gels
Article
Effective Carbon/TiO2 Gel for Enhanced Adsorption and
Demonstrable Visible Light Driven Photocatalytic Performance
Anam Safri
and Ashleigh Jane Fletcher *
Department of Chemical and Process Engineering, University of Strathclyde, Glasgow G1 1XJ, UK;
anam.safri@strath.ac.uk
* Correspondence: ashleigh.fletcher@strath.ac.uk; Tel.: +44-141-5482-431
Abstract: A new strategy to synthesise carbon/TiO2 gel by a sol–gel method is proposed. Textural,
morphological, and chemical properties were characterised in detail and the synthesised material
was proven to be an active adsorbent, as well as a visible light photocatalyst. Homogenously
distributed TiO2 is mesoporous with high surface area and, hence, exhibited a high adsorption
capacity. The adsorption equilibrium experimental data were well explained by the Sips isotherm
model. Kinetic experiments demonstrated that experimental data fitted a pseudo second order
model. The modification in electronic structure of TiO2 resulted in a reduced bandgap compared to
commercial P25. The absorption edge studied through UV-Vis shifted to the visible region, hence,
daylight photocatalytic activity was efficient against degradation of MB dye, as an example pollutant
molecule. The material was easily removed post treatment, demonstrating potential for employment
in industrial water treatment processes.
Keywords: adsorption; carbon/TiO2 gels; resorcinol formaldehyde RF/TiO2 gels; photocatalysis;
adsorption kinetics; methylene blue dye degradation
Citation: Safri, A.; Fletcher, A.J.
Effective Carbon/TiO2 Gel for
Enhanced Adsorption and
Demonstrable Visible Light Driven
Photocatalytic Performance. Gels
2022, 8, 215. https://doi.org/
10.3390/gels8040215
Academic Editors: Hiroyuki Takeno
and Avinash J. Patil
Received: 11 February 2022
Accepted: 29 March 2022
Published: 1 April 2022
Publisher’s Note: MDPI stays neutral
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Copyright: © 2022 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1. Introduction
Adsorption of carbon is perhaps the most widely used water treatment technique.
However, there is an ongoing effort to develop efficient adsorbents with reduced regeneration costs. Currently, the combination of carbon and titanium dioxide (TiO2 ) appears to
offer a promising route to obtain an adsorbent with self-regeneration properties. Additionally, the synergistic effect of both carbon and TiO2 enhances the degradation process due to
respective adsorptive and photocatalytic properties. Literature reports several studies to
address the synergy of adsorption and photodegradation by experimental demonstration
of various carbon/TiO2 composite materials [1,2]. However, there is still a need to better
understand the phenomenon of pollutant-adsorbent interactions in order to have a good
knowledge to design an efficient water treatment process. Additionally, the improvement
in design involves the type of materials and synthesis process employed to attain maximum
efficiency of the system.
Previously, carbon has been combined with TiO2 through various approaches, in different forms, such as carbon nanotubes [3–5], graphene [6–8], and activated carbon [9,10].
Lately, focus has been shifted to highly porous carbon materials as support matrix for
industrial applications, due to the high surface area and tuneable porosity. Ideally, welldeveloped mesoporous structures with large pore volumes and uniform pore size distributions are preferred, due to enhanced accessible surface sites contributing to superior
adsorption capacity of pollutants from the aqueous phase. However, the preparation
process of these mesoporous carbons is costly and complicated, usually resulting in materials with moderate or low surface area. The efficiency of the material is also limited,
since most TiO2 nanoparticles incorporated in the pores of the carbon are unavailable for
photocatalysis [11].
Gels 2022, 8, 215. https://doi.org/10.3390/gels8040215
https://www.mdpi.com/journal/gels
Gels 2022, 8, 215
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Amongst mesoporous carbon materials, carbon gels are a new type of nanocarbon
with potential applications in photocatalysis [2,12,13]. Carbon gels produced by polycondensation of resorcinol (R) with formaldehyde (F) are highly porous and have flexible
properties. A comprehensive review of sol–gel synthesis of RF gel reveals that the material
can be easily tailored to attain desired properties, mainly tuneable porosity, and acts as a
support for metals [14]. Hence, RF gels can be promising materials for water treatment
applications, mainly due to their stability, owing to aromatic resorcinol rings and their
overall interconnected mesoporous carbon structure. For industrial applications where
a continuous process system is often required, carbon derived from RF gels can be more
efficient and cost-effective than commercial adsorbents, which are in the form of granules
or powders and are unsuitable for use in continuous systems.
The aim of this study is to synthesize an adsorbent with visible light driven photocatalytic activity by incorporating TiO2 nanoparticles into RF gels. A typical synthesis route of
an RF gel [15] is modulated in this study to integrate TiO2 nanoparticles by formulating
a twostep synthesis scheme. In addition to enhancement in textural properties of this
newly synthesised adsorbent, improvement in photocatalytic properties is expected by
(i) modification in electronic structure of TiO2 , due to the presence of RF gel as a carbon
source, shifting the absorption edge to the visible light region, hence, enabling TiO2 to
activate under visible light irradiation; (ii) the carbon phase can entrap the photogenerated
electron and hole pairs, which would otherwise recombine and dissipate heat energy; and
(iii) the porous RF gel helps facilitate dispersion of TiO2 and easy post treatment removal
of the adsorbent/photocatalyst.
Here, we report a study of the textural and optical characteristics of the adsorbent/photocatalyst. Detailed adsorption experiments were carried out to study the effect of
several parameters on adsorption capacity. Additionally, the interaction behaviour between
potential pollutants and the material were investigated, using methylene blue (MB) as a
model adsorptive. Equilibrium sorption data were modelled using Langmuir, Freundlich,
Sips, and Toth isotherm models. Kinetic analyses were carried out by comparing the
experimental data with pseudo first order and pseudo second order expressions, as well
as a diffusion model to better understand the transfer behaviour of the adsorbate species.
Further, photocatalytic application tests were performed under visible light irradiation and
the data were modelled to study the kinetics of photocatalysis.
2. Results and Discussion
2.1. Morphology
The morphology of sample, studied using FESEM, is shown in Figure 1. Figure 1a
shows a heterogenous nature of synthesised RF/TiO2 with homogenously distributed TiO2 ,
as represented in Figure 1b. The overall structure shows the nanospheres connected to form
a three-dimensional porous network, as represented in Figure 1c [14]. The heterogenous
surface is more evident in Figure 1d where organic and inorganic phases can be differentiated. The diameter of microspheres ranged around 0.76–1.66 µm, indicating that the
size of the primary particles was slightly larger than pristine RF, which generally is in the
nanometre range [16]. Energy dispersive X-ray (EDX) spectra of the microspheres is shown
in Figure 1e, (EDX zone shown in supplementary information, Figure S1) which evidently
corresponds to the recorded spectra.
2.2. FTIR Analysis
The IR absorption bands of RF/TiO2 overall resembled those of the pristine RF gel,
as also observed through FESEM images with clear uniform spheres illustrating a porous
network and the retention of the gel structure even after addition of TiO2 . Typical characteristic peaks, such as the previously reported C=C stretching, CH2 , and C-O-C of aromatic
rings, methylene bridges, and methylene ether bridges [17,18], were observed. The broad
peak at 3300 cm−1 is characteristic of stretching vibrations associated with phenolic OH
groups. Weak vibrations in the range of 2000–1700 cm−1 are attributed to CH bending of
Gels 2022, 8, 215
3 of 19
aromatic compounds. The absorption bands at 1605 and 1473 cm−1 correspond to aromatic
ether bridges, attributed to condensation of resorcinol to form the RF gel network. A
strong IR peak, expected in the range 1740–1700 cm−1 , associated with C=O stretching of
aldehyde, was not observed, which confirms that the sol–gel reaction was complete. In
comparison to a spectrum of pristine RF, a few additional peaks were observed that verify
the chemical linkages between RF and TiO2 , as marked in Figure 2. It has been established
that the oxygenated surface groups of carbon materials support the attachment of TiO2 . [19].
