ArticlePDF Available

MgAl-Layered Double Hydroxide-Coated Bio-Silica as an Adsorbent for Anionic Pollutants Removal: A Case Study of the Implementation of Sustainable Technologies

Authors:

Abstract

The adsorption efficiency of Cr(VI) and anionic textile dyes onto MgAl-layered double hydroxides (LDHs) and MgAl-LDH coated on bio-silica (b-SiO2) nanoparticles (MgAl-LDH@SiO2) derived from waste rice husks was studied in this work. The material was characterized using field-emission scanning electron microscopy (FE-SEM/EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopic (XPS) techniques. The adsorption capacities of MgAl-LDH@SiO2 were increased by 12.2%, 11.7%, 10.6%, and 10.0% in the processes of Cr(VI), Acid Blue 225 (AB-225), Acid Violet 109 (AV-109), and Acid Green 40 (AG-40) dye removal versus MgAl-LDH. The obtained results indicated the contribution of b-SiO2 to the development of active surface functionalities of MgAl-LDH. A kinetic study indicated lower intraparticle diffusional transport resistance. Physisorption is the dominant mechanism for dye removal, while surface complexation dominates in the processes of Cr(VI) removal. The disposal of effluent water after five adsorption/desorption cycles was attained using enzymatic decolorization, photocatalytic degradation of the dyes, and chromate reduction, satisfying the prescribed national legislation. Under optimal conditions and using immobilized horseradish peroxidase (HRP), efficient decolorization of effluent solutions containing AB-225 and AV-109 dyes was achieved. Exhausted MgAl-LDH@SiO2 was processed by dissolution/precipitation of Mg and Al hydroxides, while residual silica was used as a reinforcing filler in polyester composites. The fire-proofing properties of composites with Mg and Al hydroxides were also improved, which provides a closed loop with zero waste generation. The development of wastewater treatment technologies and the production of potentially marketable composites led to the successful achievement of both low environmental impacts and circular economy implementation.
Citation: Abduarahman, M.A.;
Vuksanovi´c, M.M.; Kneževi´c, N.;
Banjanac, K.; Miloševi´c, M.; Veliˇckovi ´c,
Z.; Marinkovi´c, A. MgAl-Layered
Double Hydroxide-Coated Bio-Silica
as an Adsorbent for Anionic
Pollutants Removal: A Case Study of
the Implementation of Sustainable
Technologies. Int. J. Mol. Sci. 2024,25,
11837. https://doi.org/10.3390/
ijms252111837
Academic Editor: Shaojun Yuan
Received: 15 October 2024
Revised: 30 October 2024
Accepted: 1 November 2024
Published: 4 November 2024
Copyright: © 2024 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/).
Article
MgAl-Layered Double Hydroxide-Coated Bio-Silica as an
Adsorbent for Anionic Pollutants Removal: A Case Study of the
Implementation of Sustainable Technologies
Muna Abdualatif Abduarahman 1,2, Marija M. Vuksanovi´c 3,* , Nataša Kneževi´c 3, Katarina Banjanac 4,
Milena Miloševi´c 5, Zlate Veliˇckovi´c 6and Aleksandar Marinkovi´c 1
1Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade, Serbia;
munsabdalla@gmail.com (M.A.A.); marinko@tmf.bg.ac.rs (A.M.)
2Faculty of Science, University of Sabratha, Sabratha 240, Libya
3
“VIN ˇ
CA” Institute of Nuclear Sciences—National Institute of the Republic of Serbia, University of Belgrade,
Mike Petrovi´ca Alasa 12-14, 11351 Belgrade, Serbia; natasa.knezevic@vin.bg.ac.rs
4Innovation Center of Faculty of Technology and Metallurgy Ltd., Karnegijeva 4, 11120 Belgrade, Serbia;
kbanjanac@tmf.bg.ac.rs
5
Institute of Chemistry, Technology and Metallurgy—National Institute of the Republic of Serbia, University of
Belgrade, Njegoševa 12, 11000 Belgrade, Serbia; milena.milosevic@ihtm.bg.ac.rs
6Military Academy, University of Defense, Veljka Luki´ca Kurjaka 33, 11000 Belgrade, Serbia;
zlatevel@yahoo.com
*Correspondence: marija.vuksanovic@vin.bg.ac.rs
Abstract: The adsorption efficiency of Cr(VI) and anionic textile dyes onto MgAl-layered double
hydroxides (LDHs) and MgAl-LDH coated on bio-silica (b-SiO
2
) nanoparticles (MgAl-LDH@SiO
2
)
derived from waste rice husks was studied in this work. The material was characterized using field-
emission scanning electron microscopy (FE-SEM/EDS), X-ray diffraction (XRD), Fourier transform
infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopic (XPS) techniques. The adsorption
capacities of MgAl-LDH@SiO
2
were increased by 12.2%, 11.7%, 10.6%, and 10.0% in the processes of
Cr(VI), Acid Blue 225 (AB-225), Acid Violet 109 (AV-109), and Acid Green 40 (AG-40) dye removal
versus MgAl-LDH. The obtained results indicated the contribution of b-SiO
2
to the development of
active surface functionalities of MgAl-LDH. A kinetic study indicated lower intraparticle diffusional
transport resistance. Physisorption is the dominant mechanism for dye removal, while surface
complexation dominates in the processes of Cr(VI) removal. The disposal of effluent water after
five adsorption/desorption cycles was attained using enzymatic decolorization, photocatalytic
degradation of the dyes, and chromate reduction, satisfying the prescribed national legislation. Under
optimal conditions and using immobilized horseradish peroxidase (HRP), efficient decolorization of
effluent solutions containing AB-225 and AV-109 dyes was achieved. Exhausted MgAl-LDH@SiO
2
was processed by dissolution/precipitation of Mg and Al hydroxides, while residual silica was
used as a reinforcing filler in polyester composites. The fire-proofing properties of composites
with Mg and Al hydroxides were also improved, which provides a closed loop with zero waste
generation. The development of wastewater treatment technologies and the production of potentially
marketable composites led to the successful achievement of both low environmental impacts and
circular economy implementation.
Keywords: MgAl-LDH; adsorption; enzymatic decolorization; silica reinforcement; UPR composites
1. Introduction
Water pollution, resulting from industrial activities, necessitates the continuous devel-
opment of new, tailored wastewater treatment methods. Pollutants, such as heavy metals
and organic compounds, e.g., pharmaceuticals1, pesticides, dyes, etc., are undesirable
pollutants. The widely recognized toxic and carcinogenic effects of Cr(VI) [
1
] underscore
Int. J. Mol. Sci. 2024,25, 11837. https://doi.org/10.3390/ijms252111837 https://www.mdpi.com/journal/ijms
Int. J. Mol. Sci. 2024,25, 11837 2 of 20
its significant impact on human health [
2
]. Chromium, frequently present in industrial
discharge from sectors like chemicals, paints, metal finishes, stainless steel manufacturing,
alloy cast irons, chrome, and wood treatment, poses a significant threat due to its high
mobility in ecosystems and adverse effects on human health [
3
]. Hexavalent chromium in
aqueous systems exists in various pH-dependent oxoanionic forms, including hydrogen
chromate (HCrO4), chromate (CrO42), and dichromate (Cr2O72) [4], that are linked to
severe health conditions, such as lung cancer, nasal irritation, nasal ulcers, hypersensitivity
reactions, and contact dermatitis [
5
]. Recognizing the health risks, the World Health Orga-
nization (WHO) has set a maximum allowable concentration of Cr(VI) in drinking water at
0.05 mg L1.
Organic dyes play a pivotal role in industries such as paper, paint, plastic, and tex-
tiles, featuring intricate molecular structures like azo, anthraquinonoid, and heterocyclic
groups; however, due to their extensive application and resistance to degradation, they
pose significant hazardous potential [
6
]. Their environmental persistence results in their
accumulation in the environment, leading to contamination of food chains as potential
threats to humans [7], and the combined presence of heavy metal ions and dyes generally
imparts greater toxicity with respect to living organisms [8].
The current challenge in wastewater treatment lies in the coexistence of different
pollutants, including anionic [
9
] and cationic [
10
] pollutants, markedly amplifying the
difficulty and cost of water treatment. Conventional technologies face inefficiencies in
simultaneously removing these diverse pollutants due to their distinct physicochemical
properties, including their molecular size and chemical structure [
11
]. Therefore, it becomes
imperative to devise effective approaches for the removal of coexisting pollutants.
Various techniques have been explored, including electrochemical precipitation [
12
],
ion exchange [
13
], membrane ultrafiltration [
14
], and adsorption. Adsorption stands out
as a cost-effective technique. Additionally, when coupled with an effective desorption
process, adsorption can address the sludge-related challenges commonly encountered in
precipitation methods. Various adsorbents, including clay [
15
], zeolite [
16
], carbon-based
materials [17], and layered double hydroxide (LDH), have been used [18].
Layered double hydroxides represent a versatile class of two-dimensional (2D) inor-
ganic layered matrices, attracting considerable attention owing to their distinctive physical
and chemical properties. These properties have been translated into outstanding perfor-
mance across diverse applications, including catalysis [
19
], photochemistry [
20
], electro-
chemistry [
21
], biotechnology [
22
], medicine [
23
], adsorption in wastewater treatment [
24
],
and support for enzyme immobilization due to good enzyme retention capacity [
25
]. LDH
helps in the preservation of enzyme activity and supports charge transport in the immobi-
lized system [
26
]. Three approaches, including coprecipitation methods, direct exchange
methods, and rehydration methods, are commonly applied for LDH synthesis [27].
The main idea and novelties of the study are reflected in the development of sus-
tainable water purification technologies that result in the minimization of negative envi-
ronmental impacts. Using the 3R approach (reduce, reuse, recycle), bio-based materials
replaced commercial ones, adsorbents were reused, and spent adsorbents were repurposed.
The use of silica (SiO
2
) from waste rice husk as a support for MgAl layered double hy-
droxides precipitation (LDHs are well-known adsorbents for anionic pollutants removal
from water) was used to improve the applicability and adsorption performance of newly
synthesized MgAl-LDH@SiO
2
adsorbent. Efficient water purification, desorption, and
the proper disposal of effluent desorption water and discharged adsorbent into valuable
materials were achieved. Adsorption studies were performed in relation to isotherm, ki-
netic, and thermodynamic performances in a batch system at moderate and low initial
pollutants concentration. An adsorption/desorption study in a flow system with subse-
quent environmentally friendly technologies developed for the treatment of effluent waters
was proposed. Cr(VI) was transformed to solidified material, AG-40 was subjected to
photocatalytic decomposition, and the decolorization of effluent water containing AB-225
and AV-109 dyes using immobilized horseradish peroxidase (HRP) was performed. All
Int. J. Mol. Sci. 2024,25, 11837 3 of 20
of the parameters of the treated water were below the values prescribed by the national
regulations, as outlined in the “Official Gazette of RS” nos. 67/2011 and 48/2012, regarding
limit values of the emission of pollutants and deadlines for achieving them, as well as the
Water Framework Directive of the European Commission [
28
]. The implementation of
the principles of sustainable development was realized in a novel way by either provid-
ing valuable products or minimizing the negative environmental impacts of discharged
treated water.
