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. 2023 Feb 15;8(8):7626–7638. doi: 10.1021/acsomega.2c07214

Facile Strategy for Fabricating an Organosilica-Modified Fe3O4 (OS/Fe3O4) Hetero-nanocore and OS/Fe3O4@SiO2 Core–Shell Structure for Wastewater Treatment with Promising Recyclable Efficiency

Mohamed A Habila †,*, Mohamed Sheikh Moshab †, Ahmed Mohamed El-Toni ‡,§, Abdulrhman S Al-Awadi ∥,⊥, Zeid A ALOthman †
PMCID: PMC9979343  PMID: 36872962

Abstract

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The development of a sustainable process for heavy metal ion remediation has become a point of interest in various fields of research, including wastewater treatment, industrial development, and health and environmental safety. In the present study, a promising sustainable adsorbent was fabricated through continuous controlled adsorption/desorption processes for heavy metal uptake. The fabrication strategy is based on a simple modification of Fe3O4 magnetic nanoparticles with organosilica in a one-pot solvothermal process, carried out in order to insert the organosilica moieties into the Fe3O4 nanocore during their formation. The developed organosilica-modified Fe3O4 hetero-nanocores had hydrophilic citrate moieties, together with hydrophobic organosilica ones, on their surfaces, which facilitated the further surface coating procedures. To prevent the formed nanoparticles from leaching into the acidic medium, a dense silica layer was coated on the fabricated organosilica/Fe3O4 (OS/Fe3O4). In addition, the prepared OS/Fe3O4@SiO2 was utilized for the adsorption of cobalt(II), lead(II), and manganese(II) from the solutions. The data for the adsorption processes of cobalt(II), lead(II), and manganese(II) on OS/(Fe3O4)@SiO2 were found to follow the pseudo-second-order kinetic model, indicating the fast uptake of heavy metals. The Freundlich isotherm was found to be more suitable for describing the uptake of heavy metals by OS/Fe3O4@SiO2 nanoparticles. The negative values of the ΔG° showed a spontaneous adsorption process of a physical nature. The super-regeneration and recycling capacities of the OS/Fe3O4@SiO2 were achieved, comparing the results to those of previous adsorbents, with a recyclable efficiency of 91% up to the seventh cycle, which is promising for environmental sustainability.

1. Introduction

The rapid pollution of water with heavy metals has become the most prevalent environmental concern.1−5 Heavy metals may alter the water surface as a result of industrial activities which produce polluting effluents that are discharged to the water surface.6,7 Heavy metals have a high ability for persistence and can be transferred to various environmental components, such as soil, plants, food, and drinking water. Human exposure to heavy metals, even with very low traces, is considered hazardous due to the fact that heavy metal accumulation, over time, can lead to higher and dangerous concentrations, and the symptoms start to appear late in the human lifecycle.8−11 For example, manganese is considered as essential element for humans; however, the reported risk of manganese pollution is now reaching higher levels, especially among people who drink manganese-polluted water or those who work in mining and are exposed to air polluted with manganese through direct inhalation.12−14 The commonly reported health effects of excessive exposure to manganese include depression, changes in memory ability, as well as irritability and, in some cases, tremors.15,16 Another example of a heavy metal pollutant is cobalt, which is an essential element at trace levels but is toxic at higher levels of exposure, leading to hematological effects as well as immune system disturbance.17−20 Lead is reported to have toxic effects on humans at high exposure levels in both children and adults. The symptoms include renal and degenerative effects as well as intellectual disabilities.21−25 There are many treatment methods for the remediation of heavy metals from wastewater, such as reverse osmosis, chemical precipitation, ion exchange, filtration, and adsorption.5,26−30