Here, crosslinking of TiO2 with RF, via the hydroxyl groups, can be observed through the
peaks in the vicinity of 1400 cm−1 , attributed to OH groups of RF, which appeared weak
in the spectrum of RF/TiO2 , signifying the reaction of OH and TiO2 . Meanwhile, new
signals observed at 1200 and 1084 cm−1 suggest formation of Ti-O-C functionalities. Similar
crosslinking has previously been reported in TiO2 /phenol resol hybrid structures, where
chemical interactions between TiO2 and phenol resol form Ti-O-C complexes. This heterojunction is responsive to visible light due to formation of a charge complex between the
interface of TiO2 and mesoporous phenol resol producing new electronic interactions [20].
Hence, it can be concluded that the interactions between RF and TiO2 are chemical in nature.
Additional signals below 1000 cm−1 , such as bands at 963 and 880 cm−1 , are associated
with titanium ethoxide functional groups. Additionally, the broad band observed in the
range of 600 cm−1 corresponds to the vibration of Ti-O-Ti bonds [21].
2.3. Surface Area Analysis
A nitrogen sorption isotherm was measured to determine the specific surface area and
pore volume of RF/TiO2 . Figure 3 shows N2 sorption isotherm and pore size distribution
(inset Figure 3). As can be seen, the isotherm of RF/TiO2 is of Type IV classification [22]
with a sharp capillary condensation at P/Po = 0.4–0.9 and a well-defined hysteresis loop
of Type H1, associated with open ended pores whilst suggesting a mesoporous structure [16]. Pore filling occurs at low relative pressure and the calculated mesoporosity in
the structure was ~94%. The SBET , corresponding pore size and total pore volume of as
prepared RF/TiO2 is 439 m2 g−1 , 9.4 nm and 0.71 cm3 g−1 , respectively. The SBET value
of pristine RF gel obtained in this study is 588 m2 g−1 . The reason in reduced SBET value
for RF/TiO2 is attributed to blockage of pores of RF gel matrix with inclusion of TiO2
nanoparticles. Meanwhile, in comparison with pristine TiO2 , the SBET value is significantly
higher for the synthesised RF/TiO2 . Additionally, noteworthy SBET value for pristine TiO2
(i.e., 111 m2 g−1 ) is obtained in this study, contrary to commercial P25 with SBET value of
57 m2 g−1 .
2.4. Effect of pH
The influence of MB sorption was studied by varying the solution pH from 2–12
(25 mL, 100 mg L−1 , 0.01 g of adsorbent). The adsorption capacities at different pH values
are shown in Figure 4. The efficiency of uptake increases from 47.24 to 65.96 mg g−1 when
the pH increases from 2–5. Thereafter, a sharp increase in adsorption capacity is observed
at pH ≥ 6. The variation in adsorption behaviour of MB on RF/TiO2 can be explained by
considering the structure of MB and evaluated point of zero charge (pzc). The pHpzc value
for RF/TiO2 is determined to be 7.2 (Figure S2).
RF/TiO2 can be amphoteric having both positively and negatively charged surface
sites in aqueous solution due to the varying amount and nature of surface oxygen [23].
At pH lower than the pHpzc, the surface of RF/TiO2 is positively charged, which repels
the cationic dye (MB), and resultant interactions are hindered in acidic media due to
electrostatic repulsion between the competing H+ ions on the surface of adsorbent and MB
dye molecules. As the pH increases, the surface of RF/TiO2 becomes deprotonated and the
adsorption sites available for interaction with cationic species increase, therefore, increased
adsorption capacity is observed. This suggests that the electrostatic forces of attraction
between MB and the surface of RF/TiO2 increases due to increased ion density and positive
Gels 2022, 8, 215
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Gels 2022, 8, x FORcharges
PEER REVIEW
on the
surface. Further, the OH groups on the surface of RF/TiO2 can also attract
MB dye molecules under higher pH conditions.
Figureof1.RF/TiO
Morphology
of RF/TiO
2 sample
of RF/TiO
2, (b) TiO
2 distribution
Figure 1. Morphology
(a) FESEM
image(a)ofFESEM
RF/TiOimage
deter2 sample
2 , (b) TiO
2 distribution
by EDX
the sample,
(c) distinct
appearance of (d)
micro/nanospheres,
(d) isolated
mined by EDX onmined
the sample,
(c) on
distinct
appearance
of micro/nanospheres,
isolated microsphere
sphere
with differentiation
between
phase,EDX
andspectra.
(e) corresponding EDX sp
with differentiation
between
organic–inorganic
phase,organic–inorganic
and (e) corresponding
2.2. FTIR Analysis
The IR absorption bands of RF/TiO2 overall resembled those of the pristine RF
also observed through FESEM images with clear uniform spheres illustrating a p
Gels 2022, 8, 215
junction is responsive to visible light due to formation of a charge complex between the
interface of TiO2 and mesoporous phenol resol producing new electronic interactions [20].
Hence, it can be concluded that the interactions between RF and TiO2 are chemical in nature. Additional signals below 1000 cm−1, such as bands at 963 and 880 cm−1, are associated
with titanium ethoxide functional groups. Additionally, the broad band observed in5the
of 19
range of 600 cm−1 corresponds to the vibration of Ti-O-Ti bonds [21].
Pristine RF
RF/TiO2
C=O
C=C
C-H
Gels 2022, 8, x FOR PEER REVIEW
5 of 19
OH 2 is 439 m2 g−1, 9.4 nm and 0.71 cm3 g−1
Ti-O-Ti
prepared RF/TiO
, respectively. The SBET value of
pristine RF gel obtained in this study is 588 m2 OH
g−1. The reason in reduced SBET value for
RF/TiO2 is attributed to blockage of pores of RF gel
matrix with inclusion of TiO2 nanoTi-O-C
particles. Meanwhile, in comparison with pristine TiO2, the SBET value is significantly
higher
the synthesised
RF/TiO
2. Additionally,
S500
BET value for pristine TiO2
4000 for3500
3000
2500
2000
1500 noteworthy
1000
-1
(cmcontrary
)
(i.e., 111 m2 g−1) is obtainedWavenumber
in this study,
to commercial P25 with SBET value of 57
2 g−1.
m
Figure
2 gel
compared
to to
pristine
RFRF
gel.
Figure2.2.FTIR
FTIRspectra
spectraofofsynthesised
synthesisedRF/TiO
RF/TiO
gel
compared
pristine
gel.
2
600
2.3. Surface
Area Analysis
Adsorption
Desorption
2.0
dV/dlog(w) Pore Volume (cm³g -1 )
-1
Quantity Adsorbed (cm³g )
A nitrogen sorption isotherm was measured to determine the specific surface area
500
and pore
volume of RF/TiO2. Figure 3 shows N2 sorption isotherm and pore size distribution (inset Figure 3). As can be seen, the isotherm of RF/TiO2 is of Type IV classification
[22] with
a sharp capillary condensation at P/Po = 0.4–0.9 and a well-defined hysteresis
400
loop of Type H1, associated with open ended pores whilst suggesting a mesoporous structure [16]. Pore filling occurs at low relative pressure and the calculated mesoporosity in
300
the structure
was ~94%. The SBET, corresponding pore size and total pore volume of as
1.5
1.0
0.5
0.0
10
20
30
40
50
60
70
80
90
100
Pore width (nm)
200
100
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (p/p0)
Figure
Figure3.3.Nitrogen
Nitrogensorption
sorptionisotherms
isothermsand
andpore
poresize
sizedistribution
distributionof
ofsynthesised
synthesisedRF/TiO
RF/TiO2 2gel.
gel.