2. Results and Discussion
2.1. Characterization of MgAl-LDH and MgAl-LDH@SiO2Particles
2.1.1. Morphological Study
The morphological features of bio-silica, MgAl-LDH, and MgAl-LDH@SiO
2
particles
were analyzed according to results of SEM microscopy (Figure 1).
Int. J. Mol. Sci. 2024, 25, x FOR PEER REVIEW 3 of 21
proposed. Cr(VI) was transformed to solidied material, AG-40 was subjected to photo-
catalytic decomposition, and the decolorization of euent water containing AB-225 and
AV-109 dyes using immobilized horseradish peroxidase (HRP) was performed. All of the
parameters of the treated water were below the values prescribed by the national regula-
tions, as outlined in the Ocial Gazee of RS” nos. 67/2011 and 48/2012, regarding limit
values of the emission of pollutants and deadlines for achieving them, as well as the Water
Framework Directive of the European Commission [28]. The implementation of the prin-
ciples of sustainable development was realized in a novel way by either providing valua-
ble products or minimizing the negative environmental impacts of discharged treated wa-
ter.
2. Results and Discussion
2.1. Characterization of MgAl-LDH and MgAl-LDH@SiO2 Particles
2.1.1. Morphological Study
The morphological features of bio-silica, MgAl-LDH, and MgAl-LDH@SiO2 particles
were analyzed according to results of SEM microscopy (Figure 1).
(a) (b)
(c) (d)
Figure 1. Morphology of (a) SiO2, (b) MgAl-LDH, (c) and MgAl-LDH@SiO2 particles and (d) TEM
images of MgAl-LDH@SiO2.
Figure 1a shows that the silica particles have an irregular shape, while the MgAl-
LDH and MgAl-LDH@SiO2 particles are in the form of akes (Figure 1b,c).
A mapping image of the elemental composition of MgAl-LDH@SiO2 particles was
obtained from EDS analysis on a signicant portion of the SEM sample (Figures 2 and S1).
Figure 1. Morphology of (a) SiO
2
, (b) MgAl-LDH, (c) and MgAl-LDH@SiO
2
particles and (d) TEM
images of MgAl-LDH@SiO2.
Figure 1a shows that the silica particles have an irregular shape, while the MgAl-LDH
and MgAl-LDH@SiO2particles are in the form of flakes (Figure 1b,c).
A mapping image of the elemental composition of MgAl-LDH@SiO
2
particles was
obtained from EDS analysis on a significant portion of the SEM sample (Figures 1and S1).
The EDS analysis (Figure 2) shows that the elemental content is as follows: Si 70.8%, O
25.9%, Mg 2.5%, and Al 0.8%. The diameter distribution of MgAl-LDH@SiO
2
(Figure S2)
indicates that the mean diameter is 42 ±9 nm.
Int. J. Mol. Sci. 2024,25, 11837 4 of 20
Int. J. Mol. Sci. 2024, 25, x FOR PEER REVIEW 4 of 21
The EDS analysis (Figure 2) shows that the elemental content is as follows: Si 70.8%, O
25.9%, Mg 2.5%, and Al 0.8%. The diameter distribution of MgAl-LDH@SiO2 (Figure S2)
indicates that the mean diameter is 42 ± 9 nm.
Figure 2. (a) SEM images of MgAl-LDH@SiO2 particles, (b) merged image of MgAl-LDH@SiO2, (c)
EDS mapping results, and (dg) elemental mapping of Si, O, Mg, and Al.
2.1.2. XRD and FTIR Structural Characterization
The XRD paern and FTIR spectra of bio-silica, MgAl-LDH, and MgAl-LDH@SiO2
are given in Figure 3. In addition, FTIR spectra of MgAl-LDH@SiO2 after dyes adsorption
are given in Figure S3.
(a) (b)
Figure 3. (a) XRD paerns of bio-silica, MgAl-LDH, and MgAl-LDH@SiO2 particles and related (b)
FTIR spectra.
The SiO2 diraction paern displays the amorphous silica’s reecting property [29].
The MgAl-LDH diraction peaks correspond to planes (003), (006), (009), (015), (012),
(110), and (113), which indicate a layered structure. Because the MgAl-LDH@SiO2 was
synthesized intentionally with 7.6 wt.% of LDH deposit on low-crystalline SiO2, the peaks
for the LDH structure are smaller in Figure 3a. In addition, because the layer is small, the
crystals are also small [30].
Two bands, observed at 3423 and 1644 cm1 in the FTIR spectrum of MgAl-LDH (Fig-
ure 3b), are related to the stretching and bending vibrations of hydroxyl groups in the
MgAl hydroxide layer and water in the interlayer, respectively. Vibrations of the Mg-O
and Al-O groups were noticed in the range 550–760 cm1 [31]. After coprecipitation at b-
Figure 2. (a) SEM images of MgAl-LDH@SiO
2
particles, (b) merged image of MgAl-LDH@SiO
2
,
(c) EDS mapping results, and (dg) elemental mapping of Si, O, Mg, and Al.
2.1.2. XRD and FTIR Structural Characterization
The XRD pattern and FTIR spectra of bio-silica, MgAl-LDH, and MgAl-LDH@SiO
2
are given in Figure 3. In addition, FTIR spectra of MgAl-LDH@SiO
2
after dyes adsorption
are given in Figure S3.
Int. J. Mol. Sci. 2024, 25, x FOR PEER REVIEW 4 of 21
The EDS analysis (Figure 2) shows that the elemental content is as follows: Si 70.8%, O
25.9%, Mg 2.5%, and Al 0.8%. The diameter distribution of MgAl-LDH@SiO2 (Figure S2)
indicates that the mean diameter is 42 ± 9 nm.
Figure 2. (a) SEM images of MgAl-LDH@SiO2 particles, (b) merged image of MgAl-LDH@SiO2, (c)
EDS mapping results, and (dg) elemental mapping of Si, O, Mg, and Al.
2.1.2. XRD and FTIR Structural Characterization
The XRD paern and FTIR spectra of bio-silica, MgAl-LDH, and MgAl-LDH@SiO2
are given in Figure 3. In addition, FTIR spectra of MgAl-LDH@SiO2 after dyes adsorption
are given in Figure S3.
(a) (b)
Figure 3. (a) XRD paerns of bio-silica, MgAl-LDH, and MgAl-LDH@SiO2 particles and related (b)
FTIR spectra.
The SiO2 diraction paern displays the amorphous silica’s reecting property [29].
The MgAl-LDH diraction peaks correspond to planes (003), (006), (009), (015), (012),
(110), and (113), which indicate a layered structure. Because the MgAl-LDH@SiO2 was
synthesized intentionally with 7.6 wt.% of LDH deposit on low-crystalline SiO2, the peaks
for the LDH structure are smaller in Figure 3a. In addition, because the layer is small, the
crystals are also small [30].
Two bands, observed at 3423 and 1644 cm1 in the FTIR spectrum of MgAl-LDH (Fig-
ure 3b), are related to the stretching and bending vibrations of hydroxyl groups in the
MgAl hydroxide layer and water in the interlayer, respectively. Vibrations of the Mg-O
and Al-O groups were noticed in the range 550–760 cm1 [31]. After coprecipitation at b-
Figure 3. (a) XRD patterns of bio-silica, MgAl-LDH, and MgAl-LDH@SiO
2
particles and related
(b) FTIR spectra.
The SiO
2
diffraction pattern displays the amorphous silica’s reflecting property [
29
].
The MgAl-LDH diffraction peaks correspond to planes (003), (006), (009), (015), (012),
(110), and (113), which indicate a layered structure. Because the MgAl-LDH@SiO
2
was
synthesized intentionally with 7.6 wt.% of LDH deposit on low-crystalline SiO
2
, the peaks
for the LDH structure are smaller in Figure 3a. In addition, because the layer is small, the
crystals are also small [30].
Two bands, observed at 3423 and 1644 cm
1
in the FTIR spectrum of MgAl-LDH
(Figure 3b), are related to the stretching and bending vibrations of hydroxyl groups in the
MgAl hydroxide layer and water in the interlayer, respectively. Vibrations of the Mg-O and
Al-O groups were noticed in the range 550–760 cm
1
[
31
]. After coprecipitation at b-SiO
2
,
the new peaks observed at 1048, 801, 586, and 443 cm
1
are assigned to the stretching
vibrations of Si-O-Si, Si-O, Mg-O-Mg/Al-O-Al/Si-O-Si, and O-Si-O in MgAl-LDH@SiO
2
,
respectively [
31
,
32
]. The spectrum of amino-MgAl-LDH@SiO
2
showed additional bands at
2921–2850 and 1482–1360 cm
1
related to C-H stretching and the deformation of methyl
Int. J. Mol. Sci. 2024,25, 11837 5 of 20
and methylene groups. In addition, bands at 1570 and 690 cm
1
were assigned to N-H
bending and deformations of amino groups, respectively.
2.1.3. XPS Analysis
The XPS spectra of the MgAl-LDH@SiO
2
adsorbent before and after adsorption are
given in Figure 4and Figures S4–S6, respectively. The survey spectrum of MgAl-LDH@SiO
2
(Figure S4) displayed the presence of Mg, O, C, Al, and Si in 1s, 2s, and 2p orbit states,
and two additional S 2p and N 1s XPS signals are observed in the survey spectra after dye
adsorption. The Mg 1s, Mg 2p, and Al 2p spectra (Figure 4a,c) demonstrate the presence of
Mg
2+
(1302.4 and 1305.4 eV), Mg
2+
(49.1 and 51.8 eV), and Al
3+
(73.2 and 75.3 eV) valent
states, respectively, corresponding to Mg(Al)-OH/Mg(Al)-O [
33
]. In addition, the Si 2p
spectrum (Figure 4e) is characterized by three deconvoluted peaks at 97.9, 100.9, and
103.8 eV binding energy, which are related to elementary Si and its oxide [
33
]. The O
1s spectrum (Figure 4g) was deconvoluted into three overlapping peaks at 528.0, 531.8,
and 533.3 eV, associated with Mg(Al)-O, Mg(Al)-OH/Si-O, and Si-OH/OH(adsorbed
H
2
O) eV, [
31
] respectively, while the small C1s spectrum (Figure 4i) was deconvoluted into
three peaks at 284.4, 285.8, and 289.2 eV corresponding to C-Si/C-C, C-C/C-H, and C=O
functional groups, respectively.
