Sigma Aldrich and Merck, Karachi Sindh, Pakistan, provided methylene blue (C16H18ClN3S, Mm 319.85 g/mol), sodium hydroxide (NaOH, Mm 39.997 g/mol), hydrochloric acid (HCl), ascorbic acid (C6H8O6) and sodium borohydride (NaBH4). Throughout the experiment, high analytical grade chemicals and ultrapure water were used.

As described in Scheme 1, carbon material was synthesized by using an aqueous chemical growth method at low temperatures followed by mild pyrolysis. To begin with, Citrullus Colocynthis was collected during the summer season from a deserted area in district Qamabar Shahdad Kot Sindh, Pakistan. Afterwards, Citrullus Colocynthis was sliced and juiced using a juicer machine. Following that, 100 mL of Citrullus Colocynthis juice was mixed with 100 mL of deionized water, covered with aluminum sheet and boiled at 98 °C for 4 hours. It was necessary to treat the mixture at low temperatures in order to obtain a desired amount of carbon with a favorable active surface for the application of photocatalysis. In the second step, 50 mL of the boiled mixture was placed into Pyrex glass of 100 mL and kept in a muffle furnace at 250°C for 2 h at a ramp rate of 15°C per minute. After sonicating for 50 min, the solution was centrifuged at 5,000 rotations per minute for 30 min to remove the larger carbon particles. We were able to recover 0.4 g of dark brown carbon for further characterization and analysis. The carbon yield percentage based on the use of 50 mL of the boiled mixture and recovery of 0.4 g of carbon were estimated about 0.8%. The following is a brief description of the steps involved in producing photocatalytic carbon material (Scheme 1).

Using X-ray diffraction (XRD), the crystal structure of as prepared carbon was analyzed (Cu Kα radiation with a wavelength of 0.15406 nm at a scanning range of 10–80, Bruker D8 advance). The morphology of carbon material prepared from Citrullus Colocynthis was examined using a scanning electron microscope (Hitachi Regulus 8,100, Tokyo, Japan), and the functional groups were identified using an FT-IR spectrometer (Tensor 27, Bruker Optics, Ettlingen, Germany). Carbon and dye degradation were assessed using UV-visible spectroscopy (Lamda 365, PerkinElmer, Waltham, MA, United States) in the wavelength range of 200–800 nm.

As obtained, carbon material from Citrullus Colocynthis was evaluated under UV light irradiation for its photocatalytic activity using UV-visible absorbance spectrometry. In a quartz jar containing 250 mL of deionized water, different concentrations of MB were prepared, including 2.3 × 10⁻⁵ M, 1.5 × 10⁻⁵ M and 0.6 × 10⁻⁵ M. The MB solutions were consistently mixed with 5 mg of carbon material until the dye was absorbed and fully mixed into the carbon surface. A well-established equilibrium between adsorption and desorption was observed after constant stirring for 30 min. Following irradiation of dye solutions with UV light, every 15 min interval was recorded for the decrease in absorbance. A spectrophotometer associated with a 300 W Xenon lamp with a wavelength range of 200–800 nm was used to detect the decrease in UV-visible absorbance. The homemade UV box was built using six light emitting diodes having a wavelength of 365 nm and power of 12 W. In controlled conditions, the dye removal was monitored by observing the decrease in color intensity, the decrease in UV light absorbance, and the time factor. Throughout the irradiation of ultraviolet light, the reaction jar was constantly stirred. All sources of light were cut off during the photodegradation of MB, and the reaction jar was primarily irradiated with UV light in the UV box. Following is a formula for estimating the removal percentage of MB: %   D = A 0 − A t A 0 × 100 % (1)

Herein, A0 and A are initial and after degradation concentrations of MB, while the %D indicate the degradation percentage of MB.

