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Journal of the Taiwan Institute of Chemical Engineers 151 (2023) 105138
1876-1070/© 2023 Published by Elsevier B.V. on behalf of Taiwan Institute of Chemical Engineers.
Adsorption of Black MNN reactive dye from tannery wastewater using
activated carbon of Rumex Abysinicus
Jemal Fito Nure
a
,
*
, Ashagrie Mengistu
b
,
*
, Mikiyas Abewaa
c
, Kenatu Angassa
d
,
Welldone Moyo
a
, Zebron Phiri
a
, Potlako J. Mafa
a
, Alex T. Kuvarega
a
, Thabo T.I. Nkambule
a
a
Institute for Nanotechnology and Water Sustainability (iNanoWS), College of Science, Engineering and Technology, Florida Science Campus, University of South Africa,
1710, Johannesburg, South Africa
b
The Federal Democratic Republic of Ethiopia, Manufacturing Industry Development Institute, Leather and Leather Products Industry Research and Development Center,
P.O. Box 1180, Addis Ababa, Ethiopia
c
Department of Chemical Engineering, College of Engineering and Technology, Wachemo University, Hossana, Ethiopia
d
Department of Environmental Engineering, Addis Ababa Science, &Technology University, Addis Ababa, Ethiopia
ARTICLE INFO
Keywords:
Adsorbent
Biomass
Industrial efuent
Kinetics
Wastewater treatment
Water pollution
ABSTRACT
Background A high concentration of Black MNN reactive dye in untreated tannery wastewater can cause adverse
effects on public health and environmental sustainability. Therefore, this study aimed to investigate the
adsorptive performance of Rumex Abyssinicus-based activated carbon (RAAC) to remove Black MNN reactive
dye from tannery wastewater.
Method The stem of Rumex Abyssinicus was impregnated with 50 % diluted phosphoric acid at the ratio of 1:3
(w/w biomass to acid) and then it was carbonized in the mufe furnace at 600◦C. The Box-Behnken experimental
design of 3
4
was used to optimize the removal efciencies under the response surface methodology.
Signicant ndings Characteristics of the adsorbent were described by SEM for surface morphology with high
cracks, FTIR spectrometry for multi-functional groups (O
–
H at 3443.08 cm
−1
, aromatic C =C 1633.78 cm
−1
,
vibrational and stretching motion of -OH 1205.65 cm
−1
and vibrational motion of C
–
O-C 1045.46 cm
−1
), BET
with specic surface area of 3619.7 m
2
/g, and X-RD with amorphous structure. The maximum dye adsorption of
99.9 % was observed at experimental conditions of 150 mg/L, 0.25 mg/100 mL, pH 2, and 90 min. The
experimental data were evaluated in terms of Langmuir, Freundlich, Temkin, and Spis isotherms which showed
the Langmuir model was found to be the best t with the experimental data at R
2
0.99. This shows that the
adsorbent surface is homogeneous and monolayer. Furthermore, the kinetic study conrms that the Pseudo
second-order model best describes the experimental data at R
2
0.99. Finally, statistical analysis showed that the
experimental values are almost the same as the predicted data which were indicated by the Adjusted R
2
0.99 and
predicted R
2
0.97. In conclusion, Rumex Abyssinicus is a good precursor material for adsorbent development to
remediate industrial and municipal wastewater.
1. Introduction
Nowadays, freshwater consumption is growing alarmingly due to
uncontrollable fast industrialization, agricultural boosting, and sudden
world population explosion [1]. Industrialization has a vital role in the
economic development of the nations. It is seen as a motor behind many
of the changes usually termed social transformation and modernization.
However, the environmental crisis due to industrial wastewater
discharge has reached an unprecedented level across the globe which is
exerting considerable negative pressure on the biosphere. Particularly,
surface and groundwater quality depletion and deterioration have been
a global challenge to achieving sustainable development [2]. Gaol 6 of
sustainable development aimed to realize safe, affordable, clean, and
sufcient water for all mankind by 2030 [3]. However, water scarcity
and quality deterioration have become global challenges to meeting
sustainable development goals [2]. Basically, water scarcity refers to the
availability of freshwater below 1000 L per person per year, and it is
estimated that 40 % of the world’s population will end up with water
scarcity by 2030. In line with water scarcity, 4.8–5.7 billion people will
be at risk by 2050 [4]. This needs a call not only for action for the
* Corresponding authors.
E-mail addresses: tojemal120@gmail.com (J.F. Nure), ashagmen2017@gmail.com (A. Mengistu).
Contents lists available at ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers
journal homepage: www.journals.elsevier.com/journal-of-the-taiwan-institute-of-chemical-engineers
https://doi.org/10.1016/j.jtice.2023.105138
Received 13 July 2023; Received in revised form 12 September 2023; Accepted 13 September 2023
Journal of the Taiwan Institute of Chemical Engineers 151 (2023) 105138
2
efcient allocation of water resources but also for the potential reuse of
wastewater streams from different industrial setups. Industries extract
materials from natural resources and release both products and wastes
into the environment. Industries are causing serious environmental di-
sasters, which have yet to be brought under control. Tanning industries
are well known to be one of the most water-intensive industries that
discharge a considerable volume of wastewater toward the nearby
ecosystem. The beam house, tan yard, and retaining operations in the
tanning industry consume a considerable amount of freshwater making
it a water-intensive industry [5]. Tannery wastewater consists of a sig-
nicant number of toxic pollutants that have the capability of affecting
human health and damaging the ecosystem [6]. Pollutants commonly
discharged from the tanning industries at unit operations include toxic
dyes, harmful and toxic chemicals like chromium, limes, suldes, acids,
bases, salts, and other chemicals [7]. Particularly, a considerable num-
ber of reactive dyes are being used during the dyeing and fat liquoring
stages of the tanning processes. Studies reported that only 85–90 % of
the dye applied is exhausted by the material and the rest 10–15 % is
being lost with the efuent imparting color and toxicity to the natural
receiving water bodies [8]. Some dyes like Black MNN reactive have
carcinogenic properties and they are recalcitrant, resistant to aerobic
digestion, and stable to light (heat) and other oxidizing agents. This
phenomenon can make a difcult condition for wastewater treatment.
Therefore, searching for advanced and new wastewater treatment
technologies is essential.
The removal of dyes from industrial efuents using conventional
wastewater treatment methods is inefcient due to the complex nature
of wastewater. On the other hand, many advanced wastewater treat-
ment techniques such as advanced oxidation process, reverse osmosis,
ion exchange, and electro dialysis are attracting the attention of scholars
to overcome the shortcomings of conventional wastewater treatment
techniques. However, these technologies on the other hand require large
capital investment and are non-sustainable. Contrary to those technol-
ogies, adsorption technology has been used in a variety of important
industrial applications to remediate wastewater [9]. Adsorption is
low-cost, easy to apply, and economically and environmentally friendly.
Consequently, several investigative studies on the development of acti-
vated carbon have been conducted for the removal of pollutants using
locally available plant materials and plant byproducts. However, the
most commonly used activated carbon is the coal-based adsorbent which
is expensive and not sustainable. Hence, researchers striving to examine
locally available materials and their treatment efciencies in water and
wastewater remediation. For instance, the removal of chemical oxygen
demand (COD) from distillery using bagasse y ash [10], adsorption of
uoride from aqueous solution and groundwater using avocado seed
[5], chromium removal from tannery wastewater, and methylene blue
removal from textile industrial wastewater using activated carbon
derived from parthenium hysterophorus have been investigated [2].