In the spectra after adsorption (Figures 4, S5 and S6), a shift in the binding energy
and a change in the shape and intensity of the deconvolution peaks were observed. In
addition, the appearance of new peaks in the following spectra were noticed: Mg 2p at
50.3 eV (Figure 4b) and 50.9 (Figures S5a and S6c); Al 2p at 71.2 eV (Figure 4d), 72.7/77.7 eV
(Figure S5b), and 72.9eV (Figure S6b); and Si 2p at 100.1/104.0/104.9 eV (Figure 4f), 104.8 eV,
(Figure S5c) and 102.4 eV (Figure S6c) attributable to Mg(Al, Si)-O, Al-Metal, and Al-O. This
indicates that these functional groups that contain Mg/Al/Si participate in the interactions
between the adsorbent and dye.
(a) (b)
(c) (d)
Figure 4. Cont.
Int. J. Mol. Sci. 2024,25, 11837 6 of 20
Int. J. Mol. Sci. 2024, 25, x FOR PEER REVIEW 6 of 21
(e) (f)
(g) (h)
(i) (j)
(k) (l)
Figure 4. The core level XPS spectra of (a and b)Mg 1s, Mg 2p, (c and d) Al 2p, (e and f) Si 2p, (g and
h) O 1s, (I and j) C 1s, (k) S 2p, and (l) N 1s of the MgAl-LDH@SiO2 and MgAl-LDH@SiO2 + AG-40.
Further, in the O 1s spectra after adsorption, a notable decrease in the intensity of the
deconvoluted peaks is observed, and the peaks are signicantly shifted to higher binding
energy values, suggesting an interaction between dyes and sorbent. The peaks at 530.7,
533.1, and 534.7 eV after AG-40 adsorption (Figure 4h); 531.0, 533.3, and 534.9 after AB-
225 adsorption (Figure S5d); and 531.0, 532.9, and 534.2 after AV-109 adsorption (Figure
S6d) are aributed to Mg(Al)-hydroxide, Si-hydroxide/O-C-OH/C-O, and O-C=O/OH (ad-
sorbed H2O), respectively [31,33]. Deconvolution of the C1s spectrum of MgAl-LDH@SiO2
after adsorption indicates the presence of considerable functional groups from the dyes,
including C-N, (285.6 eV) (Figures 4j, S5e, and S6e), N-C=O (288.2 eV) (Figures S5e and
S6e), C-Cl (287.4 eV) (Figure 4j), C-Br (286.2/286.3 eV) (Figures S5e and S6e), and C=C from
the aromatic structure of dyes at ~284 eV, which overlaps with C-Si from the sorbent (Fig-
ures 4j, S5e, and S6e). Finally, the S2p [31,32] in the spectra of all adsorbent samples can
be ed by four deconvoluted peaks (Figures 4k, S5g, and S6g), and a peak for N 1s
around ~400 is also observed (Figures 4l, S5f, and S6f). These results indicate the interac-
tion of the sulfonate groups from the dyes with the MgAl-LDH@SiO2 surface.
2.1.4. Determination of Zero Point Charge (pHPZC)
Figure 4. The core level XPS spectra of (aand b) Mg 1s, Mg 2p, (cand d) Al 2p, (eand f) Si 2p, (gand
h) O 1s, (iand j) C 1s, (k) S 2p, and (l) N 1s of the MgAl-LDH@SiO
2
and MgAl-LDH@SiO
2
+ AG-40.
Further, in the O 1s spectra after adsorption, a notable decrease in the intensity of the
deconvoluted peaks is observed, and the peaks are significantly shifted to higher binding
energy values, suggesting an interaction between dyes and sorbent. The peaks at 530.7,
533.1, and 534.7 eV after AG-40 adsorption (Figure 4h); 531.0, 533.3, and 534.9 after AB-225
adsorption (Figure S5d); and 531.0, 532.9, and 534.2 after AV-109 adsorption (Figure S6d)
are attributed to Mg(Al)-hydroxide, Si-hydroxide/O-C-OH/C-O, and O-C=O/OH (ad-
sorbed H
2
O), respectively [
31
,
33
]. Deconvolution of the C1s spectrum of MgAl-LDH@SiO
2
after adsorption indicates the presence of considerable functional groups from the dyes,
including C-N, (285.6 eV) (Figures 4j, S5e and S6e), N-C=O (288.2 eV) (
Figures S5e and S6e
),
C-Cl (287.4 eV) (Figure 4j), C-Br (286.2/286.3 eV) (
Figures S5e and S6e
), and C=C from
the aromatic structure of dyes at ~284 eV, which overlaps with C-Si from the sorbent
(Figures 4j, S5e and S6e)
. Finally, the S2p [
31
,
32
] in the spectra of all adsorbent samples can
Int. J. Mol. Sci. 2024,25, 11837 7 of 20
be fitted by four deconvoluted peaks (Figures 4k, S5g and S6g), and a peak for N 1s around
~400 is also observed (Figures 4l, S5f and S6f). These results indicate the interaction of the
sulfonate groups from the dyes with the MgAl-LDH@SiO2surface.
2.1.4. Determination of Zero Point Charge (pHPZC)
The values of pH
pzc
at different ionic strengths (Figure S7) were found to be 7.9 and
8.1 for MgAl-LDH and MgAl-LDH@SiO
2
, respectively. The extent of positive charge at
pH < pH
pzc
at each adsorbent surface depends on operative pH and surface properties,
but has a low dependence on ionic strength. This parameter strongly indicates the high
applicability of both adsorbents for anionic pollutant removal at pH < pHpzc.
2.2. Adsorption Studies
2.2.1. Adsorption Isotherm Study
The results of the processing of adsorption data using the Langmuir (Equation (S1)) and
Freundlich isotherm models (Equation (S2)) are given in Tables 1, S1–S3 and Figure S8 [9].
Table 1. The results of Langmuir non-linear fitting for Cr(VI) (Ci = 10 mg dm
3
) and dyes
(Ci = 25 mg dm3) adsorption onto MgAl-LDH@SiO2.
Langmuir Model qm(mg g1)KL(dm3mg1) R2
Cr(VI)
25 C 100.3 ±13.5 2.31 ±0.76 0.927
35 C 105.4 ±14.9 2.41 ±0.82 0.927
45 C 111.3 ±16.9 2.56 ±0.92 0.925
AB-225
25 C 307.5 ±32.5 3.11 ±0.76 0.958
35 C 306.6 ±32.3 3.51 ±0.85 0.958
45 C 304.7 ±32.4 4.03 ±1.01 0.956
AV-109
25 C 243.3 ±33.1 1.69 ±0.62 0.905
35 C 244.0 ±32.0 1.86 ±0.67 0.901
45 C 244.4 ±30.9 2.08 ±0.73 0.911
AG-40
25 C 537.2 ±63.8 4.18 ±1.30 0.938
35 C 548.4 ±65.4 4.49 ±1.42 0.941
45 C 560.2 ±66.9 4.90 ±1.53 0.944
The capacities of single-layer coverage (q
m
) for chromate and dyes increase as the
temperature increased (Table 1) and confirm the high applicability of MgAl-LDH@SiO
2
for
anionic pollutant removal. The results obtained indicate that the synergetic effect of the
specific morphology of b-SiO
2
, despite its lower surface area, combined with the deposition
of MgAl-LDH, provides an active surface. This configuration offers a high availability
of surface-active sites able to interact with anionic pollutants (see Section S3.2.2). The
comparative study of the adsorption results for MgAl-LDH@SiO
2
(Table 1) and MgAl-LDH
(Table S1) showed an almost linear relationship in q
m
values for these two adsorbents,
demonstrating the value of the applied methodology. In addition, the results demonstrate
an appropriate relation between q
m
and the structural properties of the dyes, specifically
the availability of the basic/proton donating sites. This indicates a higher availability
of the sulfonate group in AG-40 dye for interactions with adsorbent’s surface charge
and functionalities (Figure S8). Otherwise, higher steric hindrance of the neighboring
groups in AB-225 and AV-109 contribute to the decrease in the sulfonate group availability.
Additionally, the adsorption study results at Ci= 1 mg dm3are given in Table S4.
2.2.2. Thermodynamic Study
To analyze the thermodynamic aspect of the adsorption process, the Gibbs free energy
(
G
Θ
), enthalpy (
H
Θ
), and entropy (
S
Θ
) were calculated using Van’t Hoff equations, i.e.,
Equations (S3) and (S4) [34]. The obtained results are given in Tables 2and S5.
Int. J. Mol. Sci. 2024,25, 11837 8 of 20
Table 2. Calculated thermodynamic parameters for Cr(VI), AB-225, AV-109, and AG-40 adsorption
onto MgAl-LDH@SiO2.
Pollutant 25 CGΘ(kJ mol1)
35 C45 CHΘ
(kJ mol1)
SΘ
(J mol1K1)R2
Cr(VI) 38.95 40.37 41.84 4.16 144.5 0.994
AB-225 39.69 41.32 43.03 10.15 167.1 0.996
AV-109 37.17 39.71 41.29 8.30 155.9 0.998
AG-40 40.42 41.96 43.55 6.25 156.5 0.994
The negative
G
Θ
values at all temperatures demonstrate the feasibility and spon-
taneity of the adsorption processes (Tables 2and S5). The dependence of
G
Θ
and
H
Θ
values versus temperature confirms more effective desolvation of the studied ion/dyes and
diffusional processes. Low influences of the structural and physico-chemical properties of
Cr(VI) oxyanions and dye molecules on the state of equilibrium can be observed.
2.2.3. Adsorption Kinetics
Process kinetics were analyzed using pseudo-first order (PFO), pseudo-second order
(PSO), i.e., the Ho–Mackay model, and a second-order model [
35
], and the results of
the statistically reliable PSO model fitting and the activation energy (Ea) are shown in
Tables 3and S6.
Table 3. PSO model parameters and activation energy (Ea) for the adsorption of Cr(VI), AB-225,
AV-109, and AG-40 onto MgAl-LDH@SiO2at 25, 35, and 45 C.
