Highly Effective Lead Ion Adsorption by Manganese-Dioxide-Supported Core-Shell Structured Magnetite

In this research, a magnetic core-shell composite, consisting of a Fe₃O₄ core and a silica shell (called Fe₃O₄@SiO₂), was developed and then functionalized via MnO₂ grafting at different MnO₂ deposition levels (termed Fe₃O₄@SiO₂-MnO₂). The resulting materials were characterized by X-ray fluorescence, X-ray diffraction, a vibration sample magnetometer, transmission electron microscopy, N₂ adsorption-desorption, zeta-potential studies and X-ray photoelectron spectroscopy. Visualizations showed that Fe₃O₄@SiO₂-MnO₂ had a magnetite core with size of 100 nm, overlaid by a rough silica shell and a relatively loose MnO₂ deposition. The Pb(II) adsorption onto the composites was also assessed. It was found that MnO₂ deposition on the Fe₃O₄@SiO₂ surface enhanced Pb(II) adsorption, and the Pb(II) adsorption amount was highly correlated to the MnO₂ deposition level. The adsorption kinetics of Pb(II) followed pseudo-second-order kinetics, and the adsorption rate could be decreased by increasing the initial concentration of Pb(II). A higher pH resulted in enhanced Pb(II) adsorption, which slightly increased with the coexistence of Na⁺ and Ca²⁺, along with the presence of dissolved humic acid. The adsorbent could easily be separated and recovered under the action of the external magnetic component and it displayed stable adsorption behaviour over four adsorption-desorption periods. The results emphasize the high potential of Fe₃O₄@SiO₂-MnO₂ materials for the adsorptive removal of Pb(II) in water.

Introduction

Lead (Pb(II)) pollutant in municipal and industrial wastewater poses a serious threat to public health and ecological systems (Liu et al., 2018). Different treatment techniques, involving chemical precipitation (Li et al., 2022), ion exchange (Li et al., 2017), adsorption (Hussain et al., 2021), and membrane separation (Hamid et al., 2020), have been used to eliminate Pb(II) pollution, among which adsorption has the highest removal efficiency and ease of operation. Therefore, the design and preparation of effective adsorbents for aqueous Pb(II) removal are being widely researched.

Some metal oxides–such as Fe₂O₃, Al₂O₃, and MnO₂–have been evaluated for adsorbing Pb(II) from wastewaters, among which MnO₂ has exhibited promising potential for the adsorptive removal of Pb(II) (Zhang et al., 2020b; Yadav et al., 2021; Ghaedi et al., 2022). Zhang et al. (2017a) and Li et al. (2021) found that MnO₂ presented a strong affinity for Pb(II) under acidic conditions and played a significant role in controlling the geochemical cycle of heavy metal ions owing to its high specific surface area and negatively charged surface over an extensive pH range. However, pure nanosized MnO₂ particles tend to aggregate in water. The separation of MnO₂ nanoparticles requires energy-intensive filtration, increasing the total cost and leading to filter clogging (Zhu et al., 2015).

Magnetic separation is quicker and more efficient than traditional filtration when used to separate suspended magnetic particles (Xia and Liu, 2021). Fe₃O₄ and γ-Fe₂O₃ are common magnetic agents, exhibiting superparamagnetic behaviour and strong magnetization. Combining their magnetic efficacy with that of MnO₂ nanoparticles can help mitigate the limitation of MnO₂ nanoparticles in terms of their slow separation from water. However, magnetic Fe₃O₄ and γ-Fe₂O₃ are not stable under acidic conditions (Bandar et al., 2021). Significant research effort has been devoted to enhancing the stability of magnetic iron oxides by coating them with polymers (Chen et al., 2018), lauric acid (Zhu et al., 2019), carbon (Wang W. et al., 2015), ZrO₂ (Gugushe et al., 2021) and SiO₂ (Vishnu and Dhandapani, 2020). Notably, SiO₂-based coatings are very stable under acidic conditions and highly resistant to oxidants and reductants (Vishnu and Dhandapani, 2020). Hence, they can act as ideal shell composites to shield the inner magnetic cores. Furthermore, SiO₂ has a high specific surface area and abundant hydroxyl groups, which facilitates facile functionalization. For instance, Huang et al. (2014) used M-Fe₃O₄@SiO₂/ZnO for the photocatalytic degradation of methyl group, finding that SiO₂ incorporation prevented the agglomeration of the Fe₃O₄ nanoparticles and enhanced the photocatalytic performance. Vishnu and Dhandapani (2020) synthesized an amino-functionalised silica-coated magnetic nanoparticle with high stability under acidic conditions. However, there have been few reports on the incorporation of MnO₂ onto the surface of magnetic core-shell nanoparticles. Zhang et al. (2015) prepared a Fe₃O₄@SiO₂@MnO₂ composite as an adsorbent for the decolouration of methyl orange, which consisted of a magnetic Fe₃O₄ core, SiO₂ shell–with thickness of 40 nm–and MnO₂ nanoparticles adhering to the surface of SiO₂ body, with a diameter of about 10 nm. However, the thick SiO₂ body and large MnO₂ nanoparticles decreased the saturation magnetisation (M_s) of Fe₃O₄ from 82.47 emu g⁻¹–29.41 emu g⁻¹, which markedly restricted the efficiency of magnetic separation and recovery of the adsorbent. Hence, we hypothesize that a nanocomposite of Fe₃O₄ core, a thin SiO₂ shell and a monolayer of active MnO₂ will exhibit a high adsorption ability and separability for Pb(II). However, to the best of our knowledge, no such study has been reported to date.

