Microstructural and Magnetic Properties of Polyamide-Based Recycled Composite with Iron Oxide Nanoparticles

1. Introduction

The textile and clothing industry’s reliance on polyamides (PAs), or Nylons, creates a significant environmental challenge due to the large volumes of solid waste generated annually [1,2]. While these synthetic fibers offer high elasticity, durability, and resistance to various environmental factors, their non-biodegradable nature results in long-term pollution as a significant portion ends up in landfills [3,4,5,6,7].

Although ongoing efforts to reduce and recycle PAs (RPAs) face obstacles, the urgency for sustainable solutions in the textile industry is undeniable. PAs are versatile, semi-crystalline thermoplastics derived from petroleum [8,9], possess excellent mechanical and chemical properties, making them indispensable across various industries [10,11,12,13].

However, the RPAs, particularly from post-consumer textile waste, is complex due to contamination and degradation during processing [3,4]. This underscores the importance of developing advanced recycling methods and incorporating RPAs into high-performance materials, which in turn can transform the textile industry, reducing its environmental burden and ensuring a more sustainable future.

RPAs, despite their potential for sustainability, often falls short in performance compared to virgin materials [14]. To address these limitations, various fillers (organic and inorganic) are commonly incorporated into PA to create composites, leading to improving their properties and reducing costs. Recently, PAs nanocomposites based on a wide range of nanometer-scale fillers, such as clay particles [15], titanium [16], cellulose nanofibers [17], graphene [18], and iron oxide (IO) [19,20], have widely been studied. Among these nanomaterials, IO polymorphs stand out as highly promising candidate for various emerging technologies, owing to their abundance, cost-effectiveness, and diverse functionalities [21]. Particularly, Fe₃O₄ (also known as magnetite) is a significant iron oxide with mixed-valence IO structure, contains both Fe²⁺ and Fe³⁺ ions within its crystal lattice, giving its characteristic ferrimagnetic properties, distinguishing it from the solely Fe³⁺-containing polymorphs [22]. The magnetite phase has an inverse cubic spinel structure (a = 8.396 Å) with Fe³⁺ ions distributed in both tetrahedral and octahedral sites, and Fe²⁺ ions exclusively in octahedral sites [23]. Under certain conditions, Fe²⁺ ions within magnetite are susceptible to oxidation, a process that can result in the transformation to the maghemite (γ-Fe₂O₃), another IO polymorph with only contains Fe³⁺ ions [24,25,26]. Both magnetite and maghemite been extensively studied for their valuable magnetic properties [27,28,29,30,31].

The combination of IO nanoparticles with polymeric matrices leads to functional materials with enhanced magnetic, mechanical, thermal, and even biological properties [19,32,33,34,35]. One key advantage is their ability to form a well-distributed network within the polymer matrix, facilitating nanoscale interactions that directly influence the overall properties of the material [36,37]. This maximizes the contact between the IO nanoparticles and the RPA matrix, improving functionality and consistency. The versatility and economic benefits of IO nanoparticles make them an attractive option for enhancing the performance of RPA composites.

This study focuses on the microstructure and magnetic properties of the RPA/IO nanocomposites derived from textile waste. The knowledge gained from this research aims to contribute to the development of sustainable and functional materials, tackling the environmental issues associated with textile waste and promoting resource conservation within the industry.

2. Materials and Methods

2.1. Chemicals

Here, the RPA was prepared according to a patent filed by the State University of Londrina, Brazil (No. BR102013032153A2). Analytical-grade chemicals were used, including ferrous sulfate heptahydrate (FeSO₄·7H₂O – Sigma-Aldrich, 99%), ferric chloride hexahydrate (FeCl₃·6H₂O – Vetec, 102%), and ammonium hydroxide solution (NH₄OH – Sigma-Aldrich, 30-33%). All chemicals were used as received without further treatment. Solutions were prepared using ultrapure water with a resistivity exceeding 18.00 MΩ·cm, obtained from a water purification system (Elga model USF CE).

