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Article

Adsorption Ability of Soft Magnetic FeCo Alloys for Microplastics

by
Shenghao Zhang
1,†,
Lan Zhang
1,†,
Xingfu Wang
2,
Pinhua Xia
1,
Zhenming Zhang
3,
Xianfei Huang
1,* and
Xuetao Guo
4,*
1
Guizhou Key Laboratory of Plateau Wetland Conservation and Restoration, Guizhou Normal University, Guiyang 550025, China
2
School of Health Management, Guiyang Healthcare Vocational University, Guiyang 550081, China
3
College of Resources and Environmental Engineering, Guizhou University, Guiyang 550025, China
4
College of Natural Resources and Environment, Northwest A&F University, Xianyang 712100, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Water 2025, 17(22), 3305; https://doi.org/10.3390/w17223305
Submission received: 16 October 2025 / Revised: 11 November 2025 / Accepted: 17 November 2025 / Published: 19 November 2025
(This article belongs to the Section Wastewater Treatment and Reuse)
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Abstract

Microplastics pose significant threats to human health and the environment, and environmentally friendly, efficient, and reliable technologies are needed to remove them from the environment. To explore the potential for metal nanoadsorbents to remove microplastics (MPs) from water, soft magnetic FeCo alloys were successfully synthesized and loaded onto carboxymethyl cellulose. The adsorption performance and mechanisms of soft magnetic FeCo alloys on typical microplastic pollutants, specifically polyethylene microplastics, were systematically studied. Characterization work performed through scanning electron microscopy, an X-ray diffraction analysis and Brunner−Emmett−Teller measurements (SEM, XRD and BET, respectively) confirmed that soft magnetic FeCo alloys possess a typical solid solution structure, abundant hydroxyl functional groups, and a high BET surface area of 142.8302 m2/g, providing sufficient active sites for MP adsorption. At 35 °C, the equilibrium adsorption capacity reached 120.92 mg/g, with a removal rate of 89.33% at a sufficient adsorbent concentration (0.5 g/L); the adsorption kinetics followed a pseudo-second-order kinetic model (R2 > 0.99). Additionally, the adsorption isotherm conformed to the Freundlich model (R2 = 0.969), demonstrating multilayer adsorption characteristics. Furthermore, X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy (XPS and FTIR analyses, respectively) indicated that the surface of FeCo is hydrophilic because of catalytic oxidation reactions, resulting in the exposure of polar groups (such as carboxyl and hydroxyl groups) on the microplastic surface, which form hydrogen bonds with oxygen or water molecules on the FeCo surface. The resulting redox vacancies provide ample active sites for the adsorption of microplastics. This study confirms the high microplastic adsorption capacity of soft magnetic FeCo alloys in aquatic environments.

1. Introduction

Iron-based alloys are primarily composed of iron, which is usually combined with elements such as carbon, chromium, and nickel to enhance properties such as its strength, durability, and corrosion resistance. Research on iron-based alloys in the field of adsorption originates from their unique material properties and broad application potential. Iron oxides such as wüstite (FeO), hematite (α-Fe2O3), maghemite (γ-Fe2O3), and magnetite (Fe3O4) have attracted great attention as efficient adsorbents and catalysts in environmental applications because of their abundant reserves [1], nontoxicity, ease of preparation, and operational safety. The research background of iron-based adsorbents can be traced back to the need to remove pollutants, such as phosphates, heavy metal ions, and organic pollutants, from water bodies [2,3,4,5]. Iron-based materials are considered among the key strategies for controlling eutrophication because of their strong affinity for phosphates, low costs, and environmental friendliness [6,7,8]. Wang et al. prepared a novel adsorbent by dispersing iron oxide on activated carbon and reported that a preoxidation treatment could significantly increase the iron loading effect, thereby increasing the phosphate adsorption capacity of the material [9].
Microplastic and nanoplastic pollution in aquatic environments is another serious environmental issue. The current research on removing microplastics from water includes flocculation, microbial degradation, photodegradation, and membrane technology. In fact, membrane technology is among the most promising methods for removing microplastics from water, with a removal rate of up to 99.99% [10]. Iron-based adsorbent materials are widely regarded as having great potential for removing organic pollutants from water because of their high efficiency levels and low costs [11]. Sun et al. [12] used ferric chloride to modify activated alumina and synthesized an iron-based alumina composite material. They reported that the adsorption performance of the iron-based alumina composite modified with a 15% ferric chloride solution reached a certain peak. To some extent, the slower the flow rate of the fluoride solution was, the better the adsorption performance of the adsorbent. Moreover, at pH = 6.5, it exhibited relatively good fluoride adsorption performance. Xu et al. [13] proposed an innovative iron–nitrogen codoped layered adsorbent derived from glucose via molten salt-assisted pyrolysis. The results showed that at low plastic concentrations and adsorbent dosages, the codoped layered adsorbent with an iron loading of 1.43% achieved a removal efficiency of 96.5% for polystyrene (PS) microplastics, with an adsorption capacity of 148.39 mg·g−1.
In addition to water treatment, iron-based alloys have also shown potential regarding the adsorption of gaseous pollutants. For example, Zhang et al. [14] prepared Nb2O5 nanorods with oxygen vacancies (OVs) through an N atom doping strategy for the visible light-driven oxidative dehydrogenation (ODH) of saturated N-heterocyclic compounds, where the OV sites could promote O2 adsorption. Although this mainly involved a catalytic process, it also highlighted the important role of defect sites in facilitating molecular adsorption. With respect to the treatment of volatile organic compounds (VOCs), metal–organic framework (MOF) materials have attracted attention because of their large specific surface areas, uniform pore sizes, and tunable functional groups [15]. Ma et al. [16] systematically synthesized three iron-based MOFs, i.e., MIL-100(Fe), MIL-101(Fe), and MIL-53(Fe), and used them to adsorb typical VOCs such as toluene; the results revealed that these materials have good adsorption performance. Research on the adsorption mechanism of iron-based alloys is crucial for understanding and optimizing their performance. The adsorption mechanism usually involves physical and chemical adsorption. Physical adsorption primarily relies on van der Waals forces, whereas chemical adsorption involves chemical bonding between the adsorbent and the adsorbate [17]. For example, a density functional theory (DFT)-based study on the adsorption energy of water molecules on iron- and cobalt-doped surfaces revealed that the distance between a water molecule and an iron atom was 2.111 Å, with an adsorption energy of −0.785 eV, whereas the distance to a cobalt atom was 2.124 Å, with an adsorption energy of −0.654 eV, indicating that different metal atoms affect water absorption differently [18].
In-depth research on the typical metal hydroxides of iron has revealed that introducing oxygen vacancies or doped metals to regulate electronic structures can enhance the catalytic activity or adsorption selectivity of materials. Han et al. [19] reported that the substitution of goethite with metal elements possessing similar radii could greatly change its physicochemical properties. The structural defects generated by the introduction of cobalt in this study significantly increased the number of hydroxyl sites and the electron transport capabilities of the catalyst. These defects can not only change the surrounding electronic structure but also serve as docking sites for capturing atomic metal species, forming new synergistic coordination structures as active sites [20]. Moreover, Lu et al. [21] summarized various typical layered bimetallic hydroxides whose electrocatalytic activity can be improved by adjusting their structure. This structure not only has a controllable layered form and an internally stable electron composition but also has been extensively applied in adsorption treatments for anionic wastewater. Cobalt substitution can stabilize the local environment of iron and promote π-symmetric bonding orbitals in the layered double-hydroxide structure (LDH), optimizing the electronic structures of NiFe-LDHs and further combining the advantages of many exposed active sites on the surfaces of hierarchical composite material structures [22]. However, owing to its poor conductivity and low utilization of internal metal sites, the LDH is generally widely used in electrochemical settings. Liu et al. [23] constructed a new type of alloy material in porous carbon fibers to provide new insights for achieving nitrate reduction, effectively promoting the adsorption of NO3- and the rapid desorption of NH3. Isocore bimetallic alloys have good soft magnetic properties, but the stability of these materials needs to be improved. Therefore, this study focused on constructing a new alloy material that combines a cobalt-iron alloy with porous carbon fibers.
In this study, cobalt and iron were incorporated into carboxymethylcellulose via the coprecipitation method, and the resulting reactants were confirmed to have a typical FeCo solid solution structure, combining the flexible porous characteristics of carboxymethyl cellulose (CMC) with the adsorption function of iron and cobalt metal active sites. For evaluating the adsorption-based removal of microplastics from water at the laboratory scale, the focus was on polymer materials represented by polyethylene microspheres, which were chosen on the basis of their significant role in promoting plastic pollution in aquatic ecosystems and freshwater environments. The adsorption performance and influencing factors of FeCo on PE-MPs were studied by changing the experimental conditions, such as the reaction time, initial concentration, pH, and competing ions. Characterizations of FeCo before and after the adsorption of PE-MPs were conducted using XRD, SEM, FTIR, and XPS methods. Furthermore, fitting models of the adsorption kinetics, adsorption thermodynamics, and adsorption isotherms were employed to preliminarily investigate the adsorption behavior and removal mechanism of the prepared FeCo for PE-MPs.