A scanning electron microscope was used to examine the shape of carbon as prepared using Citrullus Colocynthis, and the SEM images at two varying resolutions are shown in Figures 1A,B. According to the obtained morphological information, the carbon material exhibits an irregular graphitic sheet like shape. As illustrated in Figures 1A,B, sheets may have a thickness of a few microns. The functional groups information of as prepared carbon material was obtained from FTIR analysis as shown in Figure 1C. In Figure 1C, we can see that the stretching vibration frequency of 3,432 cm-1 was attributed to the hydroxyl groups (O-H) attached to the surface of the carbon material, while the C-H stretching band was positioned at 2,926 cm-1. The band at 2,509 cm-1 has been attributed to SH and the stretching band at 1808 cm-1 has been attributed to the stretching vibration band of C=O bonds. C-O and N-H are indexed to the stretching and bending vibrations at 1,578 cm-1 (Yanget al., 2012), while the stretching bands for C-N, N-H, and -COOH are observed at 1,424 cm-1. As a result of the FTIR study, several reducing groups such as -OH, -NH2, and -COO were observed to be adsorbing on the surface of the carbon material. These findings are in agreement with previous studies (Tetsuka et al., 2012; Jin et al., 2013; Sugiarti and Dhrmawan, 2015; Zhang et al., 2015). Figure 1D illustrates the spectrum generated by UV-visible spectroscopy in aqueous solution when the optical properties of the carbon material were studied. The shoulder peak at 344 nm in the absorbance spectrum has described the n–π* transition of CO and found in good agreement with the published work (Rezaei et al., 2019). It is possible to assign the absorbance peaks at 318 nm, 323 nm, and 333 nm to an sp2 hybrid configuration of aromatic carbon, which is a typical characteristic of graphite like carbon materials (Eda et al., 2009). As indicated by the UV-visible spectrum, the graphite of the as-prepared carbon material has typical characteristics, as verified by the published results (Pan et al., 2010; Zheng et al., 2013). As synthesized carbon material was examined by XRD for its crystalline properties, and the measured diffraction patterns are shown in Figure 2. In XRD analysis, reflections were observed at 19.59⁰ and 44.53⁰, respectively, and corresponded to crystal planes (001) and (101). Based on the XRD analysis, it has been confirmed that the (101) reflection peak is highly intense, which indicates that the material prepared exhibits weak crystalline characteristics. Figure 2A is accompanied by an inset explaining the behavior of synthetic carbon material made from Citrullus Colocynthis with illuminated UV light. During the day, the carbon material emits a light brown color, while when exposed to UV light, it emits a blue color, indicating zero hazards and high environmental friendliness for use in photocatalytic applications. Furthermore, the bandgap transitions, which were shown to correlate with the surface defects present in the CQDs, were mainly responsible for the luminescent features. The effect of the fluorescence features of the CQDs may also be responsible for the modest variations in particle size distribution and the sp2 hybridization of carbon clusters. Moreover, the greater excitation intensity of the CQDs resulted in larger energy emissions by the material, influencing its luminescent characteristics (Nizam, et al., 2023). Carbon material prepared in this manner is hydrophilic, which makes it a potential candidate for the photodegradation of MB. In the inset image, the carbon material was dispersed in water, then 1 mL of it was placed in quartz glass cuvette and irradiated with ultraviolet light for 3 minutes. A mobile camera was used to collect the image. Figure 2B provides information on the particle size of the carbon material as obtained using a Zeta sizer. Based on the SEM analysis, the size of the carbon material was observed in the micro dimension. The optical properties of carbon material materials have been found useful for photocatalytic applications (Yang et al., 2017; Cong et al., 2023). For this reason, we have performed the UV-visible absorption measurement and the presence of edges was noticed between 390–450 nm as shown in Figure 3A. The Kubelka-Munk model was used to estimate the optical band gap via extrapolation plot of (αh𝜈)2 vs. h𝜈, as shown in Figure 3B. Whereas, the h𝜈 as energy of photons, both n and A known as constants, E.g., corresponded to band gap energy and α as absorption coefficient. The optical band gap value of as synthesized carbon material was found about 2.52 eV. The optical band gap analysis has indicated the semiconducting nature of as synthesized carbon material.