Under this consideration, many precursor materials have been used
including avocado seed, bagasse, cassava peel, parthenium hysterophorus,
watermelon rinds, willow peat, orange waste, bagasse y ash, agricul-
tural wastes, the bark of the Vitex negundo morinda, bamboo, ramie
bars, cotton, sludge, macro-algae, sawdust, walnut shells, buffalo weed,
pine tar, orange peel, coconut husks, miscanthus straw, reed straw,
hemp stem, wheat straw, wood pellets, tinctorial bark, crocus sativus
leaves, etc., [11–21]. It was also reported that activated carbon has
gotten top attention for industrial applications due to its well-developed
pore structures and adsorption properties. However, due to minimal
adsorption capacity, poor regeneration, and long contact time, it was
found important to investigate the adsorbent with high removal ef-
ciency, short contact time, and good performance with the minimum
dosage. Hence, low-cost agricultural and weed biomasses containing
high amounts of natural polymers such as cellulose, hemicellulose, and
lignin can be taken as an alternative raw material to produce efcient,
low -cost and green adsorbents [22]. The development of the Rumex
Abyssinicus-based activated carbon (RAAC) was carried out to mitigate
such challenges. In line with this, valorization of the stem and leaves of
the Rumex Abyssinicus is limited. Moreover, no research study has been
conducted on the application of activated carbon synthesized from the
stem of Rumex abyssinicus for tannery dye decolorization. However, we
developed an adsorbent from this plant and applied it for industrial
wastewater treatment. This water treatment application was not enough
to draw a concrete conclusion [23,24]. However, the application of this
adsorbent for chromium removal was missing. Hence, the study inten-
ded to evaluate the efcacy of chromium removal from aqueous solution
under the response surface methodology approach. On the other hand,
the search for alternative raw materials for the adsorption of toxic and
persistent dyes such as Black MNN reactive dye is increasing. This
phenomenon happened due to the non-sustainability and cost of com-
mercial activated carbon. An optimization process was used to study the
interaction between the adsorbent and adsorbate under factorial
experimental design. The adsorption study includes adsorbent dose, pH,
contact time, and initial concentration with their two levels employed.
Therefore, this research work aimed to use the stem of Rumex-abyssinicus
as an adsorbent to remove reactive black MNN dye from tannery
wastewater.
Aqueous
2. Materials and methods
2.1. Materials and equipment
The sun-dried stems of the Rumex Abyssinicus plant were collected
from Addis Ababa Science and Technology University which is located
at Akaky Kaliti sub-city of Addis Ababa. Analytical grade chemicals and
reagents were used for laboratory experiments such as sodium hydrox-
ide pellet 99.0 %; hydrochloric acid 37 % w/w, phosphoric acid 88 %;
mercuric sulfate crystals 99 %; sulfuric acid,98.0 %; silver sulfate 99 %;
potassium dichromate 99.5 %. electronic balance (Shimazu-AUX220),
digital pH meter (PHS-3C), coffee grinder (model: XFYC810, 240 V,60
Hz), orbital shaker ( Model: SK-600, 230VAC, 50 Hz, 0.2A by QTEX),
UV/VIS spectrometer (Evolution 300, UV–vis by Maalab), hot air oven
(Model: J-dvo1, supplier: JISICO), mufe furnace (Carbolite AAF 1100),
testing equipment such as Kjeldahl (Buchi KjelSampler K-376), Fourier
transform infrared spectroscopy ( FT-IR), IR Afnity, Shimadzu), X-ray
power diffraction (XRD-X-ray tube cu40kv, 44 mA, Rugaku, Ultima IV)
and scanning electron microscope (model JSM840A SEM microscope
operating at 10 kV) were the equipment employed during the study.
2.2. Wastewater sampling and characterization
Wastewater samples were collected from Elico-Awash tannery and
characterization was done for some physicochemical parameters such as
pH, temperature, total suspended solids, total solids, ve days biological
oxygen demand (BOD
5
), and chemical oxygen demand (COD). The an-
alyses done are described in Table 1. The Awash tannery is located in
Addis Ababa at a geographical coordinate of 8◦56
′
33
′
’ N and 38◦45
′
41’’
E and an altitude between 2100 and 2360 m above sea level. The tannery
has a daily soaking capacity of 8991 pieces of skin and 1305 cowhides.
Data collected from the tannery showed that it has an average monthly
Table 1
Selected tannery wastewater physicochemical parameters and the analysis
methods used.
S/No Parameters Methods used for testing
1 pH Hach HQD eld case model 58,258–00
2 Temperature Hach photometer HQD eld case, Model 58,258–00
3 EC Hach photometer HQD eld case, Model 58,258–00
4 TSS APHA 5220 B, Total solid dried at 103–105 ◦C
5 TS APHA 5220 B, Total solid dried at 103–105 ◦C
6 COD APHA 5220 B, Open reux method
7 BOD
5
APHA 5210 B, 5-day BOD test
J.F. Nure et al.
Journal of the Taiwan Institute of Chemical Engineers 151 (2023) 105138
3
wastewater discharge of 3100 m
3
. The wastewater from the tannery was
composed of tan yard operation to the chrome recovery unit in addition
to the others from the beam house and post-tanning operations. A
composite wastewater sampling method was used twice for monthly
intervals and the sampled wastewater was delivered to the laboratory
for characterization. The analyses of those parameters were carried out
after the wastewater samples were collected in polyethylene plastic
bottles which were well-washed and disinfected. The wastewater sam-
ples were stored in a refrigerator at 4 ◦C and analyzed according to
standard methods for the examination of water and wastewater (APHA
1998).
2.3. Adsorbent preparation
The collected stems of Rumex Abyssinicus were taken to the research
laboratory at the Leather and Leather Products Industry Research and
Development Center and washed with tap water several times to remove
any dust particles on the surface and then rinsed with distilled water.
The plant was identied at the herbarium at Addis Ababa University’s
herbarium and the plant specimen was deposited but with no voucher
ID. All methods were performed by the relevant guidelines/regulations/
legislation by including a statement in the methods section. The washed
raw material was dried using a fan and further dried in an oven at 110 ◦C
for 12 h. The detailed procedures followed during raw material collec-
tion and sample preparation were described diagrammatically as shown
in Fig. 1 (A–H). The dried sample was ground to a reduced size to make it
suitable for the subsequent stages of sample preparation. Then, the size-
reduced samples were impregnated or soaked in 88 % phosphoric acid
diluted to 50 % solution at a 1:3 (wt/wt) ratio of Rumex Abyssinicus to
phosphoric acid (H
3
PO
4
). This helped to attain a better carbon structure
and large surface area [1,2]. The soaking activity was conducted for
about 24 h at room temperature which was followed by drying for 8 h in
a hot air oven at 105◦C. Thereafter, pyrolysis of the impregnated sample
was carried out at 600◦C in a mufe furnace for 2 h. The carbonized
sample was removed from the furnace and kept in the desiccator for
further cooling to room temperature. The cooled samples were ground
to make them suitable for washing. Washing was carried out using 0.01
N NaOH solution and distilled water separately to remove the acid used
during impregnation [1,2]. The washing was repeated until the pH of
the washing water reached pH 7. Finally, the washed activated carbon
was dried using the fan and then further dried using an oven at 105◦C for
12 h. Finally, the dried and powdered activated carbon was analyzed for
its particle size and stored in a zip-lock plastic bag to avoid moisture and
contamination by any other foreign materials till the subsequent activ-
ities of characterization and batch adsorption experiments [6].
2.4. Adsorbent characterization
Proximate analysis of the RAAC was performed to determine the four
proximate parameters, where the three of them moisture content, ash
Fig. 1. Diagrammatic description of activated carbon preparation from Rumex Abyssinicus plant (A) raw Rumex-Abyssinicus stem (B) size reduction for washing (C)
size reduced for impregnation (D) impregnation of the raw sample (E) drying after soaking (F) pyrolyzed activated carbon (G) washing of the activated carbon (H) fan
drying after washing of the activated carbon with activated carbon of 106
μ
m particle size
J.F. Nure et al.
Journal of the Taiwan Institute of Chemical Engineers 151 (2023) 105138
4
content, and volatile matter were directly analyzed based on the
experimental result whereas the xed carbon was calculated from the
result of these three parameters as per the standard methods for Amer-
ican Society for Testing and Materials (ASTM) [1,6]. Similarly, the
surface area determination was done by Brunauer, Emmett, and Teller
(BET) method [25,26], surface morphologies by SEM (model
JCM-6000PLUS BENCHTOP SEM (JOEL), Japan) [24,27], surface
functional group by FTIR spectrophotometer (SHIMADZU IR
Afnity-1S) [2,6,18], crystalline structure by XRD instrument
(XRD-X-ray tube cu 40kv, 44 mA, Rugaku, Ultima IV) [28,29] and point
of zero charges by mass titration and acid titration methods [24,30].
2.5. Design of experiment and batch adsorption process
2.5.1. BOX-Behnken experimental design method
The experimental design was conducted using the design expert of
version 7 in which the Box-Behnken Experimental design Method
(BBDM) was used under Response Surface Methodology (RSM). RSM is a
technique used for developing an empirical model by mathematical and
statistical analysis to optimize the response or output variable that is
inuenced by the input variables. Minimum numbers of experimental
runs were used to generate regression model equations and optimized
conditions [31]. Preliminary, laboratory experiments using synthetic
tannery dye and data from the previously published articles were used in
deciding the range of the selected variables [2,8]. As indicated in
Table 2, contact time (min), initial dye concentration (mg/L), pH, and
adsorbent dosage (mg/100 mL) were the considered experimental var-
iables with three levels. Finally, the removal efciency of the dye was
taken as a response variable.
