Available via license: CC BY 4.0
Content may be subject to copyright.
Page 1/20
Understanding the relationship between pore size,
surface charge density, and Cu 2+ adsorption in
mesoporous silica
Yanhui Niu
Guizhou Education University
Wenbin Yu
Chinese Academy of Sciences
Shuguang Yang
Chinese Academy of Sciences
Quan Wan
Chinese Academy of Sciences
Article
Keywords: Mesoporous Silica, Pore size, Surface Charge Density, Cu2+ adsorption
Posted Date: February 20th, 2024
DOI: https://doi.org/10.21203/rs.3.rs-3939762/v1
License: This work is licensed under a Creative Commons Attribution 4.0 International License.
Read Full License
Additional Declarations: No competing interests reported.
Page 2/20
Abstract
This research delved into the inuence of mesoporous silica's surface charge density on the adsorption of
Cu2+. The synthesis of mesoporous silica employed the hydrothermal method, with pore size controlled
by varying the length of trimethylammonium bromide (CnTAB, n = 12,14,16) chains. Gas adsorption
techniques and transmission electron microscopy characterized the mesoporous silica structure. Surface
charge densities of the mesoporous silica were determined through potentiometric titration, while surface
hydroxyl densities were assessed using the thermogravimetric method. Subsequently, batch adsorption
experiments were conducted to study the adsorption of Cu2+ in mesoporous silica, and the process was
comprehensively analyzed using Atomic absorption spectrometry (AAS), Fourier transform infrared
(FTIR), and L3 edge X-ray absorption near edge structure (XANES). The research ndings suggest a
positive correlation between the pore size of mesoporous silica, its surface charge density, and the
adsorption capacity for Cu2+. More specically, as the pore size increases within the 3-4.1 nm range, the
surface charge density and the adsorption capacity for Cu2+ also increase. Our ndings provide valuable
insights into the relationship between the physicochemical properties of mesoporous silica and the
adsorption behavior of Cu2+, offering potential applications in areas such as environmental remediation
and catalysis.
1. Introduction
Nanogeoscience is a new eld that has emerged from the intersection and integration of Nanotechnology
and Earth sciences. It combines research methods from both elds to study nanomaterials that are
present in various layers of the Earth. The aim of nanogeoscience is to uncover the relationship between
nanoscale effects and geologic phenomena occurring in the Earth's evolution1. One of the primary
research objects in nanogeoscience is mineral nanopores2. According to the International Union of Pure
and Applied Chemistry (IUPAC), nanopores are classied into three categories based on their size:
macropores (> 50 nm), mesopores (2–50 nm), and micropores (< 2 nm).
Mineral mesoporous materials are commonly found in geological environments and are closely linked to
important aspects of Earth science. For instance, minerals like diatomaceous earth and halloysite have
numerous mesopores in their structures3, 4, which provide them with strong selective adsorption
capabilities. Hochella and Baneld5 have noted that the basic properties of water, such as high viscosity
and low dielectric constant, have a signicant impact on the entire weathering process when conned to
nanoscale pores. Furthermore, the mesoporous structures in shale and tight sandstones are important
characteristics of oil and gas reservoirs6. Additionally, mineral mesopores play a crucial role in signicant
Earth science issues such as carbon cycling7, dissolution and precipitation of minerals8, and trace
element enrichment9. These issues highlight the adsorption behavior of mineral mesopores as the core
research topic of nanogeoscience.
Page 3/20
Heavy metal pollution is a serious environmental issue, and adsorption plays a crucial role in the
movement, transformation, bioavailability, bioaccumulation, and fate of heavy metals in geological
environments10. Mesoporous minerals have a large specic surface area, which makes them highly
effective at adsorbing heavy metals. Therefore, studying the adsorption mechanisms of heavy metals
onto mineral mesopores can help us better understand and predict the environmental behavior of these
metals in real geological settings. Additionally, this research can provide a scientic basis for controlling
and remediating heavy metal pollution, which is important for addressing many scientic issues, such as
the enrichment of trace/rare earth elements and metal mineralization associated with mesoporous
materials11, 12.
In recent years, researchers have been studying the mechanisms of heavy metal adsorption onto mineral
mesopores. One study investigated the impact of surface modication of mineral mesopores on the
adsorption performance of heavy metals13. Although the primary focus was on adsorption capacity,
microscale mechanisms were not considered. Another study14 examined the effect of water properties
under mesoporous connement on the adsorption performance of mineral mesopores. It was found that
the decrease in the dielectric constant of water within the mesopores reduced the binding capacity of
heavy metal ions with water. This reduction helped in the formation of stable inner-sphere complexes
between metal ions and the mineral surface. Notably, these inner-sphere complexes hindered the
migration and bioavailability of heavy metal ions within the Earth's surface. Comparative studies15 have
shown that mineral mesoporous oxides have better capacity for heavy metal adsorption than non-porous
ones.