2.2. Synthesis of IO Nanoparticles

IO nanoparticles were synthesized by co-precipitation method utilizing salts of Fe²⁺ (FeSO₄.7H₂O) and Fe³⁺ (FeCl₃.6H₂O) ions in ammonia solution [38]. About 2.78 g portion of FeSO₄.7H₂O and 5.4 g of FeCl₃.6H₂O with a molar ratio of 1:2 was dissolved in 100 mL distilled water with final concentration of 0.3 mol L-1 iron ions. The mixed iron solution was maintained under constant mechanical stirring for 10 minutes to ensure thorough mixing of the ions. Next, 75 mL of NH₄OH was slowly added to the solution at 25°C under vigorous stirring until reaching a pH of 10. Then, the solution was heated to 80°C for 30 minutes, followed by filtration and washing with distilled water. Finally, the precipitate was dried under vacuum oven at 60 °C and stored.

2.3. Synthesis of RPA/IO Nanocomposites

RPA/IO nanocomposites were synthesized following the same procedure as above, with the addition of RPA alongside the iron salts. After dissolving the iron salts in 100 mL of water, 4 g of RPA was added. The remaining steps, including NH₄OH addition, heating, filtration, washing, and drying, were performed as described for the IO nanoparticle synthesis.

3. Materials Characterization

X-ray diffraction (XRD) patterns were acquired using a Panalytical Xpert PRO MPD diffractometer with CuKα radiation (λ=0.154 nm). A 2θ scanning range of 20° to 70° was employed with a step size of 0.05°s-1. Rietveld refinement analysis [39] was performed on the XRD patterns using the High Score Plus software (version 3.0). The average crystallite size (S) was determined using the well-known Debye–Scherrer formula, as presented in Equation (1) [40,41]:

S = kλ/β Cos θ

(1)

where S is the average size of the crystallite (nm), k (0.89) is the Scherrer constant, λ is the X-ray wavelength (0.15406 nm), β is the half-peak width of the diffraction peak (in radians), and θ is the Bragg diffraction angle (in degrees). The degree of crystallinity percentage (%DOC) was calculated by applying Equation (2) [42]:

%DOC = 100 × (Ac/AT)

(2)

where Ac is the area under crystalline peaks and AT is the total area (area of crystalline and amorphous peaks).

Scanning electron microscopy (SEM) was conducted on a JEOL-FEG JSM-7100F electron microscope at 5 kV, utilizing secondary electrons for imaging. Samples were mounted on stubs with double-sided carbon tape and sputter-coated with a 20 nm gold layer (SCD 050, BAL-TEC BALZERS, Liechtenstein).

Transmission electron microscopy (TEM) was performed on a JEOL 2100F microscope equipped with a Thermo SEVEN detector for energy-dispersive X-ray spectroscopy (EDXS) operating at 200 kV. Samples were prepared by dispersing the powder in isopropyl alcohol using ultrasound for 20 minutes. A drop of the suspension was then deposited onto a copper grid with a carbon film, followed by solvent evaporation.

⁵⁷Fe Mössbauer spectroscopy in transmission mode was performed using a 57Co source embedded in an Rh matrix with constant acceleration. Calibration was done using an α-Fe foil, and measurements were taken at room temperature. Hyperfine parameters were calculated using a Lorentzian line shape and the minimum chi-square method.

Magnetic properties were assessed using a SQUID-VSM spectrometer (Quantum Design MPSM XL) at 5 and 300k, applying a magnetic field ranging from -20 to 20 kOe.

