Removal of Copper (II) from Aqueous Solutions Using Silica Xerogel as Sorbent: Adsorption Properties and Mechanism

Abstract

The contamination of water resources with heavy metals creates problems for using it as a source of drinking water. Adsorption is one of the most promising methods for heavy metal ion removal from natural and wastewater. The process of removing copper (II) from aqueous solutions using SiO₂ xerogel as an adsorbent has been studied. The xerogel was thoroughly characterized by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and argon adsorption–desorption isotherms, revealing an amorphous structure with a high surface area (~347 m²/g) and uniform mesoporosity (2–14 nm pore size). The surface chemistry, dominated by silanol groups, was confirmed by XPS analysis. The adsorption process is influenced by electrostatic interactions between the positively charged Cu(II) ions and the negatively charged surface groups, with the optimal performance near neutral pH. Batch adsorption experiments demonstrated that the silica xerogel effectively removes Cu(II) ions from aqueous solutions, with removal efficiency exceeding 99% at pH values above 4.0. The maximum adsorption capacity of copper (II) ions on SiO₂ xerogel is 67.5 mg/L.

1. Introduction

Currently, the contamination of water resources with various pollutants, including heavy metals, pesticides, pharmaceuticals, dyes, etc., and the resulting problem of the inability to use contaminated water as a source of drinking water, are some of the most pressing issues of modern times [1,2,3,4]. The main sources of various pollutants entering natural waters are industrial discharges, agricultural activities, and urban wastewater containing numerous toxic inorganic and organic pollutants. The presence of various pollutants in drinking water above permissible levels leads to increased toxicity and health problems for the population when such water is used for the drinking water supply [5,6,7].

Heavy metals, when present in drinking water at concentrations exceeding permissible levels, have a serious impact on the human body. Various methods (such as ion exchange, solvent extraction, adsorption, and precipitation) are currently used for removing heavy metals to meet the growing demand for drinking water from natural sources [8,9,10]. Among the methods of removing heavy metals, adsorption is recognized as an effective and economical method [11,12,13,14,15]. A large number of different materials have been proposed for use as sorbents for removing heavy metals, including activated carbon [16,17], zeolites [18,19], silica gel [20,21,22], aluminum oxide [23,24], and natural clay minerals (kaolinite, bentonite, illite, stevensite, rectorite, calcite, etc.) [25,26,27,28,29]. The adsorption process is flexible in operation and supplies high-quality purified drinking water. Additionally, adsorption is sometimes reversible, and adsorbents can be regenerated by a suitable desorption process [30,31,32].

Among the known methods for removing copper from natural water and wastewater, adsorption is the most attractive, effective, and economically feasible method [33,34]. Various inorganic and organic adsorbents, including some of natural origin [35], have been used to extract copper. Among the most studied materials as adsorbents for removing heavy metal ions are carbon-based adsorbents [36,37] including biochar [38], chitosan [39], natural zeolites [40], industrial waste [41], cellulose [42,43], graphene oxide [44], carbon nanotubes [45], alginate [46], metal oxides [47], etc.

Silica xerogel and aerogel-based sorbents have attracted considerable attention from researchers due to their non-toxic nature, high specific surface area, regular pore structure, and the possibility of surface modification [48,49,50,51,52,53,54]. The use of SiO₂ xerogel synthesized from various precursors for removing heavy metal ions from aqueous solutions is considered promising due to its high porosity, low density, and high adsorption capacity [55,56,57]. SiO₂ xerogel-based sorbents are also used to remove a variety of more persistent pollutants, including PFAS [58,59,60], pharmaceuticals [61,62], and personal care items [63,64]. The advantages and disadvantages of SiO₂ xerogel use as a sorbent are presented in Table S1.

The most widely used precursors in the sol–gel synthesis of SiO₂ xerogel are tetramethyl orthosilicate (TMOS) and tetraethyl orthosilicate (TEOS) [65]. Their use in the sol–gel synthesis of SiO₂ has been well studied [66]. Researchers have paid particular attention to obtaining SiO₂ aerogels using SiO₂ xerogel as a precursor, as they possess a large specific surface area [67,68,69]. However, their production involves the use of complex equipment associated with drying SiO₂ xerogel in a supercritical carbon dioxide environment [70,71].