Magnetic nanomaterials, such as ferric(III) oxides, are reported to be promising adsorbents; however, their application to the adsorption of heavy metals is limited due to their instability in acidic media.31 Therefore, the trends in this research field are oriented toward efforts to improve the stability of iron-oxide-based magnetic materials by coating with an additional layer or the incorporation of supporting materials to enhance their stability and dispersion.27,32−34 Furthermore, a wide range of valuable applications are reported for Fe3O4-based nanocomposites, including examples in the medical field,35−38 catalysis,39−42 and energy.43−47 Amiri et al. modified Fe3O4@SiO2-polyglycerol for corrosion inhibitor applications.48 Fe3O4 nanoparticles were modified with l-lysine amino acid to improve their application as an adsorbent.49 Wang et al. fabricated water-soluble Fe3O4 nanoparticles (NPs) for heavy metal ion adsorption with a high efficiency and fast process, achieving a 90% rate of removal from 10 ppm of Pb(II) solution in 2 min.50 Lewandowski et al. fabricated magnetic materials, including Fe3O4@SiO2@meso-SiO2 and Fe3O4@SiO2@meso-SiO2-NH2, as adsorbents for the detection of trace organic compounds, and they recommended their effectiveness as an inexpensive method of detection.51 Weijiang et al. applied chitosan to modify magnetic nanoparticles for the removal of Pb(II), with an adsorption capacity of 83.33 mg/g.52 Maiti et al. prepared iron oxide with fly ash as a cementitious composite by heating at 1000 °C. They applied the material as an adsorbent to lead, chromium, cadmium, copper, and dyes.53 Li et al. implemented the hydrothermal process for the preparation of Nano-PY-MCM-41@Fe3O4 as a novel nanomaterial with a high capacity for U(VI) uptake.54 Ren et al. fabricated Fe3O4 @C by solvothermal synthesis, and it exhibited a tendency to adsorb Cr(VI) and Congo red with a maximum adsorption capacity of 33.35 and 262.72 mg/g, respectively.55 Sheikhmohammadi et al. functionalized Fe3O4@SiO2 with NH2 for the removal of ethylparaben (EtP), with an efficiency of 93% at pH 7, adsorbent dosage of 0.4 g/L, EtP concentration of 50 mg/L, and reaction time of 90 min.56

Fabrication of core–shell structures enables the selection of the core properties and provides the possibility for building suitable shell/shells to produce amazing multifunctional and multilayer materials for various applications including extraction,57 photocatalysis,58 solar energy,59 and hydrogen production.60 The challenges involved in the controlled fabrication of the iron-oxide-/silica-based magnetic core/shell structures is an important and ongoing research topic comprising efforts to enhance the magnetic materials’ stability, functionality and recycling potential for various applications.61,62 The recycling and reuse of core–shell structures are important for the development of sustainable applications.63,64 Most of the previous strategies for the formation and stabilization of Fe3O4-based magnetic core–shell structures focused on the functionalization of the shell and/or shells around the Fe3O4 core.62,65,66 These include Fe3O4@SiO2/ Pr–N = Mo[Mo5O18],67 Fe3O4@SiO2@-SC,68 Fe3O4@SiO2@DMSA,69 Fe3O4@SiO2@HPG-OPPh2-PNP,70 Fe3O4@SiO2@rGO,71 Fe3O4@SiN/β-CD,72 Fe3O4@SiO2@IL-PMO/Pd,73 and Fe3O4–SiO2-2DMDPS.74 Therefore, in this research, we built a strategy for core stabilization and functionalization. The novelty of this work was that it aimed to develop a heterogeneous/magnetic core–shell structure as a stable magnetic adsorbent through efficient controlled adsorption/desorption processes in order to overcome recycling-associated problems. In detail, this work aimed to prepare a novel organosilica-modified Fe3O4 (OS/Fe3O4) and OS/Fe3O4@SiO2 core–shell structure. First, the cores of iron oxide nanoparticles were functionalized and stabilized with organosilica and citrate moieties. Then, the dense silica layer was coated using the Stöber method. The fabricated OS/(Fe3O4)@SiO2 was characterized by XRD, SQUID, FTIR, SEM, and TEM. The sustainable performance of the OS/(Fe3O4)@SiO2 core–shell structure in the adsorption of heavy metal ions was evaluated. Its wastewater application for the removal of cobalt(II), lead(II), and manganese(II) was investigated for sustainable regeneration purposes.

2. Experimental Section

2.1. Synthesis of the Organosilica-Modified Fe3O4 (OS/Fe3O4) and OS/Fe3O4@SiO2 Core–Shell Structure

The magnetic core was formed as described in reviews,75,76 with some modifications. In detail, FeCl3·6H2O was homogenized with sodium citrate, sodium acetate, and 1,4-bis(triethoxysilyl) benzene (organosilica precursor) in an ethylene glycol solvent. The mixture was continuously stirred for approximately 6 h. The mixture was slowly transferred to an autoclave and heated in oven at 200 °C for a certain time. At the end of this process, after reaching room temperature, the formed organosilica-modified Fe3O4 hetero-nanocores were collected, washed with ethanol, and subjected to the dense silica coating using the Stöber method.77,78 The organosilica-modified Fe3O4 hetero-nanocores were dispersed in 210 mL of ethanol/water solution at a ratio of 2:1. Then, 2 mL of ammonia solution was added, followed by the slow addition of TOES (2 mL through drop-by-drop addition). The reaction mixture was stirred for 8 h. Finally, the formed OS/(Fe3O4)@SiO2 nanoparticles were isolated using a magnet and washed three times with ethanol. The unmodified Fe3O4 was prepared by the same procedure as Fe3O4 hetero-nanocores but without adding 1,4-bis(triethoxysilyl) benzene, and the unmodified Fe3O4 was subjected to the same dense silica coating procedure to produce unmodified Fe3O4@SiO2 core–shell for comparison purposes.