4. Results and Discussion

4.1. Microstructural Properties

Figure. 1 (a-f) presents the XRD patterns, Rietveld refinement and SEM images of these samples under investigation. The peak positions (2θ) and β values were extracted from these XRD patterns to characterize the microstructure (Table 1). As show in Figure 1(a), the diffraction peaks at the 2θ values of 18.4° (111), 30.3° (220), 35.6° (311), 37.4° (222), 43.3° (400), 54.1° (422), 57.3° (511), and 63.3° (440), correspond to the face-centered cubic structure (belonging to the space group Fd3 ̅m, No. 227) of both Fe₃O₄ and γ-Fe₂O₃ phases, aligning with the Joint Committee on Powder Diffraction Standards (JCPDS) card no. 01-088-0315 for magnetite [43] and card no. 01-076-3168 for maghemite [44], respectively. Similar findings have been reported in the literature [27,45,46]. The peak with the most intense at 2θ = 35.5° (311) in IO sample was chosen to calculate the S value, which is about 10.0 nm and the %DOC was determined in 69.2%. Additionally, the Rietveld refinement method was used to analyze the purity and confirmed the long-range structure of the mixed-phase IO nanostructure (with experimental lattice values of a = 8.384 Å for Fe₃O₄ and a = 8.362 Å for γ-Fe₂O₃) of this sample. The proportions of the Fe₃O₄ and γ-Fe₂O₃ phases were found to be 79.4 wt% and 20.6 wt%, respectively. Herein, the GOF = 1.52 indicates a good quality of the fit. During synthesis, iron ions (Fe²⁺ and Fe³⁺) precipitate in an alkaline medium, forming Fe₃O₄, which has an inverse spinel structure belonging to the space group Fd3 ̅m (No. 227). However, exposure to oxygen in the air oxidizes the Fe²⁺ ions on the surface of magnetite, converting it to γ-Fe₂O₃, which also has a spinel structure but with all iron ions in the Fe³⁺ state.

Figure 1(b) reveals an amorphous halo ranging from 2θ = 18° to 26°, indicative of the amorphous nature of the RPA material. Superimposed on this halo are two distinct crystalline peaks at 20.1° and 23.6°, corresponding to the (200) and (002) planes, respectively. These peaks are characteristic of the α-form crystal structure commonly observed in PA materials [47,48], suggesting the presence of some crystalline regions within the predominantly amorphous RPA structure [9,49]. For the RPA, the calculated S values for the (200) and (002) planes were found to be 3.2 and 2.9 nm, respectively, while the %DOC of RPA was measured at 35.8% (Table 1). These findings are consistent with the literature on various PAs. For example, Qianhui et al. [50] used CaCl2/ethanol/water-based solvents to analyze waste PA and RPA. Both materials showed two crystalline peaks at 20.2° and 23.5°, indicating the α crystal form. The %DOC was 52.0% for waste PA and 35.9% for RPA. Similarly, Colucci et al. [51] studied the effect of mechanical recycling on the microstructure and properties of PA composites reinforced with carbon fibers. They identified the α crystal form in PA with 2θ peaks near 20° and 24°, respectively. The %DOC calculated at 26.2% for PA66 and 24.6% for RPA. These studies suggest that recycling, whether through solvents or mechanical processes, may decrease the crystallinity of PAs. This is likely due to partial degradation or structural changes during recycling.

As shown in Figure 1(e), the SEM micrographs reveals that the RPA sample exhibits an irregular morphology, characterized by a rough surface and several pores in the micrometer range. High porosity surfaces are desired for multifunctional nanocomposites, because it facilitates molecule entry, making it a promising material for adsorbing emerging pollutants due to its porous nature [52,53]. Additionally, RPA’s surface of is abundant in oxygen and nitrogen atoms, which enables interactions with positively charged ions, further enhancing its adsorption capabilities [54,55].

Regarding RPA/IO nanocomposites, the XRD patterns revealed the presence of characteristic RPA peaks alongside multiple crystalline peaks attributable to the mixed-phase IO nanostructure (Figure 1c). The Bragg intensities were characterized at 2θ angles of 20.3° (200), 23.9° (002), 30.2° (220), 35.6° (331), 43.4° (400), 54.4° (422), 57.3° (511), and 63.5° (440). A prominent IO crystalline peak was observed at 2θ = 35.6, corresponding to the (311) plane. The impact of incorporating IO into the RPA/IO nanocomposites was analyzed by calculating the S values and %DOC from the peaks at 20.3° and 23.9°, which were found to be 4.4 nm and 4.6 nm, respectively, with a %DOC of 11.5% (Table 1). These results indicated a slight increase in the S value compared to the bare RPA sample. Conversely, the %DOC decreased by approximately 24%, indicating a significant influence of IO on the crystalline structure of the resulting nanocomposite.

