Magnetic Carbon Porous Polymer Prepared from a New Suspended Emulsion for the Absorption of Heavy Metal Ions

Abstract

In this study, magnetic carbon nanopolymers (Fe₃O₄/C@PM) were synthesized by suspension polymerization using magnetic carbon nanoparticles as the matrix, 2-thiophene formaldehyde and acrylamide as the monomers, and ethylene glycol dimethacrylate (EGDMA) as the crosslinking agent. The obtained material was characterized using multiple techniques, including scanning electron microscopy (SEM), infrared spectroscopy (FTIR), X-ray diffraction (XRD), N₂ adsorption–desorption, and thermogravimetric analysis (TGA). The adsorption effects of Zn²⁺, Cd²⁺, and Pb²⁺ in the mixed solution were evaluated using magnetic carbon nanoparticles (Fe₃O₄/C) and Fe₃O₄/C@PM as adsorbents. The adsorption isotherms, kinetic models, and cyclic regeneration of various metal ions, including Zn²⁺, Cd²⁺ and Pb²⁺, were studied. The results showed that the Fe₃O₄/C@PM maintained a slightly aggregated spherical morphology similar to Fe₃O₄/C and exhibited excellent adsorption capacity for all of Zn²⁺, Cd²⁺, and Pb²⁺, with maximum adsorption capacities of 343.3, 250.7, and 177.6 mg·g⁻¹, respectively. The adsorption mechanisms were mainly based on the chemical interactions between metal ions and functional groups on the surface of polymers. The kinetic study revealed that the adsorption process followed a pseudo-second-order kinetic model. When Fe₃O₄/C@PM was reused five times, its adsorption rates for Zn²⁺, Cd²⁺, and Pb²⁺ remained above 81%, indicating its great potential for the treatment of wastewater containing Zn²⁺, Cd²⁺, and Pb²⁺.

1. Introduction

The contamination of water by metals has emerged as a significant environmental concern worldwide, posing risks to both marine life and human well-being [1,2]. Research has indicated that metal ions are resistant to biodegradation and cannot be processed by living organisms, leading to their accumulation. This in turn is detrimental to their central nervous system, reproductive systems, and organs such as the cardiovascular system, kidneys, and liver. In extreme situations, these substances may result in disorders related to cognitive development [3], potentially causing disabilities that last a lifetime [4]. Consequently, exploring methods to eliminate heavy metals from water systems is crucial for mitigating their detrimental impacts on the environment and ecological balance. To tackle this issue, various methods have been developed by scientists to effectively treat wastewater for the removal of heavy metals. Relevant methods encompass processes such as chemical precipitation [5], electrochemical treatment [6], ion exchange [7], adsorption [8], membrane filtration [9], and phytoremediation [10]. All these approaches have demonstrated considerable efficacy in managing wastewater contaminated with heavy metals, while the adsorption technique is particularly favored due to its straightforward operational process, high removal efficiency, significant impact, recyclability, eco-friendliness, low expenses, and strong adaptability to biological systems, garnering considerable interest in both academic and industrial circles [11,12]. The utilized adsorbent materials include carbon-based materials, such as nanoporous activated carbon, carbon nanotubes, and graphene oxide [13,14], which have attracted great interest. These materials have been popular research targets for their role as effective adsorbents in the removal of heavy metals from aqueous solutions, primarily owing to their cost-effectiveness, highly porous configuration, extensive surface area, and active surface functionalities. Biochar has become highly popular in recent years within the context of treating water contaminated by heavy metals, primarily owing to its economic efficiency and effectiveness in metal ion retention [15]. However, while biochar has demonstrated a notable ability to remove heavy metal ions from contaminated waters, its adsorption performance is often not as high as that of other established biosorbents like activated carbon. As a result, research has focused on enhancing the properties and structures of biochar through various modifications to improve its effectiveness in environmental remediation and its overall usefulness [16]. On the other hand, the separation of these non-magnetic carbon nanomaterials from water is challenging, leading to high costs and low recyclability, which are factors that hinder the overall effectiveness of biochar compounds in treating wastewater contamination.

The integration of magnetic nanomaterials leverages the benefits of both magnetic separation techniques and nanotechnology advancements, which can result in remarkable reusability after magnetic separation processes. Additionally, these materials are appreciated for their relatively large surface area, compatibility with biological systems, inert chemical properties, minimal toxicity, and ease of dispersion, features that render them highly effective for treating wastewater [17,18]. Researchers have engineered magnetic carbon nanomaterials along with their modified versions to facilitate the extraction of heavy metal ions from contaminated sources [19,20,21]. In Gabalda’s study, magnetic carbon nanomaterials were synthesized using electromagnetic induction heating and then utilized as nanoadsorbents for the extraction of Cr (VI) from water [22]. In a 25 mL aqueous solution with pH = 6 and 1 mg·L⁻¹ Cr (VI), 25 mg of magnetic carbon nanomaterials were used as adsorbents for 30 min, and the efficiency of eliminating Cr (VI) exceeded 98%. In a study by Guo [23], Cd²⁺ was chosen as the template ion, and a magnetic ionic polymer (MIIP) consisting of graphitic carbon nitride (g-C₃N₄) was enhanced by incorporating polyn-isopropyl acrylamide through grafting. The results showed that in MIIP adsorbate aqueous solution with pH = 6 and an initial concentration of Cd²⁺ 250 mg·L⁻¹, over a period of 180 min, the maximum adsorption capacity reached 184 mg·g⁻¹. The mechanism of adsorption indicated that the initial specific uptake of Cd²⁺ by the ionic polymer is influenced by both its functional groups and the presence of ionic channels. Molecularly Imprinted Polymer has shown enhanced selectivity towards template ions, but its fabrication is a complex procedure, the elution of template ions proves challenging, the polymers may leak during the adsorption phase, and there remains a need to boost their adsorption capacity further. In order to address the above shortcomings, a series of polymeric molecularly imprinted polymers prepared from high internal phase emulsions (HIPEs) [24,25] or Pickering emulsions [26] were used as adsorbents to remove ions from aqueous solutions.

