Sonochemically Synthesized Pure and Gd₂O₃-Modified ZnO Nanoneedles for Enhanced Degradation of Paracetamol

Abstract

Pure ZnO and ZnO/Gd₂O₃ (1 and 2 mol %) nanoneedles were synthesized via a sonochemical route and evaluated as catalytic materials for the degradation of paracetamol using glass and PTFE (Teflon) stirring rods. The morphology and elemental composition of the obtained nanostructures were investigated by SEM and EDS analyses, confirming the formation of anisotropic rod-like architectures and the successful incorporation of gadolinium species into the ZnO matrix. The optical and defect-related properties were further examined by photoluminescence and UV–Vis spectroscopy, revealing defect-related modifications in the electronic structure and improved charge carrier behavior in the gadolinium-modified samples. Comparative catalytic experiments showed higher degradation efficiencies in the system employing the glass stirring bar compared to the PTFE. However, the differences between these two setups are not limited solely to the stirring bar material, but also involve variations in interfacial contact conditions during operation. Therefore, the observed differences in catalytic activity cannot be attributed to a single mechanistic origin such as mechanically induced effects, but rather reflect the combined influence of catalyst–surface interactions and the specific nature of the stirring medium. The influence of inorganic ions on paracetamol degradation was also investigated using distilled water and aqueous solutions containing sodium chloride, sodium sulfate, and sodium hydrogen carbonate. In both systems, the ZnO/Gd₂O₃ samples exhibited higher degradation efficiency than pristine ZnO, indicating that Gd incorporation plays a key role in enhancing catalytic performance. This improvement can be associated with a modified defect structure and more favorable charge carrier dynamics in the doped material. The mineralization efficiency of the treated solutions was additionally evaluated through chemical oxygen demand (COD) measurements, confirming a significant reduction in organic load after treatment.

1. Introduction

The continuous accumulation of pharmaceutical contaminants in aquatic environments has emerged as a major global environmental issue due to their persistence, bioaccumulation potential, and resistance to conventional wastewater treatment technologies [1,2]. Among the most frequently detected pharmaceutical residues, paracetamol (acetaminophen) is extensively consumed worldwide and is commonly identified in hospital, municipal and industrial wastewater streams [3]. Even at relatively low concentrations, paracetamol and its degradation intermediates may induce toxic effects on aquatic organisms and negatively affect ecosystem stability [4]. Consequently, the development of sustainable and energy-efficient advanced oxidation technologies for pharmaceutical degradation has attracted considerable scientific attention in recent years [5].

Semiconductor catalytic processes have demonstrated remarkable potential for environmental remediation applications owing to their ability to generate reactive oxygen species (ROS) capable of mineralizing persistent organic pollutants [6]. In particular, ZnO has emerged as one of the most promising catalytic materials due to its low toxicity, chemical stability, low cost, wide band gap, and intrinsic piezoelectric properties [7]. Furthermore, ZnO can be synthesized in various morphologies, including nanoparticles, nanosheets, nanoflowers, nanoneedles and nanorods, where one-dimensional nanostructures exhibit superior charge transport and enhanced polarization behavior under mechanical stimulation [8]. ZnO nanoneedles are especially attractive because their anisotropic morphology facilitates directional separation of charge carriers and promotes the formation of ROS during mechanically induced catalytic processes [9].

Tribocatalysis has recently gained increasing interest as an emerging advanced oxidation process capable of converting mechanical energy into chemical energy through the triboelectric charge generation originating from frictional contact between materials [10]. During tribocatalytic activation, charge transfer processes occurring at the contact interface lead to ROS generation and subsequent degradation of organic contaminants [11]. Although tribocatalysis has demonstrated considerable potential for wastewater purification, its catalytic efficiency strongly depends on catalyst morphology, surface area, and charge separation capability [12]. ZnO materials have shown promising tribocatalytic activity due to their semiconducting and piezoelectric characteristics, while morphology-controlled nanostructures provide additional active sites and improved interfacial charge transfer [13].

Recent research efforts have focused on improving the catalytic efficiency of ZnO through rare-earth modification and defect engineering strategies [14]. Rare-earth oxides are known to influence the electronic structure, oxygen vacancy concentration, and charge carrier dynamics of ZnO-based systems [15]. Among them, gadolinium-containing compounds are particularly attractive due to their ability to introduce defect-related energy states and suppress electron–hole recombination processes [16]. Gd modification may additionally enhance ROS generation and facilitate interfacial electron transfer during mechanically induced catalytic reactions [17]. Despite the growing number of studies on tribocatalytic systems, comparative investigations on pure and Gd2O3-modified ZnO nanoneedles for paracetamol degradation remain limited.

In the present work, pure ZnO and ZnO/Gd₂O₃ (1 and 2 mol %) nanoneedles were synthesized via a facile sonochemical route and comparatively evaluated as catalysts for paracetamol degradation in aqueous solution. The morphology, elemental composition, and optical properties of the obtained nanomaterials were systematically characterized by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), photoluminescence (PL), and UV–Vis spectroscopy. In addition, the degradation efficiency and mineralization degree were assessed through paracetamol removal and chemical oxygen demand (COD) measurements. Particular emphasis was placed on elucidating the influence of anisotropic nanorod morphology and Gd₂O₃ modification on the mechanically induced catalytic performance of ZnO-based materials.

2. Results and Discussion

2.1. Structural, Morphological and Elemental Characterization

The X-ray diffraction (XRD) patterns of pristine ZnO and ZnO/Gd₂O₃ (2 mol %) nanoneedles are presented in Figure 1. The diffraction peaks indexed to the (100), (002), (101), (102), (110), (103), (200), (112), and (201) crystallographic planes are characteristic of the hexagonal wurtzite ZnO structure and are consistent with the standard JCPDS card No. 36-1451. No secondary impurity phases were detected in the pristine ZnO sample, confirming the successful synthesis of phase-pure ZnO nanoneedles via the sonochemical route. The sharp and intense diffraction peaks further indicate the high crystallinity of the synthesized material [18].

For the ZnO/Gd₂O₃ sample, the XRD pattern retains all characteristic reflections of the hexagonal ZnO phase, demonstrating that the modification of gadolinium oxide does not disturb the ZnO crystal lattice. However, slight peak broadening and a small reduction in diffraction intensity were observed after Gd₂O₃ modification. Such behavior may be associated with lattice distortion, defect generation, and suppression of crystal growth induced by the presence of gadolinium ions [16]. In addition, several weak diffraction peaks corresponding to cubic Gd₂O₃ were detected, confirming the successful modification of gadolinium oxide into the ZnO sample. Similar structural behavior has previously been reported for rare-earth modified ZnO nanostructures, where the introduction of Gd-containing phases led to increased defect concentration and modified crystallization kinetics [9]. The average crystallite size of the synthesized nanomaterials was estimated using the Scherrer equation. The calculated average crystallite size for pure ZnO nanoneedles was approximately 65 nm, whereas the modified ZnO/Gd₂O₃ sample exhibited a reduced crystallite size of approximately 55 nm. The decrease in crystallite size after gadolinium oxide modification suggests that Gd₂O₃ acts as a crystal growth inhibitor during the sonochemical synthesis process, limiting grain coalescence and promoting the formation of finer nanocrystalline structures [19]. The reduction in crystallite size is also associated with increased surface defect density and larger active surface area, which may positively influence charge separation efficiency and reactive oxygen species generation during catalytic processes [11].