Overall,
2.4. Effect
of pHa good adsorption capacity for MB is observed at pH higher than the pHpzc
due to an increased number of negative sites in the higher pH range. This is in good
The influence of MB sorption was studied by varying the
solution pH from 2–12 (25
agreement with
the fact that, due to the presence of COO− and OH- functional groups,
−1
mL, 100 mg L , 0.01 g of adsorbent). The adsorption capacities at different pH values are
MB dye adsorption is favoured at pH > pHpzc [24]. The same trend has been observed in
shown in Figure 4. The efficiency of uptake increases from 47.24 to 65.96 mg g−1 when the
previous studies with activated carbon and TiO2 composites where reduced activity was
pH increases from 2–5. Thereafter, a sharp increase in adsorption capacity is observed at
observed at acidic pH and maximum activity was observed in the pH range 6–10 [25–27].
pH ≥ 6. The variation in adsorption behaviour of MB on RF/TiO2 can be explained by considering
of MB and evaluated point of zero charge (pzc). The pHpzc value
2.5. Effectthe
of structure
Contact Time
for RF/TiO
2 is determined to be 7.2 (Figure S2).
Figure
5 shows the effect of contact time on the amount of MB molecules adsorbed
-1
qe (mg g )
by RF/TiO2 gel under different initial MB concentrations. As shown, the adsorption
220
capacity increases with increase in initial concentration. The equilibrium adsorption ca200
pacity increases from 102 mg g−1 to 207 mg g−1 by increasing the initial concentration
180
of MB
from 50 mg L−1 to 200 mg L−1 . Initially, the adsorption capacity overall is rapid
160
for timeframes up to 30 min. This trend is expected, due to the greater driving force of
MB140dye molecules and immediate availability of vacant adsorption sites, hence resulting
120
100
80
60
600
Adsorption
Desorption
-1
Quantity Adsorbed (cm³g )
500
400
1.5
6 of 19
1.0
in increased in frequency of collisions between MB dye molecules and the RF/TiO2 gel.
300
Additionally, mesoporosity throughout the RF/TiO2 gel structure provides a high surface
area for greater adsorption of MB molecules. It is noteworthy that at higher MB concenPore width (nm)
200 the adsorption rate is greater and adsorption capacity attains equilibrium faster
tration
than at low concentration. The reason is attributed to immediate occupancy of available
active
sites by a large amount of adsorbate molecules. This rapid occurrence of sorption
100
is due to the presence of mesoporosity within the RF/TiO2 gel, which corresponds to a
large portion of the adsorption sites. In this case, the mesoporous structure provides a
large 0surface
area
to solution
volume
within
the porous
network of the adsorbent gel.
0.0
0.2
0.4
0.6
0.8
1.0
Additionally, within the mesopores, MB dye molecules are confined to be in close proximity
Relative Pressure (p/p )
to the surface. Such observations have 0been reported in previous research, particularly
for activated
carbons
[28].
Over time,
saturation
of active
occurs,
and adsorption
Figure
3. Nitrogen
sorption
isotherms
and pore
size distribution
of sites
synthesised
RF/TiO
2 gel.
becomes difficult on the fewer available active sites due to repulsive forces between the
MBEffect
molecules
2.4.
of pH and the RF/TiO2 gel surface. Additionally, the blockage of pores and charge
repulsion
of MB dye
may decelerate
theby
adsorption
progress.
phenomena
The influence
of species
MB sorption
was studied
varying the
solutionSimilar
pH from
2–12 (25
have
been
explained
for
porous
TiO
and
other
carbon/TiO
porous
composite
materials,
2
2
−1
mL, 100 mg L , 0.01 g of adsorbent). The adsorption capacities at different pH values
are
where it may have taken longer for the adsorbate to diffuse deeper in the fine pores [29].
shown in Figure 4. The efficiency of uptake increases from 47.24 to 65.96 mg g−1 when the
Thereafter, the adsorption capacity increases gradually until 90 min, and equilibrium is
pH increases from 2–5. Thereafter, a sharp increase in adsorption capacity is observed at
attained for the entire concentration range. Thus, equilibrium time was considered as
pH ≥ 6. The variation in adsorption behaviour of MB on RF/TiO2 can be explained by con90 min which was considered sufficient for removal of MB ions by RF/TiO2 gel. Hence,
sidering the structure of MB and evaluated point of zero charge (pzc). The pHpzc
value
the contact time was set to 90 min in the remaining experiments to ensure equilibrium
for RF/TiO2 is determined to be 7.2 (Figure S2).
was achieved.
0.5
0.0
10
20
30
40
50
60
70
80
90
100
220
200
180
160
140
-1
qe (mg g )
Gels 2022, 8, 215
dV/dlog(w) Pore Volume (cm³g -1 )
2.0
120
100
80
60
40
20
0
0
2
4
6
8
10
12
pH
Figure4.
4. Effect
Effectof
ofpH
pHon
onthe
theadsorption
adsorption of
of MB
MB dye
dye by
by RF/TiO
RF/TiO2 2gel.
gel.
Figure
2.6. Effect of Sorbent Dose
The percentage removal of MB dye increased with increase in the adsorbent dose from
0.005 to 0.01 g but remained almost constant with further increase in the dose range 0.01 to
0.1 g, as represented in Figure 6. Percentage removal was calculated using Equation (2),
and showed an increase with increase in adsorbent dose, due to greater availability of
vacant active sites, a large surface area, and a greater number of adsorptive sites present on
the surface of RF/TiO2 . With further increase in adsorbent dose (>0.01 g), the rate of MB
removal becomes low, as the concentrations at the surface and solution reach equilibrium.
The resultant reduction in adsorption rate is attributed to unoccupied adsorbent sites, as
well as overcrowding or aggregation of adsorbent particles [30]. Hence, the surface area
available for MB adsorption per unit mass of the adsorbent reduces, whereby percentage
removal was not significantly enhanced with further increase adsorbent dose.
-1
qe (mg g
Gels
FOR PEER REVIEW
Gels2022,
2022,8,8,x215
100
7 of
7 of1919
50
qt (50 mg/L)
qt (100 mg/L)
qt (150 mg/L)
qt (200 mg/L)
0
0
50
100
200
150
200
250
time (min)
Figure 5. Effect of adsorption on contact time and initial concentration of MB dye by RF/TiO2 gel.
150
-1
qe (mg g )
2.6. Effect of Sorbent Dose
The percentage removal of MB dye increased with increase in the adsorbent dose from
0.005
100to 0.01 g but remained almost constant with further increase in the dose range 0.01 to
0.1 g, as represented in Figure 6. Percentage removal was calculated using Equation (2), and
showed an increase with increase in adsorbent dose, due to greater availability of vacant
active
50 sites, a large surface area, and a greater number of adsorptive sites present on the
qt (50 mg/L)
surface of RF/TiO2. With further increase in adsorbent
dose (>0.01 g), the rate of MB reqt (100 mg/L)
qt (150 mg/L)
moval becomes low, as the concentrations at the
surface and solution reach equilibrium.
qt (200 mg/L)
The resultant
reduction in adsorption rate is attributed to unoccupied adsorbent sites, as
0
50
100
150
200 particles
250 [30]. Hence, the surface area
well as0 overcrowding
or aggregation
of adsorbent
time
(min)
available for MB adsorption per unit mass of the adsorbent reduces, whereby percentage
removal
not
significantly
increase adsorbent
dose.
Figure 5. was
Effect
of adsorption
on enhanced
contact timewith
and further
initial concentration
of MB dye
by RF/TiO2 gel.
Figure 5. Effect of adsorption on contact time and initial concentration of MB dye by RF/TiO2 gel.
300
2.6. Effect of Sorbent Dose
qe
% Removal
75
The percentage removal of MB dye increased with increase
in the adsorbent dose from
0.005 to 0.01 g but remained almost constant with further increase in the dose range 0.01 to
0.1 g, as represented in Figure 6. Percentage removal was calculated
using Equation (2), and
70
showed
an
increase
with
increase
in
adsorbent
dose,
due
to
greater
availability of vacant
200
active sites, a large surface area, and a greater number of adsorptive sites present on the
surface of RF/TiO2. With further increase in adsorbent dose65(>0.01 g), the rate of MB re150
moval becomes low, as the concentrations at the surface and solution reach equilibrium.
