PTFE-Enhanced Tribocatalytic Degradation of High-Concentration (100–500 mg/L) Rhodamine B Solutions Using TiO₂ Nanoparticles

Abstract

Dye wastewaters are produced in huge amounts every year world-widely and they pose serious threats to ecosystems and human health if not treated properly. High-concentration organic dye wastewaters are usually treated through multiple steps including pretreatment and advanced treatment. In this work, TiO₂ nanoparticles suspended in high-concentration rhodamine B (Rh B) solutions up to 500 mg/L have been subjected to magnetic stirring to initiate tribocatalytic degradation of Rh B. While 100 and 200 mg/L Rh B solutions can be rather thoroughly degraded in beakers with four kinds of bottom/coatings (glass, Al₂O₃, Ti, and PTFE), 300 mg/L Rh B solutions can only be degraded effectively in a beaker with PTFE coating. Even 400 and 500 mg/L Rh B solutions can also be degraded rather thoroughly by TiO₂ nanoparticles in a PTFE-coated beaker. EPR analyses revealed that PTFE coating enhanced the formation of both hydroxyl radicals and superoxide radicals by magnetic stirring-stimulated TiO₂ nanoparticles. These findings highlight the importance of the synergism between materials in friction pairs for tribocatalysis and demonstrate a one-step tribocatalytic degradation of Rh B solutions with concentrations of real Rh B wastewaters.

1. Introduction

The contamination of water resources, caused by industrial effluents, agricultural runoff (loaded with fertilizers and pesticides), municipal wastewater, and plastic pollution (notably marine microplastics), is a major environmental challenge of the 21st century, jeopardizing both drinking water security and aquatic ecosystem integrity [1,2,3,4]. This has made water pollution control and remediation a subject of extensive global concern. Textile dye effluent constitutes a major environmental pollutant [5,6,7]. The complex matrix of this wastewater, characterized by highly toxic and non-biodegradable organic compounds, demands sophisticated treatment solutions. While some highly efficient green technologies like specialized enzymes and advanced oxidations are available, considerable capital and operational expenditures associated with them hinder their widespread implementation [8,9,10,11].

The treatment of high-concentration organic dye wastewater is a complex and challenging industrial issue [12,13,14]. Single conventional wastewater treatment methods are often inadequate for effective remediation, necessitating the adoption of integrated multi-technology process routes [15,16] and adhering to the principle of “stepwise treatment and gradual reduction”. First, pretreatment: initially remove suspended solids, adjust pH, and reduce toxicity to create favorable conditions for subsequent treatment stages. Second, main treatment: Focus on the core degradation or separation of dye molecules and auxiliaries, significantly reduce chemical oxygen demand (COD) and color intensity. Finally, advanced treatment: Perform a “finishing process” on the effluent from the main treatment to ensure compliance with discharge standards or facilitate water reuse [17,18]. The treatment of high-concentration dye wastewater poses significant challenges, where photocatalysis, as a green and clean energy technology, is often hindered by the intense coloration of dyes at elevated concentrations [19,20]. This coloration impedes the effective utilization of photogenerated electrons, thereby substantially limiting the efficiency of photocatalytic degradation in such contexts [21]. Mechanical energy, as another abundant and clean energy source in nature, has gained intensive research attention in recent years. In particular, tribocatalysis that converts mechanical energy into chemical energy through friction has emerged as a promising approach for treating wastewater of low transparency [22,23,24,25,26].

