A novel adsorbent (ZA/SiO2) was prepared by blending urea mixture of ZnSO4 and Al2(SO4)3 while using SiO2 as a support form. The adsorption properties of ZA/SiO2 for the removal of toxic metal ions (Cu(II) and Cr(VI))from water were evaluated. By batch experiment method to investigate the ZA/SiO2 adsorption of Cu(II) and Cr(VI) solution treatment effect. The sorption kinetics curves of Cu(II) and Cr(VI) on ZA/SiO2 were L-shaped. What's more, the solid concentration effect was found in the process of sorption kinetics. Langmuir and Freundlich sorption isotherm models were used to analyze the adsorption data. The results showed that the adsorption conforms to Langmuir and Freundlich adsorption isotherm models. However, the adsorption capacity of ZA/SiO2 compounds for Cu(II) and Cr(VI) is greatly improved. The adsorption capacity of Cu(II) is 158 mg·g− 1 and of Cr(VI) is 176 mg·g− 1, which were 3.6 and 1.8 times of ZA, respectively. Density functional theory (DFT) was utilized for the analysis of intrinsic mechanism and specific pathways. It primarily involved isomorphic substitution of Cr(VI) and Zn(II) and the intercalation of Cr2O72−, with the exception of Cr(OH)3 precipitation. Conversely, Cu(II) removal's primary mechanism in water was isomorphic substitution, except for Cu(OH)2 precipitation.
Article
Preparation of ZnAl layered double hydroxides supported by silica for the treatment of Cr(VI) and Cu(II) in aqueous solution
https://doi.org/10.21203/rs.3.rs-5008652/v1
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Layered double hydroxides
SiO2
Adsorption
Solid concentration effect
With the development of industry and the continuous improvement of the level of social productivity, the material wealth of human society is unprecedentedly prosperous, but various environmental problems come one after another [1, 2]. Among them, global water resource pollution is one of the main environmental problems faced by mankind. Especially with the growth of population and the rapid expansion of various industries such as petrochemicals, printing and dyeing, and textiles, water pollution problems have become increasingly prominent [3–5]. This not only aggravates the shortage of freshwater resources, threatens human survival and health, destroys the natural balance, but also reduces the availability of precious water resource [6]. Therefore, researchers are increasingly focusing on reducing water pollution and treating and reusing contaminated water resources.
As one of the main pollutants in sewage, heavy metals got serious concerned because of their accumulate through the food chain and cause unacceptable environmental problems [7, 8]. Heavy metals can form a variety of harmful complex compounds that affect biological functions [9]. Within the environment, chromium (Cr) predominantly is found in two states of oxidation: Cr(VI) and Cr(III). These two states exhibit distinct toxicity levels and chemical properties, contributing to their diverse environmental impacts. Cr(VI) [10, 11] has been reported to be toxic to biological systems and can easily penetrate biofilms. Cr(VI) can damage DNA and has also been linked to cancer in some organs. Although Cu(II) is one of the essential trace minerals of the human body. But if the body ingests too much, it may also cause great harm to organisms and even endanger aquatic organisms and human lives by binding with enzymes and proteins [12, 13]. Therefore, the removal of heavy metal ions from wastewater is of great significance.
At present, a variety of water treatment technologies [14] have been developed and widely used, including adsorption, membrane filtration, electrochemical treatment, chemical precipitation, bioremediation, in order to effectively solve the problem of water pollution. However, due to capital requirements and high operating costs, the application of these processes has been limited, especially for small and medium-sized industries. Among all these methods, adsorption is a reliable and commonly used method for water purification. With excellent adsorption performance, low operating cost, simple adsorbent design, recyclable features, adsorption technology is an economical and effective waste treatment method [15].
Layered double hydroxides (LDHs) [16–18] is a general term for hydrotalcite compounds (HT) and hydrotalcite-like compounds (HTLCs). It is a kind of anionic clay with layered structure. Because LDHs has many kinds, large layer spacing, strong ion exchange ability, large specific surface area and low cost, it shows good adsorption potential and has broad research and application prospects, and has been widely concerned by scholars [19–21]. The LDHs is composed of metal ions and hydroxy-octahedral lamellae. In the middle of the lamellae, hydrogen bonds and electrostatic gravity connect can exchange anions to make up for the lamellae charge, so that LDHs is electrically neutral [22]. The basic chemical formula of LDHs is [M2 + 1−x M3 + x (OH)2]x+[An−]x/n·mH2O, of which, M2+ and M3+ represent bivalent and trivalent metal ions respectively, An− represents the n-valent anion, x is the molar ratio, and m represents the count of water molecules in a hydrated state [23]. The unique structure and tunability of LDHs determine the diversity of its species. There are more charges on the surface of LDHs laminates, so the layers are easy to agglomerate, resulting in uneven pore size and reduced porosity, which ultimately affects the adsorption efficiency and was not conducive to material recovery [24]. So LDHs composite material gradually into people's vision [25]. The core-shell material is composed of the central particle and the coating layer. Selecting the appropriate material through the combination of physical or chemical forces can make up for the deficiency of a single component material, and reach 1 + 1>2 [26]. Silicon-based nanomaterials provide advantages such as affordability, extensive specific surface area, robust stability, and multiple modification options, making them ideal for use as the central component in core-shell structures [27, 28].