Although previous studies have made signicant progress, there has been little focus on the surface
charge density during heavy metal ion adsorption onto mineral mesopores. The surface charge density
plays a vital role in the adsorption properties of substances and their interactions with ions and
molecules16–18. Improving the quantitative control over the surface chemistry of mineral mesopores,
including surface functional group density, surface charge density, and acidity/alkalinity ratio, is still
necessary. This has led to debates on the nano-connement effects on ion or molecule adsorption.
The intricate nature of natural compositions and structures pose a challenge in explaining the adsorption
mechanisms of heavy metal ions onto mineral nanopores. For this reason, synthetic porous materials,
like mesoporous silica, are often used as model systems to simulate the adsorption of ions or molecules
onto mineral nanopores19.
The aim of this research is to understand how heavy metal ions are adsorbed onto mineral surfaces. To
do this, the study investigates the relationship between pore size, surface charge density, and Cu2+
adsorption in mesoporous silica. The research employs gas adsorption, acid-base titration,
thermogravimetry, and IR spectroscopy to analyze the mesoporous silica structure. The focus of the
study is on adsorption experiments and spectroscopy measurements, which help to establish a
correlation between surface charge density and the adsorption capacity of Cu2+. The data obtained from
Page 4/20
these experiments provide valuable insights into the reactivity of mineral interfaces under mesoporous
connement.
2. Results and Discussion
2.1 Structural characterization analysis of mesoporous
silica.
The N2 adsorption method is an effective way to examine mesoporous materials. In Fig. 1a, we can see
the nitrogen adsorption/desorption isotherm of MPS at 77 K. According to the International Union of Pure
and Applied Chemistry (IUPAC) classication, the plot matches a type IV adsorption isotherm, which is
typical of mesoporous structures20. The plot shows that capillary condensation causes a sharp increase
in adsorption. At low pressures, the adsorption grows linearly, indicating the monolayer adsorption of N2
on the pore walls. The steeper change in adsorption during the capillary condensation region suggests a
more uniform pore size distribution. After capillary condensation, a long and stable plateau is reached at
higher pressures, indicating the multilayer adsorption on the pore surface. Furthermore, the specimen with
a pore size of 4.1 nm demonstrated higher adsorption than those with pore sizes of 3.2 nm and 3.7 nm.
This means that the pore size inuences the adsorption of N2 according to physical adsorption. However,
the variation in chemical adsorption was not consistent with pore size changes because chemical
adsorption generally occurs at chemically active sites21. The large specic surface area and uniform pore
size distribution (as shown in Fig. 1b) of the two-dimensional hexagonal mesoporous structure also
agreed with the TEM results (as shown in Fig. 2A).Therefore, these MPS samples with different pore sizes
were further selected for Cu2+ adsorption experiments.
2.2 Analysis of the relationship between pore size and
surface charge density
Amphoteric oxides carry positive charges at low pH levels and negative charges at high pH levels22. This
allows us to determine the solid amphoteric oxide's point of zero charge (pHPZC). In this study, we used
the titration method proposed by Kosmulski23 to obtain the titration curves of the blank solution and the
sample suspension. By calculating the difference between the acid and base titration curves at the same
pH, we determined the surface charge density σ0 using the formula: σ0 = F c∆V/(mA) (where, σ0 is in
C/m², F is the Faraday's constant (96487 C/mol), c is the molar concentration of the added base (mol/L),
m is the solid quality, and A is the solid specic surface area in m²/g). The intersection of these two
titration curves is considered the apparent PZC. The titration and surface charge curves are shown in Fig.
2B and Fig. 2C, respectively. The apparent pH PZC was found to be between 2.6-3. The surface charge
densities calculated for MPS-1, MPS-2, and MPS-3 are listed in Table 1, showing an increasing trend with
the increase in pore size.
Page 5/20
In Fig. 2D, we can see the TGA curve that was obtained after mixing MPS with deionized water for 24
hours. Based on various reports23, the dehydration process of mesoporous silica can be described as
follows: the weight loss before 200°C is because of the desorption of physically adsorbed water. On the
other hand, the weight loss between 200°C and 1200°C is caused by the dehydroxylation of silicon
hydroxyl groups. Therefore, we can calculate the density of silicon hydroxyl groups using a formula:
. The silicon hydroxyl densities of MPS-1, MPS-2,
and MPS-3 are presented in Table1. The TGA curve shows a distinct weight loss starting at 200°C,
indicating an increase in the silicon hydroxyl density with increasing pore size. Although this method
does not involve measurements in an in-situ adsorption system at pH 5, it indirectly explains the variation
in surface hydroxyl contents of MPS with different pore sizes. This discrepancy can be attributed to the
fact that a portion of the solution remains trapped within the mesopores and cannot be eliminated from
the system.
