Preparation of porous silica materials using a eucalyptus template method and its efficient adsorption of methylene blue

Abstract

Mesoporous silica has become one of the primary adsorbent materials for solving dye wastewater pollution due to its high specific surface area and good adsorption properties. However, the high cost of the traditional chemical synthesis method limits its wide application. In this thesis, low-cost and high-efficiency porous silica adsorbent materials (PSAM) were successfully prepared by dissolving quartz powder in NaOH solution and depositing and growing in the pores of eucalyptus wood under hydrothermal conditions using eucalyptus wood as a templating agent. The experimental results showed the prepared materials have a loose, porous slit pore structure and many active adsorption sites. The adsorption efficiency of methylene blue was high, reaching more than 85% within 10 min, and the maximum adsorption amount was 90.01 mg/g. The adsorption process was by the pseudo-first-order，pseudo-second-order, and Langmuir models. The analysis of thermodynamic data showed that the adsorption of methylene blue by PSAM was a heat-absorbing process and spontaneous. Therefore, PSAM can be effectively used for the application of methylene blue dye removal in water.

Full text

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Preparation of porous silica materials using a
eucalyptus template method and its ecient
adsorption of methylene blue
Wenxin Zhu
Nanning Normal University
Leping Liu (  130078@nnnu.edu.cn )
Nanning Normal University
YuanXia Lao
Nanning Normal University
Yan He
Guangxi University
Research Article
Keywords: Quartz powder, Eucalyptus, Adsorption, Organic dye
Posted Date: April 26th, 2023
DOI: https://doi.org/10.21203/rs.3.rs-2844761/v1
License:   This work is licensed under a Creative Commons Attribution 4.0 International License. 
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Abstract
Mesoporous silica has become one of the primary adsorbent materials for solving dye wastewater
pollution due to its high specic surface area and good adsorption properties. However, the high cost of
the traditional chemical synthesis method limits its wide application. In this thesis, low-cost and high-
eciency porous silica adsorbent materials (PSAM) were successfully prepared by dissolving quartz
powder in NaOH solution and depositing and growing in the pores of eucalyptus wood under
hydrothermal conditions using eucalyptus wood as a templating agent. The experimental results showed
the prepared materials have a loose, porous slit pore structure and many active adsorption sites. The
adsorption eciency of methylene blue was high, reaching more than 85% within 10 min, and the
maximum adsorption amount was 90.01 mg/g. The adsorption process was by the pseudo-rst-order
pseudo-second-order, and Langmuir models. The analysis of thermodynamic data showed that the
adsorption of methylene blue by PSAM was a heat-absorbing process and spontaneous. Therefore,
PSAM can be effectively used for the application of methylene blue dye removal in water.
1. Introduction
Water is a vital material resource for human health and development. Water pollution from industrial
production is an increasingly severe threat to freshwater resources, causing toxic damage to plants,
animals, and humans. Industrial organic dyes are one of the leading causes of surface and groundwater
pollution. Organic dyes typically have complex structures and are not readily biodegradable. Most contain
elements such as nitrogen and sulfur, which are highly toxic and carcinogenic [1, 2]. One such dye is
methylene blue (MB). This cationic thiazine dye can remain in aquatic systems long if left untreated,
endangering marine organisms' survival and destroying the ecosystem's balance [3]. Industrial
wastewater must be effectively treated before discharge to reduce pollution of inowing water sources.
Current methods for treating industrial wastewater and organic dyes can be classied into physical,
biological, and chemical treatments. Commonly used methods include electrochemical degradation,
photocatalytic degradation, Fenton oxidation, adsorption, and biological anaerobic degradation [4–6].
Each method has its advantages and disadvantages. The adsorption method has received extensive
research attention owing to its low cost, convenience, low energy consumption, and low temperature and
pressure requirements [7].
In recent years, the commonly used adsorbents for treating organic dyes in wastewater by adsorption
include commercial activated carbon, but commercial activated carbon has a high cost, which limits its
application [8]. So other materials have better research and application in adsorption, such as biomass
carbon materials and nano metal oxides, which are widely used in dye adsorption and degradation.
Biomass carbon materials (e.g., lychee shell, sugarcane peel, wood chips, etc.) and nano oxides (e.g.,
TiO2, ZnO, SiO2, etc.) possess the advantages of large specic surface area and pores, which make them
excellent adsorbent materials. [9–13].