Furthermore, SEM micrographs in Figure 1(f) provide insights into the morphology and distribution of the IO with the nanocomposite. These results show that these nanostructures, exhibiting an almost spherical shape, are unevenly dispersed on the RPA surface but instead form aggregates. This creates a unique nanocomposite structure with several potential advantages. First, the porous nature of the RPA surface offers a high surface area for IO nanostructure to adhere to. This enhanced the interaction between the RPA matrix and the IO can lead to stronger interfacial interactions, improving overall stability and mechanical properties of nanocomposite. Second, the uneven dispersion and formation of IO aggregates can create localized regions of concentrated magnetic activity. This could significantly enhance the nanocomposite’s performance in applications that rely on magnetic properties, such as magnetic separation, catalysis, or sensors [19,56,57].

Figure 2 depicts the TEM images, particle size distributions, and selected area electron diffraction (SAED) patterns for both IO and the RPA/IO nanocomposite. The spherical morphology of the IO is confirmed by the TEM image (Figure 2a). The SAED pattern confirms the polycrystalline nature of the inverse spinel IO structure (Figure 2c), as indicated by the rings corresponding to the (111), (220), (311), and (400) planes [58]. From the SAED results, the lattice parameter was calculated to be 8.369 Å, which is consistent with Rietveld refinement of the XRD data. Figure 2(b) displays a well-ordered (311) lattice plane is attributed to the principal crystalline peak of the IO phase with an average particle size of about 15.3 ± 0.3 nm. Hence, the agglomerated IO can be observed in Figure 2(d), which is consistent with the SEM results. HRTEM image reveals the crystalline nature of IO (Figure 2e), while SAED and XRD patterns confirms the IO phase (Figure 2f). The bright field TEM images of the RPA/IO nanocomposite, presented in Figure 3g, indicate the nanoparticle nature of the synthesized IO powder, which has an average particle size of approximately 14.1 ± 0.2 nm and an irregular quasi-spherical morphology. Additionally, the EDX mapping indicates that the RPA/IO nanocomposite is rich in Fe and O, with the C content associated with both IO and RPA phases, and no other contaminating elements. Hence, the presence of the IO in nanocomposite suggests that this functional material likely exhibits magnetic properties.

4.2. Magnetic Properties

To study the magnetic behavior of the IO and RPA/IO nanocomposites, magnetization measurements as a function of the magnetic field were performed. In the VSM measurements, the saturation magnetization (Ms), remnant magnetization (Mr), and coercivity (Hc) were extracted from the hysteresis curves at 5 K and 300 K (see Table 2 and Figure 3). The mixed-phase IO nanostructure exhibited a MS of 50.0 emu g⁻¹ at 5K, lower than that of bulk magnetite [45,59,60], likely due to the presence of both Fe₃O₄ and γ- Fe₂O₃ phases as identified by Rietveld refinement of XRD data. This presence of multiple phases could lead to a decrease in the overall Ms value observed for the IO, as the magnetic properties of the mixed phases can result in a lower effective magnetization compared to pure bulk Fe₃O₄ [48,61,62]. At 300 K, the Ms, Mr, and Hc values were calculated as follows: Ms = 44.1 emu g⁻¹, Mr = 0.7 emu g⁻¹, and Hc = 2.5 × 10⁻⁴ kOe, respectively. Hu et al. [60] suggest that the magnetic properties of these materials are significantly influenced by their crystallinity and the method used for their synthesis. As observed in Figure 3(a), the curve for the IO at 300 K does not show any hysteresis loop and exhibits low Hc (2.5×10⁻⁴ KOe) and Mr (0.7 emu g⁻¹) values. At 300 K, the absence of a hysteresis loop and significantly reduced Mr and Hc suggest a transition to superparamagnetic behavior [63]. Conversely, the presence of a hysteresis loop at 5 K suggests ferrimagnetic behavior, as the reduced thermal energy allows the magnetic moments to align and remain aligned. These findings are consistent with previous studies in the literature [63,64,65]. Chirita et al. [64] synthesized magnetite nanoparticles and studied the magnetic properties, particularly the effect of temperature. They recorded magnetization curves within the 5–300 K, observing a decreased in hysteresis loop as the temperature increased. Above 150 K, the cubic inverse spinel phase exhibited a nearly hysteresis-free magnetization cycle. Based on these findings, they concluded that the IO demonstrate a superparamagnetic-like behavior.

