Journal of Hazardous Materials 428 (2022) 128238
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Journal of Hazardous Materials
journal homepage: www.elsevier.com/locate/jhazmat
Research Paper
Selective adsorption of anionic dye from wastewater using
polyethyleneimine based macroporous sponge: Batch and
continuous studies
Mohd Arish Usman , Anees Y. Khan *
Department of Chemical Engineering, Manipal University Jaipur, Dehmi Kalan, Off. Jaipur-Ajmer Expressway, Jaipur 303007, Rajasthan, India
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
• Method of Ice Templating used to
fabricate macro-porous PEI sponge
(S100).
• S100 utilized in both static and dynamic
adsorption experiments
• Achieved remarkably higher adsorption
capacity of the S100 (1666.67 mg/g) for
Congo red among PEI based sorbents.
• Selectively separate anionic dyes from
binary mixtures
• S100 retain the original capacity up to
five consecutive adsorption–desorption
cycles.
A R T I C L E I N F O
A B S T R A C T
Editor: Dr. R. Maria Sonia
Dyes are well known for their hazardous impacts on public health and the environment. Dye removal using
monolithic adsorbents is an attractive approach for industrial applications and process design owing to their
utilization in both static and dynamic adsorption experiments. In the present work, polyethyleneimine (PEI)
based macroporous monolithic sponge (S100) was engineered by ice-templating method and used as an adsor­
bent. Both batch and continuous operations for dye removal were studied. The effect of various parameters such
as pH, adsorbent amount, flow rate, influent dye concentration, and adsorbent bed height on adsorption per­
formance of S100 was studied and modelled using Langmuir/Freundlich isotherms for static operations and
Adam-Bohart/Thomas model in packed-bed column experiments. Under optimum conditions, the adsorbent
showed a remarkably higher adsorption capacity towards CR (1666.67 mg/g), which is considerably higher than
most PEI-based adsorbents. Amine groups in S100 offered exceptional selectivity for anionic Congo red (CR)
against cationic Methylene blue (MB) dye (separation factor of 208 and 87 in absence and presence of sodium
chloride, respectively). It can be easily regenerated in alkaline medium without a significant loss in percent
adsorption capacity and shows good thermal and mechanical stability. Notably, in column studies, a relatively
smaller percentage of unused bed height (32.3%) was observed with higher dye uptake for 16 mg S100 at flow
rate 10 mL/h and inlet concentration 300 mg/L. Thus, the adsorbent displays an outstanding physiochemical
characteristic, excellent selectivity for anionic dye, ease of regeneration and high adsorption performance in both
batch and continuous studies.
Keywords:
Polyethyleneimine
Selective adsorption
Batch adsorption
Continuous flow adsorption, porous materials
* Correspondence to: Chemical Engg. Department, MUJ, India.
E-mail address: anees.khan@jaipur.manipal.edu (A.Y. Khan).
https://doi.org/10.1016/j.jhazmat.2022.128238
Received 10 October 2021; Received in revised form 3 January 2022; Accepted 5 January 2022
Available online 7 January 2022
0304-3894/© 2022 Elsevier B.V. All rights reserved.
M.A. Usman and A.Y. Khan
Journal of Hazardous Materials 428 (2022) 128238
selective removal of anionic dyes using PEI blended adsorbents such as
metal organic frameworks, fibre, bio-based polymer (bio-char, cellulose,
membranes) etc (Yang and Liu, 2019; Kumari et al., 2016; Saratale et al.,
2011). Min et al. (2012) achieved adsorption capacity of 1000 mg/g for
Sunset yellow FCF, by fabricating nano-fibrous structure membrane of
poly (ether sulfones) blended with PEI. Huang et al. (2020) fabricated
PEI blended platinum nanomaterials onto bio-based polymer targeting
acid black ATT dye reaching adsorption capacity of 1157.9 mg/g while
13.5 mg/g for methylene blue (MB). Sui et al. (2013) synthesize 3D
graphene oxide (GO)-PEI based lightweight adsorbent demonstrating
high adsorption uptake for both acidic dyes and carbon dioxide. Zhao
et al. (2018) prepare GO-PEI 3D aerogel via sol-gel method to selectively
remove anionic MO from its mixed solution with MB. Zhu et al. (2016)
chemically oxidise cellulose molecules using hyperbranched PEI
achieving adsorption capacity of 2100 mg/g for CR and 1860 mg/g for
cationic basic yellow 28 dye. Ahmad et al. (2021) synthesised bifunc­
tional polyethyleneimine-based sponges for selective and efficient sep­
aration of ionic pollutants from wastewater. Yan et al. perform both
batch and continuous flow adsorption experiments for efficient removal
of lead Pb (II) and acidic red AR 18 from wastewater employing poly­
dopamine (PDA) modified PEI based sponges (Liu et al., 2020).
PEI blended crosslinked three-dimensional monoliths which were
synthesized using other substances (such as graphene oxide, gelatin, and
silk fibroin), are usually costly and required sophisticated fabrication
method (Huang et al., 2020; Sui et al., 2013; Godiya et al., 2019).
Although, batch adsorption studies for selective removal of anionic dyes
using PEI-based adsorbent have been widely investigated (Zhu et al.,
2016; Jiang et al., 2020; Guo et al., 2018; Wang et al., 2020), innovative
route to selectively remove the anionic dyes from the mixture of
anionic-cationic dyes from wastewater using both batch and continuous
adsorption methods with a high separation factor is rarely discussed.
Therefore, it is highly desirable to synthesize a simple PEI-based
monolithic adsorbent for high selectivity and efficient adsorption
capability towards carcinogenic anionic dyes.
In the present work, the most frequently used anionic azo dyes CR/
MO and cationic MB were utilised as model pollutants to investigate
selective and competitive adsorption capability of S100 towards anionic
pollutants. Ice-templated macroporous PEI based polymeric sponges
selectively and efficiently remove anionic contaminants from a binary
mixture of anionic-cationic dyes. The reason behind selecting targeted
dyes as model contaminants for adsorption studies is their wide appli­
cability in industrial sector and also the environmental damage they
cause to the aquatic environment (Zhou et al., 2018; Faysal Hossain
et al., 2020; Liu et al., 2020). The main novelty of these sponges lies in
their cost effective, single step preparation process, with high selectivity
and exceptional adsorption capacity towards anionic pollutants as
compared to other PEI based sorbents. The positive surface charge
created on the monolithic surface led to a high separation performance
of CR/MO in comparison to cationic MB. The adsorption equilibrium
and kinetic studies for CR were carried out for both batch and contin­
uous flow adsorption experiments with various parameters such as
initial concentration of CR, solution temperature, initial solution pH,
contact time, flow rate, bed height etc.
1. Introduction
Polluted water causes various health hazards, threatens aquatic di­
versity, restricts photosynthetic activity, and disturbs the production of
different crops (Gavrilescu et al., 2015). Stringent rules and legislations
have been passed till date to safeguard the quality of water, but the
wastewater disposal methods and the facilities to treat wastewater are
not adequate (Drumond Chequer et al., 2013). Dyeing industry is one of
the largest water consuming industry (after agriculture) in India
(Krishnan and George, 2016). The streams coming out of the dyeing
industries contain various chemicals and colouring compounds and the
spout requires proper treatment before discharge (Tavangar et al.,
2020). Consumption of dyed water even at low concentration ˂ 100
mg/L causes waterborne diseases like typhoid, amoebiasis, anaemia,
neutropenia, ascariasis, respiratory infections, hepatitis, vomiting,
stomach aches and even death (Lellis et al., 2019). More than 3600
variety of textile dyes are available in the market (Kant, 2012). Many
classical dyes such as congo red (CR), methylene blue (MB), rhodamine
B (RhB) are utilized in textile dyeing and printing business due to their
suitable bright colour and fastness properties (Wong et al., 2020). It has
been reported that 30 out of 72 toxic chemicals generated in water from
textile dyeing cannot be removed (Kant, 2012).
Generally, dyes are classified based on their chemical structures
which determine their properties, colour, and application (Kiernan,
2001). According to the molecular structure these are sub classified into
cationic or anionic dye (Bartošová et al., 2017). Among these, removal
of anionic dyes required proper attention as these are highly carcino­
genic, damage DNA and nervous system if consumed (Bentahar et al.,
2017; Gong et al., 2013). For instance, Methyl orange (MO) and CR are
two most stable azo dyes, widely used in industrial segments as a syn­
thetic colorant, which are difficult to degrade by biological treatment
due their complex structure (Gong et al., 2013; Huang et al., 2019; Liu
et al., 2019). Long-term exposure of CR in polluted water can cause
jaundice, skin irritation, cancer, vomiting in human beings (Bentahar
et al., 2017). Consequently, intensive research efforts and effective
strategies are undertaken for the decontamination of these ionic pol­
lutants from wastewater.
Various conventional methods such as flocculation/coagulation,
membrane filtration, electrospinning, photocatalytic degradation, sedi­
mentation, osmosis, ion exchange, biological degradation etc. are
available (Krishnan and George, 2016; Tawfik et al., 2014; Rao et al.,
2012). However, due to their limitations of low selectivity, low recov­
ery, high maintenance and operational cost, researchers are looking for
an effective, efficient, and economical method for dyed water treatment
(Mousavi et al., 2018; Wang et al., 2013, 2020). Adsorption is recog­
nised as an economically viable, technically acceptable approach hing­
ing on its simplicity, high selectivity, regeneration capability, low
energy dissipation and eco-friendly nature (Mecca et al., 2020; Yang and
Liu, 2019).