Rietveld refinement of the XRD patterns was performed to determine the structural parameters of pristine ZnO and ZnO/Gd₂O₃ samples. The refinement confirmed that the hexagonal wurtzite structure of ZnO is preserved after Gd₂O₃ modification, with only minor variations in lattice parameters. A slight shift in the diffraction peaks toward lower 2θ values, particularly for the dominant (101) reflection, was observed in the modified sample. This shift may be attributed to lattice distortion induced by the modification of larger Gd³⁺ ions into or near the ZnO lattice. In addition, very weak additional reflections can be observed in the XRD pattern of the Gd-modified sample. However, due to the low dopant concentration (1–2 mol %) and the very low intensity of these features, they cannot be unambiguously assigned to a separate crystalline Gd₂O₃ phase based on XRD analysis alone. Therefore, no definitive conclusion regarding the presence of a distinct Gd₂O₃ phase can be drawn from the diffraction data. To provide complementary compositional information, EDS elemental mapping analysis (Figure 3) was performed. The results confirm the presence of Gd and its homogeneous distribution throughout the ZnO/Gd₂O₃ nanoneedle structures. However, while EDS mapping verifies the presence and spatial distribution of gadolinium species, it does not by itself distinguish between lattice incorporation and highly dispersed Gd-containing phases.

The refined structural parameters of pristine and 2 mol % modified ZnO synthesized via the sonochemical route are summarized in

Table 1. Structural parameters obtained from the Rietveld refinement of the XRD data for pristine ZnO and ZnO/Gd₂O₃ samples.

Catalysts	Crystal.		Microstrain
	Size	Parameters
ZnO	65 nm	a, b: 3.2437 c: 5.2017
			1.05 × 10−3
ZnO/Gd2O3, 1 mol %	58 nm	a, b: 3.2477 c: 5.2041
			1.25 × 10−3
ZnO/Gd2O3, 2 mol %	55 nm	a, b: 3.2487 c: 5.2048	1.88 × 10−3

. Although Gd incorporation leads to only marginal changes in lattice constants, a slight increase in lattice strain is observed, suggesting the presence of local structural distortions.

The average crystallite sizes estimated from the Scherrer equation were approximately 65 nm for pristine ZnO and 55 nm for ZnO/Gd₂O₃. The reduction in crystallite size suggests that Gd incorporation may have influenced the crystal growth process during sonochemical synthesis, leading to finer crystallites. Such subtle structural modifications may contribute to the catalytic performance by influencing surface properties and charge-transfer behavior without significantly altering the overall crystal structure [11,19].

Although the Rietveld refinement revealed only marginal changes in the lattice parameters after Gd₂O₃ incorporation, such subtle structural variations do not preclude the formation of point defects or oxygen vacancies. Since XRD is primarily sensitive to long-range crystallographic order, localized defects introduced by Gd species may remain undetectable by diffraction analysis but can significantly influence the electronic structure and charge carrier dynamics, as evidenced by the PL measurements.

Figure 2 presents the SEM micrographs of the sonochemically synthesized pure ZnO and ZnO/Gd₂O₃ (2 mol %) samples. Both materials exhibit well-defined anisotropic nanorod morphology with relatively homogeneous distribution and dense aggregation of structures. The obtained ZnO nanoneedles exhibit an elongated hexagonal morphology, characteristic of wurtzite ZnO synthesized under alkaline conditions and promoted by ultrasonically assisted crystal growth. In the case of the Gd₂O₃ modified sample, the nanoneedles appear slightly finer and more densely packed, indicating that gadolinium oxide incorporation influences the nucleation and growth kinetics during the sonochemical synthesis process. The observed decrease in rod dimensions is consistent with the XRD results, which showed a reduction in crystallite size after Gd₂O₃ modification. Furthermore, the high density of interconnected nanoneedles may provide an increased active surface area and enhanced availability of catalytic active sites, which are beneficial for mechanically induced catalytic processes. The anisotropic needle-like architecture is also expected to facilitate efficient polarization and charge separation under mechanical stimulation, contributing to the improved catalytic degradation performance toward paracetamol removal [20].

The elemental composition and chemical purity of the synthesized ZnO and ZnO/Gd₂O₃ nanoneedles were analyzed by energy-dispersive X-ray spectroscopy (EDS), as presented in Figure 3. The EDS spectra of both samples reveal the presence of Zn and O as the predominant constituent elements, confirming the successful formation of ZnO-based materials. In the case of the ZnO/Gd₂O₃ sample, additional characteristic gadolinium-related signals were detected, demonstrating the successful incorporation of gadolinium oxide species into the ZnO system. The obtained results additionally suggest that gadolinium ions are predominantly distributed over the surface of the ZnO nanoneedles, contributing to surface defect formation and modification of the interfacial properties of the material. Furthermore, the absence of impurity-associated peaks in the EDS spectra indicates the high chemical purity of the synthesized materials and suggests that no undesirable secondary phases or contaminant elements were introduced during the sonochemical synthesis process.

2.2. Optical Properties and Photoluminescence Analysis

Figure 4 presents the diffuse reflectance UV–Vis spectra (Figure 4a) and the corresponding Tauc plots (Figure 4b) of pure ZnO and ZnO/Gd₂O₃ nanoneedles synthesized via the sonochemical method. The diffuse reflectance spectra show strong absorption in the ultraviolet region, characteristic of ZnO semiconductors due to intrinsic band-to-band electronic transitions within the hexagonal wurtzite structure [21]. The sharp absorption edge observed for both samples confirms the preservation of semiconducting properties and the high crystallinity of the synthesized nanoneedles. Compared with pure ZnO, the ZnO/Gd₂O₃ sample exhibits a slight red shift in the absorption edge toward longer wavelengths, together with reduced reflectance intensity, indicating enhanced absorption capability after gadolinium oxide incorporation.

The observed optical behavior may be associated with defect formation, oxygen vacancies, and localized electronic states introduced by gadolinium-related species distributed over the ZnO surface [8]. Rare-earth modification is known to influence the electronic structure of ZnO through defect engineering and interfacial electronic interactions, resulting in improved charge transfer properties and suppression of electron–hole recombination [22]. Furthermore, gadolinium-related surface centers may act as electron-trapping sites, prolonging the lifetime of generated charge carriers and facilitating reactive oxygen species formation during catalytic processes.