In this study, the synthesis of Fe₃O₄/C@PM, a type of magnetic carbon nanopolymer, achieved through a new suspension emulsion polymerization, was described. This process utilizes magnetic carbon nanoparticles as a foundational matrix, combined with 2-thiophenaldehyde and acrylamide serving as functional monomers, while ethylene glycol dimethacrylate (EGDMA) acts as the crosslinking agent. The fabrication of magnetic carbon nanopolymers, characterized by consistent particle size, effective dispersion, and stability, is achieved by utilizing Fe₃O₄/C nanoparticles as a stabilizing agent during the suspension emulsion polymerization process. The metal ion adsorption capability is enhanced by leveraging the strong binding properties of 2-thiophenaldehyde and acrylamide. A series of batch experiments are conducted to explore the influences of various factors, including pH, contact duration, starting ion concentration, and presence of competing ions, on the adsorption performance. To enhance the comprehension of the adsorption mechanisms, various parameters related to the adsorption behaviors of numerous metal ions, such as Zn²⁺, Cd²⁺, and Pb²⁺, are examined through isotherm analysis, kinetics models, and the assessment of recycling processes. The obtained data provide insights into the enhancement of future uses of innovative magnetic carbon nanopolymer materials.

2. Materials and Methods

2.1. Starting Materials

Hexahydrate of ferric chloride, heptahydrate of ferrous sulfate, sodium sulfite, disodium hydrogen phosphate, aqueous ammonia, sodium citrate in its dihydrate form, anhydrous ethanol, polyvinyl alcohol (PVA), sodium dodecyl sulfate, toluene, lead nitrate, nickel nitrate, zinc nitrate, and copper nitrate, all of analytical purity, were sourced from Guangzhou Chemical Reagent Factory. Cadmium nitrate, cobalt nitrate, 2-thiophenaldehyde, acrylamide, EGDMA, ammonium persulfate, and potassium persulfate were all of analytical grade and sourced from Aladdin Shanghai Co., LTD. (Shanghai, China). Solutions of Zn²⁺, Cd²⁺, Cu²⁺, Pb²⁺, Co²⁺, and Ni²⁺ at a concentration of 1000 μg·mL⁻¹ were obtained from the National Nonferrous Metals and Electronic Materials Analysis and Testing Center located in Beijing, China. This study utilized Mill-Q ultra-pure water along with deionized water for all procedures.

2.2. Instruments

The instruments used in this work included the following: SQP precision scale (Sardorius Scientific Instrument (Beijing) Co., Ltd., Beijing, China), KXN-6450 DC regulated power supply (Shenzhen Zhaoxin Electronic Instrument Equipment Co., Ltd., Shenzh, China), JB50-S electric mixer (Shanghai Spectrum Biotechnology Co., Ltd., Shanghai, China), DF-101S constant temperature magnetic stirrer (Gongyi Yuhua Instrument Co., Ltd., Gongyi, China), DZF-6020 vacuum drying chamber (Shanghai Yiheng Scientific Instrument Co., Ltd., Shanghai, China), WE-2 constant temperature oscillator water bath (Tianjin Onuo Instrument Co., Ltd., Tianjin, China).

The material compositions were analyzed using an FTIR-8400S Fourier transformer infrared spectrometer. Differential thermogravimetric analysis was conducted using a DTG-60H (Shimadzu, Kyoto, Japan). The D8 Advance X-ray diffractometer (Bruker, Karlsruhe, Germany) was utilized for characterizing and analyzing the phases present in the materials. The materials’ surface characteristics were examined using a SUPRA 55 SAPPHIRE field emission scanning electron microscope (Carl Zeiss, Oberkochen, Germany). The materials were evaluated for their specific surface characteristics and pore configuration using a Brunauer–Emmett–Teller (BET) specific surface area and pore size analyzer (Autosorb-iQ, Quantachrome, Boynton Beach, FL, USA). Graphite furnace atomic absorption (GF-AAS, AA-7000, Shimadzu, Kyoto, Japan) was implemented to determine metal ion concentrations.