Functional groups such as amine, amide, ketone, carboxylic acid
present in the guest molecules determine the surface capacity, efficacy,
selectivity and regeneration capability of the adsorbent material (Dru­
mond Chequer et al., 2013; Yang et al., 2019). Adsorbent containing
amine groups display versatile dual nature to adsorb both anionic and
cationic host molecules at varying pH values in liquid media (de Farias
et al., 2018; Min et al., 2012). Monolithic adsorbents due to their high
surface area, porous nature, structure stability, and tuneable function­
ality display outstanding performance in comparison to traditional
granular adsorbents (Kumari et al., 2016; Liu et al., 2020). Further, these
monolithic adsorbents can be employed for continuous flow adsorption
experiments, a mechanism similar to traditional packed beds (Saratale
et al., 2011; Malakhova et al., 2020). Polyethyleneimine (PEI), a
repeating linear (C2H5N)n polymer, has emerged as an ideal adsorbent
additive due to the presence of abundant primary and secondary amine
groups along its polymeric chain (Krishnamoorthi et al., 2021; Liu et al.,
2020). Already many research papers are available in relation to
2. Experimental
2.1. Materials and instruments
Congo red (CR), methyl orange (MO), and methylene blue (MB) dyes
were purchased from Loba Chemie and used as received. Branched
polyethyleneimine (PEI, MW 60,000 Da, 50%), crosslinker 1–4 buta­
nediol diglycidyl ether were obtained from Sigma Aldrich. Salts such as
potassium chloride (KCl), sodium hydroxide (NaOH), aqueous hydro­
chloric acid (HCl 33 wt%) are of analytical grade and were obtained
from Loba Chemie. Sodium chloride (NaCl) was purchased from Merck.
The stock solutions of 1 litre of 2000 mg/L of CR and 1000 mg/L of MB
2
M.A. Usman and A.Y. Khan
Journal of Hazardous Materials 428 (2022) 128238
were made by dissolving the required quantities in de-ionised (DI)
water. Whenever needed, contaminant desired concentration was
formulated by diluting from stock solution. Calibration curves were
plotted for respected dye standard solutions (5–20 mg/L for CR, 5–15 for
MO, and 1–5 mg/L for MB) analysed spectrophotometrically using
HACH UV-Vis-spectrophotometer. Elemental analyses of pristine sponge
(S100), sponge dyed with CR (S100/CR) and MB (S100/MB) were performed
using EDAX spectrometer (AMETEK Instruments) equipped with scan­
ning electron microscope (SEM). The electron micrographs of gold
coated sponges before and after adsorption with respective dyes were
captured by SEM (JEOL JSM-7610 FPLUS model). The Fourier transform
infra-red (FTIR) analysis was performed using Bruker ALPHA FTIR
Spectrophotometer in KBr pellets (spectroscopic grade) in the wave­
length range of 500–4000 cm− 1 for S100, S100/CR and S100/MB. Thermo
Gravimetric Analyser (TGA, Shimadzu) was utilised to test the thermal
stability in nitrogen atmosphere at a heating rate of 10 ℃/min in the
range of 30–750 ℃. The solution pH was measured using pH meter
(MxRady Instruments). In a continuous flow adsorption experiment,
uniform flow rates were maintained by a syringe pump.
the residual dye was analysed using UV-Vis- spectrophotometer at the
maximum wavelengths of (λmax) 497 nm, 464 nm, and 664 nm for CR,
MO, and MB, respectively. The CR/MO/MB dye removal percentage and
adsorption capacity of S100 (mg/g) were determined using Eqs. 1 and 2
given as (Shakoor and Nasar, 2017; Kumar and Tamilarasan, 2013):
(
)
Co − Ct
% removal = 100
(1)
Co
qe = (Co − Ce )
V
m
(2)
where, Co is the initial concentration of the adsorbate, Ct is adsorbate
concentration at any time t, Ce is the adsorbate concentration at equi­
librium (mg/L), and qe is the adsorbate adsorbed efficiency at equilib­
rium. The time is measured in h, V represents volume in litres, and m is
the weight of S100 in g.
To explore the effect of solution pH on the adsorption performance of
S100, the solution pH was varied in the range 6–10 for both CR and MB
adjusted using 0.1 M HCl or 0.1 M NaOH solution. These experiments
were performed for 1000 mg/L of CR and 300 mg/L MB, respectively.
The adsorption isotherm experiments were conducted at constant
temperature of 30, 40 and 50 ℃ with initial dye concentrations ranging
from 800 to 1400 mg/L for CR and 50–300 mg/L for MB, respectively.
However, adsorption performance for MO was obtained at temperature
40 ℃ with initial dye concentrations ranging from 800 to 1400 mg/L at
pH 6. The kinetic experiments for CR adsorption were carried out at
50 ℃ by analysing residual dye concentration at predetermined time
intervals at four distinct initial CR concentrations of 800, 1000, 1200
and 1400 mg/L, respectively. The effect of mass of S100 adsorbent on CR
sorption capacity and removal efficiency was investigated by adding
different amounts of S100 (4, 8, 12 and 16) mg into 10 mL of 1000 mg/L
CR solution at 50 ℃.
2.2. Synthesis of crosslinked PEI sponges
Synthesis of PEI based sponge has been reported in our previous
work (Usman and Khan, 2021). In separate 2 mL Eppendorf tubes, 470
µL de-ionised water was taken. To each tube either 90, 100, or 120 µL of
PEI solution (100 mg PEI/mL water) was added and the contents were
vortexed for 15 min. Subsequently, 10 µL of crosslinker was added to
each tube and vortexed again. Afterwards, the tubes were placed inside
refrigerator at − 16 ℃ for ice-templating. After 24 h, the ice templated
sponges were taken out from tubes, washed multiple times with water to
remove the sol fraction and dried at 50◦ C. The sponges were names as
S90, S100, and S120 based on the aliquots of PEI solution used in the
experiment. These sponges were kept separately in airtight sample
containers for further studies. All the static/dynamic experiments were
performed using sponge S100 as it exhibits the maximum dye uptake
efficiency (data not shown).
2.5. Selective adsorption performance of S100
The selective performance of S100 was evaluated using the procedure
discussed elsewhere (Wang et al., 2020). Initially, the calibration curves
for binary mixtures of CR and MB were prepared by varying the molar
concentrations of one dye while fixing the other. This step was crucial to
avoid any mutual interference between the CR and MB molecules (Yang
et al., 2019). For instance, the concentration of MB was fixed at
0.015 mM, and CR was dissolved to prepare a solution ranging from
0.07 to 0.35 mM. Similarly, the concentration of CR was fixed at
0.014 mM and MB concentration was varied between range 0.06 –
0.18 mM.
Known amounts of CR and MB were taken from the prepared stock
solution to formulate 10 mL of binary mixture placed in 15 mL glass vial
with molar ratios of CR/MB varying as: 9:1, 3:7, 5:5, 7:3, and 1:9,
respectively. Then S100 was added into each vial, stirred at 250 rpm for
24 h at 50 ℃. Afterwards, the supernatant was analysed using UV-Vis
spectrophotometer. Evaluation of selective performance of S100 for
both anionic and cationic dye is based on separation factor (α) deter­
mined using Eq. 3.
(
)(
)
QCR
CMB
αCR/MB =
(3)
QMB
CCR
2.3. PZC determination
Point of zero charge (PZC) was determined using salt addition
method as reported in literature (Usman et al., 2021). Initially, 0.1 M
KCl stock solution was made and 10 mL from this stock solution was
transferred into five different 15 mL glass vials. The initial pH of each
vial was adjusted between 4 and 12 using 0.1 M HCl and 0.1 M NaOH
solutions. Afterwards, the sponge S100 was added to each of these vials.
Similarly, the above-mentioned step was repeated with 10 mL of
de-ionised (DI) water with pH varying in the same range (4− 12). After
24 h with intermittent stirring (50 rpm), the final pH of the solutions was
analysed using pH meter and a graph was plotted between initial pH and
ΔpH (change from initial to final pH readings) for both KCl and DI so­
lutions. The adsorbent surface charge (pHZPC) was the point of inter­
section at which ΔpH= 0 (Rao et al., 2016).
To further prove electrical neutrality of S100 surface, mass titration
experiments were performed at constant ionic strength of 0.1 M NaNO3
at 30 ⁰C as reported in literature (Mahmood et al., 2011). Aqueous so­
lution of 10 mL NaNO3 containing different amounts of S100 were
equilibrated for 24 h (100 rpm), and then the pH of each was measured.
The PZC was determined from the appearance of a plateau in the pH
versus mass of S100 curve.
Where, Qi and Ci (i: CR or MB) represents the equilibrium adsorbed dye
quantity and equilibrium concentration in the supernatant, respectively.
Selective adsorption of sponge S100 was further checked by formu­
lating 10 mL of 1:9 molar MO/MB aqueous solution kept at pH 6. Ac­
cording to the UV spectrum result, selective performance of adsorbent
for binary mixture is evaluated using same procedure as discussed
above.
2.4. Batch adsorption experiment with S100
Typically, the sponge S100 (16 mg in weight) was put into a 15 mL
glass vial containing 10 mL of CR, MO, or MB aqueous solution with
known concentrations. The glass vial was kept on a multistage magnetic
stirrer (250 rpm) for adsorption until equilibrium was achieved. Then,
3
M.A. Usman and A.Y. Khan
Journal of Hazardous Materials 428 (2022) 128238
2.6. CR saturated sponge (S100/CR) regeneration
bed (cm), respectively.