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Mesoporous silica is usually prepared using the sol-gel method with surfactants as soft templating
agents; the high preparation cost limits its large-scale application. Karine et al. [14] prepared
mesostructured silica materials via a synergistic self-assembly and liquid crystal templating mechanism
using polyoxyethylene uoroalkyl ether and quaternary ammonium surfactants as substrates. Su et al.
[15] used ZnO nanorod arrays as macroporous templates and CTAB as a mesoporous template to
synthesize graded mesoporous silica lms. However, most surfactants are petrochemicals, which are
costly and do not meet the requirements of a low-carbon environment. Current applications of
mesoporous silica include the adsorption of heavy metal ions and organic pollutants. Zhang et al. [16]
used an improved oil–water biphasic layered coating to synthesize silica with a brous structure. The
resulting particles were modied with CPBA and studied as cis-diol adsorbents for adsorption
performance. The simple, green, and ecient preparation of mesoporous silica materials with good
adsorption properties is still a problem worthy of further investigation.
Eucalyptus is the world's most widely distributed hardwood tree and a relatively abundant timber resource
in Guangxi [17]. Due to its low cost and abundance, it is commonly used in papermaking and building
materials for home use and as a carbon source for fuel and other materials [18–20]. Eucalyptus wood
has many pore structures, and some lignin is dissolved under robust alkaline solutions. The pore size
increases, providing a suitable environment for SiO2 deposition. Quartz has the most stable and
abundant crystal structure of SiO2. Quartz powder can be quickly dissolved in a NaOH solution in a
hydrothermal environment at a specic temperature and pressure.
This paper aims to obtain a low-cost and ecient mesoporous silica adsorbent material using eucalyptus
wood as a templating agent. Under hydrothermal conditions, quartz powder is rapidly dissolved in NaOH
solution and deposited and grown in the pores of eucalyptus wood after calcination to remove the
templating agent. Moreover, the prepared mesoporous silica material showed highly ecient adsorption
performance for methylene blue dye in water. In addition, the lignin-containing waste solution and the
precipitated SiO2 can be recycled as a bio-binder after concentration. For example, Juan et al. [21]
prepared high-density berboard (HDF) from wheat straw under alkaline conditions using lignin as a
natural adhesive, Yang et al. [22] rst Lignin was catalyzed by sodium hydroxide and phenolized by
phenol at high temperature. Lignin was washed by ether and precipitated by acid to prepare phenolic
resin by replacing part of the phenol with this product. In the present study, a report on preparing
mesoporous silica using eucalyptus wood as a template has yet to be prepared. This thesis focuses on
the structure of PSAM and its adsorption performance and adsorption mechanism on methylene blue dye
to provide a new idea for preparing and applying ecient dye adsorbents.
2. Materials And Methods
2.1. materials
The quartz powder in this experiment was purchased from Chengde Group Company in Beihai, Guangxi,
and the eucalyptus wood was from Chongzuo City, Guangxi. Sodium hydroxide is an analytically pure