Rhodamine B (Rh B) is an important member of a highly significant family of fluorescent dyes with extensive applications in life sciences and industrial sectors [27,28,29]. Notably, Rh B has been substantiated to exhibit certain toxicity and carcinogenic properties. Recent research findings indicate that TiO₂ nanoparticles (P25) achieve photocatalytic Rh B degradation at concentrations around 10~20 mg/L under visible light irradiation [11,23,30,31,32,33]. As for tribocatalysis, Rh B solutions of much higher concentrations have been degraded, such as 40 mg/L Rh B by P25 [23,34], and 50 mg/L RhB by BaTiO₃ [35,36,37] and CdS [38,39] nanoparticles. However, these concentrations are still much lower than that of practical Rh B wastewaters. As a matter of fact, as high as 100 mg/L MO solutions have been successfully degraded by BaTiO₃ nanoparticles in beakers with PTFE coatings through magnetic stirring recently, which suggests that the synergism between catalysts and vessel bottoms is effective for enhancing tribocatalytic degradation of high-concentration organic dyes [40]. In this context, an exploration has been conducted to degrade high-concentration Rh B solutions by TiO₂ nanoparticles through magnetic stirring in beakers with glass bottom, Al₂O₃, Ti, and PTFE coatings, separately. While 100 and 200 mg/L Rh B solutions can be rather thoroughly degraded in all the four kinds of beakers, 300 mg/L Rh B solution can only be degraded efficiently in a PTFE-coated beaker. Moreover, even 400 and 500 mg/L Rh B solutions have also been degraded rather thoroughly in the PTFE-coated beaker. These results represent a significant breakthrough and advancement compared to previous photocatalytic and tribocatalytic degradation of organic dyes, and highlight the vital role of the synergism between catalysts and vessel bottoms in tribocatalysis. And to the best of our knowledge, this is the first time that organic dyes with concentrations as high as 500 mg/L are rather thoroughly degraded via a single catalytic technology using a green energy source, which may bring a reform to current wastewater treatment strategies.

2. Materials and Methods

2.1. Materials and Characterization

TiO₂ nanoparticles used in the experiments were procured commercially from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). XRD measurements were conducted to determine the crystal structure using a Bruker AXS D8 ADVANCE diffractometer (Karlsruhe, Germany) equipped with a Cu Kα source. The microstructure and morphology of the TiO₂ nanoparticles were further examined using an FEI Tecnai G2 F30 high-resolution transmission electron microscope (HRTEM) (Jena, Germany). The specific surface area of the TiO₂ nanoparticles was measured through a high throughput surface area and porosity analyzer (TriStar II 3020, Micromeritics, USA) and calculated with the Brunauer–Emmett–Teller (BET) equation.

2.2. Formation of Coatings on the Bottoms of Glass Beakers

Coating disk-shaped materials on vessel bottoms has emerged as an effective method to enhance tribocatalysis through regulating friction pairs [41]. Commercial flat-bottom glass beakers with a diameter of 45 mm and a height of 60 mm were employed as reaction vessels. The beakers were divided into four groups based on substrate treatment: the first group was left untreated with the original glass bottom; Disks of Ti, Al₂O₃, and PTFE (40 mm in diameter, 1 mm thick) served as modified substrates for the other three groups and were bonded to the beaker bottoms using a strong adhesive. This procedure yielded experimental beakers with four distinct substrate surfaces: glass, titanium, alumina, and PTFE.

2.3. Rh B Degradation Tests

In this study, TiO₂ nanoparticles were employed as the catalytic material for the degradation of Rh B solutions. The experimental procedure was as follows: Rh B solutions with concentrations of 100, 200, 300, 400, and 500 mg/L were prepared, each with a volume of 30 mL, and placed in separate beakers. To each beaker, 0.30 g of TiO₂ nanoparticles was added [23].

A custom-made PTFE magnetic rotary disk, which was described in detail in a previous paper [20], was immersed in a suspension. When a beaker with a magnetic rotary disk in the suspension in it was placed on an ordinary magnetic stirrer, this disk was driven to rotate at 400 rpm in the beaker through the magnetic stirrer. The beaker was kept in dark and room temperature was kept at 25 °C. The groove structure in the disk surface facilitated TiO₂ nanoparticles into the interface between the disk surface and the beaker bottom in the course of magnetic stirring, thereby subjecting the TiO₂ nanoparticles to friction.