2.2. Characterizations

The organosilica-modified Fe3O4 (OS/Fe3O4) and OS/Fe3O4@SiO2 core–shell nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR). TEM characterization was conducted using a 200 kV (JEOL JEM-2100F-UHR) field-emission instrument. The X-ray diffraction measurements were conducted using the PANalyticalX’Pert PRO MPD (Netherlands) with Ni-filtered Cu Kα radiation (45 kV, 40 mA). The FTIR spectra were acquired by means of a Unicam 4000 FTIR spectrometer using a KBr plate in the range of 400–4000 cm–1. The magnetic character of the samples was measured using a superconducting quantum interference device (SQUID) magnetometer. The unmodified Fe3O4, unmodified Fe3O4@SiO2 core–shell and amorphous silica were investigated for comparison in some characterization tools.

2.3. Stability Assessment

An amount of 0.1 g of the prepared structures including unmodified Fe3O4, unmodified Fe3O4@SiO2 core–shell, OS/Fe3O4 hetero-nanocores, OS/Fe3O4@SiO2 core–shell structure, and OS/Fe3O4@SiO2 after 7 uses were mixed with 10 mL of HCl with various concentrations such as 0.05, 0.1 and 0.2 M in separate tubes. The mixtures were shaken for 5 h. Then the phases were separated by an external magnetic field. The liquid solutions were analyzed by ICP-MS to evaluate the possible release of Fe ions in relation to the materials’ stability due to coating.

2.4. Application and Regeneration of the OS/Fe3O4 and OS/Fe3O4@SiO2 Core–Shell Structure for Sustainable Wastewater Treatment

A batch process was applied to evaluate the heavy metal uptake from the solutions on the unmodified Fe3O4, unmodified Fe3O4@SiO2 core–shell, OS/Fe3O4 hetero-nanocores, and OS/Fe3O4@SiO2 core–shell structure, as described in previous literature.1,79 The stalk heavy metal mixture solution of cobalt(II), lead(II), and manganese(II) was prepared in deionized water with a concentration of 500 mg/g, which was diluted daily for the desired concentration during the adsorption process evaluation. A known weight of the Fe3O4 (OS/Fe3O4) and OS/Fe3O4@SiO2 core–shell samples was mixed with the heavy metal ion solution containing cobalt(II), lead(II), and manganese(II) in 50 mL tubes. The mixture was then shaken for a certain time, and finally, the adsorbents were separated using a magnetic field. The remaining heavy metal mixture was diluted to adjust the range of detection by ICP-MS before and after the adsorption. The adsorption experiments were conducted in three replicates. Further experiments were conducted with a blank to study the influences of the pH, time of contact, pollutant concentrations, and temperature. The adsorption data were analyzed through kinetic, isotherm, and thermodynamic investigations. The adsorption capacity of the OS/Fe3O4 and OS/Fe3O4@SiO2 core–shell structure was calculated using eq 1:

2.4. 1

where C0 represents the initial concentration of metal ions, Ce is the equilibrium concentration of metal ions, V is the solution volume in liters, and M is the adsorbent mass in grams.

For the regeneration of the OS/Fe3O4@SiO2, real wastewater samples were applied as a matrix during the evaluation of the adsorption of cobalt(II), lead(II), and manganese(II). The wastewater samples were collected from the Damam eastern zone in Saudi Arabia, which falls on the coast of the Arabian Gulf. The wastewater samples were acidified upon acquisition, filtered, and kept in a refrigerator for daily use. The previously described batch method was repeated to investigate the regeneration and reuse of OS/Fe3O4@SiO2. The OS/Fe3O4@SiO2-loaded samples were washed with 0.1 M HCl twice and with deionized water once, and then the washed OS/Fe3O4@SiO2 was separated using an external magnetic field and applied in the next cycle. The efficiency was calculated as a ratio based on the first application cycle, as shown in eq 2:

2.4. 2

where REx% is the recyclable efficiency of cycle x, qx is the adsorption capacity of cycle number x, and q1st is the adsorption capacity of the first cycle (x = 1).

3. Results and Discussion

3.1. Characterizations of the Organosilica-Modified Fe3O4 (OS/Fe3O4) and OS/Fe3O4@SiO2 Core–Shell Structure

The X-ray diffraction patterns of the Fe3O4 nanoparticles prepared using the solvothermal method by either citrate or citrate/organosilica modifications (hetero-nanocore) are shown in Figure 1a. The spectra indicate that the as-prepared citrate-modified Fe3O4 nanoparticles were formed of a spinel phase of Fe3O4, with related peaks such as (200), (311), (400), (511), and (440) (JCPDS card no. 19-0629).80 However, upon modification with organosilica (Figure 1b), we observed similar diffraction peaks of Fe3O4, together with a broad one, at 2θ = 22, that can be attributed to the amorphous silica obtained from the organosilica precursor. The XRD patterns for the amorphous silica (Figure 1c) confirm the broad peak at 2θ = 22, for comparison. The X-ray patterns indicate that the existence of the organosilica precursor during the solvothermal synthesis of the magnetic nanoparticles did not affect their phase formation.