The resultant reduction in adsorption rate is attributed to unoccupied adsorbent sites, as
60
well100
as overcrowding or aggregation of adsorbent particles [30]. Hence, the surface area
available for MB adsorption per unit mass of the adsorbent reduces, whereby percentage
removal
was not significantly enhanced with further increase
55 adsorbent dose.
50
-1
qe (mg g )
% Removal
250
300
0
0.00
qe
0.02
250
0.04
0.06
0.08% Removal0.10
50
75
Dose (g)
Figure6.6.Effect
Effectof
ofadsorbent
adsorbentdose
doseon
onthe
theremoval
removaland
andadsorption
adsorptionof
ofMB
MBdye
dyeby
byRF/TiO
RF/TiO2 2gel.
gel.
70
Figure
200
2.7. Adsorption Kinetics
% Removal
-1
qe (mg g )
The adsorption kinetics were studied using a contact time
65 of 240 min in the concentration150range 50–200 mg L−1 . The experimental data obtained for MB dye adsorption capacity
vs. time (t) were fitted with PFO and PSO, as presented in Figure 7a–d. The parameters
60
determined,
including measured equilibrium adsorption capacity
qe (experimental), theo100
retical equilibrium adsorption capacity qe (calculated), first order rate constant K1 , second
order rate constant K2 , and regression coefficient R2 , are presented
in Table 1.
55
50
As observed from the data, the correlation factor R2 deviates significantly from 1 for
PFO and, therefore, pseudo first order model does not exhibit good compliance with the
0
50 implies that the adsorption
experimental
data for the entire concentration range. This
0.00
0.02
0.04
0.06
0.08
0.10
reaction
is not inclined
towards
physisorption,
and the
MB dye molecules adsorb to specific
Dose (g)
sites on the surface of RF/TiO2 gel. The argument regarding the failure of the pseudo
first order
model
suggests
that
otherand
interactions
responsible
for the
sorption
Figure
6. Effect
of adsorbent
dose
onseveral
the removal
adsorptionare
of MB
dye by RF/TiO
2 gel.
mechanism. Hence, the correlation coefficients R2 of the pseudo second order model were
compared with pseudo first order parameters. R2 values for pseudo second order behaviour
are approximately 0.99 for the entire concentration range, indicating that the system is
more appropriately described by the pseudo second order equation. The dependence on
qe experimental (mg
g−1)
112.75
175.98
201.46
212.56
Pseudo first order
167.45
183.10
206.99
0.11886
0.1261
0.17078
8 of 19
0.9632
0.948
0.985
Pseudo second order
−1
qe, mg g
116.97
178.54
203.58
217.59
initial concentration
of MB dye is verified by good compliance of the experimental data
K2 (×10−3 g mg−1 min−1)
1.01
1.10
1.06
1.48
with the pseudo
second order equation, where the adsorption capacity is affected by the
2
R
0.998
0.989
0.987
0.996
initial MB dye concentration, subsequent surface-active sites, and adsorption rate (Other
error analyses are represented in Table S1).
qe, mg g−1
K1 (min−1)
R2
Gels 2022, 8, 215
107.65
0.08087
0.9758
200
120
(b)
180
160
100
140
120
-1
qe (mg g )
-1
qe (mg g )
80
100
60
40
80
60
40
20
-1
qe (50 mg L )
PFO
PSO
0
-1
20
qe (100 mg L )
PFO
PSO
0
0
50
100
150
200
250
0
50
100
time (min)
150
200
250
time (min)
(a)
(b)
250
200
200
-1
qe (mg g )
-1
qe (mg g )
150
100
150
100
50
50
-1
150 (mg L )
PFO
PSO
-1
0
0
50
100
150
200
200 (mg L )
PFO
PSO
0
250
0
50
100
time (min)
150
200
250
time (min)
(c)
(d)
−1 mg L−1, (c) 150 mg
−1 L−1, (d) 200 mg −
Figure7.7. MB
onon
RF/TiO
2 gel at (a) 50 mg L−1, (b) 100
L1−1,
Figure
MBuptakes
uptakes
RF/TiO
2 gel at (a) 50 mg L , (b) 100 mg L , (c) 150 mg L ,
and
fitted
data
for pseudo
firstfor
order
and first
pseudo
second
order kinetic
−1 , and
(d)
200
mg L
fitted data
pseudo
order
and pseudo
secondmodels.
order kinetic models.
Table 1. Kinetic parameters obtained by fitting kinetic data for MB adsorption to RF/TiO2 .
Model
qe experimental (mg
g−1 )
50 mg L−1
100 mg L−1
150 mg L−1
200 mg L−1
112.75
175.98
201.46
212.56
183.10
0.1261
0.948
206.99
0.17078
0.985
203.58
1.06
0.987
217.59
1.48
0.996
Pseudo first order
g−1
qe , mg
K1 (min−1 )
R2
107.65
0.08087
0.9758
167.45
0.11886
0.9632
Pseudo second order
g−1
qe , mg
K2 (×10−3 g mg−1 min−1 )
R2
116.97
1.01
0.998
178.54
1.10
0.989
The equilibrium sorption capacity increased from 116.97 to 217.59 mg g−1 when
initial dye concentration was increased from 50 to 200 mg g−1 confirming that MB dye
removal is dependent on initial concentration, where the rate limiting step is determined
by both adsorbate (MB) and adsorbent (RF/TiO2 ) concentration. This signifies that the
sorption mechanism is chemisorption. Previous studies have explained theoretically that if
200
150
-1
qe (mg g )
Gels 2022, 8, 215
initial MB dye concentration, subsequent surface-active sites, and adsorption rate. (Other
error analyses are represented in Table S1)
The equilibrium sorption capacity increased from 116.97 to 217.59 mg g−1 when initial
dye concentration was increased from 50 to 200 mg g−1 confirming that MB dye removal
is dependent on initial concentration, where the rate limiting step is determined by 9both
of 19
adsorbate (MB) and adsorbent (RF/TiO2) concentration. This signifies that the sorption
mechanism is chemisorption. Previous studies have explained theoretically that if diffusion is not the rate limiting factor, then higher adsorbate concentrations would give a good
diffusion is not the rate limiting factor, then higher adsorbate concentrations would give a
pseudo first order fit whereas, for low concentrations, pseudo second order better repregood pseudo first order fit whereas, for low concentrations, pseudo second order better
sents the kinetics of sorption, analogous to the observations made here [31]. Previously,
represents the kinetics of sorption, analogous to the observations made here [31]. Previously,
the adsorption processes of MB on TiO2/carbon composites have also exhibited strong dethe adsorption processes of MB on TiO2 /carbon composites have also exhibited strong
pendencies of pseudo second order fitting parameters on initial concentrations [27].
dependencies of pseudo second order fitting parameters on initial concentrations [27]. 0.5
Figure 8 shows a plot of MB dye uptake (qe) on synthesised RF/TiO2 against (time)0.5.
Figure 8 shows a plot of MB dye uptake (qe) on synthesised RF/TiO2 against (time) .
The plots exhibit multi-linearity, rather than two straight lines, indicating that the adsorpThe plots exhibit multi-linearity, rather than two straight lines, indicating that the adsorption
influenced by
by several
several steps.
steps. The
tion process
process is
is influenced
The initial
initial segment
segment of
of the
the plots
plots shows
shows that
that
diffusion
comparison
diffusion across
across the
the boundary
boundary of
of the
the adsorbent
adsorbent only
only lasts
lasts for
for aa short
short time
time in
in comparison
to
whole adsorption
adsorption process.
to the
the whole
process. This
This second
second section
section is
is attributed
attributed to
to diffusion
diffusion into
into the
the
mesopores
of
the
adsorbent,
i.e.,
the
MB
dye
molecules
enter
less
accessible
pore
mesopores of the adsorbent, i.e., the MB dye molecules enter less accessible pore sites.
sites.
Resultantly,
the diffusion
diffusion resistance
resistance increases,
increases, and
and the
the diffusion
diffusion rate
rate decreases.
Resultantly, the
decreases. This
This stage
stage
is
a
slow
and
gradual
stage
of
the
adsorption
process.