In this paper, ZnAl-LDHs/SiO2 composites (for short ZA/SiO2) are prepared by modifying ZnAl-LDHs with SiO2, and is used to treat wastewater containing Cu(II) and Cr(VI). The objectives of this research are: (i) ZA/SiO2 are prepared by the hydrothermal treatment in zinc sulfate, aluminum sulfate and urea aqueous solution; (ii) the adsorption properties of ZA/SiO2 to Cu(II) and Cr(VI) were investigated; (iii) DFT analysis of Cu(Ⅱ) and Cr(Ⅵ) removal mechanism by ZA/SiO2.
2.1 Reagents
Urea, ZnSO4·7H2O, Al2(SO4)3·18H2O and Cu(SO4)2 were produced by China Aladdin Reagent (Shanghai) Co., Ltd. China, K2Cr2O7 was purchased from Tianjin Damao Chemical Pharmacy Factory, and ethyl silicate was from Tianjin Guangfu Fine Chemical Research Institute. All chemicals were used without further purification. Water was purified with a Hitech-Kflow water purification system (Hitech, China).
2.2 Preparation of ZA and ZA/SiO2 composites
SiO2 was prepared by an improved Stober method. Firstly, mixed 24 mL NH3·H2O and 81 mL anhydrous ethanol in a beaker, then 4.2 mL ethyl silicate was slowly added dropwise withing a glass rod stirring. After standing for 1 hour, the milky liquid was washed by deionized water and anhydrous ethanol repeatedly then centrifuged every time, till the pH of supernatant reached to 7. After washing, the centrifuged sediment was placed in an oven at 70 ℃, completely dried and ground in a mortar with appropriate and average strength to obtain white solid powder, namely SiO2 material with a particle size of about 300 nm.
Pour 60 mL of deionized water into the beaker, add 0.0068 mol ZnSO4·7H2O, 0.0011 mol Al2(SO4)3·18H2O and 0.0225 mol urea in sequence, then stir at room temperature for 30 mins to create a cloudy white solution. Subsequently, the solution was moved into a high-pressure reactor, react at 120 ℃ for 12 hours. After the reactor was fully cooled, centrifuged it and washed it repeatedly with deionized water three times to remove impurities; dried the centrifuged sediment in an oven at 70 ℃ for 6 hours, and then grinded it with a mortar to obtain a white solid powder, briefly referred to as ZA.
Using the same method, the prepared SiO2 was ultrasonic treated and added to the mixed solution of ZnSO4, Al2(SO4)3·18H2O and urea through the above-mentioned preparation steps of ZA were repeated. The prepared sample was recorded as ZA/SiO2.
2.3 Characterization
The crystal structure analysis of ZA and ZA/SiO2 was performed by X-ray diffractometer (XRD, D/max-rA, Bruker AXS, Co., Ltd., Germany) at 40 kV and 40 mA in the 2θ range of 10–70° at a scanning rate of 10 °/min. The morphology of ZA and ZA/SiO2 was observed with scanning electron microscope (SEM, Gemini 300, Germany). The specific surface area (SBET) and pore volume (Vp) of the sample were calculated using the Brunauer-Emmet-Teller (BET) approach and the Barrett-Joyner-Halenda (BJH) approach respectively. The particle size distribution and average particle size values of samples were represented by mastersizer 3000 laser particle size analyzer (Malvern, United Kingdom). The analysis of surface functional groups in the samples was conducted using Fourier transform infrared spectra (FTIR) (Nicolet IS50, Thermo Scientific) documented within the spectrum of 400–4000 cm− 1. Spin polarization density functional theory (DFT) was utilized to analyzed the binding energies of Cu(II) and Cr(VI) on ZA/SiO2.
2.4 Batch adsorption experiments
The adsorption capacity of Cu(II) and Cr(VI) on SiO2, ZA and ZA/SiO2 samples were measured by batch experiments. The stock solution of Cu (II) and Cr(VI) were created through dissolving Cu(SO4)2 and K2Cr2O7 into deionized water and diluted to the desired concentration prior to application. The initial pH values of the solutions were adjusted to 5.5 with 1 mol/L of HCl and 1 mol/L of NaOH solution. The concentration of Cu was tested by atomic absorption spectrophotometer (AAS-3600, Shanghai Metash Instruments Co., Ltd., China), and visible spectrophotometer (V-5600PC, Shanghai Yuan analysis Instrument Co., LTD) was utilized to quantify the capacity of Cr(VI) at 540 nm.
In order to investigate the time required to reach adsorption equilibrium, samples were taken at different time from 0 to 5 hrs. The adsorption capacity (qt) at different times were calculated as follows:
q t = (C0 – Ct) /Cs (1)
of which, the C0 (mg·L− 1) is the initial concentrations, and Ct (mg·L− 1) is the concentration of the solution at time t. Cs (g·L− 1) is the sorbent dosage.
