Abstract
In this study, we report a facile hydrothermal synthesis of strontium-doped SnS nanoflowers that were used as a catalyst for the degradation of antibiotic molecules in water. The prepared sample was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and ultraviolet–visible absorption spectroscopy (UV–Vis). The photocatalytic ability of the strontium-doped SnS nanoflowers was evaluated by studying the degradation of metronidazole in an aqueous solution under photocatalytic conditions. The degradation study was conducted for a reaction period of 300 min at neutral pH, and it was found that the degradation of metronidazole reached 91%, indicating the excellent photocatalytic performance of the catalyst. The influence of experimental parameters such as catalyst dosage, initial metronidazole concentration, initial reaction pH, and light source nature was optimized with respect to metronidazole degradation over time. The reusability of the strontium-doped SnS nanoflowers catalyst was investigated, and its photocatalytic efficiency remained unchanged even after four cycles of use.
HIGHLIGHTS
Sr-doped SnS is synthesized by the hydrothermal process.
Sr-doped SnS nanoflowers were selected to remove dyes from water.
The removal percentage of Sr-doped SnS could reach 91%.
The reusability of our catalyst Sr-doped SnS was also explored.
INTRODUCTION
Water pollution is a serious environmental problem that occurs when harmful substances, such as chemicals, waste materials, and sewage, contaminate bodies of water like oceans, rivers, and lakes. Human activities such as industrialization, agricultural practices, and improper waste disposal are major contributors to water pollution (Bashir et al. 2020; Qadri et al. 2020; Akhtar et al. 2021; Karri et al. 2021; Sarker et al. 2021). The consequences of water pollution can be devastating for both aquatic life and humans who rely on these water sources for drinking, irrigation, and recreational purposes. It can lead to the depletion of fish populations, the spread of waterborne diseases, and the contamination of food sources. Addressing water pollution requires a concerted effort from individuals, governments, and industries to adopt sustainable practices and reduce harmful pollutants in our water sources (Weldeslassie et al. 2018; Chowdhary et al. 2020; Kiliç 2021; Morin-Crini et al. 2022).
Water pollution caused by antibiotics, including metronidazole (MNZ), is a growing concern worldwide. When antibiotics are used in human or animal medicine, a portion of the drug is excreted from the body and ends up in wastewater. Treatment plants are not designed to remove antibiotics from wastewater, so they are discharged into rivers, lakes, and oceans. This discharge of antibiotics into water bodies can lead to the development of antibiotic-resistant bacteria, which can pose a serious threat to public health (Polianciuc et al. 2020; Baaloudj et al. 2021; Jovanovic et al. 2021; Tian et al. 2021). MNZ, in particular, is an antibiotic used to treat infections such as bacterial vaginosis and periodontitis. It has been found to persist in the environment for long periods and can be toxic to aquatic organisms, even at low concentrations. The accumulation of MNZ in the environment can have detrimental effects on aquatic ecosystems and pose a risk to human health (Bashiri et al. 2020; Ighalo et al. 2020; Wang et al. 2022a; Aoudjit et al. 2023).
Eliminating organic pollutants from water is a complex process that requires the use of various techniques. One of the most effective methods is activated carbon filtration, which involves passing water through a bed of activated carbon to remove organic contaminants (Castiglioni et al. 2022; Zioui et al. 2022; Huang et al. 2023; Yang et al. 2023). Another technique is reverse osmosis, which uses a semi-permeable membrane to filter out pollutants (Hu et al. 2023; Liao et al. 2023; Pezeshki et al. 2023; Shen et al. 2023). Biological treatment, which uses bacteria or other microorganisms to breakdown organic compounds, is also an effective method for eliminating pollutants (Yang et al. 2022; Rokkarukala et al. 2023; Tian et al. 2023). Membrane process represent an efficient, straightforward, and low-cost alternative as a pretreatment step for continuous treatment processes for simultaneous organic and inorganic contaminants' remediation in real industrial effluent sources as reported by Zioui et al. (2023). Additionally, advanced oxidation processes (AOPs) such as UV oxidation or ozonation can be used to breakdown organic contaminants into less harmful substances. Overall, a combination of these techniques may be necessary to completely eliminate organic pollutants from water (Liu et al. 2022; Li et al. 2023; Saoud et al. 2023; Wardighi et al. 2023). Photocatalysis is a powerful technique used to eliminate organic pollutants from water and air. It involves the use of a catalyst that can convert light energy into chemical energy to breakdown organic molecules into harmless substances such as water and carbon dioxide. This technique is highly effective and environmentally friendly as it does not produce any toxic byproducts or require any additional chemicals. Photocatalysis is becoming an increasingly popular technique of choice for eliminating organic pollutants due to its high efficiency, low cost, and versatility in treating a wide range of contaminants (Sun et al. 2023; Yin et al. 2023; Yu et al. 2023; Wang et al. 2023a, 2023b). The decontamination of wastewater (degradation of oil in wastewater) using AOP photocatalysts and solar wastewater treatment (SOWAT) has been reported as possible techniques for wastewater purification using solar radiation for reuse in agriculture and industry (Igoud et al. 2019, 2021, 2022; Zioui et al. 2019; Martins et al. 2021).