The
third
segment
represents
is a slow and gradual stage of the adsorption process. The third segment represents the
the
final
equilibrium
stage
where
intra-particle
diffusion
slows
down
to
an
extremely
low
rate
final equilibrium stage where intra-particle diffusion slows down to an extremely low rate
due
remaining concentration
concentrationof
ofthe
theMB
MBdye
dyemolecules
moleculesininthe
thesolution.
solution.This
Thisimplies
impliesa
due to
to the
the remaining
aslow
slowtransport
transportrate
rateofofMB
MBdye
dyemolecules
moleculesfrom
fromthe
the solution
solution (through
(through the
the gel–dye
solution
gel–dye solution
interface)
sites. Here,
the surface
surface of
of the
the RF/TiO
RF/TiO22 gel,
interface) to
to available
available sites.
Here, the
gel, and
and micropores,
micropores, may
may be
be
responsible
MB dye
dye molecules.
molecules.
responsible for
for the
the uptake
uptake of
of MB
100
50
-1
50 (mg L )
-1
100 (mg L )
-1
150 (mg L )
-1
200 (mg L )
0
0
2
4
6
8
0.5
time
10
12
14
16
0.5
(min )
Intra-particle diffusion kinetics of MB dye adsorption on RF/TiO
RF/TiO2.2 .
Figure 8. Intra-particle
2.8. Adsorption Isotherms
The equilibrium data were analysed using Langmuir, Freundlich, Sips, and Toth
isotherm equations to obtain the best fit. The isotherm data plots, and fitting model
parameters are shown in Figure 9 and Table 2, respectively. Comparison of the correlation
factor R2 indicates that qe,exp fitted well to the Sips model with the lowest χ2 value. The
qe,cal value, calculated using the Sips model, is closest to qe,exp with R2 closest to 1.
The Sips model is a combination of the Langmuir and Freundlich adsorption isotherms,
hence, the model suggests both monolayer and multilayer adsorption. At low MB dye
concentrations, the model predicts Freundlich adsorption isotherms as a heterogenous
adsorption system and localised adsorption without adsorbate–adsorbate interactions,
whereas at high concentrations the model predicts monolayer adsorption as in Langmuir
isotherm [32,33]. In the present study, the value of constant ns from Equation (11), the
heterogeneity factor, is greater than 1 (i.e., ns = 1.91), hence, the adsorption system is
predicted to be heterogenous [33]. Further, the Toth isotherm model validates multilayer
and heterogeneous adsorption, where the factor nT determines heterogeneity. Here, again
the value of nT is greater than 1, and, therefore, the system confirms heterogeneity. It is
evident that the equilibrium uptakes follow the Sips model according to the correlation
factor R2 (other error analyses are represented in Table S2) and the isotherm models fit the
data in the order Sips > Toth > Langmuir > Freundlich.
erogenous [33]. Further, the Toth isotherm model validates multilayer and heterogeneous
adsorption, where the factor nT determines heterogeneity. Here, again the value of nT is
greater than 1, and, therefore, the system confirms heterogeneity. It is evident that the
equilibrium uptakes follow the Sips model according to the correlation factor R2 (other
error analyses are represented in Table S2) and the isotherm models fit the data in
the
10 of
19
order Sips > Toth > Langmuir > Freundlich.
Gels 2022, 8, 215
250
200
-1
qe (mg g )
150
100
50
qe
Langmuir isotherm
Freundlich isotherm
Sips isotherm
Toth isotherm
0
0
20
40
60
80
100
120
-1
Ce (mg L )
Figure 9. Adsorption
Adsorption data
data for
for RF/TiO
RF/TiO2 2onto
ontoMB
MBdye
dyecorresponding
correspondingfits
fitsto
toLangmuir,
Langmuir, Freundlich,
Freundlich,
Figure
Sips,
Sips, and
and Toth
Toth equation.
Table
2. Isotherm
Isotherm parameters
parameters obtained
obtained by
by fitting
fitting MB
MB adsorption
adsorption data
data for
for RF/TiO
RF/TiO22to
tothe
theLangmuir,
Langmuir,
Table 2.
Freundlich,
equations.
Freundlich, Sips,
Sips, and
and Toth
Toth equations.
Langmuir
Freundlich
Sips
Toth
qm (mg g−1)
qm (mg g−1 )
KL (L mg−1)
Langmuir
KL (L mg−1 )
R2
R2
KF mg g−1 (L mgK-1)1/n
−1 (L mg-1 )1/n
mg
g
F
nF
nF
Freundlich
R2
R2
qs (mg g−1)
qs (mg g−1 )
KS
KS
Sips
ns
ns
2
2
R
R
qm (mg g−1 )
qm (mg g−1)
KT
KT
Toth
nT
nT
R2
254.65
254.65
0.0732
0.0732
0.960
0.960
54.85 54.85
3.19933.1993
0.865 0.865
218.71218.71
0.010 0.010
1.913 1.913
0.994 0.994
558.47558.47
0.02950.0295
1.403
1.403
0.991
2.9. Thermodynamic Study
Thermodynamic parameters for the adsorption system are recorded in Table 3. Negative values of free energy changes are evident from the data, which signifies the spontaneous
adsorption of MB dye molecules on the sample for the studied temperature range. Adsorption capacity increases with an increase in temperature and a positive ∆H0 (Table 3)
suggests that the adsorption is endothermic in nature. Positive ∆S0 indicates some structural changes in the MB dye and RF/TiO2 gel causing an increase in the degree of freedom
of the MB dye species and consequently increased randomness at the adsorbent–adsorbate
interface. At high temperature, the release of high-energy desolvated water molecules from
the MB dye molecules and/or aggregates arise after adsorption on RF/TiO2 gel, which
relates to a positive ∆S0 [34]. Before sorption begins, the MB ions are surrounded by highly
ordered water clusters strongly bound via hydrogen bonding. Once MB ions come in close
contact with the surface of RF/TiO2 , the interaction results in agitation of the ordered
water molecules, subsequently increasing the randomness of the system. Although, the
adsorption of MB dye onto RF/TiO2 gel may reduce the freedom of the system, the entropy
increase in water molecules is much higher than the entropy decrease in MB ions. Therefore, the driving force for the adsorption of MB on RF/TiO2 is controlled by an entropic
effect rather than an enthalpic change. Similar phenomena have previously been reported
Gels 2022, 8, 215
11 of 19
in order to explain the fact that thermodynamic parameters are not only related to the
properties of the adsorbate but also to the properties of other solid particles [35,36].
Table 3. Thermodynamic data for MB adsorption onto RF/TiO2 at various temperatures.
T (K)
lnk
∆G0 (KJ/mol)
∆S0 (J/mol)
112
281
296
305
313
1.29
2.20
2.40
2.50
−3.01
−5.41
−6.09
−6.51
∆H0 (KJ/mol)
28.2
3. Photocatalytic Tests
Absorbance (a.u.)
0 min
30 min
60 min
90 min
500
600
700
800
Wavelength (nm)
Figure10.
10.UV-Vis
UV-Visspectra
spectraofofMB
MBdye
dyedegradation
degradationusing
usingRF/TiO
RF/TiO
Figure
2 gel.
2 gel.
(h) (eVcm)
1/2
Within
studied systems,
no photodegradation
(reduction
in concentration
The
dye the
degradation
data obtained
after treatmentactivity
with RF/TiO
2 showed ~73% MB
andremoval
decolourisation
of MB
dye)
was observed
insynergy
the absence
adsorbent/catalyst,
dye
after 90 min.
This
is attributed
to the
of RFofand
TiO2, enabling anas
well
as
in
the
presence
of
pristine
RF,
indicating
that
the
properties
of
MB
are11).
more
stable.
absorption shift to a higher wavelength, as λmax is detected at 410 nm (Figure
Further
Additionally,
RF
solely
may
not
be
recommended
for
photocatalysis
due
to
slow
charge
analysis indicates modification in the electronic structure and a subsequent reduction in
transfer occurs
properties,
has also
been
proven by the study carried out by Zang, Ni, and
bandgap
due which
to doping
of TiO
2 similar to when combined with carbon [39]. The
Liu,
where
the
researchers
employed
pristine
RFin
resins
for11
visible
light
photocatalysis
[37].
calculated band gap energy is 2.97 eV, as shown
Figure
(inset).