The optical bandgap energies were estimated from the Tauc plots using the Tauc relation for direct bandgap semiconductors (Figure 4b). The calculated bandgap value for pure ZnO was 3.21 eV, whereas the ZnO/Gd₂O₃ nanoneedles exhibited slightly lower bandgap energy of 3.19 eV. The observed narrowing of the bandgap after Gd₂O₃ incorporation may be attributed to lattice distortion, defect- related states, and oxygen vacancy formation induced by gadolinium species [23]. Such electronic modifications can facilitate charge carrier migration and improve the separation efficiency of carrier pairs while reducing their recombination rate.

The reduced bandgap energy and enhanced defect- assisted electronic transitions observed for the ZnO/Gd₂O₃ sample are expected to positively influence mechanically induced catalytic processes. Improved charge separation and prolonged carrier lifetime may promote the generation of reactive oxygen species, thereby contributing to enhanced catalytic degradation efficiency toward paracetamol removal.

Figure 5 presents the photoluminescence (PL) spectra of pure ZnO and ZnO/Gd₂O₃ nanostructures recorded at room temperature. Both samples exhibit a near-band-edge emission centered at approximately 375 nm, a visible emission band around 446 nm, and a broad visible emission centered around 531 nm. The near-band-edge emission at 375 nm is attributed to the radiative recombination of free excitons, whereas the visible emission at 446 nm and the broad visible emission centered around 531 nm are associated with intrinsic defect states, particularly oxygen vacancies, zinc interstitials, and other defect-related recombination centers.

A significant decrease in PL intensity is observed after Gd₂O₃ incorporation across the entire emission range. The reduced luminescence intensity indicates a suppression of recombination resulting from the introduction of Gd-related trapping centers and defect states, which facilitate charge separation and prolong the lifetime of charge carriers. The presence of these additional trapping sites promotes charge migration toward the catalyst surface, where the charges participate in redox reactions responsible for reactive oxygen species generation.

The lower recombination rate evidenced by the PL spectra is consistent with the enhanced catalytic performance of the Gd₂O₃-modified ZnO samples observed during both catalytic degradation experiments with PTFE and glass rods.

2.3. Degradation of Paracetamol with PTFE Rod, Using ZnO and ZnO/Gd₂O₃ Nanoneedles

Catalytic degradation experiments were conducted using paracetamol as a model pharmaceutical contaminant at an initial concentration of 15 ppm under dark conditions to exclude any contribution from photocatalytic processes. The reaction suspensions were continuously stirred at 500 rpm using a PTFE (Teflon) stirring bar to induce mechanical agitation and interfacial contact between the catalyst particles and the reactor components. The degradation process was monitored by UV–Vis spectroscopy at the characteristic absorption maximum of paracetamol (λ_max = 243 nm). In addition, a control experiment conducted under identical conditions in the absence of a catalyst showed no measurable decrease in paracetamol concentration, indicating that the observed degradation is attributable to the presence of the ZnO materials.

Figure 6 presents the evolution of the UV–Vis absorption spectra of paracetamol during treatment over pure ZnO (Figure 6a), ZnO/Gd₂O₃ (1 mol %) (Figure 6b), and ZnO/Gd₂O₃ (2 mol %) (Figure 6c) nanoneedles. The progressive decrease in the intensity of the characteristic absorption peak with increasing reaction time indicates the gradual removal of paracetamol from the aqueous solution under mechanical stirring conditions.

For the pure ZnO sample (Figure 6a), a gradual reduction in absorption intensity at 243 nm was observed, corresponding to degradation efficiencies of 22.87%, 38.00%, 47.06%, and 59.63% after 1, 2, 3, and 4 h, respectively. The incorporation of 1 mol % Gd₂O₃ led to improved catalytic performance (Figure 6b), resulting in degradation efficiencies of 33.43%, 48.50%, 64.81%, and 73.12% over the same time intervals. The highest activity was observed for the ZnO/Gd₂O₃ (2 mol %) sample (Figure 6c), where degradation efficiencies reached 38.31%, 59.68%, 73.75%, and 80.21% after 1, 2, 3, and 4 h, respectively.

The continuous decrease in absorbance at λ_max = 243 nm confirms the progressive decomposition of paracetamol and is consistent with the kinetic analysis based on the pseudo-first-order model. The degradation efficiencies derived from UV–Vis measurements (

Table 2. Summary of the degradation efficiency after 4 h of catalytic treatment and the corresponding kinetic parameters for pristine ZnO and ZnO/Gd₂O₃ nanoneedles.

Catalysts	Rate Constant, h−1	Degradation Rate, %
ZnO	0.2251	59.63
ZnO/Gd2O3, 1 mol %	0.3375	73.12
ZnO/Gd2O3, 2 mol %	0.4267	80.21

) show the same overall trend as the apparent rate constants, which increase from 0.2251 h⁻¹ for pure ZnO to 0.3375 h⁻¹ and 0.4267 h⁻¹ for the 1 mol % and 2 mol % Gd₂O₃ modified samples, respectively. This agreement indicates that Gd₂O₃ incorporation leads to enhanced catalytic performance, which may be associated with improved charge transfer processes and defect-mediated effects reported in similar ZnO systems in the literature.

Figure 7 presents the temporal evolution of the normalized concentration (C/C₀) during tribocatalytic treatment (Figure 7a) together with the corresponding pseudo-first-order kinetic plots (Figure 7b) for pure ZnO, ZnO/Gd₂O₃ (1 mol %), and ZnO/Gd₂O₃ (2 mol %) nanoneedles.

As shown in Figure 7a, all investigated materials exhibit catalytic activity toward paracetamol degradation under mechanical stirring; however, noticeable differences in performance are observed depending on the Gd₂O₃ content. Pure ZnO shows the lowest degradation efficiency, with approximately 60% of the initial paracetamol concentration remaining after 4 h of treatment. In contrast, the incorporation of Gd₂O₃ leads to a clear enhancement in activity. The ZnO/Gd₂O₃ (1 mol %) sample demonstrates improved degradation behavior, while the ZnO/Gd₂O₃ (2 mol %) nanoneedles exhibit the highest performance. The improved performance of the Gd-modified samples may be related to changes in the structural and defect properties induced by Gd incorporation, as reported in similar ZnO-based systems. These modifications can influence the electronic environment of the material and may affect charge carrier dynamics during the mechanically assisted process. Such changes are likely to contribute to the observed differences in degradation efficiency.

