Porous-suspended graphite carbon doped TiO₂ improves the band structure of photocatalysts to enhance photocatalytic degradation of tetracycline in water

Tetracycline (TC) antibiotics, recognized as one of the four new types of pollutants by the Chinese government, have garnered significant attention and research (Sauvé & Desrosiers 2014; Morin-Crini et al. 2022; Zhang et al. 2023b). Studies indicate that antibiotics are extensively used and administered in large quantities for the treatment of infectious diseases (Zhang et al. 2023a). However, following their use, 5–90% of antibiotics are excreted from the human or animal body in their parent form or as metabolites through urine or feces (Zeng et al. 2023). Due to the frequent occurrence of influenza in recent years, the misuse of antibiotics has become normalized (Baran et al. 2023; Gu et al. 2023). Some researchers have collected water samples from various locations, detecting the presence of TC. Han et al. (2020) detected antibiotics in various substrates (seawater, sediment/biofilm, organisms, and feed) in different culture modes (greenhouse and outdoor culture) during the rainy and dry seasons in the coastal area of Shandong Province. A study by Liu et al. (2019) investigated the extent of contamination by a variety of antibiotics and antibiotic resistance genes (ARGs) in hospital wastewater, groundwater, and the Wenyu River, and 11 antibiotic concentrations of the detected ranged from undetectable to 16,800 ng/L in various water samples. TC antibiotics have been detected at the highest concentrations, especially in the hospital water samples. The health effects and ecological hazards associated with TC residues in water have been substantiated, and further research is ongoing (National Toxicology 1989). The stable fused ring structure of TC contributes to its environmental stability, rendering it difficult for microorganisms to degrade. Moreover, it is not easy to be removed from wastewater by common water treatment processes, such as the common activated sludge method for wastewater treatment, which is less effective in treating TC wastewater (Li et al. 2022a). Consequently, research into novel treatment technologies for wastewater containing low concentrations of antibiotics has emerged as a new area of interest.

Researchers have explored a variety of methods for removing antibiotics from the aquatic environment, which can be broadly categorized into physical, chemical, and biological approaches (Oberoi et al. 2019; Zhu et al. 2021; Li et al. 2022b). A comprehensive comparison of these research methods revealed that photocatalytic oxidation is an effective and environmentally friendly technology for the removal of antibiotics from water. Furthermore, it represents a superior green and sustainable water treatment solution. It is also a better green and sustainable water treatment technology. Tayyab, Muhammad et al. found that sulfur group compounds are more effective for photocatalytic hydrogen production. In parallel, MoS 2-tipped CdS nanowires photocatalysts were efficiently synthesized by a two-step hydrothermal method using a soft template for the efficient production of H₂ by selective oxidation of benzyl alcohol under visible light irradiation (Muhammad Tayyab et al. 2023; Tayyab et al. 2023, 2024). Photocatalysts have better results in catalytic H₂ production. Meanwhile, the degradation of antibiotics using load-modified nanophotocatalysts has also achieved better results in a series of studies. In addition to this, the application of photocatalysts to optical fibers is extremely promising and of high research value. Ling et al. (2017) explored the aqueous-phase light-emitting diode (LED) irradiation of optical fibers for the treatment of organic pollutants and proposed for the first time a dual-mechanism approach for light transmission and photocatalytic performance. Wang et al. (2024) developed a low-cost, physically flexible catalytic polymer optical fiber structure called photoelectrode fiber to remove organic pollutants in water up to 90% or more. Utilizing the strength of ultraviolet light (UV-C) in combination with photocatalysts for the treatment of pollutants is also an effective method. Zhao et al. showed that emitting low-flux UV-C light into Surface Electron-Transfer Oxygen Functionalization (SEOF) on membrane surfaces is a promising nonchemical method for attenuating biofouling formation on reverse osmosis membranes while inhibiting biofilm buildup on surfaces in small-diameter piping or other complex geometries (Rho et al. 2022; Zhao et al. 2023). Zhao et al. (2021) used silica nanoparticle coatings on quartz optical fibers to promote the lateral emission of germicidal UV-C, which is expected to disinfect contaminated air, water, and surfaces. Li et al. (2019) developed nanoparticles-decorated TiO₂ nanorods embedded in a cellulose acetate (Au_0.1Ag_0.9/TiO₂/CA) membrane for enhanced photocatalytic degradation of TC and killing pathogenic bacteria. The TC degradation rate of the Au_0.1Ag_0.9/TiO₂/CA membrane under visible light irradiation for 120 min could reach about 90%. Meanwhile, nanostructured C–TiO₂-x microspheres were prepared by a facile modified solvothermal method and used as an efficient visible light photocatalyst for photocatalytic elimination of antibiotics (Nie et al. 2020). The visible light photocatalytic activity of C–TiO₂-x-180 microspheres is about 3.14 times higher than that of commercial TiO₂. However, there is still a research gap for improvement in the preparation process and recycling of these loaded nanocatalysts.