To ensure sorption equilibrium, the adsorption time (t) was selected as 24 h in the equilibrium sorption tests. The equilibrium adsorption capacity (qe, mg·g− 1) were calculated as follows:
q e = (C0 − Ce) /Cs (2)
of which, the Ce (mg·L− 1) refers to the concentration of solution at adsorption equilibrium.
The pseudo-first order and pseudo-second order dynamic models were commonly used to analyze the adsorption kinetic parameters, as expressed by Eqs. (3) and (4):
ln (qe - qt) = ln qe – k1t (3)
t/qt = 1/(k2 qe2) + t/qe (4)
where k1 and k2 are the rate constants of pseudo-first order and pseudo-second order kinetics, respectively.
Sorption isotherms are commonly described using the Langmuir or Freundlich isotherms. The Langmuir model can be expressed in a nonlinear form as,
$$\:{q}_{e}=\frac{{K}_{L}{q}_{m}{C}_{e}}{1+{K}_{L}{C}_{e}}$$
5
or in a linear form as,
$$\:\frac{{C}_{e}}{{q}_{e}}=\frac{{C}_{e}}{{q}_{m}}+\frac{1}{{K}_{L}{q}_{m}}$$
6
The Freundlich model can be expressed in a nonlinear form as,
$$\:{q}_{e}={K}_{F}{C}_{e}^{{n}_{F}}$$
7
or in a linear form as,
lg qe = lg KF + nF lg Ce (8)
In these equations, qm is the maximum sorption capacity, KL is the Langmuir equilibrium constant, and KF and nF are the Freundlich constants.
2.5 Density functional theory (DFT) calculations
The Perdew-Burke-Ernzerhof (PBE) formulation within the generalized gradient approximation (GGA) exchange correlation functionals was used in the geometry optimization and single point energy calculations. The dispersion was corrected by the Grimme scheme to avoid the limitations in handling weak interactions by DFT. Double numerical plus polarization basis set (DNP) was used for the geometry optimization tasks and energy calculations. DFT Semi-Core Pseudopots (DSPP) was used to describe the core electrons. The global orbital cutoff value was set to 5.0 angstrom. The COSMO solvent model of pure water (dielectric constant = 78.54) was applied to simulate an actual solvent environment. The setting values of the convergence thresholds for energy change, maximum force, maximum displacement and the threshold for SCF density convergence were specified for 1.0×10− 5 Ha, 0.002 Ha/Å, 0.005 Å and 1.0×10− 5, respectively.
3.1 Characterizations
In Fig. 1, the patterns of X-ray diffraction (XRD) for SiO2, ZA, ZA/SiO2 were presented. The XRD results of all samples revealed no other phases, affirming the purity of all samples. The ZA and ZA/SiO2 samples display the same characteristic diffraction peaks as the Zn-Al layered double hydroxide [29, 30]; the diffraction peaks (003), (006), (009), (012), (015), (110) and (113) at 10.04o, 20.02o, 30.38o, 33.12o, 33.48o 60.44o and 61.55o were shown, indicated ZA and ZA/SiO2 were prepared successfully via the hydrothermal treatment. While SiO2 only has a wide peak near 23o, demonstrating its amorphous morphology [27, 28]. The addition of SiO2 did not change the layered structure of Zn-Al layered double hydroxide, but only acts as a supporting template. At the same time, it increased its specific surface area, which would be conducive to the improvement of adsorption performance (see the section of 3.2).
Field emission scanning electron microscope (SEM) was utilized to detect the microscopic morphology of samples, and the result was showed in Fig. 2. The Fig. 2a illustrate that the silica spheres, measuring approximately 300 nm in size, exhibit a smooth surface. In the absence of SiO2 templates, the synthesized ZA sample consists of self-assembled nanosheets that form an aggregated spherical structure (Fig. 2d). It can be seen from the TEM (Fig. 2b) and SEM (Fig. 2c) images of ZA/SiO2 that with the addition of SiO2, a part of the silica pellets was wrapped in thin sheets, forming a core-shell structure. The SiO2 pellets, which were not coated by sheets, supported the smaller pores in the ZA in the outer layer, making the structure bulkier, which was consistent with BET analysis. Simultaneously, this phenomenon contributed to the augmentation of the specific surface area of ZA/SiO2 composites. This indicates that the spatial structure of ZA/SiO2, which were acquired through SiO2 loading, has been effectively optimized.