The kinetic behavior of the mechanically assisted degradation process was further analyzed using the pseudo-first-order model. As shown in Figure 7b, the degradation process fits well to the pseudo-first-order kinetics with high correlation coefficients (R² > 0.99). The calculated apparent rate constants were 0.2251 h⁻¹ for pure ZnO, 0.3375 h⁻¹ for ZnO/Gd₂O₃ (1 mol %), and 0.4267 h⁻¹ for ZnO/Gd₂O₃ (2 mol %). These results indicate that increasing the Gd₂O₃ content leads to an enhancement of the apparent degradation rate of paracetamol. The rate constant obtained for the 2 mol % Gd₂O₃-modified sample is approximately 1.9 times higher than that of pure ZnO, highlighting the beneficial effect of gadolinium incorporation on the overall catalytic performance.

The improved activity of the Gd₂O₃-modified nanoneedles may be associated with a combination of structural and electronic factors, including variations in crystallite size, an increased density of defect-related states, and the possible formation of Gd-related trapping sites. These features may influence charge carrier behavior and interfacial processes occurring under mechanical stirring, thereby affecting the overall degradation kinetics. The formation of reactive oxygen species, including superoxide (•O₂⁻) and hydroxyl (•OH) radicals, confirmed in this study through scavenger experiments, indicates that radical-mediated oxidation pathways play a key role in the degradation process. These reactive species are likely generated through interfacial charge transfer processes occurring under mechanical stimulation. However, the exact contribution of triboelectric and/or piezoelectric effects to the initial charge generation cannot be independently quantified in the present experimental setup; therefore, the mechanistic origin is considered to involve a combination of mechanically induced interfacial effects and subsequent ROS-driven degradation pathways [24,25,26].

Furthermore, the gradual increase in the mechanically stirred catalytic activity with increasing Gd₂O₃ content suggests that the influence of gadolinium incorporation becomes more pronounced at higher dopant concentrations. Previous studies have reported that Gd incorporation into ZnO may promote the formation of defect-related states, including oxygen vacancies, which can influence charge carrier dynamics and surface reactivity [27]. Such defect sites may act as charge trapping centers, potentially affecting electron–hole recombination processes and the lifetime of charge carriers involved in interfacial reactions. In addition, the formation of heterointerfaces between ZnO and Gd₂O₃ phases has been suggested in similar systems, where it may facilitate interfacial charge transfer processes and modify surface reaction pathways. Comparable effects of Gd-related modifications on charge carrier behavior in ZnO systems have also been reported [7]. With increasing Gd₂O₃ loading from 1 to 2 mol %, a higher concentration of dopant-induced defect states and trapping sites can be expected, which may further influence the utilization of mechanically induced charge carriers. Literature reports indicate that increasing rare-earth dopant concentration can enhance catalytic performance up to an optimal level, mainly due to modifications in defect chemistry and electronic structure [28]. As a result, an increased generation of reactive oxygen species (•OH and •O₂⁻) can be inferred from the scavenger experiments, contributing to the observed enhancement in degradation performance. Overall, the increased degradation efficiency and apparent rate constant with higher Gd₂O₃ content indicate that gadolinium incorporation plays an important role in tuning the defect structure and surface properties of ZnO nanoneedles, thereby influencing their mechanically assisted catalytic behavior.

Radical scavenging experiments were conducted to identify the reactive species involved in the degradation of paracetamol and to gain further insight into the reaction pathway. For this purpose, ascorbic acid (AA), isopropanol (IPA), and EDTA-2Na were used as scavengers for superoxide radicals (•O₂⁻), hydroxyl radicals (•OH), and holes (h⁺), respectively. The obtained results are presented in Figure 8. In the absence of scavengers, the degradation efficiency increased progressively from pure ZnO to the Gd₂O₃-modified samples, consistent with the trend observed in the catalytic performance. Upon the addition of IPA, a noticeable decrease in degradation efficiency was observed for all investigated catalysts, suggesting that hydroxyl radicals play a significant role in the oxidation process of paracetamol. Similarly, the addition of AA led to a considerable reduction in catalytic activity, indicating that superoxide radicals also contribute importantly to the degradation pathway. In contrast, the presence of EDTA-2Na resulted in a comparatively less pronounced inhibition effect, implying that holes are likely involved mainly through secondary pathways, such as the generation of reactive oxygen species, rather than direct oxidation of paracetamol molecules.

The inhibition pattern observed for all samples suggests that the mechanically assisted degradation process proceeds predominantly via radical-mediated pathways. Moreover, the more pronounced effect of both IPA and AA for the Gd₂O₃-modified catalysts may indicate a higher involvement of hydroxyl and superoxide radicals under these conditions. This behavior can be reasonably associated with the influence of gadolinium incorporation on the surface and electronic properties of ZnO, which may affect charge carrier dynamics and the formation of reactive species under mechanical stimulation. Such effects are in line with previous reports suggesting that rare-earth doping can modify defect structures and oxygen-vacancy-related states, which in turn may influence charge separation behavior and reactive oxygen species generation. The presence of defect centers is suggested to influence electron transfer processes and interfacial charge carrier dynamics, which may contribute to variations in the generation and utilization of reactive oxygen species responsible for paracetamol degradation [29,30,31].

2.4. Degradation of Paracetamol with a Glass Rod, Using ZnO and ZnO/Gd₂O₃ Nanoneedles

Following the tribocatalytic investigation, the degradation of paracetamol was further examined under other mechanical conditions using ZnO and ZnO/Gd₂O₃ nanoneedles. In this system, mechanical energy was introduced through the movement of glass rods, resulting in continuous agitation, collision events, and interfacial contact between the catalyst particles, the stirring medium, and the reactor walls. These conditions generate a complex mechanical environment in which interfacial charge transfer processes may occur. In addition, ZnO is a piezoelectric material; therefore, the possible contribution of mechanically induced polarization cannot be completely excluded. These effects are generally considered to influence charge carrier behavior and may contribute to the formation of reactive oxygen species involved in pollutant degradation. To evaluate the effect of Gd₂O₃ incorporation on the catalytic performance, degradation experiments were conducted using a 15 ppm aqueous paracetamol solution, while the reaction progress was monitored through the evolution of the characteristic UV–Vis absorption band at 243 nm.

Figure 9 presents the changes in the UV–Vis absorption spectra of paracetamol during the process for pure ZnO (Figure 9a), ZnO/Gd₂O₃ (1 mol %) (Figure 9b), and ZnO/Gd₂O₃ (2 mol %) (Figure 9c) nanoneedles. For all investigated samples, a progressive decrease in the absorption intensity at 243 nm was observed with increasing reaction time, indicating gradual degradation of paracetamol under mechanically stimulated conditions.

The pure ZnO sample exhibited a gradual reduction in the absorption peak intensity, corresponding to degradation efficiencies of 29.68%, 48.94%, 65.43%, and 73.75% after 1, 2, 3, and 4 h of treatment, respectively. The incorporation of 1 mol % Gd₂O₃ improved the catalytic performance, yielding degradation efficiencies of 40.37%, 61.50%, 74.69%, and 85.25% over the same time periods. The highest activity was observed for the ZnO/Gd₂O₃ (2 mol %) nanoneedles, where the absorption band decreased most rapidly, resulting in degradation efficiencies of 47.31%, 70.82%, 85.13%, and 91.74% after 1, 2, 3, and 4 h of treatment, respectively.