Figure 1.

Figure 1

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X-ray diffraction spectra of (a) unmodified Fe3O4 (b) OS/Fe3O4 hetero-nanocores and (c) and amorphous silica.

To estimate the magnetic character of the citrate- and OS-modified Fe3O4, the magnetization was measured against the magnetic field, and the curve is shown in Figure 2. It is clear that hysteresis occurred, which indicates that both nanoparticles had a superparamagnetic character, with magnetic saturations of 67.8 and 66 emu/g for the citrate- and OS-modified Fe3O4 nanoparticles (hetero-nanocore), respectively. There was no considerable loss of magnetic strength with the addition of the organosilica precursor in the solvothermal synthesis of the Fe3O4 nanoparticles.

Figure 2.

Figure 2

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Room temperature magnetization curve of Fe3O4 nanocores prepared using the solvothermal method, including (a) unmodified and (b) organosilica-modified hetero-nanocores.

The fabricated structures including unmodified Fe3O4, unmodified Fe3O4@SiO2 core–shell, OS/Fe3O4 hetero-nanocores, and OS/Fe3O4@SiO2 core–shell were examined by TEM for morphology and structure identification (Figure 3). The fabricated organosilica-modified Fe3O4 (OS/Fe3O4) nanoparticles as a core (Figure 3C) showed that particle size ranged from 50 to 100 nm, with a spherical shape. A higher-magnification image of OS/Fe3O4 (Figure 3D) showed the existence of bumps originating from the Fe3O4 hetero-nanocores and appearing clearly on the surface, while the unmodified Fe3O4 exhibits the same spherical structure with same range (Figure 3A and B). The OS/Fe3O4@SiO2 core–shell structure was examined by TEM (Figure 3E and F), which showed the uniform coating of a silica layer of approximately 150 nm in thickness. The images also suggest that all the coated nanoparticles possessed a highly monodisperse and spherical shape, while the unmodified Fe3O4@SiO2 (Figure 3 G and H) showed thinner dense silica layer of about 50 nm, under the same coating conditions. These results indicate that the presence of organosilica species that formed during the solvothermal formation of Fe3O4 provide a center for condensation with the silica species during the silica coating step, which facilitated silica shell formation and produced a thicker shell. Magnetic nanomaterial-based adsorbents have been applied for the removal of pollutants from aqueous solutions with a promising efficiency; however, the problem associated with adsorption applications is the instability of the Fe3O4 magnetic nanoparticles in acidic media in which the adsorption of heavy metals occurs.33,81

Figure 3.

Figure 3

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TEM image of the unmodified Fe3O4 (A and B), organosilica-modified Fe3O4 nanoparticles (C and D), silica-coated OS/Fe3O4 nanoparticles (E and F), and unmodified Fe3O4@SiO2 core–shell (G and H).

To assess the surface functional groups of the unmodified Fe3O4, unmodified Fe3O4@SiO2 core–shell, OS/Fe3O4 hetero-nanocores, and OS/Fe3O4@SiO2 core–shell structure, the samples were characterized by FTIR, and the spectra are shown in Figure 4. The organosilica framework vibration (Figure 4c) represented by Si–C stretching modes appeared at 1270 cm–1.82 The peak at 1458 cm–1 is attributed to the C=C vibration of the aromatic ring, which is related to the benzene bridging group within the organosilica.83 These two peaks were observed, together with the peak related to the Fe3O4 nanoparticles, at approximately 563 cm–1 due to Fe–O bond in the structure of the Fe3O4 nanoparticles, whereas the unmodified Fe3O4 showed the same peaks but without silica related ones (Figure 4a). The existence of peaks characteristic of organosilica, together with those of Fe3O4, suggested that organosilica modification was accomplished. After the silica coating step, peaks related to the silica nanoparticles were detected for both the unmodified Fe3O4@SiO2 core–shell (Figure 4b) and OS/Fe3O4@SiO2 core–shell structure (Figure 4d) at 1065 cm–1, with a shoulder at 1210 cm–1, which can be assigned to the Si–O–Si vibration mode. In addition, weak bands were identified at 795 and 960 cm–1, which can be associated with Si–O–Si and Si–OH stretching, respectively.81 In addition, a hydroxyl group was detected through the presence of a peak between 3300 and 3450 cm–1, which was related to O–H stretching bonding.

Figure 4.

Figure 4

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FTIR spectra of the (a) unmodified Fe3O4, (b) unmodified Fe3O4@SiO2 core–shell, (c) OS/Fe3O4 hetero-nanocores, and (d) OS/Fe3O4@SiO2 core–shell structure.