Semiconductor photocatalysts, particularly those based on sulfur, have gained significant attention in recent years due to their unique properties and potential applications in various fields. Sulfur-based photocatalysts have a narrow bandgap, allowing them to absorb light in the visible range and generate charge carriers that can facilitate chemical reactions. This property makes them particularly useful in photocatalytic applications, such as environmental remediation, hydrogen production, and organic synthesis. Additionally, sulfur-based photocatalysts are cost-effective and environmentally friendly, making them a promising alternative to traditional photocatalytic materials. Ongoing research in this area aims to optimize the performance of sulfur-based photocatalysts and explore new applications for these versatile materials (Lincho et al. 2023; Liu et al. 2023; Saoud et al. 2023; Zhang et al. 2023).
SnS (tin sulfide) is a semiconductor material with a tunable bandgap energy that makes it suitable for various optoelectronic and photovoltaic applications, including photocatalysis. The bandgap energy of SnS ranges from 0.9 to 1.3 eV, which corresponds to the visible to near-infrared light spectrum, making it an efficient absorber of solar radiation. In photocatalysis, SnS-based materials have been used for the degradation of organic pollutants, water splitting, and CO2 reduction. The unique electronic and optical properties of SnS, such as its high absorption coefficient, long carrier lifetime, and high quantum efficiency, make it a promising candidate for photocatalytic applications. Additionally, the abundance, low toxicity, and earth-abundant constituent elements of SnS make it a sustainable and environmentally friendly alternative to other photocatalytic materials. Further research is needed to optimize the photocatalytic performance of SnS and to develop efficient and stable SnS-based photocatalysts for practical applications (Nengzi et al. 2020; Hegde et al. 2021; Zhang et al. 2021; Alikarami et al. 2022; He et al. 2023; Katoch et al. 2023).
The process of doping photocatalysts involves adding impurities to improve their photocatalytic properties by altering their electronic structure and enhancing their ability to absorb light and generate charge carriers. Metals like silver, gold, and platinum and nonmetals like nitrogen and sulfur are common dopants used. Doping with strontium (Sr) has shown to significantly enhance photocatalytic activity by improving visible light absorption and charge carrier separation, leading to applications in water splitting, pollutant degradation, and hydrogen production. Sr doping also improves stability and durability, making it a promising strategy for developing high-performance photocatalysts (Tran et al. 2020; Iqbal et al. 2021; Yarahmadi et al. 2021; Uma et al. 2022; Sharma et al. 2023).
In this work, we have successfully synthesized nanoflowers of SnS and Sr-doped SnS, which demonstrate excellent performance for the degradation of MNZ. The unique morphology of the nanoflowers provides a large surface area for the active sites to interact with the target pollutant, while the Sr doping enhances the catalytic activity and stability of the material. Our results suggest that the synthesized nanoflowers have great potential for the treatment of MNZ -contaminated wastewater, and may also inspire the development of new nanomaterials for environmental remediation.
EXPERIMENTAL
Catalyst preparation
In this study, a hydrothermal technique was employed to synthesize SnS and Sr-SnS NPs. Initially, SnS NPs were prepared by dissolving 2.25 g of SnCl2 2H2O in 20 mL of distilled water (625 mM) and 4.5 g of SC(NH2)2 in 40 mL of distilled water (1.5M) separately. Each solution was stirred for 30 min. Next, the SnCl2 2H2O solution was added to the SC (NH2)2 solutions, and the resulting mixture was stirred for a duration of 2 h, while introducing 20 mL of distilled water. The mixed solution is then transferred to a 100-mL Teflon-lined stainless steel autoclave and maintained at 180 °C for 12 h. The Sn0.9Sr0.1S samples were synthesized using 62.5 mM of SrCl2 6H2O. After the reaction was completed, the autoclave was allowed to cool down to room temperature. To eliminate any remaining impurities, the resulting powder underwent multiple washes with ethanol and distilled water. Finally, the powder was air-dried for duration of 2 h at 50 °C.