The surface charge density of silica is usually controlled by protonation and deprotonation reactions,
which belong to the category of solid surface ionization. These reactions are inuenced by counterions
and are referred to as "charge screening," which implies that co-ions accumulate and counter ions are
repelled24. Normally, the interfacial region is electrically neutral. However, it maintains its electrical
neutrality under the inuence of counter ions from the surface layer (the inert electrolyte) when protons
are adsorbed or dissociated. The surface charge of mesoporous silica is directly related to the surface
acidity/basicity ratio, namely the pH level and ionic strength, and can be evaluated using acid-base
titration methods.
When a solid surface is charged in a solvent, according to electroneutrality, an equal amount of the
opposite charge should exist on the other side of the surface, namely the solvent side. This forms what is
known as a double layer25. In the case of silica, the ionization in water can provide information on its
surface charge properties in the solution. At pH 5, the pore walls of mesoporous silica (MPS) carry
negative charges through the dissociation of silanol groups. This generated potential attracts positively
charged counterions from the aqueous solution, making the entire system electrically neutral and forming
the double layer. The Debye layer, which is the negative charge layer, combines with the pore wall surface.
Its primary function is to generate negative charges on the pore walls, thus increasing their potential.
When the thickness of the Debye layer is larger than the mesopore radius, the potential to the pore wall
and the double layer overlap, causing the central potential of the mesopores to no longer be zero. Silica
microchannels with hydroxyl groups have an isoelectric point of 2–3, and in solutions with a pH higher
than 3, the silica surface carries negative charges. When the diameter of MPS is at the nanoscale (2–5
nm), the surface double layer may overlap. In summary, the charge density on the solid surface of MPS is
related to the pore size, surface functional groups, pH of the solution, and ionic strength. With
quantitative data on mesoporous silica's surface charge and hydroxyl density26, one can explain the
mechanism of heavy metal adsorption on mesoporous materials at a certain pH level.
δ
OH(Si − OH/nm2) = × 2 × 6.02 × 103
Δ
mH2O
18SBET
Page 6/20
2.3 Analysis of the relationship between pore size and
copper ion adsorption.
2.3.1The thermodynamics and kinetics of Cu2+ adsorption
in MPS.
A series of experiments were conducted to investigate the adsorption of Cu2+ using three different pore-
sized MPS (3.2nm, 3.7nm, and 4.1nm) under pH = 5 conditions. The experiments were carried out by
considering theoretical aspects concerning surface charge density and the various forms of Cu2+. Figures
3a and 3b show the Langmuir and Freundlich nonlinear tting isotherms, respectively, which demonstrate
the correlation between Qe and Ce. According to the ndings, the adsorption of Cu2+ by MPS increases
with the enlargement of pore size. Table 2 provides the model parameters for the Langmuir and
Freundlich isotherms. The Kf value of the Freundlich isotherm indicates a rise in relative adsorption
capacity with an increase in pore size. Similarly, the n value follows a similar trend with the enlargement
of pore size. The n value ranges from 1 to 10, indicating that adsorption is relatively easy. The
experimental ndings suggest that the adsorption of Cu2+ becomes more favorable as the pore size
increases. The Langmuir equation is based on three assumptions: monolayer adsorption, no interaction
between adsorbate molecules, and uniform adsorption energy on the adsorbent surface. Meanwhile, the
Freundlich equation is used to model multi-layer adsorption on heterogeneous surfaces27. Based on the
calculation results of R2, the primary mechanism driving the enhanced adsorption of Cu2+ with an
increase in pore size may be attributed to the formation of a monolayer through chemical adsorption
between Cu2+ and the -OH groups of MPS or physical adsorption resulting from surface electrostatic
forces.
The results of the thermodynamic calculation are presented in Table 3. The table demonstrates that for
all three pore size samples, the ΔGθ values are negative at 278 K and 298 K, which indicates that the
adsorption process is spontaneous. As the temperature increases, the ΔGθ values decrease, suggesting
an inverse relationship between spontaneity and temperature. The enthalpy change (ΔHθ) values for all
three pore sizes are positive, indicating an endothermic adsorption process. However, it is essential to
note that there is no established standard in literature regarding the relationship between ΔHθ values and
adsorption28. The ΔHθ is related to the adsorption process in the aqueous solution system. The formation
of copper ion-water and water-water bonds in hydrated copper ions, the formation of copper ion
complexes, and the creation of new bonds on the adsorbent surface may all contribute to a positive ΔHθ.
The ΔSθ values for all three pore sizes are positive, indicating an increase in disorder at the solid-liquid
interface during the adsorption process of metal ions onto MPS. At higher temperatures, the adsorption
capacity of MPS for Cu2+ is increased due to the activation of the adsorbent surface29 and the
enlargement of surface pore size. This is in alignment with earlier experimental ndings which indicate
that the adsorption capacity of MPS for Cu2+ is improved with larger pore sizes.