2. Materials and Methods

2.1. Chemicals and Materials

All the reagents used in this study were analytically pure. Potassium hydroxide (NaOH) and hydroxylamine hydrate (NH3OH·H2O) (Jin Shan Chemical Reagent Co. Ltd., Chengdu, China). Ammonium oxalate ((NH4)2C2O4·H2O), iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O), and carboxymethyl cellulose (CMC) (Kermel Chemical Reagent Development Center Co. Ltd., Tianjin, China). Cobalt(II) chloride hexahydrate (CoCl2·6H2O) (Zhiyuan Chemical Reagent Co. Ltd., Tianjin, China). PE microplastics (Sinopec Maoming Petrochemical Company Co. Ltd., Maoming, China).

2.2. Materials Preparation Methods

Fe(NO3)3·9H2O and Co(Cl)2·6H2O were dissolved in 500 mL of ultrapure water, 2.5 mol/L KOH solution was added at a rate of 5 mL/min under stirring, the suspension was maintained at a pH > 12, and the samples were then aged for 5 days at room temperature. During this period, the container lid was opened, and the suspension was stirred for half an hour each day for gas exchange purposes. After performing aging, the supernatant was discarded, and the remaining slurry was washed with 0.2 mol/L ammonium oxalate solution at room temperature at 80 r/min for 2 h in the dark. After undergoing centrifugation and washing, the obtained solid was dried in a 60 °C oven, ground through a 100-mesh sieve, and stored for later use. The solid was dissolved in a CMC solution (1.0% w/v, 200 mL) and heated with vigorous stirring to 40 ± 2 °C. Afterward, the mixture was heated to 80 ± 2 °C under a nitrogen atmosphere, 12 mL of 25% NH4OH solution was added to the reaction flask, and the mixture was homogenized for 20 min with moderate stirring. Upon undergoing purification for 24 h, the solution was cooled to room temperature and centrifuged at −20 °C for 10 min (1900 r/min), after which the solid sample was collected, freeze-dried, and stored at 6 ± 2 °C for further use.

2.3. Material Characterization and Analysis

X-ray diffraction analysis (XRD, X-Pert PRO powder diffractometer, Anton Paar XRDynamic500, AT) was for crystal structure and material composition determination purposes. A Cu target, Kα radiation (k = 1.5418 Å), a voltage of 40 kV, a current of 40 mA, a scanning range of 2θ = 10–90°, and a scanning speed of 2°/min were used. Fourier-transform infrared spectroscopy (FTIR, Thermofisher Nicolet iS 10, Thermo Fisher Scientific Nicolet iS20, Waltham, MA, USA, USA) was used to analyze the functional groups and chemical bonds of the samples, with a scanning range of 500–4000 cm−1. Scanning electron microscopy (SEM, ZEISS GeminiSEM 300, DE) analysis was carried out to analyze the surface structure and morphology of the material, with an accelerating voltage of 5 kV and a magnification of 10,000–30,000 times. The specific surface area and pore volume distribution of the material were analyzed using an automatic surface area and porosity analyzer (BET, Quantachrome Autosorb-iQ, USA, Boynton Beach, FL, USA), with high-purity nitrogen employed as the adsorption medium. The samples were degassed under vacuum conditions at 150 °C for 12 h; this was followed by nitrogen adsorption–desorption measurements at 77 K in a liquid nitrogen environment. The specific surface areas, pore volumes, and pore sizes of the samples were calculated using theoretical models such as BET and BJH. The weight loss integral curve of the samples was obtained through a thermogravimetric analysis.