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reagent purchased from Chongqing Chuandong Chemical Group Co. Methylene blue is also an
analytically pure reagent, purchased from Tianjin Zhiyuan Chemical Reagent Co. Deionized water was
homemade in the laboratory and used for the preparation of chemical solutions. The XRD spectrum of
raw quartz powder is shown in Fig.1.
2.2 Synthesis of PSAM
Sodium hydroxide (30 g) and quartz powder (15 g) with a mass ratio of 2:1 were mixed and stirred well,
and the concentrations of sodium hydroxide were 2 mol/L (2M), 4 mol/L (4M), 6 mol/L (6M) and 8 mol/L
(8M), respectively. After the hydrothermal treatment, the waste solution and precipitate were concentrated
and recovered for the bioadhesive. The eucalyptus slices were sonicated for 5 min in an ultrasonic
cleaner with an operating frequency of 37 kHz three times. 80°C oven drying and calcination at 600°C for
2 h. The calcined material was ground to obtain a black powder and collected for subsequent testing.
2.3 Characterization
The crystal structures of the prepared samples were characterized using X-ray diffraction (XRD, Rigaku
MiniFlex 600 diffractometer, Japan) with Cu Kα radiation at a scanning rate of 5°/min at a voltage of 40
kV and a current of 15 mA at 5–80°. The structure of the substances was analyzed by recording Fourier
transform infrared spectrograms (FTIR, Thermo Scientic Nicolet IS50, USA) using the KBr particle
method. Scanning electron microscopy (SEM) images of the studied catalysts were acquired on a eld
emission scanning electron microscope (FE-SEM, Hitachi SU8220, Japan) device. The specic surface
area and the prepared samples' pores were tested by adsorption and desorption in N2 with a surface area
porosity analyzer (BET, Micromeritics ASAP2460, USA). The samples were degassed at 300°C for 6 h
before measurement. The zeta potential measurements were carried out using a nanoparticle analyzer
(Horiba, SZ-100, Japan).
2.4 Adsorption performance
The adsorption kinetics were performed at room temperature (20 ± 2°C) in the dark. Adsorbent (50 mg)
was added to 50 mL of a 20 mg/L MB solution. The supernatant was shaken at 230 rpm and ltered
through a needle lter (4 mL) at predetermined time intervals before measurement with a double-beam
UV spectrophotometer at a wavelength of 664 nm.
The adsorption isotherms were also obtained at room temperature (20 ± 2°C) in the dark. Adsorption
equilibrium was reached by adding 50 mg of adsorbent to 50 mL of MB solution at different
concentrations (20–60 mg) and shaking at 230 rpm for 24 h. The supernatant (4 mL) was ltered and
measured at 664 nm using a double-beam UV-Vis spectrophotometer (Shimadzu UV2600, Japan).
Isothermal adsorption curves were tted from Langmuir (5) and Freundlich (6) models.
The thermodynamics of attachment was measured at 20°C, 30°C, and 40°C by adding 50 mg of
adsorbent to a solution of MB at a concentration of 20 mg/L. After shaking at 230 rpm for 24 h, 4 mL of
the supernatant was ltered, and the absorbance was measured at a wavelength of 664 nm using a dual-

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beam UV-Vis spectrophotometer (Shimadzu UV2600, Japan). The thermodynamic parameters were
obtained by tting Eqs.(8), (9), and (10).
The adsorbent's effect on MB solution's adsorption was investigated at different temperatures (20–60℃)
and pH (pH = 2–8).
2.4 Related formulas
The adsorption amount (qt, mg/g) and equilibrium adsorption capacity (qe, mg/g) of MB for the samples
at different adsorption times (t) are expressed as
1
2
where C0 is the starting concentration of the adsorbent solution (MB solution) (mg/L); Ct is the
concentration of the adsorbent (MB solution) at a particular moment t (mg/L); Ce is the concentration of
the adsorbent (MB solution) at the moment of equilibrium (mg/L); V is the volume of the adsorbent
solution (MB solution) (L), and W is the mass of the adsorbent (g).
· Adsorption kinetic
Pseudo-rst order [23] (3)
Pseudo-second order [24] (4)
where k1 is the adsorption rate constant (min− 1) for the pseudo-rst-order model; k2 is the adsorption rate
constant (g/mg/min) for the pseudo-second-order model, and qe and qt are the MB uptakes (mg/g) at
equilibrium and at time t (min), respectively.
·Adsorption isotherm
qt= (C0-Ct)V
m
qe= (C0-Ce)V
m
qt=qe(1-e-k1t)
qt=k2qet
1+k2qet