T(C) qe(mg g1)k2(g (mg min)1)R2Ea (KJ mol1)
Cr(VI)
25 C 90.01 ±2.61 0.00179 ±0.0001 0.998
35 C 91.60 ±2.62 0.00202 ±0.0002 0.998 9.13
45 C 93.26 ±2.87 0.00226 ±0.0001 0.999
AB-225
25 C 254.5 ±10.4 0.00120 ±0.0002 0.999
35 C 252.8 ±9.09 0.00150 ±0.0001 0.999 18.4
45 C 251.4 ±8.96 0.00192 ±0.0002 0.999
AV-109
25 C 232.5 ±7.86 0.00117 ±0.0001 0.988
35 C 232.9 ±8.32 0.00126 ±0.0001 0.998 6.18
45 C 233.3 ±8.51 0.00137 ±0.0001 0.998
AG-40
25 C 468.5 ±6.15 0.00129 ±0.0001 0.999
35 C 469.9 ±6.06 0.00134 ±0.0001 0.999 5.61
45 C 470.4 ±5.91 0.00149 ±0.0001 0.999
Diffusional resistance was evaluated by fitting kinetic data with the Weber–Morris
(W–M) model, the Dunwald–Wagner model (D-W), and the homogeneous diffusion model
(HSDM) [
9
] (Tables 4and S7). The LDH structure exhibits a gallery pathway that facilitates
carrier diffusion and transportation throughout the entire particle bulk [36].
Table 4. Kinetic parameters of the W–M, D–W, and HSDM models for the adsorption of Cr(VI) and
dyes onto MgAl-LDH@SiO2.
Model Parameter Cr(VI) AB-225 AV-109 AG-40
W–M
(Step 1)
k
p1
(mg g
1
min
0.5
)
9.726 ±0.30 23.39 ±0.94 21.07 ±1.05 47.59 ±1.88
C(mg g1)21.81 110.9 97.47 248.23
R20.999 0.999 0.999 0.998
Int. J. Mol. Sci. 2024,25, 11837 9 of 20
Table 4. Cont.
Model Parameter Cr(VI) AB-225 AV-109 AG-40
W–M
(Step 2)
k
p2
(mg g
1
min
0.5
)
0.435 ±0.01 0.264 ±0.02 0.352 ±0.02 0.527 ±0.0
C(mg g1)79.08 237.1 214.3 467.9
R20.998 0.998 0.998 0.998
D–W K0.0215 ±0.00 0.0319 ±0.00 0.0312 ±0.00
0.0217 0.0348 ±0.00
R20.845 0.819 0.826 0.736
HSDM Ds2.48 ×1011 ±0.00 3.52 ×1011 ±0.00 3.46 ×1011 ±0.00 3.72 ×1011 ±0.00
R20.835 0.816 0.824 0.731
2.2.4. Continuous Flow Experiments
The calculated column parameters (see theoretical background in S2.2.4) using the Bohart–
Adams (B–A) [
37
] and Yoon–Nelson (Y–N) [
38
] models are given in
Tables 5, S8 and S9
and
Figure S9.
Table 5. Results of pollutant removal using MgAl-LDH@SiO
2
(C
i
[AB-225] = C
i
[AG-40] = C
i
[AV-109]
= 25 mg dm3, Ci[Cr(VI)] = 10 mg dm3, mads = 0.782 g, T = 25 C, pH = 6).
Model and Parameters Q(cm3min1)
0.5 1.0 1.5
B–A
KBA (dm3mg1min1)
Cr(VI)
0.028 ±6.99 ×1040.053 ±0.002 0.091 ±0.003
qo(mg g1)90.02 ±0.65 74.15 ±0.93 60.83 ±0.89
R20.999 0.997 0.997
KBA (dm3mg1min1)
AB-225
0.011 ±4.45 ×1040.021 ±8.69 ×1040.032 ±0.00
qo(mg g1)294.7 ±2.46 271.2 ±2.84 226.6 ±2.32
R20.992 0.990 0.993
KBA (dm3mg1min1)
AV-109
0.014 ±3.54 ×1040.027 ±9.33 ×1040.040 ±0.001
qo(mg g1)233.9 ±1.34 207.4 ±1.85 178.8 ±1.52
R20.997 0.995 0.997
KBA (dm3mg1min1) 0.007 ±2.07 ×1040.014 ±4.34 ×1040.020 ±3.22 ×104
qo(mg g1)AG-40 555.5 ±2.99 503.5 ±3.23 438.0 ±1.75
R20.992 0.992 0.998
kYN (min1)0.564 ±0.01 0.533 ±0.02 0.603 ±0.02
θ(min) Cr(VI) 6.952 ±0.05 5.787 ±0.07 4.744 ±0.07
R20.999 0.997 0.996
KYN (min1)0.566 ±0.02 0.527 ±0.02 0.529 ±0.02
θ(min) AB-225 9.194 ±0.08 8.377 ±0.09 7.071 ±0.07
R20.992 0.990 0.993
Y–N KYN (min1)0.688 ±0.02 0.674 ±0.02 0.680 ±0.02
θ(min) AV-109 7.299 ±0.04 6.470 ±0.06 5.525 ±0.05
R20.997 0.995 0.997
KYN (min1)0.347 ±0.01 0.349 ±0.01 0.332 ±0.01
θ(min) AG-40 17.33 ±0.09 15.55 ±0.01 13.67 ±0.05
R20.992 0.992 0.998
2.3. Desorption Study in a Column System
Adsorption/desorption cyclability provides a relevant indicator of adsorbent longevity
and cost-effectiveness, which are crucial criteria for assessing potential applicability. The
type and strength of adsorbate/adsorbent interactions, as well as the regenerant’s displace-
ment power, primarily govern the efficacy of adsorption/desorption processes. Meanwhile,
the choice of regenerant dictates the degree of material erosion.
Int. J. Mol. Sci. 2024,25, 11837 10 of 20
Due to sensitivity of the MgAl-LDH deposit to strong acidic medium, a brief study
related to regenerant type and desorption condition selection (desorption efficiency versus
concentration and time) was performed (SM S2.3). Accordingly, two adsorption/desorption
technologies were developed:
-
A method using a moderate dye inlet concentration (25 mg dm
3
) (S2.4.2) with a low
volume of regenerator (Table S10) was developed to promote dye regeneration (less
favorable).
-
A method using a low dye inlet concentration (1 mg dm
3
) (Table 6) with a larger vol-
ume of desorption solution at higher desorption efficiency (Table S10) was developed
with potential for wastewater purification (highly favorable).
Table 6. The results of adsorption/desorption of the studied pollutants onto/from MgAl-LDH@SiO
2
(C
i
=1mgL
1
, Q
des
= 0.50 cm
3
min
1
; m
ads
~ 0.40 g) using 3.0 and 1 dm
3
of 1 wt.% NaOH/2 wt.%
NaCl for dyes and Cr(VI) desorption, respectively.
Pollutants Adsorption
(mg g1) *
Desorption
(mg g1) *
Desorption
Efficiency (%) C(mg dm3) ** q, (mg g1) *** Σ***
I
Cr(VI)
71.3 70.1 98.2 28.1 1.3
13.5
III 63.3 61.0 96.4 24.4 2.3
V 48.1 44.9 93.4 17.9 3.2
I
AB-225
204.1 198.3 97.2 26.4 5.7
43.9
III 189.1 179.8 95.1 23.9 9.3
V 168.9 157.7 93.4 21.1 11.2
I
AV-109
189.8 181.8 95.8 24.3 7.9
48.6
III 175.5 166.5 94.9 22.2 8.9
V 168.2 156.1 92.8 20.8 12.1
I
AG-40
374.2 368.2 98.4 49.1 5.9
52.9
III 357.9 347.8 97.2 46.4 10.1
V 332.4 317.2 95.4 42.3 15.3
* Adsorbed and desorbed pollutants; ** concentration of the pollutant in effluent water; *** quantity of the
irreversibly bonded pollutants in the 1st, 3rd, and 5th cycles, and overall for five cycles, respectively.
The establishment of circular technologies, after pollutant desorption, was achieved
using different treatments of the effluent water: enzymatic decolorization of AV-109 and
AB-225 (Section 2.4) as well as photocatalytic degradation of AG-40 (Section 2.5).
2.4. Decolorization of Wastewater Using Immobilized HRP on an Amino-MgAl-LDH@SiO
2
Support
2.4.1. Immobilization of HRP on an Amino-MGAl-LDH@SiO2Support
First, HPR immobilization on amino-modified MgAl-LDH@SiO
2
particles was exam-
ined in order to determine if this support has potential for use for enzyme immobilization.
Herein, the initial enzyme concentration was varied to examine the support capacity for
enzyme attachment and the activity of immobilized preparations. The impact of the ini-
tial enzyme concentration on the mass of bound protein and the activity of immobilized
peroxidase during the time period are presented in Figure 5.
Immobilization was performed at different initial enzyme concentrations, ranging
from 1.3 to 57 mg/g of support. The enzyme was immobilized in a Na-phosphate buffer at
pH 7 because, at this pH, the amino groups on the enzyme are expected to be positively
charged. In contrast, the enzyme molecules will carry an overall negative charge at pH
values above the isoelectric point. This difference in charge promotes attractive interactions
between the enzyme molecules and the support [39].
Int. J. Mol. Sci. 2024,25, 11837 11 of 20
Int. J. Mol. Sci. 2024, 25, x FOR PEER REVIEW 11 of 21
(a) (b)
(c)
Figure 5. (a) Inuence of the initial enzyme concentration on protein loading (mg/g of support) and
(b) protein immobilization yield (%), and (c) the eect of immobilization time on the activity of
immobilized peroxidase at dierent protein concentrations. The activity of immobilized HPR on
amino-MgAl-LDH@SiO2 was determined based on a reaction with commercial substrate pyrogallol.
Immobilization was performed at dierent initial enzyme concentrations, ranging
from 1.3 to 57 mg/g of support. The enzyme was immobilized in a Na-phosphate buer at
pH 7 because, at this pH, the amino groups on the enzyme are expected to be positively
charged. In contrast, the enzyme molecules will carry an overall negative charge at pH
values above the isoelectric point. This dierence in charge promotes aractive interac-
tions between the enzyme molecules and the support [39].
