Introduction

Access to clean and safe water is essential for public health and sustainable development. Rapid industrialization and population growth have introduced numerous synthetic compounds into the environment, many of which enter aquatic systems through wastewater discharge. Major contributors to water contamination include agricultural activities and industrial sectors such as oil and petrochemical, pharmaceutical, pulp and paper, and food processing industries1,2,3,4.

The global consumption of antibiotics has risen markedly due to their essential role in preventing and treating microbial infections. Because these compounds are poorly absorbed in humans and animals, a substantial portion is excreted and released into the environment, where they persist in aquatic systems and disrupt ecological balance. Tetracycline (TC), one of the most commonly used broad-spectrum antibiotics in both human and veterinary medicine, interferes with photosynthesis in aquatic plants and bioaccumulates in aquatic organisms, facilitating its transfer through the food chain. Even at low concentrations, TC in water can promote the development of antibiotic-resistant bacteria. In humans, TC exposure has been linked to gastrointestinal disturbances, reduced appetite, nausea, vomiting, diarrhea, tooth discoloration in children, and adverse effects on fetal bone development4,5,6,7,8,9.

Conventional methods for tetracycline removal from wastewater include precipitation, activated sludge, adsorption, biological treatment, membrane filtration, and advanced oxidation processes (AOPs) such as Fenton, photocatalysis, ozonation, and UV/H2O210,11. Although membrane filtration can achieve complete removal, it is costly and susceptible to membrane fouling (Table 1). Adsorption methods suffer from adsorbent saturation and the need for regeneration. Biological treatment is often slow and less reliable (Table 1)10,11,12,13,14,15,16,17,18.

Table 1 Tetracycline removal methods: advantages and disadvantages10,11,12,13,14,15,16,17,18.
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Photocatalysis, a widely studied AOP, involves irradiating a photocatalyst with light to transfer electrons from the valence band to the conduction band, generating e−/h+ pairs. When the absorbed photon energy exceeds the semiconductor band gap, these charge carriers are produced; however, rapid e−/h+ recombination dissipates energy as heat, limiting the degradation efficiency. Effective photocatalysis requires separation of e−/h+ pairs, enabling their interaction with water and dissolved oxygen to form reactive species (e.g., ·OH, O2−·, HOO·), which degrade pharmaceutical pollutants19,20,21.

Numerous studies have demonstrated high-efficiency photocatalytic degradation of tetracycline in aqueous media. Table 2 summarizes some of these findings14,15,22,23,24,25,26,27,28,29,30.

Table 2 Reviews of works for the photocatalytic removal of tetracycline.
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Graphene, discovered in 2004, has attracted considerable attention due to its exceptional mechanical, electrical, thermal, and optical properties, along with its two-dimensional structure composed of sp2 hybridized carbon atoms. Each carbon atom forms three sigma bonds in a single atom-thick honeycomb lattice. Graphene’s common synthesis methods include chemical vapor deposition, mechanical exfoliation, liquid phase exfoliation, and pyrolysis31,32,33,34,35. In the Hummer’s method, a type of liquid phase exfoliation, graphite oxide is first exfoliated into graphene oxide, which is subsequently reduced to graphene36. With its high surface area and low band gap, graphene facilitates charge transfer and visible light absorption, suppresses e−/h+ recombination, and enhances photocatalytic performance31,32,33,34,35. The photocatalytic activity of graphene is also enhanced by coupling it with semiconductor materials, which facilitate charge separation through a photosensitization mechanism37. Tin (IV) oxide (SnO2), a semiconductor with a band gap of 3.6 eV, has been widely studied for photocatalytic applications38. Several previous studies incorporated graphene onto a semiconductor structure, such as SnO2, to degrade organic contaminants39,40. In contrast, the present study investigates the deposition of SnO2 on reduced graphene oxide (RGO) support for the photocatalytic removal of tetracycline from water. Deposition of SnO2 markedly enhanced the photocatalytic activity of RGO, leading to improved tetracycline efficiency. The SnO2/RGO composite was characterized using FTIR, SEM, XRD, TEM, BET, PL, and DRS techniques. Its removal efficiency was evaluated under various operating conditions to identify the optimum parameters for maximum tetracycline removal.