To address the issues arising from the aforementioned materials, some researchers have developed a floatable photocatalyst material tailored to the specific conditions of the water environment. This innovation enhances both the utilization of visible light and photocatalytic efficiency. Floating photocatalysts, constructed from melamine sponge and polyurethane sponge, were synthesized using the sodium alginate immobilization method by Wang et al. (2023). The degradation rates of rhodamine B, crystal violet, and malachite green had reached 94.02, 92.1, and 97.13%, respectively. Xu et al. (2022) prepared different floating photocatalytic spheres by loading TiO₂-based photocatalysts to degrade TC in simulated seawater. Foamed melamine photocatalyst had the largest specific surface area of 28.47 m²/g and the best algal inactivation rate of 98.68% within 6 h. These floating photocatalysts have a better degradation effect but the preparation process is more complicated and still needs further improvement. However, the above studies show that it is an effective idea to develop new floating photocatalysts with low material prices, simple preparation steps, and a high recycling rate.

Our research team has discovered that graphitic carbon enhances the efficiency of titanium dioxide photocatalysis, which has the potential to streamline the preparation of floatable photocatalysts. Through examination of TC antibiotic degradation, we have developed a one-pot method for creating a floating TiO₂ photocatalyst. By utilizing our earlier research on graphitic carbon materials with high adsorption and photocatalytic efficiency, we propose a novel floating TiO₂ graphite carbon (FTDGC) sphere that uses polyvinyl alcohol as a binder for TC removal (Zhang et al. 2022). We have optimized the conditions for FTDGCs based on adsorption, strength, and recyclability, with the aim of introducing a new catalyst for TC degradation in surface water.

Peanut shells, fly ash floating beads, and electrode graphite were purchased from Zhengzhou's local market. Chemical reagents such as polyvinyl alcohol, sodium hydroxide, and butyl titanate were purchased from Komeo Chemical Reagent Co. Phosphoric acid and hydrochloric acid were purchased from Luoyang Chemical Reagent Factory. All reagents were analytically pure, and the experimental water was deionized water.

Peanut shells and spent electrode graphite were cleaned with deionized water and 10% nitric acid and then dried in an oven at 110°C.

Preparation process: 0.17 g of peanut shell powder, 0.5 g of electrode graphite powder, and 2.5 g of bleaching beads were mixed well and added to 200 mL of water, to which 20 mL of butyl titanate was slowly added dropwise, stirred on the magnetic stirrer for 12 h and then filtered, and the mixed powders were dried at 80°C. A certain amount of powder was added, different doses of phosphoric acid and 10% polyvinyl alcohol solution was added, mixed and stirred evenly, and molded into shape. The pellets were formed into small balls with a diameter of about 5 mm and dried in an oven at 110°C for 16 h. After cooling, the ball was pyrolysised at 500°C in a tube furnace under a nitrogen atmosphere for 2 h (heating rate of 10°C/min and N₂ flow rate of 45 mL/min). According to the different proportions of added raw materials, the samples were labeled as FTDGC-1, FTDGC-2, FTDGC-3, FTDGC-4, and FTDGC-5, respectively.

For photocatalytic experiments at room temperature, 0.45 g pellets were put into 100 mL of TC solution (30 mg/L), and after a dark reaction for 1 h to reach adsorption equilibrium, photodegradation experiments were carried out under visible light irradiation generated by a 300 W xenon lamp (wavelength range of 200–800 nm). When the degradation time was 0, 30, 60, 90, 120, 150, and 180 min, 5 mL of sample was taken, the sample solution was passed through a 0.45 μm membrane and the concentration of TC hydrochloride solution was detected by UV spectrophotometer. The initial pH was adjusted to 3.0, 5.0, 7.0, and 9.0 to study the pH changes during TC degradation by FTDGC.