The lower level of each factor was designed as ‘‘−1,’’ the middle
value was represented by ‘‘0,’’ and the higher level by ‘‘+1.’’ The
experiment was designed expressed as 3
4
and is anticipated to generate
81 runs; however, the number of experimental runs was reduced to 30
using design expert. Interactions between the four variables and their
impacts on the response variable were evaluated using the response
surface method. A random combination of each variable from their three
levels was done. The interaction effects of all four factors on the response
variable were studied. Variance analysis of the model (ANOVA) was
used to check the adequacy of the model. Experimental design results
with three-level were tted using the model represented by the desired
response Eq. (5).
Y=bo+∑
n
i=1
biXi+∑
n
i=1
biiX2
i+∑
n−1
i=1∑
n
j=2
bijXiXj+
ε
(5)
where the X
1
X
2
… X
n
is the input factors that can inuence the response
Y; n is the number of variables, b
o
is the constant intercept, b
ii
(i =1, 2…
n) is the quadratic coefcient, b
ij
(i =1, 2… n; j =1, 2… n) is the
interaction of the coefcient, and
ε
represents the random error. The
regression model and response surface methods were used to investigate
the highest projected dye removal.
2.5.2. Batch adsorption experiment and optimization
Synthetic wastewater was used to conduct batch adsorption based on
the designed experiments and dye adsorption from real tannery waste-
water. The adsorption was performed under the optimization process. A
stock solution of synthetic wastewater (1000 mg/L) was prepared from
the anionic tannery dye. Then, the required concentrations of the
experimental solutions were made using the dilution principle, in which
distilled water was used to dilute the stock solution. pH Adjustments for
the experimental solutions were done using 0.1 M HCl and 0.1 M NaOH
solutions depending on the basicity or acidity of the required experi-
mental solutions. In general, the dilution principle was used to prepare
150, 120, and 90 mg/L solutions of synthetic anionic dye [6]. The
adsorption experiment was done in a 250 mL conical ask, in which the
required amount of adsorbate and adsorbent was added based on the
designed experiment in 100 mL solution mixtures. Thereafter, the asks
with the experimental solutions were agitated using an orbital shaker
(Model: SK-600; Lab-companion, by QTEX) at 125 rpm. After comple-
tion of the adsorption contact time, the experimental solutions were
ltered using the Whatman lter paper 42. Finally, ltrate solutions
were taken for further analysis to determine the percentage of dye
removed. The absorbance of dyes remaining in the solutions was
determined using Uv–visible spectrophotometer (Agilent technology,
Cary 100 UV–visible Spectrophotometer) at 665 nm. The amount of dye
that remained in the puried water was determined using a calibration
curve drawn using known concentrations of synthetic wastewater con-
taining the Black MNN reactive dye. Six solutions having dye concen-
trations of 60, 80,100,120, 140, and 160 mg/L were used to develop the
calibration curve. In real wastewater analysis, the tannery efuent was
collected from the retannig and dyeing section of leather processing. The
wastewater sample was ltered to remove interfering from dissolved
and suspended solid materials. The concentration of the dye was
determined using the calibration curve. The adsorbent’s adsorption
capacity and efciency of removing pollutants were determined using
Eqs. (1) and (2), respectively. In addition, the study ndings were
documented in the form of means plus standard deviation [1].
qt= (Co−Ct) × V
M(1)
%R=(Co−Ct)
Co
×100 (2)
where R is the dye removal efciency, q
t
is the adsorption capacity at the
time ‘‘t’’ (mg/g), C
o
is the initial dye concentration, C
t
is the dye con-
centration at the time ‘‘t’’ in (mg/L), V is the volume of the aqueous
solution (mL) and M is dry mass of the adsorbent (g) [32,33].
2.6. Adsorption isotherm
Adsorption isotherm models were used to describe how the adsor-
bate molecules of dye interact with the surface adsorption sites and to
study the relationship between the amounts of pollutant adsorbed at
equilibrium and the mass of the sorbent utilized, in which dye concen-
trations ranging from 90 to 150 mg/L. However, the other three pa-
rameters were kept at their optimum values of adsorbent dose of 0.25 g/
100 mL, solution pH 2, and contact time of 90 min. The tness of data
and determining whether the adsorbent surfaces are heterogeneous or
homogenous were studied using Langmuir and Freundlich isotherms.
Langmuir isotherm assumes that adsorbates accumulate forming a
monolayer on binding sites. The simplied and linearized model is
shown in Eq. (3).
1
qe
=1
qmax
+1
KLqmax
×1
Ce
(3)
where q
max
(mg/g) is the maximum monolayer adsorption capacity of
the adsorbent, C
e
(mg/L) is the equilibrium value of the initial concen-
tration,q
e
(mg/L) is the equilibrium adsorption capacity and K
L
(L/mg) is
the Langmuir constant which is related to the free energy of adsorption.
Furthermore, the dimensionless separation factor constant (R
L
) shown in
Eq. (4) is used to estimate Langmuir’s isothermal feasibility [34,35].
Table 2
Experimental design using the Box-Behnken design method for the selected
factors with their levels.
Variables Units Low (-) Middle (0) High (+)
pH 2 5.5 9
Time min 30 60 90
Adsorbent dosage g/100 mL 0.15 0.2 0.25
Dye concentration mg/L 90 120 150
J.F. Nure et al.
Journal of the Taiwan Institute of Chemical Engineers 151 (2023) 105138
5
RL =1
1+KL×Ce
(4)
where C
o
(mg/L) is the initial concentration, R
L
is the afnity between
the adsorbate and the adsorbent, and its value for favorable adsorption
should be between 0 and 1, and for unfavorable adsorption, the RL value
should be more than 1, whereas RL values of 1 and 0 suggest linear and
irreversible adsorption processes, respectively [36].
In addition, the Freundlich isotherm model has an assumption that in
the adsorption process, the total surface of the activated carbon is
multilayered and has a linearized isotherm model equation as in Eq. (5).
logqe=logKf+1
n×logCe(5)
Where K
f
is the Freundlich constant showing adsorption capacity
(mg/g), 1/n is the empirical parameter related to the intensity of
adsorption indicating favorable conditions if its value is laid between 0.1
and 1 [8,37,38].
Temkin adsorption isotherm model states that there is a linear
reduction of the heat of heat instead of logarithmic during the course
adsorption. The linear form of the Temkin equation is shown by Eq. (6).
Qe =RT
bln KT +RT
Bln Ce (6)
Where K
T
is the equilibrium binding constant (L/mg), b is the Temkin
heat of adsorption constant, R is the universal ga constant (8.314 J/
Kmol), and T(K) is the system temperature. Sips adsorption isotherm is
the combination of the Freundlich and Langmuir model used for pre-
dicting the adsorption on heterogeneous surfaces and its linear form is
shown by Eq. (7).
ln (Qe
Qmax −Qe)=1
ns lnCe +ln(ks)ns (7)
where 1/ns and ks are the sips constant related to intensity and
adsorption capacity, respectively
2.7. Adsorption kinetics
Adsorption kinetic models were used to determine the uptake rate of
adsorbate, and the kinetic data were tted to the pseudo-rst-order,
pseudo-second-order, and intraparticle diffusion models to study the
kinetics of tannery anionic dye adsorption. The linear equations of the
three-adsorption equilibrium kinetic models were shown in Eqs. (8), (9),
and (10), respectively. To evaluate the kinetics, the time of contact
varied between 40 and 90 min with a constant incremental change of 10
min.
log(qe−qt) = logqe−Kf×t
2.303 (8)
t
qt
=1
Ks×q2
e
+t
qe
(9)
qt=Kp×t0.5+C(10)
where q
t
andq
e
(mg/g) are the adsorption capacity at time t and equi-
librium adsorption capacity, respectively; K
f
andK
s
are the pseudo-rst-
order (g/(mg min)) and pseudo- second-order (g/(mg min)) adsorp-
tion rate constants, respectively; K
P
(mg/(g min
0.5
)) is the rate constant
for intraparticle diffusion and C (mg/g) is intraparticle diffusion model
intercept [39–41].