Fourier transform infrared spectroscopy (FT-IR) could be used to detect the surface functional groups of samples and to analyze the interactions between adsorbent and adsorbate molecules. Figure 3 shows the FT-IR spectra of SiO2, ZA, ZA/SiO2. In the spectra of each sample, the wide absorption peaks of 3200 ~ 3400 cm− 1 were the stretching vibration peak of the -OH on the laminate and the water molecule -OH between the laminate; the absorption peaks around 1400 cm− 1 were the vibration absorption of interlayer CO32− [28] The low frequency absorption peaks between 400 and 900 cm− 1 were the stretching and bending vibration peak of M-O and M-OH.
|
Sample |
SBET (m2·g− 1) |
Dx(10) (µm) |
Dx(50) (µm) |
Dx(90) (µm) |
D [3,2] (µm) |
D [4, 3] (µm) |
|---|---|---|---|---|---|---|
|
SiO2 |
13.3 |
2.87 |
16.3 |
45.8 |
7.54 |
23.4 |
|
ZA |
62.4 |
2.80 |
16.5 |
56.4 |
7.67 |
29.1 |
|
ZA/SiO2 |
62.5 |
2.25 |
8.94 |
76.5 |
5.11 |
28.1 |
Figure 4 shows the N2 adsorption-desorption curves and pore size distribution curves of SiO2, ZA and ZA/SiO2 samples, respectively. It could be seen that the adsorption curves and desorption curves of all samples both form a closed curve, which was called the H3 type adsorption hysteresis ring, and its appearance further proves that SiO2, ZA and ZA/SiO2 samples are porous structures formed by the accumulation of flaky particles [31]. Table 1 shown the characterization results of the specific surface area (SBET), particle size distribution and average particle size values of SiO2, ZA and ZA/SiO2 samples. It was worth noting that the SBET values of ZA and ZA/SiO2 were very similar. As can be seen from Fig. 4b, compared with ZA and SiO2, the particle size distribution of ZA/SiO2 is relatively concentrated, indicating that the particle size distribution is uniform. The appearance of SiO2 provides a landing point for the lamellar structure of ZA, making it grow evenly. Generally, the smaller the particle size of the adsorbent, the larger the adsorption specific surface energy. And the particle size distribution is concentrated, indicating that the prepared adsorbent is more stable [31]. DX (10), DX (50) and DX (90) can reflect the uniformity of the powder sample, which is called the cumulative distribution or frequency distribution of the powder. Where DX (50) is called the median diameter or median particle size, it is an important parameter to measure the particle size distribution, representing the corresponding particle size when the cumulative particle size distribution percentage reaches 50%. A smaller DX (50) value means that the particle size is more evenly distributed and the particle size is smaller. From Table 1, it shown that the particle size of ZA/SiO2 (8.94 µm) is the most uniform, and the particle size of ZA (16.5 µm) and SiO2 (16.3 µm) is similar. Both D [3, 2] and D [4, 3] represent the average particle size on the basis of volume. Where D [3, 2] is called "volume area mean diameter", referred to as the area mean diameter, which is the ratio of the total volume of particles to the total area. D [4, 3] is called "mass moment volume mean diameter" and referred to as volume mean diameter. Both of them are the average diameters calculated from particle size distributions on different benchmarks. D [3, 2] is calculated by area distribution and D [4, 3] is calculated by volume distribution. It can be seen from Table 1 that ZA/SiO2 has the smallest average particle size by volume area, but its average particle size is similar to ZA.
3.2. Adsorption performance of samples
The kinetic study is one of the main reasons to reveal the adsorption reaction rate. Figure 5 respectively shown the adsorption kinetics curves of Cu(II) and Cr(Ⅵ) on ZA/SiO2 samples with different solid content. The adsorption rates of ZA/SiO2 to Cu(II) and Cr(Ⅵ) were fast in the first 30 min, and the adsorption equilibrium states were reached in about 180 min. This was because with the progress of the reaction, the adsorption site on the adsorbent was continuously occupied and gradually reaches a near saturation state. What’s more, it can be seen that under each solid content, the adsorption kinetics curves of ZA/SiO2 samples were L-shaped, and the adsorption capacity decreased with the increase of solid content, which indicates that there is an obvious effect of adsorbent concentration in the adsorption system, that is, the adsorption capacity decreases with the increase of adsorbent concentration [32, 33]. The sorption kinetics of Cu(II) and Cr(Ⅵ) on ZA/SiO2 samples were fitted with the pseudo-first order model and pseudo-second order model (Fig. 5a, b, a′ and b′), and the fitting parameters are listed in Table 2. At different solids content, adsorption kinetics data of ZA/SiO2 samples were well fitted by a pseudo-second-order kinetic model, indicating that chemical adsorption played a leading role in the adsorption process of Cu(II) and Cr(Ⅵ) by ZA/SiO2 [34].