Increasing the initial enzyme concentration up to 57 mg/g of support resulted in an
increase in protein loading to 5 mg/g of support (Figure 5a). Immobilized peroxidase ex-
hibited the lowest activity of 1189.6 IU/g after 24 h at an initial protein content of 1.3 mg/g
of support (Figure 5b). The enzyme activity of the immobilized preparation was in the
range of 1750 to 2158 IU/g at the initial protein concentrations above 14 mg/g of support,
indicating that the optimal immobilization time is 4 h and the optimal initial enzyme con-
centration is 29 mg/g of support. The plot (Figure 5c) shows that at the optimal initial
enzyme concentration of 29 mg/g of support, the immobilized enzyme expressed a maxi-
mum immobilized enzyme activity of 2158 IU/g of support, a protein immobilization yield
of 15% (Figure 5b), and a specic activity of 431.6 IU/mg of protein (Table S11). The pre-
sented results have shown that HRP immobilized on mesoporous silica via adsorption
exhibits higher activities at lower enzyme loadings, suggesting that amino-MgAl-
LDH@SiO2 has the potential to be used as support for HPR immobilization.
Figure 5. (a) Influence of the initial enzyme concentration on protein loading (mg/g of support)
and (b) protein immobilization yield (%), and (c) the effect of immobilization time on the activity
of immobilized peroxidase at different protein concentrations. The activity of immobilized HPR on
amino-MgAl-LDH@SiO
2
was determined based on a reaction with commercial substrate pyrogallol.
Increasing the initial enzyme concentration up to 57 mg/g of support resulted in an
increase in protein loading to 5 mg/g of support (Figure 5a). Immobilized peroxidase
exhibited the lowest activity of 1189.6 IU/g after 24 h at an initial protein content of
1.3 mg/g of support (Figure 5b). The enzyme activity of the immobilized preparation
was in the range of 1750 to 2158 IU/g at the initial protein concentrations above 14 mg/g
of support, indicating that the optimal immobilization time is 4 h and the optimal initial
enzyme concentration is 29 mg/g of support. The plot (Figure 5c) shows that at the
optimal initial enzyme concentration of 29 mg/g of support, the immobilized enzyme
expressed a maximum immobilized enzyme activity of 2158 IU/g of support, a protein
immobilization yield of 15% (Figure 5b), and a specific activity of 431.6 IU/mg of protein
(Table S11). The presented results have shown that HRP immobilized on mesoporous
silica via adsorption exhibits higher activities at lower enzyme loadings, suggesting that
amino-MgAl-LDH@SiO2has the potential to be used as support for HPR immobilization.
2.4.2. Activation of Support with Glutaraldehyde
After demonstrating that amino-MgAl-LDH@SiO
2
can be used for the immobilization
of HRP via adsorption, the amino group was activated with glutaraldehyde (GA) to obtain a
more stable immobilized preparation by forming covalent bonds between the GA-activated
support and the amino groups of enzyme molecules. For support activation, a 1% solution
of GA was used (SM S3.5). The effect of the GA activation on protein loading and enzyme
activity was examined (Table S11). The results indicated that activating the support led to a
22% decrease in activity and a 28% reduction in protein loading. However, activation of
the support with GA did not result in the deactivation of the enzyme molecules during
Int. J. Mol. Sci. 2024,25, 11837 12 of 20
immobilization, which could have occurred [
40
], as evidenced by the fact that the specific
activity remained unchanged after activation.
On the other hand, a desorption assay with 1 M CaCl
2
and 1% triton demonstrated that
covalent bonds between the enzyme molecules and GA activated amino-MgAl-LDH@SiO
2
are formed. In the case of the HPR-immobilized preparation, 86% of all bonds formed
between HPR and GA-amino-MgAl-LDH@SiO
2
were be covalent. It can be presumed that
the stable covalent immobilized preparation will have much more prospects for use in the
decolorization of wastewater in comparison with the enzyme immobilized by adsorption.
2.4.3. Decolorization Efficiency and Reusability Study
To fully exploit the potential of the immobilized peroxidase produced in the decoloriza-
tion reaction, the decolorization of AV-109 dye was conducted using both GA-activated
and non-activated supports (Figure 6a). The decolorization efficiency of the obtained
HPR-immobilized preparations was examined (Figure 6a) under reaction conditions of the
textile dye AV-109 at a concentration of 25 mg L
1
and pH 4.0 in the presence of H
2
O
2
(concentration of 0.08%) [
41
] (Table S12). The anthraquinone dye AV-109 was chosen as
the model dye in order to preliminary determine if the immobilized preparations could
be used for wastewater treatment. Subsequently, AG-40 dye, AB-225 dye, and a mix-
ture of dyes (AV-109, AG-40, AB-225) were treated with HPR covalently immobilized on
GA-amino-MgAl-LDH@SiO2(Figure 6b).
Int. J. Mol. Sci. 2024, 25, x FOR PEER REVIEW 12 of 21
2.4.2. Activation of Support with Glutaraldehyde
After demonstrating that amino-MgAl-LDH@SiO2 can be used for the immobiliza-
tion of HRP via adsorption, the amino group was activated with glutaraldehyde (GA) to
obtain a more stable immobilized preparation by forming covalent bonds between the
GA-activated support and the amino groups of enzyme molecules. For support activation,
a 1% solution of GA was used (SM S3.5). The eect of the GA activation on protein loading
and enzyme activity was examined (Table S11). The results indicated that activating the
support led to a 22% decrease in activity and a 28% reduction in protein loading. However,
activation of the support with GA did not result in the deactivation of the enzyme mole-
cules during immobilization, which could have occurred [40], as evidenced by the fact
that the specic activity remained unchanged after activation.
On the other hand, a desorption assay with 1 M CaCl2 and 1% triton demonstrated
that covalent bonds between the enzyme molecules and GA activated amino-MgAl-
LDH@SiO2 are formed. In the case of the HPR-immobilized preparation, 86% of all bonds
formed between HPR and GA-amino-MgAl-LDH@SiO2 were be covalent. It can be pre-
sumed that the stable covalent immobilized preparation will have much more prospects
for use in the decolorization of wastewater in comparison with the enzyme immobilized
by adsorption.
2.4.3. Decolorization Eciency and Reusability Study
To fully exploit the potential of the immobilized peroxidase produced in the decol-
orization reaction, the decolorization of AV-109 dye was conducted using both GA-acti-
vated and non-activated supports (Figure 6a). The decolorization eciency of the ob-
tained HPR-immobilized preparations was examined (Figure 6a) under reaction condi-
tions of the textile dye AV-109 at a concentration of 25 mg L1 and pH 4.0 in the presence
of H2O2 (concentration of 0.08%) [41] (Table S12). The anthraquinone dye AV-109 was cho-
sen as the model dye in order to preliminary determine if the immobilized preparations
could be used for wastewater treatment. Subsequently, AG-40 dye, AB-225 dye, and a mix-
ture of dyes (AV-109, AG-40, AB-225) were treated with HPR covalently immobilized on
GA-amino-MgAl-LDH@SiO2 (Figure 6b).
(a) (b)
Int. J. Mol. Sci. 2024, 25, x FOR PEER REVIEW 13 of 21
(c)
Figure 6. (a) Decolorization eciency of HPR immobilized on amino-MgAl-LDH@SiO2 and GA-
amino-MgAl-LDH@SiO2. (b) Decolorization of AG-40 and AB-225 dye and a mixture of dyes using
HPR-immobilized GA-amino-MgAl-LDH@SiO2 support. (c) Reusability of the produced GA-
amino-MgAl-LDH@SiO2-HPR preparation in the decolorization of the textile dye AV-109 and the
adsorption of AV-109 dye on GA-amino-MgAl-LDH@SiO2.
The highest decolorization eciency (90%) was achieved within 300 min with HPR
immobilized on GA-activated support (GA-amino-MgAl-LDH@SiO2) (Figure 6a). On the
other hand, degradation with HPR immobilized on amino-MgAl-LDH@SiO2 remained
stable after 50 min of treatment, with a maximum decolorization eciency of only 30%.
Preliminary experiments showed that the adsorption capacity of both supports is up
to a maximum of 50% for 5 h in the case of the AV-109 dye. This indicates that the greater
than 50% decolorization eciency of the immobilized enzyme could be ascribed only to
the enzyme’s activity, with the highest impact observed after 125 min in case of the HPR
immobilized on the GA-activated support.
A comparison of the results for GA-activated and amino supports clearly demon-
strates that activation with GA positively impacted the anity of peroxidase toward AV-
109 decolorization. Therefore, the decolorization potential of HPR immobilized on GA-
activated support was examined in reactions with two more anthraquinone dyes as well
as a mixture of dyes under similar reaction conditions as noted for the AV-109 dye.
The results showed that GA-amino-MgAl-LDH@SiO2 is capable of decolorizing the
AB-225 dye by 92% within 175 min (Figure 6b), while it decolorized the mixed dye solu-
tion by 49% in the same timeframe. For AG-40 dye, a low decolorization eciency of ~10%
over 240 min was detected. Based on the presented results, it can be concluded that HPR
immobilized on GA-amino-MgAl-LDH@SiO2 could be used for the degradation of AV-109
and AB-225 with high eciency. In the case of the dye mixture, the eciency is approxi-
mately 50% due to the presence of AG-40. The poor performance of the HPR immobilized
preparation with AG-40 dye can be aributed to the dye’s chemical structure, which af-
fects its ability to approach the enzyme’s active site eectively. Since HPR is unable to
degrade AG-40 dye, photodegradation will be applied.
Based on reusability research, the potential for using the derived biocatalyst on an
industrial scale was also assessed. The ndings are displayed in Figure 6c.
The biocatalyst applied under optimal reaction conditions was separated from the
reaction mixture after each cycle, and its remaining catalytic activity was assessed in com-
parison to the rst cycle (100%). Over the course of ve cycles, the HRP immobilized prep-
aration maintained a decolorization eciency above 90%. However, a gradual decrease in
decolorization eciency was noted in the 5th and 6th cycles. In contrast, for the GA-acti-
vated support, dye adsorption was observed only after the initial cycle. Given that 50% of
the dye is adsorbed after the initial cycle and only 5% is adsorbed in the subsequent ve
cycles, it can be concluded that the performance of immobilized HPR preparation in dye
decolorization during reuse is solely due to the enzyme.
Figure 6. (a) Decolorization efficiency of HPR immobilized on amino-MgAl-LDH@SiO
2
and GA-
amino-MgAl-LDH@SiO
2
. (b) Decolorization of AG-40 and AB-225 dye and a mixture of dyes using
HPR-immobilized GA-amino-MgAl-LDH@SiO
2
support. (c) Reusability of the produced GA-amino-
MgAl-LDH@SiO
2
-HPR preparation in the decolorization of the textile dye AV-109 and the adsorption
of AV-109 dye on GA-amino-MgAl-LDH@SiO2.