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Freundlich [25] (5)
Langmuir [26] (6)
 (7)
where KL (L/mg) and KF [(mg/g)/(mg/L)n] represent the Langmuir and Freundlich adsorption coecients,
respectively; n is the exponential coecient, and RL (L/mg) is the Langmuir constant.
·Adsorption thermodynamics [13]
(8)
(9)
(10)
where KL is the partition coecient, ∆S (J/K mol) is the standard entropy change; ∆H (kJ/mol) is the
standard enthalpy; ∆G (kJ/mol) is the standard free energy change; R is the ideal gas constant (8.314
J/mol/ K), and T is the temperature (K).
3. Results And Discussion
3.1. Characterization
3.1.1 XRD
Figure 2 shows the XRD patterns of specimens hydrothermally heated at 120°C for 24 h with different
NaOH solution and quartz powder concentrations. Peaks at 2θ = 20.86°, 26.64°, and 50.05° were observed
for all specimens with complete coincidence with the characteristic peaks of quartz crystals (PDF#89-
1961), corresponding to crystallographic planes (100), (011), and (112). As the NaOH concentration
increased, the intensity of the characteristic peaks of the quartz crystals decreased. After the
hydrothermal reaction, the surface of the quartz powder particles reacted with sodium hydroxide solution
to form many silica hydroxyl groups. Quartz particles with silica hydroxyl groups on the surface were
deposited in the pore structure of eucalyptus chips via a condensation reaction between them. The PSAM
was obtained after calcination to remove the template. With an increase in the NaOH solution
concentration, many non-bridging oxygen active sites were formed in the pore channels of PSAM to
balance its charge with Na+. The calcined PSAM formed Na2CO3 crystals after the adsorption of CO2
logqe= logKF+ logCe
1
n
= +
Ce
qe1
KLqm
Ce
qm
RL=1
1+RL+C0
KL= mqe
Ce
ΔG = -RTlnKL
lnKL= +
ΔS
R-ΔH
R

Page 7/27
from the air. With NaOH concentrations of 2M and 4M, Na2CO3 crystals were not detected in the XRD
spectra; the presence of Na2CO3 crystals was determined from later FTIR analysis. With NaOH
concentrations of 6M and 8M, characteristic peaks of Na2CO3 were observed at 2θ = 20.06°, 34.19°,
35.16°, 37.93°, 39.85°, 40.99°, 46.28°, and 48.10° in the spectra of the PSAM specimens, in addition to the
characteristic peaks of quartz crystals (PDF#72–0628).
3.1.2 FTIR
Figure 3 shows the FTIR spectra of wood specimens treated with different concentrations of NaOH
solutions. The gure shows that all four materials correspond to the anti-integrated stretching vibration of
H-O-H at 3440 cm− 1 and the bending vibration peak of O-H at 1590 cm− 1. [27]. As can be seen from the
gure, the characteristic peaks are weak mainly because the water is removed from PSAM after
calcination at 600°C. The PSAM specimens correspond to the antisymmetric stretching vibration of Si-O-
Si and the bending vibration of Si-O in quartz crystals at 1090 cm− 1 and 461 cm− 1, respectively. The
characteristic peaks of quartz crystals gradually weakened with increasing NaOH concentration, which is
consistent with the trend of the characteristic peaks of quartz crystals in the XRD patterns. The
specimens corresponded to the antisymmetric stretching vibration and symmetric stretching vibration of
CO32− at 1440 cm− 1 and 877 cm− 1 [28, 29]. Respectively, the peak of the stretching vibration of CO32−
gradually increased with increasing NaOH concentration, indicating that the Na2CO3 crystals gradually
increased in PSAM, which is consistent with the XRD results.
3.1.3 SEM
Figure 4 shows SEM images of the materials treated with different NaOH concentrations after
magnication at different multiples. It is observed in the gure that all four materials have a very loose
internal structure with many widely spaced voids, providing more active sites for dye adsorption. The pore
size of the PSAM decreased with increasing NaOH concentration. The NaOH solution had two leading
roles in preparing the materials: (1) dissolving part of the lignin in the pore channels of eucalyptus chips
and (2) reacting with the surface of quartz particles. The surface hydroxylated particles were condensed
and deposited in the pore channels of eucalyptus chips. From the microstructural analysis, the second
effect was dominant; the number of quartz particles deposited into the pore channels by dissolution–
condensation increased with increasing NaOH concentration, and the pore size formed after calcination
to remove the template were smaller. Analysis of XRD and FTIR data indicates the mechanism of action
and microstructure change pattern of the NaOH solution during the preparation of the materials. However,
all four materials generally had more pores and better adsorption performance for MB.
3.1.4 BET