3. Results and discussion
3.1. Tannery wastewater characteristics
The two-discharge lines (the discharging points from unit operation)
of Awash-Elico Tannery wastewater were analyzed for their physico-
chemical properties and the results were tabulated in the form of mean
plus standard deviation as shown in Table 3. The pH for efuent
discharge from the tan yard operation was found to be 3.90±0.67 which
is a direct discharge from the tanning operation being carried out at pH
(4) and the discharge from other sections (beam house and post tanning)
was found to be 9.1 ±0.48. The variation in pH is an indication that
different process operations need different pH values. A higher pH of 9.1
±0.48 for the second line was generated from the mixtures of soaking,
liming, batting, pickling, re-chroming, neutralization, and dyeing (fat
liquoring) which had a pH of 10, 12, 8, 2.8, 4, 5.5, and 3.6, respectively.
The pH value tested at the mixing point of the two lines after chrome
recovery was found to be pH (7.68±0.70) which is within the range of
Ethiopian industrial permissible discharge limits for pH of 6–9 [6,42].
The result shows that discharge of the wastewater does not affect the
biochemical reactions in the water bodies due to its pH.
An average temperature of 22.4 ±1.8 ◦C was recorded which is
below the standard for Ethiopian tannery industrial efuent discharge
limit of 40 ◦C [2]. High-temperature industrial efuent discharge is not
allowed due to its accelerating effect on the biochemical reaction rate in
the biosphere and hydrosphere. In this specic case, the recorded tem-
perature is much lower than the temperature limit set for Ethiopian
wastewater efuent from tanneries, 40 ◦C [6]. This implies that dis-
charging the wastewater into the ecosystem or water bodies does not
have any effect on the biochemical and physicochemical reactions in the
water bodies due to their temperature. COD and BOD
5
are the two sig-
nicant parameters used to identify the extent of organic matter present
in wastewater. Untreated Elico-Awash has values of 1820.0 ±28.67
mg/L and 1350.0 ±35.36 mg/L which are beyond the allowable
discharge limits of 250 mg/L and 60 mg/L, respectively [2]. Besides this,
the ratio of BOD
5
to COD, which refers to the biodegradability index,
was found to be 0.74, indicating either biological or chemical treatment
to be applied to decrease the organic matter level in the tannery efuent
[43]. The presence of such organic matter in the wastewater can cause
severe pollution of the receiving water bodies and dry land. This in-
dicates that measures have to be taken on the efciency of the chrome
recovery plant and the reuse of the organic matter as resource energy,
biogas.
The results of physical wastewater analysis showed that the efuent
has TS of 12,522.5±265.20 mg/L and TSS of 1840.0 ±28.67 mg/L.
Such a high TS in the efuent is the result of lterable and non-lterable
particulate matter in the wastewater samples. Discharging efuents with
such high TDS and TSS into water bodies can cause high turbidity and
reduce the transparency of the water bodies, which disturbs the natural
aquatic ecosystem. Furthermore, such a high concentration of TS causes
depletion of dissolved oxygen in the water bodies, which disturbs the
interaction of living organisms and biochemical reactions endangering
aquatic life. Black MNN dye in the tannery wastewater was analyzed to
Table 3
Physicochemical properties of Elico-Awash tannery untreated wastewater.
Parameters Value for line-1 Value for line-
2
Value for the
mixture
pH 3.9±0.67 9.1±0.48 7.7±0.70
Temperature (
○
C) 28.0 ±1.20 20.5±2.00 22.4±1.80
Conductivity
(mScm
−1
)
– – 15.7±3.85
COD (mg/L) 2823.0 ±212.13 891.9±2.40 1820.0 ±28.67
TSS (mg/L) 2122.5±24.75 1680.0 ±
113.14
1840.0 ±141.42
TS (mg/L) 57,317.5±11,
982.00
8895.0 ±
650.54
12,522.5±
265.20
Cr
2
O
3
(g/L) 6.2±0.87 0 3.4±0.85
BOD
5
(mg/L) – – 1350.0 ±75.36
Original dye
concentration
(mg/L)
0 92.76±4.26 92.76±4.26
J.F. Nure et al.
Journal of the Taiwan Institute of Chemical Engineers 151 (2023) 105138
6
have an average concentration of 92.8 ±4.26 mg/L. Such high con-
centrations of reactive dye in the efuent can severely affect the envi-
ronment and public health. Some dyes are carcinogenic, recalcitrant,
resistant to aerobic digestion, and stable to light, heat, and other
oxidizing agents which makes the situations difcult to remediate [8,
44].
3.2. Characteristics of activated carbon
3.2.1. Proximate analysis
Proximate analysis of moisture, volatile matter, xed carbon, and ash
content was done by the prescribed methods according to ASTM D 121.
The results were tabulated in the form of mean and standard deviation as
shown in Table 4. The adsorbent has percentage moisture, volatile
matter, ash, and xed carbon content of 2.97±0.75, 15.71±3.33, 9.83
±0.68, and 71.50±1.58 %, respectively. The moisture content of an
activated carbon indicates its hygroscopic nature. Therefore, the more
hygroscopic the adsorbent is the less will be its adsorption capacity
because when the adsorbent is left open to the atmosphere the moisture-
laden atmospheric air will penetrate the pore space of the adsorbent and
get absorbed. Normally, High moisture content adds mass to the acti-
vated carbon without contributing to adsorption hence reducing the
share of the carbonous material. This condition forces us to utilize more
adsorbent dosage to attain the desired degree of adsorption, which
indirectly affects the adsorption capacity. Therefore, the less amount of
moisture in this specic adsorbent indicates the hygroscopic nature of
the adsorbent is less which makes It has more free active sites for the
pollutants to be adsorbed and leads to an increase in adsorption ca-
pacity. obtained moisture content was in agreement with the standard
quality of activated carbon based on SNI 06–3730–1995, which set the
maximum allowable water content as 15 % but fewer results were re-
ported [1,2]. The proximate analysis of this adsorbent is in good
agreement with the previous study. In another study, it was reported
that proximate analysis bagasse y ash was composed of ash (16.8 %)
moisture content (5 %), volatile substances (16.8 %), and xed carbon
(42.2 %) [28].
Volatile matter is used as a parameter to determine the amount of
organic volatile materials (H
2
, CO,CO
2
,CH
4
,N
2
and Hydrocarbons) that
escaped during the pyrolysis process of carbonization at the pre-
determined temperature [45]. A volatile matter of 15.71 ±3.33 % was
obtained, which is below the maximum volatile matter allowed by the
Indonesian National Standard (SNI) (25 %). The result is very low
compared with the one recorded from kenaf core ber [46]. However, it
is higher compared to those results obtained from parthenium hyster-
ophorus stem and oil palm empty fruit bunch [2,18,47]. The larger
percentage of volatile matter is an indication that the precursor material
has a larger specic surface area due to the creation of many pores and
pore spaces compared with the above-reported values. On the contrary,
the solid yield of activated carbon production will be affected due to
much loss of materials during the carbonization stage. However, having
a larger percentage yield does not mean that the activated carbon has a
high adsorption capacity. Fixed carbon and ash content are used as
important parameters to determine adsorbent quality, where precursors
are suitable for mass production and should contain a high percentage of
xed carbon and a very small amount of ash. The xed carbon that
determines the yield of the activated carbon resulted from the
non-volatile carbon in the original treated raw material whereas the ash
content refers to the undesired inorganic oxides. In general, all the
proximate analysis parameters are within the standard range of acti-
vated carbon quality standards based on SNI [48].
3.2.2. Determination of specic surface area
The BET theory was used to determine the total surface area of the
material per unit mass of the activated carbon, which is based on the
physical adsorption phenomenon of gasses on the overall surfaces of the
adsorbent material. The adsorbent was degassed using an N
2
atmo-
sphere at a temperature of 200 ◦C to study the BET isotherm. BET test
result revealed the specic surface area for carbon precursor to being
3619.40 m
2
/g when sample washing was done using distilled water.
However, a reduction in specic surface area to 550 m
2
/g was observed
for the sample washed using 0.01 M NaOH solution. This shows that
distilled water best suit for pH adjustment. The recorded specic surface
area is far more than the largest value obtained from bagasse of sugar
cane, 3554.82 m
2
/g) [49]. Such a signicantly large area indicates that
the material is rich in active sites for the adsorption of pollutants.
Thereby, making the adsorption process more economical and
cost-effective by increasing its adsorptive capacity. However, washing
the sample using a dilute NaOH solution affects the surface character-
istics of the precursor material. Many researchers have also reported
adsorbents with surface areas of 90.00, 253.25, 1359.50, 1000.00, and
5.73 m
2
/g from sewage sludge, eucalyptus biochar, mesopore-rich
badam-shell biochar, and onopordom heteracanthom pyrolysis,
respectively. However, the current specic surface area is extremely
large compared to those previously reported research ndings [50]. This
indicates that the adsorbent material can signicantly improve the
adsorption capacity and is more economical to scale up to an industrial
scale.