2.4. Adsorption of FeCo on PE-MPs

To determine the optimal parameters for removing polyethylene microplastics from water suspensions, batch adsorption studies were conducted using FeCo composite materials. A certain amount of PE suspension was placed in a glass bottle containing FeCo composite materials, and adsorbent concentration gradients were established. Adsorption experiments were conducted by placing mixtures with different concentrations on a shaker at room temperature, shaking them for a specific amount of time until a uniformly dispersed body was obtained, collecting the solid samples via vacuum filtration, drying them in an oven, and separating the in situ adsorbent material using a magnetic stirrer. The formed complexes were separated from the suspension by applying a permanent magnet after static equilibrium was reached. The remaining unadsorbed microplastics in the solid were weighed, and the weight change between the states before and after the calculation was calculated. The concentration of the remaining polyethylene microplastics in the suspension was determined, and the solution concentration (Ct) and adsorption amount (qt) at time t were calculated by fitting the adsorption curve using Equation (1) [24]:
q t = C 0 − C e m × V
where C0 represents the initial mass concentration of the solution to be adsorbed in mg/L; Ce represents the mass concentration of the supernatant at adsorption equilibrium in mg/L; m represents the mass of the adsorbent; and V represents the volume of the solution to be adsorbed.
The adsorption obeying the kinetic equation is shown in Equation (2) [25]:
d q t d t = k n q e − q n
In the formula, n is the adsorption order, kn is the pseudo-first-order adsorption rate constant, and qe is the adsorption capacity at equilibrium. Usually, n is taken as 1 or 2, corresponding to pseudo-first-order and pseudo-second-order kinetics, respectively, and integration yields pseudo-first-order and pseudo-second-order kinetic modeling Equations (3) and (4) [26,27]:
q t = q e 1 − e − k 1 t
q t = q e 2 k 2 t 1 + q e k 2 t
The adsorption isotherms were fitted using the Langmuir model, Freundlich model, and intraparticle diffusion model (5)–(7) [28,29,30].
Langmuir model:
q e = q m C e K L 1 + C e K L
Freundlich model:
q e = K f C e n
Elovich model:
q t = 1 β ln α β t + 1
where KL represents the Langmuir constant (L/mg), qm represents the maximum adsorption capacity (mg/g), Kf represents the affinity coefficient of the Freundlich equation [(mg/g)/(mg/L) n], and n represents the nonlinear index of the Freundlich equation; the smaller the n value is, the greater the nonlinearity of the sorption isotherm. α is the initial sorption rate constant (mg/g), and β is the desorption constant (mg/g).

3. Results and Discussion

3.1. Characterization Analysis of Adsorption Materials

The X-ray diffraction technique was integrated to analyze the phase structures and crystallinity of the synthesized samples, and the refined XRD spectrum and standard card of the FeCo nanocomposite materials, obtained using GSAS-II software 5455 (python 3.10.8 64-bit), are shown in Figure 1. The results indicate that the characteristic peaks of FeCo were located at 2θ = 41.419° (d110 = 1.1983, belonging to the cubic crystal system of the cobalt-iron alloy phase), 35.072°, 50.463°, 62.871°, 67.353°, and 74.195°, and minor impurity peaks formed due to the incompleteness of the solid solution at 13.326° and 24.849°. The refined XRD spectrum of the material shows that the main component of FeCo was the alloy phase of (Co1.4Fe0.6)HO, whose peaks corresponded to the cubic crystal system of FeCo, which was formed mainly because of the formation of a Co-doped Fe3O4 solid solution in the material [31]. Owing to the similar atomic radii and crystal structures of cobalt and iron, they formed substitution solid solutions during the precipitation process, with cubic lattice parameters that were close to the atomic ratio of the FeCo alloy [32]. The lattice parameters gradually changed with increasing Co content during the formation of solid solutions, thus maintaining the stability of the structure. The hydroxide oxide existed as a surface layer, and its structure was further studied in combination with an SEM analysis.
SEM scanning electron microscopy images obtained before and after adsorption process are shown in Figure 2. The surface presented a porous layered structure with significant defect vacancies, embedded nanoparticles, and a relatively uniform pore distribution. At a scale of 20 μm, the macroscopic morphology of the material appeared as a lamellar structure covered with nanoparticles, indicating that the material may have formed a composite structure through self-assembly or deposition [33]. Moreover, the particles were uniformly distributed at the microscopic level without regional aggregation; at a scale of 500 nm before adsorption, the particles were nearly spherical in shape and evenly distributed without obvious agglomeration, and the surface was smooth, indicating good dispersibility during the sample preparation stage. Significant particle deformation was observed at the microscopic scale before and after the adsorption process, with microplastic particles adhering to the surface and the outer surface being damaged, exposing internal fibrous-like parts. An SEM characterization analysis revealed that the sample had a nanoparticle–porous composite matrix structure with a narrow particle size distribution, good dispersibility, and strong structural stability, with Fe and Co nanoparticles distributed within the diameter range of 130.501–332.185 nm. The structural morphology of the mixed FeCo nanostructure, where iron and cobalt nanoparticles were uniformly dispersed and anchored on the CMC, confirming the formation of the FeCo nanocomposite material, is shown in Figure 2. Additionally, to confirm the formation of the nanocomposite material, an EDS analysis of the FeCo nanocomposite material was performed, as shown in Figure 2.
EDS clearly revealed the presence of all the elements except for H (including C, O, Fe, and Co), indicating that the material was an iron–cobalt composite material (Figure 3). The introduction of Fe and Co provided potential catalytic or adsorption active sites for the material, where Fe and Co partially overlapped at low spectral numbers due to EDS resolution limitations. The atomic ratio of Fe to Co is close to 2:1, and an XRD analysis of the material revealed the formation of an alloy phase of a cubic FeCo core, where iron ions were in a cubic closest-packed structure and cobalt ions filled half of the tetrahedral voids. Furthermore, face scanning revealed a uniform distribution of iron and cobalt, indicating that the material was an iron core bimetallic complex, with both metals exhibiting synergistic catalytic activity [34]. The results of the EDX elemental analysis before and after the material was adsorbed are shown in Figure 4, where significant changes in the element contents are observed (Table 1). Increases in the carbon and oxygen contents indicated the presence of oxygen-containing functional groups attached to the surface or interior of the material during the adsorption of microplastics, whereas decreases in the cobalt and iron contents were attributed to a relative decrease in the proportion of these groups. It is also possible that carbon and oxygen functional groups covered these sites. However, the elemental distribution spectrum showed an increase in the number of cobalt sites, whereas the number of iron sites decreased only slightly. In summary, the adsorption of microplastics by FeCo materials is related not only to the generated oxygen-containing functional groups but also to the types and quantities of metal sites.
The nitrogen adsorption–desorption isotherms of FeCo are shown in Figure 5, and the pore size information of the materials is listed in Table 2. The BET surface area of the material was 142.8302 m2/g. The micropore specific surface area determined according to the t-plot method was 10.5186 m2/g, while the external specific surface area calculated according to the t-plot method was 132.3116 m2/g. Owing to the appearance of a false desorption peak at 3.8 nm, the average pore diameter reflected by the adsorption curve was 7.7283 nm, and the average pore diameter of the mesopores was 7.9508 nm, indicating that the material was mesoporous. The curve shows that FeCo is a microporous material with a hysteresis loop structure formed by capillary condensation before and after the adsorption of microplastics. Many irregular mesopores are loaded on the surface, and the origin of the formation process is considered slit pores formed by the accumulation of flake-like particles [35]. The utilization rate of surface sites reached 66.83% according to the BET results. FeCo mainly fills micropores as its main adsorption mechanism, and adsorbate molecules preferentially enter the micropores with high affinity. When the micropores are close to saturation, adsorption may shift to mesopores or multilayer adsorption on the surface.