Materials and methods

Materials

TC was obtained from Sigma-Aldrich. Graphite powder, nitric acid (HNO3), potassium permanganate (KMnO4), sulfuric acid (H2SO4), ascorbic acid (C6H8O6), hydrogen peroxide (H2O2), ethanol (C2H5OH), hydrochloric acid (HCl), ammonia solution (NH4OH), and tin (II) chloride (SnCl2) were purchased from Merck and used.

SnO2/RGO photocatalyst synthesis

RGO synthesis

RGO was synthesized by Hummer’s method. Graphite powder (1 g) was added to a mixture of 18 mL HNO3 and 46 mL H2SO4, and the solution was stirred in an ice bath for 45 min. KMnO4 (5 g) was gradually added while keeping the temperature below 20 °C, followed by stirring for 90 min. The reaction continued at 35 °C for 15 min. Subsequently, 100 mL of deionized water was added, the temperature was increased to 98 °C, and stirring was continued for 15 min. After 1 h, the suspension turned dark brown. Then, 120 mL of deionized water and 15 mL of H2O2 (30 wt%) were added simultaneously, forming a yellow dispersion. The resulting precipitate was centrifuged and washed with 100 mL of dilute HCl (1:100 v/v) to remove residual metal ions, followed by rinsing with deionized water until the supernatant pH reached 5–6. The final product (graphene oxide, GO) was dried in an oven at 70 °C for 12 h36.

To obtain RGO, the dried intermediate (graphene oxide, GO) was ultrasonically dispersed in 500 mL of deionized water. Ascorbic acid (5 g) was added, and the pH was adjusted to 10 using NH4OH. The suspension was stirred at 95 °C for 2 h, then centrifuged to recover the precipitate. After sequential washing with deionized water to neutral pH and ethanol, the precipitate was dried at 70 °C for 12 h36,41.

SnO2/RGO synthesis

Tin chloride (0.05 g) was dissolved in 100 mL of deionized water, after which 0.1 g of RGO powder was added to the solution. The mixture was sonicated for 20 min, centrifuged, and the precipitate was dried at 100 °C for 12 h. The dried material was then calcinated at 400 °C for 2 h to yield the SnO2/RGO photocatalyst.

Characterization instrument

Fourier transform infrared spectroscopy (FTIR) was conducted using a Vector 22 spectrometer (Bruker) to identify and analyze surface functional groups. X-ray diffraction (XRD) was employed to examine the crystalline structure of the photocatalyst. Diffraction patterns were recorded over a 2θ range of 10 to 80° with a step size of 0.02°, using a Cu kα anode (λ = 0.154 nm, 40 kV, 40 mA) on an X’Pert MPD diffractometer (Philips). The structural features of the synthesized graphene were assessed using Raman spectroscopy (Unidorn). Elemental analysis was performed by inductively coupled plasma optical emission spectroscopy (ICP-OES) using a 730-ES instrument (Varian). Surface morphology was visualized using scanning electron microscopy (SEM, MIRA3, TESCAN). Transmission electron microscopy (TEM, Zeiss-EM) images were captured at an electron voltage of 100 kV. The photoluminescence (PL) spectrum was recorded using Varian spectrometer to evaluate e−/h+ recombination rates. Diffuse reflectance spectroscopy (DRS) was carried out using an AvaSpec-2048TE spectrometer (Avantes).

Photocatalyst tests

The experimental setup has been described in detail in the previous study42. Photocatalytic degradation tests were carried out under visible light irradiation using a 125 W LED lamp. In a typical experiment, the SnO2/RGO photocatalyst (4 g/L) was added to 50 mL of TC solution with an initial concentration of 5 mg/L. The suspension was stirred for 10 min, followed by centrifugation to separate the photocatalyst. The remaining TC concentration was determined using a TG 80 + spectrophotometer (PG Instruments) based on the Beer-Lambert law. The maximum absorption wavelength (λmax) for TC was 357 nm. The removal efficiency was calculated using Eq. (1)42,43:

$${\text{TC removal}}\left( {\%} \right)=\frac{{{{\text{C}}_0} - {\text{C}}}}{{{{\text{C}}_0}}} \times 100$$
(1)

where C0 is the initial TC concentration (mg/L), and C is the concentration after photocatalytic treatment.