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Table 1
Surface area and pore characteristics of PSAM
samples.
Samples SBET(m2/g) Vtotal(cm3/g) Dave(nm)
2M 88.05 0.082 3.707
4M 69.17 0.065 3.786
6M 54.07 0.041 3.017
8M 61.75 0.041 2.645
Figure 5(a) shows the four materials' N2 adsorption–desorption isotherms. According to the IUPAC
classication, the prepared materials' N2 adsorption–desorption isotherms are type II isotherms with an
H3 lagging loop [12]. As observed in the gure, the relative pressure increased gently with increasing
pressure between 0 and 0.8. When the relative pressure was more signicant than 0.8, the adsorption
increased rapidly with increasing pressure, mainly due to capillary condensation in the mesopores and
macropores. Different types of hysteresis loops represent different types of pore structures. The H3
hysteresis loop in the gure indicates the presence of mesopores and large slit pores in the prepared
material [30]. Figure 5(b) shows the pore size distribution of the four materials; the range of the pore
centers of the four materials was between 3–4 nm, and the pore size was approximately 3.7 nm. Table 1
shows the four materials' specic surface area and pore size; the specic surface area, cumulative pore
volume, and pore size showed an overall decreasing trend with increasing sodium hydroxide
concentration. Formation of the pore size of PSAM is based on two factors: (1) the surface hydroxylation
of quartz particles and the residual pores of the unlled eucalyptus pore channels formed by the
condensation reaction deposited in the pore channels of eucalyptus chips and the pores between the
piles of particles, and (2) the slit pores left by the removal of the eucalyptus chip template. When the
NaOH concentration is low, there are fewer hydroxyl groups on the surface of the quartz particles; the
quartz deposited in the pore channels is not lled, and there are pores between the pile of particles. The
eucalyptus pore channels dissolve less under the action of NaOH solution, and more slit holes remain
after calcination. When the concentration of NaOH increased, the surface of the quartz particles dissolved
and produced a large number of hydroxyl groups, and the amount deposited in the structure of
eucalyptus pore channels through the condensation reaction increased. The accumulation between
particles was denser, whereas the pore walls of the eucalyptus chips were thinner under the action of a
high NaOH concentration. Smaller slit holes were formed after calcination. Overall, the specic surface
areas and pore sizes of all four materials were not signicantly different, and they showed good
adsorption properties for MB. The variation pattern of the pore structure can be determined from
microstructure analysis.
3.1.5 Zeta potential

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Figure 6. As the pH of the solution increases from 2 to 10, the zeta potential of PSAM shows a decreasing
trend, and the value changes from positive to negative. The data in the gure shows that the zero point
charges (pHpzc) of PSAM are 2.12, 2.09, 2.11, 2.12. Since MB is a cationic dye, when the pH of the
solution is lower than the pHpzc value (pH < pHpzc), the surface of PSAM is positively charged, which is
not favorable for the adsorption of MB; on the contrary, when the pH of the solution is higher than the
pHpzc value (pH > pHpzc), the surface of PSAM is negatively charged, which will be favorable for MB
adsorption [31].
3.2. Adsorption of MB
Figure 7(a) shows the adsorption rates of different materials on MB. It can be observed from the gure
that the adsorption rates of the four prepared materials on MB increased sharply in the rst 10 min, and
the adsorption amount could reach 17.64 mg/g in the 10th min. With the increase of time, the adsorption
rates increased slowly and gradually stabilized to reach the equilibrium, and the adsorption amount at the
equilibrium was 20 mg/g. When the NaOH concentration was 2 M and 4 M, the pore size of PSAM was
more signicant, and there were more slit pores in When methylene blue adsorption was carried out, more
dyes entered the internal active site dyes through internal diffusion, in addition to those adsorbed on the
surface of PSAM. When the concentration of sodium hydroxide is 6M and 8M, the pore size of PSAM is
small, the slit pores are small, more active sites on the surface, and more dyes are adsorbed.
Meanwhile, due to the signicant molecular weight of dyes, the spatial site resistance is considerable,
and the amount of active sites entering inside and outside through internal diffusion is reduced. The total
active adsorption sites of PSAM prepared from different NaOH solutions were consistent; therefore, the
adsorption rates of methylene blue were the same. The trend analysis of PSAM adsorption rates was
consistent with the previous XRD, FTIR, and SEM analysis that PSAM is a porous material with abundant
active sites. The main reason for the different trends from the BET data is that the adsorption properties
of the dye are related to the active adsorption sites in the pore channels and the active adsorption sites
on the surface of the material. In this study, as the specic surface area and pore size decreased with the
increase of sodium hydroxide concentration, the amount of dye entering the internal pore channels
decreased due to the spatial location. However, the active adsorption sites on the material surface
increased, so the total adsorption rate was unchanged. Figure 7(b) shows the effect of different
temperatures on MB adsorption. With the increase in temperature, the adsorption rate of MB increased for
the four materials. The adsorption process is heat absorption, and the temperature increase accelerates
the binding rate of dye molecules and materials [32]. From the following thermodynamic analysis, this
speculation can be proved. Figure 7(c) shows the effect of different pH on the adsorption of MB, from
which it can be seen that the adsorption of MB was not satisfactory under highly acidic conditions (pH =
2). However, between pH 4–10, PSAM was suitable for the adsorption of MB on the four materials
because the pH was higher than pHpzc, which was favorable for adsorption, indicating that the material
can be applied to the treatment of weakly acidic, neutral and alkaline dye wastewater, which is consistent
with the previous zeta potential analysis.