3.2. Isothermal Adsorption Experiment

The FeCo isotherm adsorption curves of the PE-MPs produced at 288, 298, and 308 K are shown in Figure 6. The amount of PE-MPs adsorbed by the material rapidly increased as the initial mass concentration of the PE-MPs in the solution ranged from 0 to 800 mg·L−1.
The calculation parameters of the corresponding model are listed in Table 3. The fitting results indicate that the FeCo composite materials were more consistent with the Freundlich isotherm model at different temperatures and that the correlation coefficients were greater than those of the Langmuir model at different temperatures, which could be attributed to the synergistic effect and interlayer heterogeneity of the metal structure. Owing to the nonuniformity of the adsorbent surface, the energy distribution of the adsorption sites decreased exponentially, which was highly consistent with the characteristics of nanocomposite materials. High porosity and a multilevel pore structure are conducive to the multilayer accumulation of adsorbates on surfaces, and the unique multilevel pore structure, surface functional group modification, and metal oxide doping process led to an uneven energy distribution for the adsorption sites. Adsorbates may continue to accumulate on the adsorbed layer through physical and weak chemical adsorption in the mid-low concentration range [36].
Porous adsorbent materials (such as modified iron-based composites) theoretically have site heterogeneity [37], but in experiments, the introduced functional groups (such as PDA and hydroxyl groups) may be uniformly distributed on the surfaces, making the originally heterogeneous sites appear “pseudohomogeneous [38].” According to the SEM observations, the adsorbate appeared to accumulate on the material surface, which can preliminarily support the notion that it better conforms to the Freundlich model in a physical sense.
To explain the thermodynamic characteristics of the adsorption reactions, the Gibbs free energy equation was used to describe the Gibbs free energy (ΔG0), enthalpy (ΔH0), and entropy (ΔS0). The parameters were calculated on the basis of the curve shown in Figure 7, and the activation energy was fitted through temperature-variable adsorption experiments, which revealed that the micropore diffusion activation energy should be significantly lower than the chemical adsorption activation energy; when the adsorption entropy change for thermodynamic consistency was calculated, the entropy reduction in the micropore filling region was significantly greater than that in the chemical adsorption region (Table 4). A positive value of ΔG corresponds to the feasibility and nonspontaneity of the system, indicating that the reaction in the system is nonspontaneous and dominated by an endothermic effect and that absorbing heat is required for the adsorbate to bond with the adsorbent by breaking the solvation layer and overcoming the energy barrier of the adsorption site. High temperatures may lead to desorption, indicating that an increase in entropy significantly enhances the contribution to the reaction with increasing temperature, thus becoming the main driving factor for adsorption [39]. A positive value of ΔS indicates that solvent molecules are released or that the conformational degrees of freedom of the adsorbate and adsorbent increase during the adsorption process, leading to a decrease in the order of the adsorbent surface after the adsorption sites are occupied; this results in a disorderly arrangement (in the solution, because of hydrogen bonding or electrostatic constraints, more rotation and vibration degrees of freedom may be obtained after adsorption due to surface hydrophobic interactions). A positive value of ΔH indicates that the adsorption process is essentially endothermic, where the chemical bonds on the surface are broken to form coordination bonds with microplastic particles, and the distortion of the microplastic molecules or the need to overcome the entropy decrease resistance barrier during the adsorption process (such as entropy losses of polymer chains) occurs [40]. Combining the results of the XPS and SEM analyses, the surface-modified hydroxyl groups continuously adjust their conformations to fit the adsorbates, while microplastics entering FeCo mesopores may have caused curvature or skeletal distortions of the pore walls [41]. Both processes require energy involvement, and furthermore, the decrease in the ΔG value with increasing temperature confirms that increasing the temperature benefits the progress of the reaction, which is consistent with the results of the adsorption isotherm and adsorption kinetic studies.

3.3. Adsorption Kinetics Experiment

The pathways, mechanisms, and rates of PE–MP adsorption onto FeCo nanocomposites were evaluated. Ten, 50, and 100 mg/L of FeCo were added to separate vials containing 10 mL of PE suspension, where the PE microspheres adsorbed onto the adsorbent and underwent continuous oscillation for 400 min. Aliquots were collected at regular intervals; the vials were mixed on a magnetic stirrer for 10 min to separate the complexes; the remaining unadsorbed microplastic solids were fileted, dried, and weighed; and the equilibrium adsorption capacity qe and removal efficiency at that time were calculated. The removal efficiency of the FeCo nanocomposites increased significantly, as shown in the figure, and the system reached equilibrium in terms of removal efficiency after 200 min. The adsorption behavior of the FeCo nanocomposites was evaluated by fitting the experimental data to a kinetic model.
The fitting graphs of the pseudo-first-order kinetics, pseudo-second-order kinetics, and intraparticle diffusion models are shown in Figure 8, with the calculated kinetic parameters listed in Table 3. The kinetic parameters indicate that the adsorption of PE microplastics on FeCo at medium to low concentrations follows the assumption of pseudo-second-order kinetic fitting, which fits the experimental data better than the first-order kinetics model does. This suggests that the adsorption kinetics of PE microplastics on FeCo are controlled by surface chemical adsorption reactions (such as coordination and electrostatic interactions) rather than diffusion or physical adsorption processes. Additionally, the fitting of the intraparticle diffusion model also yielded a high R2, indicating that PE-MPs may diffuse into pores during the adsorption process [42]. The fitting results of the intraparticle diffusion experiment conducted at different concentrations are segmented: the first stage of adsorption is close to the origin and is mainly controlled by intraparticle diffusion, which governs the adsorption rate; the trend of the second stage of adsorption becoming slower suggests that the main mechanism is membrane diffusion; and the third stage approaches adsorption equilibrium, indicating that the adsorption of FeCo on the PE-MPs tends to saturate. The decreasing trend of the slope in the predicted results stages eventually reaches a plateau. The above mechanisms are supported by the experimental evidence mentioned above: when the initial concentration of the adsorbent is changed, the equilibrium adsorption amount qe tends toward saturation, and the apparent rate constant k1 does not change significantly with the concentration, which is consistent with the assumptions of the pseudo-second-order kinetic model. Furthermore, the adsorption rate is positively correlated with the number of active sites on the adsorbent surface that are not occupied rather than directly depending on the concentration gradient of the adsorbate in the liquid phase. This characteristic contrasts sharply with mass transfer processes that are dominated by physical adsorption, which usually follow Fick’s diffusion law, where the adsorption rate is directly related to the concentration gradient of the solution and the difference in the interface concentration; i.e., the hydrophobic interactions on the metal surface play a dominant role in adsorption [43].