Supplementary Figure S1 depicts the breakdown of MB dye at 2.3 × 10⁻⁵ M using UV light, without the use of manufactured carbon material from Citrullus Colocynthis extract. Supplementary Figure S1A shows a restricted kinetic response with minor absorbance variations, demonstrating that UV radiation has a negligible impact on the breakdown process of MB. The kinetic parameters were also studied to illustrate the reaction process, as illustrated in Supplementary Figure S1B,C. These results show that the rate constant value was very low, and UV radiation had a minor effect on the degradation process. The deterioration efficiency of MB utilizing simply UV light exposure was found to be around 12%, defining the low performance of MB oxidation as illustrated in Supplementary Figure S1D. At fixed intervals of 15 min, 10 min and 5 min, Figures 4–6 illustrates the degradation profile for three different concentrations of MB, 2.3 × 10⁻⁵ M, 1.5 × 10⁻⁵ M and 0.6 × 10⁻⁵ M, respectively, resulting in a decrease in UV-visible absorbance spectra and percent (%) dye removal. Based on the results of this study, we found that dye concentration plays an important role in the evaluation of the performance of the proposed carbon material. Figure 6 shows that a lower MB concentration significantly reduced the UV-visible absorbance of the carbon material. Similarly, the % removal of MB was highly increased when MB 0.6 × 10⁻⁵ M concentration was utilized, which suggests that the carbon material as prepared has a dye removal capacity of almost 99.8%. Figures 4, 5 indicate that the dye removal efficiency for MB concentrations of 2.3 × 10⁻⁵ M and 1.5 × 10⁻⁵ M is close to 97.1% and 97.9%, respectively. Furthermore, the reaction kinetics of MB onto the surface of as synthesized carbon material was determined using a pseudo-first order kinetic equation that included irradiation time intervals (min) and k as a first order velocity constant. The value of k measures the photocatalytic activity, with a higher value of k indicating increased photocatalytic efficiency (Katheresan et al., 2018; Panagopoulos, 2022a). Based on the results presented in Figure 4A–C, Figure 5A–C, Figure 6A–C with their linear fitted lines, Figure 7 shows the relationship between concentration and irradiation time. As a result of a linear fit, the value of slope corresponding to the first order reaction was used to estimate the rate constant. From the kinetics study, it was observed that the degradation rate was higher in the lowest dye concentration compared to the highest dye concentration. However, the reaction rate at higher MB concentrations was decreased due to the significant barriers provided by the large adsorbed dye molecules on the surface of carbon material for UV photons to react. This resulted in the highest removal % of MB at the lowest concentration of MB.