Experimental design

Experimental design involves systematic planning and execution of tests to evaluate the effects of multiple independent variables on a response. Once a reliable model is established, it can be used to predict responses within defined parameter ranges44,45. In this study, Design Expert software (version 11) was used. Response surface methodology (RSM) based on a Box-Behnken design was employed to model and optimize the TC degradation process. The independent variables included TC concentration (5–55 mg/L), photocatalyst dosage (0.2–4 g/L), and pH (2–10). Reaction time (10 min) and light intensity (125 W LED lamp) were kept constant. The response was TC removal efficiency. Table 3 summarizes the experimental conditions and corresponding responses.

Table 3 Experimental design and responses.
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Results and discussions

Optimization of SnO2/RGO

During the synthesis of the photocatalyst, SnCl2 solutions with concentrations of 0, 0.05, 0.07, and 1 wt% were tested. As shown in Fig. 1A, the 0.05 wt% SnCl2 concentration resulted in the highest TC removal. Increasing the SnCl2 content from 0 to 0.05 wt% enhanced TC degradation, likely due to the increased number of tin active sites. However, concentrations above 0.05 wt% led to interactions between adjacent active sites, reducing TC adsorption and consequently diminishing photocatalytic efficiency.

Following the optimization of tin content, the effect of calcination temperature was evaluated at 300, 350, 400, 450, and 500 °C. As depicted in Fig. 1B, the highest TC removal was observed at 400 °C. Based on these findings, all subsequent SnO2/RGO syntheses were conducted using 0.05 wt% SnCl2 as the precursor and a calcination temperature of 400 °C.

Fig. 1
figure 1

Effects of (A) SnCl2 concentration and (B) photocatalyst calcination temperature on TC removal; Operating conditions: initial TC concentration 10 mg/L, pH 7, catalysts dosage 2.1 g/L, reaction time 10 min; in (A) calcination temperature 400 °C; (B) SnCl2 concentration 0.05 wt%.

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Characterization of SnO2/RGO

The weight content in the composite (SnO2/RGO), determined by ICP-OES, and was 4.43%. FTIR, XRD, and Raman spectroscopy were done to identify the functional groups and structure of the SnO2/RGO. In the FTIR spectrum of RGO (Fig. 2A), characteristic absorption bands appeared at 3446 cm−1 (O–H stretching), 2921.16 cm−1 (C–H stretching), 1578.84 cm−1 (C = C stretching), and 1027.53 cm−1 (C–O stretching). In the spectrum of SnO2/RGO, these bands exhibited higher intensities, attributed to the presence of Sn species. The bands at 573.78, 537.17, and 474.80 cm−1 were assigned to Sn-O bonds46,47,48. The FTIR spectrum of SnO2/RGO after reaction (Fig. 2B) showed absorption bands at 3445.64 cm−1 (O–H stretching), 2920.96 and 2851.59 cm−1 (C–H stretching), 1624.48 cm− 1 (C=C stretching), 1024.76 cm−1 (C–O stretching), and 548.21, 475.10, and 415.90 cm−1, corresponding to Sn–O vibrations. The decreased intensities relative to the fresh sample can be related to the tetracycline adsorption.