At the same time, there is little attention paid in the literature to studying the removal of copper (II) ions by adsorption in SiO₂ xerogels. The presence of a large number of hydroxyl groups on the surface of the synthesized SiO₂ xerogel may lead to the precipitation of heavy metal ions as hydroxides [72]. The most promising materials for the removal of heavy metals, including copper (II) ions, are mesoporous silica-based sorbents (SiO₂, such as silicate and silica gel) [48,49]. These materials have garnered attention from researchers as effective adsorbents for the removal of heavy metal ions [50]. The surface and pores of silica exhibit a substantial number of hydroxyl groups, which facilitate the efficient removal of Cu²⁺ ions [35]. The removal of Cu²⁺ ions to silica-based adsorbents is primarily associated with electrostatic interactions and ion exchange processes [73].

There is little information in the literature on the adsorption of SiO₂ xerogel obtained from TMOS or TEOS without modification. Unmodified SiO₂ xerogel has a low adsorption capacity to heavy metal ions. Table S2 shows the comparative characteristics of the adsorption of copper ions by SiO₂ xerogel-based sorbents with and without modification.

This work investigates the process of copper (II) ion adsorption on freshly prepared SiO₂ xerogel, which ensures a consistently high pH level. The structural characteristics and the influence of various parameters on the copper (II) ion adsorption process in SiO₂ xerogel have been studied.

2. Materials and Methods

2.1. Synthesis of SiO₂ Xerogel

A sample of SiO₂ xerogel was prepared from TEOS in two stages of the sol–gel process, involving acid hydrolysis using hydrochloric acid in an alcohol medium, followed by gel formation with an ammonium hydroxide solution [74]. The molar ratios at the first stage were 1 mol TEOS: 3.9 mol ethanol: 1 mol H₂O: 12 × 10⁻⁴ mol HCl (1.5 h at 60 °C, pH = 3). At the second stage, the ratio was 2.5 mol H₂O: 7 × 10⁻³ mol NH₄OH. After gel formation, the samples were soaked in ethanol twice for 24 h [75]. Subsequently, to obtain xerogels, the sample was dried in a drying oven at a temperature of 60 °C for 24 h, then at 150 °C (1 h) and 170 °C (1 h) [76].

2.2. Characterization of SiO₂ Xerogel

Morphological studies, structural analysis of samples, and elemental mapping were carried out using a transmission electron microscope JEOL JEM-F200 (JEOL, Tokyo, Japan) with an electron accelerating voltage range from 20 to 200 kV. X-ray diffraction (XRD) studies were performed using an Empyrean PANalytical X-ray diffractometer (Almelo, The Netherlands) in the radiation of a copper anode with a nickel filter, with radiation wavelength λ(CuKα) = 0.154051 nm. Data processing was performed using the High Score Plus application program, included in the instrument software, and the diffraction database ICSD (PDF-2). The surface composition was carried out by an AXIS SupraTM X-ray photoelectron spectrometer (XPS) (Kratos Analytical Ltd., Manchester, UK). The data were processed by CasaXPS v.2.3.23 software (Casa Software Ltd., Wilmslow, UK). The changes that occurred in the surface properties of silica xerogels using different synthesis methods, as well as before and after adsorption, were identified using Fourier-transform infrared spectroscopy in the range from 400 cm⁻¹ to 4000 cm⁻¹ using an IR spectrometer Varian 640 (Varian, Inc., Palo Alto, CA, USA). The samples were prepared in the form of pellets with potassium bromide. The specific surface area was determined using the “Chemisorb-2750” instrument (Micromeritics, Norcross, GA, USA). Prior to measurement, the sample was heated at 200 °C to remove moisture and other adsorbed gases. Measurements were carried out using the Brunauer–Emmett–Teller method by adsorbing Ar at a temperature of −196 °C. High-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDX) analyses were conducted using JEOL JEM-F200 (JEOL Ltd., Tokyo, Japan), which is operated at a 200 kV accelerating voltage and equipped with a cold field-emission gun (CFEG) and high-resolution CMOS AMT camera.