At predetermined time intervals, 3 mL aliquots were extracted from the reaction mixture. These samples were centrifuged at 8000 rpm for 5 min to separate the TiO₂ nanoparticles. The resulting supernatant was then analyzed using a UV-Shimadzu 2550 ultraviolet–visible spectrophotometer (Kyoto, Japan). The measurement parameters were set as follows: wavelength range from 200 to 800 nm, wavelength interval of 2.0 nm, and a slit width of 2 nm. The degradation efficiency of organic dyes is typically defined by the following equation:

D = 1 − A/A₀,

(1)

where A and A₀ represent the temporal and initial concentrations, respectively, as determined from the characteristic absorption peak of the dye.

2.4. Detection of Active Species

For the detection of hydroxyl radicals (•OH), beakers (45 mm diameter × 60 mm height) with four different substrate materials (glass, alumina-coated, titanium-coated, and PTFE-coated) were used. Each experimental system comprised 10 mL of deionized water, 0.15 g of TiO₂ nanoparticles, and 50 μL of DMPO (a spin-trapping agent) in individual beakers. Detection of superoxide radicals (•O₂⁻) was carried out using the same four-beaker setup, where the 10 mL deionized water was substituted with methanol, keeping the amounts of DMPO and TiO₂ nanoparticles constant. Under dark conditions at room temperature, each beaker was stirred using a PTFE magnetic rotary disk at 400 rpm for 15 min. Detection of the generated hydroxyl (•OH) and superoxide radicals (•O₂⁻) was performed on a Bruker A300-10/12 model EPR spectrometer (Karlsruhe, Germany).

3. Results and Discussion

An XRD pattern of the TiO₂ nanoparticles used in this tribocatalytic study is presented in Figure 1a. By comparing with the standard diffraction peaks from the TiO₂ XRD reference database (PDF#21-1272), it is clear that the TiO₂ is of anatase phase [21]. The most intense diffraction peak in the sample appears at 25°, corresponding to the (101) crystal plane. Other prominent peaks are observed at approximately 48° and 37°, which are indexed to the (200) and (004) planes, respectively [42,43]. The N₂ adsorption–desorption isotherm of the TiO₂ nanoparticles at 77 K is displayed in Figure 1b. Analysis of the curve’s variation trend reveals that the isotherm conforms to the classical Type IV model, indicating a mesoporous structure of the material [39]. Calculation results demonstrate that the specific surface area is 338 m²/g, the average pore size is 4.36 nm, and the pore volume is 0.33 cm³/g. The large specific surface area is consistent with the broad full width at half maximum observed in XRD patterns (shown in Figure 1a). The TEM micrograph presented in Figure 1c reveals significant agglomerations of the powder, which could not be effectively dispersed even with ultrasonication. Nevertheless, some discrete TiO₂ nanoparticles can be observed in Figure 1d, showing a size below 5 nm.

Figure 2 presents the UV-Vis absorption spectra illustrating the tribocatalytic degradation of 100 mg/L Rh B solutions in beakers with glass bottom, titanium, alumina, and PTFE coatings separately. Nearly 100% degradation efficiency was achieved for the glass substrate after 12 h of magnetic stirring (Figure 2a). A direct and visually apparent color change, demonstrating the degradation process, is presented in the inset. Similarly, complete degradation (close to 100%) was also observed for alumina (Figure 2b) and titanium substrates (Figure 2d), requiring approximately 12 and 8 h of magnetic stirring, respectively. Surprisingly, for the PTFE substrate the RhB solution was almost completely degraded within just 3.5 h of magnetic stirring (Figure 2c). This represents a significant reduction in processing time by 8 h and 6 h compared to alumina and glass substrates, respectively. These results clearly indicate that the PTFE substrate plays a significant modulating role in enhancing the tribocatalytic process of TiO₂ nanoparticles.

Figure 2e graphically compares the degradation efficiencies attained by TiO₂ nanoparticles when deposited on the four different substrate materials. For a more direct performance comparison, a histogram summarizing the degradation efficiencies after 2 h of magnetic stirring is presented in Figure 2f. The measured efficiencies on glass, Teflon, titanium, and alumina substrates are 17%, 95.1%, 28%, and 16.4%, respectively. These results unequivocally demonstrate that the PTFE substrate exerts a significant modulating effect, markedly enhancing the tribocatalytic degradation of Rh B by TiO₂ nanoparticles.