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3.2.1. Adsorption kinetics

Table 2
Kinetics parameters for MB Adsorption on PSAM samples.
Samples pseudo-rst-order pseudo-second-order
qe(mg g− 1) k1(min− 1) R2qe(mg/g) k2(mg− 1 g− 1 L− 1) R2
2M 19.77 0.83 0.998 19.88 0.32 0.998
4M 20.06 0.67 0.995 20.26 0.15 0.997
6M 19.49 0.71 0.994 19.66 0.18 0.996
8M 18.93 0.43 0.986 19.34 0.06 0.995
Figure 8 shows a t to the adsorption kinetics of PSAM on MB, where (a) is a pseudo-rst-order kinetic
model and (b) is a pseudo-second-order kinetic model. Table 2 presents the parameters of the kinetic ts
for the four materials. The kinetic t curves show that the adsorption of MB by the materials increased
gradually with time, with a faster increase in the rst 10 min, followed by a attening until equilibrium.
This is because the material provided more adsorption sites at the beginning of the adsorption process.
However, with increasing time, the adsorption sites on the material were occupied by dye molecules. With
an increase in adsorption time, the dye concentration decreased; the adsorption eciency decreased, and
the adsorption reached saturation [11]. The tted data show that the R2 values of the pseudo-rst-order
and pseudo-second-order kinetic models are close to 1; thus, both models apply to the adsorption of MB
by PSAM [33, 34]. In a previous study, the same adsorption kinetic trends were observed for the synthesis
of nano-silica-coated magnetic carbonaceous adsorbents for the adsorption of MB in water using a low-
temperature hydrothermal carbonization technique (HCT), indicating that the adsorption of MB by PSAM
is controlled by multiple processes and not by a single process [13, 35].
3.2.2. Adsorption isotherms


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Table 3
Adsorption isotherm parameters for MB Adsorption on PSAM samples.
Samples Langmuir Freundlich
qm
(mg g− 1)
KL
(L mg− 1)
RL
(L mg− 1)
R2n KF
(L mg− 1)1/n(mg g− 1)
R2
2M 90.01 1.56 0.03 0.981 1.74 55.74 0.946
4M 80.97 1.02 0.05 0.990 1.92 38.57 0.988
6M 64.52 0.49 0.09 0.983 2.48 23.19 0.977
8M 61.16 1.01 0.05 0.978 2.85 29.40 0.917


Table 4
Comparison of the adsorption capacity of MB by different adsorbents
Adsorbent qm(mg/g) Reference
Acrylated composite hydrogel (ACH) 56.61 [36]
Activated lignin-chitosan extruded pellets (ALiCE) 36.25 [4]
Carboxymethyl chitosan-modied magnetic-cored dendrimers (CCMDs) 96.31 [37]
Mesoporous silicon carbon (MSC) 156.56 [38]
Magnetic starch-based composite hydrogel microspheres (SCHMs) 88.33 [39]
Hydrophobic (surface modied) silica aerogel (MSA) 65.74 [40]
Paintosorp 44.64 [41]
Sago-grafted silica 80:20 10.31 [42]
PSAM 90.01 This work
Figure 9 shows the results of the PSAM t to the MB adsorption isotherm, where Fig. (a) and Fig. (b) are
the Langmuir model t curves, and Fig. (c) is the Freundlich model t curve. Table 3 shows the
parameters of the adsorption isotherm t. From the data of the graphs, it can be seen that the R2 of the
Langmuir model is close to 1, so the Langmuir model is more applicable to the adsorption of PSAM on
MB [43]. The Langmuir model applies to the adsorption of the monomolecular layer, so the adsorption of
PSAM on MB is of monomolecular layer adsorption. From the data in Table 3, the maximum adsorption
amounts of the four materials are not signicantly different. However, in comparison, the adsorption
amount of 2M PSAM reaches 90.01 mg/g, the most considerable adsorption amount among the four