2.3. Synthesis of the Magnetic Carbon Nanopolymer

2.3.1. Formulation of Carbon Nanosuspensions

The preparation of magnetic carbon nanosolutions was performed according to the reference [27]. Two carbon rods with a diameter of 1 cm were washed ultrasonically in distilled water and inserted into 40 mL PBS buffer with a concentration of 6 × 10⁻⁵ mol·L⁻¹ and pH = 7. After connecting the DC regulated power supply, a method involving a constant current was utilized for ultrasonic electrolysis, with an electrode gap of 5 mm, current of 0.25 A, and ultrasonic power of 80 W, and the duration of electrolysis was set to 3 h. This resulted in a brown carbon nanosolution that was evenly distributed.

2.3.2. Synthesis of Magnetizable Carbon Nanoparticles

Amounts of 0.3503 g FeCl₃·6H₂O, 0.1853 g FeSO₄·7H₂O, and 0.1388 g sodium citrate dihydrate were added into 40 mL carbon nanosolution and dissolved by ultrasound for 20 min to ensure they dispersed evenly. The mixture was placed in a 70 °C water bath, with the pH adjusted to 11 with ammonia water, stirred for 3 h, cooled, and magnetically separated. The products were cleaned with deionized water and anhydrous ethanol to a pH of 7, followed by vacuum drying at 60 °C for 12 h to obtain magnetic carbon nanoparticles (Fe₃O₄/C).

2.3.3. Fabrication of Carbon-Based Magnetic Polymer Nanocomposites

Phase A for the aqueous solution involved introducing 1 g of PVA into a 500 mL round-bottom flask containing 200 mL of ultra-pure water. This mixture was heated to 90 °C while being stirred mechanically at 500 rpm. Once the PVA had dissolved entirely, 20 mL of 20% sodium dodecyl sulfate solution was added along with 2 g of sodium sulfite. After 30 min of stirring, the mixture was allowed to cool to 45 °C.

In a suitable 50 mL vessel, a mixture was prepared by gradually adding 2.5 mL of toluene, 1.2 mL of 2-thiophenaldehyde, 0.57 g of acrylamide, 4.0 mL of EGDMA, and 0.04 g of ammonium persulfate. The vessel was then subjected to ultrasonic treatment at 60 °C for a duration of 20 min to ensure full solubilization of the components.

The synthesis of the magnetic carbon nanopolymer was achieved by the suspension polymerization technique with a water-to-oil ratio of 1:40. Specifically, 1.00 g Fe₃O₄/C and 0.04 g potassium persulfate were added to the aqueous phase A at 45 °C, to which approximately 5.4 mL of oil phase B was added progressively while maintaining mechanical stirring at a rate of 500 rpm for an hour to ensure the formation of a stable emulsion. Next, the polymerization process took place at 85 °C for 16 h to obtain a black solid product. The magnetic carbon nanopolymer (Fe₃O₄/C@PM) was then isolated after fine crushing and thorough rinsing in hot water for 4–5 cycles. To eliminate the remaining emulsion, Soxhlet extraction was performed using ethanol and an ethanol–acetic acid mixture for a duration of 24 h each. The magnetic carbon nanopolymer (Fe₃O₄/C@PM) was obtained by vacuum drying at 60 °C for 12 h. The aforementioned method of synthesizing magnetic carbon nanopolymer composites is displayed in Figure 1.

2.4. Adsorption Experiment

The efficacy of Fe₃O₄/C and Fe₃O₄/C@PM adsorbents in the removal of mixed metal ions from an aqueous solution was explored through competitive adsorption trials. A 10 mL standard solution containing a mixture of Zn²⁺ ions, Cd²⁺, Cu²⁺, Pb²⁺, Co²⁺, and Ni²⁺ in a concentration of 500 mg·L⁻¹ and pH = 4, and 30 mg of either Fe₃O₄/C or Fe₃O₄/C@PM was added into a 100 mL conical flask, which was then placed in a temperature-controlled oscillator at 25 °C at 180 revolutions per minute for oscillatory adsorption for 1 h. After magnetic separation, the supernatant containing metal ions was subjected to analysis using flame atomic absorption spectrometry. The removal efficiency R (%) of each ion was determined by the following equation:

$$R={\displaystyle \frac{c_0-c_t}{c_0}}\times 100\%$$

(1)

where c₀ and c_t (measured in mg·mL⁻¹) denote the initial metal ion concentration within the solution and its concentration post-adsorption after one hour, respectively; V (in mL) indicates the solution’s volume; m (in g) refers to the mass of the adsorbent used.

Batch experiments were conducted to investigate the ability of the adsorbents Fe₃O₄/C or Fe₃O₄/C@PM to adsorb metal ions, namely Zn²⁺, Cd²⁺, and Pb²⁺, while varying different conditions, such as the pH of the solution, the initial concentration of the metal ions, and the duration of adsorption.

The influence of the pH level of the solution was investigated by the following experiment: First, 30 mg of adsorbent was introduced into 10 mL of a solution that varied in pH from 2 to 7 adjusted with different concentrations of HNO₃ and had a concentration of 500 mg·L⁻¹ of Zn²⁺, Cd²⁺, and Pb²⁺. The mixture was then subjected to an adsorption reaction at 25 °C while being stirred at 100 rpm for one hour, followed by magnetic separation.