Reusability studies were performed by immersing the CR-saturated
sponge S100/CR (~26 mg) into 10 mL of (0.05 M) NaOH solution with
magnetic stirring at 250 rpm for 6 h at 30 ℃. Afterwards, the regener­
ated sponge (S100/CR) was picked up and thoroughly cleaned with deionised water, followed by drying in an oven at 55 ℃ for the next
cycle. Removal performance of each recycle was evaluated using Eq. 4
given as:
3. Results and discussion
%Removal =
C0 − Ce
× 100
C0
3.1. Synthesis and textural characterization
The process of synthesis of macroporous monolithic structure has
been discussed in our previously published work (Usman and Khan,
2021). Water with dissolved PEI and a crosslinker turns to ice crystal at
− 16 ◦ C and in the process, throws PEI and the crosslinker towards its
boundaries. This results in their very high local concentrations leading
to crosslinked, self-standing monolithic sponge (Deville, 2018). Since
the water content is very high (~95% by volume), the ice crystals on
thawing and drying translates to macropores in the sponge. Being hy­
drophilic, the as-prepared sponge (named S100) swells in water and
absorbs 30 times of its dry weight (Chatterjee et al., 2016). Fig. 1a shows
SEM images of S100 (before adsorption) with many interconnected pores
with an average pore diameter of 40 µm (calculated using ImageJ soft­
ware). While SEM images in Fig. 1b-c reveal the porous structure after
the adsorption of CR (named as S100/CR) or MB (S100/MB). Further, Inset
in Fig. 1b show that CR is uniformly adsorbed at both internal and
external surfaces of S100 while trace amount of MB is noticed only on its
external surface (Inset Fig. 1c). Fewer opened pores visible in S100/CR is
due to penetration of CR into the pores causing pore blockage and a
decrease in surface area. Comparatively, fewer cavities are noticed in
S100/MB and the average pore diameter drops down to 12 µm in size.
Stacking of original pores might be due to the resistance offered to the
cationic MB by the protonated surface of the amine-based sponge,
indirectly affecting the surface morphology (closed basic sites) (Li et al.,
2018). The swelling ratio and the sponge size decreased after adsorp­
tion, which may attribute to the change in surface morphology as
observed in Fig. 1 (Ahmad et al., 2021). Average pore diameter of
sponge before and after sorption were evaluated using ImageJ and
Origin Software and shown in Fig. S1 in SI.
Energy-dispersive X-ray (EDX) analysis was done to analyse and
compare the elementary composition of sponges with different elements
C, N, O, and S before and after dye adsorption (Gao et al., 2013). The
results in Table 1 show a high N content in S100 owing to its amine-rich
PEI-colloid hybrid structure (Li et al., 2018; Liu et al., 2020). On
adsorption of CR, the N content in S100/CR decreases from 26% to 20%
while S content increases 0.5% to 4.5%. This can be attributed to
adsorption of negatively charged CR on positively charge amine-rich
S100. Further, S content is higher in S100/CR than S100/MB, we ascribe
this to the inherent S content in CR and MB which adhere to the poly­
meric sponge during adsorption experiments. The S peaks in the EDS
spectrum after adsorption as shown in Fig. 1 indicate that sponge was
successfully endowed with pollutants (Yap et al., 2020). The distribution
of elements on the surface of sponge obtained by elemental mapping in
EDX has been shown in Fig. S2 in SI.
FT-IR analysis was performed to elucidate the changes in functional
groups before and after dye adsorption in S100, S100/CR, S100/MB along
with as received CR and MB (Fig. S3, SI). Tailored sponge S100 display
prominent peaks at 3400 cm− 1, 2936 cm− 1, 2849 cm− 1, and 1478 cm− 1,
attributed to the stretching vibration of O-H, (CH2)n stretching band, CH stretching vibration, and C-N-H stretching vibration, respectively (Yap
et al., 2020). The intense peaks at 1115 cm− 1 in spectra of S100 corre­
sponds to C-N-C bond stretching vibration, which confirm successful
crosslinking of PEI (Tang et al., 2020). The displayed FTIR spectra in Fig
S3 showed characteristic peaks of diazo CR at 1612 cm− 1 and
– N-)
1040 cm− 1 which are indicative to the vibrations of azo group (-N–
and unsymmetrical wagging vibrations of S-O(SO3-H) group (Vimonses
et al., 2009; Kim et al., 2019). The characteristic intense peak at
– N/C–
– C wagging and
1598 cm− 1 and 1336 cm− 1 corresponds to the C–
C-N stretching vibrations of dimethyl amino groups in MB (Shakoor and
Nasar, 2017; Li et al., 2018). With CR penetration, two new adsorption
peak appears at 1612 cm− 1 and 1050 cm− 1 for S100/CR.. This suggest
(4)
where, Co and Ce are the initial and equilibrium concentrations (mg/L),
respectively of the dye molecules.
2.7. Continuous flow adsorption experiments
Sorption performance of S100 for CR removal was studied by using a
glass column of 10 cm height and inner diameter of 0.6 cm. Initially, the
dried S100 sponge was fitted at the middle of the column and saturated
with deionised water. Due to its hydrophilic nature, it expands to a
desired height of 0.9 cm after 24 h and gets tightly packed. This helps in
preventing channelling during continuous flow experiments. The influ­
ence of inlet dye concentration, flow rate and adsorbent mass on the
removal efficiency was studied. The sorption capacity of S100 towards
CR was examined by varying flow rates (10, 15, and 20 mL/h) and
influent concentration (300, 525, and 700 mg/L) at room temperature.
The effect of adsorbent dosage (leading to different packing heights) was
studied by varying adsorbent masses (8, 12 and 16 mg) for fixed
300 mg/L CR solution and 10 mL/h flow rate. The dye solution with
known concentration was pumped upward through the stationary
sponge using a syringe pump to avoid additional channelling due to
gravity. At a certain time interval, the column effluent was collected in
an Eppendorf tube and finally analysed using UV-Vis
spectrophotometer.
The breakthrough curves were plotted based on the ratio of the outlet
to influent concentration (Ceff/C0) versus time profile (Afroze et al.,
2016; Liu et al., 2020). The breakthrough time (tb) and exhaustion time
(te) were calculated from the plot when the value of Ceff/C0 reached 5%
and 95%, respectively (Jain et al., 2020). The total dye adsorbed (qtotal,
mg) for a particular discharge rate, influent concentration and packing
height in a fixed bed was determined by the total area in breakthrough
plot (Liu et al., 2020). Maximum CR adsorbed was evaluated using Eq. 5
expressed as:
)
∫ t=ttotal (
Q
Ceff
qtotal =
1−
dt
(5)
1000 t=0
C0
Where, Q is the pump discharge rate (mL/min), (1-Ceff/C0) is the con­
centration of CR adsorbed (mg/L) and ttotal is the total experimental time
(min).
Moreover, the equilibrium adsorption uptake (qeq) of CR onto S100
was evaluated by Eq. 6:
qeq =
qtotal
m
(6)
Fraction of unused bed length (FUNB) is computed by the following
equation (Charola et al., 2018):
FUNB
⎧
⎪
⎪
⎪
⎨
LUNB
=
× 100 = 1 −
⎪
LC
⎪
⎪
⎩
tb
⎛∫ (
) ⎞⎫
Ceff
dt ⎪
⎪
C0
⎬
⎟⎪
⎜
⎟
⎜ t=0
× 100
⎜ ttotal (
⎟
) ⎠⎪
⎝∫
⎪
⎪
Ceff
1−
dt ⎭
C0
1−
(7)
t=0
Where, LUNB and LC represent unused bed length and total height of the
4
Journal of Hazardous Materials 428 (2022) 128238
M.A. Usman and A.Y. Khan
Fig. 1. SEM images of (a) S100, (b) S100/CR, and (c) S100/MB. Insets show the adsorbent appearance before and after dye adsorption. Figures on the right show the
higher magnification images and inset illustrates EDX spectrum.
strong interaction occur between functional groups of CR creating
complexes on S100 suggesting chemisorption of CR molecules (Li et al.,
2020). Change in intensity and the peak positions of spectra bands at
3400 cm− 1 after CR sorption stipulates that S100 and CR may interact
through hydrogen bonds as well (Tang et al., 2020). Furthermore,
disappearance of peak was recorded at 1478 cm− 1 and 1115 cm− 1 for
S100/CR suggest that the adsorption of dye occurred through interaction
of N-H groups (amines) and C-N groups between S100 and dye (Quan
Table 1
Elemental compositions of S100, S100/CR, and S100/MB.
Characteristics
S100
S100/CR
S100/MB
C content (%)
N content (%)
O content (%)
S content (%)
53.4 ± 1.04
25.7 ± 0.99
19.9 ± 0.59
0.6 ± 0.08
55.2 ± 0.95
19.7 ± 0.4
20.3 ± 0.3
4.5 ± 0.2
53.7 ± 0.5
23.0 ± 0.45
20.9 ± 0.85
2.7 ± 0.05
5
M.A. Usman and A.Y. Khan
Journal of Hazardous Materials 428 (2022) 128238
et al., 2019). However, the spectral band assigned for S100/MB remain
intact with an additional intense peak at 1660–1714 cm− 1 represent the
Chet=N+(CH3)2 stretching vibrations (Bartošová et al., 2017). The band
is shifted to low frequency region by 50 cm− 1 in S100/MB relative to its
position from procured MB suggesting formation of one bridging
H-bonds between MB monomers and H2O molecules of S100 (Ovchin­
nikov et al., 2016). The broadening of adsorption band at
3200–3700 cm− 1 observed in SMB may occur by the overlap of absorp­
tion peaks of O-H and N-H (Tang et al., 2020). It was reasonable to
deduce that CR is strongly adsorbed on S100 as compared to that of MB
based on captured digital photographs (Fig. 1) and FTIR results.