3.2.3. The SEM analysis
The SEM analysis was used to examine the surface morphology of the
adsorbent and the result was depicted in Fig. 2A and B before and after
adsorption respectively. The SEM analysis of the activated carbon pro-
vided information about the form of the material comprising shape, size,
structure, and pore spaces generated during the activation and pyrolysis
processes. During SEM analysis of examining the surface morphology,
the surface of activated carbon was magnied at a resolution of 600
times (X600), with pore size 50
μ
m at 15 kV, and pores of different sizes
and volumes. Generally, heterogeneous and non-uniform shapes were
observed on the activated carbon which is caused by chemical activation
done using phosphoric acid and pyrolysis at a high temperature of
600◦C. The different pore sizes and distributions were observed from the
magnied image of the RAAC sample, showing some irregularity in the
shape of the pores, which is important for creating a binding site for
adsorbates varying in size and distribution. After adsorption, it seems
the voids and irregularities are decreased showing the successful
attachment of the pollutant onto Rumex Abyssinicus-derived activated
carbon. In addition, the broad spectrum of the adsorbent porosity such
as large pores, roughage, and cracks could result in the conducive op-
portunity to interact with multi pollutants in real wastewater simulta-
neously [6].
3.2.4. The FTIR analysis results
The FTIR spectra of the precursor material and raw Rumex abyssi-
nicus sample are shown in Fig. 3, respectively. The FTIR spectrum was
used to identify the functional groups of the raw Rumex Abyssinicus
sample and RAAC. It was observed that a large number of peaks were
displayed at the raw stage; however, some of the peaks disappeared or
diminished after activation, which is due to the escaping of volatile
components of the original sample as shown in Fig. 4. The FTIR spec-
trum under transmittance mode is expected to have peaks due to the
characteristic structure and specic motion of important functional
groups on the surface of the activated carbon such as –CH
2
, -CH
3
,-C =O,
C =C, C
–
–
–
C, and -C-H was found [51,52]. The peaks at the wavelengths
Table 4
Proximate analysis results of activated carbon.
Parameters of proximate analysis Analysis result ( %) in terms of mean±SD
Moisture content 2.97±0.75
Volatile matter 15.81±3.33
Ash content 9.83 ±0.68
Fixed carbon 71.50±1.14
J.F. Nure et al.
Journal of the Taiwan Institute of Chemical Engineers 151 (2023) 105138
7
of 3329.29, 2893.35, 1724.44, 1624.13, 1313.58, 1234.50, 1024.25,
and 785.06 cm−1 were observed from raw Rumex Abyssinicus sample.
However, after activation of the sample, the number of peaks observed
was reduced to the wavelength values of 3443, 1634, 1316, 1206, 1045,
and 546 cm−1. The relatively broad peak at 3443.08 cm
−1
corresponds
to the intermolecular bound hydroxyl group i.e., due to
hydrogen-bonded -O-H motion of stretching vibrations (polyphenolic
group), absorbed atmospheric moisture, and -NH group. A shift to
3404.51cm
−1
due to the change in stretching motion as shown in the
spectrum of Fig. 4 was also observed which might be due to the for-
mation of intra and intermolecular hydrogen bonds [35]. The peak at
2893.3cm
−1
is attributed to the O
–
H and C
–
H stretching vibrations of
carboxylic acid, alcohol, or alkene groups Fig. 4. The clear sharp peaks
located at 1724.44 cm
−1
and 1624.19 cm
−1
(Fig. 4) indicate the exis-
tence of C =O or C =C stretching vibration of acid derivatives, which
are characteristics of carbonyl group stretching from aldehyde and ke-
tones [33]. Smaller peaks observed between 15,000–1600 cm
−1
are
associated with the stretching motion of aromatic rings. The relatively
less intense peak observed in the region of 1500 –1210 cm
−1
corre-
sponds to the stretching motions of -C-H groups. However, the intense
sharp peak at 1647.37 cm
−1
is due to the stretching vibration of the
carbonyl (-C =O) functional group [35]. The stretching vibration of
-C-C- functional groups was observed at 1205.65 cm−1. The diminished
peak displayed between 1200 and 1500.52 cm−1 was due to the
bending vibration of the -CH
2
functional
group.
The long sharp peak at
1024.24cm
−1
was an indicator of the presence of single C
–
O bonds. In
general, the phenol (OH), amine (NH), carboxyl (C =O) and carboxylic
(COOH) groups are the main contributors to the peaks observed on the
FTIR spectrum [33].
3.2.5. The XRD results
The existence of crystalline structures and the amorphous nature of
the matrix of the RAAC were studied using X-ray diffraction and the XRD
power diffraction spectra for the adsorbent was presented as shown in
Fig. 4. The analysis was undergone at a scan speed of 6 rev/min over an
angle range of 10 to 80
○
. According to the XRD power diffraction result,
Fig. 2. SEM surface image of Rumex Abyssinicus activated carbon (A) before adsorption (B) after adsorption.
Fig. 3. The FTIR spectrum for the raw Rumex-Abyssinicus sample and the activated carbon.
J.F. Nure et al.
Journal of the Taiwan Institute of Chemical Engineers 151 (2023) 105138
8
the RAAC showed two peaks at 2θ =23
o
and2θ =43.74
o
.In general, the
X-ray diffraction pattern showed that the activated carbon has a natural
amorphous structure. However, there is a special place on (2θ) with a
peak where the crystalline nature of the activated carbon was observed.
Normally, amorphous structures create favorable conditions for the
adsorption of different pollutants varying in size and shape, whereas, in
crystalline structures, the adsorption capacity becomes difcult due to
the crystalline structure’s uniform shape [24]. This material showed
amorphous properties and insignicant peaks were not observed [28].
3.2.6. pH point of zero charge
The point at which the adsorbent’s surface becomes neutral was
determined using a pH point of zero charges (pH
pzc
) as depicted in Fig. 5.
Therefore, the surface of the adsorbent has an equal number of posi-
tively and negatively charged surface functions [24]. The pH
pzc
for
RAAC was found to be 7.2, showing that the activated carbon adsorbs
more of anionic pollutants like Black MNN dye in tannery efuent below
this pHpzc and cationic dyes are more likely to adsorb on the positively
charged surface of the adsorbent above pH 7.2. The adsorbent of the pH
point of zero charges became neutral. It was reported that the pH
pzc
of
the adsorbent was found to be 5.03. The surface of the adsorbent is
negatively below and positively above the point, respectively [6].
Fig. 4. The XRD pattern for activated carbon.
Fig. 5. Determination of the pH point of zero charge.
J.F. Nure et al.
Journal of the Taiwan Institute of Chemical Engineers 151 (2023) 105138
9
3.3. Batch adsorption performances
3.3.1. Box-Behnken design method results
The most important parameters affecting the dye removal efciency
are adsorbent dosage (g/100 mL), dye concentration (mg/L), and con-
tact time (min). The adsorption experiments were carried out for
different combinations of the physical parameters using statistically
designed experiments to study the combined effect of these factors. The
batch adsorption experiment on anionic dye removal from synthetic
tannery wastewater was studied using the Box-Behnken design method
under Response Surface Methodology (RSM). Known concentrations of
synthetic anionic tannery dye solutions were used to draw the calibra-
tion curve. Concentrations of the remaining dyes in the solution after the
batch adsorption process were determined using the drown calibration
curve with a correlation coefcient of 0.98 as indicated in Table 5.
In general, the lower removal efciency of 47.6 % was observed at
pH 5.5, a dye concentration of 150 mg/L, an adsorbent dose of 150 mg/
100 mL, and a contact time of 90 min, whereas the maximum dye
removal efciency of 99.9 % was observed at 150 mg/L, pH 2, adsorbent
dose of 250 mg/100 mL, and contact time of 90 min. Several studies
have been conducted on the biomass adsorbent-based removal of tan-
nery and textile reactive dyes. Table 6 depicts the comparison of the
performances of adsorbent in different adsorption studies on reactive
dyes.