The highest decolorization efficiency (90%) was achieved within 300 min with HPR
immobilized on GA-activated support (GA-amino-MgAl-LDH@SiO
2
) (Figure 6a). On the
Int. J. Mol. Sci. 2024,25, 11837 13 of 20
other hand, degradation with HPR immobilized on amino-MgAl-LDH@SiO
2
remained
stable after 50 min of treatment, with a maximum decolorization efficiency of only 30%.
Preliminary experiments showed that the adsorption capacity of both supports is up
to a maximum of 50% for 5 h in the case of the AV-109 dye. This indicates that the greater
than 50% decolorization efficiency of the immobilized enzyme could be ascribed only to
the enzyme’s activity, with the highest impact observed after 125 min in case of the HPR
immobilized on the GA-activated support.
A comparison of the results for GA-activated and amino supports clearly demonstrates
that activation with GA positively impacted the affinity of peroxidase toward AV-109 de-
colorization. Therefore, the decolorization potential of HPR immobilized on GA-activated
support was examined in reactions with two more anthraquinone dyes as well as a mixture
of dyes under similar reaction conditions as noted for the AV-109 dye.
The results showed that GA-amino-MgAl-LDH@SiO
2
is capable of decolorizing the
AB-225 dye by 92% within 175 min (Figure 6b), while it decolorized the mixed dye solution
by 49% in the same timeframe. For AG-40 dye, a low decolorization efficiency of ~10%
over 240 min was detected. Based on the presented results, it can be concluded that
HPR immobilized on GA-amino-MgAl-LDH@SiO
2
could be used for the degradation of
AV-109 and AB-225 with high efficiency. In the case of the dye mixture, the efficiency
is approximately 50% due to the presence of AG-40. The poor performance of the HPR
immobilized preparation with AG-40 dye can be attributed to the dye’s chemical structure,
which affects its ability to approach the enzyme’s active site effectively. Since HPR is unable
to degrade AG-40 dye, photodegradation will be applied.
Based on reusability research, the potential for using the derived biocatalyst on an
industrial scale was also assessed. The findings are displayed in Figure 6c.
The biocatalyst applied under optimal reaction conditions was separated from the
reaction mixture after each cycle, and its remaining catalytic activity was assessed in
comparison to the first cycle (100%). Over the course of five cycles, the HRP immobilized
preparation maintained a decolorization efficiency above 90%. However, a gradual decrease
in decolorization efficiency was noted in the 5th and 6th cycles. In contrast, for the GA-
activated support, dye adsorption was observed only after the initial cycle. Given that 50%
of the dye is adsorbed after the initial cycle and only 5% is adsorbed in the subsequent five
cycles, it can be concluded that the performance of immobilized HPR preparation in dye
decolorization during reuse is solely due to the enzyme.
2.5. Photodegradation of Effluent Water Containing AG-40
Photocatalytic tests were conducted using zinc oxide at 0.08 g L
1
and an initial dye
concentration (effluent solution, Table 6) of 22.7 mg/L for 210 min (S2.5). The decrease in
absorbance at 615 nm versus time is given in Figure 7.
Figure 7. Time-dependent UV spectra in the course of the photodegradation of AG-40 dye.
Int. J. Mol. Sci. 2024,25, 11837 14 of 20
The efficiency of photodegradation, demonstrating a rate of 82.5% after 210 min,
depends on the reaction conditions, dye structure, and irradiation efficiency [
42
]. The rate of
the photocatalytic reaction is determined by pseudo-first-order kinetics:
k1= 0.0073 min1
and t
1/2
= 94.9 min [
43
,
44
]. The determination of the COD value was performed to evaluate
the potential environmental threat of the effluent and treated water (S2.5) providing one
general parameter of water quality (Table S13). The trend of COD values showed a nearly
linear decrease, reaching 178 mg O
2
/L after 210 min and 126 mg O
2
/L after 4 h of irradiation.
Both values are lower than 200 mg O
2
/L, as prescribed by the Serbian national regulation
on sewage water from the textile industry (S2.5). Results from photolysis experiments
and quantum yield determination (S2.5; Table S14) confirm the photocatalytic degradation
potential for future optimization in real water. The effective treatment and purification of
water provide effluent waters that are able to be safely discharged into water-courses.
2.6. Recycling of Exhausted MgAl-LDH@SiO2
Exhausted MgAl-LDH@SiO
2
particles were transformed into a native form of bio-silica
through acid washing and used as reinforcements in b-UPR [
45
]. A SEM micrograph of
recycled bio-silica is given in Figure S10. Acidic washing was further processed by selective
precipitation of Al-hydroxide and Mg(OH)
2
(Figure S11) (S2.6). The XRDs of those materials
are given in Figure S12. The obtained composite Al-based materials, named c-Al(OH)
3
,
and Mg(OH)2were used as fire retardant fillers in b-UPR at 20, 40, and 60 wt.% addition.
2.6.1. Mechanical Properties of the b-UPR/bio-Silica Composites
The highest reinforcing effect of bio-silica addition was obtained at 2.5 wt.% of both un-
modified and vinyl (SiO
2
-V) modified bio-silica (S2.6.1), while the Charpy impact strength
peaked at 1 wt.% of SiO
2
-V addition. The colored test specimens are given in Figure S13.
The values of tensile strength, elongation, and modulus of elasticity are provided in Figure 8
and Table S15.
Int. J. Mol. Sci. 2024, 25, x FOR PEER REVIEW 15 of 21
(a) (b)
(c)
Figure 8. Tensile strength, Youngs modulus, and the Charpy impact strength of b-UPR/SiO2 and b-
UPR/SiO2-V composites.
The SEM micrograph of b-UPR/2.5 wt.% SiO2 composites is given in Figure S14.
Moreover, the results of the mechanical properties of b-UPR/c-Al(OH)3 are given in Figure
9 and Table S16.
(a) (b)
Figure 9. Tensile strength and Youngs modulus of b-UPR/c-Al(OH)3 composites.
Similar results were obtained for b-UPR/Mg(OH)2 composites (Table S17).
Figure 8. Tensile strength, Young’s modulus, and the Charpy impact strength of b-UPR/SiO
2
and
b-UPR/SiO2-V composites.
Int. J. Mol. Sci. 2024,25, 11837 15 of 20
The SEM micrograph of b-UPR/2.5 wt.% SiO
2
composites is given in Figure S14.
Moreover, the results of the mechanical properties of b-UPR/c-Al(OH)
3
are given in Figure 9
and Table S16.
Int. J. Mol. Sci. 2024, 25, x FOR PEER REVIEW 15 of 21
(a) (b)
(c)
Figure 8. Tensile strength, Youngs modulus, and the Charpy impact strength of b-UPR/SiO2 and b-
UPR/SiO2-V composites.
The SEM micrograph of b-UPR/2.5 wt.% SiO2 composites is given in Figure S14.
Moreover, the results of the mechanical properties of b-UPR/c-Al(OH)3 are given in Figure
9 and Table S16.
(a) (b)
Figure 9. Tensile strength and Youngs modulus of b-UPR/c-Al(OH)3 composites.
Similar results were obtained for b-UPR/Mg(OH)2 composites (Table S17).
Figure 9. Tensile strength and Young’s modulus of b-UPR/c-Al(OH)3composites.
Similar results were obtained for b-UPR/Mg(OH)2composites (Table S17).
2.6.2. Thermal Stability of b-UPR-Based Composites
The rating of the thermal stability of the produced composites, including b-UPR/SiO
2
,
b-UPR/c-Al(OH)
3
, and b-UPR/Mg(OH)
2
, was determined according to the UL-94 stan-
dard vertical test (Table 7) using the apparatus presented in Figures S15 and S18. In the
flammability test, after exposing the samples to a flame [
9
], the dripping was more domi-
nant during the exposure of neat b-UPR. The speed of dripping, i.e., the sample bursting,
differed depending on the ratio of cross-linked b-UPR and fire-retardant addition.
Table 7. Results of the flammability test UL-94V.
Sample b-UPR with Filler First Flaming Second Flaming Cotton Indicator Ignited Category
Clean b-UPR 42 59 Yes V-2
20 wt.% c-Al(OH)318 27
No
V-1
20 wt.% Mg(OH)223 30
40 wt.% c-Al(OH)33 6
V-0
40 wt.% Mg(OH)25 10
60 wt.% c-Al(OH)32 3
60 wt.% Mg(OH)22 5
When materials reach the end of their useful lives after several cycles, they can be
used again as either non-reactive fillers in freshly developed UPR matrices or by evaluating
the biodegradability of the composites they form.
2.7. Literature Survey of Adsorption Data for LDH-Based Adsorbents
The efficiency of the MgAl-LDH and MgAl-LDH@SiO
2
particles for dyes removal are
similar or higher than those of other LDH-based adsorbents (Table S18) [33,4654].
3. Materials and Methods
3.1. Materials
Supplementary Material S3.1 lists all of the materials employed.
Int. J. Mol. Sci. 2024,25, 11837 16 of 20
3.2. Syntheses of the MgAl-LDH and MgAl-LDH@SiO2
The methods used for the preparation of bio-silica particles and MgAl-LDH and
modification of MgAl-LDH@SiO
2
with amino-silane are given in S3.2.1, S3.2.2, and S3.2.4,
respectively [55].
Optimization of MgAl-LDH@SiO2Synthesis
The optimization goal was to produce an optimal amount of uniform coating of MgAl-
LDH onto the bio-silica surface. The synthesis process was conducted using six consecutive
steps [
56
]. The first modification step was as follows: bio-silica (100 g) was wetted with
an aqueous solution of MgCl
2×
4H
2
O (33 mmol) and Al
2
(OH)
5
Cl
×
2.5 H
2
O (11 mmol)
in 30 cm
3
deionized water and added to the reactor [
56
]. The test tube was filled with
xylene as a non-solvent, and gentle mixing of the media was achieved by nitrogen/air
bubbling in an upstream flow (for 30 min). Aqueous solutions of 1M NaOH were used
to adjust the pH to 10, and the solution was left overnight. The obtained material was
washed with deionized water until a neutral pH of washing was obtained and used in
subsequent deposition experiment (five cycles). All materials obtained were used in the
adsorption study. The optimal mass ratio of bio-silica to MgAl-LDH was ~12:1 (~7.6 wt.%)
(Figure S17).
3.3. Synthesis of Bio-Based Unsaturated Polyester Resin (b-UPR)
The methods for the production of bio-based unsaturated polyester resin, b-UPR, as
the matrix for composites preparation are given in S3.3 [57].
3.4. Adsorption/Desorption Study in a Batch and Fixed-Bed Column System
Details of the adsorption/desorption experiments are provided in S3.4 [58].