The enhanced mechanically assisted catalytic activity observed after Gd₂O₃ incorporation may be associated with the combined effects of defect engineering and modified charge carrier dynamics. The introduction of gadolinium oxide can promote the formation of oxygen vacancies and lattice distortions, which are commonly reported in doped ZnO systems and may act as charge trapping sites, thereby influencing recombination processes. Under mechanical deformation, ZnO nanoneedles, as piezoelectric materials, may generate polarization charges; however, the exact contribution of piezoelectric effects cannot be independently isolated in the present experimental configuration. In addition, gadolinium-related defect states may further affect the lifetime and mobility of charge carriers. As a result, a higher fraction of charges may participate in interfacial redox processes, which is consistent with the observed formation of reactive oxygen species and the enhanced degradation performance toward paracetamol. The increase in degradation efficiency with increasing Gd₂O₃ content therefore suggests a beneficial role of gadolinium incorporation in tuning the defect structure and surface properties of ZnO nanostructures, in agreement with trends reported for similar doped ZnO systems.

To further quantify the mechanically assisted catalytic performance of the synthesized materials, the degradation data obtainsed from UV–Vis absorption measurements were analyzed using a pseudo-first-order kinetic model. Figure 10a presents the variation of the normalized concentration ratio (C/C₀) as a function of reaction time, while Figure 10b shows the corresponding linear plots of −Ln(C/C₀) versus time.

As shown in Figure 10a, a continuous decrease in the concentration of paracetamol was observed during the mechanically catalytic process with a glass rod for all investigated samples. Among the studied materials, the ZnO/Gd₂O₃ (2 mol %) nanoneedles exhibited the most pronounced decrease in C/C₀ values, followed by ZnO/Gd₂O₃ (1 mol %), while pure ZnO showed the lowest degradation rate. This trend is consistent with both the UV–Vis absorption results (Figure 9), which show a progressive decrease in absorbance at 243 nm corresponding to increased paracetamol removal with higher Gd₂O₃ content, and the kinetic analysis (Figure 10), which confirms the same overall behavior.

The kinetic plots presented in Figure 10b reveal an approximately linear relationship between −ln(C/C₀) and reaction time, indicating that the degradation process can be reasonably described by a pseudo-first-order kinetic model. The apparent rate constants (k) determined from the slopes of the fitted lines were 0.3411 h⁻¹ for pure ZnO, 0.4734 h⁻¹ for ZnO/Gd₂O₃ (1 mol %), and 0.6285 h⁻¹ for ZnO/Gd₂O₃ (2 mol %). The corresponding correlation coefficients (R² = 0.9978–0.9996) indicate a very good fit and support the applicability of the pseudo-first-order model for describing the degradation process under mechanically stirred conditions with a glass rod. The increase in the apparent rate constant with increasing Gd₂O₃ content provides quantitative evidence of enhanced degradation kinetics upon gadolinium incorporation. Compared with pure ZnO, both ZnO/Gd₂O₃ samples exhibit faster reaction rates, which is consistent with the higher degradation efficiencies observed in the UV–Vis spectroscopic analysis. The increase in rate constants may be associated with more efficient utilization of charge carriers generated under mechanical stimulation, potentially arising from improved charge separation and the presence of gadolinium-related defect states that can facilitate interfacial charge transfer processes.

To gain further insight into the degradation behavior of paracetamol, trapping experiments were conducted using specific scavengers capable of selectively quenching the main reactive species potentially involved in the process. Ascorbic acid (AA), isopropanol (IPA), and EDTA-2Na were introduced into the reaction system as scavengers for superoxide radicals (•O₂⁻), hydroxyl radicals (•OH), and holes (h⁺), respectively. The corresponding results are summarized in Figure 11.

The addition of scavengers led to a noticeable decrease in the catalytic properties with a glass rod for all investigated samples, suggesting the involvement of multiple reactive species in the degradation pathway. Among the tested scavengers, IPA induced the strongest inhibition effect, indicating that hydroxyl radicals play a major role in the oxidation of paracetamol. A significant reduction in degradation efficiency was also observed in the presence of AA, implying an important contribution of superoxide radicals, which are likely generated through interfacial electron transfer processes involving dissolved oxygen. In comparison, the influence of EDTA-2Na was less pronounced, suggesting that holes contribute to the overall process mainly through indirect pathways.

The inhibition behavior observed for the Gd₂O₃-modified samples followed a similar trend to that of pure ZnO, while the overall degradation efficiencies remained higher in all cases. This suggests that Gd₂O₃ incorporation does not significantly alter the dominant reactive species involved in the process but may enhance the overall efficiency of their generation or utilization.

Under mechanical stirring, ZnO nanoneedles, as piezoelectric materials, may generate polarization charges; however, the exact contribution of piezoelectric effects cannot be independently isolated in the present system. In addition, gadolinium-related defect states and oxygen-vacancy-like centers may act as trapping sites that influence charge carrier dynamics and recombination behavior. Similar enhancement of charge separation and ROS generation through defect engineering in rare-earth-modified ZnO systems has been reported in the recent literature [32]. As a result, a higher fraction of charge carriers may participate in interfacial redox reactions, which is consistent with the observed formation of reactive oxygen species (•OH and •O₂⁻) and the enhanced degradation performance.

2.5. Chemical Oxygen Demand (COD) Reduction During Degradation of Paracetamol

At an initial paracetamol concentration of 15 ppm (15 mg L⁻¹) and an initial chemical oxygen demand (COD) of 31.65 mg O₂ L⁻¹, the catalytic performance was evaluated based on the reduction in COD during the degradation process. Under mechanical stirring using the PTFE rod, ZnO exhibited a degradation efficiency of 59.63%, corresponding to a decrease in COD from 31.65 to 12.77 mg O₂ L⁻¹. The ZnO/Gd₂O₃ (1 mol %) composite demonstrated an enhanced degradation efficiency of 73.12%, resulting in a residual COD of 8.51 mg O₂ L⁻¹. The highest activity was achieved by the ZnO/Gd₂O₃ (2 mol %) composite, which attained a degradation efficiency of 80.25% and reduced the COD to 6.25 mg O₂ L⁻¹.

A more pronounced reduction in COD was observed under the second mechanical stirring with a glass rod. Pure ZnO achieved a degradation efficiency of 73.75%, yielding a residual COD of 8.31 mg O₂ L⁻¹. The ZnO/Gd₂O₃ (1 mol %) composite further improved the degradation efficiency to 85.25%, accompanied by a decrease in COD to 4.67 mg O₂ L⁻¹. The most effective catalyst was ZnO/Gd₂O₃ (2 mol %), which exhibited a degradation efficiency of 91.81% and reduced the COD to 2.59 mg O₂ L⁻¹.