2.3. Adsorption Properties

The adsorption properties of the obtained samples were investigated in relation to copper (II) ions depending on pH, temperature, and contact time. Adsorption and kinetic curves were plotted. To study the effect of pH on the removal of Cu²⁺ ions, 7 samples of copper (II) sulfate solutions with different pH values ranging from 2 to 8, an initial concentration of Cu²⁺ C₀ = 300 mg/L, and a solution volume of 100 mL were used. The pH value was determined using the Multitest IPL-101 pH meter (Moscow, Russia) with a built-in temperature sensor. The pH meter was calibrated using standard buffer solutions with pH values: 1.65; 3.56; 6.86; and 9.18. After adding 0.2 g of xerogel samples, the sample was kept for 30 min and filtered, followed by determination of the residual concentration of Cu²⁺ ions. The initial copper sulfate solution had pH = 5. The solution was acidified with a 0.1 M HCl solution and alkalized with 0.1 M NaOH. To prevent the absorption of carbon dioxide from the air, the flasks with solutions were tightly closed with stoppers. To determine the point of zero charge, the powder addition method was used, as described in [77,78].

The copper (II) content in the solution was determined via copper (II) complexes with ammonia using a photometric colorimeter SF-2000 (Krasnoobsk, Russia) [73]. The adsorption capacity (q_e, mg/g) was calculated using Equation (1) and the percentage removal (α, %) using Equation (2):

$${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}})·\mathrm{V}}{\mathrm{m}}}$$

(1)

$$\mathsf{\alpha }={\displaystyle \frac{{(\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}})}{{\mathrm{C}}_0}}·100\%$$

(2)

where C₀ is the initial concentration of Cu²⁺ ions, mg/L, C_e is the equilibrium concentration of Cu²⁺ ions, and mg/L and m are the mass of the sorbent, g.

3. Results

3.1. Characterization of the Adsorbent

The morphology of the synthesized SiO₂ xerogel samples was characterized using transmission electron microscopy (Figure 1). As seen from the obtained data, the SiO₂ xerogel samples represent an agglomeration of non-spherical particles. In this case, SiO₂ particles of arbitrary shapes are formed, different from Stoeber-like synthesis, where spherical silica particles are formed. A single SiO₂ particle in Figure 1a has an irregularly formed spherical shape. The micrograph of a SiO₂ xerogel segment, shown in Figure 1b, indicates the absence of any crystalline structures. The SiO₂ xerogel obtained from TEOS is characterized by an amorphous structure. Elemental mapping of the isolated microparticle with a size of approximately 2 µm and energy-dispersive spectra shows the presence of only elements characteristic of SiO₂ (Figure 1d–f). The data obtained indicate that the investigated microparticle of the SiO₂ xerogel only exhibits the presence of oxygen and silicon (Figure 1d,e). The EDX spectra (Figure 1f) only showed the presence of silicon and oxygen in the synthesized xerogel sample. The presence of hydroxide ions on the surface of the SiO₂ xerogel is an advantage in this case and will contribute to good binding of copper (II) ions.

The porosity of the SiO₂ xerogel is fairly uniform in the size range of 2–14 nm, as illustrated in Figure S1, which shows the nitrogen sorption isotherm (Figure S1a) and particle size distribution (Figure S1b). The specific surface area of silica xerogel synthesized from TEOS was determined as 346.75 m²/g. The pore volumes of silica xerogel were determined as 0.796 cm³/g.

The absence of crystalline structures in the synthesized SiO₂ xerogel is confirmed by the X-ray diffraction spectra presented in Figure 2. The X-ray diffraction pattern exhibits a broad peak between 17° and 30° with a maximum at 22°, characteristic of typical amorphous silica [79]. As confirmed by the data of other researchers, SiO₂ synthesized using sol–gel technology from TEOS has an amorphous structure, and the maximum absorption peak can be in the range from 21.8° to 23° for different researchers [80,81]. At the same time, depending on the ratio of TEOS and water, quartz-like or cristobalite-like structures can also be formed during the synthesis of SiO₂ [80].