To further validate the efficacy of the PTFE substrate in the tribocatalytic degradation of high-concentration Rh B solutions using TiO₂ nanoparticles, experiments were conducted under identical conditions with an initial concentration of 200 mg/L. As illustrated in Figure 3, with the increased dye concentration, the time required for complete degradation on glass, PTFE, Ti, and Al₂O₃ substrates was 16, 10, 16, and 18 h, respectively. Direct visual evidence of the color change associated with the dye degradation is presented in the corresponding inset images. Photocatalysis faces inherent limitations in Rh B dye degradation due to decreased light penetration efficiency at elevated dye concentrations, with typical optimum degradation concentrations only around 10 mg/L [27,31]. It is surprising that TiO₂, being an Earth-abundant oxide material, demonstrates such an exceptional tribocatalytic performance to degrade 200 mg/L Rh B for all the four substrates. A comparative analysis of the degradation efficiencies is clearly presented in Figure 3e. The histogram in Figure 3f displays the degradation rates achieved after 10 h of magnetic stirring, which were 72.4% for glass, 97% for PTFE, 51.7% for Ti, and 46.0% for Al₂O₃. These results consistently re-affirm the superior and significant enhancing effect of the PTFE substrate on the tribocatalytic process.

For 300 mg/L Rh B solutions, the tribocatalytic degradation became much more difficult for substrates of glass, Al₂O₃, and Ti. In contrast to the higher efficiencies observed in Figure 3, the degradation rates for glass, titanium, and alumina substrates after 16 h of magnetic stirring (Figure 4) were considerably lower, reaching only 40.4%, 35.4%, and 55.1%, respectively. Fortunately, a rather thorough degradation, 96.2%, was observed for PTFE substrate, suggesting that 300 mg/L RhB can still be effectively degraded by TiO₂ with the help of PTFE. It seems that for Rh B solution with a higher concentration, PTFE coating shows a more decisive effect on its tribocatalytic degradation by TiO₂ nanoparticles than glass, Ti, or Al₂O₃. Much attention should be paid to coating material optimization on vessel bottoms in future tribocatalytic degradation of high-concentration organic dyes.

For the beaker with PTFE coating, tribocatalytic degradation of 400 and 500 mg/L Rh B by TiO₂ nanoparticles had been further conducted, and the results are shown in Figure 5a,b. It is highly impressive that 400 and 500 mg/L Rh B were degraded by nearly 100% after 23 and 25 h of magnetic stirring, respectively. Figure 5c presents curves of C/C₀ versus magnetic stirring time for tribocatalytic degradation of 100, 200, 300, 400, and 500 mg/L Rh B solutions by TiO₂ nanoparticles in a PTFE-coated beaker. It is clear that all these solutions can be rather thoroughly degraded, though the magnetic stirring time needed increases with the initial concentration of Rh B solutions. In particular, the 500 mg/L Rh B solution was degraded much more slowly in the first 10 h than the other four solutions, as shown in Figure 5d. Remarkably, it still has been thoroughly degraded in 25 h. It is worthy to note that the power of magnetic stirring under this condition was below 10 W [40], which suggests that such a treatment of Rh B solutions with a concentration as high as 500 mg/L is not only simple but also energy-efficient.

It should be pointed out that a concentration of 500 mg/L actually approximates the levels encountered in practical textile industry effluent of Rh B wastewaters. As shown in Figure 5, Rh B solutions around 500 mg/L are highly opaque, for which the photocatalysis by TiO₂ nanoparticles is obviously impotent. Magnetic stirring alone, in the presence of TiO₂ nanoparticles, effectively degraded these solutions to a colorless state. This directly demonstrates the considerable potential of harnessing mechanical energy for environmental remediation. And as pointed out previously, high-concentration organic dye wastewaters are currently being treated step by step, with different technologies adopted in different steps. For example, in a previous investigation on treatment of 160 mg/L methylene blue (MB) solution [44], such multiple procedures have been suggested: Ag₂O-modified TiO₂-based nanosheets were first immersed in the solution to absorb MB, then they were collected through centrifugal treatment, and finally were placed in clear water to have absorbed MB molecules degraded through UV irradiation. The results obtained in this study should be the first time for organic dyes around 500 mg/L to be rather thoroughly treated in a single step, which should be meaningful for simplifying wastewater treatment strategies in future.