3.5. Technologies Developed for Desorbed Pollutant and Exhausted Adsorbent Disposal
To ensure environmentally friendly disposal of the effluent/treated water after des-
orption and exhaustion of the adsorbent, several methods for stabilizing spent adsorbent
and aqueous solutions containing pollutants have been developed (S3.5).
3.5.1. Disposal of Exhausted Adsorbent
After five adsorption/desorption cycles, the exhausted adsorbent was subjected to
acid washing to remove the MgAl-LDH deposit, leaving b-SiO
2
nanofiller that was used as
reinforcement for biobased unsaturated polyester resin (b-UPR) (S3.5.1) [45].
3.5.2. Preparation of Immobilized Enzyme on Amino-MgAl-LDH@SiO2Support
After introducing amino groups onto MgAl-LDH@SiO
2
[
29
], immobilization of HPR
onto amino-MgAl-LDH@SiO
2
was performed. In order to obtain a more stable immobilized
preparation, amino-MgAl-LDH@SiO
2
was activated with glutaraldehyde (GA). The protein
concentration was determined using the Bradford method, and the activities of free and
immobilized peroxidase were calculated. All methods, including the decolorization of dyes
and the HPR recycling potential in consecutive cycles, are described in S3.5.2 [5963].
3.5.3. Dye Decolorization Procedure
A description of the decolorization procedure is given in S3.5.3.
3.5.4. Photocatalytic Experiment
The photocatalytic protocol is presented in S3.5.4 [6468].
3.5.5. Disposal of Cr(VI)
The procedures for heavily soluble Cr(III)-oxide formation are given in S4.5.5 [
69
71
].
Int. J. Mol. Sci. 2024,25, 11837 17 of 20
3.6. Characterization Methods
Comprehensive details regarding the characterization methods used are provided in
S3.6 [72,73].
4. Conclusions
The sustainable development agenda related to the preservation of the planet is a
rapidly evolving area, and a wealth of results have been achieved. Nevertheless, there are
still many challenges, such as increasing the use of bio-based raw materials, decreasing
energy consumption, simplifying technologies, enhancing end-of-life material recycling,
and improving environmental friendliness, that should be addressed. In line with this,
herein, information on the achievement of some these goals through the development of
sustainable technologies for wastewater purification and the valorization of generated
waste materials into useful products is presented. Three main goals were achieved:
-
The production of effective MgAl-LDH and MgAl-LDH@SiO
2
useful for anionic
pollutant removal was attained. Adsorption capacities of 89.39, 275.4, 219.9, and
488.4 mg g
1
as well as 100.3, 307.6, 243.3, and 537.2 mg g
1
for Cr(VI), Acid Blue 225
(AB-225), Acid Violet 109 (AV-109), and Acid Green 40 (AG-40) dye removal using
MgAl-LDH and MgAl-LDH@SiO2adsorbent, respectively, were obtained.
-
Effluent water obtained from desorption was successfully treated either by photocat-
alytic or enzymatic methods using regenerated bio-silica as support.
-
The COD values decreased nearly linearly, reaching 178 mg O
2
/L after 210 min and
126 mg O2/L after 4 h of irradiation.
-
Exhausted adsorbent MgAl-LDH@SiO
2
was transformed to bio-silica reinforcement,
and c-Al(OH)
3
and Mg(OH)
2
fire retardants were used in the production of b-UPR-
based composites with improved mechanical and fire-proofing properties.
-
The addition of 2.5% silica particles raises the composite’s tensile strength by 61.6%
compared to the pure matrix. Young’s modulus exhibits a similar increasing trend,
reaching 37.3% of that of the pure matrix. Adding c-Al(OH)
3
to the polymer matrix
reduces the composite’s mechanical characteristics. Tensile strength is reduced by
43.6% with the addition of 60 wt.% c-Al(OH)3.
At the same time, it is necessary to continue work on the development of low-cost,
high-efficiency, and pollution-free technologies, promoting their widespread application in
industrial production, in line with sustainable development goals.
Supplementary Materials: The following supporting information can be downloaded at:
https://www.mdpi.com/article/10.3390/ijms252111837/s1. Refs. [
9
,
33
,
34
,
37
,
38
,
41
,
45
73
] are cited
in Supplementary Materials.
Author Contributions: Conceptualization, A.M. and M.M.V.; methodology, K.B. and Z.V.; software,
M.M.V.; validation, M.M.V., M.M., and A.M.; formal analysis, M.M.; investigation, M.A.A., M.M.V.,
N.K., and K.B.; resources, M.M.V. and A.M.; data curation, M.M. and Z.V.; writing—original draft
preparation, M.A.A.; writing—review and editing, M.M.V., N.K., M.M., Z.V., and A.M.; visualization,
M.M.V. and M.M.; supervision, A.M.; project administration, A.M.; funding acquisition, M.M.V. and
A.M. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the Ministry of Science, Technological Development and
Innovation of the Republic of Serbia funded the research (Contracts No. 451-03-65/2024-03/200135,
451-03-66/2024-03/200017, and 451-03-66/2024-03/200026).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available on request from the
corresponding author or co-authors. The data are not publicly available.
Int. J. Mol. Sci. 2024,25, 11837 18 of 20
Conflicts of Interest: Author Katarina Banjanac was employed by the company Innovation Center
of Faculty of Technology and Metallurgy Ltd. The remaining authors declare that the research was
conducted in the absence of any commercial or financial relationships that could be construed as a
potential conflict of interest.
References
1.
Standeven, A.M.; Wetterhahn, K.E. Chromium(VI) Toxicity: Uptake, Reduction, and DNA Damage. J. Am. Coll. Toxicol. 1989,8,
1275–1283. [CrossRef]
2.
Rybka, K.; Matusik, J.; Marzec, M. Mg/Al and Mg/Fe Layered Double Hydroxides Derived from Magnesite and Chemicals: The
Effect of Adsorbent Features and Anions Chemistry on Their Removal Efficiency. J. Clean. Prod. 2022,332, 130084. [CrossRef]
3.
Camargo, F.A.O.; Bento, F.M.; Okeke, B.C.; Frankenberger, W.T. Chromate Reduction by Chromium-Resistant Bacteria Isolated
from Soils Contaminated with Dichromate. J. Environ. Qual. 2003,32, 1228–1233. [CrossRef]
4.
Ramsey, J.D.; Xia, L.; Kendig, M.W.; McCreery, R.L. Raman Spectroscopic Analysis of the Speciation of Dilute Chromate Solutions.
Corros. Sci. 2001,43, 1557–1572. [CrossRef]
5.
Costa, M. Toxicity and Carcinogenicity of Cr(VI) in Animal Models and Humans. Crit. Rev. Toxicol. 1997,27, 431–442. [CrossRef]
6.
Jha, P.; Jobby, R.; Desai, N. Remediation of Textile Azo Dye Acid Red 114 by Hairy Roots of Ipomoea Carnea Jacq. and Assessment
of Degraded Dye Toxicity with Human Keratinocyte Cell Line. J. Hazard. Mater. 2016,311, 158–167. [CrossRef]
7.
Ali, H.; Khan, E.; Ilahi, I. Environmental Chemistry and Ecotoxicology of Hazardous Heavy Metals: Environmental Persistence,
Toxicity, and Bioaccumulation. J. Chem. 2019,2019, 6730305. [CrossRef]
8.
Chen, Y.-Y.; Yu, S.-H.; Jiang, H.-F.; Yao, Q.-Z.; Fu, S.-Q.; Zhou, G.-T. Performance and Mechanism of Simultaneous Removal of
Cd(II) and Congo Red from Aqueous Solution by Hierarchical Vaterite Spherulites. Appl. Surf. Sci. 2018,444, 224–234. [CrossRef]
9.
Kneževi´c, N.; Milanovi´c, J.; Veliˇckovi´c, Z.; Miloševi´c, M.; Vuksanovi´c, M.M.; Onjia, A.; Marinkovi´c, A. A Closed Cycle of
Sustainable Development: Effective Removal and Desorption of Lead and Dyes Using an Oxidized Cellulose Membrane. J. Ind.
Eng. Chem. 2023,126, 520–536. [CrossRef]
10.
Yang, L.; Zhang, Y.; Liu, X.; Jiang, X.; Zhang, Z.; Zhang, T.; Zhang, L. The Investigation of Synergistic and Competitive Interaction
between Dye Congo Red and Methyl Blue on Magnetic MnFe2O4.Chem. Eng. J. 2014,246, 88–96. [CrossRef]
11.
Holkar, C.R.; Jadhav, A.J.; Pinjari, D.V.; Mahamuni, N.M.; Pandit, A.B. A Critical Review on Textile Wastewater Treatments:
Possible Approaches. J. Environ. Manag. 2016,182, 351–366. [CrossRef] [PubMed]
12.
Kongsricharoern, N.; Polprasert, C. Electrochemical Precipitation of Chromium (Cr) from an Electroplating Wastewater. Water Sci.
Technol. 1995,31, 109–117. [CrossRef]
13.
Xing, Y.; Chen, X.; Wang, D. Electrically Regenerated Ion Exchange for Removal and Recovery of Cr(VI) from Wastewater. Environ.
Sci. Technol. 2007,41, 1439–1443. [CrossRef]
14.
Korus, I.; Loska, K. Removal of Cr(III) and Cr(VI) Ions from Aqueous Solutions by Means of Polyelectrolyte-Enhanced Ultrafiltra-
tion. Desalination 2009,247, 390–395. [CrossRef]
15.
Hu, B.; Luo, H.; Chen, H.; Dong, T. Adsorption of Chromate and Para-Nitrochlorobenzene on Inorganic–Organic Montmorillonite.
Appl. Clay Sci. 2011,51, 198–201. [CrossRef]
16.
Barquist, K.; Larsen, S.C. Chromate Adsorption on Bifunctional, Magnetic Zeolite Composites. Microporous Mesoporous Mater.
2010,130, 197–202. [CrossRef]
17.
Ko, Y.-J.; Choi, K.; Lee, S.; Cho, J.-M.; Choi, H.-J.; Hong, S.W.; Choi, J.-W.; Mizuseki, H.; Lee, W.-S. Chromate Adsorption
Mechanism on Nanodiamond-Derived Onion-like Carbon. J. Hazard. Mater. 2016,320, 368–375. [CrossRef]
18.
Kim, Y.; Son, Y.; Bae, S.; Kim, T.-H.; Hwang, Y. Particle Size and Interlayer Anion Effect on Chromate Adsorption by MgAl-Layered
Double Hydroxide. Appl. Clay Sci. 2022,225, 106552. [CrossRef]
19.
Wang, Z.; Liu, F.; Lu, C. Mg–Al–Carbonate Layered Double Hydroxides as a Novel Catalyst of Luminol Chemiluminescence.