3.3.2. Statistical analysis and t summary
The experimental data were statistically analyzed using RSM and the
analysis of Variance (ANOVA) of the percentage removal is presented in
Table 7. The output of different models was compared and focus was
given to the non-aliased model maximizing the Adjusted R
2
(0.9923)
and the predicted R
2
(0.9746) values and additional terms being sig-
nicant. Furthermore, the two values are in agreement with a difference
of less than 0.2. Therefore, the quadratic model was suggested.
The quadratic regression model was found to be the model relevant
for the response prediction, in which an F-value of 141.64 indicates that
the model is signicant. P-values below 0.05 indicate that model terms
are signicant for terms A, AB, AC, BC, A
2
, B
2
and C
2
are signicant
model terms. However, p-values greater than 0.100 indicates that the
model terms are not signicant in which the model terms B, C, AD, and
D
2
are insignicant. This implies linear interaction between pH and dye
concentration, pH and adsorbent dosage, adsorbent dosage, and dye
concentration, adsorbent dosage and contact time, dye concentration
and contact time, and the quadratic interaction effect of pH, adsorbent
dosage and dye concentration affect signicantly the removal efciency
of dye. The quadratic polynomial model was used to develop the
mathematical relationship between the response and the independent
process variables. The output of the t summary of three different
models is presented in Table 8. The selection of the best-t model
focused on non-aliased model maximizing, the Adjusted R
2
and the
predicted R
2
values being large and additional terms being signicant.
Therefore, the quadratic model was suggested. The signal-to-noise ratio
of 47.05, which is greater than 4 indicates that the model has an
adequate signal and can be used to navigate the design space.
3.3.3. Empirical model by response surface estimation
The input data for variables from the experimental design based on
the selected range and experimental response variables are shown in
Table 5. The Box-Behnken Design was used to develop an empirical
model for the maximum removal of reactive dye from wastewater by
building a relationship between the four independent variables and dye
removal efciency from an aqueous solution using a second-order
polynomial equation. Based on the model analysis, a quadratic model
was tted to the data model to predict the response variable. An
empirical relationship between the response and the independent vari-
ables is shown by the following quadratic model Eq. (11).
Table 5
Batch dye adsorption experiment results for synthetic tannery aqueous solution.
Run No. pH Dye con. (mg/L) Adsorbent dosage (g/100 mL) Contact time (min) Absorbance @665nm Actual removal efciency ( %) Adsorption capacity
(mg/g)
2 120 0.25 90 0.004002 98.85 47.448
9 90 0.2 60 0.065381 74.95 7.525
2 150 0.25 30 0.006003 98.62 59.172
2 90 0.15 90 0.003002 98.85 59.310
5.5 90 0.2 30 0.094012 63.98 28.791
5.5 90 0.25 30 0.108132 58.57 21.085
2 150 0.2 30 0.004002 99.08 74.31
2 120 0.25 60 0.005986 98.28 47.174
5.5 150 0.2 60 0.172999 60.23 45.173
2 90 0.15 60 0.003993 98.47 59.082
5.5 120 0.2 90 0.094969 72.71 43.626
9 120 0.2 90 0.074994 78.45 47.07
9 90 0.25 60 0.056011 78.54 28.274
5.5 120 0.25 90 0.057342 78.03 40.091
9 150 0.15 30 0.211019 51.49 51.490
2 120 0.15 30 0.005985 98.28 78.624
2 150 0.25 90 0.000044 99.99 59.994
2 90 0.15 60 0.015008 94.25 56.550
5.5 90 0.2 60 0.092003 64.75 29.138
2 150 0.25 60 0.009005 97.93 58.758
2 150 0.2 90 0.000087 99.98 44.985
5.5 120 0.25 30 0.111952 67.83 32.558
2 120 0.2 60 0.006995 97.99 58.794
9 150 0.15 60 0.222894 48.76 48.760
2 90 0.15 30 0.006003 97.7 58.620
5.5 150 0.15 90 0.217239 50.06 50.060
9 120 0.25 30 0.188929 75.71 21.941
9 150 0.25 90 0.105270 75.8 45.480
5.5 150 0.15 90 0.227985 47.59 47.590
9 90 0.15 90 0.076995 70.5 42.300
J.F. Nure et al.
Journal of the Taiwan Institute of Chemical Engineers 151 (2023) 105138
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Table 6
Comparison of the adsorptive performance of biobased adsorbents on tannery and textile reactive dyes.
No Adsorbent Mechanism Pollutant Removal efciency
( %) and Adsorption capacity(mg/g)
Reference
1 Rumex-Abyssinicus H
3
PO
4
/Thermal Black MNN reactive dye 99 %,
60.00
mg/g
Present study
2 Palm shell powder Sulphuric acid Reactive red-141 13.95
mg/g
[18,53]
3 Marine sea weed Thermal decomposition Reactive red-120 85 %,
107.13 mg/g
[54]
4 Bagasse pith H
3
PO
4
/ Pyrolysis Reactive orange – [55]
5 Coconut shell Coagulant and pyrolysis Reactive Black-5 96 % [56]
6 Thespesia7populnea pod H
2
SO
4
/ Pyrolysis Orange-G reactive dye 96 %, 9.13
mg/g
[57]
7 Palm shell Acid/pyrolysis Remazol reactive dye 90 %, 7.00 mg/g [57]
8 Periwinkle shell CO
2
activation/pyrolysis Remazol Brilliant blue 83 %, 312.00
mg/g
[58]
9 Saw dust Carbonization/ Al
2
(SO
4
)
3
Brilliant blue reactive dye 98 % [59]
10 Jatropha Curcas H
2
SO
4
/Pyrolysis Remazol brilliant blue 95 % [57]
11 Tea leaf Potassium acetate Methylene blue 83 % [57]
12 Tea leaf Potassium acetate Acid Blue-29 80 % [57]
13 Lignocellulosic waste Hydrothermal Reactive Blue-19 97 %,
71.60
mg/g
[57]
14 Lignocellulosic waste Hydrothermal Reactive red-218 94 %,
63.30
mg/g
[57]
Table 7
Analysis of variance for adsorption of dye.
Source Sum of Squares Df Mean Square F-value p-value
Model 9753.21 14 696.66 141.64 0.0001 Signicant
A-pH 1170.77 1 1170.77 238.04 0.0001
B-Adsorbent dose 12.68 1 12.68 2.58 0.1292
C-dye concentration 2.16 1 2.16 0.4387 0.5178
D
–
Contact time 99.82 1 99.82 20.29 0.0004
AB 182.91 1 182.91 37.19 <0.0001
AC 65.91 1 65.91 13.40 0.0023
AD 3.41 1 3.41 0.6927 0.4183
BC 105.64 1 105.64 21.48 0.0003
BD 43.29 1 43.29 8.80 0.0096
CD 28.15 1 28.15 5.72 0.0303
A
2
1631.67 1 1631.67 331.75 <0.0001
B
2
33.91 1 33.91 6.89 0.0191
C
2
57.37 1 57.37 11.66 0.0038
D
2
11.15 1 11.15 2.27 0.1529
Residual 37.37 15 2.49
Lack of t 6.28 11 2.83 1.67 0.263
Pure error 6.28 4 1.57
Cor Total 9826.99 29
Table 8
Fit summary and lack of t for the three models.
Source Sequential p-value Lack of t p-value Adjusted R
2
Predicted R
2
Adeq. Precision
Linear <0.0001 0.0005 0.5898 0.4864
2FI 0.0555 0.0007 0.0.701 05,254
Quadratic <0.0001 0.3294 0.9923 0.9746 47.048 Suggested
Cubic 0.8126 – 0.9737 Aliased
Source Sum of squares Df Mean square F-value P-value
Linear 2953.49 21 140.64 64.53 0.0005
2FI 1886.5 15 125.77 57.70 0.0007
Quadratic 31.09 11 2.83 1.67 0.263 Suggested
Cubic 0.000 0 Aliased
Pure Error 6.28 4 1.57
J.F. Nure et al.
Journal of the Taiwan Institute of Chemical Engineers 151 (2023) 105138
11
Removal efficiency(%) = 69.22 −10.88 ×A+0.8411 ×B−0.5584 ×C
+2.97 ×D+2.25 ×AB −3.44 ×AC +0.5864
×AD +2.05 ×BC +0.9832 ×BD −1.83 ×CD
+19.15 ×A2−0.7091 ×B2−4.29 ×C2+1.43
×D2
(11)
where A is the solution pH, B is the adsorbent dose, C is the dye con-
centration, and AB, AC, BC, A
2,
and B
2
are their linear interaction effect
and quadratic effect, respectively. A, D, AB, AC, BC, BD, CD, A
2
, B
2,
and
C
2
have a signicant effect on the response variable as the P-value for all
is less than 0.05 (P <0.05). However, P-values for B, C, AD, and D
2
are
greater than 0.1000 indicating that the model terms are not signicant.