For RPA/IO nanocomposites, the Ms values at 5 K and 300 K were measured at 22.2 and 19.1 emu g⁻¹, respectively, which are 50% lower compared to those of the pure IO. Additionally, the RPA/IO nanocomposites exhibit hysteresis loops at both temperatures, with the loops being less pronounced at 300 K. This reduction in magnetic properties relative to pure IO is likely due to the interaction between the polymer matrix and the IO. The presence of RPA may be disrupting the Fe-O-Fe interactions by altering the local structure or surrounding environment of the IO nanostructure. We hypothesize that RPA could interfere with the formation or stability of the oxide layer on the IO, potentially modifying the interactions between iron atoms and, consequently, impacting magnetization.

As shown in Table 2, the magnetic moment ratio (Mr/Ms) was found to be below 0.3 for all samples. At the elevated temperature of 300 K, these Mr/Ms ratios further decreased to 0.02 for IO and 0.04 for RPA/IO nanocomposite. This reduction is likely associated with the material’s magnetic anisotropy [66], which can diminish at higher temperatures, leading to a more disordered state [67]. This disorder results in a lower Mr relative to the Ms. Additionally, the distribution and density of IO on the RPA surface significantly influence the magnetization behavior of the nanocomposites [66]. SEM and TEM analysis confirmed the aggregation of IO nanostructure, which can influence magnetic anisotropy and exchange interactions, thereby contributing to the observed variations in Ms values in RPA/IO nanocomposites.

In our study, the magnetic IO and the RPA/IO nanocomposite exhibit superparamagnetic behavior at 300 K, a characteristic of systems with single-domain particles [68]. In such systems, the absence of domain walls means that the magnetization reversal mechanism is primarily driven by magnetic anisotropy [69]. This anisotropy arises from a combination of magnetocrystalline, shape, strain, and surface anisotropies, with its magnitude represented by the magnetic anisotropy constant (K), which quantifies anisotropy energy per unit volume. For spherical magnetic nanoparticles, the dominant contribution typically comes from magnetocrystalline anisotropy, expressed by the magnetocrystalline anisotropy constant (K₁) [70,71]. As the SEM micrographs and TEM images illustrated that the IO in our samples are almost spherical, we can reasonably approximate K as K₁ in this study, making the magnetocrystalline anisotropy the primary focus for understanding the anisotropic properties of these nanoparticles.

To investigate magnetic anisotropy, the initial magnetization curves of the samples were fitted using the law of approach to saturation (LAS), which facilitates the estimation of the constant, K₁. This approach was utilized to analyze magnetic behavior in the saturation region, illustrating the relationship between magnetization and the applied magnetic field when H≫Hc. The magnetization near saturation can be expressed as follows [72]:

M = Ms [1 – b/H²] + kH,

(3)

The term b/H² represents the rotation of magnetization against the magnetocrystalline anisotropy energy, where b = 8/105× ${\displaystyle \frac{{\mathrm{K}}_1^2}{{\mathsf{\mu }}_0^2{\mathrm{M}}_{\mathrm{s}}^2}}$ (8/105 is a coefficient related to cubic anisotropy of random polycrystalline samples, K₁ is the cubic anisotropy constant, MS is the saturation magnetization, and μ₀ is the permeability of free space). H is the applied magnetic field, and kH, referred to as the forced magnetization, behaves similarly to a paramagnetic component, resulting from a linear increase in spontaneous magnetization with the applied magnetic field. Forced magnetization becomes significant under elevated temperatures and very high magnetic fields, necessitating its inclusion in such analyses. However, for the purpose of estimating K₁ value in this study, the term kH was omitted from equation (3). The observed magnetization data for applied magnetic field above 2 kOe at 300 K were fitted using equation (3). The values of Ms and b were then used to estimate the K₁ constant, as shown in Equation (4):

$${\mathrm{K}}_1={\mathsf{\mu }}_0{\mathrm{M}}_{\mathrm{s}}\sqrt{105\mathrm{b}/8}$$