Magnetic nano-adsorbents were synthesized in this study by grafting MnO₂ with various of concentrations onto the surface of Fe₃O₄@SiO₂ core-shell nanoparticles. The characterization of the adsorbents included X-ray fluorescence (XRF), powder X-ray diffraction (XRD), transmission electron microscopy (TEM), vibration sample magnetometry (VSM), N₂ adsorption-desorption, X-ray photoelectron spectroscopy (XPS) and zeta-potential tests. The Pb(II) adsorption capabilities of the materials including the adsorption ability, kinetics, the impact of water chemistry on adsorption, and the regeneration of the saturated adsorbents, were investigated to evaluate the performance of the novel adsorptive materials.

Materials and Methods

Adsorbent Preparation

The Fe₃O₄ nanoparticles were prepared using the procedure described by Huang et al. (2014) and Shi et al. (2021). In brief, 3.0 g of FeCl₃·6H₂O, 2.0 g of polyvinylpyrrolidone and 7.2 g of sodium acetate were dissolved in 60 ml ethylene glycol at 50°C. This was followed by transferring the compound to a 100 ml stainless steel autoclave lined with Teflon and heating it to 200°C for 8 h. Once the mixture’ temperature dropped to room temperature, the black solid was gathered through magnetic separation, repeatedly washed in ethanol and deionized water, and then dried at 60°C in a vacuum for 12 h.

After dispersing 1.0 g of Fe₃O₄ nanoparticles in 200 ml of deionized water at 80°C, 20 ml of 1.0 mol L⁻¹ Na₂SiO₃ solution was added to the suspension with vigorous agitation. The pH was adjusted to 6.0 using 2.0 mol L⁻¹ HCl solution within 1 h, followed by 3 h of vigorous agitation at 80°C. It was necessary to utilize magnetic separation to collect the resulting nanoparticles, which were then thoroughly washed with deionized water, and followed by drying for 12 h in a vacuum at 60°C. The material obtained is referred to as Fe₃O₄@SiO₂.

KMnO₄ as the source of manganese was used to prepare MnO₂-functionalized Fe₃O₄@SiO₂ nanoparticles. One gram of Fe₃O₄@SiO₂ nanoparticles was suspended in 100 ml of deionized water, and the predicted amount of KMnO₄ solution was added. The mixture was then added dropwise to 40 ml of 30% H₂O₂ solution. Within 0.5 h of mechanical stirring, the pH of this solution was adjusted to 7.0 by adding 1.0 mol L⁻¹ HNO₃. Following that, the reaction mixture was agitated at 25°C for 3 h. The final compound was collected through magnetic separation and repeatedly washed with deionized water before drying at 60°C in a vacuum for 12 h. The subsequent components were designated as Fe₃O₄@SiO₂-MnO₂(X), where X denotes the concentration of MnO₂ (wt. per cent).

Material Characterization

The amount of MnO₂ present in the adsorbents was measured using an ARL9800XP X-ray fluorescence (XRF) spectrometer (Thermo Electron, Switzerland). The X-ray diffraction (XRD) patterns of the materials were recorded using a Rigaku D/max-RA powder diffractometer connected to a Cu Kα radiation source over a wide range of angles (10–70°) (Rigaku, Japan). An electron microscopy, the JEM-2100 (JEM-2100, JEOL, Japan), was utilized to analyse the morphology of the adsorbents under investigation. Through the N₂ adsorption-desorption operations performed on a Micromeritics ASAP 2020 analyser (Micromeritics Instrument Co., Norcross, GA, United States), we evaluated the Brunauer–Emmett–Teller (BET) surface area at −196°C (77 K). In this study, an XPS examination was carried out using a PHI5000 VersaProbe equipped with a monochromatized Al Kα excitation source (hv = 1,486.6 eV) (ULVAC-PHI, Japan). The binding energy was calibrated using the C 1s peak (284.6 eV). A vibrating sample magnetometer was used to determine the magnetic characteristics of the sample (Lake Shore VSM 7410, United States). A Zeta Potential Analyser was utilized to determine the nanoparticles’ surface Zeta (ζ) potentials (Zeta PALS, Brookhaven Instruments Co., United States). To ensure proper dispersion 24 h before the zeta potential measurement, 40 mg of the material was spread evenly across 400 ml of 1.0 mmol L⁻¹ NaNO₃ solution, and various solution pH values were obtained by adding 0.1 mol L⁻¹ HNO₃ or NaOH.