Page 7/20
The study investigated the kinetics of Cu2+ adsorption in mesoporous silica and modeled it using the
pseudo-rst-order and pseudo-second-order equations. The mechanism of adsorption depends on the
properties of the adsorbate and adsorbent and their relationship with the adsorption capacity and contact
time. To assess the suitability of the models, the logarithm of (Qe-Qt) against time (t) and (t/Qt) against
time (t) were plotted, as shown in Figs. 4a and 4b, respectively. The coecient of determination (R2) was
determined to evaluate the applicability of each model. The results indicate that the pseudo-second-order
model (R2 > 0.9937) provides a better t to the experimental data than the pseudo-rst-order model
(Table4). This suggests that the adsorption mechanism of Cu2+ on MPS may involve a chemical
adsorption process30. It is also evident from Table 4 that the rate constants decrease as the pore size
increases.
2.3.2 The spectroscopic analysis of Cu2+ adsorption on
MPS.
During the initial investigation31, comprehensive experiments were conducted on MPS to explore the
interaction between Cu2+ and MPS. ATR-IR analysis was performed on MPS samples before and after the
adsorption of Cu2+. The IR spectra of MPS exhibit characteristic peaks at 1082 cm− 1, 970 cm− 1, and 800
cm− 1, as shown in Fig. 5a. The peaks at 1082 cm− 1 and 800 cm− 1 are associated with the asymmetric
stretching vibrations of Si-O-Si, while the band at 970 cm− 1 is attributed to the stretching vibration of Si-
OH groups. The features at 969 cm− 1, 967 cm− 1, 965 cm− 1, and 963 cm− 1 indicate the stretching
vibrations of Si-O- groups. The redshift observed in the stretching modes of Si-O- groups indicates the
potential replacement of Si-OH groups by Si-O-Cu groups32.
Subsequently, infrared vibrational spectroscopy experiments, utilizing the principle of spectral
difference33,34, were conducted to observe spectral changes in the materials under investigation. The
relative content of Cu2+ in the samples was determined by evaluating the integrated intensities of the Si-
O-related peaks at 970 cm− 1 following spectral normalization. Specically, the I970/I800 ratio was
employed to assess the Cu-MPS quantity qualitatively. As depicted in Fig. 5a, the overall trend of the
I970/I800 ratio correlated with the adsorption capacity. According to Figs. 5c and 5d, the Cu2+ adsorption
capacity was associated with the mesopore size, increasing the relevant IR peak intensities of Si-O-Cu
bonds.
Figure 5b shows the Cu L3 XANES spectrum of Cu-MPS. The peak in the spectrum corresponds to the
transition from the 2p3/2 level to the highest unoccupied 3d state, with the energy position representing
the difference between the 2p3/2 level and the highest unoccupied 3d state. The maximum absorption
peak of the XANES spectrum is observed at approximately 930.1 eV, while the shoulder peak is around
930.8 eV. These peaks are attributed to CuO4 tetrahedra and CuO6 octahedra35,36, respectively. According
to previous research, the XANES ndings suggest that the adsorption process of Cu2+ may involve the
formation of Si-O-Cu bonds. X-ray absorption ne structure spectroscopy (XAFS) has explored the
Page 8/20
adsorption complexes of Cu2+ on high surface area amorphous silica (Am-SiO2), revealing that Cu2+
forms inner-sphere coordination with Am-SiO237. This indicates the formation of Si-O-Cu bonds during the
adsorption process. Furthermore, Nelson et al. have shown the inner-sphere coordination of zinc ions on
Am-SiO238. Moreover, an increase in pore size leads to heightened peak intensity at 930.1eV and 930.8eV,
accompanied by slight positional shifts. This phenomenon may be attributed to the increased adsorption
capacity of MPS for Cu2+ with an expansion in pore size.
2.4 The analysis of the adsorption mechanism of Cu2+ in
MPS.
The study provides valuable insights into the ability of MPS to absorb Cu2+ by analyzing the
thermodynamics, kinetics, and equilibrium behavior involved in the process. The results suggest that the
adsorption mechanism may involve electrostatic interactions, coordination bonds, and the effects of
nano-connement, as seen in Fig. 6 and Fig. 7.
The study also examines the relationship between surface charge density, mesoporous diameter, and the
adsorption of metal ions. It nds that there is a direct correlation between an increase in surface charge
density (Fig. 5e) and mesoporous diameter (Fig. 5f) with an increase in Cu2+ adsorption. Additionally, the
adsorption capacity rises with pore volume (Fig. 5d), indicating that the lling of Cu2+ in mesopores is
crucial when the Cu2+ is not in a monolayer. The amount of copper ions adsorbed is closely related to the
mesoporous surface charge density, which is determined by the mesoporous diameter and specic
surface area.
When MPS is mixed with water, its silicon hydroxyl groups break apart and create a negatively charged
silica surface, which generates a surface potential. In the experiment, small mesopores were used, which
caused the potentials/double layers to overlap effectively within the mesopores, as shown in Fig. 6. This
overlapping affected the ionization of silicon hydroxyl groups within the pores. Generally, the pKa
decreases as the pore size increases, leading to increased dissociation and higher adsorption capacity for
Cu2+. This phenomenon is known as the potential overlapping mechanism.