The XRD pattern of RGO (Fig. 3A) displayed peaks at 11.85°, 25°, and 42.85°, which is consistent with the literature46. In the XRD pattern of fresh SnO2/RGO (Fig. 3B), peaks at 26.39°, 34.04°, and 51.89° corresponded to the tetragonal SnO2 phase (JCPDS File No. 72-1147)49. RGO peaks were absent, likely due to the dominance of SnO2 crystallinity and strong diffraction intensity. Crystallite size was calculated using the Scherrer equation: \(\:\text{L}=\frac{\text{k}{\uplambda\:}}{{\upbeta\:}\text{c}\text{o}\text{s}\left({\uptheta\:}\right)}\), where λ is the X-ray wavelength (nm), β is the full width at half maximum in radians, k is the shape factor (typically 0.9), and θ is the diffraction angle50. The crystalline size of SnO2 in the fresh composite was 10.82 nm. The XRD pattern of the used SnO2/RGO (Fig. 3C) showed peaks at 26.29°, 33.79°, and 51.74°, indicative of tetragonal SnO2 (JCPDS File No. 72-1147). The intensities of these peaks remained unchanged relative to the fresh sample, confirming no Sn leaching. The crystalline size of SnO2 in the used photocatalyst was 12.37 nm, showing a negligible structural change of SnO2.

Fig. 2
figure 2

FTIR spectra of (A) RGO and fresh SnO2/RGO; (B) fresh and used SnO2/RGO.

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Fig. 3
figure 3

XRD pattern of (A) RGO; (B) fresh SnO2/RGO; and (C) used SnO2/RGO.

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Raman spectra of RGO and SnO2/RGO are presented in Fig. 4. The spectrum of RGO (Fig. 4A) featured a D band at 1352.17 cm−1, associated with defects, and a G band at 1599.58 cm−1, attributed to in-plane vibrations of sp2 hybridized carbon atoms. For SnO2/RGO (Fig. 4B), D and G bands were observed at 1350.03 and 1529.97 cm−1, respectively, with decreased intensity due to SnO2 loading and partial reduction of the RGO structure49. The ID/IG ratios were 0.988 (RGO) and 1.02 (SnO2/RGO), indicating increased defect density during SnO2 deposition and calcination.

Fig. 4
figure 4

Raman spectra of (A) RGO and (B) SnO2/RGO.

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Nitrogen adsorption/desorption isotherms (Figs. 5A and B) exhibited type IV behavior with H3 hysteresis loops, indicative of slit-shaped, non-uniform pores51. Pore size distribution analysis (Figs. 5C and D) revealed a multimodal distribution for RGO and a unimodal distribution for SnO2/RGO, with the most frequent pore diameters at 1.21 nm. Specific surface area, porosity, and average pore diameter values are reported in Table 4.

Fig. 5
figure 5

(A, B) N2 adsorption/desorption isotherm for RGO and SnO2/RGO; (C, D) pore size distribution.

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Table 4 Specific surface area, porosity, and average pore diameter of RGO and SnO2/RGO.
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SEM images of RGO with magnifications of 88,500 and 259,000 (Figs. 6A and B) revealed a thin, wrinkled, layered morphology52. SEM images of SnO2/RGO with magnifications of 90,100 and 185,000 (Figs. 6C and D) displayed bright surface features corresponding to SnO2 nanoparticles.

Fig. 6
figure 6

SEM images of (A, B) RGO and (C, D) SnO2/RGO.

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TEM images (Figs. 7A–D) confirmed the layered, transparent morphology of RGO (Fig. 7A, B) and the uniform distribution of SnO2 nanoparticles on the graphene surface (Figs. 7C, D) 49, 52.

Fig. 7
figure 7

TEM images of (A, B) RGO and (C, D) SnO2/RGO.

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The high electron mobility of RGO suppresses e−/h+ recombination53,54. Photoluminescence spectroscopy of SnO2/RGO (Fig. 8), excited at 327.63 nm, exhibited an emission peak at 394 nm, indicating that recombination was maximized. Electron transfer in RGO effectively suppressed the recombination of e−/h+ and enhanced the photocatalytic activity54,55,56.

Fig. 8
figure 8

Photoluminescence spectrum of SnO2/RGO.

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UV-Vis diffuse reflectance spectra (Figs. 9A and B) confirmed light absorption across both UV and visible regions. Tauc plots (Figs. 9C and D), constructed with photon energy (hν, calculated as 1240/λ) on the x-axis and (αhν)2 on the y-axis48,56,57,58, and revealed band gaps of 0.615 eV for RGO and 0.24 eV for SnO2/RGO, indicating improved photocatalytic activity of SnO2/RGO due to its lower band gap.