Static adsorption experiment: First, 15 mg adsorbent was dispersed in 10 mL solution with pH = 6.0 and containing Zn²⁺, Cd²⁺, and Pb²⁺ in the concentration range of 50~1000 mg·L⁻¹. The adsorbent underwent reciprocating oscillation for 1 h at 25 °C and 100 rpm. The resulting adsorbent was collected by a magnet. The supernatant was analyzed by GF-AAS.

Dynamic adsorption experiment: First, 15 mg adsorbent was dispersed in 10 mL solution with pH = 6.0 and 400 mg·L⁻¹ Zn²⁺, Cd²⁺, and Pb²⁺ at 25 °C, which was subjected to 0~60 min of oscillating adsorption at 100 rpm. The metal ion content in the solution was analyzed at different time points, allowing the adsorption quantity (Q) to be calculated at different times, and the adsorption kinetics curve was then drawn. The adsorption capacity Q (mg·g⁻¹) was calculated by the following formula:

$$Q={\displaystyle \frac{(c_0-c_t)V}{m}}$$

(2)

where c₀ and c_t (in mg·mL⁻¹) denote the original concentration of metal ions present in the solution and the concentration recorded after t minutes of adsorption, respectively; V (in mL) signifies the volume of the solution; m (in grams) indicates the mass of the adsorbent used.

2.5. Regeneration Analysis

The reusable performance of the Fe₃O₄/C@PM adsorbent prepared with 0.1 mol·L⁻¹ ethylenediaminetetraacetic acid (EDTA) as eluent was studied. For this experiment, 15 mg of the adsorbent was dispersed in 10 mL solution with pH = 6 and 400 mg·L⁻¹ concentration of Zn²⁺. The adsorbent was treated with oscillations at a temperature of 25 °C and a frequency of 100 rpm for a duration of 30 min to achieve adsorption equilibrium. Subsequently, the levels of Zn²⁺ in this process were analyzed. The levels of Cd²⁺ and Pb²⁺ in the solution were determined using flame atomic absorption spectrometry. After the adsorbent became saturated with Zn²⁺, deionized water was used to rinse Cd²⁺ and Pb²⁺. After the addition of 30 mL of 0.1 mol·L⁻¹ EDTA, the desorption of eluents occurred at a temperature of 45 °C while being agitated at 180 rpm for a duration of 20 min. This process was repeated with fresh eluents every 20 min until Zn²⁺ was no longer present. The presence of Cd²⁺ and Pb²⁺ was not observed. The regenerated material was subsequently rinsed with deionized water to prepare it for reuse in future applications. To assess the effectiveness of the synthesized adsorbent, the same adsorbent underwent an adsorption–desorption procedure six times.

3. Results and Discussion

3.1. Characterization of Fe₃O₄/C and Fe₃O₄/C@PM

Figure 2 displays the scanning electron microscopy (SEM) images pertaining to Fe₃O₄/C and Fe₃O₄/C@PM. Figure 2a (Fe₃O₄/C) illustrates that the carbon nanoparticles produced through electrolysis exhibit an irregular lamellar structure, featuring pores that extend between the layers of carbon and housing spherical Fe₃O₄ particles within these carbon nanoparticles. Figure 2b (Fe₃O₄/C@PM) reveals that the polymer-coated Fe₃O₄/C has a dispersive arrangement of uneven layered granular structures characterized by a textured surface, promoting effective mass transfer.

The spectra for Fe₃O₄/C and Fe₃O₄/C@PM are illustrated in Figure 3. In Figure 3a, the spectrum corresponding to carbon nanoparticles synthesized through electrolysis is presented. The notable wide peaks observed in the range of 3400~2900 cm⁻¹ correspond to the indicative stretching vibrations associated with -OH and -COOH groups, and 1638 cm⁻¹ is the stretching vibration peak of C=O in the carboxyl group, which suggests that the carbon nanoparticles synthesized through electrolysis possess an abundance of -OH and -COOH functional groups [27]. The spectra in Figure 3b,c reveal a distinct Fe-O absorption band located between 567 and 580 cm⁻¹, suggesting that ferric oxide is integrated within the carbon nanoparticles and polymers. The spectrum presented in Figure 3c is that of the Fe₃O₄/C@PM sample, where the broad peak observed at 1088 cm⁻¹ corresponds to the stretching vibration associated with C-O-C, CS, and OH bonds. The characteristic absorption band at 1394 cm⁻¹ is associated with CH₂, while the broad and intense peak around 1628 cm⁻¹ is linked to the -NH₂, -C=C, -C=N, and -C=O stretching vibrations. The absorption signal observed between 2350 and 2600 cm⁻¹ is associated with the stretching vibration of the sulfonyl group, and the stretching vibration peak at 3183 cm⁻¹ is the stretching vibration peak of -CH=C on the thiophene ring. The strong absorption peaks in the range of 3300~3550 cm⁻¹ correspond to the stretching vibration peaks of O-H and -NH₂. The above results confirm the cross-linking reaction of thiophenic formaldehyde and acrylamide with EGDMA. It can be seen from the FTIR results that Fe₃O₄/C@PM has been successfully prepared.