The chemical composition and surface state of SiO₂ play a crucial role in the adsorption process of heavy metal ions. Therefore, the surface of the sorbent was investigated using X-ray photoelectron spectroscopy (XPS) in the range of 0 to 1400 eV. An overview spectrum of the SiO₂ surface is presented in Figure 3a, showing the presence of peaks C1s, O1s, Si2s, and Si2p. High-resolution XPS spectra of Si2p demonstrate only one peak at 103.6 eV (Figure 3b), corresponding to SiO₂ [82]. Deconvolution of the experimental XPS spectra for Si 2p ionization revealed two component peaks at 103.2 eV and 103.8 eV, attributed to the bonds between silicon and oxygen (Si–O–Si) and hydroxyl groups adsorbed on the SiO₂ surface (–Si–OH) [83,84].

XPS spectra for oxygen O1s ionization are presented in Figure 3c. Oxygen is the most abundant element in the SiO₂ xerogel sample. The center of the oxygen O1s spectrum was located at 533 eV. The O1s spectra were deconvoluted using two peaks, also showing contributions of oxygen atoms in the hydroxyl group (Si–OH) at 534.8 eV [85] and those associated with oxygen in silicon (–Si–O–Si–) or similar SiO₂ at 533.1 eV [86]. The small area of the O1s peak at 534.8 eV, corresponding to hydroxyl groups, indicates that there are few hydroxyl groups on the xerogel surface.

3.2. Adsorption of Cu²⁺

The removal efficiency for Cu²⁺ ions, calculated based on the experimental data, for each pH value is shown in Figure 4. As seen in Figure 4a, the removal efficiency of copper ions sharply increases in the pH range transition of 2–3 and remains consistently high around pH 5. The residual concentration of Cu²⁺ ions averaged 0.2 mg/L. The obtained dependence of removal efficiency on pH shows that the optimal pH value for removing Cu²⁺ ions fall within a broad range, starting from 4 to 6. At low pH values, the effect of competition between H⁺ and Cu²⁺ ions for the active centers of the adsorbent leads to low values of adsorption efficiency. At pH increases, the concentration of H⁺ ions decreases, reducing competition between metal ions and hydrogen ions for active sites on the xerogel surface. Moreover, the surface charge also affects the adsorption process of Cu²⁺ ions.

As shown in Figure 4a (insert), the pH_pzc for SiO₂ xerogel was close to 2.9 and the adsorbent surface was positively charged, which promotes the adsorption of copper ions at pH > pH_pzc. Hence, at low pH values, low adsorption levels are observed. Increasing the pH can significantly improve the adsorption property of xerogel for removing Cu²⁺ ions. At pH levels of 2–6, the metal is present in the solution as an ion. Further increases in the solution pH above 6 leads to the precipitation of Cu²⁺ ions as poorly soluble hydroxides. To avoid sediment formation, a pH value of 5.0 was chosen for the subsequent study of adsorption processes. The selected pH value of 5 is above the point of zero charge (pzc) for the SiO₂ xerogel (pH_pzc = 2.9). Due to the negatively charged surface of the xerogel, electrostatic interactions between Cu²⁺ ions and the surface in addition to ion exchange can also be the driving forces leading to adsorption. The mechanism of the physical and chemical adsorption of Cu²⁺ is shown in Figure S2.

To study the dependence of Cu²⁺ ion adsorption on temperature, experiments were conducted with three samples at temperatures of 25, 30, and 40 °C. After adding xerogel, allowing the samples to stand for 30 min, and further filtering the solutions, the residual concentration of Cu²⁺ ions was determined. The research results showed that with increasing temperature, the degree of Cu²⁺ ion adsorption on silicon dioxide xerogel increased by an average of 0.45% per degree [74].

The adsorption capacity of xerogels was determined by immersing them in a copper-containing solution for 120 min. Samples were taken every 20 min. After separating the xerogel by filtration, the solutions were analyzed for Cu²⁺ ion content. As shown in the results (Figure 4b), there is an increase in adsorption capacity with increased contact time between phases, especially up to 20 min of contact for the SiO₂ xerogel sample. With further increases in contact time between phases, the increase in the adsorption capacity is insignificant. The optimal contact time for the studied sample was found to be 80 min.