Batch adsorption Experiments

We determined the Pb(II) adsorption isotherms by conducting a series of batch adsorption tests. A 40 ml glass vial was filled with 40 ml of Pb(II) solution with various initial concentrations and capped with a screw cap containing 20.0 mg of adsorbent. The pH of the combination was maintained at 4.0 ± 0.2. The samples were then oscillated in an orbital shaker at 25°C for 24 h. Preliminarily kinetic trials (data available upon reasonable request) indicated that the adsorption time (24 h) was sufficient to reach an apparent adsorption equilibrium. Following the equilibrium adsorption, the adsorbent was magnetically isolated from the aqueous solutions. When determining the residual Pb(II) concentrations in aliquots, atomic absorption spectroscopy (AAS) (Hitachi Z-8100, Japan) was utilized. The equilibrium amount (mg g⁻¹) of Pb(II) adsorption was determined as following:

$q_e=\frac{\left(C_0-C_e\right)V}{M}(1)$

The initial and equilibrium concentrations of Pb(II) are C₀ and C_e (mg L⁻¹), respectively. The total volume of Pb(II) solution is V (L) and the adsorbent mass M is measured in grams.

The Pb(II) adsorption data were fitted to the Langmuir and Freundlich models, which were often used to describe the interaction between the adsorbate and the adsorbent. The Langmuir and Freundlich models could be expressed as Eqs. 2, 3, respectively:

$\frac{C_e}{q_e}=\frac{C_e}{q_m}+\frac{1}{q_{m}b}(2)$

$\log q_e=\log K_f+\frac{1}{n}\log \left(C_e\right)(3)$

The maximum adsorption capacity is represented by q_m (mg g⁻¹), and b (L mg⁻¹) is the Langmuir equilibrium constant. K_f (mg g⁻¹) is the Freundlich coefficient, which indicates the adsorbent’s adsorption capacity, and n is the linearity index, which indicates the adsorption intensity.

Adsorption Kinetics

To investigate the Pb(II) adsorption kinetics, 250.0 mg of Fe₃O₄@SiO₂-MnO₂(1.54) was put into a round-bottom flask with 500 ml of aqueous solution with different Pb(II) concentrations (10.0 mg L⁻¹ and 25.0 mg L⁻¹) under vigorous stirring. About 5 ml of the solution was sampled using a pipette at preset time separations. The suspended magnetic nanoparticles were fast filtered by a 0.22 μm filter membrane. AAS was used to determine the concentration of Pb(II) in the filtrate. The quantity of Pb(II) adsorption was calculated using the following formula:

$q_t=\frac{\left(C_0-C_t\right)V}{M}(4)$

An adsorbing quantity, known as “q_t” (mg g⁻¹) is dependent on t (min). C₀ and C_t are the Pb(II) solution concentration (mg L⁻¹) at initial and time t of (mg L⁻¹). The volume of Pb(II) solution is measured by V (L) and the adsorbent mass M is measured in grams.

Effect of Water Chemistry on Pb(II) adsorption

Using Fe₃O₄@SiO₂-MnO₂(1.54) at a temperature of 25°C, the influence of pH on Pb(II) adsorption was investigated. A 25.0 mg L⁻¹ Pb(II) solution with variable pH was added to Teflon-lined, cap-on glass tubes containing 20.0 mg of Fe₃O₄@SiO₂-MnO₂(1.54), and the tubes were shaken at 25°C for 48 h. Equation 1 was used to compute the amount of Pb(II) adsorption, and the residual Pb(II) concentration was evaluated by AAS. The effects of co-existing ions on Pb(II) adsorption onto Fe₃O₄@SiO₂-MnO₂(1.54) were carried out with the presence of various NaCl or CaCl₂ concentrations (0.01–0.20 mol L⁻¹) at pH 4.0 ± 0.2, and at 25°C. The effect of dissolved humic substances on Pb(II) adsorption was studied by comparing the adsorption isotherms of Pb(II) in the presence and absence of dissolved humic acid (Sigma-Aldrich) at a concentration of 10.2 mg L⁻¹.