Further, to know the surface charge of the sponge and the adsorption
mechanism, the pHzpc was estimated and found to be 8.45 (Fig. S4, SI).
Thus, the sponge surface functions as positively charged when solution
pH < 8.45, neutral for pH = 8.45, and negatively charged for pH > 8.45
(Usman et al., 2021). Furthermore, Fig. S4 show that the curves of both
KCl and DI intersect at the same point on the horizontal axis suggesting
that the presence of univalent ions (Na+ or K+) will not intrude in the
sorption experiments (Rao et al., 2016).
Moreover, PZC value determined using mass titration method from
the appearance of a plateau approximated to be 8.55 as shown in Fig. S4
in SI. Calculated values are almost similar to the point of zero charge
values estimated using salt addition method. Hence, the surface of S100 is
positively charged under circumneutral conditions favouring CR/MO
adsorption (Zheng et al., 2021).
PEI which protonate into -NH3+ and -NH2+- according to the solution pH
(Min et al., 2012). This effect of pH can also be explained based on the
estimated pHzpc value (8.45). At a pH < 8.45 the adsorbent S100 gets
positively charged against anionic CR. At lower pH values, relatively
high protonated amines would attract more anionic sulfonate groups of
CR, leading to a strong electrostatic attractions and relatively high
adsorption capacity as shown in Fig. 2a. Similar kind of pH dependency
trend was observed when CR was adsorbed onto polypyrrole–polyani­
line nanofibers (Bhaumik et al., 2013).
On the other hand, the observed poor dependence of MB adsorption
on solution pH (Fig. 2b) is due to the constituents such as amines and
amides, which inhibit the penetration of MB towards polymeric material
and lead to low adsorption capacity. Moreover, relatively high basic
condition does not increase the sorption performance as suggested by
Fig. 2b. With an increase in pH from 6 to 10, the adsorption capacity of
MB slightly increased from 2 mg/g to 5.5 mg/g, respectively. Adsorbent
surface functions negatively charged at pH > 8.45 as deprotonation of
nitrogen atoms occur above this value (de Farias et al., 2018). As illus­
trated in Fig. 2b majority of the protonated amines disfavour MB
adsorption at pH < 8.45 and only adsorption uptake of 3.5 mg/g was
achieved at pH 8. However, the competition between protons and
cationic MB do not weaken much after the neutralization of amines at
higher pH and the optimum adsorption capacity achieved was only
5.5 mg/g for an initial MB concentration of 300 mg/L at 30 ℃.
Competitive interaction between protonated and positively charged dye
molecules led to an extreme lower adsorption performance of our
adsorbent towards MB. A similar trend of pH dependency towards
anionic acid black ATT and cationic MB uptake was reported earlier
using PEI based bio adsorbent (Huang et al., 2020). Since a relatively
higher adsorption of CR at its concentration (1000 mg/L) at 30 ℃ and at
pH 6 was obtained, these conditions were fixed for subsequent studies.
3.2. Batch adsorption performance
Generally, the sorption performance of an adsorbent majorly de­
pends on its porous structure and surface properties. Our tailored sponge
has a 3D macroporous crosslinked structure with primary and secondary
amine groups along its polymeric chain, indicating its potential as highly
effective adsorbent for anionic dyes selectively. To check these antici­
pations, static adsorption performance of S100 towards CR, and MB was
thoroughly investigated.
3.2.2. Adsorption isotherms
In general, equilibrium adsorption isotherms are invaluable curves
used to describe the adsorptive characteristics of a certain adsorbent and
essential for design and optimization of adsorption processes (Vimonses
et al., 2009; Binupriya et al., 2007). Penetration or release of the
contaminant from aqueous media to a solid phase at given operating
conditions occurred through a mathematical correlation, expressed
graphically by solid phase against its residual concentration (Rao et al.,
2012, 2016). For this purpose, isothermal experiments were carried out
for both CR and MB by varying dye concentrations described in Section
2.4 at three different temperatures 30, 40, and 50 ℃, respectively. Fig. 3
shows that as the initial concentrations of CR and MB increases from 800
to 1400 mg/L and 50–300 mg/L, respectively, the adsorption capacity
of S100 increases and plateaus after attaining saturation. This may be
attributed to the fact that for fixed adsorbent amount, the number of
3.2.1. Influence of solution pH
Adsorbent-adsorbate interaction is favoured or opposed based on the
solution pH solution and surface charge of the adsorbent (Li et al., 2020;
Zúñiga-Zamora et al., 2016). Since azo dyes display similar absorption
peaks, and solubility in pH range 6–10, this pH range was selected for
studying the adsorption performance of S100 (Zúñiga-Zamora et al.,
2016). Fig. 2 shows that the anionic CR is strongly adsorbed on positively
charged S100 as compared to cationic MB (distinct ordinate values).
Further, as the pH decreases from 10 to 6, the adsorption capacity of CR
subsequently increases from 541.18 mg/g to 603.22 mg/g, respectively.
Such dependency occurred due to the amine and imino groups present in
Fig. 2. Effect of solution pH on adsorption capacity of S100 at 30 ℃ for initial concentrations of (a) 1000 mg CR/L, and (b) 300 mg MB/L.
6
M.A. Usman and A.Y. Khan
Journal of Hazardous Materials 428 (2022) 128238
isotherm constants were determined (Foo and Hameed, 2010). The
well-established Langmuir model assumes homogeneous adsorption of
guest molecules onto adsorbent with each molecule possessing similar
enthalpies and adsorption activation energy with no lateral and steric
hindrance between neighbouring sites (Foo and Hameed, 2010). While
Freundlich model signifies multilayer adsorption, with unsteady distri­
bution of enthalpies and affinities between adsorbent and adsorbate
widely applied in heterogeneous systems (Foo and Hameed, 2010).
qe =
KL qm Ce
1 + KL Ce
(8)
(9)
qe = KF Ce 1/n
Where, qm is the maximum monolayer adsorption capacity (785.95 mg/
g for CR, 666.02 mg/g for MO, and 7.25 mg/g for MB achieved using
adsorbent dose 16 mg) for complete homogenous coverage; KL is
Langmuir binding constant in L/mg; Ce and qe are the equilibrium
concentration in mg/L and adsorbent uptake capacity at equilibrium in
mg/g, respectively; KF is the Freundlich sorption equilibrium constant in
mg/g; and n denotes characteristic surface heterogeneity.
The isotherm curves generated using theoretical models at different
temperatures are shown in Fig. S5 in SI. Furthermore, the validity of
both isotherm models was evaluated using root mean square error of
calibration (RMSEC) expressed as (Zhao et al., 2018):
√̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
√∑
2
√n
√ (Y exp
− Y pred
)
i
√i=1 i
RMSEC =
(10)
N
Where, Yiexp and Yipred are the obtained and predicted value of the ith
sample; and N denotes number of samples, respectively. Lower RMSEC
value indicate good prediction capability of the used model.
Smaller values of RMSEC for Langmuir isotherm (compared to
Freundlich isotherm) suggested that the interaction between S100 and
anionic dye (CR) occurred mainly through monolayer adsorption for
temperature ranging from 30 to 50 ℃ (Table 2) (Foo and Hameed,
2010). In contrast, lower values of RMSEC for Freundlich isotherm
(compared to Langmuir isotherm) suggested that uptake of MB on the
sorbent surface follows multilayer adsorption process for the similar
temperature range. The calculated isotherm parameters (qm, KL, KF, and
1/n) along with correlation coefficient (R2) are shown in Table 2. The
batch adsorption experiments performed with S100 for MO adsorption
resulted a high adsorption capacity of 666.02 mg/g, at 40 ℃ (Fig. S6 in
SI). Fig. S6 also shows that Langmuir adsorption isotherm nicely fits the
experimental data. Therefore, Fig. 3 and Fig.S6 suggest that S100 can
adsorb both CR as well as MO, and thus is effective in adsorbing anionic
dyes from aqueous solutions.
In-order to investigate the inherent energetic changes occurred
during CR adsorption on S100, thermodynamic studies were performed
in temperature range of 30 ◦ C – 50 ◦ C. The thermodynamic parameters
Fig. 3. Effect of temperature for different initial concentration of dyes (a) CR,
and (b) MB sorption onto S100.
vacant sites in S100 could accommodate specific pollutant quantity and
no more active sites remain available to adsorb more (Gündüz and
Bayrak, 2017). Furthermore, pollutant molecules of lower concentration
experience high affinity due to low competition felt in attaching to
surface sites for similar adsorbent quantity as compared to higher con­
centration leading to lower removal efficiency with an increase in con­
centration (Wang et al., 2013). Distinctly higher loading of CR is
observed as compared to that of MB (Table 2). Besides, it could be seen
that adsorption capacities for both dyes increased with an increase in
temperature revealing dye uptake by S100 occurred through endo­
thermic process (Usman et al., 2021).
The adsorption isotherm data were fitted with Langmuir and
Freundlich adsorption isotherm models (Eqs. 8 and 9 given below) and
Table 2
Fitting parameters of Langmuir and Freundlich adsorption equations for CR and MB sorption onto S100.