During adsorption, removing water molecules from hydrated Cu2+ is an endothermic process (as shown
in Fig. 7). At the same time, forming new bonds between Cu2+ and the adsorbent's surface is an
exothermic process. The previous thermodynamic analysis (as depicted in Fig. 4) suggests that the
overall adsorption of Cu2+ on MPS is a spontaneous process characterized by an increase in entropy (ΔS
> 0) and endothermicity (ΔH > 0). As the pore size increases, the degree of potential overlap in the double
layer decreases, which reduces the interaction between the pore wall and Cu2+. Additionally, the ability of
the pore wall to bind with water molecules and hydrated ions weakens, causing an increase in the
endothermic ΔH and the disorder of entropy change ΔS in the system. Therefore, the thermodynamic
adsorption mechanism suggests that adsorption is a spontaneous process.
Page 9/20
3. Conclusions
This research delves into the thermodynamic and kinetic processes of Cu2 + adsorption on MPS with
varying pore sizes. It investigates the micro-mechanism of the adsorption process by integrating surface
charge density and spectroscopic analysis. The experimental data were tted and analyzed using the
Freundlich and Langmuir isotherm models, with the ndings indicating that the adsorption isotherm
aligns more closely with the Langmuir model. The uctuations in thermodynamic parameters (ΔG, ΔH,
and ΔS) were assessed, revealing that the adsorption is an endothermic and entropy-increasing
spontaneous process, with ΔH and ΔS escalating with increasing mesopore size. The adsorption kinetics
data suggest that the adsorption of Cu2+ on MPS adheres to a pseudo-second-order kinetic model, with
the adsorption constant diminishing as the pore size increases. The results imply that the adsorption
primarily constitutes a chemical adsorption process.
Spectroscopic evidence from FTIR and XANES supports the forming of inner-sphere coordination
complexes between Cu2+ and MPS, providing insights from various perspectives. Integrating the observed
adsorption patterns with the micro-mechanism analysis in this study conrmed that Cu2+ could adhere to
MPS in both inner-sphere and outer-sphere complex forms. With the potential overlap in the pore wall, the
rise in surface charge density as pore size expands may signicantly contribute to the heightened
adsorption capacity of copper ions within the pore size range of 3.1 to 4.1.
In brief, the surface charge density of MPS is correlated with the mesoporous diameter and specic
surface area of the material. At the same time, the adsorption capacity for Cu2+ is inuenced by the pore
size and surface charge density. The infrared spectroscopy results demonstrated that an increase in the
surface charge density of MPS resulted in a higher adsorption capacity for Cu2+ within the 3.1 to 4.1 nm
range. Understanding the relationship between the surface charge density of MPS and its adsorption
capacity for Cu2+ is advantageous for designing and optimizing MPS materials for environmental
remediation and other applications. It is important to note that the Cu2+ adsorption capacity of
mesoporous silica is determined by multiple factors, with surface charge density being just one of them.
Therefore, further research is still required to elucidate the impact of other parameters.
4. Experiment and methods
4.1 The Preparation of Mesoprous silica.
Mesoporous silica (MPS) was synthesized using a method proposed by Grün et al39. The surface-active
agents used were long-chain alkyl trimethylammonium bromides (CnH2n+1N(CH3)3+Br−, CnTAB) with
varying chain lengths of n = 12, 14, and 16. Mesoporous silica specimens with various pore sizes and
improved structural order were obtained by modifying the Grün method and then heating for 18 days at
105℃. The three samples, namely MPS-1, MPS-2, and MPS-3, underwent further calcination at a
temperature of 550℃. All reagents with a purity of 99% were procured from Shanghai Aladdin Bio-
Page 10/20
chemical Technology Co., Ltd., China. Ultrapure water with a resistivity of 18.25MΩ·cm was obtained
using a Millipore water purication system ( Molsheim, Alsace, France).
4.2 The Characterization Methods.
Detailed information was obtained on the pore structure, surface properties, and Cu2+ interactions of MPS
using the methods below.
Automated Gas Adsorption: The Autosorb-iQ2-MP equipment (Quantachrome, USA) was used to conduct
N2 adsorption experiments on the samples. During the experiments, N2 with a purity of 99.999% and a
cross-sectional area of 0.162 nm2 was used, and the pressure range (p/p0) was 10− 6 to 0.99. The MPS
samples were degassed at 200°C for 20 hours before testing. The BET model was used to calculate the
specic surface area and the NLDFT model was applied to determine the pore size and volume of the
samples.
Transmission Electron Microscopy (TEM): The pore structure of the samples was examined by means of
a FEI Tecnai G2F20 S-TWIN TMP TEM instrument (FEI, USA) at an accelerating voltage of 200 kV.