|
Adsorption object |
Adsorbents |
Langmuir isotherm: qe = KLqmCe/(1 + qmKL) |
Freundlich isotherm: qe = KFCenF |
|||||
|
qm (mg·g− 1) |
KL (L·mg− 1) |
R2 |
nF |
KF (mg1 − nFLnF·g− 1) |
R2 |
|||
|
Cu(II) |
SiO2 |
8.82 |
0.0185 |
0.977 |
0.339 |
1.09 |
0.926 |
|
|
ZA |
43.6 |
0.0432 |
0.923 |
0.223 |
11.0 |
0.998 |
||
|
ZA/SiO2 |
158 |
0.0137 |
0.959 |
0.592 |
5.35 |
0.995 |
||
|
Cr(VI) |
SiO2 |
16.8 |
0.00432 |
0.892 |
0.701 |
0.176 |
0.831 |
|
|
ZA |
98.6 |
0.00366 |
0.980 |
0.729 |
0.835 |
0.978 |
||
|
ZA/SiO2 |
176 |
0.00644 |
0.989 |
0.551 |
4.96 |
0.966 |
||
The adsorption isotherm would describe the relationship between the adsorption capacity and the equilibrium concentration of the adsorbent solution at 25°C and pH = 5.5. Figure 6 respectively shown the adsorption isotherms of Cu(II) and Cr(Ⅵ) on SiO2, ZA, ZA/SiO2 with 1 g·L− 1 solid content. It can be seen that the adsorption amount of ZA/SiO2 was the largest, followed by ZA, and the adsorption capacity of SiO2 was the smallest. Both Langmuir model and Freundlich model could fit the adsorption data well. The nonlinear-fit data of model parameters for Cu(II) and Cr(VI) sorption on SiO2, ZA, ZA/SiO2 was shown in Table 3 and the linear-fit data was shown in Table 4. As can been seen, the equilibrium adsorption capacity of ZA/SiO2 were 158 mg·g− 1 for Cu(II) and 176 mg·g− 1 for Cr(VI) from nonlinear-fit, which were 3.6 and 1.8 times of ZA, respectively. However, the equilibrium adsorption capacity of ZA/SiO2 were 208 mg·g− 1 for Cu(II) and 233 mg·g− 1 for Cr(VI) from linear-fit. Under both fitting conditions, the adsorption effect of ZA/SiO2 on Cr(VI) is better than that of Cu(II).
|
Adsorption object |
Adsorbents |
Linear form of Langmuir isotherm: \(\:\frac{{C}_{e}}{{q}_{e}}=\frac{{C}_{e}}{{q}_{m}}+\frac{1}{{K}_{L}{q}_{m}}\) |
Linear form of Freundlich isotherm: lg qe = lg KF + nF lg Ce |
||||||
|
qm (mg·g− 1) |
KL (L·mg− 1) |
R2 |
nF |
KF (mg1 − nFLnF·g− 1) |
R2 |
||||
|
Cu(II) |
SiO2 |
9.04 |
0.017 |
0.986 |
0.423 |
1.40 |
0.858 |
||
|
ZA |
43.4 |
0.037 |
0.898 |
0.218 |
11.3 |
0.996 |
|||
|
ZA/SiO2 |
208 |
0.008 |
0.964 |
0.640 |
4.27 |
0.990 |
|||
|
Cr(VI) |
SiO2 |
19.1 |
0.003 |
0.902 |
1.18 |
0.014 |
0.866 |
||
|
ZA |
120 |
0.003 |
0.576 |
0.720 |
0.855 |
0.932 |
|||
|
ZA/SiO2 |
233 |
0.004 |
0.944 |
0.739 |
2.04 |
0.977 |
|||
3.3 DFT analysis of Cu(Ⅱ) and Cr(Ⅵ) removal mechanism by ZA/SiO2
The distinct ionic radii of Cu (II) and Cr (VI) give rise to inherent variations in the adsorption process of ZA/SiO2 for these two ions. Therefore, the adsorption mechanism at electronic scale was explored in-depth by density functional theory.
Figure 7a, b, and c depict that when the Zn atom was replaced with Cr(VI), the electrons lost by Cr(VI) were transferred to the neighboring oxygen atom. In comparison to the pre-replacement model, a greater proportion of the chemical bonds formed by Cr(VI) with the surrounding O atoms assumed the form of ionic bonds. This observation suggests that Cr(VI) has a propensity to preserve its positive charge by relinquishing its own electrons. Consequently, Cr(VI) was drawn towards the surrounding electronegative O atoms, where it existed in a stable state. Simultaneously, Cr(III) formed chemical bonds with adjacent O and H atoms. Moreover, taking into account the extent of the electron cloud, it can be deduced that electrons from Cr(III) were primarily moved to the hydroxyl atoms within the interlayer, leading to the formation of a stable Cr(OH)3 precipitate. Additionally, the elimination of Cr(VI) from water might also be linked to the incorporation of Cr2O72−.
Figure 7c and d depict the substitution of the Zn atom by Cu(II), where the transfer of electrons occurred towards the neighboring O atoms. However, in the case of Cu(II), the quantity of transferred electrons was smaller compared to what was observed for Cr(VI), and the ratio of ionic bonds following Cu(II) substitution was A bit greater than that in the case of Cr(VI). Despite the presence of an electron cloud surrounding Cu(II), the most extensive area was observed near the oxygen (O) atoms adjacent to Zn(II) (as shown in Fig. 7c), indicating a predominant transfer of electrons to the O atoms of Zn-O. This presents a contrasting electron transfer pathway compared to the ion exchange process observed in Cr(VI) removal. Additionally, the energy needed for Cu(II) removal via ion exchange was less than that required for Cr(VI) removal. This distinction can be attributed to the notable difference in the ionic radius of Cu(II), which was considerably smaller compared to that of Cr(VI) and was closer in size to that of Al(III). Therefore, not only did the Cu(II) ion exchange process have a lower energy requirement, but it also displayed superior structural stability in comparison to Cr(VI).