Isotherm
Parameters
Temperature (℃)
CR
30
Langmuir
qm (mg/g)
KL (L/mg)
χ2
Freundlich
R2
RMSEC
1/n
KF (L/mg)
χ2
R2
RMSEC
MB
40
759.9±10.1
0.199
0.0224
0.996
13.876
0.119
410.6±27.6
0.0887
0.987
27.654
774.6±11.2
0.226
0.0823
0.986
27.898
0.107
448.2±45.1
0.2641
0.963
47.61
7
50
30
40
50
785.9±13.8
0.488
0.0436
0.994
19.343
0.099
493.1±33.3
0.2112
0.962
42.579
10.9±1.8
0.003
0.0244
0.992
0.142
0.708
0.08±0.02
0.0207
0.993
0.131
6.3±0.39
0.01
0.0744
0.982
0.574
0.427
0.49±0.08
0.024
0.994
0.142
7.2±0.24
0.018
0.0113
0.994
0.649
0.315
0.93±0.11
0.0168
0.995
0.118
M.A. Usman and A.Y. Khan
Journal of Hazardous Materials 428 (2022) 128238
such as change in Gibbs free energy (ΔGΟ), standard enthalpy (ΔHΟ),
and standard entropy (ΔSΟ) were obtained using the Van’t Hoff’s plot
(Fig. not shown) are summarised in Table 3 (Min et al., 2012). Standard
free energy change was correlated with each equilibrium concentration
and thermodynamic relations are evaluated using Eqs. 11, 12 and 13
expressed as:
KC =
qAC
Ce
time (h), respectively; K1 (1/min) and K2 (g/mg-min) are adsorption rate
constants for pseudo-first-order and pseudo-second-order models,
respectively. The fitted plots for all the three theoretical models pre­
sented in Fig. 4b-d provide rate constants and obtained values are re­
ported in Table 4.
To predict the rate-controlling step involved in penetration of CR
onto adsorbent, kinetic data was analysed using Weber and Morris intraparticle diffusion model (WM) described as:
(11)
qe = Kid t1/2 + I
(12)
ΔG◦ = − RTlnKC
ΔSO
ΔH O
logKC =
−
2.303R 2.303RT
Where, Kid (mg/g-min− 1) is WM constant and I is the intercept (mg/g)
proposing boundary layer thickness.
Obtained adsorption data fitted well with both pseudo first order
(Fig. 4b) and pseudo-second order (Fig. 4c) kinetic models based on
higher correlation coefficients R2 and lower RMSEC values (Table 4).
Further, qe,cal values differed appreciably for pseudo-first-order equa­
tion. In contrast the experimental data is well matched to predicted
value estimated using pseudo-second order model with significantly
higher R2 (0.981–0.997) and lower RMSEC values. The results clearly
demonstrate that pseudo-second-order model is better in defining ki­
netic CR sorption process rather than pseudo-first-order kinetic model
suggesting chemisorption between the guest-host molecules (Solgi et al.,
2017).
The WM plot represented by Fig. 4d indicate multi step adsorption,
driven by three sequential stages. Rapid adsorption rate was observed in
stage 1 indicating external surface diffusion. This is pursued by lower
adsorbate transfer in stage 2 where steepness in slope decreased due to
interior diffusion. Finally parallel lines suggest contaminant is utmost
penetrated into the pores of S100 in stage 3 (Jain et al., 2020; Enene­
beaku et al., 2017). The values of Kid, Ii, and R2 for four different initial
concentrations are listed in Table 4. The obtained trend in I values with
an increment in dye concentration as described in Table 4, propose
thickness of boundary layer decrease in stage 1 and increase in later
stages as experiment progress (Usman et al., 2021).
(13)
Where, qAC and Ce are quantity of CR adsorbed in mg/g and equilibrium
concentration in mg/L, respectively; R denotes gas constant
(0.008314 kJ/mol-K); T is the absolute temperature K; KC denotes
equilibrium constant of adsorption. The Van’t Hoff’s plot exhibits linear
relationship (R2 > 0.93) and the thermodynamic parameters were
quantified using slope and intercept of this plot. Evaluated parameters
are summarised in Table 3. The positive values of ΔHº confirms endo­
thermic process of CR adsorption on S100 (Gündüz and Bayrak, 2017).
The positive values of ΔSº indicate enhanced chaos (randomness) at the
guest-host interface during dye penetration (Rao et al., 2016).
3.2.3. Adsorption kinetics
Adsorption equilibrium is attained when no more contaminants
could be accommodated inside the adsorbent surface due to the un­
availability of vacant sites. As the optimum adsorption performance of
S100 towards CR was achieved at pH 6, all kinetic experiments were
performed at this pH with different CR concentrations and time. Fig. 4a
shows that apparently more than 50% CR removal occurs in initial 5 h
for all concentrations and the rate decreased gradually until equilibrium
was achieved. This is due to the availability of abundant active sites for
dye uptake initially, which at later stage decreases on saturation (Ene­
nebeaku et al., 2017). The equilibrium time has a positive correlation
with CR concentration and longer time is required to attain saturation
for high concentrations (Kim et al., 2019). Further, the adsorbed amount
of CR increases with increase in initial concentration. A complete satu­
ration of S100 for a CR concentration of 1400 mg/L was observed at 20 h.
Adsorption kinetics was established by fitting the kinetic data of CR
to pseudo-first order (Ong et al., 2020), pseudo-second order (Ho and
Mckay, 1999), and intraparticle diffusion (Jain et al., 2020) kinetic
models. Linear expression for pseudo-first-order and pseudo-second
order kinetic model is described by Eqs. 14 and 15 respectively:
(
)
K1
log(qe − qt ) = logqe −
t
(14)
2.303
( )
t
1
1
=
+
t
qt K2 qe 2
qe
3.2.4. Influence of adsorbent dosage on CR removal
The effect of different mass of S100 on CR removal was investigated,
by adsorbing a fixed concentration of CR on different weights of S100 as
shown in Fig. S7 in SI. It shows that when the amount of S100 decreases
from 16 to 4 mg, its adsorption capacity for CR increases by twofold
from 619.92 mg/g to 1425.15 mg/g. Such a higher adsorption capacity
achieved at relatively lower amount of S100 can be due to two main
factors. Primarily, for a given concentration and low adsorbent mass
(4 mg S100), the probability of collision with the adsorbent surface is
much higher and all active vacant sites are completely saturated with CR
molecules (Usman et al., 2021). Secondly, with an increase in S100 dose
from 4 to 16 mg, excessive adsorbent sites are exposed for CR penetra­
tion, which on adsorption might aggregate onto the monolithic pore
walls (Rao et al., 2016). As a result, CR molecules experience higher
resistance to diffuse and reach the available active sites and hence less
surface area becomes accessible for further adsorption. Additionally,
optimum CR uptake by 4 mg S100 (shown in Fig S7c) comes out to be
1550 mg/g (CR concentration 1200 mg/L) which is higher than most
reported PEI based sorbents. However, a dosage of 16 mg was chosen for
further investigations as our interest was to explore the potential of
amino-crosslinked sponge in selective removal of anionic contaminant,
easy regeneration, and fixed bed column CR sorption.
(15)
Where, qe and qt represent CR uptake (mg/g) at equilibrium and at any
Table 3
Thermodynamic data of CR adsorption onto S100 at different initial
concentration.
Ci (mg/L)
ΔH˚(KJ/mol)
ΔS˚(J/mol-K)
800
900
1000
1100
1200
1300
1400
41.34
38.478
34.71
42.167
52.664
17.367
14.078
167.53
158.46
141.08
162
193
72
56
ΔG˚(J/mol)
303.15 K
313.15 K
323.15 K
9811.82
9780.9
8140.84
7312.01
5869.15
4569.75
3205.66
10323.79
10673.51
9291.99
8423.01
8333.36
5010.37
3514.09
13213.17
12980.29
10973.75
10595.8
9727.99
6026.5
4354.38
(16)
3.3. Selective adsorption of anionic dye from anionic-cationic dyes
mixture
Typically dye molecules are charged species which are present in
wastewater from textile and allied industries. Similarly, most of the
adsorbents are also charged (Yang et al., 2019; Min et al., 2012; Fang
et al., 2018; Lv et al., 2019; Fan et al., 2015). Therefore, selective
8
M.A. Usman and A.Y. Khan
Journal of Hazardous Materials 428 (2022) 128238
Fig. 4. (a) Effect of contact time on adsorption capacity for CR onto S100 at pH 6.0 and 50 ℃. Kinetic data was modelled using (b) pseudo first-order (c) pseudosecond-order, and (d) WM diffusion model, respectively.
adsorption of dyes in batch studies helps in selection of adsorbent for
continuous studies. In addition, on an industrial scale of separation,
selectivity of adsorbent guides in sequential removal of both anionic and
cationic dyes from wastewater. Further, regeneration of such selective
adsorbents provides opportunity of recycling dyes, which otherwise
creates additional burden on the environment for disposal.
To test selectivity of S100 for an anionic dye (CR) from a mixture of
anionic-cationic dye solutions (CR/MB solutions), a separation factor
(Section 2.5) was calculated and shown in Fig. 5a. It shows that as the
molar ratio of MB in CR/MB mixture increases from 9:1–1:9, the sepa­
ration factor (αCR/MB) increases from 21.41 to 208.76. This indicates that
anionic molecules have higher probability to occupy positively charged
adsorbent sites as compared to cationic MB molecules (Wang et al.,
2020). Therefore, anionic dye penetrates into S100 through electrostatic
attraction even before the sponge reaches the saturation and blackish
mixture of dyes as illustrated in Fig. 5c turns to dark blue (indicating
adsorption of CR and rejection of MB by S100). Moreover, protonated
amines led to a strong electrostatic repulsion for cationic MB led to lower
or no diffusion on adsorbent surface.
Furthermore, influence of solution pH on the separation factor was
also explored at equimolar dye concentrations of both CR and MB
(Fig. S8, in SI). Interestingly, the results of pH-responsiveness for se­
lective adsorption are same as that of batch adsorption experiments. As
represented in Fig. S8, with an increment in solution pH from acidic (pH
4) to basic (pH 12), the separation factor subsequently decreased from
142.7 to 51.1. This is ascribed to the fact that at higher pH, protonated
amines deprotonates (less protons available) and surface affinity to­
wards anionic CR decrease significantly leading to a lower separation
performance for binary mixture (Wang et al., 2020). However, optimum
separation factor was achieved at pH 6 (152.6) ascribed to the screening
of electrostatic interaction by both H+ and -NH2+ ions at pH below 6,
resisting diffusion of anionic dye towards less positive surface.