Catalyst characterization
The X-ray diffraction (XRD) patterns and scanning electron microscope (SEM) micrographs of the samples prepared as mentioned above were recorded. The XRD patterns were recorded on a Siemens D-5000 diffractometer with Cu-Kα radiation (λ = 1.5418 Å) and surface morphology was studied using SEM (Thermo Fisher Scientific). The UV–Vis absorption spectra were measured by UV–Vis spectrophotometer (Shimadzu UV-2401).
Photocatalytic removal studies
RESULTS AND DISCUSSION
Characterization of the catalyst
XRD analysis
Structural parameters of SnS and Sr-SnS nanoflowers
Samples . | Diffraction angles . | Crystallite size (nm) . | Microstrain 10−3 . | Dislocation density 10−3 (nm−2) . | Lattice parameters (Å) . | ||
---|---|---|---|---|---|---|---|
a . | b . | c . | |||||
SnS | 31.54 | 44 | 0.2151 | 0.5593 | 11.20 | 3.99 | 4.31 |
Sr-SnS | 31.57 | 40 | 0.2309 | 0.6594 | 11.19 | 3.99 | 4.30 |
Samples . | Diffraction angles . | Crystallite size (nm) . | Microstrain 10−3 . | Dislocation density 10−3 (nm−2) . | Lattice parameters (Å) . | ||
---|---|---|---|---|---|---|---|
a . | b . | c . | |||||
SnS | 31.54 | 44 | 0.2151 | 0.5593 | 11.20 | 3.99 | 4.31 |
Sr-SnS | 31.57 | 40 | 0.2309 | 0.6594 | 11.19 | 3.99 | 4.30 |
Optical properties and band gap estimation
UV–Visible spectra for SnS and doped Sr-SnS nanoflowers. The inset shows a shift toward longer wavelengths.
UV–Visible spectra for SnS and doped Sr-SnS nanoflowers. The inset shows a shift toward longer wavelengths.
The doping also led to a change in the gap energy of SnS NPs. Gap energy is an essential characteristic that directly influences the electronic and optical properties of semiconductor materials. Commonly, the direct bandgap energy (Eg) is estimated via Tauc's relation (F(R)hν)2=A (hν−Eg), which hν and A are incident photon energy and ‘A’ constant, respectively (Arefi-Rad & Kafashan 2020). The interception of the linear fit of (F(R) hν)2 plot with hν axis shows the Eg (Salima et al. 2023).
Estimation of band gap for SnS and doped Sr-SnS nanoflowers using the Kubelka–Munk approach.
Estimation of band gap for SnS and doped Sr-SnS nanoflowers using the Kubelka–Munk approach.
SEM analysis
Photodegradation of MNZ
Evaluation of the treatment strategy for MNZ degradation
Degradation of MNZ under different processes (CMNZ = 20 mg/L, Ccatalyst = 1 g/L and free pH = 6, 4).
Degradation of MNZ under different processes (CMNZ = 20 mg/L, Ccatalyst = 1 g/L and free pH = 6, 4).
The results of the experimental tests clearly indicate that photocatalysis has a substantial impact on the photodegradation of MNZ, while the effects of adsorption and photolysis are relatively insignificant. This finding is consistent with prior research in the field, which has already demonstrated the effectiveness of photocatalytic methods in removing pollutants from water. The use of a catalyst in combination with sunlight leads to a significant increase in the degradation rate of MNZ, highlighting the potential of photocatalysis as an efficient and sustainable method for water treatment. These results provide valuable insights into future research in the field of environmental engineering and water treatment (Chekir et al. 2017; Aoudjit et al. 2018; Bouarroudj et al. 2021).
Comparison of SnS and Sr-doped SnS photocatalytic activities under sunlight irradiation (CMNZ = 20 mg/L, Ccatalyst = 1 g/L and free pH).
Comparison of SnS and Sr-doped SnS photocatalytic activities under sunlight irradiation (CMNZ = 20 mg/L, Ccatalyst = 1 g/L and free pH).
Radiation source
Comparison of degradation of MNZ exposed to different sources of light (CMNZ = 20 mg/L, Ccatalyst = 1 g/L and free pH = 6, 4).
Comparison of degradation of MNZ exposed to different sources of light (CMNZ = 20 mg/L, Ccatalyst = 1 g/L and free pH = 6, 4).
The photocatalyst's ability to breakdown MNZ can be explained by its reaction with light. When the photocatalyst is exposed to light, electrons from the valence band move to the conduction band and holes are created in the valence band. This process generates photons that can reduce oxygen to create superoxide radicals (O2–). In addition, the holes in the valence band can oxidize water molecules or (OH–) ions to create hydroxyl radicals (OH•). These superoxide and hydroxyl radicals are involved in the breakdown of MNZ drugs. The photogenerated electrons that move to the conduction band are typically unstable and quickly return to the valence band, as has been demonstrated previously (Bouarroudj et al. 2021, 2023).