3.4. Study on the Adsorption Behaviors Observed Under Different Driving Factors

To explore the factors that can affect the adsorption process, the adsorption behaviors of the materials on the microplastics were comprehensively described by changing the experimental conditions of the initial reaction and reaction process and calculating the adsorption amount and removal efficiency. As shown in Figure 9, at 298 K and an adsorption time of 3 h, FeCo was added to a solution of 25-mg/L PE-MPs at a specific concentration. As shown in the figure, the adsorption efficiency increased from an initial value of 7.91% to 89.33%, whereas the amount of PE adsorbed gradually decreased with increasing addition of FeCo. This is because the increase in the amount of adsorbent added increases the number of effective active sites, which is advantageous for the removal of pollutants [44]. The influence of the initial pH on the adsorption and removal of microplastics is shown in Figure 10. The solution pH can affect the adsorption effect by influencing the surface charge distribution and functional groups of the adsorbent. With an adsorption time of 1 h, a temperature of 298 K, and an initial FeCo concentration of 2.5 mg/L for the PE-MPs, the pH was set to 1–12 to explore its effect on the adsorption process. Significant change patterns were not observed at medium and low concentrations, indicating that the adsorption of FeCo on PE-MPs is less affected by the initial pH value, possibly because the material is a corrosion-resistant alloy. Iron-based composite alloy materials adsorb microplastics, and their adsorption capacities are unstable with changing pH, mainly because of complex multifactor interactions occurring between the surface chemical properties of the composite materials, the surface characteristics of the microplastics, and the chemical environment of the solution [45]. The equilibrium adsorption capacity of the material sharply decreased from approximately 9 mg/g to approximately 7 mg/g when the pH increased from 1.0 to 4.0, sharply increased from 7.0 to 9.0, and gradually decreased from 10–12 (Figure 10). Combined with the removal efficiency, these results indicate that the material is more conducive to the adsorption of microplastics in neutral and weakly alkaline environments. This is because the surface charges of iron-based composite materials vary with respect to the pH, and a change in the surface charge directly affects the electrostatic interactions between the material and microplastics [46]. The other chemical components contained in the solution, including the form of the metal ions, the dissolved organic matter (DOM), and the presence of competitive ions, are all regulated by the pH and indirectly affect the instability of adsorption [47]. In a low-acidity environment with a pH ≤ 2, surface protonation of iron-based materials occurs, and microplastics are adsorbed in the form of smaller particulate NPs through hydrogen bonding. At pH values of 3–6, the surface hydroxyl groups form inner-layer complexes with the MPs; at pH values of 7–10, the MPs exist in a negatively charged form and generate electrostatic repulsion with negatively charged iron-based surfaces; at pH values of 9–12, the surface forms a complex with the adsorbate through its surface hydroxyl groups, and the functional groups in the iron-rich matrix precipitate through the dissolution of Fe(III). This reaction is inhibited by the pH [48].
Iron-based alloys are materials with face-centered or body-centered cubic lattice structures that form substitutional solid solutions with other metallic alloying elements and can affect surface properties. Reports on iron-based alloy composites have shown that some materials exhibit excellent adsorption performance. Li et al. [49] reported on an Fe3O4@PDA adsorbent made from magnetite (Fe3O4) nanoparticles coated with adhesive polydopamine (PDA). The adsorption efficiency of the Fe3O4 nanoparticles themselves for microplastics was less than 70%, whereas the Fe3O4@PDA adsorbent could significantly increase the microplastic removal efficiency of the system to 98.5%. This is attributed to the PDA shell, which enhances the degree of adhesion to microplastics through the formation of hydrogen bonds, π–π stacking, and hydrophobic interactions. Cao et al. [50] produced two-dimensional ultrathin iron oxide nanodisks, which exhibited a significant adsorption capacity of 188.4 mg/g. Combined with pseudo-first-order kinetics and the Sips model, the adsorption of MPs/NPs was driven by electrostatic and magnetic forces originating from the vortex domain of the nanodisks (NDs). Xia et al. [51] prepared a functionalized magnetic nanoiron oxide-biochar composite (MFe@BC) via impregnation-pyrolysis to simulate the removal of microsized poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) microplastics from wastewater. At the optimal dosage (1.5 g/L), the adsorption capacity reached 13.14 mg/g, with a removal efficiency of 98.53%. After four adsorption–regeneration cycles, its removal efficiency for PHBV microplastics remained above 92%. To gain a more comprehensive understanding of the adsorption potential of iron-based alloy composites in real water environments, future research should focus on addressing their crystallographic plane dependence while elucidating the mechanisms by which alloying elements enhance catalytic activity through electron redistribution. The results of studies conducted on the removal of MPs using iron-based alloy composites and reported above are summarized in Table 5. These findings indicate that iron-based alloy composites have great potential for use in future research on adsorbing and removing microplastics.