Desorption and Regeneration of the Saturated Adsorbent

To determine the reusability of the synthesized sorbent, eight successive periods of Pb(II) adsorption and regeneration on Pb(II)-saturated Fe₃O₄@SiO₂-MnO₂ (1.54) were conducted. Specifically, 50.0 mg of Fe₃O₄@SiO₂-MnO₂(1.54) was suspended in 100 ml of 25.0 mg L⁻¹ Pb(II) solution and maintained at 25°C for 24 h with vigorous stirring. The Pb(II)-saturated adsorbent was then split magnetically, and the residual Pb(II) concentration was measured by methods described previously. The resultant Pb(II)-saturated adsorbent was regenerated by suspending it in 20 ml of 1.0 mol L⁻¹ HCl solution and keeping it for 24 h. Before the next adsorption-desorption period, the regenerated adsorbent was repeatedly cleaned using deionized water until the pH of the supernatant was almost neutral.

Results and Discussion

Characterization of the Adsorbents

The XRD diagrams of Fe₃O₄, Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂-MnO₂ with different MnO₂ concentrations are shown in Figure 1. The major diffraction peaks of Fe₃O₄ were determined with 2θ of 18.3°, 30.1°, 35.5°, 37.1°, 43.1°, 53.5°, 57.1°, and 62.7°, this indicated the existence of magnetite with a cubic spinel framework (Bandar et al., 2021; Panahandeh et al., 2021). For Fe₃O₄@SiO₂, no prominent and new crystalline peak was observed, revealing that SiO₂ component in Fe₃O₄@SiO₂ was amorphous (Bandar et al., 2021). Upon MnO₂ functionalization, a broad peak at 2θ of 23.7° was detected (except for the diffraction peaks of Fe₃O₄@SiO₂), the intensity of which enhanced with increasing MnO₂ concentration, reflecting the presence of amorphous MnO₂ moieties on the Fe₃O₄@SiO₂ surface (Yin et al., 2011; Zhang et al., 2017b). Additionally, the few diffraction peak features of crystalline MnO₂ reflected the low MnO₂ content in the Fe₃O₄@SiO₂-MnO₂ adsorbents.

FIGURE 1

FIGURE 1. XRD patterns of the samples.

The BET surface areas of the adsorbents identified through N₂ adsorption-desorption are summarized in Table 1. These specific surface areas of the Fe₃O₄@SiO₂-MnO₂ adsorbents were higher than that of Fe₃O₄@SiO₂, revealing an increase in the surface area upon functionalization. The XRF analysis results of MnO₂ loading amounts are listed in Table 1. The MnO₂ contents of Fe₃O₄@SiO₂-MnO₂ adsorbents were 0.32, 0.62, 0.97, 1.54, and 1.92 (wt%), reflecting the successful grafting of MnO₂ onto the Fe₃O₄@SiO₂ surface.

TABLE 1

TABLE 1. BET surface areas and contents of Fe₃O₄, SiO₂ and MnO₂ of the sorbents.

The TEM images of Fe₃O₄, Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂-MnO₂ are displayed in Figure 2. The Fe₃O₄ image (Figure 2A) showed the aggregation of spherical clusters with an average diameter of 100 nm. In Figure 2B, rough SiO₂ with a thickness of approximately 3 nm outside the Fe₃O₄ core was identified, reflecting that Fe₃O₄@SiO₂ consists of Fe₃O₄ cores and SiO₂ shells. After MnO₂ functionalization (see Figures 2A,C,D relatively loose body was obviously observed on the Fe₃O₄@SiO₂ surface, reflecting the successful deposition of MnO₂ onto the Fe₃O₄@SiO₂ surface.

FIGURE 2

FIGURE 2. TEM images of (A) Fe₃O₄, (B) Fe₃O₄@SiO₂, and (C,D) Fe₃O₄@SiO₂-MnO₂(1.54).

The magnetization curves determined for Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂-MnO₂(1.54) are presented in Figure 3A. The saturation magnetisation (M_s) of Fe₃O₄@SiO₂ was determined to be 84.1 emu g⁻¹, whereas the M_s of Fe₃O₄ nanosphere was reported to around 100.3 emu g⁻¹ (Shi et al., 2021). The lower saturation magnetization of Fe₃O₄@SiO₂ than that of Fe₃O₄ could be attributed to the SiO₂ shell. After MnO₂ modification, the saturation magnetization of Fe₃O₄@SiO₂-MnO₂(1.54) continuously reduced to 56.8 emu g⁻¹, further confirming the successful deposition of MnO₂ onto the Fe₃O₄@SiO₂ surface. VSM results also showed that the samples were superparamagnetic with no remanent coercivity (Xu et al., 2018). As evident in Figure 3B, the effective separation and recovery of Fe₃O₄@SiO₂-MnO₂(1.54) could be achieved in seconds with the assistance of an external magnetic field. Further, the nanoparticles quickly redispersed while the magnetic field was removed, reflecting the potential for using MnO₂-functionalized Fe₃O₄@SiO₂ magnetic nanoparticles as recyclable adsorbent in water and wastewater treatment.