The complexation model for Cu(Ⅱ) and Cr(VI) adsorption by ZA/SiO2 was optimized. Figure 7e illustrates the adsorption energy calculated post-structural optimization, where Eads = − 2.12 eV and − 3.27 eV for ZA/SiO2–Cu (Ⅱ) and Cr(VI) complex form and Eads <0. The results show that under mild condition, ZA/SiO–Cu(Ⅱ) and Cr(VI) complexation cannot occur spontaneously. The deduction can be made that the -OH groups within ZA/SiO- were involved in the complexation of Cu(Ⅱ) and the removal of Cr(VI). To acquire a more comprehensive understanding of the interfacial interactions, the d-band centers (εd) and projected d-band densities of Cu(II) and Cr(VI) atoms at their stable adsorption locations in pristine ZA/SiO2 with hydroxide interfacial layer slabs were calculated. Figure 7f illustrates that the (εd) of Cu(II) in pure ZA/SiO2 was lower than the (εd) of Cr(VI) in ZA/SiO2 with hydroxide interfacial layer slabs. Consequently, the adsorption affinity of Cr(VI) on the surface of ZA/SiO2 with a hydroxide interface layer was stronger than that of pure ZA/SiO2. This was primarily due to the interface interaction, which mainly involves the modification of electron transfer and the shift in the D-band center.
Upon conducting comprehensive characterization analysis and batch adsorption experiments, it becomes evident that the adsorption of Cu(II) and Cr(VI) by ZA/SiO2 entails a multifaceted process marked by diverse mechanisms.
ZA/SiO2 possess an exceptional porous structure and a significantly expanded specific surface area, which provides numerous effective adsorption sites within the pore channels for Cu(II) and Cr(VI) uptake. The binding of Cu(II) and Cr(VI) to ZA/SiO2 may result from multilayer physical adsorption. Regarding the saturated adsorption capacity (qm) of ZA-LDHs for pollutants, the respective adsorption capacities for each contaminant can be ranked as follows: Cr(VI) > Cu(II). In summary, the primary mechanism for removing Cr(VI) from water involves isomorphic substitution of Cr(VI) and Zn(II), as well as the intercalation of Cr2O72−, with the exception of Cr(OH)3 precipitation.
The preceding phenomenon can be explained by the smaller size of Cr(VI) (0.044 nm) compared to Zn(II) (0.074 nm), whereas the latter can be understood as a result of the successful incorporation of SiO2 (0.86 nm × 0.59 nm × 0.18 nm) at the initial stages, thus creating a pathway for the entry of Cr2O72− (0.55 nm × 0.25 nm × 0.21 nm). Regarding the primary mechanism for eliminating Cu(II) from water, it involves isomorphic substitution as the predominant process, except for the formation of Cu(OH)2 precipitates. This phenomenon can mainly be attributed to the very similar ionic radii of Cu(II) (0.073 nm) and Zn (II) (0.074 nm).
In this study, the crystal structure of LDHs was effectively fine-tuned through the addition of SiO2 to the urea mixture of ZnSO4 and Al2(SO4)3. This optimization resulted in the formation of Zn-Al/SiO2-based LDHs (referred to as ZA/SiO2), which exhibited excellent adsorption properties for Cu(II) and Cr(VI), effectively mitigating LDHs accumulation. When compared to SiO2 and ZA, the crystal structure optimization of ZA/SiO2 had the most significant impact, causing a substantial increase in the adsorption capability for Cu(II) and Cr(VI). The equilibrium adsorption capacity of ZA/SiO2 were 158 mg·g− 1 for Cu(II) and 176 mg·g− 1 for Cr(VI), which were 3.6 and 1.8 times of ZA, respectively. Analyzing the mechanism of ZA/SiO2 in eliminating Cu(II) and Cr(VI) at the electronic level revealed that for Cr(VI) removal, it primarily involved isomorphic substitution of Cr(VI) and Zn(II) and the intercalation of Cr2O72−, with the exception of Cr(OH)3 precipitation. Conversely, Cu(II) removal's primary mechanism in water was isomorphic substitution, except for Cu(OH)2 precipitation. What's more, the solid concentration effect was found in the process of sorption kinetics. Both Langmuir model and Freundlich model could fit the adsorption data well. Furthermore, this work established a facile technique in developing multifunctional adsorbents.
Acknowledgements
This work is supported financially by the project of Shandong Province higher educational science and technology program (No. J18KA104), Hubei Province central government guide local science and technology development project (No. 2204-429006-04-02-813827) and the horizontal research project of process optimization of febuxostat API for Hubei Huashitong (No. 010008002040030).