The XRD patterns of Fe₃O₄/C and Fe₃O₄/C@PM are presented in Figure 4, and the prepared carbon nanoparticles are shown in Figure 4a. The two sharp peaks at 2θ = 26.603° and 54.69° correspond to the (002) and (004) crystal plane diffraction peaks (JCPDS 75-1621) of graphitic carbon, respectively [28], suggesting that the structure of the prepared carbon nanoparticles resembles that of graphite. Regarding the graphs illustrated in Figure 4b,c, when compared to the reference diffraction pattern JCPDS No.19-062 for Fe₃O₄, diffraction peaks are observed at 2θ = 35.60°, 43.17°, 54.61°, and 62.65°, corresponding to the (311), (400), (422), and (440) planes of Fe₃O₄, respectively [23], suggesting that the synthesized composite materials incorporate Fe₃O₄ nanoparticles. In Figure 4c, a wide diffraction peak at 2θ = 19.43° is present, which aligns with the observed characteristic peak of the non-crystalline polymer mentioned in the reference [29], signifying the successful synthesis of Fe₃O₄/C@PM. The diminished intensity of the diffraction peaks associated with Fe₃O₄ in both Fe₃O₄/C and Fe₃O₄/C@PM can be explained by the presence of the carbon nanosheet and polymer coating [30]. These findings are in good agreement with the FTIR analysis results.

Utilizing TGA allows for the quantitative analysis of the proportions of carbon nanoparticles and polymers present in the composite material. In Figure S1, the TGA profiles of Fe₃O₄/C and Fe₃O₄/C@PM are illustrated. The TGA profile corresponding to Fe₃O₄/C (shown in Figure S1a) indicates a mass reduction of 9.65% when analyzed over the temperature interval of 25–240 °C, which results from the loss of both absorbed moisture and structural water present within the sample. At 240–700 °C, the loss of weight gradually accelerates as the temperature rises. This phenomenon can be largely attributed to the elimination of carboxyl and hydroxyl groups that are attached to the surface. Once the temperature exceeds 700 °C, a notable reduction in weight occurs due to the thermal oxidative breakdown of carbon nanoparticles. Within the temperature range of 240–800 °C, the reduction in mass observed for carbon nanoparticles is recorded at 12.93%. In comparison to Fe₃O₄/C, in the range of 25–240 °C, Fe₃O₄/C@PM experiences a weight reduction of 10.03% (Figure S1b). Within the range of 240–800 °C, a considerable mass reduction (41.43%) occurs owing to the breakdown of the polymer and the thermal degradation of carbon nanoparticles. These findings indicate that Fe₃O₄/C@PM exhibits strong thermal stability when subjected to a decomposition temperature of 240 °C.

The isotherms for nitrogen adsorption and desorption of Fe₃O₄/C and Fe₃O₄/C@PM are depicted in Figure S2. These are characteristic of type IV, displaying a distinct hysteresis loop at elevated pressures, suggesting a mesoporous architecture within the materials. The pore size distributions for Fe₃O₄/C and Fe₃O₄/C@PM are illustrated in Figure S3, where the primary pore dimensions for Fe₃O₄/C range from 0.3 nm to 5 nm, with a mean pore dimension of 0.622 nm. The primary pore size of Fe₃O₄/C@PM is noted to vary between 0.5 nm and 10 nm, with a significant concentration of mesoporous sizes observed around 3.5 nm. The average pore diameter is 0.823 nm, with mesopores predominantly found near 5 nm, suggesting the presence of numerous micropores and mesopores within the structure. According to the findings in Figure S3, the measured values of specific surface area for Fe₃O₄/C and Fe₃O₄/C@PM, obtained through multi-point BET, are 239.2 m²·g⁻¹ and 348.5 m²·g⁻¹, respectively, in line with total pore volumes recorded at 0.228 and 0.3017 cm³·g⁻¹, respectively. It is evident from these data that Fe₃O₄/C@PM exhibits a greater specific surface area, while Fe₃O₄/C has a larger total pore volume and finer pore dimensions. This phenomenon can be attributed to the processes involved in suspension emulsion polymerization: the dispersion of Fe₃O₄/C becomes more uniform, resulting in an improved dispersion of Fe₃O₄/C@PM, which leads to a growth in the available surface area. The enlargement of pore dimensions and overall pore capacity is the result of voids created by tiny organic solvent particles and dispersed emulsified droplets.