EPR tests have been widely used to reveal radicals formed in tribocatalysis [24]. Some EPR results obtained in this study are presented in Figure 6. The hydroxyl radical EPR signal intensity for the glass substrate was approximately 63.4% of the intensity observed for the PTFE coating, and the peak intensity of superoxide radical for the glass substrate was about 62.4% of that for the PTFE coating. These results clearly demonstrate that the synergistic effect between TiO₂ and PTFE in the course of magnetic stirring promotes the generation of more reactive species such as superoxide and hydroxyl radicals, which accounts for the vital role that PTFE coating plays in the tribocatalytic degradation of high-concentration Rh B solutions.

A mechanism for tribocatalytic degradation of organic dye by TiO₂ nanoparticles has been well established in some previous papers, which can be outlined as follows [20,22]:

$$Ti{O}_{2}NP + Friction \to Ti{O}_{2}NP + h^{+} + e^{-}$$

(2)

$$O{H}^{-}+ h^{+} \to •0H$$

(3)

$$O_2+{\mathrm{e}}^{-} \to •O_2^{-}$$

(4)

$$OH \left(or·O_2^{-}\right)+Dye \to Decomposition$$

(5)

As the tribocatalytic degradation by TiO₂ nanoparticles has been greatly enhanced through PTFE coating, and this enhanced performance is attributed to the particularly effective friction between TiO₂ and PTFE, which promotes the generation of electron–hole pairs in TiO₂. It is well known that PTFE has an extremely strong affinity for electron. Given the enhancement observed for PTFE coating, we would like to revise the processes for tribocatalytic degradation of organic dye by TiO₂ nanoparticles as follows to take the effect of PTFE into consideration when it forms friction with TiO₂ nanoparticles:

$$Ti{O}_{2}NP \stackrel{Friction}{\to } Ti{O}_{2}NP·h^{+}·e^{-}$$

(6)

$$Ti{O}_{2}NP·h^{+}·e^{-}+PTFE \stackrel{Friction}{\to } Ti{O}_{2}NP·h^{+}+PTFE·e^{-}$$

(7)

$$Ti{O}_{2}NP·h^{+}+O{H}^{-}\to Ti{O}_{2}NP+·OH$$

(8)

$$·OH \left(or·O_2^{-}\right)+Dye \to Decomposition$$

(9)

Namely, electrons are first attached to PTFE, in this way the recombination of excited electrons and holes can be greatly suppressed, and subsequent degradation of organic dyes is efficiently enhanced. The underlying tribocatalytic degradation mechanism within the PTFE-coated beaker system is schematically presented in Figure 7, which illustrates the interaction between catalysts and coatings on vessel bottoms in tribocatalysis.

4. Conclusions

Tribocatalytic experiments employed a series of concentrated Rh B solutions and TiO₂ nanoparticles in beakers with glass, Ti, Al₂O₃, and PTFE bottoms to assess substrate-dependent performance. While 100 and 200 mg/L Rh B solutions can be rather thoroughly degraded in beakers featuring the four distinct substrates/coatings, 300 mg/L Rh B solution can only be effectively degraded in a PTFE-coated beaker. Moreover, 400 and 500 mg/L Rh B solutions can also be degraded by nearly 100% in the PTFE-coated beaker after 23 and 25 h of magnetic stirring, respectively. EPR study revealed that much more hydroxyl and superoxide radicals were generated by magnetic stirring-stimulated TiO₂ in a PTFE-coated beaker than in a glass-bottomed beaker. This work demonstrates that the synergistic effect between materials in friction pairs is vital for enhancing tribocatalysis. This study reports, for the first time, the complete degradation of Rh B solutions with concentrations as high as 500 mg/L via a one-step tribocatalytic process, which should be meaningful for simplifying the complex and multi-step integrated treatment steps in current high-concentration dye wastewater treatment.