In addition, pH has been found to play an essential role in evaluating the photocatalytic performance of photocatalysts due to the accumulation of charges on the surface of the catalysts and the depth of activation towards specific pollutants (Yang et al., 2021). As shown in Supplementary Figure S1, the carbon material was used in order to understand the role of pH at pH values ranging from 5 to 11 in 0.6 × 10⁻⁵ M MB. In comparison to pH 7, nine and 11, there is a considerable decrease in the absorbance at pH 5, indicating that the surface of the synthesized carbon material is highly functional and activated. There is also a significant effect of pH variation on the dye removal percentage in dye solutions. A pH adjustment of 2M HCl and 2M NaOH was used to study the pH effect in the 2.3 × 10⁻⁵ M MB. With no acid or base present, the pH of dye solution was approximately 6.5. Lower and higher pH values were adjusted using 2M HCl and NaOH solutions, respectively. In comparison with other pH values of dye solution, the degradation of MB was dominated by the acidic pH of 5. As a result of the catalyst generating more holes, it can effectively participate in the degradation of MB. In contrast, at pH 11, dye molecules are largely adsorption on the surface of carbon material, providing less surface area for interactions with UV photons and hydroxyl radical generation, thus reducing the oxidation rate of dye (Marszewski et al., 2016). A kinetic analysis of the reaction rate was also conducted using Eq. (1); this analysis is shown in Figures 7A,B. It was found that the reaction rate constant of MB degradation was higher at pH five than at other pH values of MB solution under the influence of different pH values. According to Figure 7C, the dye removal % was almost 100% according to an estimate of degradation efficiency. The different pH values of the MB solution indicate that pH plays a significant role in the removal of dye from aqueous solutions, and the proposed carbon material has the potential to nearly remove MB at pH 5. The pH of dye solution may change the point of charge on the surface of photocatalyst and favoring the degradation of MB, hence it might play a role for the efficient of degradation of MB at pH 5. It is essential to understand the role of radicals in the degradation of MB in order to propose a mechanism for its degradation. To accomplish this, we conducted a scavenger test using ascorbic acid, ethylene-ediamine tetramine (EDTA) and sodium borohydride in an environment of 2.3 × 10⁻⁵ MB, and Figure 8D illustrates the degradation of carbon material in this environment. Previously, it has been shown that the hydroxyl (˙OH), photogenerated holes (h+) and superoxide radical ions (˙O2−) radicals have been responsible for the degradation of dyes. In this study, the selected scavengers generated hydroxyl and oxygen radicals (Shelar et al., 2020). These radicals were primarily responsible for the degradation of MB in the laboratory (Molla et al., 2017; Mondol et al., 2021). In the scavenger analysis, it was found that EDTA significantly decreased the degradation of MB, indicating that the hydroxyl radicals (OH) are mainly responsible for the degradation of MB when using the prepared carbon material in Figure 7D. The reusability test was also conducted to confirm the long-term durability of the carbon material and five reusable dye degradation tests of 2.3 × 10⁻⁵ M MB were conducted as shown in Figure 9A. The results of one controlled test and four reusable tests are presented in Figure 8A under UV irradiation. Following the proper washing and storage of the synthesized carbon material, it was evident that it could be used for several measurements with nearly the same degradation efficiency. According to Figure 8A, the degradation efficiency for cycle 0, cycle 1, cycle 2, cycle 3, and cycle four was 99.81%, 98.11%, 96.43%, 93.51%, and 90.73%, respectively. Even after four reusable tests, the degradation efficiency did not significantly decrease, indicating that the proposed carbon material could serve as an effective photocatalyst for wastewater treatment. Furthermore for the description of point of difference between the photocatalytic and adsorption properties of as synthesized carbon, the adsorption experiment was conducted but the results of dye removal percentage were noticed very poor. Whereas using photocatalytic approach the removal percentage of MB was highly efficient, supported the aims of proposed study about the design of photocatalytic carbon materials using green synthesis. Hence, the dye removal performance of as prepared carbon material based on the photocatalytic oxidation are reported in this study. The rapid and high efficiency of photocatalytic method have been seen in the literature, therefore the design of low cost, efficient, simple, scale up and environment friendly photocatalysts are highly desirable. According to Figure 7B, electrochemical active surface area was estimated using cyclic voltammetry with non-faradic region at different scan rates to demonstrate improved performance of the synthesized carbon material towards degradation of MB under UV illumination. The calculation of electrochemical active surface area (ECSA) was done using reported work (Laghari et al., 2023). An analysis of the linear plot of the difference between the current density on the anodic and cathodic sides against various scan rates has yielded a value of ECSA around 0.0017 μ cm−2 as shown in Figure 8C. According to the estimated value of ECSA, the carbon material displayed a significant amount of surface active sites which could play a vital role in the degradation of MB under UV light irradiation. As shown in Figure 8D, electrochemical impedance spectroscopy (EIS) was used to evaluate the charge transfer rate in order to enhance the degradation performance of the carbon material. Z-view software was used to fit the raw data of EIS, and the equivalent circuit is shown in the inset of Figure 9D. According to the equivalent circuit, there was a constant phase element (CPE), a solution resistance (Rs), and a charge transfer resistance (Rct). There is good agreement between the results presented in the EIS and those published in the literature (Hartanto et al., 2022). The small semicircle of the Nyquist plot indicates that the carbon material exhibited good conductivity and a high charge transfer rate. ECSA and EIS results reveal that the enriched active surface area and rapid charge transfer rate influenced the degradation of MB under the illumination of UV light. Hence, CV and EIS were used to illustrate the exposed active surface area and charge transfer rate for dynamic degradation of dye. Because the exposure of active surface area and the charge transfer could provide two aspects, active surface area for high interaction of dye molecules and charge transfer for causing the creation of more radicals during the interaction with water, hence rapid dye degradation could be expected. Scheme 2 describes the degradation mechanism of MB on photocatalytic carbon material can be described generally as follows. As a result of the UV light interaction with the carbon material, electron and hole pairs are transferred from the valence band to the conduction band. The electrons and holes then interact with the water molecules, generating radicals like superoxide and hydroxyl that contribute to dye degradation. MB dye is then converted into harmless products. Supplementary Table S1 summarizes the performance of photocatalytic carbon towards MB degradation, whereas Supplementary Table S2 compares the photocatalytic activity of carbon material with those of several recently reported catalysts. In comparison with previously reported catalysts, the proposed carbon material is facile, low cost, highly efficient, and environmentally friendly, which makes it potentially useful for wastewater treatment.