The obtained results demonstrate that Gd₂O₃ incorporation enhances the catalytic performance of ZnO under both mechanical stirring. Increasing the dopant content to 2 mol % resulted in the highest degree of paracetamol mineralization, as reflected by the lowest residual COD values. In addition, differences in performance between the two experimental configurations are consistently observed across all samples, indicating that the nature of the mechanical environment influences the overall degradation efficiency.

Compared with the PTFE system, the glass-based stirring exhibited higher degradation rates. This improvement may be associated with differences in mechanical energy transfer and interfacial contact conditions between the catalyst particles and the contacting surfaces. The nanoneedle morphology of the synthesized ZnO materials, with its high aspect ratio and sharp features, may further contribute to localized stress distribution during mechanical contact, which can influence interfacial charge transfer processes. However, due to the coexistence of multiple interacting factors, a clear separation of individual contributions cannot be definitively established based on the present data.

The enhanced catalytic behavior of ZnO-based nanoneedles under mechanical stimulation may be interpreted as arising from a combination of interfacial charge-related processes and material-dependent electronic effects. Mechanical activation under different contact conditions is generally considered to induce charge redistribution at solid interfaces; in the case of ZnO, possible piezoelectric contributions under deformation may also play a role. These effects may facilitate the formation of reactive oxygen species, which are commonly associated with the degradation of organic pollutants such as paracetamol. The incorporation of Gd₂O₃ may further influence the observed activity through modifications in the defect structure of ZnO. In particular, gadolinium-related states and oxygen-vacancy-like defects, as reported in similar systems, may act as charge trapping centers that affect recombination dynamics and charge carrier lifetimes. Such changes may in turn influence the efficiency of interfacial redox reactions under mechanical stimulation.

Scavenger experiments suggest the involvement of multiple reactive species, including hydroxyl and superoxide radicals, indicating that the degradation process proceeds through a complex reaction network rather than a single dominant pathway. Kinetic analysis further shows that the degradation behavior can be reasonably described by a pseudo-first-order model under the investigated conditions.

2.6. Control Experiments for Decoupling Mechanical and Material-Dependent Effects

2.6.1. Reactor Material (Glass vs. PTFE Vessel)

Control experiments were conducted using both glass and PTFE vessels under otherwise identical experimental conditions in order to evaluate the influence of the reaction environment on the mechanically induced catalytic performance [33]. All reaction parameters, including catalyst loading, initial paracetamol concentration, stirring speed, reaction volume, and reaction time, were strictly maintained to ensure direct comparability between systems. The results show that the glass vessel leads to consistently higher degradation efficiencies compared to the PTFE vessel for both ZnO and ZnO/Gd₂O₃ (2 mol %) catalysts. This difference suggests that, in addition to the nature of the stirring bar–catalyst interaction, the overall reactor environment may influence interfacial charge-related processes occurring during mechanical agitation. However, since the reactor material can simultaneously affect multiple factors, including frictional interactions, surface contact characteristics, and collision dynamics, the observed differences cannot be attributed to a single isolated mechanism. Therefore, the results indicate that the degradation performance is governed by coupled mechanically induced effects arising from the combined interaction of the catalyst with the reactor environment. In this context, it is more appropriate to describe the system in terms of mechanically assisted catalytic processes rather than assigning strictly separated catalytic contributions.

2.6.2. Adsorption Control Experiment

To evaluate the contribution of adsorption to the overall degradation process, a control experiment was performed under identical experimental conditions in the absence of mechanical stirring and light irradiation. The catalyst was dispersed in the paracetamol solution and maintained under continuous stirring in the dark to approach adsorption–desorption equilibrium. After a predetermined time interval, the suspension was analyzed to determine the residual concentration of paracetamol.

The results showed that only a negligible amount of paracetamol was removed by adsorption, indicating that adsorption contributes only marginally to the overall removal process. This suggests that the observed decrease in paracetamol concentration under mechanically stirring conditions is not driven primarily by surface adsorption but is associated with catalytic processes activated under mechanical agitation. Therefore, adsorption is not a significant pathway contributing to the high degradation efficiencies observed in the present system.

2.6.3. Effect of Stirring Speed

Under identical experimental conditions, control experiments were additionally performed at stirring speeds of 100, 300, and 500 rpm to evaluate the influence of mechanical energy input on the catalytic performance. For both types of catalysis, the degradation efficiency increased progressively with increasing stirring speed.

In the system with PTFE, the degradation efficiency of pure ZnO increased from 24.53% at 100 rpm to 43.26% at 300 rpm and 59.63% at 500 rpm, while ZnO/Gd₂O₃ (2 mol %) exhibited efficiencies of 48.57%, 66.42%, and 80.21%, respectively. Similarly, in the stirring with a glass rod, pure ZnO achieved degradation efficiencies of 36.74%, 61.13%, and 73.75%, whereas ZnO/Gd₂O₃ (2 mol %) reached 56.24%, 73.86%, and 91.81% at 100, 300, and 500 rpm, respectively.

These results demonstrate that increasing mechanical energy input significantly enhances the catalytic performance, likely by promoting more frequent interfacial contact events and improving mass transport and catalyst–surface interactions, which can facilitate charge-related processes and the formation of reactive oxygen species.

Although the precise contributions of different mechanically induced effects cannot be independently quantified, the consistently higher efficiencies observed in the glass-based configuration indicate that the overall reaction environment plays an important role in modulating the catalytic response under dynamic conditions.

2.6.4. TiO₂ as a Reference Catalyst

Identical experimental conditions were used for an additional control experiment employing commercial TiO₂ to assess the contribution of the intrinsic electromechanical properties of ZnO. Commercial TiO₂ was selected as a reference material to provide a benchmark for comparison under identical experimental conditions. Although tribocatalytic activity has been reported for some nanostructured TiO₂ materials, the commercial TiO₂ used in this work exhibited only limited catalytic activity compared with ZnO, allowing the intrinsic performance of the synthesized ZnO-based catalysts to be evaluated.

In the catalysis with the PTFE rod, TiO₂ achieved a degradation efficiency of 18.6%, which increased only slightly to 24.3% in the with a glass rod. In comparison, pure ZnO exhibited degradation efficiencies of 59.63% and 73.75%, while ZnO/Gd₂O₃ (2 mol %) reached 80.25% and 91.81% for the PTFE-coated and glass bars, respectively.

The considerably lower activity of the commercial TiO₂ reference material compared with ZnO indicates that mechanical agitation alone cannot account for the high degradation efficiencies observed for the ZnO-based catalysts. Although tribocatalytic activity has been reported for certain nanostructured TiO₂ materials, the commercial TiO₂ used in the present study exhibited only limited degradation efficiency under identical experimental conditions, confirming its suitability as a reference material. Instead, the enhanced performance of ZnO may be related to its more favorable electronic and surface properties under mechanical stimulation, which can facilitate more efficient charge-related processes. Although different mechanically induced effects may coexist in both experimental configurations, these results suggest that the intrinsic properties of ZnO play an important role in its overall mechanically assisted catalytic activity.