3.5. Adsorption Mechanism Analysis

Since the strength of chemical adsorption reflects the activity of surface reactions, the surface coordination bonds formed between adsorbed molecules and limited surface atoms often act on the adsorption surface. The adsorption performance of materials is dominated by the number of active sites produced by radicals active on the surface, so the adsorption capacity can be effectively improved by regulating the surface defect structures of materials or increasing the exposure levels of active sites (nanoscale, doping, etching, etc.). Moreover, the strength of the coordination bonds determines the irreversibility and selectivity of adsorption, where strong coordination can enhance the directional adsorption of specific molecules [52].
To further elucidate the interaction mechanisms between the materials and adsorbents, the materials were characterized before and after the adsorption step. The surface functional groups of the catalyst were identified by an FTIR analysis (Figure 11). The strong absorption peak at 582 cm−1 was attributed to the in-plane bending vibration of C-C and C-O, which was caused by the medium-to-strong absorption peak shifting from 580 to 560 cm−1 when the α position was substituted. After adsorption, this peak shifted to 548 cm−1, possibly because of changes in the coordination environment or lattice distortion leading to a redshift. The weak peak at 1333 cm−1 can be attributed to the in-plane deformation vibration of -OH, but the dilution caused the band to shift toward lower wavenumbers. After executing a combined XRD and XPS analysis, the absorption peak at 1637 cm−1 was attributed to the coordination vibration between metal ions (such as Co2+ and Fe3+) and organic ligands, indicating the chemical bonding between metal ions and ligands as well as the C-O stretching vibration in organic ligands, especially when coordinated with metal ions (such as cobalt or iron). The broadening or shifting of the hydroxyl free radical stretching vibration peak near 3400 cm−1 in the FTIR spectrum indicates a hydrogen bonding enhancement [53]. Therefore, the medium peak observed at 3360 cm−1 can be attributed to the metal–hydroxyl bond formed by the stretching vibration of hydroxyl groups, leading to coordination with the metal ions, whereas the medium peak at 3653 cm−1 is likely due to the stretching vibration of free hydroxyl groups (O-H) in the sample. Compared with that before adsorption, the intensity of the main peak after adsorption increased and shifted to varying degrees, possibly as a result of the synergistic effect of the oxygen functional groups and hydroxyl coordination bonds in PE-MPs [54].
XPS can provide information about the elemental compositions and chemical states of material surfaces (Figure 12). The full spectrum (Figure 12a) shows that the FeCo surface contains C, O, Co, and Fe. No significant changes were observed in the peak positions before and after adsorption, indicating that FeCo was successfully loaded with Fe and Co and that the sites were stable. The O1s core level spectrum (Figure 12b) shows four peaks at 529.1, 531.35, 530.4, and 531.95 eV, corresponding to Fe-O-Fe, Fe-OH, Co-OH, and Co-O, respectively. Changes occurred in the surface states of FeCo before and after undergoing adsorption, with the O1s and C1s orbital peaks shifting toward higher binding energies. The Fe-O-Fe, Co-OH, and Co-O contents decreased by 22.34%, 18.89%, and 7.59%, respectively, while the Co-OH content increased by 48.82%. This suggests that the PE in the solution may have been adsorbed by replacing the -O on the template layer to form oxides [55]. The C1s characteristic peaks (Figure 12c) correspond to C-C and C-O/C=O, and the carbon–oxygen bond content increased by 10.95% after adsorption, indicating that the increase in C-C content is related to the oxygen functional groups coordinated with the hydroxyl groups on the surface; this is consistent with the FTIR characterization results.
Representations of different oxidation forms of Fe2p1/2 and Fe2p3/2 are shown in Figure 12d, with their absorption peak positions at 709.94 and 723.56 eV, respectively. Within the range of 700.4~739.1 eV, four peaks are present. The higher binding energy of Fe2p1/2 (723.5~724 eV) is caused by the increase in the oxidation state (such as Fe3+), which enhances the effective nuclear charge. Significant changes in the conduction band electron content occur in the adsorbed Fe2p3/2 as a result of the conversion of partially mixed Fe2+ and Fe3+ states into predominantly Fe3+ after adsorption. Compared with that of Fe2+, the decrease in the number of 3d orbital electrons of Fe3+ leads to a decrease in the electron density of the conduction band, causing an increase in the binding energy of Fe2p3/2 in XPS [56]. The crystal field splitting energy of Fe3+ in an octahedral field (such as Fe2O3) is different from that of Fe2+ in a tetrahedral field (such as Fe3O4), and the local electron density of Fe changes along with the restructuring of the coordination environment [57]. This phenomenon is consistent with the increasing proportions of Fe2O3 and Fe3O4 in the O1s spectrum, further confirming that the mechanism of the adsorption-induced surface oxidation reaction dominates the chemical state transformation of the material. The formation of peaks at 709.6~711.1 eV is attributed to the increase in the number of positively charged surface atoms, as well as the localized strain and quantum confinement resulting from the presence of defect sites, leading to the densification of the charge and energy near the defect and the enhancement of the Hamiltonian, causing a positive shift in the binding energy of small clusters [58].
The Co2p spectrum in Figure 12e shows three peaks at 771.4~812.2 eV. The higher binding energy at Co2p1/2 (789.04~795.02 eV) is attributed to the binding energy changes exhibited by Co2+ and Co3+, as well as the electronic structure influenced by differences in coordination [59]. The chemical shift in Co2p3/2 (779.45~781.16 eV) is possibly related to the covalent strengthening of the metal–oxygen bonds in the mixed oxidation states of Co(II) and Co(III) in CoFe2O4, leading to slight decreases in the binding energy and peak shift [60]. The significant shift exhibited by the characteristic binding energies detected via the XPS analysis before the occurrence adsorption further supports the role of chemical bonding in the adsorption properties of materials.

4. Discussion

4.1. The Impact of Material Site Substitution Behaviors on Adsorption Performance

Site replacement refers to the behavior of changing the local geometric structures and electronic properties of a material by introducing heteroatom metal atoms to replace the original metal sites in the matrix material. This regulation can significantly affect the adsorption capacities of material surfaces for gas molecules, ions, or organic pollutants [61], which essentially involves changing the geometric configuration and electronic state distribution of the material interface atoms at the atomic scale, thus affecting the interaction strength and selectivity between the adsorbate and the material surface. Adjusting the adsorption energy of reaction intermediates by tuning the d-band center of transition metal atoms (i.e., introducing impurities, alloying, or introducing vacancy defects to change the coordination number) introduces impurities and radicals to disrupt the periodicity of the lattice, enhancing the covalent binding of functional groups and forming new catalytically active sites and coordination environments [62]. Moreover, in the BCC structure of iron, the doping of cobalt atoms leads to changes in the surrounding coordination number and lattice constant, affecting the binding mode between the adsorbate and surface and resulting in the reconstruction of the coordination environment. Cobalt and iron form an amorphous solid alloy solution structure in this environment, and the synergistic effect between the two metals can optimize the intermediate radical to enhance the resulting adsorption performance; this structure can be controlled to a certain extent and have different catalytic centers [63]. Moreover, cobalt doping, as a key catalytic active site, can change the 3d orbital structure of iron and become an activator of iron sites, thereby enhancing the inherent reduction activity of the catalyst.

4.2. Adsorption Kinetics and Thermodynamic Processes

Adsorption kinetics describe the rate of the transfer and binding of an adsorbate from the solution or gas phase to the surface of the target material. This study reveals the underlying mechanism by which the diffusion rate is manipulated and the activation energy of microplastics in the material is transformed (formation of a transition state structure and exposure of dynamic binding sites). The FeCo material is more in line with the quasi-second-order kinetic model because of its chemical adsorption mechanism. The three-stage diffusion characteristics of the PE-MPs adsorbed on the FeCo material are as follows: initial rapid adsorption dominated by boundary layer diffusion, slowed adsorption dominated by membrane diffusion, and the process of adsorption eventually reaching saturation. The kinetic model shows that the adsorption process mainly involves surface chemical adsorption, whereas the diffusion model inside the particles suggests mass transfer between the membrane and particles, indicating the presence of hydrophobic interactions between the metal surface and the microplastics.
The thermodynamic description process reveals that the spontaneity level, the heat of the adsorption/desorption characteristics, and the driving force of the adsorption process and site substitution affect the thermodynamic equilibrium by altering the strength of the adsorbate–surface interactions. The adsorption process described in this study involves an increase in entropy and endothermic steps; namely, after adsorption, the degrees of freedom of the active sites increase, and most microplastics adsorb in an undirected manner on the material surface and inside the pores [64]. Moreover, there is a competition between the ordering of adsorbates and the disordering of solvent molecules, and the adsorption of microplastics is often attributed to the increased interfacial water molecule rearrangement entropy caused by multiple element sites under high-entropy contributions; as the adsorption process is multilayered, an increase in temperature favors the adsorption process [65].

