEPs) products as an effec-
tive adsorbent. The activant (acetylene) used improved the
O. C. TAIWO et al.: Recycling of Waste Expanded Polystyrene as an Effective Adsorbent of ..., Kem. Ind. 70 (9-10) (2021) 519−534 531
surface characteristics of the AWEPs, and this inuenced
its adsorptive properties, particularly for the removal of
naphthalene from aqueous solution. The adsorption pro-
cess was chemically driven, as suggested by the tness of
the data generated to Dubinin-Radushkevich isotherm
and pseudo-second-order kinetic models. This, therefore,
opens more opportunities to explore the WEPs for the ad-
sorption of organic pollutants from aqueous solution and
real-life wastewater.
List of abbreviations and symbols
a_k – Khan model exponent
b_k – Khan constant
A– Temkin isotherm constant
ANOVA – analysis of variance
ARE – average relative error at monolayer coverage
AWEPS – acetylated waste expanded polystyrene
b– heat of adsorption constant
B– Temkin isotherm constant
BET – Brannuer–Emmet–Teller method
d– interlayer spacing, m
DOE – design of experiment
EPS – expanded polystyrene
FTIR – Fourier transform infrared spectroscopy
H– standard enthalpy, J mol−1
h– initial adsorption rate as t→0, mg g−1 min−1
IMR – impregnation ratio
K1– rate constant of pseudo-rst order adsorption, 1/min
K2– rate constant of the pseudo-second, g mg−1 min−1
kdiff – rate constant for intraparticle diffusion, m g−1 min−1
kF– Freundlich constant
KL– free energy of adsorption constant
MBN – methylene blue number
MW – microwave
n– Hill coefcient of binding interaction of the adsorbate
OFAT – one factor at a time
PAHs – polycyclic aromatic hydrocarbons
qe– adsorption capacity at equilibrium, mg g−1
qm– maximum adsorption capacity, mg g−1
qs– theoretical isotherm saturation capacity, mg g−1
qt– adsorption capacity at time t, mg g−1
r– inverse power of distance from the surface, m−1
R– universal gas constant, J mol−1 K−1
RE – removal efciency
RSM – response surface methodology
SSE – sum of square error
T– absolute temperature, K
V– volume, dm3, cm3
WEPS – waste expanded polystyrene
∆G– change in Gibbs free energy, J mol−1
∆S– standard entropy change, J K−1
β– free energy of adsorption per mole, KJ2 mol2
ε– Polanyi potential, J mol−1
γe– equilibrium concentration, mg l−1
γo– adsorbate initial concentration, mg l−1
θ– degree of surface coverage
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534
SA ŽETAK
Recikliranje ekspandiranog polistirena
kao učinkovitog adsorbensa naftalena iz vodene otopine
Oluwayemisi Christiana Taiwo
,a,b
Tinuade Jolaade Afolabi
,a,b
Funmilayo Nihinlola Osuolale
,a
Ayobami Olu Ajani
,a
Olufunmilayo Abiola Aworanti
,a
Olabanji Raphael Ogunleye
,a,b and
Abass Olanrewaju Alade
a,b,c*
Šaržni faktori procesa adsorpcije [vrijeme kontakta (20 – 150 min), doziranje adsorbenta
(0,5 – 1,5 g), koncentracija adsorbata (5–30 mg l−1) i brzina miješanja (100–250 min−1)] optimizi-
rani su na temelju D-optimalnog dizajna primjenom metodologije odzivne površine (RSM) progra-
ma Design-Expert (7.6.8) za uklanjanje naftalena iz vodene otopine pomoću adsorbenta razvije-
nog iz acetiliranog otpadnog ekspandiranog polistirena (AWEP). Ostvareni maksimalni adsorpcijski
kapacitet (5,6608 mg g−1) dobro je prilagođen izotermi Dubinin-Radushkevich (R2 = 0,9949). SSE
(< 0,05) i ARE (< 4,0 %) označili su pseudo-drugi red kao najprikladniji model. Ovo istraživanje
pokazalo je učinkovitost WEP-a za uklanjanje naftalena iz vodene otopine.
Ključne riječi
Adsorpcija, D-optimalnost, naftalen, otpadni ekspandirani polistiren
Izvorni znanstveni rad
Prispjelo 25. prosinca 2020.
Prihvaćeno 26. lipnja 2021.
a Department of Chemical Engineering,
Ladoke Akintola University of Technology,
Ogbomoso, Nigerija
b Bioenvironmental, Water and Engineering
Research Group, (BWERG), Ladoke Akintola
University of Technology Ogbomoso, Nigerija
c Science and Engineering Research Group,
(SEARG), Ladoke Akintola University of
Technology Ogbomoso, Nigerija