The calculation showed that, over a contact time of 80 min, the maximum adsorption capacity for Cu²⁺ ions was 67.5 mg/g, with an initial concentration of Cu²⁺ ions in the solution being 500 mg/L (Figure 5a).

By its appearance, the adsorption isotherm for Cu²⁺ ions can be classified as type I Langmuir isotherms. For isotherms of this class, the initial part is curved relative to the concentration axis, as with an increase in the fraction of occupied adsorption sites, it becomes more difficult for molecules of the adsorbate to find vacant space. At a certain concentration of the adsorbate, adsorption reaches a limit. In this case, physical adsorption takes place. The almost vertical portion of the isotherm at low concentrations indicates a strong affinity of the adsorbate for the adsorbent and extremely strong adsorption [87]. According to the Langmuir isotherm, only one layer of molecules can be adsorbed on the surface of the adsorbent. As the concentration of copper ions exceeds 300 mg/L, the surface of the xerogel becomes saturated with adsorbate ions.

The adsorption data of copper ions by silica xerogel in Figure 5a were investigated by Langmuir and Freundlich adsorption isotherm models. The Langmuir and Freundlich isotherm plots for the adsorption of Cu²⁺ onto SiO₂ xerogel are presented in Figure 6a,b. Of the two linearized plots of the Langmuir and Freundlich isotherms, the Langmuir isotherm provides the best fit to the experimental data with a regression coefficient R2 of 0.9949, while for the Freundlich isotherm value is 0.9026.

This model assumes that the adsorption process was carried out by the monolayer adsorption of copper ions on the active sites of silica xerogel with equal affinity. The parameters of the adsorption isotherms are listed in

Table 1. Parameters of the adsorption isotherm of copper (II) ions.

Langmuir Model			Freundlich Model
KL, l/mg	Am, mg/g	R2	KF, (mg/g)·(l/mg)1/n	1/n	R2
0.07	71.4	0.9949	17.8	0.21	0.9026

.

The Langmuir model is based on the fact that a monomolecular layer of adsorbate is formed on the surface of the sorbent, and all active centers have equal energy and enthalpy.

The linear form of the Langmuir equation is as follows:

$${\displaystyle \frac{C_e}{A}}={\displaystyle \frac{1}{K_L·A_m}}+{\displaystyle \frac{C_e}{C_m}}$$

K_L and A_m—the constants of the Langmuir equation; A, A_m—the amount of Cu²⁺ sorbed per 1 g of sorbent at equilibrium and the adsorption capacity of the sorbent at saturation, respectively, mg/g; and C_e—the equilibrium concentration of copper in the solution, mg/L.

The constants of the equation were calculated from the slope and intersection of the lines on the graph in the corresponding coordinates of the linear equation C_e/A from C_e.

Using the Langmuir adsorption model, we estimated the dimensionless equilibrium parameter (R_L) shown in Equation (3).

$$R_L={\displaystyle \frac{1}{1+K_L·{\left[{\mathit{Cu}}^{2+}\right]}_{\mathit{in}}}}$$

(3)

The adsorption favorability of Cu²⁺ by silica xerogel was evaluated based on the R_L value, which describes the adsorption as unfavorable (R_L > 1), linear (R_L = 1), favorable (0 < R_L < 1), and irreversible (R_L = 0). In this study, the R_L value was 0.75, indicating a favorable adsorption process [36,88]. Using the Langmuir adsorption isotherm model, the maximum adsorption capacity was calculated to be 71.4 mg/g, which is not much different from the practically obtained value. There is a dependence of the dimensionless parameter R_L on the initial concentration of copper (II) ions for TEOS silica xerogel (Figure S3). According to Figure S3, it is evident that the values of the parameter R_L are between 0.17 and 0.39, which means that the adsorption process of copper (II) ions is favorable and follows monolayer adsorption [50,51].