Page 12/27
materials, consistent with the previous BET data. KL denotes the Langmuir constant, which indicates the
anity of the adsorbate for the binding site. The separation factor RL can be calculated by Eq. (7) when 0
< RL < 1 favors the adsorption, as observed from the Langmuir t data, so all four materials prepared are
favorable for MB adsorption [44]. Moreover, the tted data from the Freundlich model also reveals that
the n values of all four materials are between 1 and 10, n is a parameter describing the adsorption
strength, and when 1 ≤ n ≤ 10, the adsorption is favored, so all the four prepared PSAMs show good
adsorption on MB [34, 45]. Meanwhile, the maximum adsorption of different adsorbents on MB dyes is
listed in Table 4, which shows that PSAM can effectively remove organic dyes from water.
3.2.3. Adsorption thermodynamics

Table 5
Various thermodynamics parameters for the Adsorption of MB onto 2M PSAM.
Samples ∆G0(kJ mol− 1)∆H0(kJ mol− 1)∆S0(kJ mol− 1 K− 1) R2
2M -1.128 21.364 76.765 0.946
Adsorption thermodynamics is an essential basis for studying whether adsorption occurs spontaneously.
Figure 10 shows the lnKL-1/T curve for 2M PSAM; the relevant thermodynamic parameters can be
calculated from the curves in the gure, as shown in Table 5. From the data in the table, ∆H > 0 indicates
that the adsorption of PSAM on MB is heat-absorbing. If the temperature of the reaction is increased, the
adsorption rate of PSAM on MB can be enhanced, which is consistent with the effect of temperature on
PSAM adsorption. Based on a value of ∆H between 0 kJ mol-1 and 84 kJ mol-1, it can be judged that the
adsorption is mainly a physical adsorption process [46]. A positive value of ∆S indicates that the
adsorption process is disorderly, increasing at the solid–liquid interface. The value of ∆G is less than 0,
indicating that the adsorption of MB by PSAM is spontaneous [47, 48].
3.3. Adsorption mechanism of MB on PSAM
The adsorption effect of adsorbents on adsorbates is not only determined by the physical structure and
chemical properties of the adsorbent material itself, such as the specic surface area and pore size of the
material but also by other inuences, such as the interaction between the adsorbent and the adsorbate
and the charge on the adsorbent surface [13]. When adsorption occurs, the adsorbent and adsorbent are
bound in different ways, usually through ion exchange, hydrogen bonding, electrostatic interactions,
dipole-dipole interactions, hydrophobic interactions, and surface metal cation coordination between
adsorbent and adsorbent. Figure 11 shows the FTIR spectra of 2M PSAM before and after the adsorption
of MB. It can be observed that the peaks at 1440 cm− 1 and 877 cm− 1 disappear before and after the
adsorption, and the peaks at these two locations are the characteristic peaks of CO32−, thus indicating
that MB+ can be adsorbed with CO32− by electrostatic gravitation. At the same time, the characteristic
peaks of SiO2 also changed slightly, indicating that the material's structure did not change before and