FIGURE 3

FIGURE 3. (A) Magnetization curves of Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂-MnO₂(1.54) and (B) the magnetic separation and redispersion of Fe₃O₄@SiO₂-MnO₂(1.54).

The surface zeta potentials of Fe₃O₄, Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂-MnO₂(1.54) are displayed in Figure 4, as a function of pH. The surface potential gradually decreased with increases in the pH for all of the samples, likely due to the deprotonated surface hydroxyl groups. The isoelectric point (IEP) of the Fe₃O₄ nanoparticles was found to be 5.8, which was in line with the value found in literature (Bandar et al., 2021). The IEP of Fe₃O₄@SiO₂ was found to be 2.4, further confirming the successful amorphous silica coating (Wang et al., 2013). More specifically, the IEP of Fe₃O₄@SiO₂-MnO₂(1.54) was found to be approximately 1.4. It is important to note that the IEP of MnO₂ has previously been found to lie within the range of 1–2 (McBride, 1997; Huang et al., 2014). Hence, the lower IEP of Fe₃O₄@SiO₂-MnO₂(1.54) compared with Fe₃O₄@SiO₂ was attributed to the MnO₂ grafting on the Fe₃O₄@SiO₂ surface.

FIGURE 4

FIGURE 4. Surface zeta potentials of the samples as a function of pH.

The XPS spectra of Mn 2p and O 1s are presented in Figures 5A,B, respectively. In the spectra of Fe₃O₄@SiO₂-MnO₂ adsorbents, Mn 2p peaks were found, reflecting the successful grafting of the MnO₂ functional groups onto the Fe₃O₄@SiO₂ surface. The typical Mn 2p XPS spectrum of Fe₃O₄@SiO₂-MnO₂ nanoparticles revealed a spin-orbit doublet with Mn 2p_1/2 (653.3 eV) and Mn 2p_3/2 (641.3 eV), which was separated into a mixed-valent manganese system comprising Mn³⁺ and Mn⁴⁺ (Zhang et al., 2015; Zhang et al., 2017b). Owing to the low MnO₂ content and high MnO₂ dispersion, the intensity of the Mn 2p_1/2 and Mn 2p_3/2 peaks of Fe₃O₄@SiO₂-MnO₂(0.32) and Fe₃O₄@SiO₂-MnO₂(0.62), respectively, were not the typical intensities reported for manganese oxides in literature (Zhang et al., 2020a). In the case of the O 1s spectrum (Figure 5B), two peaks at 529.8 and 532.1 eV appeared in the spectra of the samples, indicative of the lattice oxygen (i.e., Mn-O, Si-O and Fe-O) and surface oxygen (O-H) (Wan et al., 2020; Minale et al., 2021), respectively. Additionally, the area ratio of surface oxygen increased with increasing MnO₂ deposition levels, which was ascribed to the abundance of hydroxyl groups on the functionalized manganese oxides (Wang et al., 2013).

FIGURE 5

FIGURE 5. XPS spectra of Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂-MnO₂ in (A) the Mn 2p and (B) the O 1s region.

Adsorption Isotherms

Figure 6A shows the isotherms of Pb(II) adsorption onto the adsorbents. Very low quantities of Pb(II) adsorption was detected for Fe₃O₄ and Fe₃O₄@SiO₂, indicating that the Fe₃O₄ and Fe₃O₄@SiO₂ exhibited a negligible affinity for Pb(II) adsorption. In comparison to Fe₃O₄ and Fe₃O₄@SiO₂, Fe₃O₄@SiO₂-MnO₂ exhibited a significant increase in Pb(II) adsorption, which increased as the MnO₂ deposition level rose, indicating that MnO₂ was the active species for Pb(II) adsorption.

FIGURE 6

FIGURE 6. (A) Pb(II) adsorption isotherms and (B) MnO₂ content-normalized Pb(II) adsorption isotherms of the samples. (Adsorbent dosage = 0.5 g L⁻¹, Temperature = 25°C, pH = 4.0 ± 0.2).

Table 2 summarizes the fitting parameters. For Fe₃O₄@SiO₂-MnO₂(0.32), the Adj. R² value (0.976) of the Freundlich model was slightly higher than that of the Langmuir model (0.965), likely because the Fe₃O₄@SiO₂ surface was not entirely covered by MnO₂ at the 0.32% (wt.) deposition level, leading to significant adsorption heterogeneity (Feheem, et al., 2016). In contrast, much higher R² values (0.990–0.994) of the Langmuir model were obtained when the MnO₂ deposition levels increased, indicating homogeneous adsorption sites for Pb(II) adsorption at high MnO₂ deposition levels. Additionally, the b and K_f values increased with increasing MnO₂ deposition levels, confirming that the active adsorption species was MnO₂.