Further, the selective adsorption study was extended to another
anionic-cationic dye mixture (MO and MB) under the optimum condi­
tion (pH 6 and mole ratio of MO/MB as 1:9). Similar phenomenon
occurred in the adsorption of the two dyes and the result of separation
factor (αMO/MB) comes out to be 152.3 proving good removal perfor­
mance of S100 for selected dye pollutant (Fig. 5b and S8b). Compared to
MO, CR has one more sulphonic group and hence bears higher anionic
properties, and thus facilitate relatively higher adsorption selectivity.
Hence, dyes can be selectively removed based on their charge differ­
ences and our sponge exhibits great potential in selective separation of
anionic dyes from waste stream. For comparison, Table 7 enlists the
adsorption selectivity of recently reported PEI based sorbents and results
indicate that relatively higher selectivity was achieved by S100 than
other porous materials.
Wastewater generated from textile industries contains chloride and
bicarbonate in higher range due to their utilization in dying process,
9
M.A. Usman and A.Y. Khan
Journal of Hazardous Materials 428 (2022) 128238
Further, FTIR analysis was performed to illustrate the variation of
functional groups on S100 before and after dye adsorption (Fig. S3). It is
found that the characteristic absorption peaks of N-H at 1478 cm− 1 for
S100/CR display lower intensity as compared to pristine S100 facilitating
effective CR ion exchange interaction towards -NH2+ groups of PEI
(Bartošová et al., 2017). In the spectrum of CR adsorbed S100, new
adsorption peaks at 1612 cm− 1 and 1050 cm− 1 appears, attributing to
the azo and sulfonate groups of CR.
Since pHZPC study of S100 showed the ionic nature of sponge, the
sulfonate groups of CR are readily adsorbed under acidic conditions.
S100 brimmed with amine groups repel alkaline MB molecules to such an
extent that they do not adsorb onto the exposed vacant sites of S100 at
relative higher pH values. Typically, CR a dipole anionic molecule is
negatively charged when pH > 5.5 (pKa = 5.5) and MO an anionic dye is
negatively charged when pH > 3.4 (pKa = 3.4) (Huang et al., 2019). At
high solution pH > 8.45, S100 deprotonates; at neutral pH (~7) all the
primary amines are protonated, while at low pH < 4 almost all amines
are protonated (Huang et al., 2019). At pH = 6, optimum adsorption
capacity of CR on S100 was observed due to strong electrostatic attrac­
tion between SO3- groups of CR and NH3+ groups of S100. Similar ionic
attraction occurred between negatively charged groups of MO and
positively charged S100. However, when pH increases the OH- increase
gradually and compete with anionic molecules for penetrating into the
positively charged adsorbent. Furthermore, due to strong electrostatic
repulsion between positively charged groups of MB and positively
charged groups of PEI, feeble adsorption capacity was achieved for MB
in this pH range. Based on above mentioned results, it is evident that
surface charge (amines) present on adsorbent surface governed the
adsorption behaviour towards both anionic and cationic dyes and the
possible adsorption mechanism was described in Fig. 6.
Table 4
Kinetic parameters for the adsorption of CR onto S100 at different initial
concentrations.
Kinetic models
Pseudo-first order
Pseudo-second
order
Intraparticle
diffusion
Parameters
C0 (mg/L)
800
1000
1200
1400
qe (mg/g)
K1 (1/h)
qe,cal (mg/g)
R2
Δqa (%)
RMSEC
K2 (g/mg-h)
497.71
0.085
429.53
0.948
5.17
124.84
0.0012
618.49
0.091
542.62
0.954
4.63
150.52
0.0007
737.66
0.107
645.65
0.983
4.71
151.6
0.0005
778.75
0.121
762.07
0.996
0.8
169.255
0.0003
qe,cal (mg/g)
R2
Δqb (%)
RMSEC
Ki-1 [mg/g(h)1/
2
]
R12
I1
Ki-2 [mg/g(h)1/
2
]
R22
I2
Ki-3 [mg/g(h)1/
2
]
R32
I3
458.71
0.997
2.9
23
177.02
632.91
0.981
0.8
36.62
222.72
806.45
0.993
3.52
51.95
269.05
977.19
0.994
9.63
153.76
289.9
0.987
-15.49
100.252
0.975
- 22.48
95.966
0.979
- 32.89
145.62
0.973
- 40.695
177.02
0.947
117.86
47.913
0.972
228.2
49.02
0.977
199.55
33.06
0.977
210.82
22.039
0.958
270.52
0.895
389.44
0.965
557.27
0.989
692.16
improving fibre quality, and mechanical properties of the dyed fibres
(Ru et al., 2018; Hussain et al., 2004). The sodium compounds are
extensively used as dyeing promoters as it assists in maintaining pH,
energy saving and lower dyeing temperature (approximately 10 ℃
compared to traditional dyeing process) (Ru et al., 2018). To get deeper
insight, the impact of NaCl on selective performance of S100 towards 1:9
molar CR/MB mixture was also studied. As pH of textile wastewater
majorly lies in the range 7–9, a solution pH > 8 was taken (Hussain
et al., 2004). Typically, 0.1 g of NaCl was added into 10 mL binary
mixture of CR/MB, and 16 mg S100 was added. The results αCR/MB of S100
obtained is 87 (Fig. 5b) which is remarkable even in presence of such a
high salt concentration. However, αCR/MB in presence of salt is lower
when compared with that in absence of the salt (208.76). This could be
attributed to the fact that the negative surface of S100 (at pH > 8.45) led
to a reduction in the removal efficiency of CR, thus resulting in lower
αCR/MB(NaCl) for binary mixtures. Although, the selectivity of S100 to­
wards anionic dye decreased in presence of salt, it is still higher than
recently reported porous hybrid materials (Wang et al., 2020). Hence,
S100 shows a promise to remove selected pollutants from industrial
wastewater.
3.5. Reusability and stability studies of S100
In this study, reusability of CR-saturated S100 (S100/CR) was tested by
initially desorbing CR via treatment with 0.05 M NaOH solution. As
ionic interaction is the governing mechanism for CR penetration onto
S100, desorption of adsorbed CR from S100/CR could only be achieved by
strengthening its exhausted amine groups (Guo et al., 2018). An alkaline
solution weakens the interactive bonds between surface sites of S100/CR
and adsorbed CR leading to easy regeneration. After successive
desorption, regenerated S100 was again used for CR uptake. The reus­
ability of S100 for five consecutive adsorption/desorption experiment
was studied and percentage removal attained after each cycle was
shown in Fig. 7. It shows that CR adsorption percentage remains the
same for two cycles (~99%) and drops slightly to 92% after 5 cycles. A
slight decrease in percent adsorption of CR is attributed to the delami­
nation of the polymeric chain after repeated acid/base treatment during
regeneration. The obtained trends appeared better compared to recently
synthesized PEI based adsorbents. For instance, for poly(ether sulfones)
PES /PEI nanofibrous membrane the removal efficiency of fast green
FCF and amaranth decreased to 70.6% after three cycles (Min et al.,
2012), for melamine formaldehyde MF-PEI/CS2 sponge removal effi­
ciency of Cu (II) decreased to 50% of its initial value after five cycles
(Huang et al., 2018), for persimmon tannin PTP bio-adsorbent, the
removal efficiency of methyl orange decreased by 81.47% after six cy­
cles (Li et al., 2018). In this context, S100 displays relatively better
regeneration ability. This suggests that S100 can be utilised for repetitive
use in anionic contaminants removal from wastewater.
Sufficient anchor of negative charge sulphonate groups of CR can be
easily removed by regeneration. However, CR saturated sponges can be
aimed to remove MB pollutant from aqueous media as well (Fig S9 in SI).
Further, TGA of S100 before adsorption (Fig. S10 in SI) was carried
out. Fig. S10 shows two stages of weight losses, around 100 ℃ signifying
moisture evaporation and within temperature range from 250 to 400 ℃
which is attributed to the degradation of PEI backbone. TGA study
suggests a good thermal stability of S100 sponge, which in agreement
3.4. Possible adsorption and separation mechanisms
Earlier reports suggest that functional groups on the adsorbent sur­
face, mass transfer process, specific surface area generally determine the
adsorption capacity facilitating higher chances of guest-host or re­
actant’s interaction (Huang et al., 2019; Faysal Hossain et al., 2020).
The driving force for CR or MB adsorption onto S100 are bifunctional
amino and imino groups which are uniformly distributed along the
polymeric assembly illustrated in EDS results Fig. 1 (Chatterjee et al.,
2016; Liu et al., 2020). It could be one reason for the remarkably higher
adsorption performance towards CR/MO molecules in comparison to
MB. To explore the possible reason for remarkable high adsorption
performance of S100 towards selective adsorption of anionic CR, a
detailed characterization studies were performed. EDS spectra show
significant decrease in N (~5 wt%) content with a concurrent increase
in S, ascribed to the utilization of amino groups for adsorption. Subse­
quently, the presence of sulfonate groups (azo dyes) further confirmed
the dye uptake (Li et al., 2020).
10
M.A. Usman and A.Y. Khan
Journal of Hazardous Materials 428 (2022) 128238
Fig. 5. Selectivity of S100 achieved by varying (a) molar ratio of CR/MB in binary mixture, and (b) Comparison of adsorption selectivity of S100 towards different 1:9
molar anionic/cationic dye solutions (represented namely) [Conditions: T = 50 ℃, total dye concentration= 2 mM, V = 10 mL, adsorbent mass = 16 mg].