The performance of the catalyst was evaluated through five successive cycles. Following the photocatalytic reaction, the supernatant was allowed to settle and was subsequently removed to obtain TiO₂–GC, which was subsequently washed ultrasonically with distilled water for 4 h, dried at 80°C, and then employed in the reusability experiments that were conducted over five cycles, employing the same procedures as specified in Section 2.4.

The specific surface area and porosity of graphite carbon also affect the adsorption and photodegradation of TC, and graphite carbon with adsorption ability can accelerate the migration of pollutants to the surface of composite materials, especially the graphite carbon with larger porosity, which is more conducive to the utilization of pollutant migration. In addition, the C/Ti ratio of the photocatalytic material also affects the photocatalytic effect, the higher the Ti content in the material, the better the photocatalytic effect, and the higher the C/Ti ratio after pyrolysis at high temperature, the more Ti will be exposed, and the better the photocatalytic effect will be.

When graphitic carbon is combined with TiO₂ and different pore sizes, the pore size and structure of the graphitic carbon can impact the performance of the composite photocatalyst. The results of the photodegradation tests on FTDGC-1, FTDGC-4, and FTDGC-5 demonstrate that the addition of graphitic carbon can effectively enhance the adsorption capacity, thanks to an increase in the content of highly conductive graphite, which functions as a photogenerated electron-absorbing cell. This facilitates the transport of electrons to the surface of the charcoal-based material, preventing direct electron–hole pair complexation and improving the photovoltaic performance of the material. As a result, the catalytic efficiency of the material is increased. However, the photocatalytic activity of the composites decreases rapidly with further increases in carbon content, emphasizing the importance of optimizing the carbon content for optimal photocatalytic activity.

Seventeen sets of experiments were conducted according to the experimental design of Design Expert 13 software to explore the effects of photocatalyst dosage (A: 3.0, 4.0, and 5.0 g/L), initial TC concentration (B: 5, 10, and 15 mg/L), and solution pH (C: 3, 6, and 9) on the efficiency of TC removal from water by FTDGC. Table 1 shows the results of analysis of variance (ANOVA). The ‘Prob > F’ values of A, B, and C were 0.0004, 0.0007, and 0.0011, respectively, which were less than 0.0500, indicating that A, B, and C had a significant effect on the TC degradation efficiency, and the effect of A was the most significant. The ‘Prob > F’ value for AB was 0.0052 compared to AC and BC, indicating that the interaction between the dosage and the initial concentration had a significant effect on the TC degradation efficiency. The ‘Prob > F’ value of <0.0001 (<0.0500) in this model indicates that the model has a significant effect on the response values under the selected conditions.

The ANOVA in Figure S1 shows that the model has a high correlation coefficient (R² = 0.9802) and indicates a good correlation between the actual and predicted values.

Figure S2 shows the effect of the interaction between the three variables on TC degradation efficiency, and the interaction between the two independent variables had different effects on TC removal. AB (dosage and initial concentration of TC) was 0.0052, indicating that the interaction between dosage and initial concentration of TC had a significant effect on TC removal. AC (dosage and pH) and BC (initial concentration of TC and pH) were 0.125 and 0.7667, respectively, indicating that the effects of AC and BC on TC degradation were not significant. The increase in the initial concentration of the solution caused more TC molecules to be adsorbed by the FTDGC, and the probability of the molecules colliding with the active groups on the surface of the FTDGC increased, so the photodegradation efficiency increased. However, at higher concentrations, the TC molecules may restrict the transmission of light and interfere with the interaction between TC molecules and FTDGC, resulting in a lower rate of desorption of the degradation products from the surface of the material, and there is adsorption competition with TC molecules, so the photodegradation efficiency decreases. Different pH values of the solution affect the surface charge of FTDGC and the morphology of TC molecules, which are positively charged when the pH is acidic and negatively charged when alkaline. Therefore, under acidic and alkaline conditions, FTDGC and pollutants have the same charge and repel each other, which is unfavorable for the contact between catalyst and target pollutants. The model predicted the optimal degradation conditions as follows: the dosage of 4.3 g/L, the initial concentration of 6.65 mg/L, and the pH of 6.95. Under these conditions, the removal efficiency of TC under xenon lamp irradiation for 3 h reached 90.08%.