4.3. Study on the Adsorption Mechanism and an Analysis of the Driving Factors

The adsorption process of FeCo on PE-MPs is complicated. The results of XPS and FTIR analyses reveal that the characteristic binding energy of metal ions is significantly greater than that of carbon–carbon double bonds and carbon–oxygen single bonds, indicating that the main effect on the material surface comes from functional groups, as well as the combined action of metal covalent bonds and hydroxyl ions, and that some carbon–oxygen single bonds and carbon–oxygen double bonds have influences [66]. Moreover, a BET analysis reveals that the high specific surface area of the material provides more adsorption sites, allowing it to bind more adsorbate molecules simultaneously and significantly increasing the adsorption capacity. Based on an XRD analysis and the use of JADE6 software (6.5), the FeCo alloy has a low crystallinity (57.22%), which results in the greater exposure of Fe2+/Co2+ on the surface, resulting in the formation of more coordination bonds with the functional groups on the surfaces of the microplastics. Therefore, the adsorption mechanism may include the following. (1) The hydroxyl groups on the surfaces of FeCo and PE-MPs collectively form a hydrogen bond network, whereas heteroatomic sites such as Co2+, Fe3+, and oxygen vacancies on the FeCo surface disrupt the hydrogen bond network of interface water molecules, causing the ordered water molecules on the surface to undergo disordered recombination. Moreover, the sites can enrich electrons, allowing them to combine the polar functional groups on the surfaces of microplastics with nonpolar chain segments. (2) Surface chelation leads to hydrogen bonding between the hydroxyl radicals on metal oxide surfaces and the oxygen-containing functional groups on microplastic surfaces, whereas the hydrophobic polyethylene segments in PE-MPs bind to the hydrophobic regions on the FeCo surface through van der Waals forces. The coordination between Co2+ and Fe3+ on the FeCo layer and that between the hydroxyl groups and carbon double bonds in the PE-MPs result in the formation of metal–oxygen coordination bonds and Π bonds, increasing the adsorption rate through the combined effect of bonds.
The effect of pH on the adsorption of FeCo is not significant, possibly because Co2+ and Fe3+ are uniformly dispersed in a layered structure to form a solid solution. The solid solution structure may reduce the exposure of highly active edge sites (such as coordinatively unsaturated metal atoms), decrease the reactivity of surface redox reactions, and weaken the regulatory effect of pH on electron transfer processes [67]. Owing to the presence of hydrophobic chain segments in the adsorbate (such as the polyethylene backbone), the van der Waals adsorption between it and the hydrophobic microzone on the FeCo surface is insensitive to the pH.

5. Conclusions

(1)
The present study employs the coprecipitation method to integrate cobalt and iron into porous, high-surface-area CMC precursors at a Co:Fe ratio of 7:3, thereby constructing high-quality soft magnetic FeCo materials with iron–cobalt alloys. SEM and XRD characterizations reveal that the prepared FeCo sample exhibits the characteristic peaks of soft magnetic iron–cobalt alloy materials and a typical solid solution structure. Its abundant pore structure is conducive to the surface and pore adsorption of polyethylene microplastics.
(2)
The quasisecond-order kinetic fitting model can partially describe the adsorption kinetic process of FeCo on PE-MPs. The adsorption behavior of PS-MPs on the adsorbent conforms to the Freundlich isotherm model. The adsorption of FeCo on PE-MPS is a chemical adsorption process, with a maximum adsorption capacity of 120.92 mg/g for FeCo on PE-MPs at 45 °C. The influence of the pH on the adsorption of FeCo is relatively small.
(3)
The adsorption process of FeCo on PE-MPs involves the formation of hydrogen bonds, the coordination complexation of metal with hydroxyl groups, and hydrophobic interactions. The site substitution of cobalt for iron changes the structure and electronic properties of the material, enhancing the adsorption effect of the material on PE-MPs, and the formation of a solid alloy solution stabilizes the material structure. In summary, the process of preparing FeCo materials is simple and cost-effective and is capable of achieving efficient PE-MP adsorption; thus, the toxicity of PE-MPs in water can be effectively controlled, and these materials have good application prospects in water treatment and pollution remediation scenarios.

6. Environmental Implications

Microplastics have always been widely present in the environment as persistent pollutants, and microplastics with different functional groups can interact with other pollutants in the environment and act as carriers for each other, thus affecting their environmental fate. In addition, the adsorption properties of a promising homonuclear bimetallic alloy with good soft magnetic properties on porous carbon fibers were studied, and the adsorption mechanism and the main driving force of the adsorption process were revealed. The work accounts for the porous flexibility of carbonaceous material and the stability of the solid alloy solution, and the chemical heterogeneity of the alloy is dominated by the active site of the metal and hydroxyl group due to the change in the adsorption surface site.

Author Contributions

Conceptualization, X.H.; Methodology, X.W., P.X., Z.Z. and X.G.; Software, S.Z., L.Z. and X.G.; Validation, X.W., P.X., Z.Z., X.H. and X.G.; Formal analysis, L.Z., P.X., Z.Z. and X.H.; Investigation, S.Z., L.Z., X.H. and X.G.; Resources, P.X., Z.Z. and X.H.; Data curation, S.Z.; Writing—original draft, S.Z.; Writing—review and editing, X.W., P.X., Z.Z., X.H. and X.G.; Visualization, S.Z., L.Z., X.W., P.X. and X.G.; Supervision, X.W., P.X. and X.H.; Project administration, X.W., P.X., Z.Z. and X.H.; Funding acquisition, X.H. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Guizhou Provincial Science and Technology Projects (no. QKHZC[2023]YB215) and the Guizhou Key Laboratory of Plateau Wetland Conservation and Restoration (no. ZSYS[2025]015).

Data Availability Statement

This study was carried out using publicly available data from Open Science Framework at APA with Huang, X. (29 September 2025). Adsorption performance of soft magnetic FeCo alloys on microplastics. https://doi.org/10.17605/OSF.IO/JMD5A.

Acknowledgments

The authors extend their gratitude Luo Dan (from Scientific Compass www.shiyanjia.com) for providing invaluable assistance with the XPS and SEM analysis, they would like to thank Wang Yuqing from SCI-GO (www.sci-go.com) for the BET and FTIR analysis.