The appearance of silica peaks together with those of organosilica and Fe3O4 indicated the formation of the OS/Fe3O4@SiO2 core–shell structure. The presence of such effective surface groups on the OS/Fe3O4 magnetic hetero-nanocores and OS/Fe3O4@SiO2 core–shell led to high dispersion and facilitated their application as magnetic-based adsorbents. The dense silica layer which is coated on the surfaces of OS/Fe3O4 magnetic hetero-nanocores supports the stability of magnetic materials and protects the magnetic core during adsorption applications.75

3.2. Stability and Adsorption Capacity Assessment

The unmodified Fe3O4, unmodified Fe3O4@SiO2 core–shell, OS/Fe3O4 hetero-nanocores, OS/Fe3O4@SiO2 core–shell structure, and OS/Fe3O4@SiO2 after seven uses were investigated for the Fe release by ICP-MS analysis. Results presented in Table 1 indicate the noticeable stability improvement in case of Fe3O4@SiO2 due to silica coating, while higher stability is reported for OS/Fe3O4 hetero-nanocores, OS/Fe3O4@SiO2 core–shell structure, and OS/Fe3O4@SiO2 after seven uses samples. Therefore, the organosilica modification, as well as the silica coating, of the magnetic hetero-nanocores achieved in this work can provide the required stability, even after seven uses.

Table 1. Effects of Acid Conditions on Iron Dissolution from the Prepared Adsorbents.

  detected iron concentration (ppm)
dissolution 0.05 M HCl 0.1 M HCl 0.2 M HCl
unmodified Fe3O4 1.510 ± 0.034 8.140 ± 0.108 9.964 ± 0.114
unmodified Fe3O4@SiO2 0.140 ± 0.005 0.236 ± 0.021 1.954 ± 0.046
OS/Fe3O4 0.005 ± 0.001 0.027 ± 0.005 1.270 ± 0.005
OS/Fe3O4@SiO2 0.000 ± 0.000 0.000 ± 0.000 0.015 ± 0.001
OS/Fe3O4@SiO2 After 7 uses 0.000 ± 0.000 0.000 ± 0.000 0.020 ± 0.002
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The adsorption capacities of unmodified Fe3O4, unmodified Fe3O4@SiO2 core–shell, OS/Fe3O4 hetero-nanocores, and OS/Fe3O4@SiO2 core–shell structure were compared (Figure 5). The higher adsorption capacity was in the order OS/Fe3O4@SiO2> OS/Fe3O4 > unmodified Fe3O4@SiO2 > unmodified Fe3O4. This improvement may be attributed to the silica modification and coating which introduce Si–O–Si and Si–OH groups to the adsorbents surfaces as indicated from FTIR analysis (Figure 4b–d). Therefore, the OS/Fe3O4 hetero-nanocores and OS/Fe3O4@SiO2 core–shell structure were subjected for further adsorption and regeneration investigations.

Figure 5.

Figure 5

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Comparison of the adsorption capacity of the prepared adsorbents for the removal of cobalt(II), lead(II), and manganese(II).

3.3. Application of the Fe3O4 (OS/Fe3O4) and OS/Fe3O4@SiO2 Core–Shell Structure to the Adsorption of Heavy Metals

The adsorption capacity of the organosilica-modified Fe3O4 and OS/Fe3O4@SiO2 core–shell structure for the removal of cobalt(II), lead(II), and manganese(II) was investigated to identify the suitable conditions, such as the pH, contact time, heavy metal concentrations, and temperature.84−86 The weak acidic medium (pH between 6 and 7) was the most suitable for achieving a high adsorption capacity, compared to the strong acidic medium (pH 2), as presented in Figure 6. It can be seen that the organosilica-modified Fe3O4 showed a noticeable adsorption removal capacity for the Co(II), Pb(II), and Mn(II) cations. However, the OS/Fe3O4@SiO2 core–shell exhibited a higher adsorption capacity than the organosilica-modified Fe3O4. The weak acidic medium is reported to enhance metal adsorption, as it allows for less competition between H+ and heavy metals. These results are in agreement with the findings of Pang et al. and Hashem et al., who reported that heavy metal cations are completely released from adsorption sites under strongly acid conditions.87,88 The adsorption capacities were 144, 146, and 154 mg/g for Co(II), Pb(II), and Mn(II), respectively, on the organosilica-modified Fe3O4 nanoparticles and 192, 184, and 196 mg/g for Co(II), Pb(II), and Mn(II) on the OS/Fe3O4@SiO2 core–shell. The organosilica-modified Fe3O4 showed promising adsorption capacities for different metal cations, which can be attributed the existence of organosilica moieties within the magnetic hetero-nanocores. On the other hand, after coating with 150 nm silica, it was expected that adsorption capacity would be elevated. These results indicate the effectiveness of the prepared organosilica-modified Fe3O4 and OS/Fe3O4@SiO2 core–shell as adsorbents for wastewater treatment through the uptake of heavy metals.