The degradation of TOC by both is shown in Figure 8(b). The removal rate of TOC by commercial TiO₂ is only about 40%, while the removal rate of TOC by FTDGC-1 reaches 73%, which is a clear advantage compared with commercial TiO₂.

We can see from Table 2 that the degradation of TC with other newly prepared photocatalysts, the reusability of FTDGC-1 was investigated by reusing the photocatalyst sample in five consecutive photocatalytic processes. The removal rates of TC were 90.26, 86.25, 81.67, 76.60, and 72.45% for five consecutive photocatalytic processes, as shown in Figure S3. The degradation efficiency of the material on TC solution showed a slow decreasing trend, but still had a good removal effect after five times of use, which indicated that the material has the advantages of high efficiency, reusability, and good stability, and has potential practical applications in wastewater treatment.

By comparing the degradation of TC with other newly prepared photocatalysts, we found that all of them can degrade about 90% of TC within 3 h, but there are very few molded and floating photocatalysts, which shows our advantage, and our application scenario is clear and suitable for industrial production.

The calculation of electrical energy per order reacts to the reduction of the concentration of organic pollutants by one logarithmic order of the electric power consumption in 1 m³ of wastewater during the water treatment process, and its value can evaluate and compare the efficiency of the energy utilization of different photocatalysts, so as to assess their performances and advantages and disadvantages. Furthermore, it can help researchers understand the input and output of energy in the photocatalytic reaction so as to optimize the reaction conditions and improve the reaction efficiency.

After calculating the answer is E_EO = 389.61. The magnitude of this value also reflects the concept of saving electricity for economic green development in photocatalytic degradation. We will pay more attention to the improvement of light utilization in the future.

In this paper, the molding of floating TiO₂ graphitic charcoal and its degradation performance on TC were investigated. Floating titanium dioxide graphitic charcoal spheres had good photodegradation performance on TC in water. Structural and morphological characterization confirmed that the crystal structure on FTDGC is a pure anatase phase, with a uniform layer of TiO₂ nanoparticles loaded on a carbon-based surface. We found experimentally that the catalytic effect of different C/Ti catalysts, the greater the influence of the C/Ti ratio of the material on the photocatalytic effect, and the higher the Ti content, the better the photocatalytic effect. This may be that the combination of GC and TiO₂ makes the effect better. The FTIR and XPS peaks clearly indicate the generation of the Ti–C–Ti valence bond and the further enhancement of the Ti–C–O bond generated on the carbon-based material and TiO₂ surface. The removal efficiency of TC reached 90.08% by irradiation with xenon lamp for 3 h at the dosage of 4.5 g/L in the reaction system. The degradation process has a self-regulating effect on the pH value of the wastewater within a certain range, which greatly avoids further acid-base adjustment of the wastewater. The recycling experiments showed that FTDGC still had a good degradation ability for TC after five times of reuse. Our advantage over other literature is that the application scenarios are clearer and the green economy is environmentally friendly and recyclable. The floatable nature of FTDGC can be utilized in open-air oxidation ditches to efficiently remove pollutants from wastewater by combining hydrodynamic drive with sunlight. Our next steps are centered on developing more floatable photocatalysts and developing different materials for transporting more solid wastes. In conclusion, the successful preparation of FTDGC composite photocatalysts provides a new route for efficient TC removal. It also follows the principle of green chemistry, provides low-cost for large-scale production, and is in line with China's dual-carbon policy.

This project is supported by a special fund of Henan Key Laboratory of Water Pollution Control and Rehabilitation Technology (CJSZ2024009) and funded by the Henan Provincial Joint Fund (232103810070).

Z.W. developed the software, rendered support in formal analysis, validated and investigated the whole process, wrote the original draft. H.C. rendered support in funding acquisition, supervised the work, wrote the review and edited the article, and developed the project administration. K.Z. developed the software, rendered support in data curation. B.Z. and K.Z. supervised the work and developed the project administration. H.Z. and W.H. rendered support in data curation. All authors have read and agreed to the published version of the manuscript.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.