2.7. Evaluation of Catalyst Performance in Simulated Water Matrices Containing Common Inorganic Anions

The effect of common inorganic anions on the catalytic degradation of paracetamol was investigated using 5 mM solutions of NaCl, Na₂SO₄, and NaHCO₃. As shown in Figure 12, the degradation efficiency decreased in the presence of dissolved inorganic ions compared to distilled water. For pristine ZnO, the degradation efficiency decreased from 73.75% in distilled water to 72.23%, 67.54%, and 58.67% in the presence of Cl⁻, SO₄²⁻, and HCO₃⁻ ions, respectively. Similarly, the ZnO/Gd₂O₃ composite exhibited degradation efficiencies of 90.14%, 87.31%, and 79.15% in the corresponding solutions, compared to 91.81% in distilled water. The inhibitory effect followed the order Cl⁻ < SO₄²⁻ < HCO₃⁻ for both catalysts.

The pH of the reaction media was also influenced by the presence of inorganic ions. The initial pH values were 6.8, 6.7, and 8.2 for the NaCl, Na₂SO₄, and NaHCO₃ solutions, respectively, compared to 6.9 for distilled water. After the catalytic process, the pH values decreased to 6.4, 6.5, and 8.0, respectively, while the pH of distilled water decreased from 6.9 to 6.5. The observed decrease in pH after treatment may be attributed to the formation of intermediate oxidation products during paracetamol degradation. The relatively small pH variation in the bicarbonate-containing solution can be explained by the buffering capacity of the HCO₃⁻/CO₃²⁻ system.

The observed decrease in degradation efficiency in the presence of inorganic ions can be associated with changes in the reaction medium induced by dissolved electrolytes, which may influence the interaction between the catalyst surface and paracetamol molecules. These ions may affect adsorption–desorption equilibria and interfacial processes occurring during the degradation reaction, leading to variations in the overall catalytic performance.

Among the investigated ions, bicarbonate exhibited the most pronounced effect. This behavior can be related to the higher initial pH and the buffering capacity of the bicarbonate system. Changes in pH may influence the surface properties of ZnO and the interfacial behavior of paracetamol molecules, resulting in reduced degradation efficiency. In contrast, chloride ions showed only a minor effect on the degradation process, while sulfate ions led to an intermediate decrease in activity, suggesting a moderate influence of these species on the overall reaction environment.

Despite the decrease in degradation efficiency in the presence of inorganic ions, the ZnO/Gd₂O₃ composite consistently outperformed pristine ZnO in all tested media. The improved performance of the composite suggests that Gd₂O₃ incorporation enhances the robustness of ZnO-based catalysts, allowing them to maintain relatively high activity even in the presence of common inorganic constituents. These results indicate good potential applicability of the developed catalyst in water matrices containing dissolved salts.

2.8. Proposed Degradation Mechanism

The results obtained from the control experiments provide a basis for discussing the possible mechanistic aspects of the process. The degradation of paracetamol over ZnO and ZnO/Gd₂O₃ nanoneedles proceeds through a mechanically stirring catalytic process involving multiple interfacial phenomena. Mechanical agitation induces complex solid–liquid–solid interactions between catalyst particles and reactor components, which may lead to charge redistribution at interfaces. In the case of ZnO, possible electromechanical (piezoelectric) effects may also contribute under mechanical deformation; however, these contributions cannot be independently isolated within the present experimental configuration.

The incorporation of Gd₂O₃ modifies the defect structure of ZnO, introducing oxygen-vacancy-related and dopant-induced states that may influence charge carrier dynamics and interfacial electron transfer processes. These defect states may facilitate charge separation, suppress charge carrier recombination, and consequently enhance the formation of reactive oxygen species.

Scavenger experiments confirm the involvement of reactive oxygen species, including hydroxyl (•OH) and superoxide (•O₂⁻) radicals, indicating that radical-mediated oxidation plays a key role in the degradation pathway. The proposed mechanism is consistent with recent reports on mechanically induced catalytic systems, where charge generation is attributed to a combination of interfacial electron transfer, defect-mediated charge separation, and electromechanical effects in semiconductor materials. In ZnO-based systems, mechanically induced polarization and surface charge redistribution may facilitate the separation of charge carriers and promote electron transfer to dissolved oxygen, resulting in the formation of reactive oxygen species such as •O₂⁻ and subsequently •OH radicals. Furthermore, oxygen-vacancy-related defects and dopant-induced trapping states can prolong charge carrier lifetimes and suppress recombination, thereby enhancing the efficiency of interfacial redox reactions. Similar defect-assisted ROS generation pathways and mechanically stimulated charge-transfer mechanisms have been reported for rare-earth modified ZnO systems in recent studies [32]. Overall, the observed catalytic performance arises from a combination of mechanically induced interfacial effects, defect engineering, and ROS-driven degradation pathways, rather than a single dominant mechanism.

3. Materials and Methods

Zinc acetate dihydrate (Zn(CH₃COO)₂ × 2H₂O, >99.5%), sodium hydroxide (NaOH, >99.0%), and gadolinium oxide (Gd₂O₃, >99.0%) were sourced from Fluka (Buchs, Switzerland). Commercial TiO₂ powder (rutile, Sigma-Aldrich, (Merck), Burlington, MA, USA), Cat. No. 224227, ≥99.9%, particle size < 5 μm) was used as the reference material. Distilled water was used in all experiments, and all other chemicals and reagents were of analytical reagent grade.

Paracetamol (C₈H₉NO₂), a widely used analgesic and antipyretic pharmaceutical frequently detected in wastewater and surface waters, was employed as a model emerging contaminant in all degradation experiments. The concentration changes were monitored spectrophotometrically at its characteristic absorption maximum (λ_max = 243 nm). To investigate the applicability of the catalytic system under realistic environmental conditions, degradation studies were performed in both distilled water and tap water, allowing the effect of background water constituents on the catalytic process to be evaluated.

Pure and gadolinium-modified ZnO nanoneedles containing 1 and 2 mol % Gd₂O₃ were synthesized via a sonochemical precipitation method using aqueous solutions. For the synthesis of pure ZnO, 2.19 g of zinc acetate dihydrate (Zn(CH₃COO)₂ × 2H₂O) was dissolved in 100 mL of distilled water under continuous magnetic stirring at room temperature until a clear homogeneous solution was obtained. Subsequently, 100 mL of 0.2 M NaOH was added dropwise under vigorous stirring, resulting in the formation of a white suspension attributed to the precipitation of zinc hydroxide species (Zn(OH)₂) at pH ≈ 10. The mixture was then subjected to ultrasonic irradiation in a bath operating at 40 kHz and 150 W for 60 min, promoting nucleation and anisotropic growth through acoustic cavitation, which favored the formation of hydroxide-based precursor nanostructures. After ultrasonication, the precipitate was aged for 12 h at room temperature, collected by centrifugation, and washed several times with distilled water to remove residual ions. The obtained precursor was then dried at 80 °C for 12 h and finally calcined at 400 °C for 2 h, leading to dehydration and crystallization into ZnO nanoneedles with improved crystallinity.