3.2. Adsorption of Fe₃O₄/C@PM

3.2.1. Influence of pH Value

The acidity level of the medium is crucial in the process of substance adsorption, as it influences not only the reactions occurring with metal ions within the medium but also the charge properties of the adsorbent material. The effectiveness of 30 mg Fe₃O₄/C@PM composite in removing 500 mg/L concentrations of Zn²⁺, Cd²⁺, and Pb²⁺ was evaluated within a pH range of 2 to 7 in a 10 mL solution. As shown in Figure 5a, the highest removal efficiencies for Zn²⁺, Cd²⁺, and Pb²⁺ are observed at pH levels of 6, 7, and 6, respectively. Between pH levels 2 and 4, a notable increase in the Zn²⁺, Cd²⁺, and Pb²⁺ removal rate occurs with the rising pH. Between pH levels of 4 and 6, the rate of removal for each ion continues to rise gradually, ultimately stabilizing. As the pH of the solution increases at lower pH levels (pH < 6), the amino and thiophene functional groups on the Fe₃O₄/C@PM surface become highly protonated, which inhibits the entry of Zn²⁺, Cd²⁺, and Pb²⁺ ions into the adsorbent’s surface. At the same time, a reduction occurs in the protonation of the functional groups present on the Fe₃O₄/C@PM surface, leading to a decrease in electrostatic repulsion of adsorbent’s surface with Zn²⁺, Cd²⁺, and Pb²⁺, which subsequently reduces the removal efficiency. When the pH exceeds 6, the formation of insoluble compounds such as Zn (OH)₂ and Pb (OH)₂ occurs in the solution, which leads to a decrease in the levels of Zn²⁺ and Pb²⁺ attached to the adsorbent, ultimately resulting in a notable drop in the removal efficiency for these ions. According to the above observations, a solution pH of 6 was selected for the subsequent experiments.

3.2.2. Impact of Varying Concentrations of Fe₃O₄/C@PM on Removal Efficacy

The efficacy of Fe₃O₄/C@PM, in amounts ranging from 5 to 30 mg, in eliminating Zn²⁺, Cd²⁺, and Pb²⁺, separately, in 10 mL 500 mg·L⁻¹ solutions was assessed. The pH was set to 6 for reasons discussed above, as illustrated in Figure 5b. With increasing amounts of Fe₃O₄/C@PM added, there was a substantial rise in removal rates of Zn²⁺, Cd²⁺, and Pb²⁺. Nevertheless, once the amount of Fe₃O₄/C@PM surpassed 10 mg, the increase in the ion removal efficacy noticeably diminished, which could be attributed to the merging or clustering of adsorption sites with the increasing quantity of adsorbents, leading to their reduced utilization efficiency. Beyond a dose of 15 mg adsorbent, the increase in the ion removal efficiency remained minimal, nearing its optimal level, suggesting that the adsorption had achieved a state of saturation equilibrium. Consequently, the ideal concentration of Fe₃O₄/C@PM was determined as 1.5 g·L⁻¹.

3.2.3. Effect of Initial Concentration of Heavy Metal Pollutants

In order to assess the capability of Fe₃O₄/C@PM in adsorbing Zn²⁺, Cd²⁺, and Pb²⁺, the influence of varying initial concentrations of Zn²⁺ on the adsorption process was investigated, and the influence of both Cd²⁺ and Pb²⁺ present on the adsorption ability of Zn²⁺ was examined at a temperature of 25 °C and a pH level of 6.0. The results are illustrated in Figure 5c, spanning an initial concentration spectrum of 50 to 200 mg·L⁻¹ Zn²⁺. It can be seen that as the initial levels of Zn²⁺ increase, the capacity for its adsorption by Fe₃O₄/C@PM rises, while the removal of Cd²⁺ and Pb²⁺ shows a linear increase until the removal rate of Pb²⁺ is nearly complete, at which point there is still and Zn²⁺ and Cd²⁺ in the solution. This observation is attributed to the ample active sites present on the surface of Fe₃O₄/C@PM, and that the insufficient amount of metal ions present fails to occupy all these active sites effectively. When the starting concentration of Zn²⁺ ranges from 200 to 500 mg·L⁻¹, the maximum capacity for its adsorption, the adsorption levels of Cd²⁺ and Pb²⁺ display a gradual and slow increase due to the gradual reduction in the number of available active sites on Fe₃O₄/C@PM. Upon raising the initial concentration of Zn²⁺ to 600 mg·L⁻¹, the adsorption process appears to reach a level of completion, with the capacity stabilizing at a consistent value. This shows that metal ions have fully occupied the active sites of Fe₃O₄/C@PM. The peak adsorption capacities for Zn²⁺, Cd²⁺, and Pb²⁺ in connection with Fe₃O₄/C@PM reach 343.3, 250.7, and 177.6 mg·g⁻¹, respectively.

3.2.4. Selectivity of Adsorption

Various types of heavy metal ions, such as Zn²⁺, Cd²⁺, Cu²⁺, Pb²⁺, Co²⁺, and Ni²⁺, frequently exist together in both natural and industrial effluents. The impacts of 30 mg Fe₃O₄/C and Fe₃O₄/C@PM on the simultaneous presence of these metal ions were evaluated in a volume of 10 mL, pH = 4.0, with concentrations set at 500 mg·L⁻¹. As illustrated in Figure 5d, in comparison to Fe₃O₄/C, the Fe₃O₄/C@PM composite exhibits an enhanced ability to adsorb various metal ions present simultaneously, and it demonstrates significant effectiveness in eliminating them from contaminated water. This is particularly the case for the uptake and elimination of Zn²⁺ ions, and the removal of Cd²⁺ and Pb²⁺ can also be enhanced. This might result from the increased specific surface area of Fe₃O₄/C@PM, a greater number of active sites, along with a richer presence of surface functional thiophene and amide groups compared to Fe₃O₄/C; therefore, it displays enhanced effects of electrostatic interactions and coordination with Zn²⁺, Cd²⁺, and Pb²⁺. The adsorbent chosen for the optimization of adsorption conditions and the investigation of the adsorption mechanism concerning Zn²⁺, Cd²⁺, and Pb²⁺ in water was Fe₃O₄/C@PM.