Figure 6.

Figure 6

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Effect of pH on the adsorption of cobalt(II), lead(II), and manganese(II) on the organosilica-modified Fe3O4 (A) and OS/Fe3O4@SiO2 core–shell structure (B).

3.3.1. Investigation of the Adsorption Mechanism

3.3.1.1. Kinetic Evaluation

The uptake of heavy metals on the organosilica-modified Fe3O4 and OS/Fe3O4@SiO2 core–shell structure involved the migration of ions from the solutions to the active surfaces and their arrangement in layers under equilibrium conditions and continuous dynamic movement. The equilibrium state occurred after a certain time, and the equilibrium concentration of the ions in the solution appeared to be constant, and there was no further uptake of metals from the solution to indicate the maximum adsorption capacity.89−91 In this work, the effect of time was studied in the range of 10 to 300 min, and the adsorption capacity is presented in Figure 7. The equilibrium for the adsorption of cobalt(II), lead(II), and manganese(II) on the organosilica-modified Fe3O4 and OS/Fe3O4@SiO2 core–shell structure took place at 100 min. The capacity for the uptake of cobalt(II), lead(II), and manganese(II) was 137.2, 146, and 174.3 mg/g for the organosilica-modified Fe3O4, respectively. On the other hand, the OS/Fe3O4@SiO2 core–shell structure showed uptake values of cobalt(II), lead(II), and manganese(II) of 206, 164, and 271 mg/g, respectively.

Figure 7.

Figure 7

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Effect of contact time on the adsorption of cobalt(II), lead(II), and manganese(II) on the and organosilica-modified Fe3O4 and OS/Fe3O4@SiO2 core–shell structure.

To investigate the kinetics of the adsorption of metal cations on the organosilica-modified Fe3O4 and OS/Fe3O4@SiO2 core–shell structure, the pseudo-first-order model, in its integrated form (eq 3),92,93 was applied for the adsorption data of the cobalt(II), lead(II), and manganese(II):

3.3.1.1. 3

Figure 8A and C reveals the plot of the log(qe – qt) and t, and using the slope and intercept, the values of k1 and qe were calculated (Table 2). The obtained results showed that poor agreement was obtained for the values of the q experiment and qe calculation, which suggested that the pseudo-first-order model is not sufficient to describe the adsorption process of metal cations on the synthesized organosilica-modified Fe3O4 and OS/Fe3O4@SiO2 core–shell structure.

Figure 8.

Figure 8

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Kinetic models for the adsorption of cobalt(II), lead(II), and manganese(II) on organosilica-modified Fe3O4 nanoparticles, pseudo-first-order (A) and pseudo-second-order models (B), and on the OS/Fe3O4@SiO2 core–shell structure, pseudo-first-order (C) and pseudo-second-order models (D).

Table 2. Pseudo-First-Order and the Pseudo-Second-Order Kinetic Constants for the Adsorption of Cobalt(II), Lead(II), and Manganese(II) on the Organosilica-Modified Fe3O4 and OS/Fe3O4@SiO2 Core-–hell Structure.
      pseudo-first order
 pseudo-second order
    qe, exp (mg/g) K1 (min-1) qe, calcd (mg/g) R2 k2 (g/mg·min) qe, calcd(mg/g) R2
OS/Fe3O4 Mn(II) 174.3 0.044 240 0.91 3.38 × 10–4 185 0.99
Co(II) 137.2 0.037 224 0.87 1.32 × 10–4 164 0.96
Pb(II) 146 0.023 169 0.94 9.48 × 10–5 192 0.96
OS/Fe3O4@SiO2 Mn(II) 271 0.065 646 0.90 1.37 × 10–4 303 0.98
Co(II) 206 0.044 288 0.80 3.86 × 10–4 217 0.99
Pb(II) 164 0.016 169 0.81 9.29 × 10–5 208 0.96
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On the other hand, the pseudo-second-order kinetic model was applied in its integrated form, as shown in eq 4,94 to examine the adsorption data of the metal cations on the synthesized nanoparticles:

3.3.1.1. 4

Figure 8B and D shows the plot of the t/qt and t, from which qe and k can be determined. The values of the experimental and calculated q showed a good correlation, revealing that the pseudo-second-order assumption is the most consistent in describing the uptake of cobalt(II), lead(II), and manganese(II) on the organosilica-modified Fe3O4 and OS/Fe3O4@SiO2 core–shell structure.