Chem. Commun. 2011,47, 5479–5481. [CrossRef]
20.
Zhao, Y.; Li, B.; Wang, Q.; Gao, W.; Wang, C.J.; Wei, M.; Evans, D.G.; Duan, X.; O’Hare, D. NiTi-Layered Double Hydroxides
Nanosheets as Efficient Photocatalysts for Oxygen Evolution from Water Using Visible Light. Chem. Sci. 2014,5, 951–958.
[CrossRef]
21.
Li, M.; Liu, F.; Cheng, J.P.; Ying, J.; Zhang, X.B. Enhanced Performance of Nickel–Aluminum Layered Double Hydroxide
Nanosheets / Carbon Nanotubes Composite for Supercapacitor and Asymmetric Capacitor. J. Alloys Compd. 2015,635, 225–232.
[CrossRef]
22.
Kapusetti, G.; Mishra, R.R.; Srivastava, S.; Misra, N.; Singh, V.; Roy, P.; Singh, S.K.; Chakraborty, C.; Malik, S.; Maiti, P. Layered
Double Hydroxide Induced Advancement in Joint Prosthesis Using Bone Cement: The Effect of Metal Substitution. J. Mater.
Chem. B 2013,1, 2275. [CrossRef] [PubMed]
23.
Liang, R.; Wei, M.; Evans, D.G.; Duan, X. Inorganic Nanomaterials for Bioimaging, Targeted Drug Delivery and Therapeutics.
Chem. Commun. 2014,50, 14071–14081. [CrossRef]
24.
Abduarahman, M.A.; Vuksanovi´c, M.M.; Miloševi´c, M.; Egelja, A.; Savi´c, A.; Veliˇckovi´c, Z.; Marinkovi´c, A. Mn-Fe Layered
Double Hydroxide Modified Cellulose-Based Membrane for Sustainable Anionic Pollutant Removal. J. Polym. Environ. 2024,32,
3776–3794. [CrossRef]
Int. J. Mol. Sci. 2024,25, 11837 19 of 20
25.
Ribeiro, L.N.M.; Alcântara, A.C.S.; Darder, M.; Aranda, P.; Araújo-Moreira, F.M.; Ruiz-Hitzky, E. Pectin-Coated Chitosan–LDH
Bionanocomposite Beads as Potential Systems for Colon-Targeted Drug Delivery. Int. J. Pharm. 2014,463, 1–9. [CrossRef]
26.
Djebbi, M.A.; Braiek, M.; Hidouri, S.; Namour, P.; Jaffrezic-Renault, N.; Ben Haj Amara, A. Novel Biohybrids of Layered Double
Hydroxide and Lactate Dehydrogenase Enzyme: Synthesis, Characterization and Catalytic Activity Studies. J. Mol. Struct. 2016,
1105, 381–388. [CrossRef]
27.
Olfs, H.-W.; Torres-Dorante, L.O.; Eckelt, R.; Kosslick, H. Comparison of Different Synthesis Routes for Mg–Al Layered Double
Hydroxides (LDH): Characterization of the Structural Phases and Anion Exchange Properties. Appl. Clay Sci. 2009,43, 459–464.
[CrossRef]
28.
Available online: https://pravno-informacioni-sistem.rs/eli/Rep/sgrs/vlada/uredba/2011/67/4/reg (accessed on 30 October 2024).
29.
Alazreg, A.; Vuksanovi´c, M.M.; Mladenovi´c, I.O.; Egelja, A.; Jankovi´c-Mandi´c, L.; Marinkovi´c, A.; Heinemann, R.J. Dental
Material Based on Poly(Methyl Methacrylate) with Magnesium-Aluminum Layered Double Hydroxide (MgAl-LDH) on Bio-Silica
Particles. Mater. Lett. 2024,354, 135354. [CrossRef]
30.
Vuksanovic, M.; Mladenovic, I.; Tomic, N.; Petrovic, M.; Radojevic, V.; Marinkovic, A.; Jancic-Heinemann, R. Mechanical
Properties of Biomass-Derived Silica Nanoparticles Reinforced PMMA Composite Material. Sci. Sinter. 2022,54, 211–221.
[CrossRef]
31.
Yang, D.; Song, S.; Zou, Y.; Wang, X.; Yu, S.; Wen, T.; Wang, H.; Hayat, T.; Alsaedi, A.; Wang, X. Rational Design and Synthesis
of Monodispersed Hierarchical SiO
2
@layered Double Hydroxide Nanocomposites for Efficient Removal of Pollutants from
Aqueous Solution. Chem. Eng. J. 2017,323, 143–152. [CrossRef]
32.
Zheng, G.; Wu, C.; Wang, J.; Mo, S.; Zou, Z.; Zhou, B.; Long, F. Space-Confined Effect One-Pot Synthesis of
γ
-AlO(OH)/MgAl-LDH
Heterostructures with Excellent Adsorption Performance. Nanoscale Res. Lett. 2019,14, 281. [CrossRef] [PubMed]
33.
Dai, X.; Jing, C.; Li, K.; Zhang, X.; Song, D.; Feng, L.; Liu, X.; Ding, H.; Ran, H.; Zhu, K.; et al. Enhanced Bifunctional Adsorption
of Anionic and Cationic Pollutants by MgAl LDH Nanosheets Modified Montmorillonite via Acid-Salt Activation. Appl. Clay Sci.
2023,233, 106815. [CrossRef]
34.
Lombardo, S.; Thielemans, W. Thermodynamics of Adsorption on Nanocellulose Surfaces. Cellulose 2019,26, 249–279. [CrossRef]
35.
Major, G.H.; Chatterjee, S.; Linford, M.R. Resolving a Mathematical Inconsistency in the Ho and McKay Adsorption Equation.
Appl. Surf. Sci. 2020,504, 144157. [CrossRef]
36.
Xu, D.-M.; Guan, M.-Y.; Xu, Q.-H.; Guo, Y. Multilayer Films of Layered Double Hydroxide/Polyaniline and Their Ammonia
Sensing Behavior. J. Hazard. Mater. 2013,262, 64–70. [CrossRef]
37.
Bohart, G.S.; Adams, E.Q. Some aspects of the behavior of charcoal with respect to chlorine
1
.J. Am. Chem. Soc. 1920,42, 523–544.
[CrossRef]
38.
Yoon, Y.H.; Nelson, J.H. Application of Gas Adsorption Kinetics I. A Theoretical Model for Respirator Cartridge Service Life. Am.
Ind. Hyg. Assoc. J. 1984,45, 509–516. [CrossRef]
39.
Salih, R.; Banjanac, K.; Vukoiˇci´c, A.; Gržeti´c, J.; Popovi´c, A.; Veljkovi´c, M.; Bezbradica, D.; Marinkovi´c, A. Acrylic Modified Kraft
Lignin Microspheres as Novel Support for Immobilization of Laccase from M. Thermophila Expressed in A. Oryzae (Novozym®
51003) and Application in Degradation of Anthraquinone Textile Dyes. J. Environ. Chem. Eng. 2023,11, 109077. [CrossRef]
40.
Monsan, P. Optimization of Glutaraldehyde Activation of a Support for Enzyme Immobilization. J. Mol. Catal. 1978,3, 371–384.
[CrossRef]
41.
Jamal, F.; Singh, S.; Qidwai, T.; Singh, D.; Pandey, P.K.; Pandey, G.C.; Khan, M.Y. Catalytic Activity of Soluble versus Immobilized
Cauliflower (Brassica Oleracea) Bud Peroxidase-Concanavalin A Complex and Its Application in Dye Color Removal. Biocatal.
Agric. Biotechnol. 2013,2, 311–321. [CrossRef]
42.
Gaya, U.I.; Abdullah, A.H. Heterogeneous Photocatalytic Degradation of Organic Contaminants over Titanium Dioxide: A
Review of Fundamentals, Progress and Problems. J. Photochem. Photobiol. C Photochem. Rev. 2008,9, 1–12. [CrossRef]
43.
Tran, H.D.; Nguyen, D.Q.; Do, P.T.; Tran, U.N.P. Kinetics of Photocatalytic Degradation of Organic Compounds: A Mini-Review
and New Approach. RSC Adv. 2023,13, 16915–16925. [CrossRef] [PubMed]
44.
Jovanovi´c, A.; Stevanovi´c, M.; Barudžija, T.; Cvijeti´c, I.; Lazarevi´c, S.; Tomaševi´c, A.; Marinkovi´c, A. Advanced Technology for
Photocatalytic Degradation of Thiophanate-Methyl: Degradation Pathways, DFT Calculations and Embryotoxic Potential. Process
Saf. Environ. Prot. 2023,178, 423–443. [CrossRef]
45.
Embirsh, H.S.A.; Stajˇci´c, I.; Gržeti´c, J.; Mladenovi´c, I.O.; An ¯
delkovi´c, B.; Marinkovi´c, A.; Vuksanovi´c, M.M. Synthesis, Characteri-
zation and Application of Biobased Unsaturated Polyester Resin Reinforced with Unmodified/Modified Biosilica Nanoparticles.
Polymers 2023,15, 3756. [CrossRef] [PubMed]
46.
Pan, X.; Zhang, M.; Liu, H.; Ouyang, S.; Ding, N.; Zhang, P. Adsorption Behavior and Mechanism of Acid Orange 7 and Methylene
Blue on Self-Assembled Three-Dimensional MgAl Layered Double Hydroxide: Experimental and DFT Investigation. Appl. Surf.
Sci. 2020,522, 146370. [CrossRef]
47.
Extremera, R.; Pavlovic, I.; Pérez, M.R.; Barriga, C. Removal of Acid Orange 10 by Calcined Mg/Al Layered Double Hydroxides
from Water and Recovery of the Adsorbed Dye. Chem. Eng. J. 2012,213, 392–400. [CrossRef]
48.
Abbasi, M.; Sabzehmeidani, M.M.; Ghaedi, M.; Jannesar, R.; Shokrollahi, A. Synthesis of Grass-like Structured Mn-Fe Layered Double
Hydroxides/PES Composite Adsorptive Membrane for Removal of Malachite Green. Appl. Clay Sci. 2021,203, 105946. [CrossRef]
49.
Lu, L.; Li, J.; Ng, D.H.L.; Yang, P.; Song, P.; Zuo, M. Synthesis of Novel Hierarchically Porous Fe
3
O
4
@MgAl–LDH Magnetic
Microspheres and Its Superb Adsorption Properties of Dye from Water. J. Ind. Eng. Chem. 2017,46, 315–323. [CrossRef]