Furthermore, (c) and (d) illustrates 20 times diluted 5:5 molar CR/MB (without salt) and 1: 9 molar CR/MB (with salt) dye solutions (1) before, and (2) after se­
lective separation.
with Ahmad et al. (2021), and Meng et al. (2018). Thermal stability at
higher temperature is required in gas phase adsorption systems, but
adsorption study in aqueous phase is majorly done within the temper­
ature range 15 – 50 ℃ for which S100 is stable. In addition, S100 is also
mechanically stable and shows remarkable elastic property by subject­
ing the sponge to more than 70% of the compressive strain (Usman and
Khan, 2021).
investigated and finally modelled using Adam’s Bohart and Thomas
kinetic models (Jain et al., 2020). Measured experimental values were
reproducible (with ± 10% accuracy). Captured video SV1 presented in
SI describe the saturation of sponge S100 with time under dynamic ex­
periments displaying adsorptive effectiveness of our sponge to treat
dye-containing wastewater.
Supplementary material related to this article can be found online at
doi:10.1016/j.jhazmat.2022.128238.
3.6. Continuous flow adsorption performance
3.6.1. Effect of flow rate
The effect of flow rate (10, 15 and 20 mL/h) on the shape of BTC with
a packing height of 0.9 cm (pre-water saturated S100) of CR inlet con­
centration (300 mg/L; pH 6) at ambient temperature was investigated
and shown in Fig. 8a. Initially higher dye uptake was observed till
breakthrough time (tb) at all three flow rates due to availability of fresh
active sites. However, in later stages due to the occupancy of active sites,
the attachment of dye molecules becomes less effective (Mousavi et al.,
2018). Therefore, the concentration of dye in the effluent was observed
to increase after the breakthrough time. Finally, the curves become
almost flat after reaching exhaustion time indicating lowest CR uptake
Fabrication of 3-D architectures can be easily employed in contin­
uous flow fixed bed column, thus expanding its practical applications. In
addition, continuous adsorption studies can be helpful where interaction
time is usually not long enough to reach saturation and much needed to
scale up the process in industries dealing with contaminant removal
(Zhou et al., 2018; Faysal Hossain et al., 2020). Therefore, continuous
flow experiments for CR adsorption onto S100 were performed in a glass
column (fixed bed pilot plant) and schematically shown by Fig. S11 in SI.
The impact of design process parameters such as CR concentration, flow
rate, and packing length on breakthrough curves (BTC) were
11
M.A. Usman and A.Y. Khan
Journal of Hazardous Materials 428 (2022) 128238
Fig. 6. Schematic adsorption mechanism for the adsorption of CR, and MB, respectively.
(Li et al., 2018). As shown in Fig. 8a, the increase in flow rate
(10–20 mL/h) caused steepness in BTC with a decrease in tb from 125 to
52 min for CR, respectively (Table 5). The reason could be ascribed to
the reduction of impregnation time of S100 in the packed bed with an
increase in flow, which limits the dye molecules to properly adhere to
the adsorbent active sites (pores) and hence the exhaustion occurred
early (Saratale et al., 2011). BTC curves shown in Fig. 8a shifted towards
left along with time with an increase in flow rate. CR adsorbed quantity
was reported to drop from 140.76 mg/g at 10 mL/h to 84.68 mg/g at
20 mL/h, respectively. A similar tendency was reported earlier for
remediation of AR18 and Pb (II) using PEI@PDA/MS adsorbent (Liu
et al., 2020). All the relevant parameters of the obtained BTC are re­
ported in Table 5.
3.6.2. Effect of initial concentration
The effect of initial concentration of CR (300, 525 and 700 mg/L) on
the shape of BTC was studied at a constant flow rate of 10 mL/h and
depicted in Fig. 8b. It is observed from Fig. 8b that with an increase in
influent concentration, the S shaped BTC curves turn steeper and both tb
and exhaustion time (te) at concentration (700 mg/L) was achieved
early as compared to lower concentration 300 mg/L (Table 5). Appar­
ently, rapid filling of sorption sites occurs at higher concentration
generating high electrostatic force for molecules to penetrate resulting
Fig. 7. Percent adsorption of anionic CR on S100 from five adsorption/
desorption cycles.
12
M.A. Usman and A.Y. Khan
Journal of Hazardous Materials 428 (2022) 128238
BTC curves was investigated and depicted in Fig. 8c. The obtained BTC
trend represents steepness and early exhaustion of bed at lower adsor­
bent mass for same influent pollutant concentration 300 mg/L at 10 mL/
h, respectively. Table 5 revealed that with an increase in the height from
0.3 to 0.9 cm, tb increases from 70 min to 125 min, leading to an in­
crease in residence time inside the adsorption column. However,
maximum dye uptake evaluated do not increase much as anticipated
with an increase in the packing height. Apparently at lower flow rate, CR
molecules get sufficient time to diffuse into the entire S100 sites (Afroze
et al., 2016). This leads to similar axial dispersion mass transfer, and as a
result, similar sorption efficiency was achieved ( ± 10 mg/g) for
different adsorbent heights (Jain et al., 2020). All these results suggest
that lengthening of mass transfer zones in the column do not affect dye
uptake at low flow rates.
As sorption progress, formation of mass transfer zone (MTZ)
occurred at the front of the column which is dependent on influential
parameters such as nature of adsorbate-adsorbent, particle size of
adsorbent, pollutant inlet concentration, solution flowrate, etc (Charola
et al., 2018). We observed that the length of unused bed (LUNB) increases
with increase in high influent concentration, higher flowrate and lower
adsorbent dose as summarised in Table 5. A relatively smaller percent­
age of unused bed height (FUNB) observed with higher dye uptake for
16 mg S100 at flow rate 10 mL/h and inlet concentration 300 mg/L.
Therefore, the column operation at lower values of both the inlet con­
centrations and the flowrates and a higher values of bed heights are
recommended for appropriate utilization of the bed.
3.6.4. Continuous flow adsorption modelling
The BTC experimental data were fitted using Adam-Bohart and
Thomas model based on the nonlinear regression approach. AdamBohart model is conceptualised on the surface reaction hypothesis and
suggest that the residual concentration of pollutant and sorption ca­
pacity of solid phase determine the adsorption rate (Li et al., 2017). The
Adam-Bohart model, employed to fit initial BTC, generally determines
the characteristic parameters such as N0 and KAB and the model equation
is described below (Di Natale et al., 2015):
Ceff
= eKAB C0 t−
C0
KAB N0 FZ
(17)
Where, Ceff and C0 and are the outlet and influent concentrations (mg/
L), KAB denotes the Adam-Bohart model constant (L/min-mg), N0 de­
notes the optimum sorption capacity (mg/mL), Z denotes packed col­
umn height (cm), t and F denotes time (min) and solution velocity in
axial direction (cm/min), respectively.
The ideal Thomas model behaves like Langmuir isotherm (Di Natale
et al., 2015). It assumes that diffusion in axial direction do not affect the
equilibrium sorption rate constant, instead obeys pseudo second-order
reversible reaction kinetics (Liu et al., 2020). Mostly BTC data in
continuous flow systems are better fitted using Thomas model (given in
Eq.18):
Fig. 8. BTC for CR adsorption at (a) different flow rates, (b) different pollutant
concentration, and (d) different S100 dosage, respectively.
Ceff
=
C0
1+
in shortening of mass transfer zone and an early depletion of active sites
(Malakhova et al., 2020). Maximum CR adsorption quantity decreases
significantly from 140.76 mg/g at 300 mg/L to 40.81 mg/g at 700 mg/L
of influent concentration, agreeing with previously reported works on
different fixed bed adsorption systems (Saratale et al., 2011; Afroze
et al., 2016). Established BTC results and operating parameters are listed
in Table 5.
Where KT denotes Thomas model constant (L/min-mg), q0 denotes op­
timum pollutant uptake (mg/g), Q denotes flow rate (mL/min), and m is
the S100 dosage packed inside column (g), respectively.
It appears that the simulation of BTC for varying flow rates, inlet
concentration and packing height is fitted well with the Thomas model
(mean R2 = 0.989) in comparison to Adam-Bohart model (mean R2 =
0.807). In general, the predicted adsorption capacity using Thomas
model for all operating column parameters in Table 6 was close to the
experimental values summarised in Table 5. Thus, continuous fixed bed
adsorption of CR onto S100 is governed by external mass transfer at the
interface (Sharma and Singh, 2013). The value of KT was found to in­
crease with an increase in flow rate and decreased with a surge in
3.6.3. Influence of packing height
The design of adsorption column obtained through concept of BTC
also depends on used bed height representing the mass transfer zone.
The effect of different packing heights of S100 (0.3, 0.6, and 0.9 cm) on
13
1
KT q 0 m
Q KT C0 t
e
(18)
M.A. Usman and A.Y. Khan
Journal of Hazardous Materials 428 (2022) 128238
Table 5
Breakthrough parameters for column adsorption of CR onto S100.
Q mL/min
C mg/L
0.167
0.25
0.334
0.167
0.167
0.167
0.167
0.167
0.167
a
Z (mm)
300
300
300
300
525
700
300
300
300
M (g)
9
9
9
9
9
9
9
6
3
tb (min)
0.016
0.016
0.016
0.016
0.016
0.016
0.016
0.012
0.008
te (min)
125
75
52
125
65
40
125
80
70
qtotal (mg)
–
320
225
–
330
250
–
360
250
qeq (mg/g)
FUNB %
a
32.33a
39.95
40.9
32.33
51.85
65.76
32.33
64.65
84.13
140.76
130.375
84.687
140.76
78.375
40.812
140.76
139.17
130.99
–
2.086
1.355
–
1.254
0.653
–
1.67
1.047
estimated by Thomas model; Z-packed column height
Table 6
Operating parameters utilised in Adam-Bohart and Thomas modelling of CR adsorption onto S100.