(4)

Figure 3c and 3d show the typical LAS fitting curves at 300 K for the IO and RPA/IO nanocomposites, respectively. Table 2 presents the K₁ values of the samples at 300 and 5 K. The K₁ values for the IO and RPA/ IO nanocomposite was estimated to be 4.77×10⁵ and 3.53×10⁵ at 5 K, respectively, whereas at 300 K, the values were 2.90 × 10⁵ and 1.26×10⁵, respectively. Notably, in both cases, the K₁ constant increased as the temperature decreased. According to Paswan et al. [70], magnetic anisotropy in spinel ferrite is caused by spin-orbit interactions and unquenched orbital magnetic moments. At 5 K, the higher K₁ values suggest restricted moment orientations owing to stronger interactions and reduced thermal energy, resulting in stable magnetization. In contrast, the lower K₁ values at 300 K indicate weaker spin-orbit interactions and suppressed orbital magnetic moments. In the superparamagnetic state, particles easily switch magnetization directions, consistent with the lower anisotropy constant observed at 300 K. Therefore, the relationship between K₁, temperature, and magnetic behavior illustrates the transition from a stable ferrimagnetic state at low temperatures to a dynamic superparamagnetic state at higher temperatures.

In the study by Mamiya et al. [73], the magnetic anisotropy constant K₁ ranged from 1.0×10⁵ – 2.0×10⁵ erg cm^–3, values corresponding to pure magnetite nanoparticles. In contrast, our IO sample consisted of both magnetite and maghemite phases, potentially altering the K₁ value owing to differences in crystal structure and cation distribution. Magnetic anisotropy in magnetite results from a combination of factors, including magnetocrystalline anisotropy, shape anisotropy, surface anisotropy, and mechanical stress [73]. Specifically, magnetocrystalline anisotropy is influenced by strong super-exchange interactions between tetrahedral and octahedral sites. The presence of maghemite may affect these interactions, leading to variations in the magnetic anisotropy constant compared to pure magnetite nanoparticles. Moreover, for nanoparticles of volume V, the magnetic anisotropy constant can be related to the blocking temperature (TB) by the following equation (5) [74]:

T_B = KV/25KB,

(5)

where K is the magnetic anisotropy constant; V is the volume of the IO; and KB is the Boltzmann constant. This equation demonstrates that the blocking temperature is directly proportional to the magnetic anisotropy constant K₁ and the volume V of the nanoparticle. This blocking temperature reflects the point at which thermal energy overcomes the energy barrier KV, allowing magnetization to fluctuate. Below TB, magnetization remains “blocked” in a stable direction, while above this temperature, the system transitions to a superparamagnetic state, where magnetization can easily reorient due to thermal fluctuations.

To determine T_B for the samples, K₁ at 300 K was used to investigate temperature-dependent magnetic characteristics (see Table 2). The volume of the nanoparticles was calculated from the diameter obtained through TEM analysis (Figure 2a and 2d). The TB for the IO and RPA/IO nanocomposites was found to be 149 and 147 erg cm^–3, respectively. At TB, the IO exhibits ferrimagnetism in the blocked state, whereas above this temperature, it transitions to superparamagnetic behavior. The T_B can vary with particle size, synthesis methods, and interactions [75]. For instance, spherical magnetic nanoparticles with a size of approximately 5.7 nm exhibited a T_B of approximately 28 K under an applied field of 100 Oe [76]. In another study, magnetite nanoparticles with a mean size of 20 nm embedded in polyvinyl alcohol exhibited a T_B of approximately 300 K [77]. Under more specific conditions, including the field and particle size, similar blocking temperatures between 100-200 K have been reported for nanoparticles in the range of 11–18 nm [78,79]. Understanding the temperature dependence of superparamagnetic behavior is crucial for applications requiring particle stability across a range of temperatures.