Author Contribution
Conceptualization, Fengrong Zhang; methodology, Fengrong Zhang and Cuilan Zhang; validation, Fengrong Zhang and Luxing Liu; formal analysis, Fengrong Zhang, Cuilan Zhang, Lishun Wu and Dawei Shang; investigation, Cuilan Zhang and Luxing Liu; resources, Fengrong Zhang, Luxing Liu and Lishun Wu; data curation, Cuilan Zhang and Luxing Liu; writing—original draft preparation and the drawing of the figures, Luxing Liu; writing—review and editing, Fengrong Zhang, Cuilan Zhang and Lishun Wu; visualization, Fengrong Zhang, Cuilan Zhang and Lishun Wu; supervision and the design of the manuscript and the theoretical framework, Cuilan Zhang; project administration, Fengrong Zhang and Cuilan Zhang; funding acquisition, Fengrong Zhang. All authors have read and agreed to the published version of the manuscript.
Conflict of Interest Statement
On behalf of all authors, the corresponding author states that there is no conflict of interest.
Data Availability Statement
The authors confirm that the data supporting the findings of this study are available within the article.
- X. Xue, Y. Han, X. Wu, H. Wang, S. Wang, J. Zheng, R. Ran, C. Zhang. Review: Phytate modification serves as a novel adsorption strategy for the removal of heavy metal pollution in aqueous environments. J. Environ. Chem. Eng. 2023, 11: 111440.
- B. Wang, K. Wu, T. Liu, Z. Cheng, Y. Liu, Y. Liu, Y. Niu, Feasible synthesis of bifunctional polysilsesquioxane microspheres for robust adsorption of Hg(II) and Ag(I): Behavior and mechanism. J. Hazard. Mater., 2023, 442: 130121.
- S. Kurwadkar, Occurrence and distribution of organic and inorganic pollutants in groundwater. Water Environ. Res. 2019, 91: 1001–1008.
- T. Liu, Y. Lawluvy, Y. Shi, J.O. Ighalo, Y. He, Y. Zhang, P.S. Yap. Adsorption of cadmium and lead from aqueous solution using modified biochar: A review. J. Environ. Chem. Eng. 2022, 10: 106502.
- Q.Y. Wang, L.P. Li, L.C. Kong, G.Y. Cai, P. Wang, J. Zhang, W. Zuo, Y. Tian, Compressible amino-modified carboxymethyl chitosan aerogel for efficient Cu(II) adsorption from wastewater. Sep. Purif. Technol. 2022, 293: 121146.
- Y. Yang, Y. Wang, C. Xue, Y. Lin, J. Lee, X. Yi, Z. Dang. Efficient removal of heavy metals from acid mine drainage by ε-MnO2 adsorption. J. Clean. Prod. 2024, 452: 141936.
- D. Mani, C. Kumar, Biotechnological advances in bioremediation of heavy metals contaminated ecosystems: an overview with special reference to phytoremediation. Int. J. Environ. Sci. Technol. 2014 (11): 843–872.
- A. Ashraf, J. Dutta, A. Farooq, M. Rafatullah, K. Pal, G. Kyzas. Chitosan-based materials for heavy metal adsorption: Recent advancements, challenges and limitations. J. Mol. Struct. 2024 (1309): 138225.
- K. Usman, M. Abu-Dieyeh, N. Zouari, M. Al-Ghouti, Lead (Pb) bioaccumulation and antioxidative responses in Tetraena qataranse. Sci. Rep. 2020 (10): 17070.
- X. Yuan, S. Deng, D. Fang, Y. Li, X. Yu, L. Huang, Layer-by-layer adsorption behavior of Cr(VI) on MoS2@Mt induced by sulfamethoxazole. Sep. Purif. Technol. 2025, 354: 129261.
- S. Zhao, F. Lv, Y. Zhao, J. Feng, H. Tang, Effective strategy for Cr(VI) removal from water in evolution process of ferroan brucite by redox and intercalation. J. Water Process Eng., 2024, 66: 105999.
- X. Zeng, J. Zhu, G. Zhang, Z. Wu, J. Lu, H. Ji. Molecular-level understanding on complexation-adsorption-degradation during the simultaneous removal of aqueous binary pollutants by magnetic composite aerogels. Chem. Eng. J. 2023, 468: 143536.
- Q.Y. Wang, L.P. Li, L.C. Kong, G.Y. Cai, P. Wang, J. Zhang, W. Zuo, Y. Tian. Compressible amino-modified carboxymethyl chitosan aerogel for efficient Cu(II) adsorption from wastewater. Sep. Purif. Technol. 2022, 293: 121146.
- R. Mishra, A. Kumar, E. Singh, S. Kumar, V.K. Tripathi, S.K. Jha, S.K. Shukla. Current status of available techniques for removal of heavy metal contamination in the river ecosystem, in: Proc. Ecol. Significance River Ecosyst., 2022, 217–234.
- N. A. Qasem, R.H. Mohammed, D.U. Lawal. Removal of heavy metal ions from wastewater: a comprehensive and critical review. NPJ Clean Water. 2021 (4): 36.
- A. Farhan, A. Khalid, N. Maqsood, S. Iftekhar, H. M. A. Sharif, F. Qi, M. Sillanpää, M.B. Asif. Progress in layered double hydroxides (LDHs): Synthesis and application in adsorption, catalysis and photoreduction. Sci. Total Environ. 2024, (912): 169160.