TABLE 2

TABLE 2. Fitting parameters of the Langmuir and Freundlich isotherm models for adsorption of Pb(II) onto the adsorbents.

The calculated q_m of the sorbents suggested that Pb(II) adsorption was not strongly affected by the MnO₂ deposition levels. To obtain deeper insight into the adsorption mechanism, MnO₂-content-normalized adsorption isotherms of Fe₃O₄@SiO₂ with various MnO₂ deposition levels are compared in Figure 6B. The calculated maximum adsorption capacities Q_m upon MnO₂ content normalization are listed in Table 2. For Fe₃O₄@SiO₂-MnO₂ with various MnO₂ contents, the adsorption capacities Q_m decreased with increasing MnO₂ deposition levels, and the highest Q_m was observed for Fe₃O₄@SiO₂-MnO₂(0.32). This is likely due to the high dispersion of nanosized MnO₂ particles at low deposition levels, and thus, the high utilization of MnO₂ as the active site for Pb(II) adsorption. At higher deposition levels, the MnO₂ particles may aggregate, leading to the partial inaccessibility of the adsorption sites. Notably, almost identical adsorption isotherms were acquired for MnO₂ normalization at deposition levels of 1.54 (wt%) and 1.92 (wt%), reflecting similar structural characteristics (e.g., the content of hydroxyl groups and the particle size) (Zhang et al., 2020a). The nearly identical normalized isotherms implied that the outer surface of SiO₂ shell might be entirely covered at the 1.54% (wt.) MnO₂ deposition, which was in good agreement with the XPS results.

MnO₂ is used as an effective adsorbent for Pb(II) removal in wastewater. Wan et al. (2018) found that the maximum adsorption capacity of biochar-supported hydrated manganese oxide for Pb(II) was 23.8 mg g⁻¹ at a pH of 4.5. Wang S. et al. (2015) investigated Pb(II) adsorption on MnO₂-modified biochar acquired an adsorption capacity of 47.1 mg g⁻¹ at an equilibrium concentration of 40 mg L⁻¹. Note that the adsorption capacity usually increases with the equilibrium concentration of the adsorbate, and these high adsorption capacities reported were obtained at relatively high Pb(II) equilibrium concentrations. In the current research, the maximum adsorption of Pb(II) on Fe₃O₄@SiO₂-MnO₂(1.54) was 33.6 mg g⁻¹ Fe₃O₄@SiO₂-MnO₂ at an equilibrium concentration of 24.0 mg L⁻¹ and pH of 4.0, showing that the uptake of Pb(II) at low Pb(II) concentrations with the synthesized sorbents was much higher than the reported results and, therefore, a higher efficiency of Pb(II) enrichment was achieved under acidic conditions.

Adsorption Kinetics

The kinetics of Pb(II) adsorption on Fe₃O₄@SiO₂-MnO₂(1.54) with different initial concentrations are depicted in Figure 7A. In roughly 30 min, equilibrium was reached at both initial concentrations of 10.0 and 25.0 mg L⁻¹ of Pb(II) on the adsorbent. Adsorption kinetics can be better understood using three kinetic models (Yang et al., 2018): the pseudo-first-order model (Eq. 5), the pseudo-second-order model (Eq. 6), and the Weber-Morris model (Eq. 7).

$\log \left(q_e-q_t\right)=\log q_e-\frac{k_{1}t}{2.303}(5)$

$\frac{t}{q_t}=\frac{1}{k_2{q}_e^2}+\frac{t}{q_e}(6)$

$q_t=k_d{t}^{1/2}+I(7)$

where the q_t (mg g⁻¹) is the adsorption amount at time t (min), k₁ (min⁻¹) is the pseudo-first-order rate constant, k₂ (g mg⁻¹ min⁻¹) is pseudo-second-order rate constant, k_d and I denote the diffusion constant and the thickness of the boundary layer, respectively.

FIGURE 7

FIGURE 7. (A) Adsorption of Pb(II) with various initial concentrations on Fe₃O₄@SiO₂-MnO₂(1.54) as a function of the adsorption time, and fitting of Pb(II) adsorption kinetics using the (B) pseudo-first-order model, (C) pseudo-second-order model and (D) Weber-Morris model. (C₀ (Pb(II) = 10.0 mg L⁻¹ and 25.0 mg L⁻¹, respectively, adsorbent dosage = 0.5 g L⁻¹, Temperature = 25°C, pH = 4.0 ± 0.2).