For gadolinium-modified ZnO nanoneedles, the same procedure was followed, with the addition of Gd₂O₃ corresponding to 2 mol % relative to ZnO. Before NaOH addition, the required amount of Gd₂O₃ powder was dispersed in the zinc acetate solution under continuous stirring and ultrasonic treatment for 30 min to ensure uniform dispersion of the gadolinium oxide particles within the precursor solution. Afterward, the NaOH solution was added dropwise until pH 10 was reached, and the mixture was further sonicated for 60 min under identical conditions. The resulting precipitate was washed, dried, and calcined following the same procedure as for pure ZnO. Incorporation of Gd₂O₃ into the ZnO matrix during sonochemical processing contributed to structural modification of the ZnO nanoneedles and may enhance their optical and catalytic properties.

The synthesized ZnO and ZnO/Gd₂O₃ nanoneedles were comprehensively characterized to investigate their structural, morphological, compositional, and optical properties. Surface morphology was examined by scanning electron microscopy (SEM, Zeiss EVO 15, Bruker, Resolution 126 eV, Berlin, Germany), while the elemental composition was determined using energy-dispersive X-ray spectroscopy (EDS). Phase composition and crystallographic structure were analyzed by X-ray diffraction (XRD) using Cu Kα radiation (λ = 1.5418 Å) on an Empyrean diffractometer (Malvern Panalytical, Malvern, UK). The diffraction patterns were recorded with a step size of 0.05° and a counting time of 4 s per step. The average crystallite size was estimated from the (101) diffraction peak of ZnO using the Scherrer equation. Photoluminescence (PL) spectra were collected at room temperature using a Cary Eclipse fluorescence spectrophotometer with an excitation wavelength of 325 nm to evaluate the recombination behavior of charge carriers.

Paracetamol (C₈H₉NO₂) was selected as a model pharmaceutical pollutant owing to its widespread use and frequent occurrence in aquatic environments. The catalytic performance of the synthesized materials was evaluated through catalytic degradation experiments using 50 mL of an aqueous paracetamol solution with an initial concentration of 15 mg/L. For each experiment, 50 mg of catalyst was dispersed in the reaction solution and then maintained in the dark for 30 min to establish adsorption–desorption equilibrium before catalytic treatment. A 50 mL Paracetamol solution was subjected to catalytic degradation. The solution was prepared in a 100 mL beaker using distilled water, with continuous mixing provided by a magnetic stirrer.

Catalytic experiments were performed using a PTFE-coated magnetic (2.0 cm) and glass (2.0 cm) stirring rod at a constant stirring speed of 500 rpm and ambient temperature (23 ± 2 °C). Under these conditions, they may generate triboelectric charges and other mechanically induced interfacial charge-transfer phenomena.

At predetermined reaction intervals, aliquots were withdrawn from the suspension and centrifuged at 6000 rpm to remove the catalyst particles. The residual paracetamol concentration was monitored using a UV–Vis spectrophotometer (Evolution 300, Thermo Scientific, Madison, WI, USA) in the wavelength range of 200–600 nm. The degradation efficiency and kinetic parameters were calculated from the evolution of the characteristic absorption band of paracetamol at 243 nm.

The contribution of reactive species to the catalytic degradation mechanism was investigated through scavenger experiments. Ascorbic acid (AA), isopropanol (IPA), and EDTA-2Na were employed as selective scavengers for superoxide radicals (•O₂⁻), hydroxyl radicals (•OH), and holes (h⁺), respectively. Each scavenger was introduced separately into the reaction system at a fixed dosage of 6 mg per 50 mL of paracetamol solution. The resulting changes in degradation efficiency were used to identify the dominant reactive species participating in the catalytic process.

All catalytic measurements were performed in triplicate to ensure reproducibility, and the reported degradation efficiencies and kinetic parameters correspond to the mean values obtained from three independent experiments.

Three model aqueous solutions containing 5 mM of representative inorganic salts were prepared using deionized water. Sodium chloride (NaCl), sodium sulfate (Na₂SO₄), and sodium bicarbonate (NaHCO₃) were selected as sources of Cl⁻, SO₄²⁻, and HCO₃⁻ ions, respectively, due to their widespread occurrence in natural water systems. For the preparation of 1 L of solution, 0.292 g NaCl, 0.710 g Na₂SO₄, and 0.420 g NaHCO₃ were dissolved separately in deionized water to obtain 5 mM solutions. Paracetamol was subsequently added to each solution at the same concentration used in the experiments conducted in deionized water. The initial pH values of the NaCl, Na₂SO₄, and NaHCO₃ solutions were 6.8, 6.7, and 8.2, respectively. Tribocatalytic degradation experiments were performed under identical operating conditions using pristine ZnO and Gd₂O₃/ZnO catalysts. After completion of the reaction, the final pH values were measured and the degradation efficiencies were calculated based on the residual paracetamol concentration. The effect of the selected inorganic anions on the catalytic performance of the catalysts was subsequently assessed and compared with that obtained in distilled water.

4. Conclusions

The present study demonstrates the effectiveness of defect-engineered ZnO nanoneedles for the mechanically assisted degradation of the pharmaceutical pollutant paracetamol. The incorporation of Gd₂O₃ into the ZnO matrix results in improved catalytic performance under mechanically stimulated conditions, which can be attributed to modifications in the structural and defect properties of the material.

Control experiments, including adsorption tests, ionic strength and pH variation, stirring speed dependence, comparison of catalytic processes with PTFE and glass rods, and TiO₂ reference experiments, confirm that pollutant removal is not governed by adsorption or mechanical agitation alone but is associated with mechanically assisted interfacial processes and material-dependent electronic properties. Scavenger experiments indicate the involvement of reactive oxygen species, including hydroxyl and superoxide radicals, suggesting that radical-mediated pathways contribute to the degradation process.

The degradation efficiency increases with increasing stirring speed, indicating a strong dependence on mechanical energy input. Differences between catalysis with PTFE and glass rods reflect the influence of the reaction environment on interfacial interactions and charge-related processes. The TiO₂ control sample exhibits significantly lower activity compared to ZnO-based materials under identical conditions.

The reduction in chemical oxygen demand confirms the mineralization of paracetamol during treatment. The presence of NaCl, Na₂SO₄, and NaHCO₃ influences the degradation efficiency, with ZnO/Gd₂O₃ maintaining higher performance than pristine ZnO in all tested media.