Operating Parameters
Q mL/min
0.167
0.25
0.334
0.167
0.167
0.167
0.167
0.167
0.167
Adams-Bohart Model
C mg/L
300
300
300
300
525
700
300
300
300
M (g)
Z (cm)
0.016
0.016
0.016
0.016
0.016
0.016
0.016
0.012
0.008
KAB (L/min-mg)
Thomas Model
N0 (mg/mL)
− 5
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.6
0.3
2.45×10
2.05×10− 5
1.43×10− 5
2.45×10− 5
9.43×10− 6
4.92×10− 6
2.45×10− 5
2.34×10− 5
1.7×10− 5
43,162.1
53,892.2
64,657.7
43,162.1
60,421.2
72,818.9
43,162.1
56,335.1
101,082.7
influent concentration and packing height. Further, higher flow rates
and CR concentrations led to a lower q0 values, in agreement with the
results of many researchers (Afroze et al., 2016; Liu et al., 2020; Saratale
et al., 2011). Moreover, the q0 values remain almost unchanged for
different packing heights at lower flow rate (10 mL/h).
R
2
0.929
0.879
0.681
0.929
0.818
0.671
0.929
0.933
0.738
KT (L/min-mg)
− 5
3.94×10
6.35×10−
9.84×10−
3.94×10−
3.62×10−
3.52×10−
3.94×10−
5.58×10−
9.53×10−
5
5
5
5
5
5
5
5
q0 (mg/g)
R2
140.76
131.44
91.57
140.76
86.66
65.74
140.76
142.65
144.03
0.975
0.992
0.994
0.975
0.989
0.993
0.975
0.991
0.993
adsorption performance of PEI based adsorbents for removal of dyes.
Further comparison was made based on type of study (batch and
continuous) and selectivity of the adsorbent. The results in Table 7 show
S100 has much higher adsorption capacity for anionic dyes.
Furthermore, cylindrical shape of our adsorbent display excellent
adsorption performance under both static and dynamic conditions. To
be noted that the optimum adsorption capacity achieved with a decrease
in S100 dose from initial (16 mg) to 4 mg was reported in comparison
table to show that the maximum contaminant uptake in the present
work is higher than most PEI based adsorbents.
3.7. Comparison with other adsorbents
Table 7 provides a comparison of recently reported PEI based ad­
sorbents with our S100 based on their potential to adsorb different dyes
from aqueous phase. In Table 7, we show the optimum conditions of pH,
temperature and adsorbent mass of the present work and compared the
Table 7
Separation performance of S100 and some recently reported PEI based adsorbents for different dye systems.
Guest
Adsorbates
Optimum
pH
Temperature
(℃)
Adsorbent mass
(mg)
qmax (mg/
g)
Selectivity
Continuous adsorption
studies
Ref.
Fe3O4 @SiO2/
PEI
PEI@PDA/MS
PEI_BD_1:4
Methyl
Orange
Acid red 18
Alizarin red S
4
15–35
20
231
–
Not Performed
(Huang et al., 2019)
3
–
25–45
23
5
5
464.58
1076
–
–
Performed
Performed
hPEI-CE
PEI-Pt@BC
Congo red
Acid black
ATT
Methyl orange
Methyl blue
Methyl orange
Congo red
Methyl orange
Congo red
Amaranth
Methyl
Orange
Congo red
5
5.2
25–55
25
5
13
2100
1157.9
Performed
–
Not performed
Not performed
(Liu et al., 2020)
(Malakhova et al.,
2020)
(Zhu et al., 2016)
(Huang et al., 2020)
GP
PMC
PTP
PEI Cu-BTC
PANF-g-HPEI0.6
H-PVTMS-PEI-2
PEI/PDMAEMA
PFGA
S100
2
4
4
5
5
6
2
3
25 ± 1
25–40
20–50
25–55
25
25
30–50
–
3–10
20
5–30
5
25
10
50
2
331
1333.04
225.74
2578.6
194
922.4
757
3059.2
Performed
–
–
–
Performed
Performed
–
Performed
Not performed
Performed
Not performed
Not Performed
Not performed
Not performed
Performed
Not Performed
(Zhao et al., 2018)
(Guo et al., 2018)
(Li et al., 2018)
(Quan et al., 2019)
(Fan et al., 2015)
(Yan et al., 2021)
(Liang et al., 2019)
(Shu et al., 2017)
6
30–50
4–16
1666.67
208.76
Performed
This study
Where, Fe3O4@SiO2/PEI: PEI modified magnetic-core shell nanocomposite; PEI@PDA/MS: PEI cross-linked melamine sponge; PEI_BD_1:4: 1,4-butanediol digly­
cedyl ether cryogel; hPEI-CE: cationic hyper-branched PEI onto cellulose; PEI-Pt@BC: PEI caged platinum nanomaterials onto bacterial cellulose; GP: Graphene oxide
PEI aerogel; PMC: PEI modified cellulose aerogel; PTP: persimmon tannin bioadsorbent; PEI Cu-BTC: PEI incorporated copper-1,3,5-benzentricarboxylic acid
composite; PANF-g-HPEI0.6: polyacrylonitrile fiber; H-PVTMS-PEI-2: PEI functionalised polysilsesquioxane hollow sphere; PEI/PDMAEMA: PEI copolymerised poly
(N,N dimethylaminoethyl-methacrylate composite gel; and PFGA: PEI functionalised grapheme aerogel.
14
M.A. Usman and A.Y. Khan
Journal of Hazardous Materials 428 (2022) 128238
4. Conclusions
Supplementary information
This study confirmed that abundant amine groups present in the
crosslinked monolithic sponge have a high adsorption capacity
(1666.67 mg/g for 4 mg S100, pH 6), selectivity (α = 208.7 and 81 for
CR/MB binary mixtures in absence and presence of NaCl, respectively),
recyclability for anionic dyes. The pristine sponge can be used in both
static and dynamic adsorption of anionic dyes from aqueous solution
and are strongly dependant on various parameters such as pH, adsorbent
dosage, initial CR concentration, flow rate, bed height, temperature etc.
The hydrophilic macroporous sponge allows anionic dye molecules to
diffuse freely inside the pores due to protonated amine groups in S100
(pHZPC = 8.45) and repels cationic dyes due to a strong electrostatic
repulsion between positively charged S100 sponge surface and quater­
nary ammonium ions of MB. The optimum adsorption capacity of CR as
predicted by Langmuir isotherm with 16 mg of adsorbent (pH=6) was
75 times higher as compared to that of MB (pH=10). We found that
lower pH and adsorbent dose generates high adsorption uptake towards
anionic dyes in static experiments. On the other hand, the lower values
of flow rate and influent concentration increases the sorption perfor­
mance of S100 towards CR in dynamic column experiments. We believe
that porous S100 sponge can serve as a novel adsorbent which will pro­
vide opportunity for selective recognition and removal of target mole­
cules from mixtures in both batch and continuous experiments.
The Supplementary Information contains details about the textural
characterization of sponge before and after adsorption, point of zero
charge, adsorption isotherms, influence of adsorbent dosage, selective
and competitive adsorption, thermal stability, schematic diagram
describing experimental set up in dynamic studies, and video captured
during continuous fixed-bed column studies through sponge S100.
Appendix A. Supporting information
Supplementary data associated with this article can be found in the
online version at doi:10.1016/j.jhazmat.2022.128238.
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CRediT authorship contribution statement
Mohd Arish Usman: Investigation, Data analysis, Writing – original
draft. Anees Y. Khan: Conceptualization, Methodology, Funding
acquisition, Writing – review & editing, Resources, Validation, Super­
vision, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgments
Authors gratefully acknowledge fundings received from Science and
Engineering Research Board (SERB), Government of India, grant no.
ECR/2018/000479. Central Analytical Facility (MUJ) and Sophisticated
Analytical Instruments Facility (MUJ) are also gratefully acknowledged
for their supports in materials characterization.
Novelty statement
Polyethyleneimine (PEI) after crosslinking generates a novel, cost
effective, hierarchical macroporous polymer-colloid hybrid which is
directly utilised in both batch and continuous operations without any
modification. Amine rich macropores exhibit high adsorption uptake
towards CR (1666.67 mg/g; 4 mg) at optimum conditions which is
remarkably higher than most reported PEI based adsorbents. Established
PEI blended crosslinked 3D monoliths synthesized using other sub­
stances (such as graphene oxide, silk fibroin, and gelatin), are usually
costly, required pre-treatment indicating blocked pores and sophisti­
cated fabrication method.
More importantly, utilization of anionic dye saturated sponge for the
removal of cationic dye from aqueous media is rarely discussed. Overall,
single step and easily fabricated PEI-based hybrid with exceptional
selectivity and adsorption capacity towards anionic dye is never re­
ported in literature.
15
M.A. Usman and A.Y. Khan
Journal of Hazardous Materials 428 (2022) 128238
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pollutants in the environment: present and future challenges in biomonitoring,
ecological risks and bioremediation. N. Biotechnol. 32, 147–156. https://doi.org/
10.1016/j.nbt.2014.01.001.
Godiya, C.B., Cheng, X., Deng, G., Li, D., Lu, X., 2019. Silk fibroin/polyethylenimine
functional hydrogel for metal ion adsorption and upcycling utilization. J. Environ.
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