Fig. 9
figure 9

UV–vis DRS spectra of (A) RGO, (B) SnO2/RGO; Tauc plots of (C) RGO, (D)SnO2/RGO.

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Experimental design

The independent variables in the design were photocatalyst dosage (g/L), pH, and initial TC concentration (mg/L). All experiments were conducted under a fixed irradiation time of 10 min using a 125 W LED lamp. Experimental runs were randomized, and each test was performed in triplicate with averaged values recorded. The investigated ranges were 0.2–4 g/L for catalyst dosage, 2–10 for pH, and 5–55 mg/L for TC initial concentration.

Accurate model selection is critical in experimental design analysis for ensuring predictive reliability. A fourth-order polynomial model was selected, yielding a statistically significant fit with an F-value of 349.92 and p-value < 0.0001 (Table 5)59,60. The derived model was Eq. (2):

$$\begin{aligned} {\text{TC removal }}\left( \% \right)= & - {\text{3}}0.{\text{97128}}\,+\,{\text{68}}.{\text{49}}0{\text{69A}}\,+\,{\text{18}}.{\text{92293B}}\,+\,0.0{\text{33896C}} \\ \quad & - {\text{8}}.{\text{7}}0{\text{445AB}}\,+\,0.0{\text{74187AC}} - {\text{16}}.{\text{27825}}{{\text{A}}^{\text{2}}} - {\text{1}}.{\text{53981}}{{\text{B}}^{\text{2}}} - 0.00{\text{3}}00{\text{1}}{{\text{C}}^{\text{2}}} \\ \quad & +\,{\text{3}}.{\text{31724}}{{\text{A}}^{\text{2}}}{\text{B}} - 0.0{\text{41937}}{{\text{A}}^{\text{2}}}{\text{C}}\,+\,0.{\text{386157A}}{{\text{B}}^{\text{2}}} - 0.{\text{19}}0{\text{15}}0{{\text{A}}^{\text{2}}}{{\text{B}}^{\text{2}}} \\ \end{aligned}$$
(2)

where A, B, and C represent photocatalyst dosage (g/L), pH, and TC initial concentration (mg/L), respectively. The coefficient of determination (R2) was 0.9972, and the adjusted R2 was 0.9939, indicating a strong agreement between experimental and predicted values. Adequate precision was 63.73, confirming model robustness. The model is valid within the investigated ranges, though extrapolation beyond these boundaries should be approached with caution. Among the studied variables, photocatalyst dosage exhibited the most significant effect on TC removal (F-values A: 692.41, A2: 15.20), followed by pH (F-values B: 65.64, B2: 267.51), while TC initial concentration showed comparatively the lowest influence (F-values C:20.12, C2:2.94).

Table 5 The ANOVA of the mathematical model.
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 Figure 10A depicts the interaction between photocatalyst dosage and pH. At constant pH values, TC removal increased with increasing catalyst dosage, reaching a maximum, after which further increases reduced removal efficiency. This trend was initially attributed to enhanced photogenerated charge carrier formation, which promoted degradation, and later to increased turbidity, which hindered light penetration and facilitated e−/h+ recombination61.

At fixed dosage levels, pH also influenced TC removal efficiency. TC, an amphoteric compound, has dissociation constants (pKa) at 3.2, 7.7, and 9.6. Its species distribution varies with pH: H3TC+ (cationic) below pH 3.2, H2TC (neutral), at 3.2 < pH < 7.7, and HTC− or TC2− (anionic) above pH 7.7. Under acidic conditions, photocatalyst surface protonation induces electrostatic repulsion with cationic TC, limiting adsorption. Similarly, in alkaline environments, repulsion between anionic TC species and negatively charged surfaces reduces removal. Maximum TC degradation was observed near neutral pH (Fig. 10A)58,61,62.

 Figure 10B presents the interaction between photocatalyst dosage and initial TC concentration. Increasing TC concentration decreased removal efficiency due to reduced active site availability per molecules. In both cases (Figs. 10A and B), optima for catalyst dosages were observed62.

Fig. 10
figure 10

Effects of operating parameters on TC removal: (A) interaction between catalyst dosage and pH; (B) interaction between catalyst dosage and TC initial concentration.