Figures 7B,C show change curves of log(q_e - q_t) with t and t/q_t with t; Table 3 contains the fitting parameters. The kinetic results for Pb(II) adsorption onto Fe₃O₄@SiO₂-MnO₂(1.54) fit this pseudo-second-order kinetic model very well, showing that Pb(II) adsorption is pseudo-second-order and chemisorption-limited (Panahandeh et al., 2021). Pb(II) adsorbed over Fe₃O₄@SiO₂-MnO₂(1.54) was shown to adhere to pseudo-second-order kinetics when the slopes of the t/q_t against t plots were used. The projected q′_cal significant values were very close to the experimental values. There were slower adsorption rates with higher initial Pb(II) concentrations for the Pb(II) adsorption rate constants at 10.0 and 25.0 mg L⁻¹ starting concentrations of Pb(II).

TABLE 3

TABLE 3. Fitting parameters of Pb(II) adsorption kinetics on Fe₃O₄@SiO₂-MnO₂(1.54) using pseudo-first-order, pseudo-second-order, and Weber-Morris models.

The fitted results of Pb(II) adsorption using Weber-Morris model are shown in Figure 7D, and the parameters that best fit the data are listed in Table 3. The plots display multilinearity with different initial Pb(II) concentrations, indicating that more than two procedures are involved in the adsorption. In Figure 7D, the Weber-Morris plots had two linear portions. The first is assigned to the fast adsorption of Pb(II), and the other to the function of the adsorption equilibrium. According to the model, if the adsorption process is mainly controlled by intraparticle diffusion, the q_t - t^1/2 plot will be a straight line passing through the origin (Yang et al., 2018). However, the first portion did not pass through initially indicate that external diffusion played a significant role in the adsorption procedure. Additionally, the intercepts decreased at high initial concentrations, suggesting that the diffusion was not substantially affected by the mass transfer resistance. With initial Pb(II) concentrations of 10.0 and 25.0 mg L⁻¹, respectively, the k_d were 5.5 and 7.5 mg g⁻¹ min^−1/2, indicating a higher diffusion constant at a higher initial concentration. This is likely due to the increasing diffusion driving force caused by the increasing concentration gradient.

Impact of Solution Chemistry on Pb(II) adsorption

The impact of the solution pH on Pb(II) adsorption by Fe₃O₄@SiO₂-MnO₂(1.54) is presented in Figure 8. The Pb(II) adsorption amount monotonically increased from 0.2 to 48.8 mg g⁻¹ in the pH range from 2.0 to 8.0; subsequently, it remained constant. The pH effect on Pb(II) adsorption could be explained by the electrostatic interaction between the adsorbent and Pb(II), and the precipitation of Pb(II) at high pH conditions (>6.0) (Zhang et al., 2017b). Notably, SiO₂ based shell were not stable against alkaline condition. Although the Pb(II) adsorption capacity of the Fe₃O₄@SiO₂-MnO₂(1.54) was relatively high under high pH conditions, each adsorption performance test in this study was conducted at pH = 4.0 ± 0.2. Generally, the Fe₃O₄@SiO₂-MnO₂(1.54) surface was actively charged at solution pH values under the IEP of 1.4; electrostatic repulsive interaction occurred between Pb(II) and the protonated hydroxyl groups (Wang et al., 2020). In contrast, the adsorption sites were negatively charged at solution pH values above the IEP of 1.4, leading to an electrostatic attraction between deprotonated Mn-OH groups and Pb(II) (Zhang et al., 2020b), enhancing the Pb(II) adsorption.

FIGURE 8

FIGURE 8. Effect of the solution pH on Pb(II) adsorption by Fe₃O₄@SiO₂-MnO₂(1.54). (C₀ (Pb(II) = 25.0 mg L⁻¹, adsorbent dosage = 0.5 g L⁻¹, Temperature = 25°C).

Alkaline/Earth metal ions in natural water and wastewater may compete against Pb(II) for available adsorption sites of sorbents. The impacts of co-existing Na⁺ and Ca²⁺ on Pb(II) adsorption are shown in Figure 9A. These electrolytes resulted in an increase in the Pb(II) adsorption, and Ca²⁺ had a more significant impact on adsorption than Na⁺ did, suggesting that Pb(II) adsorption was controlled mainly by a surface complexation model (Zhang et al., 2020b). Notably, the effects of co-existing Na⁺ and Ca²⁺ on Pb(II) adsorption revealed the presence of a preferable adsorbent, which exhibited great potential for Pb(II) treatment in the presence of co-existing cations at high concentrations.

FIGURE 9

FIGURE 9. (A) Effect of co-existing ions on Pb(II) adsorption over Fe₃O₄@SiO₂-MnO₂(1.54), and (B) adsorption isotherms of Pb(II) on Fe₃O₄@SiO₂-MnO₂(1.54) in the absence and presence of dissolved humic acid. (Adsorbent dosage = 0.5 g L⁻¹, Temperature = 25°C, pH = 4.0 ± 0.2).