The
value achieved
Slight photodegradation is observed in the presence of pristine TiO2 , which may be atis significantly lower than pristine TiO2 (i.e., 3.2 eV [21]), indicating photodegradation of
tributed to the potential absorbance of UV-Vis light from the surroundings confirming
MB dye under visible light irradiation. The RF matrix enables entrapment of a photogenthat the process of MB degradation is light driven. Although the TiO obtained for use
erated electron and hole pairs and, therefore, rapid generation of ROS is 2possible for effiin this study has a high surface area, which may possess good adsorption properties to
cient degradation of the MB dye. These findings are comparable to other carbon/TiO2 sysexhibit efficient adsorption of MB dye, since TiO2 only activates upon UV light irradiation
tems where synergistic effects have substantially enhanced
the performance of the system
(~280 nm), it does not produce enough reactive oxide species (ROS) to be an effective
due to improved optical properties of the material [1,40,41].
photodegradation system [38].
The dye degradation data obtained after treatment with RF/TiO2 showed ~73% MB
dye removal after 90 min. This is attributed to the synergy of RF and TiO2 , enabling an
max=410shift
nm to a higher wavelength, as λmax is detected at 410 nm (Figure 11). Further
absorption
analysis indicates modification in the electronic structure and a subsequent reduction in
bandgap occurs due to doping of TiO2 similar to when combined with carbon [39]. The
calculated band gap energy is 2.97 eV, as shown in Figure 11 (inset). The value achieved is
bsorbance (a.u.)
Gels 2022, 8, x FOR PEER REVIEW
Photocatalytic activity was determined by testing the efficiency of RF/TiO2 against
degradation of methylene blue (MB) under visible light irradiation. The maximum
ab12 of 19
sorbance vs. wavelength spectra (in the range of 550–700 nm) were collected and subsequent
activity, after 30 min, intervals was recorded, as shown in Figure 10.
Eg=2.97
0
1
2
3
4
5
6
7
8
9
10
(h) (eVcm)
1/2
max=410 nm
Absorbance (a.u.)
Gels 2022, 8, 215
The dye degradation data obtained after treatment with RF/TiO2 showed ~73% MB
dye removal after 90 min. This is attributed to the synergy of RF and TiO2, enabling an
absorption shift to a higher wavelength, as λmax is detected at 410 nm (Figure 11). Further
analysis indicates modification in the electronic structure and a subsequent reduction in
12 of 19
bandgap occurs due to doping of TiO2 similar to when combined with carbon [39]. The
calculated band gap energy is 2.97 eV, as shown in Figure 11 (inset). The value achieved
is significantly lower than pristine TiO2 (i.e., 3.2 eV [21]), indicating photodegradation of
significantly
TiO2 (i.e.,
3.2 matrix
eV [21]),
indicating
photodegradation
of MB
MB
dye under lower
visiblethan
lightpristine
irradiation.
The RF
enables
entrapment
of a photogendye
under
visible
light
irradiation.
The
RF
matrix
enables
entrapment
of
a
photogenerated
erated electron and hole pairs and, therefore, rapid generation of ROS is possible for effielectron
and hole
and,
therefore,
rapid generation
of ROS
is possible
for efficient
cient
degradation
of pairs
the MB
dye.
These findings
are comparable
to other
carbon/TiO
2 sysdegradation
of
the
MB
dye.
These
findings
are
comparable
to
other
carbon/TiO
systems
tems where synergistic effects have substantially enhanced the performance of the2system
where synergistic effects have substantially enhanced the performance of the system due to
due to improved optical properties of the material [1,40,41].
improved optical properties of the material [1,40,41].
Eg=2.97
0
1
2
3
4
5
6
7
8
9
10
h
400
450
500
550
600
650
700
750
800
Wavelength (nm)
Figure
Absorption
wavelength
spectrum
RF/TiO
ethanol
measured
through
Figure
11.11.
Absorption
vs.vs.
wavelength
spectrum
ofof
RF/TiO
2 dispersed
in in
ethanol
measured
through
2 dispersed
UV-Vis
spectrophotometer,
2. .
UV-Vis
spectrophotometer,inset
insetshows
showscalculated
calculatedband
bandgap
gapofofsynthesised
synthesisedRF/TiO
RF/TiO
2
The
photodegradation
MB
dye,
that
the
reduction
concentration
with
time
The
photodegradation
ofof
MB
dye,
that
is,is,
the
reduction
inin
concentration
with
time
is is
recorded
Figure
and
the
recorded
data
modelled
using
pseudo
first
order
kinetics,
recorded
inin
Figure
1212
and
the
recorded
data
is is
modelled
using
pseudo
first
order
kinetics,
shownininFigure
Figure12a,b.
12a,b.The
Thedata
data are
are fitted
equation
(ln(ln
(C(C
= =kt)
shown
fitted to
tothe
thefirst
firstorder
orderkinetic
kinetic
equation
o/Ce)
o /Ce)
to evaluate the value of the rate constant by slope of plot ln(Co /Ce) vs. time (t) in minutes,
where Co and Ce is the initial at t = 0 and final concentration at given time of MB concentration, respectively. The value of k here is the measure of photocatalytic performance, as
it defines the concentration of reacting substances, that is, photogenerated reactive oxide
species, therefore, a higher value of k signifies higher photocatalytic efficiency. As compared
to no catalyst (k = 2.43 × 10−6 min−1 ) pristine TiO2 (k = 1.74 × 10−3 min−1 ) and pristine
RF (k = 6.89 × 10−4 min−1 ), the rate of RF/TiO2 was the highest (k = 1.27 × 10−2 min−1 ).
Clearly, the rate constant obtained for photodegradation of MB using RF/TiO2 was the highest. Mainly, improved optical property was the most important advancement in forming
RF/TiO2 gel which is photocatalytically active under visible light (410 nm) irradiation.
The RF/TiO2 material created in this study exhibits excellent photoactivity under
visible light, which can further be explained by the mechanism of MB photodegradation represented in Equations (1)–(8). The system activates when RF/TiO2 absorbs light
with photon energy (hν) and generates conduction band (CB) electron (e− ) and valence
band (VB) hole (h+ ) pairs upon under visible light irradiation. The holes interact with
moisture on the surface of the adsorbent gel yielding hydroxy free radicals or reactive
oxide species (H+ or OH• ), which are oxidation agents that can mineralise a wide range of
organic pollutants, ultimately producing CO2 and H2 O as end products. The reaction sequence below represents the photodegradation of MB, showing a simplified mechanism of
photoactivation by a photocatalyst (Equations (1)–(4)) [19]. For the mechanism of photoinactivation of MB in the presence of RF/TiO2 , hydroxy free radicals or reactive oxide species
(H+ or OH• ) attack the aromatic ring of the MB structure, degrading it into a single ring
structure product, which then finally degrades to CO2 and H2 O (Equations (5)–(8)) [42,43].
active oxide species, therefore, a higher value of k signifies higher photocatalytic efficiency. As compared to no catalyst (k = 2.43 × 10−6 min−1) pristine TiO2 (k = 1.74 × 10−3 min−1)
and pristine RF (k = 6.89 × 10−4 min−1), the rate of RF/TiO2 was the highest (k = 1.27 × 10−2
min−1). Clearly, the rate constant obtained for photodegradation of MB using RF/TiO2 was
the highest. Mainly, improved optical property was the most important advancement in
13 of 19
forming RF/TiO2 gel which is photocatalytically active under visible light (410 nm) irradiation.