3.2.5. Analysis of Adsorption Isotherms

In order to enhance the assessment of the adsorption potential of Fe₃O₄/C@PM regarding Zn²⁺, Cd²⁺, and Pb²⁺, the experimental data on adsorption equilibrium were analyzed using the Langmuir and Freundlich isotherm models [31]. The corresponding formulas can be articulated as follows:

The equation representing the Langmuir model in linear form:

$${\displaystyle \frac{c_e}{Q_e}}={\displaystyle \frac{1}{Q_m{K}_L}}+{\displaystyle \frac{c_e}{Q_m}}$$

(3)

The Freundlich model is represented by a linear equation:

$$\mathit{ln}{Q}_e=\mathit{ln}{K}_F+{\displaystyle \frac{1}{n}}\mathit{ln}{c}_e$$

(4)

In Equation (4), c_e (mg·L⁻¹) denotes the concentration of metal ions once the adsorption process has attained equilibrium; Q_e and Q_m (mg·g⁻¹) signify the adsorption capacities at the equilibrium state and at the point of saturation, respectively; K_L and K_F (L·mg⁻¹) correspond to the isothermal adsorption constants defined by the Langmuir and Freundlich models, respectively; the variable n functions as the Freundlich adsorption constant and is indicative of the intensity of the adsorption process.

Figure 6 illustrates the adsorption isotherms derived from the Langmuir and Freundlich models. The parameters associated with both models are presented in

Table 1. Thermodynamic model fitting parameters of Zn²⁺, Cd²⁺, and Pb²⁺ by Fe₃O₄/C@PM.

Metal Ion	Langmuir			Freundlich
	Qm/(mg·g−1)	KL/(min−1)	R2	1/n	KF/(mg·g−1·min−1)	R2
Zn2+	349.65	0.0853	0.9980	0.31315	64.916	0.9488
Cd2+	256.41	0.0619	0.9972	0.32003	40.635	0.8922
Pb2+	180.83	0.0702	0.9994	0.28106	34.657	0.8401

, where the correlation coefficient (R²) derived from the linear equation of the Langmuir model exceeds that of the Freundlich model. This suggests that the Langmuir isotherm provides a more accurate representation of the adsorption dynamics of heavy metals on Fe₃O₄/C@PM. It can be concluded that the nature of adsorption follows a monolayer pattern, which encompasses both physical and chemisorption processes. The highest adsorption levels (Q_m) for Zn²⁺ and the calculated values for Cd²⁺ and Pb²⁺, derived from the Langmuir adsorption theory, amount to 349.65, 256.41, and 180.83 mg·g⁻¹, respectively, which closely matches the experimental observation of 343.3, 250.7, and 177.6 mg·g⁻¹. In addition, the Freundlich constant n falls within the range of 0 to 1, suggesting the existence of a significant attraction between the adsorbent and the metal ion, facilitating the adsorption process.

In order to assess the effectiveness of the Fe₃O₄/C@PM developed in this study in the adsorption of Zn²⁺, Cd²⁺, and Pb²⁺, a comparison of the maximum adsorption capabilities of previously studied magnetic carbon polymer adsorbents is provided in

Table 2. Comparison of properties of different magnetic carbon composite adsorbents.

Adsorbent	Preparation Method	Qm (mg·g−1)	Reference
Magnetic graphitic carbon nitride	Multi-step and surface imprinting technology	258.35 for Cd2+	[23]
Fe3O4/BC/AC	Multi-step reaction	161.78 for Pb2+	[32]
Fe3O4@C nanoparticles modified with–SO3H and –COOH Groups	Multi-step reaction	90.7, 83.1 and 39.7 mg·g−1 for Pb2+, Hg2+ and Cd2+	[33]
mGO/CS mGO/PA	Multi-step reaction	110.84 and 118.44 mg·g−1 for Pb2+ were obtained for mGO/CS and mGO/PA	[34]
Carbon coating and polyacrylamide functionalization of Fe3O4 nanoparticles	Hydrothermal technique	231.7 mg·g−1 for Cd2+	[35]
3D Fe3O4@MWCNT-CdIIP	Multi-step reaction	109 mg·g−1 for Cd2+	[36]
Fe3O4/C@PM	Suspension polymerization	349.65, 256.41, 180.83 mg·g−1 for Zn2+, Cd2+, Pb2+	This work

. The data indicate that the adsorbents developed in this research exhibit a notably higher maximum capacity for adsorbing Zn²⁺, Cd²⁺, and Pb²⁺ compared to certain other materials, showcasing their enhanced characteristics and significant potential for effective application in treating contaminated wastewater to eliminate these metal ions.