Thermogravimetric Analysis (TG): The TG analysis was performed on the samples under an Ar
atmosphere within a temperature range of 30-1200°C. The sample weight ranged from 5 to 10 mg, and
the heating rate was set between 5°C and 10°C per minute.
Attenuated Total Reectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR): The ATR-FTIR
measurements were done to investigate the interaction between mesoporous silica and Cu2+ using a
Bruker Vertex 70 setup. A total of 16 scans were performed in a range from 4000 to 400 cm− 1 at a
resolution of 4 cm− 1.
X-ray Absorption Near Edge Structure (XANES): The Cu L-edge XANES spectroscopy analyses were
conducted at the 4B7B beam-line using synchrotron radiation from the Beijing Synchrotron Radiation
Facility at the Institute of High Energy Physics of China. The Cu L-edge XANES spectra data were
obtained in the total electron yield (TEY) mode, with an energy step of 0.2 eV covering the range from 915
to 960 eV. In order to enhance the signal peak, the test sample was adsorbed using an 80 ppm Cu2+
solution at pH 5, resulting in the mesoporous silica sample obtained.
4.3 Evaluation of Surface Charge Density for MPS.
The surface charge densities (σ0) and zero point charges (PZC) of MPS were determined using an
automated potentiometric titrator (Metrohm 905 Titrando, Switzerland).
Preparation of Suspension: Prior to the testing, 0.05 g of MPS was dissolved in 50 mL of deionized water.
The mixture was then stored in a closed container for 24 hours to reach equilibrium. To remove carbon
dioxide, argon gas was bubbled through the suspension for 5 minutes. The ltered suspension was used
as a blank solution, while another portion of the suspension was used as the sample solution.
Page 11/20
pH Adjustment: To avoid dissolution of silica, the pH range was set between 1.5 and 8.0. The suspension
was initially adjusted to a pH level of 1.5 using 5% nitric acid. Subsequently, a 0.5 mol/L sodium
hydroxide solution was dropwise added until the pH value of 8.0 was reached.
Potentiometric Titrator Settings: The titrator was congured with respect to the following parameters: dV
(minimum) = 0.0005 mL, dV (maximum) = 0.2 mL, signal drift = 0.1 mV/min, dt = 150 s, minimum time =
150 s, and maximum time = 10000 s.
4.4 Cu2+Adsorption Experiments on MPS.
The Cu2+ adsorption experiments were conducted using MPS systems with different pore sizes. The
adsorbent was accurately weighed and placed in 25 mL conical asks. Then, 10 mL of the Cu2+ solution
with various concentrations was added. The pH of the solutions was adjusted to 5 using 1% HNO3. The
asks were kept in a temperature-controlled shaker at 25°C and agitated for 24 hours to reach adsorption
equilibrium. The resulting ltrates were obtained by ltering through a 0.45µm PVDF syringe lter. The
concentration of Cu2+ in each solution was measured using an atomic absorption spectrophotometer
(AAS, 990SUPER, China). Three parallel samples were prepared, and their average values were taken. The
adsorption capacity for heavy metal ions was calculated using the equation : Qe = (c0 - ce) × V / m, where
Qe is the equilibrium adsorption capacity (mg/g), c0 is the initial mass concentration of Cu2+ (mg/L), ce is
the equilibrium mass concentration of Cu2+ (mg/L), V is the volume of the solution (mL), and m is the
mass of the adsorbent used (g).
Declarations
Author contributions statement
Quan Wan and Yanhui Niu conceived the experiment(s), Yanhui Niu and Shuguang Yang conducted
the experiment(s), Yanhui Niu and Wenbin Yu analysed the results. All authors reviewed the manuscript.
Author Contribution
Quan Wan and Yanhui Niu conceived the experiment(s), Yanhui Niu and Shuguang Yang conducted the
experiment. Yanhui Niu and Wenbin Yu analyzed the results. Yanhui Niu wrote the main manuscript text.
All authors reviewed the manuscript.
Acknowledgments
This work was nancially supported by the Doctoral Program Foundation of Guizhou Education
University (2022BS010), the Youth Science and Technology Talent Development Project of Department of
Page 12/20
Education of Guizhou Province (QianJiaoHe KY [2022]299) and the Guizhou Provincial University Key
Laboratory of Advanced Functional Electronic Materials (QianJiaoji[2023]021).
References
1. Hochella Jr, M. F. J. E. Nanogeoscience: From origins to cutting-edge applications. Elements 4(6),
373–379, doi:10.2113/gselements.4.6.373(2008).
2. Wang, Y. F. Nanogeochemistry: Nanostructures, emergent properties and their control on geochemical
reactions and mass transfers. Chemical Geology 378, 1–23, doi:10.1016/j. chemgeo.2014.04.007
(2014).
3. Yuan, P. et al. Surface silylation of mesoporous/macroporous diatomite (diatomaceous earth) and
its function in Cu(II) adsorption: The effects of heating pretreatment. Microporous and Mesoporous
Materials 170, 9–19, doi:10.1016/j.micromeso.2012.11.030 (2013).