Gels 2022, 8, 215
1.2
(a)
1.0
(b)
k=1.27x10-2
RF/TiO2
no catalyst
pristine TiO2
1.0
pristine RF
0.8
ln(C0/Ce)
Ce/C0
0.8
0.6
0.6
0.4
0.4
RF/TiO2
no catalyst
pristine RF
pristine TiO2
0.2
k=1.74x10-3
0.2
k=6.89x10-4
k=2.43x10-6
0.0
0
20
40
60
time (min)
80
100
0
20
40
60
80
100
time (min)
Figure
Figure 12.
12. (a)
(a)Photocatalytic
Photocatalytic performance
performance regarding
regarding MB
MB dye
dye degradation
degradation without
without catalyst,
catalyst, and
and with
with
pristine RF, pristine TiO2 and RF/TiO2 gel (b) First-order kinetics of photoactivity without catalyst,
pristine RF, pristine TiO2 and RF/TiO2 gel (b) First-order kinetics of photoactivity without catalyst,
and with pristine RF, pristine TiO2, and RF/TiO2 gel.
and with pristine RF, pristine TiO2 , and RF/TiO2 gel.
The RF/TiO2 material created in this study exhibits
excellent
photoactivity under vis+
−
RF/TiO2 + hν = eCB
+ hvB
(1)
ible light, which can further be explained
by the mechanism
of MB photodegradation rep−
+
.
resented in Equations a–h. The system
RF/TiO
2 absorbs light with photon
hCB
+ activates
H2 O = Hwhen
+ OH
(2)
energy (hν) and generates conduction−band (CB) electron
(e−) and valence band (VB) hole
.−
eCB + O2 = O2
(3)
(h+) pairs upon under visible light irradiation.
The holes interact with moisture on the
.−
+
.
= HO
surface of the adsorbent gel yieldingOhydroxy
radicals
or reactive oxide species (H+ (4)
or
2
2 + H free
OH●), which are oxidation agents that can mineralise
a
wide
range
of
organic
pollutants,
MB + RF/TiO2 = MB.+ + e− (RF/TiO2 )
(5)
ultimately producing CO2 and H2O as end products.CB
The reaction sequence below repreO2 + e−a=simplified
O.2−
(6)
sents the photodegradation of MB, showing
mechanism of photoactivation
by a photocatalyst (Equations (1)–(4))
of
photoinactivation
of
MB
.+ [19]. For
− the mechanism
.
MB + OH = MB+OH
(7)
in the presence of RF/TiO2, hydroxy free radicals or reactive oxide species (H+ or OH●)
.+
.
OHstructure,
= H2 O +degrading
CO2 + other
products
(8)
attack the aromatic ringMB
of the+MB
it into
a single ring structure product, which then finally degrades to CO2 and H2O (Equations (5)–(8)) [42,43].
4. Conclusions
(1)
RF/TiO + hν = e + h
An RF/TiO2 gel was successfully synthesised using sol–gel techniques via a straightforH O = H excellent
+ OH˙ adsorption–photodegradation
(2)
ward route. The synergy of RF andh TiO+2 exhibited
activity due to the corresponding characteristics,
mesoporosity and photocatalysis.
(3)
e + O =mainly
O.
The synergy of contributing materials allowed modification in the electronic structure of
.
.
(4)
O linkages,
+ H = HO
TiO2 by formation of Ti-O-C chemical
responsible for a reduction in the band
gap of TiO2 for photodegradation
upon visible
(5)
MB + RF/TiO
= MB . light
+ e irradiation.
(RF/TiO ) Kinetic studies revealed
a pseudo second order reaction, signifying chemisorption phenomenon is involved in
(6)
O + eisotherm
= O. study showed that the system was
the adsorption mechanism. The adsorption
.
heterogeneous following the Sips
MBmodel
+ OHequation.
= MB +The
OH˙spontaneity of the process was
(7)
validated via thermodynamic studies, which signified an entropically driven adsorption
mechanism. Effective photodegradation results were observed due to the high adsorption
capacity and improved optical properties of the material, enabling significant MB dye
degradation within 90 min. Overall, the material possesses properties that have potential
to effectively reduce/eliminate a wide range of pollutants and, therefore, can be employed
as a low-cost photocatalytic adsorbent for water treatment Especially in the industrial
applications where post treatment separation and recovery of the adsorbent is difficult,
employing this material can reduce the costs since in this case the adsorbent precipitates
and easy separation is possible just by filtration or even decantation.
Gels 2022, 8, 215
14 of 19
5. Material and Methods
5.1. Synthesis
Synthesis of RF and TiO2 precursors was carried out in two separate systems, which
were integrated and processed further in order to obtain the final gel.
System 1: Preparation of Titania Sol
For preparation of the titania sol, 1.78 g of titanium precursor: titanium isopropoxide
(TTIP) (98+%, ACROS Organics™, Geel, Belgium) was dissolved in ethanol and stirred for
30 min. A mixture of water and HCl was added dropwise to the titania/EtOH solution
under constant stirring, at room temperature, to begin hydrolysis. After 2 h, a homogenous
solution was obtained. The molar ratios for these parameters were 1 TTIP:10 EtOH:0.3
HCl:0.1 H2 O.
System 2: Preparation of RF sol
In total, 7.74 g of resorcinol (SigmaAldrich, ReagentPlus, 99%, Poole, UK) was added to
50 mL of deionised water until completely dissolved. 0.0249 g of sodium carbonate (Na2 CO3 ,
Sigma-Aldrich, anhydrous, ≥99.5%), as a catalyst, and 4.23 g of formaldehyde (37wt%) were
added to the dissolved resorcinol under continuous stirring, at room temperature.
Finally, the prepared titania sol (system 1) was gradually transferred to the RF sol
(from system 2) under constant stirring, at room temperature. The resulting sol was stirred
at room temperature for 2 h after which the sol mixture was aged at 85 ◦ C for 72 h.
After aging, the process of solvent exchange and drying the RF/TiO2 gel, first involved
cutting the gel into smaller pieces. These pieces were then immersed in acetone for 72 h to
facilitate solvent exchange prior to drying, followed by vacuum drying at 110 ◦ C for 48 h
to obtain the final RF/TiO2 adsorbent gel. In this way, the final gel corresponded to 10 wt%
TiO2 (theoretical percentage) incorporated in the RF gel matrix.
5.2. Adsorbent Characterisation
Morphology of the synthesised sample was studied by field emission electron scanning
microscope (FESEM) TESCAN-MIRA. The functional groups on the surface of synthesised
RF/TiO2 , and the chemical linkages between the constituent RF and TiO2 components, were
verified using Fourier Transform Infrared Spectroscopy (FTIR) (MB3000 series, scan range
4000–400 nm). BET surface area measurements were carried out using a Micromeritics
ASAP 2420 to obtain N2 adsorption isotherm at 77 K and pore size was determined via BJH
theory [22]. A UV-Vis Spectrophotometer (Varian Cary 5000 UV-Vis NIR Spectrophotometer
Hellma Analytics) was used to collect absorption spectra and the data used to interpret the
change in electronic structure of RF/TiO2 [44]. The data were manipulated to calculate the
band gap energy values through the Tauc method described in previous studies [44].
6. Adsorption Experiments
6.1. Effect of pH
The effect of pH on the sorption of methylene blue (MB) dye was investigated with
0.01 g of sample by adjusting the pH of solution (25 mL, 100 mg L− 1 MB) between
2 and 12, at 23 ◦ C. The pH was adjusted using 0.01 M HCl and 0.01 M NaOH. After
2 h of agitation, the solution was centrifuged for 15 min and the supernatant solution was
collected via syringe. The initial and final concentrations were measured using a UV-Vis
spectrophotometer (Varian Cary 5000 UV-Vis NIR Spectrophotometer, Agilent, UK) and
onward calculations were performed.
6.2. Effect of Sorbent Dose
The amount of sorbent dose was gradually increased from 0.005 to 0.01 g to study the
effect of sorbent dose on the adsorption capacity. pH and temperature were maintained at
7.0 and 23 ◦ C, respectively, against 25 mL of 100 mg L− 1 MB concentrated solution. The
pH was adjusted using 0.01 M HCl and 0.01 M NaOH. After 2 h of agitation, the solution
was centrifuged for 15 min and the supernatant solution was collected via syringe. The
initial and final concentrations were measured using a UV-Vis spectrophotometer (Varian
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Cary 5000 UV-Vis NIR Spectrophotometer Hellma Analytics) and onward calculations
were performed.