In aquatic systems, dissolved humic acid may have an effect on the adsorption of Pb(II) (Esfandiar et al., 2022). Figure 9B depicts the adsorption isotherms of Pb(II) on Fe₃O₄@SiO₂-MnO₂(1.54) in the absence and presence of dissolved humic acid at a concentration of 10.2 mg L⁻¹. The results indicate the active effect of dissolved humic acid on Pb(II) adsorption at higher equilibrium Pb(II) concentrations under the tested conditions. This positive effect is attributed to the formation of weakly adsorbing complexes between Pb(II) and humic acid (Zhang et al., 2017a).

Regeneration and Reuse of the Adsorbent

Based on the finding that Fe₃O₄@SiO₂-MnO₂ had a poor adsorption capacity at low pH values, the Pb(II)-saturated adsorbent was thought to be regenerated through the acid treatment. The adsorbent was reused for eight adsorption-regeneration periods and the results are displayed in Figure 10. The adsorption capacity reduced from 31.4 to 25.6 mg g⁻¹ during the initial four adsorption-desorption cycles, likely because of partial active species loss during cycle processes or the incomplete desorption of Pb(II) from the surface of Fe₃O₄@SiO₂-MnO₂(1.54). After four cycles, the Fe₃O₄@SiO₂-MnO₂(1.54) adsorption capacity remained constant, and the sorbent could be efficiently separated from the solution within 10 s by an external magnetic field, indicating that the sorbent was stable and recyclable.

FIGURE 10

FIGURE 10. Pb(II) adsorption by virgin and regenerated Fe₃O₄@SiO₂-MnO₂(1.54) adsorbent within eight adsorption-desorption cycles. (C₀ (Pb(II) = 25.0 mg L⁻¹, adsorbent dosage = 0.5 g L⁻¹, Temperature = 25°C, Pb(II) adsorption pH = 4.0 ± 0.2).

Implication for Real-World Application

Efficient removal of Pb(II) in wastewater and rapid separation of adsorbents have always been the difficulties in the practical application of adsorbents. The as-prepared Fe₃O₄@SiO₂-MnO₂ showed high adsorption capacity and good stability for Pb(II) adsorptive removal from acidic Pb(II)-contaminated water, suggesting potential application in wastewater treatment field. Additionally, after adsorbing Pb(II) in wastewater the effective separation and recovery of Fe₃O₄@SiO₂-MnO₂ adsorbent could be accomplished within seconds with an external magnetic field. Furthermore, the Fe₃O₄@SiO₂-MnO₂ adsorbent could be rapidly redispersed while the magnetic field was removed. This means that only one external magnetic field can be used to quickly and efficiently realize the adsorption and desorption of Pb(II) by the adsorbent, which is beneficial to reduce the cost of adsorptive removal of Pb(II) in wastewater.

Conclusion

In this research, magnetic core-shell sorbents of Fe₃O₄@SiO₂-MnO₂ with various MnO₂ deposition contents were synthesized through deposition-precipitation and used for Pb(II) adsorption. The characterization results revealed that the Fe₃O₄ core was coated by a rough silica shell and a relatively loose MnO₂ deposition. Accordingly, Fe₃O₄@SiO₂-MnO₂ demonstrated increased Pb(II) adsorption in comparison to Fe₃O₄@SiO₂, and the Pb(II) adsorption amount was positively related to the MnO₂ deposition level. Fe₃O₄@SiO₂-MnO₂ could be easily divided and recovered using an outer magnetic component. The sorbent exhibited stable adsorption and regeneration performance after four consecutive adsorption-desorption periods. This adsorption was contained by surface complexation and slightly enhanced by dissolved humic acid and co-existing cations. These findings highlight that Fe₃O₄@SiO₂-MnO₂ is a promising adsorbent for use in the removal of Pb(II) from water and wastewater.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Author Contributions

HZ contributed to data curation, methodology, writing-original draft, and writing-review and editing; SC contributed to data curation and investigation; YS and JW designed all experiments and revised and examined the manuscript; XQ contributed to review and editing; YY contributed to review and editing. All authors have read and agreed to the published version of the manuscript.

Fundings

This work was supported by the National Natural Science Foundation of China (31872179, 31901447, 41701093), the Key R&D Plan Program of Jiangsu Province (BE2020319), the Innovation and Entrepreneurship Program of Jiangsu Province (JSSCBS20211062), the Excellent Doctor in Lvyangjinfeng of Yangzhou City (YZLYJFJH2021YXBS155), and the Blue Project of Yangzhou University.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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