3.3.1.2. Isotherm Studies

Langmuir Isotherm. The Langmuir assumption is correlated with the adsorption process through the monolayer formation of the adsorbate around the adsorbent. The application of the isotherm model enables one to obtain more information about the adsorption process and the behavior of the adsorbate during and after the process.7,95,96 For example, the Langmuir equation assumes that the transmigration of the adsorbed ions does not occur on the plane surface. The Langmuir equation, eq 5, was applied (Figure 9):

3.3.1.2. 5

where Qmax0 (mg/g) is the maximum saturated monolayer adsorption capacity of the organosilica-modified Fe3O4 and OS/Fe3O4@SiO2 core–shell structure, Ce (mg/L) is the adsorbate concentration at equilibrium, qe (mg/g) is the amount of adsorbate uptake at equilibrium, and KL (L/mg) is a constant associated with the affinity between an adsorbent and the adsorbate.

Figure 9.

Figure 9

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Langmuir (A) and Freundlich (B) isotherm models for the adsorption of cobalt(II), lead(II) and manganese(II) on the organosilica-modified Fe3O4, and Langmuir (C) and Freundlich (D) isotherm models for the adsorption of cobalt(II), lead(II) and manganese(II) on the OS/Fe3O4@SiO2 core–shell structure.

The correlation coefficient, R2, for the adsorption of cobalt(II), lead(II), and manganese(II) on the organosilica-modified Fe3O4 magnetic hetero-nanocores and OS/Fe3O4@SiO2 core–shell structure (Figure 9A and C) (Table 2) showed lower values, indicating that the Langmuir assumption cannot be applied to describe the adsorption process.

Freundlich Isotherm.Equation 6 represents the Freundlich model:

3.3.1.2. 6

where qe (mg/g) is the amount of adsorbate uptake at equilibrium, Ce (mg/L) is the adsorbate concentration at equilibrium, KF (mg/g)/(mg/L) is the Freundlich constant, and n (dimensionless) is the Freundlich intensity parameter, which indicates the magnitude of the adsorption driving force or the surface heterogeneity.

The values of K and n can be investigated by plotting log qe versus log Ce (Figure 9B and D). In addition, the obtained results from the application of the Freundlich model (Figure 9B and D) showed the linear form (Table 3 shows the calculated constants). By comparing the values of the qmax and correlation coefficients of the two isotherms, it can be concluded that the Freundlich model is the most consistent model that describes the adsorption of cobalt(II), lead(II), and manganese(II) on the organosilica-modified Fe3O4 and OS/Fe3O4@SiO2 core–shell structure.

Table 3. Langmuir and Freundlich Constants for the Adsorption of Cobalt(II), Lead(II), and Manganese(II) on the Organosilica-Modified Fe3O4 and OS/Fe3O4@SiO2 Core–Shell Structure.
 
 Langmuir constants
 Freundlich constants
  KL Qmax R2 KF n R2
OS/Fe3O4 Mn(II) 0.023 526 0.69 8.59 1.24 0.94
Co(II) 0.05 92 0.44 0.32 0.58 0.93
Pb(II) 0.011 112 0.39 0.37 0.61 0.90
OS/Fe3O4@SiO2 Mn(II) 1.6 × 10–3 666 0.88 13.75 1.94 0.96
Co(II) 6.0 × 10–3 769 0.37 5.79 1.17 0.91
Pb(II) 0.03 303 0.87 0.01 62 0.93
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3.3.1.3. Adsorption Thermodynamic Properties

The mechanism of the adsorption is mostly dependent on the surface features, the functional groups, and the composition of the adsorbate. These factors leads to variations in the polarity environment on the adsorbent surfaces, leading to various adsorption mechanisms in which the process may be either spontaneous or nonspontaneous adsorption and can be classified as physisorption or chemisorption.97,98 The thermodynamic properties, such as the change in the Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°), were evaluated using eqs 7 and 8:

3.3.1.3. 7
3.3.1.3. 8

By applying eq 7, plotting log Kd vs 1/T (Figure 10) enables the determination of ΔH° and ΔS°.

Figure 10.

Figure 10

Open in a new tab

Thermodynamic parameters for the adsorption of cobalt(II), lead(II), and manganese(II) on the organosilica-modified Fe3O4 (A) and OS/Fe3O4@SiO2 core–shell structure (B).

As shown in Table 4, for the adsorption on the organosilica-modified Fe3O4 magnetic hetero-nanocores, the ΔG° was in the range of (−3.3 to −5.7 kJ/mol) for cobalt(II), (−4.2 to −6.0 kJ/mol) for lead(II) and (−6.7 to −11.2 kJ/mol) for manganese(II), while the ΔH° and ΔS° values were in the range of 10.6–28.7 kJ mol–1 and 49.6–117 J mol–1.K–1, respectively. For the adsorption on the OS/Fe3O4@SiO2 core–shell structure, the ΔG° was in the range of (−6.4 to −9.3 kJ/mol) for cobalt(II), (−4.6 to −8.6 kJ/mol) for lead(II) and (−7.7 to −10.5 kJ/mol) for manganese(II), while the ΔH° and ΔS° values were in the range of 15–26.2 kJ mol–1 and 76.2–114 J mol–1.K–1, respectively.