Conflicts of Interest

The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.

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Water 17 03305 g001
Figure 1. Refined XRD spectrum of FeCo.
Figure 1. Refined XRD spectrum of FeCo.
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Figure 2. Characterization images before and after adsorption at different scales in SEMs. (a) The surface of the FeCo material observed at the 20-μm scale before adsorption, (b) at 50 μm, and (c) after adsorption, where the surface morphology of the material was observed at 20 μm and (d) 50 μm.
Figure 2. Characterization images before and after adsorption at different scales in SEMs. (a) The surface of the FeCo material observed at the 20-μm scale before adsorption, (b) at 50 μm, and (c) after adsorption, where the surface morphology of the material was observed at 20 μm and (d) 50 μm.
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Figure 3. (a) Comparison between the EDS electron images produced before and after adsorption (left to right); (b) EDS total spectrum comparison between the element distributions before and after adsorption (left to right).
Figure 3. (a) Comparison between the EDS electron images produced before and after adsorption (left to right); (b) EDS total spectrum comparison between the element distributions before and after adsorption (left to right).
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Figure 4. (a) Comparison between the electron images with element mapping produced before and after adsorption (left and right); (b) comparison between the EDX spectra with element mapping produced before and after adsorption (left and right).
Figure 4. (a) Comparison between the electron images with element mapping produced before and after adsorption (left and right); (b) comparison between the EDX spectra with element mapping produced before and after adsorption (left and right).
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Figure 5. BET before the FeCo adsorption of PE-MPS.
Figure 5. BET before the FeCo adsorption of PE-MPS.
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Figure 6. Isothermal adsorption curves and fittings of FeCo on PE-MPs.
Figure 6. Isothermal adsorption curves and fittings of FeCo on PE-MPs.
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Figure 7. Thermodynamics of the adsorption of FeCo on PE-MPs (polyethylene magnetic particles).
Figure 7. Thermodynamics of the adsorption of FeCo on PE-MPs (polyethylene magnetic particles).
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Figure 8. Adsorption kinetics of FeCo on PE-MPs at different initial concentrations: (a) pseudo-first-order and second-order kinetic model fitting; (b) intraparticle diffusion model fitting.
Figure 8. Adsorption kinetics of FeCo on PE-MPs at different initial concentrations: (a) pseudo-first-order and second-order kinetic model fitting; (b) intraparticle diffusion model fitting.
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Figure 9. Effect of the adsorbent dosage on the adsorption and removal of microplastics.
Figure 9. Effect of the adsorbent dosage on the adsorption and removal of microplastics.
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Figure 10. Influence of the initial pH on the adsorption of microplastics.
Figure 10. Influence of the initial pH on the adsorption of microplastics.
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Figure 11. FT-IR spectra of FeCo before and after adsorption on PE-MPs.
Figure 11. FT-IR spectra of FeCo before and after adsorption on PE-MPs.
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Figure 12. XPS spectra of FeCo before and after adsorption on PE-MPS: (a) survey spectrum; (b) O1s; (c) C1s; (d) Fe2p; (e) Co2p.
Figure 12. XPS spectra of FeCo before and after adsorption on PE-MPS: (a) survey spectrum; (b) O1s; (c) C1s; (d) Fe2p; (e) Co2p.
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Table 1. Element content changes before and after adsorption.
Table 1. Element content changes before and after adsorption.
ElementWt%Wt% SigmaAt%
BeforeAfterBeforeAfterBeforeAfter
C11.6355.190.210.1131.4970.48
O12.2225.280.090.1024.8424.23
Fe53.7313.840.200.0731.293.80
Co22.425.700.170.0612.381.48
Total100.00100.00 100.00100.00
Table 2. Surface areas and pore size distributions of FeCo-adsorbed PE-MPs.
Table 2. Surface areas and pore size distributions of FeCo-adsorbed PE-MPs.
Parameter Sequence NumberThermophysicalParameter Value
1BET surface area (m2g−1)142.8302 m2g−1
2Micropore volume (cm3g−1)0.004586 cm3g−1
3Micropore area (m2g−1)10.5186 m2g−1
4Average pore size (nm)7.7283 nm
Table 3. Fit parameters of the isothermal adsorption model for the adsorption of FeCo on PE-MPs.
Table 3. Fit parameters of the isothermal adsorption model for the adsorption of FeCo on PE-MPs.
Isotherm ModelT (K)
288298308
Qm84.9697.47163.40
LangmuirKL0.0110.0190.011
R20.9420.9220.920
KF17.0729.9327.15
Freundlichn4.355.773.89
R20.9690.9560.911
Table 4. Thermodynamic fitting parameters.
Table 4. Thermodynamic fitting parameters.
T (K)lnKΔG (KJ/mol)ΔH (KJ/mol)ΔS (J·mol−1·K−1)
288.15−1.603.8333.21101.63
298.15−1.263.11
308.15−0.701.79
Table 5. Potential of other iron-based composites to remove microplastics.
Table 5. Potential of other iron-based composites to remove microplastics.
MaterialPollutantsAdsorption CapacityAdsorption
Efficiency		Removal MethodReferences
Fe3O4@PDAPS4.0–10 mg/L
Not specified		98.5%Magnetic removal[49]
2D ultrathin magnetic NDsPS
PMMA		187.7 mg/g
188.4 mg/g		90%Magnetic removal[50]
MFe@BCPHBV31.96 mg/g98.53%Filtration[51]
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MDPI and ACS Style

Zhang, S.; Zhang, L.; Wang, X.; Xia, P.; Zhang, Z.; Huang, X.; Guo, X. Adsorption Ability of Soft Magnetic FeCo Alloys for Microplastics. Water 2025, 17, 3305. https://doi.org/10.3390/w17223305

AMA Style

Zhang S, Zhang L, Wang X, Xia P, Zhang Z, Huang X, Guo X. Adsorption Ability of Soft Magnetic FeCo Alloys for Microplastics. Water. 2025; 17(22):3305. https://doi.org/10.3390/w17223305

Chicago/Turabian Style

Zhang, Shenghao, Lan Zhang, Xingfu Wang, Pinhua Xia, Zhenming Zhang, Xianfei Huang, and Xuetao Guo. 2025. "Adsorption Ability of Soft Magnetic FeCo Alloys for Microplastics" Water 17, no. 22: 3305. https://doi.org/10.3390/w17223305

APA Style

Zhang, S., Zhang, L., Wang, X., Xia, P., Zhang, Z., Huang, X., & Guo, X. (2025). Adsorption Ability of Soft Magnetic FeCo Alloys for Microplastics. Water, 17(22), 3305. https://doi.org/10.3390/w17223305

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