- Z. Sun, Y. Wang, T. Liu, X. Kong, T. Pan, F. Zhang, X. Lei, X. Duan. Super-stable mineralization of Cu, Cd, Zn and Pb by CaAl-layered double hydroxide: Performance, mechanism, and large-scale application in agriculture soil remediation. J. Hazard. Mater., 2023, 447: 130723.
- G. Santos, P. Lins, L. Oliveira, E. Silva, I. Anastopoulos, A. Erto, D. Giannakoudakis, A. Almeida, J. Duarte, L. Meili. Layered double hydroxides/biochar composites as adsorbents for water remediation applications: recent trends and perspectives. J. Clean. Prod. 2021, 284: 124755.
- F. Zhang, D. Han, Q. Guo, W. Hou. Fabrication of layered double hydroxide/silica foam nanocomposites and their application for removing Pb(II) and Cr(VI) from aqueous solutions. Chem. Select. 2019, 4: 6971-6977.
- Q. Li, G. Wei, Y. Yang, Z. Li, L. Zhang, Q. Huang. Mechanochemical synthesis of Fe2O3/Zn-Al layered double hydroxide based on red mud. J. Hazard. Mater. 2020, 394: 122566.
- N. Q. Tung, D. C. Van, D. X. Thang, N. K. An, T. Trang, B. D. Nhi, N. P. Thao, L. Son, N. N. Huy, N. T. Dung. Hydrothermal synthesis of CuCoFe layered double hydroxide and its performance in the degradation of antibiotics: Influencing factors, degradation pathways, and reaction mechanism. J. Environ. Chem. Eng. 2023, 11: 110127.
- Y. Wu, L. Wu, W. Yao, Y. Chen, Y. Zhou, L. Wang, T. Wu, B. Jiang, A. Atrens, F. Pan. ZnAl-LDH@MXene modified by inhibitor as a nanofiller applied on Mg alloys coating protection. Carbon, 2023, 204: 36-49.
- M. Zubair, I. Ihsanullah, H. A. Aziz, M. A. Ahmad, M. A. Al-Harthi, Sustainable wastewater treatment by biochar/layered double hydroxide composites: Progress, challenges, and outlook, Bio. Technol. 2021, 319: 124128.
- Q. Wang，D. O’Hare. Large-scale synthesis of highly dispersed layered doable hydroxide powders containing delaminated single layer nanosheets. Chem. Commun. 2013, 49: 6301-6303.
- B. Zhang, L. Luan, R. Gao, F. Li, Y. Li, T. Wu. Rapid and effective removal of Cr(VI) from aqueous solution using exfoliated LDH nanosheets. Colloid Surf. A: Physicochem. Eng. Aspects. 2017, (520): 399-408.
- L. Tang, Y. Huang, H. Song, C. Yang, A. Bai, Q. Li. Research progress of core-shell nanocomposite materials. Material guide, 2016, 30 (1): 40-42, 51.
- C. Chen, R. Felton, J. Buffeta, D. O'Hare. Core–shell SiO2@LDHs with tuneable size, composition and morphology, Chem. Commun. 2015,51, 3462-3465.
- X. Wang, B. Cheng, L. Zhang, J. Yu, I. Normatov. Adsorption performance of tetracycline on NiFe layered double hydroxide hollow microspheres synthesized with silica as the template. J. Colloid Interface Sci. 2022, 627: 793–803.
- M. Ponce, M. Cina, C. López, S. Cerutti. Synthesis and evaluation of a Zn–Al layered double hydroxide for the removal of ochratoxin A. Greenness assessment. Anal. Methods, 2022, 14: 2841-2848.
- M. Yasaei, M. Khakbiz, E. Ghasemi, A. Zamanian. Synthesis and characterization of ZnAl-NO3(-CO3) layered double hydroxide: A novel structure for intercalation and release of simvastatin. Appl. Surf. Sci. 2019, 467-468: 782-791.
- L. Liu, M. Cheng, Z. Yang. Improved performance of flower-like ZnAl LDH growing on carbon nanotubes used in zincenickel secondary battery. Electrochim. Acta, 2018, 27767-76.
- Z. Yang, M. Yu, Y. Liu, X. Chen, Y. Zhao. Synthesis and performance of an environmentally friendly polycarboxylate superplasticizer based on modified poly (aspartic acid). Constru. Build. Mater. 2019, 202: 154-161.
- Y. Nie, C. Zhao, Z. Zhou, Y. Kong, J. Ma. Hydrochloric acid-modified fungi-microalgae biochar for adsorption of tetracycline hydrochloride: Performance and mechanism. Bioresour. Technol. 2023, 383: 129224.
- X. Su, X. Wang, Z. Ge, Z. Bao, L. Lin, Y. Chen, W. Dai, Y. Sun, H. Yuan, W. Yang, J. Meng, H. Wang, S. Pillai. KOH-activated biochar and chitosan composites for efficient adsorption of industrial dye pollutants. Chem. Eng. J. 2024, 486: 150387.
No competing interests reported.