Effect of pH
Sunlight photodegradation of MNZ at different pH values (CMNZ = 20 mg/L and Ccatalyst = 1 g/L).
Sunlight photodegradation of MNZ at different pH values (CMNZ = 20 mg/L and Ccatalyst = 1 g/L).
Effect of concentration
Photodegradation of MNZ with time at, different concentrations of MNZ, under sunlight irradiation (Ccatalyst = 1 g/L and free pH = 6, 4).
Photodegradation of MNZ with time at, different concentrations of MNZ, under sunlight irradiation (Ccatalyst = 1 g/L and free pH = 6, 4).
Effect of catalyst dose
Photodegradation of MNZ at different doses of Sr-doped SnS catalyst, under sunlight irradiation (CMNZ = 20 mg/L and free pH = 6.4).
Photodegradation of MNZ at different doses of Sr-doped SnS catalyst, under sunlight irradiation (CMNZ = 20 mg/L and free pH = 6.4).
The experimental results clearly demonstrate a positive correlation between the quantity of catalyst used and the rate of degradation of MNZ. The rate of degradation increased in proportion to the quantity of the catalyst used, with a yield of 91% after 300 min when a 1 g/L catalyst dose was utilized. This suggests that the yield of the photodegradation process is directly proportional to the amount of catalyst used. One possible explanation for this phenomenon is that the increase in active sites available for the production of OH− free radicals is responsible for the degradation of MNZ. Thus, using higher quantities of catalyst could result in more efficient and effective degradation of MNZ in wastewater treatment. However, excessive formation of free radicals can occur if the dose of photocatalyst is too high. This excessive formation of free radicals can combine and react with other species present in the solution, thereby reducing their effectiveness in degrading MNZ (Kaur & Singhal 2014; Farzadkia et al. 2015; Kumar & Kumar 2019; Bouarroudj et al. 2023).
Reuse of photocatalyst
Photodegradation of MNZ over four catalytic cycles in sunlight, (20 mg/L, free pH = 6.4, and Ccatalyst = 1 g/L).
Photodegradation of MNZ over four catalytic cycles in sunlight, (20 mg/L, free pH = 6.4, and Ccatalyst = 1 g/L).
It is possible that the efficiency of photocatalytic degradation decreases at 82% after several cycles of use, particularly after the fourth cycle of reuse. This decrease can be caused by several factors, such as the accumulation of contaminants on the surface of the catalyst, the physical or chemical deterioration of the catalyst, and the loss of active surface area of the catalyst. Organic contaminants can accumulate on the surface of the catalyst, reducing its efficiency, despite cleaning between use cycles. Additionally, the catalyst can undergo physical or chemical degradation over time, which can also reduce its efficiency. Finally, the active surface area of the catalyst can decrease due to the formation of passive layers or a decrease in specific surface area, which can also reduce the efficiency of degradation (Chakrabarti & Dutta 2004; Yu et al. 2016; Anjum et al. 2017; Zhao et al. 2018).
CONCLUSION
In conclusion, the synthesis of strontium-doped SnS nanoflowers using the solvothermal method has been successfully achieved. The structural analysis confirmed the successful substitution of Sr2+ ions into the SnS host lattice, resulting in a decrease in lattice parameter and a slight shift in diffraction peaks. Furthermore, the UV–Vis investigations revealed an increased band gap of Sr-doped SnS, with an estimated value of 1.48 eV, indicating improved photocatalytic properties.
The photocatalytic abilities of Sr-doped SnS were assessed through the degradation of MNZ under natural sunlight. The results demonstrated that the optimal conditions for MNZ degradation were a free pH of 6.4, a dye concentration of 20 mg/L, and a catalyst dose of 1 g/L. Remarkably, the Sr-doped SnS catalyst exhibited an impressive degradation efficiency, with 91% degradation of MNZ achieved under natural sunlight irradiation. Importantly, the catalyst maintained its photodegradation capacity even after multiple treatment cycles, highlighting its stability and reusability.
These findings contribute to the growing body of knowledge on the synthesis and characterization of doped semiconductor nanomaterials for environmental applications. The successful synthesis of strontium-doped SnS nanoflowers with enhanced photocatalytic properties opens up possibilities for their use in various fields, including wastewater treatment and environmental remediation. Further research and optimization of the synthesis process can lead to the development of even more efficient and sustainable photocatalytic materials for addressing environmental challenges.
ACKNOWLEDGEMENTS
The authors of this manuscript would like to thank the General Directorate for Scientific Research and Technological Development (DGRSDT) and the Ministry of Higher Education and Scientific Research (MESRS), Algeria, for funding this project.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.