Based on the values of (R_L) obtained from the approximation of the Langmuir–Koble (K_L) equation using Equation 3, there is a dependence on the initial concentration of Cu²⁺ ions (Figure S3). With increasing concentrations of Cu²⁺ ions, the R_L values decrease, indicating a reduction in the number of free active adsorption centers. At the same time, the R_L values remain within the range of 0 to 1, suggesting that active centers are still in excess compared to the Cu²⁺ ions, and adsorption can occur either as monolayer adsorption or multilayer adsorption. However, at high concentrations, precipitation is more likely to occur, which we observed visually when the concentration of Cu²⁺ ions were significantly elevated.

The maximum removal efficiency for copper (II) ions was 90%. From Figure 5b, it can be seen that at low initial metal ion concentrations (from 0 to 50–100 mg/L), removal is better, then it sharply decreases (from 100 to 200 mg/L) and gradually decreases with further increases in concentration.

Elemental mapping of the SiO₂ xerogel sample (Figure S4) and energy-dispersive spectra (Figure S4b) shows the adsorption of Cu²⁺ ions on the surface of the sorbent. The obtained data allow for a qualitative assessment of Cu²⁺ adsorption. For a more detailed understanding of the sorption mechanism and processes occurring on the surface of SiO₂ xerogel, images of the surface of the adsorbent particles after adsorption were obtained. TEM images of the SiO₂ xerogel sample after adsorption are presented in Figure 7. As can be seen from the obtained data, darker areas in the images correspond to copper ions and their compounds formed on the surface of SiO₂ (Figure 7a). The highlighted area of the sorbent shows the formation of copper compounds with a crystalline structure on the surface of the xerogel (Figure 7b).

To determine the nature of the formed compound, XPS spectra of the xerogel sample after adsorption were obtained. Figure 8a presents O1s spectra for the SiO₂ sample. Unlike the original O1s peak, a new peak at 532.7 eV appears during deconvolution, corresponding to metal hydroxides [89], and the peak corresponding to the hydroxyl group shifts by 0.6 eV. Weak adsorption occurs on the surface of the xerogel, and since the penetration depth of XPS analysis is small, it is not always possible to see the complete picture.

To study the mechanism of copper (II) ion sorption by xerogels, IR spectra of both the initial adsorbent samples and those after use were obtained. The experimental data obtained are shown in Figure 8b.

In Figure 8b, the broad peaks at 3423–3451 cm⁻¹ are due to the stretching vibrations of O-H groups on the xerogel surface and adsorbed water molecules. The peak at 1631–642 cm⁻¹ is due to the deformation vibrations of O-H groups and indicates the presence of physically sorbed water. The bands at 1079–1094 cm⁻¹ and 800 cm⁻¹ are due to the asymmetric and symmetric vibrations of Si-O-Si bonds. The bands at 975 cm⁻¹ are attributed to the stretching vibrations of Si-O groups [90]. From the analysis of the spectra, it follows that the samples correspond to silicon dioxide in the amorphous state [91,92].

The sample before adsorption shows a high transmittance at 1079 cm⁻¹, which indicates a high degree of a three-dimensional network of the Si–O–Si xerogel. After adsorption, a shift in the peak at 975 cm⁻¹, corresponding to the silanol groups Si–OH, occurs, which is associated with the adsorption process. The sample after adsorption is characterized by a decrease in the peaks at 1079–1089 cm⁻¹, which indicates the partial destruction of the silica matrix during the adsorption process [93].

3.3. Competitive Ions

It is well established that competitive cations such as Zn²⁺ and Cd²⁺ are commonly present in wastewater and can influence the adsorption process of Cu²⁺ ions. To investigate the impact of these co-existing ions on Cu²⁺ removal, Zn²⁺ and Cd²⁺ were individually added to the solution, as well as tested under combined presence with Cu²⁺. The effect of these competing cations was studied at four concentrations (50, 100, 200, and 300 mg/L) by adding them to Cu²⁺ solutions and determining the removal efficiency of copper ions.