The model equation describes how the dye removal efciency was
Fig. 6. (a) plot of actual versus predicted values and (b) residual versus predicted points for adsorption of tannery reactive dye using RAAC.
J.F. Nure et al.
Journal of the Taiwan Institute of Chemical Engineers 151 (2023) 105138
12
affected by individual variables by their (linear and quadratic) terms or
double interaction. Negative coefcients indicate that factors negatively
affect the response variable. In this specic study, the double interaction
effect of pH and dye concentration, dye concentration, and time is
negatively affecting the yield of the efciency. Similarly, the quadratic
effects of adsorbent dose and dye concentration were antagonistically
affecting the performances.
3.3.4. Adequacy checks for the developed model
The goodness of t of the model was checked using multiple corre-
lation coefcients (R
2
). The quality of model t for the adsorption
process was evaluated using Fisher’s test (F-value), the probability value
(p-value), the lack of t, the coefcient of determination (R
2
), and
adjusted R
2
(R
2
adj), and predicted R
2
(R
2
pred). F-value and P-value of
the quadratic model were found to be 141.64 and 0.0001, respectively.
This implies that the quadratic model is signicant and the model can
sufciently predict the removal efciency of reactive dye from waste-
water. The correlation coefcient (R
2
) was 0.996 indicating that the
regression is statistically signicant [31]. The quality of the t was also
examined by comparing the actual values against the predicted re-
sponses by the model of the reactive dye removal shown in Fig. 6. Fig. 6
(a) depicts that the predicted values are quite close to the actual
Fig. 7. The response surface (a) and contour map (b) show the interaction of adsorbent dose and pH.
J.F. Nure et al.
Journal of the Taiwan Institute of Chemical Engineers 151 (2023) 105138
13
experimental result both spreading closer to the straight line, which
indicates that the model ts the experimental data. Thus, it conrms that
the regression model exhibits excellent stability for dye adsorption on
activated carbon. Therefore, it can be concluded that the developed
response surface model proved to be satisfactory for the prediction of the
dye adsorption system. In addition to this, the adequacy of the model
was also checked using residual analysis as in Fig. 6(b). The zero residual
line in the plot was used to detect whether the points were scattered
around the horizontal band or clustered in a curved pattern. In this
specic study, the data points form a detectable pattern at almost the
same distance from the zero residual line. Furthermore, the residual data
points uctuate more or less in a random fashion within the horizontal
band, indicating that the model is desirable, and there are no visible
model defects.
3.4. Interaction effect of variables on dye removal efciency
3.4.1. Adsorbent dosage and pH
Response surface methodology was employed to study the combined
effect of initial solution pH and adsorbent dose on Black MNN reactive
tannery efuent dye removal efciency. The response surface and the
contour map are indicated in Fig. 7A and B. The benet of the Box-
Behnken design approach is to study the interaction effect of experi-
mental variables on response variables and develop a response surface
Fig. 8. The response surface and contour map showing the interaction of dye concentration and pH.
J.F. Nure et al.
Journal of the Taiwan Institute of Chemical Engineers 151 (2023) 105138
14
showing various conditions of interaction effects. In this specic case,
the effect of solution pH and dosage on the percentage removal of
anionic tannery dye was studied. To study the interaction effect, dye
concentration and contact time were kept constant at 150 mg/L and 90
min, respectively. The 3D view of the response surface and contour map
depicts the removal efciency of the anionic tannery dye (Black MNN)
which varied between 47.6 % at pH 5.5 and 99.9 % at pH 2 indicating
that the anionic tannery dyes are better adsorbed at pH 2. This is because
the more drop-in solution pH the more the number of protons in the
aqueous solution. This gives more positive charge to the adsorbent
which helps to adsorb more anionic dye molecules [20]. The removal
efciency was seen to drop further to 47.4 % beyond the experimental
minimum (47.6 %) at pH values between 6.8 and 7.2. Specically, there
are no opposite charges to interact with because of the neutrality of the
aqueous solution at this pH range. On the other hand, it is observed that
the percentage of dye removal increased with the amount of adsorbent.
That means increasing the adsorbent dose leads to an increase in
adsorbent surface area and the availability of more sorption sites [60].
However, the excessive addition of adsorbent beyond the required
amount makes the adsorption process uneconomical. In general, keeping
values of dye concentration and contact time constant, the optimum
value of removal efciency was not affected by the interaction between
pH and adsorbent dosage implying that the percentage removal ef-
ciency was positively affected by their interaction.
3.4.2. Concentration and pH
The interaction effect due to different levels of initial dye concen-
tration and solution pH on the response variable is shown in Fig. 8. In the
Fig. 9. The response surface and contour map showing the interaction of dye concentration and dosage.
J.F. Nure et al.
Journal of the Taiwan Institute of Chemical Engineers 151 (2023) 105138
15
gure, it was observed that both the independent variables strongly
affect the dye adsorption process. The experimental condition to study
this interaction was conducted keeping dosage and the time of contact,
at the constant values of 0.24 g/100 mL and 88 min, respectively. The
3D plot and contour map depict better responses which were obtained at
higher dye concentrations and lower pH values. The maximum removal
efciency of 99.9 % was achieved at the initial pH of 2 and dye con-
centration of 150 mg/L. From this situation, it can be inferred that
increasing dye concentration at a lower solution pH increases the
driving force, which enhances the adsorption process [61]. A relatively
strong interaction existed between initial concentration and pH, which
was reected by the corresponding P-value (P<0.001) [62]. Besides this,
an increase in protons due to the lowering of pH provides a suitable
medium for the interaction of adsorbent active sites with the anionic
tannery dye molecules. This is because the chemistry of the dye mole-
cules was affected by the solution pH and the activity of functional
groups on the adsorbent surface. Moreover, the competition of due
molecules with hydrogen ions for the binding active sites was also
observed [63]. However, a further increase of dye concentration while
the adsorbent active sites remain constant will saturate the adsorbent
which causes the driving force to remain the same until the adsorption
process reaches equilibrium. In general, the interaction between pH and
Fig. 10. The response surface and contour map showing the interaction of dye concentration and contact time.
J.F. Nure et al.
Journal of the Taiwan Institute of Chemical Engineers 151 (2023) 105138
16
dye concentration negatively affects the removal efciency.
3.4.3. Dye concentration and adsorbent dosage
The response surface and contour map depicting the effects due to
the interaction of the dye concentration and adsorbent dose on dye
removal efciency are shown in Fig. 9. Experimental data and the pre-
dicted removal efciency values were used to generate a response sur-
face showing the coexistence of the above two experimental variables at
which the pH and contact time were kept constant at 2 and 88 min,
respectively. The 3D response surface plot and contour map show that
better response was observed at the dye concentration of 125 mg/L and
adsorbent dose of 0.24 g/100 mL. Further increasing the adsorbent dose
keeps the dye concentration at 125 mg/L which will end up in the
reduction of removal efciency and adsorption capacity. This might be
due to the interference of adsorbents with each other for the dye mol-
ecules not to have direct contact with the active sites and due to
excessive active sites above the required amount [64]. That is, even if a
dose of too much adsorbent tends to have better removal efciency, the
adsorption capacity drops due to excessive utilization of the adsorbent
beyond the optimum limit [65]. On the other hand, an increase in dye
concentration in an aqueous solution at a constant dosage of 0.24 mg/L
ends up with free dye molecules due to the saturation of the adsorbent.
Hence, a drop in the response variable was observed. Therefore, the
interaction of adsorbent dosage and dye concentration positively affects
the removal of anionic tannery dye.
3.4.4. Contact time and dye concentration
The 3D plot and contour map in Fig. 10 show the combined effect of
dye concentration and contact time. The experiment was done keeping
pH 2 and adsorbent dosage (0.24 mg/100 mL) constant. It is observed
from the contour map that an optimum removal efciency of 100 % is
obtained at a contact time of 75 min and dye concentration of 135 mg/L.
The interaction effect of these parameters can be described as moving
along the 100 % contour line, which means that to get the same removal
efciency of 100 % across the dye concentration above the optimum
135 mg/. But it requires more time of contact. This is because the
amount of dye in the efuent increases the competition of dye molecules
for the active site increases so that the probability of contact between the
dye molecules and the adsorbent active site decreases. Moreover, this is
because the more concentrated the dye the better the driving force and
the frequency of contact with the adsorbent [61,64]. This can be
described considering the dye concentration of 148 mg/L which needs a
contact time of 80 min to have the dye molecules removed. Similarly,
with the drop in the concentration of dye molecules below 135 mg/L,
the time of contact required again increases due to a reduction in the
probability of dye molecules interacting with the adsorbent active sites.