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after adsorption, but MB+ could also bind to SiO2 by electrostatic interaction [13, 49]. Therefore, the dye
molecules are adsorbed on the surface of PSAM by electrostatic interaction. Moreover, PSAM can also
bind to the N atom in MB by hydrogen bonding [3, 13]. The adsorption mechanism of MB is shown in
Scheme 1. Moreover, the added wood is structurally increasing the specic surface area of the material
and providing more dye adsorption sites for the material itself.
4. Conclusion
This study successfully prepared a low-cost and high-eciency porous silica adsorbent material via a
hydrothermal synthesis method using quartz powder as the raw material and eucalyptus wood chips as
the template with the template different concentrations of sodium hydroxide. The prepared material was
used for the adsorption of MB; the experimental results are presented as follows:
1. Sodium hydroxide reacted with the surface of quartz particles. Hydroxylated quartz particles were
deposited in the pore channels of eucalyptus wood chips through a condensation reaction, and pore
channels were formed between the stacked quartz particles. After removing the template, sodium
hydroxide dissolved part of the lignin in the pore channels and formed slit pores.
2. As the concentration of NaOH increased, the characteristic peaks of quartz crystals of PSAM
weakened; the number of active sites formed on the surface increased, and the internal pore size
decreased.
3. Adsorption of MB by the four PSAMs was divided into surface and intra-pore adsorption. The pore
sizes of the PSAM materials prepared with 2M and 4M sodium hydroxide were larger, indicating
mainly intra-pore adsorption. The pore sizes of the PSAM materials prepared with 6M and 8M
sodium hydroxide were smaller. Diffusion of MB into the pores was reduced owing to the spatial site
resistance, indicating mainly surface adsorption. An adsorption rate of 85% was achieved in 10 min
for the four PSAM materials, all of which showed good adsorption performance and rate.
4. The adsorption kinetics and isotherms were tted for the four materials. The tted data shows that
they all t the pseudo-rst-order kinetic, pseudo-second-order kinetic, and Langmuir models, which
indicates that a single process controls the adsorption of MB by PSAM and that the adsorption is a
single molecular layer adsorption. Thermodynamic data show that the adsorption of MB by PSAM is
a spontaneous heat-absorbing process. PSAM has potential applications due to its low preparation
cost, wide availability of raw materials, and high eciency in the adsorption of MB from wastewater.
Environmental Implications
A new porous silica adsorbent material was synthesized and tested for its adsorption performance on
methylene blue using eucalyptus wood as a template as a simulated pollutant for industrial wastewater.
The tests showed that the porous material has high adsorption eciency for methylene blue in water and
can reach more than 85% adsorption rate in ten minutes. Moreover, the adsorption process is
spontaneous. The material can effectively remove toxic organic dyes in water.

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Declarations
CRediT authorship contribution statement
Wenxin Zhu: Conceptualization, Methodology, Software, Validation, Investigation, Data curation, Writing -
original draft; Leping Liu: Conceptualization, Supervision, Writing - review & editing; YuanXia Lao:
Investigation, Writing - review & editing. Yan He: Conceptualization, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing nancial interests or personal relationships that
could have appeared to inuence the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (51962024).
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Scheme
Scheme 1 is available in the Supplementary Files section.
Figures

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Figure 1
XRD patterns of the raw material and standard card of SiO2 (PDF#89-1961).

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Figure 2
XRD patterns of PSAM prepared with different NaOH concentrations and standard card for SiO2
(PDF#89-1961).

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Figure 3
FTIR patterns of PSAM prepared with different NaOH concentration conditions.

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Figure 4
SEM images of PSAM prepared with different NaOH concentration conditions.

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Figure 5
N2 adsorption-desorption isotherms (a), pore-size distribution (b).


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Figure 6
Zeta potential of 2M PSAM (a) 4M PSAM (b) 6M PSAM (c) 8M PSAM (d).
Figure 7

Page 25/27
Effects of different samples (a) experiment temperature (b) and solution pH (c) on adsorption eciency
with MB (adsorbent dose, 50 mg; the volume of the medium, 50 mL; initial concentration, 20 mg/L;
contact time, 24 h and temperature, 20℃).
Figure 8
Adsorption kinetics of MB on PSAM samples:
pseudo-rst-order (a); pseudo-second-order (b) (adsorbent dose, 50 mg; volume of the medium, 50 mL;
initial concentration, 20 mg/L; pH, 6 and temperature, 20℃).
Figure 9
Adsorption isotherm of MB on PSAM samples: Langmuir (a, b); Freundlich (c) (adsorbent dose, 50 mg;
the volume of the medium, 50 mL; initial concentration, 20-60 mg/L; pH, 6; contact time, 24 h and
temperature, 20℃).

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Figure 10
Vant’s Hoffplot for the determination of different thermodynamics parameters for the Adsorption of MB
onto 2M PSAM (adsorbent dose, 50 mg; the volume of the medium, 50 mL; initial concentration, 20 mg/L;
pH, 6; contact time, 24 h and temperature, 20-40℃).

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Figure 11
FTIR patterns before and after adsorption of 2M PSAM on MB.
Supplementary Files
This is a list of supplementary les associated with this preprint. Click to download.
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