4. Jin, J. Q. et al. Characterization of Natural Consolidated Halloysite Nanotube Structures.
Minerals
11,
16, doi:10.3390/min11121308 (2021).
5. Hochella, M. & Baneld, J. Chemical weathering of silicates in nature: A microscopic perspective with
theoretical considerations.
In Chemical weathering rates of silicate minerals
353–406 (De Gruyter,
2018).
. You, F. L. et al. Impacts of Pore Structure on the Occurrence of Free Oil in Lacustrine Shale Pore
Networks. Energies 16, doi:10.3390/en16207205 (2023).
7. Zimmerman, A. R., Chorover, J., Goyne, K. W. & Brantley, S. L. Protection of mesopore-adsorbed
organic matter from enzymatic degradation. Environ. Sci. Technol. 38, 4542–4548,
doi:10.1021/es035340+ (2004).
. Myndrul, V. et al. Gold coated porous silicon nanocomposite as a substrate for photoluminescence-
based immunosensor suitable for the determination of Aatoxin B1.
Talanta
175, 297–304,
doi:10.1016/j.talanta.2017.07.054 (2017).
9. Wang, Y. F., Bryan, C., Xu, H. F. & Gao, H. Z. Nanogeochemistry: Geochemical reactions and mass
transfers in nanopores.
Geology
31, 387–390, doi:10.1130/0091-7613(2003)031<03 87 :Ngramt >
2.0.Co;2 (2003).
10. Lair, G. J., Gerzabek, M. H. & Haberhauer, G. Sorption of heavy metals on organic and inorganic soil
constituents. Environmental Chemistry Letters 5, 23–27, doi:10.1007/s10311-006-0 059 – 9 (2007).
11. Piasecki, W. & Sverjensky, D. A. Speciation of adsorbed yttrium and rare earth elements on oxide
surfaces. Geochimica Et Cosmochimica Acta 72, 3964–3979, doi:https://doi.org/10.1016/j.gca.
2008.05.049 (2008).
12. Ponthieu, M., Juillot, F., Hiemstra, T., van Riemsdijk, W. H. & Benedetti, M. F. Metal ion binding to iron
oxides. Geochimica Et Cosmochimica Acta 70, 2679–2698, doi:10.1016/j.gca. 2006.02.021 (2006).
13. Vinu, A., Hossain, K. Z. & Ariga, K. Recent advances in functionalization of mesoporous silica. J.
Nanosci. Nanotechnol. 5, 347–371, doi:10.1166/jnn.2005.089 (2005).
Page 13/20
14. Van Loon, L. R. & Glaus, M. A. Mechanical compaction of smectite clays increases ion exchange
selectivity for cesium. Environ. Sci. Technol. 42, 1600–1604, doi:10.1021/es702487m (2008).
15. Wang, Y. F. et al. Interface chemistry of nanostructured materials: Ion adsorption on mesoporous
alumina.
Journal of Colloid and Interface Science
254, 23–30, doi:10.1006/jcis. 2002.8571 (2002).
1. Greathouse, J. A. et al. Effects of nanoconnement and surface charge on iron adsorption on
mesoporous silica. Environmental Science-Nano 8, 1992–2005, doi:10.1039/d1en00066g (2021).
17. Murota, K. & Saito, T. Pore size effects on surface charges and interfacial electrostatics of
mesoporous silicas. Physical Chemistry Chemical Physics 24, 18073–18082, doi:10.1039/d2cp
02520e (2022).
1. Murota, K., Aoyagi, N., Mei, H. Y. & Saito, T. Hydration states of europium(III) adsorbed on silicas with
nano-sized pores. Applied Geochemistry 152, doi:10.1016/j.apgeochem.2023.105620 (2023).
19. Vunain, E., Mishra, A. K. & Mamba, B. B. Dendrimers, mesoporous silicas and chitosan-based
nanosorbents for the removal of heavy-metal ions: A review. International Journal of Biological
Macromolecules 86, 570–586, doi:10.1016/j.ijbiomac.2016.02.005 (2016).
20. Thommes, M. et al. Physisorption of gases, with special reference to the evaluation of surface area
and pore size distribution (IUPAC Technical Report).
Pure and Applied Chemistry
87, 1051–1069,
doi:10.1515/pac-2014-1117 (2015).
21. Choi, S. W. & Bae, H. K. Adsorption of CO2 on Amine-Impregnated Mesoporous MCM-41 Silica. Ksce
Journal of Civil Engineering 18, 1977–1983, doi:10.1007/s12205-014-0229-4 (2014).
22. Schulthess, C. P. & Sparks, D. L. BACK-TITRATION TECHNIQUE FOR PROTON ISOTHERM MODELING
OF OXIDE SURFACES. Soil Science Society of America Journal 50, 1406–1411,
doi:10.2136/sssaj1986.03615995005000060007x (1986).