3.2.6. Analysis of Adsorption Kinetics

In order to assess the effectiveness of metal ion capture by the adsorbent, the effect of exposure duration on the adsorption process was examined, and the findings are illustrated in Figure S4. As the duration of adsorption increases, the adsorption capacity increases swiftly, reaching a state of equilibrium after 20 min. Once this balance has been attained, the peak adsorption capacity Q_e of Fe₃O₄/C@PM for Zn²⁺ ions, Cd²⁺, and Pb²⁺ are equal to 240, 204.6, and 139.6 mg·g⁻¹, respectively. The contents of Zn²⁺, Cd²⁺, and Pb²⁺ are as high as 90%, 76.7%, and 52.3%, respectively, showing a notable capability for absorbing Zn²⁺ ions, attributed to the significant specific surface area and the abundance of functional groups on Fe₃O₄/C@PM, enhancing its capacity to capture Cd²⁺ and Pb²⁺, as well as offering numerous active sites and a robust ability for metal coordination.

In order to investigate how Zn²⁺, Cd²⁺, and Pb²⁺ are adsorbed by Fe₃O₄/C@PM, researchers can employ a quasi-first-order and a quasi-second-order kinetic model together to analyze the obtained kinetic data. The equations representing the linear forms of the two models are as follows:

Pseudo-first-order model:

$$\ln (Q_e-Q_t)=\ln Q_e-K_{1}t$$

(5)

Pseudo-second-order framework:

$${\displaystyle \frac{t}{Q_t}}={\displaystyle \frac{1}{K_2{Q}_e^2}}+{\displaystyle \frac{1}{Q_e}}t$$

(6)

In the above equations, Q_e and Q_t (mg·g⁻¹) indicate the equilibrium adsorption capacity and the time (t, in minutes), respectively; the terms K₁ (min⁻¹) and K₂ (mg·g⁻¹·min⁻¹) denote the rate constants for quasi-first-order and quasi-second-order adsorption processes, respectively; t signifies the duration of adsorption (in minutes). The model parameters and resulting graphs can be found in

Table 3. Fitting parameters of the quasi-first-order and quasi-second-order kinetic models.

Metal Ion	Quasi-First-Order			Quasi-Second-Order
	Qe (mg·g−1)	K1 (min−1)	R2	Qe (mg·g−1)	K2 (mg·g−1·min−1)	R2
Zn2+	42.98	0.07493	0.77409	246.3	0.00291	0.99959
Cd2+	44.04	0.05279	0.89159	209.2	0.00260	0.99975
Pb2+	42.60	0.05949	0.93063	144.9	0.00258	0.99983

and Figure 7, respectively.

3.3. Desorption and Reuse

The reusability of adsorbents plays a crucial role in determining their suitability for practical applications. In this study, 0.1 mol·L⁻¹ EDTA was utilized as the eluent. The reusability of the synthesized adsorbent Fe₃O₄/C@PM was assessed, and the findings are illustrated in Figure 8. The capability of Fe₃O₄/C@PM in adsorbing Zn²⁺ was evaluated. The concentrations of Cd²⁺ and Pb²⁺ showed minimal reduction despite increasing the number of cycles of adsorption and desorption. Following six cycles of this process, during which Zn²⁺ was completely adsorbed, the removal efficiency for Cd²⁺ and Pb²⁺ on Fe₃O₄/C@PM remained above 81%. The capacity for adsorption could achieve 196.4, 162.9 and 107.9 mg·g⁻¹, respectively, for these three metal ions, showing favorable properties regarding both reusability and stability, especially in the context of Zn²⁺ adsorption. Thus, Fe₃O₄/C@PM demonstrates significant potential as an effective adsorbent for the repeated extraction of wastewater pollutants, particularly those containing Zn²⁺, Cd²⁺, and Pb²⁺.

4. Conclusions

In this study, the synthesis of polymeric materials with magnetic properties was accomplished using 2-thiophene formaldehyde alongside acrylamide as the primary monomer. Carbon nanoparticles with magnetic properties were utilized as a base material, EGDMA as a crosslinker, and the suspension polymerization technique was employed with a water-to-oil ratio of 1:40. The resulting magnetic carbon nanopolymer exhibited an irregular layered flower-like morphology characterized by a rough surface, good dispersibility, magnetic properties, and robust stability. This material boasts a specific surface area of 348.7 m²·g⁻¹, with the pore diameter predominantly falling within the range of 0.5–10 nm. The highest absorption capacities of Fe₃O₄/C@PM for Zn²⁺, Cd²⁺, and Pb²⁺ metal ions were recorded at 343.3, 250.7, and 177.6 mg·g⁻¹, respectively. For the uptake characteristics of Fe₃O₄/C@PM, the kinetics of Zn²⁺, Cd²⁺, and Pb²⁺ adsorption aligned with the quasi-second-order model, and the adsorption characteristics of heavy metal ions on Fe₃O₄/C@PM align with the Langmuir adsorption isotherm, suggesting that the adsorption mechanism is monolayer chemisorption. Overall, the Fe₃O₄/C@PM serves as a novel and highly effective material for adsorption, which holds significant promise for the remediation of wastewater contaminated with Zn²⁺, Cd²⁺, and Pb²⁺.