In another situation, a drop in removal efciency is observed due to a
decrease in dye concentration at the constant contact time, which is due
to a reduction in the frequency of contact with the xed amount of
adsorbent. Therefore, to have better interaction of dye molecules with
the active sites at constant contact time higher concentration dye mol-
ecules are required. In general, the interaction of dye concentration and
time of contact affects the response variable negatively.
3.5. Adsorption isotherms
The plot of 1/Qe versus 1/Ce resulted in the Langmuir correlation
coefcient of 0.97 shown in Fig. 12a. The Langmuir adsorption capacity
(q
max
) and binding energy constant (K
L
) were calculated to be 38.46 mg/
g and 0.32 L/mg, respectively. The Langmuir separation factor (R
L
) was
calculated using Eq. (10) and found to be (0.07). This represents a
favorable adsorption process showing the effectiveness of the interac-
tion of the adsorbent with the anionic tannery dye. On the other hand,
the model for Freundlich isotherm for the adsorption process on a het-
erogeneous adsorbent surface layer was presented in Fig. 12b. The
isotherm plot of logq
e
versus logC
e
gave a correlation coefcient of 0.96.
Using 1
n=0.1616 and logK
f
=1.3989from the isotherm model, the
Freundlich constant, n was calculated to be 6.19 at K
f
=25.05con-
rming the capacity of the adsorptive removal of synthetic tannery dye
on RAAC. Additionally, the plot of ln Ce versus Qe resulted in the
Temkin correlation coefcient of 0.96 as shown in Fig. 12c. Temkin
equilibrium binding constant (K
T
) and Temkin heat of adsorption con-
stant were determined to be 163.04 L/mg and 491.712 J/mol, respec-
tively. On the other hand, empirical intensity related to sips isotherm
and equilibrium binding constant for the sips isotherm model was found
to be 0.3629 and 5.53 respectively as shown in Fig. 12d. Values of the
determined correlation coefcient indicate that the adsorption experi-
mental data at the optimum parameters have better tness for the
Langmuir model (with a maximum R
2
of 0.97 as well as insignicance
Residual sum of squares(RSS)), which implies that the Langmuir model
predicts the adsorption trend better. Therefore, it can be inferred that
the adsorbent is homogeneous the active sites of the adsorbent have
linear distribution and the surface of the adsorbed is saturated with a
single attachment of the dye molecules [24,66] (Fig. 11).
3.6. Adsorption kinetic study
Optimum conditions from the batch experiments were used to study
the information about the kinetics of dye adsorption. Experimental
adsorption capacity (q
e,exp
), calculated adsorption capacity (q
e,calc
), and
values of parameters like K
f
, Ks, and K
p
for all three kinetic models were
calculated from the developed model curves as shown in Table 9. The
coefcient of determination (R
2
) for pseudo-second-order reaction
adsorption kinetics was found to be 0.99, However, a lower value of the
coefcient of determination R
2
0.86 was obtained for pseudo-rst-order
reaction adsorption kinetics. Therefore, due to its higher coefcient of
determination and the insignicant difference between its equilibrium
and calculated adsorption capacities (q
(e,exp)
and q
(e,calc)
) was observed.
It can be concluded that the pseudo-second-order model will describe
anionic dye adsorption kinetics better. Furthermore, chemisorption was
taken to be the rate-limiting adsorption mechanism that results from the
chemical bond between adsorbate molecules and specic surface loca-
tions (active sites) on RAAC. To study the infusibility of the adsorbate to
the interior surface of the adsorbent, a curve of the Intraparticle diffu-
sion model was developed and the Intraparticle diffusion rate constant
(K
p
) and the C (mg/g) values were determined to be 0.192 mg/(g.
min^0.5) and 57.55 mg/g, respectively. This showed that the rate of
adsorption is dependent on both reactants(the adsorbent and adsorbate )
and Intraparticle diffusion does not exclusively control the adsorption
process [67–70].
4. Conclusions
Wastewater from Elico-Awash tannery in Ethiopia was analyzed for
the concentration of Black MNN reactive dye pollutant and its concen-
tration was found to be 92.76±4.26 mg/L. Activated carbon was
developed from Rumex Abysinicus using the chemical activation method
followed by thermal activation then applied for the removal of the dye
from the aqueous solution and real tannery wastewater under batch
adsorption mode. The proximate and BET analysis results showed an
extremely large specic surface area of 3620 m
2
/g. The proximate
analysis results of low volatile matter and moisture content make the
developed activated carbon favorable for the intended adsorption pro-
cess. Furthermore, the FTIR analysis result conrmed the presence of
several functional groups for high interaction whereas the morpholog-
ical void spaces in the adsorbent described by SEM analysis could be the
reason for such a high specic surface area. The maximum dye removal
efciency of 99.9 % was obtained at the optimum working conditions of
initial dye concentration150 mg/L, pH 2, adsorbent dosage 0.25 mg/
100 mL, and contact time 90 min. However, under the same optimum
working conditions, the removal efciency from real tannery
J.F. Nure et al.
Journal of the Taiwan Institute of Chemical Engineers 151 (2023) 105138
17
Fig. 11. Langmuir (a), Freundlich isotherm (b), Temkin (c), and Sips (c) for tannery dye adsorption at the optimum conditions of parameters.
J.F. Nure et al.
Journal of the Taiwan Institute of Chemical Engineers 151 (2023) 105138
18
wastewater was found to be 94.6 %, which is lower due to other inter-
fering ions and pollutants. Freundlich and the Langmuir isotherm
models were used to t the experimental adsorption isotherm data,
where Langmuir isotherm was found to t better with the experimental
data with R
2
=0.973 showing the presence of the homogeneous
adsorbent surface. Kinetic study showed that the data is best suited to
pseudo-second-order kinetics with a correlation coefcient of 0.99. In
general, it was observed that adsorbent prepared from Rumex-Abyssini-
cus was found to be effective in removing dyes from tannery wastewater.
Therefore, it can be applied as a low-cost, locally available, and prom-
ising treatment option for dye removal from tannery industrial waste-
water which can be recommendable to be scaled up to an industrial scale
to mitigate the pollution challenges. However, additional works such as
thermodynamic study, regeneration, and reusability as well as the
activated carbon surface modication studies need to be done in the
future to commercialize the prepared adsorbent.
Declarations
Data availability
The data used to support the ndings of this study are included in the
article and can be obtained from the corresponding author.
Funding
The authors didn’t receive any funds for this research except for the
laboratory facilities
Compliance with ethical standards
Not applicable
CRediT authorship contribution statement
Jemal Fito Nure: Conceptualization, Methodology, Software, Vali-
dation, Formal analysis, Data curation, Writing – original draft, Writing
– review & editing, Supervision. Ashagrie Mengistu: Conceptualiza-
tion, Methodology, Software, Validation, Investigation, Writing – orig-
inal draft, Writing – review & editing, Visualization. Mikiyas Abewaa:
Writing – original draft, Writing – review & editing. Kenatu Angassa:
Writing – original draft, Writing – review & editing. Welldone Moyo:
Writing – original draft, Writing – review & editing. Zebron Phiri:
Writing – review & editing. Potlako J. Mafa: Writing – review & edit-
ing. Alex T. Kuvarega: Writing – review & editing. Thabo T.I. Nkam-
bule: Validation, Formal analysis, Data curation, Writing – original
draft, Visualization.
Declaration of Competing Interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Acknowledgments
The authors would like to acknowledge the Manufacturing Industry
Development Institute, Leather and Leather Products Industry Research
and Development Center, which is located in the Akaki Kality sub-city of
Addis Ababa, Ethiopia, for giving us support for the research work
including the facilities. Additionally, we would like to appreciate the
contributions from the Institute for Nanotechnology and Water Sus-
tainability (iNanoWS), University of South Africa, Johannesburg, South
Africa, and Addis Ababa Science and Technology University, Addis
Ababa, Ethiopia.
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Parameters for kinetics study tannery efuent dye adsorption.
Pseudo-rst –order Pseudo-second-order Intraparticle diffusion
Parameters Values Parameters Values Parameters Values
q
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(mg/g) 59.95 q
e,exp
(mg/g) 59.95
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