23. Kosmulski, M. The pH dependent surface charging and points of zero charge. IX. Update. Advances
in Colloid and Interface Science 296, doi:10.1016/j.cis.2021.102519 (2021).
24. Zhuravlev, L. T. & Potapov, V. V. Density of silanol groups on the surface of silica precipitated from a
hydrothermal solution. Russian Journal of Physical Chemistry 80, 1119–1128, doi:10.1134/s
0036024406070211 (2006).
25. Dove, P. M. & Craven, C. M. Surface charge density on silica in alkali and alkaline earth chloride
electrolyte solutions.
Geochimica Et Cosmochimica Acta
69, 4963–4970, doi:10.1016/j.gca. 2005.
05.006 (2005).
2. Yang, J. et al. Understanding surface charge regulation in silica nanopores. Physical Chemistry
Chemical Physics 22, 15373–15380, doi:10.1039/d0cp02152k (2020).
27. Wu, C. H. Studies of the equilibrium and thermodynamics of the adsorption of Cu2+ onto as-produced
and modied carbon nanotubes. Journal of Colloid and Interface Science 311, 338–346,
doi:10.1016/j.jcis.2007.02.077 (2007).
2. Elo, O. et al. Batch sorption and spectroscopic speciation studies of neptunium uptake by
montmorillonite and corundum. Geochimica Et Cosmochimica Acta 198, 168–181,
Page 14/20
doi:10.1016/j.gca. 2016.10.040 (2017).
29. Schmeide, K. et al. Interaction of U(VI) with Aspodiorite: A batch and in situ ATR FT-IR sorption study.
Applied Geochemistry 49, 116–125, doi:10.1016/j.apgeochem.2014.05.003 (2014).
30. Saeed, M. M. & Ahmed, M. Effect of temperature on kinetics and adsorption prole of endothermic
chemisorption process: -Tm(III)-PAN loaded PUF system. Separation Science and Technology 41,
705–722, doi:10.1080/01496390500527993 (2006).
31. Niu, Y. H. et al. Adsorption characteristics of copper ion on nanoporous silica. Acta Geochimica 38,
517–529, doi:10.1007/s11631-019-00358-6 (2019).
32. Ma, X. B. et al. Hydrogenation of dimethyl oxalate to ethylene glycol over mesoporous Cu-MCM-41
catalysts. Aiche Journal 59, 2530–2539, doi:10.1002/aic.13998 (2013).
33. Schmeide, K. et al. Interaction of U(VI) with Äspö diorite: A batch and in situ ATR FT-IR sorption study.
Applied Geochemistry 49, 116–125, doi:10.1016/j.apgeochem.2014.05.003 (2014).
34. Müller, K. et al. Sorption of U(VI) at the TiO2-water interface: An in situ vibrational spectro- scopic
study.
Geochimica Et Cosmochimica Acta
76, 191–205, doi:10.1016/j.gca. 2011. 10. 004 (2012).
35. Shimizu, K., Maeshima, H., Yoshida, H., Satsuma, A. & Hattori, T. Ligand eld effect on the chemical
shift in XANES spectra of Cu(II) compounds. Physical Chemistry Chemical Physics 3, 862–866,
doi:10.1039/b007276l (2001).
3. Persson, I. et al. EXAFS Study on the Coordination Chemistry of the Solvated Copper(II) Ion in a
Series of Oxygen Donor Solvents. Inorganic Chemistry 59, 9538–9550, doi:10.1021/acs.
inorgchem.0c00403 (2020).
37. Cheah, S. F., Brown, G. E. & Parks, G. A. XAFS spectroscopy study of Cu(II) sorption on amorphous
SiO2 and γ-Al2O3: Effect of substrate and time on sorption complexes. Journal of Colloid and
Interface Science 208, 110–128, doi:10.1006/jcis.1998.5678 (1998).
3. Nelson, J., Wasylenki, L., Bargar, J. R., Brown, G. E. & Maher, K. Effects of surface structural disorder
and surface coverage on isotopic fractionation during Zn(II) adsorption onto quartz and amorphous
silica surfaces. Geochimica Et Cosmochimica Acta 215, 354–376, doi:10.1016/j.gca.2017.08.003
(2017).
39. Grün, M., Unger, K. K., Matsumoto, A. & Tsutsumi, K. Novel pathways for the preparation of
mesoporous MCM-41 materials:: control of porosity and morphology. Microporous and Mesoporous
Materials 27, 207–216, doi:10.1016/s1387-1811(98)00255-8 (1999).
Tables
Tables 1-4 is available in the Supplementary Files section.
Figures
Page 15/20
Figure 1
See image above for gure legend.
Page 16/20
Figure 2
See image above for gure legend.
Page 17/20
Figure 3
See image above for gure legend.
Figure 4
Page 18/20
See image above for gure legend.
Figure 5
See image above for gure legend.
Page 19/20
Figure 6
See image above for gure legend.
Figure 7