The experimental data presented in Figure 9a indicate that Zn²⁺ ions do not significantly affect Cu²⁺ adsorption under the investigated concentrations, with only a slight decrease observed in the percentage of adsorbed Cu²⁺ ions. In contrast, Cd²⁺ ions exhibit a more pronounced effect on Cu²⁺ removal compared to Zn²⁺. The adsorption efficiency of copper ions decreases noticeably in the presence of Cd²⁺, and this effect is observed at lower initial concentrations of copper ions. Notably, at a Cd²⁺ concentration of 200 mg/L, the removal rate of Cu²⁺ was reduced by half.

The removal efficiency of copper ions reached only 84% and 60% in solutions with initial concentrations of 50 mg/L for Zn²⁺ and Cd²⁺, respectively. Furthermore, the removal rate decreased by 6% and 30% in solutions containing Zn²⁺–Cu²⁺ and Cd²⁺–Cu²⁺ systems.

The preferential adsorption of Cu²⁺ at high concentrations and low levels of competing ions can be attributed to several factors. Cu(II) has a smaller ionic radius and is more electronegative compared to Cd(II) and Zn(II), which may enhance its adsorption affinity under these conditions [94,95].

Based on the obtained results, the efficiency of removal for both Cu²⁺ and Zn²⁺ from model plating wastewater was investigated (Figure 9b). The use of the SiO₂ xerogel at a copper ion concentration of 38 mg/l and zinc ion concentration of 15 mg/l allows them to be completely removed in 30 min. The removal efficiency is 98%.

3.4. Regeneration

The study of the desorption of adsorbed Cu²⁺ ions provides insights into the involvement of surface and intraporous hydroxyl groups, as well as the stability and regeneration potential of SiO₂ xerogel. Practical applications of adsorbents necessitate the existence of crucial properties such as longevity and reusability after regeneration. The SiO₂ xerogel was subjected to adsorption–regeneration cycles over the course of five cycles. Saturation of the SiO₂ xerogel with Cu²⁺ ions was performed in a solution with an initial concentration of 300 mg/L under optimal adsorption conditions. Regeneration of the xerogel was conducted using a 0.1 M HCl solution. The obtained results are presented in Figure 10.

As depicted in Figure 10, the efficiency of Cu²⁺ removal by SiO₂ xerogel decreased by 20% after the first regeneration cycle. During subsequent cycles, the adsorption efficiency declined to 14–15% following the fourth and fifth usage cycles. This reduction can likely be attributed to the inadequate recovery of hydroxyl groups within the pores during the first regeneration cycle. Furthermore, hydrochloric acid regeneration may also lead to the replacement of surface hydroxyl groups with chloride ions, which significantly reduces the effectiveness of the adsorption process for the removal of Cu²⁺ ions. This confirms that surface hydroxyl groups play a critical role in the removal process of Cu²⁺ ions.

4. Conclusions

Samples of SiO₂ xerogels were synthesized using TEOS as a precursor. The SiO₂ xerogel sample has a large specific surface area of 346.75 m²/g, which is an advantage when using the sorbent to remove heavy metal ions. The study of the adsorption properties showed that the obtained samples can be used in a wide range of pH values of the treated water. With increasing temperature, the adsorption capacity of the xerogels increases by 0.45% per degree. The optimal contact time of the xerogel samples was 80–120 min. The obtained experimental data on the adsorption removal of copper (II) ions using the synthesized samples indicate a fairly high efficiency of the sorption removal of Cu²⁺ from aqueous solutions. The maximum value of the adsorption capacity of the SiO₂ xerogel is 67.5 mg/L. In their appearance, the adsorption isotherms for Cu²⁺ ions for the xerogel samples can be compared to Langmuir I isotherms. In the region of low equilibrium concentrations, a vertical initial section of the isotherms is observed, which indicates chemisorption on the surface of the sorbents. In the region of low initial concentrations of copper (II) ions, the maximum degree of removal is observed at around 90%. Analysis of SiO₂ xerogel samples after copper (II) adsorption from aqueous solutions using XPS, EDX, and IR spectroscopy showed that adsorption mainly occurs through the substitution of hydroxyl ions associated with silicon atoms. The adsorption process occurs through the process of physical sorption. However, the decrease in adsorption capacity upon repeated use suggests that further optimization—such as improving regeneration techniques or enhancing structural stability—is necessary to ensure durability and cost-effectiveness in practical applications.