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The optimum conditions, selected from the design space, were TC initial concentration 5 mg/L, pH 7, and a photocatalyst dosage of 4 g/L, with a 10 min reaction time. Under these conditions, the experimental and predicted TC removal efficiencies were 92 and 92.18%, respectively, with the model prediction within ± 5% of experimental result.

Effects of light sources

Figure 11 illustrates the effect of different light sources on TC photodegradation using the SnO2/RGO. Photocatalytic activity was observed under all tested irradiation conditions. Sunlight experiments were performed at 12:00 p.m. on January 20, 2022, in Sanandaj, Iran. Adsorption was the mechanism of TC removal in the dark conditions. The experimental data were fitted to zero, first, and second-order kinetic models. As summarized in Table 6, the photodegradation of TC followed a second-order kinetic model for all light sources. Among the tested light sources, the LED lamp yielded the highest rate constant, indicating superior photocatalytic performance for TC removal. The demonstrated activity of SnO2/RGO under artificial and natural visible light suggests promising potential for cost-effective environmental remediation applications.

Fig. 11
figure 11

Effects of different light sources on TC removal, operating conditions: TC initial concentration 10 mg/L, catalyst dosage 4 g/L, pH 7, and reaction time 10 min.

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Table 6 Kinetic model fitting for various light sources42.
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Reuse and regeneration

Photocatalyst reusability is a critical factor for practical application. Under these conditions; TC initial concentration 10 mg/L, pH 7, catalyst dosage 4 g/L, reaction time 10 min under irradiation of LED lamp 125 W; the SnO2/RGO photocatalyst was recovered by centrifugation, ultrasonically washed in ethanol for 10 min, rinsed with deionized water, and dried. As shown in Fig. 12, TC removal decreased from 91% in the first cycle to 57% in the fifth, indicating partial deactivation of SnO2/RGO over repeated use due to TC adsorption according to FTIR results.

Fig. 12
figure 12

Reuse and regeneration cycles, operating conditions: TC initial concentration 10 mg/L, pH 7, catalyst dosage 4 g/L, time 10 min.

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Mineralization and effects of other pollutants

To ensure mineralization of TC during photodegradation reaction, COD and BOD5 analyses were conducted. The photocatalytic reaction was carried out under the following conditions: TC concentration 5 mg/L, pH 7, SnO2/RGO dosage 4 g/L, 10 min reaction time, and irradiation by a 125 W LED lamp. COD and BOD5 removal were calculated using Equations. (S1) and (S2) in the Supplementary Material63,64. BOD5 and COD removal efficiencies were 100%, and 72.5%; respectively, confirming during photodegradation 72.5% of organic materials were mineralized and converted to CO2 and H2O.

To evaluate SnO2/RGO performance under realistic conditions, TC photodegradation was studied in the presence of Na+, Cl−, Mg2+, NO3− ions, and tap water. Figure 13 summarizes the results. For a 5 mg/L of TC solution containing 5 mg/L of Na+ and Cl−, TC removal of 90% was obtained. With 5 mg/L of Mg2+ and 10 mg/L of NO3−, TC removal reached 91.47%. In a mixture containing all these ions, 84.01% of TC removal resulted. When tap water was used to prepare the TC solution (5 mg/L), the removal yield was 91.09%. Tap water typically contains Na+, K+, Fe3+, Ca2+, Mg2+, Cl−, SO42−, PO43−, CO32−, and trace disinfection by-products such as trihalomethanes65,66. The presence of these species did not significantly impact the removal efficiency, indicating the photocatalyst’s robustness under real water conditions.

Additionally, the SnO2/RGO photocatalyst was tested for a mixture solution containing 3 mg/L of TC, 1 mg/L of ciprofloxacin, and 1 mg/L of cefixime. The operating conditions were a catalyst dosage of 4 g/L, pH 7, reaction time 10 min, 125 W LED lamp. The removals were 97% for TC, 97.7% for ciprofloxacin, and 97.6% for cefixime.