Mössbauer spectroscopy was used to quantitatively determine the oxidation states of Fe species and provide insights into the contributions of Fe ions at tetrahedral (FeA) and octahedral (Fe_B) sites in a sample [29,80]. The resulting ⁵⁷Fe Mössbauer spectra are shown in Figure 3e and 3f, and Table 3 lists the hyperfine parameters obtained. Mössbauer spectrum of IO nanostructure was fitted by considering a magnetic hyperfine field distribution for the FeA–sites and a doublet for the Fe_B–sites within the crystalline structure of the IO. The hyperfine magnetic field distribution for the Fe_A–sites showed an isomer shift (δ) of 0.36 mm s⁻¹, a hyperfine field (Bhf) of 44.7 T, and an intensity of 81.3%, which is characteristic of Fe⁺³ in the tetrahedral environment of magnetite [81]. The doublet component showed a δ of 0.35 mm s⁻¹ and an intensity of 18.7%, characteristic of high-spin Fe³⁺, attributed to the γ- Fe₂O₃ phase in the IO sample.

As depicted in Figure 3e, Fe³⁺ ions occupy the Fe_A sites, with oxygen coordination. Fe²⁺ and Fe³⁺ ions are distributed equally between the Fe_A and Fe_B sites, resulting in an overall occupancy of 1/8 at Fe_A and 1/2 for Fe_B sites [82]. The γ-Fe₂O₃ phase crystallizes in a similar cubic spinel structure but with a tetragonal supercell and a lattice constant of 8.33 Å [83]. Unlike magnetite, maghemite contains only Fe³⁺ ions, which are randomly distributed across 16 Fe_B and 8 Fe_A interstitial sites. These results align with those reported in previous studies [84,85,86].

The conversion of Fe²⁺ ions to Fe³⁺ ions during oxidation, leading to iron vacancies to maintain charge balance. This transformation can occur under various conditions, such as natural weathering or hydrothermal treatments, altering the crystal structure [81,82]. The presence of iron vacancies on maghemite surfaces influences its reducibility, oxidation potential, and overall properties. Mössbauer spectra of the IO nanostructure can be successfully interpreted as a mixture of magnetite and maghemite, rather than as nonstoichiometric magnetite. This interpretation aligns with previous reports [27,61,83].

For the RPA/IO nanocomposite, the spectra were fitted using a model that accounted for the magnetic hyperfine field distribution at Fe_A sites and a doublet at Fe_B sites. The δ value and Bhf for the first component were 0.34 mm s⁻¹ and 45.8 T, respectively. The second component had a δ value of 0.35 mm s⁻¹. The intensities of the first component and second components were 78.8% and 21.1%, respectively. Mössbauer spectroscopy results revealed that the incorporation of RPA did not significantly change the crystalline structure of IO in the nanocomposite. This finding is consistent with the XRD patterns shown in Figure 1c.

5. Conclusions

In conclusion, this study successfully demonstrated the fabrication of a novel RPA/IO nanocomposite using recycled textile PA. The comprehensive characterization through XRD, SEM, TEM, VSM, and Mössbauer spectroscopy confirmed the successful synthesis, microstructure and magnetic properties of this nanocomposite. XRD analysis revealed a semi-crystalline structure with an S value of 10.0 nm and a degree of crystallinity of 11.5%, while SEM and TEM showed a nanometric scale and porous morphology, with IO well-dispersed on the nanocomposite’s surface. The observed average particle size of 14 nm aligns with expectations for such materials. Magnetic measurements demonstrated temperature-dependent behavior, with Mössbauer spectroscopy indicating the presence of a maghemite phase likely due to surface oxidation.

This study highlights the potential of recycled PA as a valuable resource for developing cost-effective and functional nanocomposites, contributing to both material science and sustainable efforts. The simple preparation method and the promising properties of the RPA/IO nanocomposite suggest its suitability for a range of applications. Future research will focus on scaling up the production process and evaluation the nanocomposite’s performance in specific applications, such as those requiring magnetic properties or enhanced mechanical strength.