6.3. Effect of Initial Concentration
All adsorption experiments were performed at 23 ◦ C in 125 mL conical flasks, using
a shaker (VWR 3500 Analog orbital shaker) set to 125 rpm. The first experiment was
conducted to study the isothermal equilibrium and the effect of initial MB concentration.
Standard solutions of MB were prepared using distilled water, with initial concentrations
in the range of 20–200 mg L −1 . Then, 25 mL aliquots were distributed into each flask,
and 0.01 g of the adsorbent gel was added individually to each flask. The pH values of all
solutions were recorded and adjusted to 7.0, if required, using 1 M HCl and 1 M NaOH.
After 2 h of agitation, the solution was centrifuged for 15 min and the supernatant solution
was collected via syringe. The initial and final concentrations were measured using UV-Vis
spectrophotometer (Varian Cary 5000 UV-Vis NIR Spectrophotometer Hellma Analytics).
The equilibrium adsorption capacity, qe (mg g−1 ), was calculated using:
qe =
(Co − Ce ) × V(l)
W
(9)
while the respective percentage removal of MB was calculated as:
Removal % =
Co − Ce
× 100%
Co
(10)
where Co and Ce are the initial MB and final concentration, respectively. W is the weight (g)
of the adsorbent and V is the volume (L) of MB solution.
6.4. Effect of Contact Time
The effect of contact time was studied by adding MB solution (pH 7.0, 25 mL,
100 mg L−1 ) and 0.01 g adsorbent gel into a flask, which was then agitated for a predetermined contact time between 5 min and 4 h. The samples were prepared and treated as
described in Section 2.5 and the amount of adsorption was calculated using Equation (11):
qt =
(Co − Ct ) × V
W
(11)
where Ct is the equilibrium MB concentration at a given time, and Co , V, and W are as
previously defined. Equilibrium concentration was determined by plotting qt versus time
of aliquots collected at different time intervals. Adsorption-photodegradation (absorption)
changes of MB dye with time were also recorded via UV-Vis spectrophotometry.
6.5. Effect of Temperature
The effect of temperature on the removal of MB (pH 7.0, 25 mL, 20–200 mg L−1 ) was
investigated by adding a known concentration MB solution and 0.01 g adsorbent gel to a
flask. A hot plate with a stirrer (120 rpm) was used to maintain a constant temperature of 8,
23, 32, and 40 ◦ C, under stirring, for 120 min after which the absorbance versus wavelength
spectra were recorded to measure the final concentration, and subsequent adsorption was
calculated using Equation (9).
6.6. Kinetic Models
The kinetic-based models: pseudo first order (PFO) and pseudo second order (PSO)
were applied to study the adsorption kinetics and to explain the mode of sorption of MB
onto the synthesised RF/TiO2 . The PFO model [33] has been frequently used to describe
kinetic processes under non-equilibrium conditions. PFO is based on the assumption that
the rate of adsorption is proportional to the driving force, that is, the difference between
Gels 2022, 8, 215
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the equilibrium concentration and solid phase concentration, presented as a differential
Equation (12):
dqt
= k1 (qe − qt )
(12)
dt
Integrating Equation (13) with the initial condition of qt = 0 at t = 0, the PFO model
can be rewritten, in a linear form, as:
qt= qe 1 − e−k1 t
(13)
Several studies have also reported the use of PSO [45] to interpret data obtained for
the sorption of contaminants from water, including dyes, organic molecules, and metal
ions. The PSO model assumes that the overall adsorption rate is proportional to the square
of the driving force and can be expressed as Equation (14):
dqt
= k1 (qe − qt )2
dt
(14)
Integrating Equation (14) with the initial condition of qt = 0 at t = 0, and qt = t at t = t,
the PSO model can be rewritten as:
qt =
k2 tq2e
1 + k2 tqe
(15)
In Equations (13)–(15), qt (mg g−1 ) and qe (mg g−1 ) are the adsorption capacities
of MB dye molecules at time t and at equilibrium, respectively. k1 (mg g−1 min−1 ) and
k2 (mg g−1 min−1 ) are the PFO and PSO rate constants, respectively.
6.7. Sorption Isotherm Models
The equilibrium data for the sorption of MB on RF/TiO2 adsorbent gel as a function
of equilibrium concentration (Ce mg L−1 ) was analysed in terms of Langmuir, Freundlich,
Sips, and Toth isotherm models [2]. The nonlinear form of Langmuir’s isotherm model is
represented as:
q KL Ce
qe = m
(16)
1 + Ce KL
where qe (mg g−1 ) is the MB uptake at equilibrium, Ce (mg L−1 ) is the equilibrium concentration, qm (mg g−1 ) is the amount of adsorbate at complete monolayer coverage, and KL is
the Langmuir constant.
The Freundlich equation can be expressed as follows:
qe = KF C1/n
e
(17)
where qe and Ce are as defined in the Langmuir equation, adsorption affinity is related to
the adsorption constant KF , and n indicates the magnitude of the adsorption driving force
and the distribution of energy sites on the adsorbent surface, if n < 1, then adsorption is a
chemical process, whereas if n > 1, then adsorption maybe dependent on distribution of
the surface sites. Generally, n values within 1–10 represents good adsorption [46].
The Sips isotherm model is a combination of the Langmuir and Freundlich isotherms
and is represented as:
q Ks Cns
e
qe = s
(18)
1 + Ks Cns
e
where qe and Ce are as defined for Equation (16), Ks is the Sips isotherm model constant
(L g−1 ), and ns; is the Sips isotherm exponent.
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The Toth model also describes heterogeneous systems, considering both low- and
high-end concentrations. The Toth expression is as follows:
qe =
qm KT Ce
[1 + (KT Ce ) t ]1/t
(19)
where qe and Ce are as defined for Equation (16), qm is the maximum adsorption capacity,
t is the surface heterogeneity, and KT is the surface affinity.
6.8. Photodegradation Procedure
Photocatalytic performance of as prepared RF/TiO2 was investigated through MB
dye degradation, by recording the dye degradation spectra with time using UV-Vis Spectrophotometry. 0.01 g of the adsorbent dose were used against 25 mL of 100 mg L−1 dye
concentration at pH ~7 and a temperature of 23 ◦ C and light intensity of 111 Wm−2 . For
comparison, the measurements were also recorded in the absence of catalyst, as well as
pristine RF and TiO2 . All suspensions were stirred in the dark for 60 min to establish
sorption equilibrium before exposure to visible light.
Supplementary Materials: The following supporting information can be downloaded at: https:
//www.mdpi.com/article/10.3390/gels8040215/s1, Figure S1: Zone of Energy dispersive x-ray
(EDX) spectra; Figure S2: Point of zero charge (pHpzc) on the surface of RF/TiO2 ; Table S1: Kinetic
parameters obtained by fitting kinetic data for MB adsorption to RF/TiO2 .; Table S2: Isotherm
parameters obtained by fitting MB adsorption data for RF/TiO2 to the Langmuir, Freundlich, SIPS
and Toth equations.
Author Contributions: Methodology, A.S.; formal analysis, A.S. and A.J.F.; resources, A.J.F.; writing—
original draft preparation, A.S.; writing—review and editing, A.J.F.; supervision, A.J.F.; project
administration, A.J.F.; funding acquisition, A.J.F. All authors have read and agreed to the published
version of the manuscript.
Funding: This research received no external funding.
Acknowledgments: Anam Safri thanks Ashleigh Fletcher and the Chemical and Process Engineering
Department at the University of Strathclyde for funding this work. The authors gratefully acknowledge the Materials Science and Engineering Department at Institute of Space Technology, Islamabad,
for providing support and facilities to conduct the morphological analysis.
Conflicts of Interest: The authors declare no conflict of interest.
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