Fig. 13
figure 13

TC removal in the presence of various ions; operating conditions: TC initial concentration 5 mg/L, catalyst dosage 4 g/L, pH 7, time 10 min, and under irradiation of 125 W LED lamp; solutions: (A) TC, (B) TC, Na+, Cl−; (C) TC, Mg2+, NO3−, (D) TC, Na+, Cl−, Mg2+, NO3−, (E) TC, tap water.

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SnO2/RGO photocatalytic mechanism

Reduced graphene oxide predominantly consists of sp2-hybridized carbon networks with π-π conjugation domains, structural defects, and oxygenated functional groups. Upon light irradiation, electrons in the π systems and defect states become photoexcited. These photoexcited electrons endow pristine RGO with inherent photocatalytic activity67,34.

Tin (IV) oxide (SnO2) with a wide band gap of 3.6 eV is typically active under ultraviolet irradiation68. In contrast, the SnO2/RGO exhibits photocatalytic activity under visible light as well. Upon exposure to visible light, RGO acts as an effective photosensitizer. Its photoexcited electrons migrate into the conduction band of SnO2, thereby generating additional electron-hole (e−/h+) pairs69. This photosensitization process occurs because the Fermi level of RGO is more negative than the conduction band edge of SnO270,71,72, enabling the transfer of excited electrons from RGO to the SnO2 conduction band. Moreover, the high electrical conductivity of RGO facilitates rapid electron transport and suppresses e−/h+ recombination.

The electrons react with dissolved oxygen to generate superoxide radicals (O2−·), while the photogenerated holes oxidize water to produce hydroxyl radicals (·OH). These reactive radical species act synergistically to degrade TC molecules in water.

The reactions of O2−· and ·OH radicals with TC lead to bond cleavage, deamination, and ring-opening processes. Further oxidation ultimately results in complete mineralization of TC into carbon dioxide and water4,23. Figure 14 shows schematic of the SnO2 photocatalytic process.

Fig. 14
figure 14

Schematic of the photocatalytic mechanism of SnO2/RGO.

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Comparison with other studies

The development of photocatalysts that are efficient under visible light, stable, reusable, environmentally benign, and economically viable has been the focus of extensive research. Selected examples from previous studies are summarized in Table 7. The SnO2/RGO photocatalyst leveraged the unique properties of RGO, including high electron mobility, large surface area, visible light activity, and excellent structural stability. The photocatalytic degradation kinetics of TC by SnO2/RGO were notably fast; in contrast to earlier studies, where the reaction times typically exceeded 60 min. The present photocatalyst, SnO2/RGO, achieved comparable removal efficiencies within approximately 10 min. The significant reduction in treatment time allows for reactor size minimization and enhances the overall cost effectiveness of the process. Furthermore, recent advancements in scalable RGO synthesis have made SnO2/RGO-based systems more accessible and practical for large-scale water treatment applications.

Table 7 Comparison with previous studies.
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Conclusion

Water contamination by antibiotics, particularly TC, contributes to antimicrobial resistance, posing a critical environmental challenge. In this study, SnO2/RGO photocatalyst was synthesized via Hummer’s method and optimized for TC photodegradation under visible light. DRS analysis confirmed a reduction in the band gap, enhancing photocatalytic activity compared with pristine RGO and SnO2. ICP-OES indicated the SnO2 content of 4.34%. XRD analysis confirmed SnO2 crystallinity, with no change in SnO2 peak intensities and crystallinity size of the used catalyst. SEM and TEM imaging showed uniform dispersion of SnO2 nanoparticles over the RGO surface.

Photodegradation kinetics followed a second-order under LED, UV, tungsten, and sunlight irradiation, with LED yielding the highest efficiency. Optimum performance was achieved under 5 mg/L of TC, 4 g/L of photocatalyst, pH 7, and 10 min of LED exposure, with TC removal of 92%. Reusability tests revealed a decline in TC removal from 91 to 57% over five cycles. The photocatalyst maintained its activity in the presence of Na+, Cl−, Mg2+, NO3−, and tap water. These results demonstrate the potential of SnO2/RGO for efficient, rapid, and cost-effective wastewater treatment.