Table 4. Thermodynamic Parameters for the Adsorption of Cobalt(II), Lead(II), and Manganese(II) on the Organosilica-Modified Fe3O4 and OS/Fe3O4@SiO2 Core–Shell Structure.
    OS/Fe3O4
 OS/Fe3O4@SiO2
  T (K) ΔG° (kJ/mol) ΔS° (J/mol/K) ΔH° (kJ/mol) ΔG° (kJ/mol) ΔS° (J/mol/K) ΔH° (kJ/mol)
Mn(II) 298 –6.7 117.0 28.7 –7.7 76.2 15.0
313 –7.4 –8.9
323 –8.2 –9.4
333 –11.2 –10.5
Co(II) 298 –3.3 49.6 10.6 –6.4 79.0 17.3
313 –4.1 –7.4
323 –4.7 –8.0
333 –5.7 –9.3
Pb(II) 298 –4.2 67.2 16.8 –4.6 114.0 29.2
313 –4.8 –6.9
323 –5.3 –7.6
333 –6.0 –8.6
Open in a new tab

The negative values of ΔG° showed the spontaneous behavior of the adsorption of cobalt(II), lead(II), and manganese(II) on the organosilica-modified Fe3O4 and OS/Fe3O4@SiO2 core–shell structure. In addition, the positive value of ΔH° can be related to the endothermic process, and ΔS° indicates the increase in the degree of freedom during the adsorption of cobalt(II), lead(II), and manganese(II) on the organosilica-modified Fe3O4 and OS/Fe3O4@SiO2 core–shell structure.99−101 A positive value of ΔS° was previously reported by Radi et al. during the adsorption of heavy metals including cadmium(II), copper(II), zinc(II), and lead(II).102

3.4. Regeneration and Recycling

The regeneration and recycling of adsorbent materials are considered as added value in wastewater treatment strategies.103,104 An acid-based regeneration investigation was conducted to evaluate the multiuse of OS/Fe3O4@SiO2 as a renewable adsorbent. The cobalt(II)-, lead(II)-, and manganese(II)-loaded OS/Fe3O4@SiO2 samples were activated by washing with HCl and deionized water after each cycle. The recyclable efficiency % was no less than 91% after seven cycles, as presented in Figure 11. This regeneration process is superior to that reported by Shahriyari Far et al. for the recycling of a Cr-MOF/AC composite five times, with efficiency reductions of 3% for Co(II) and 13% for Pb(II).105 These results indicate the stability of OS/Fe3O4@SiO2 in multiuse and sustainable applications.

Figure 11.

Figure 11

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Recyclable efficiency during the regeneration process for the adsorption of cobalt(II), lead(II), and manganese(II) on the organosilica-modified Fe3O4 and OS/Fe3O4@SiO2 core–shell structure.

4. Conclusion

The strategy described in this work represents an easy and simple process for the successful formation of organosilica-modified Fe3O4 magnetic hetero-nanocores, which facilitates further coating with a dense silica layer to produce an OS/Fe3O4@SiO2 core–shell structure with uniform sample distribution, revealing a well-controlled synthetic process. The fabricated OS/Fe3O4 and OS/Fe3O4@SiO2 showed higher stability in acid medium than unmodified Fe3O4 and unmodified Fe3O4@SiO2 core–shell. This process produces two efficient adsorbents with capacities for the uptake of cobalt(II), lead(II), and manganese(II) at 137.2, 146, and 174.3 mg/g, respectively, for the organosilica-modified Fe3O4 and 206, 164, and 271 mg/g, respectively, for the OS/Fe3O4@SiO2 core–shell structure. The addition of the coated second layer to the OS/Fe3O4 nanoparticle core leads to a noticeable enhancement in the adsorption of heavy metals from aqueous solutions. The results are promising for the adsorption of various pollutants and can serve as methods for cleaning the environment, with a high recyclable efficiency. The potential to recycle OS/Fe3O4@SiO2 materials will open up space for future research aiming to assess various regeneration methods in relation to economic points of view and material stability and recyclable efficiency. The strategy used to fabricate Fe3O4 magnetic hetero-nanocores will enhance the future fabrication of various core–shell structures for various applications, which could serve fields such as imaging, wastewater treatment, energy, drug delivery, and catalysis.

Acknowledgments

The authors acknowledge the National Plan for Science, Technology, and Innovation (MAARIFAH), King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia for its grant with award number 14-WAT169-02.

The authors declare no competing financial interest.

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