Phototransformation of Graphene Oxide on the Removal of Sulfamethazine in a Water Environment

Graphene oxide (GO) is widely used in various fields and has raised concerns regarding its potential environmental fate and effect. However, there are few studies on its influence on coexisting pollutants. In this study, the phototransformation of GO and coexisting sulfamethazine (SMZ) under UV irradiation was investigated, with a focus on the role of reactive oxygen species. The results demonstrated that GO promoted the degradation of SMZ under UV irradiation. The higher the concentration of GO, the higher the degradation rate of SMZ, and the faster the first-order reaction rate. Two main radicals, ∙OH and 1O2, both contributed greatly in terms of regulating the removal of SMZ. Cl−, SO42−, and pH mainly promoted SMZ degradation by increasing the generation of ∙OH, while humic acid inhibited SMZ degradation due to the reduction of ∙OH. Moreover, after UV illumination, the GO suspension changed from light yellow to dark brown with increasing absorbance at a wavelength of 225 nm. Raman spectra revealed that the ID/IG ratio slightly decreased, indicating that some of the functional groups on the surface of GO were removed under low-intensity UV illumination. This study revealed that GO plays important roles in the photochemical transformation of environmental pollutants, which is helpful for understanding the environmental behaviors and risks of nanoparticles in aquatic environments.


Introduction
As a kind of two-dimensional layered nanomaterial, graphene oxide (GO) possesses good mechanical, electrical, and thermal properties, and is widely applied in various fields, including biology, medicine, chemistry, and electronic engineering [1]. The global production of GO is expected to reach 3800 metric tons in 2027 [2]. Due to the presence of a large number of oxygen-containing functional groups, such as hydroxyl, carboxyl, and epoxy groups, GO has excellent hydrophilicity and a high probability of being present in natural aquatic environments, thus having uncertain environmental impacts and ecological risks. It has been reported that GO and its derivatives exhibited cytotoxicity to bacteria, biofilms, and algae [3,4]. Moreover, GO could cause developmental genotoxicity in aquatic animals such as zebrafish at trace concentrations [5], and could even accumulate in humans through the food chain [6]. Therefore, an increasing number of studies on the environmental behaviors of GO have received attention.
Once released into the environment, GO can interact with other pollutants mainly through π bonds, hydrophobic interactions, hydrogen bonds, and electrostatic interactions [7][8][9][10], thus affecting the transport and fate of coexisting compounds. For example, GO exhibited a high affinity for heavy-metal ions, which improved the transport ability of Pb 2+ and Cd 2+ in saturated porous media [11]. GO also facilitated the transport of antibiotics (levofloxacin, ciprofloxacin, and tetracycline) in saturated or unsaturated porous system was operated at an average light intensity of 10.0 mW cm −2 with 5 W LED white lamps (365 nm). During the 6 h photochemical experiments, 50 mL of the reaction solutions was magnetically stirred at 300 rpm in quartz tubes that were maintained at constant temperature (22 ± 2 • C) with the circulating water bath of the reactor.

SMZ Degradation
One batch experiment was first conducted with a fixed amount of SMZ (5 µM) with GO ranging from 10 mg/L to 50 mg/L. Dark control experiments were also conducted under the same conditions. To investigate the effects of solution chemistry factors on the photochemical transformation, another three sets of experiments were also performed with 5 µM SMZ and 30 mg/L GO. The pH effect experiments were conducted with the solution pH ranging from 3.0 to 9.0, which was adjusted with 0.1 M HCl or NaOH. Ionic strength and species effect experiments were performed in the presence of 0-600 mM NaCl or 0-30 mM Na 2 SO 4 . In addition, the photochemical transformation of SMZ was tested in the presence of humic acid (HA) in the range of 0-10 mg/L. All the above experiments were performed in triplicate. During the experiments, 3 mL of solution was sampled at determined time intervals and filtered with 0.22 µm nylon membranes to remove GO. Then, SMZ was analyzed at a determination wavelength of 270 nm by high-performance liquid chromatography (HPLC, Shimadzu LC-20AT, Tokyo, Japan) with a UV detector using a C18 column (25 cm × 4.6 mm, 5 µm). The mobile phase was acetonitrile/0.05 M acetic acid (30:70, v:v) with a flow rate of 1 mL/min. The injection volume was 10 µL, and the column temperature was maintained at 40 • C.

ROS Generation
It should be noted that O 2 · − was not detected with the XTT sodium salt (probe for O 2 · − ) in this study; therefore, we only focused on the production of ·OH and 1 O 2 . Free radical quenching experiments were first carried out with L-histidine and potassium iodide (KI) as radical quenchers to identify the contribution of 1 O 2 and ·OH, respectively [37,38]. The inhibition rate of SMZ degradation was determined after introducing free radical scavengers. In addition, 200 µM terephthalic acid (TPA) and 300 µM furfuryl alcohol (FFA) were used as indicators to quantify the amount of ROS [39,40]. TPA reacted with ·OH and produced 2-hydroxyterephthalic acid (HTPA), which could be measured by a fluorescence spectrophotometer (HITACHI, F-2500, Tokyo, Japan). The excitation and emission wavelengths were 315 nm and 425 nm, respectively [23,40]. FFA was analyzed by HPLC at 218 nm. The mobile phase was 30% acetonitrile and 70% phosphoric acid and run at 1.0 mL/min.

GO Characterization
To investigate the phototransformation of GO, the changes in GO in the photoreaction system were characterized by UV-vis spectrophotometry from 200 nm to 600 nm. Additionally, the Raman spectra were measured at 1000 cm −1 to 2000 cm −1 with 532 nm excitation with a Raman spectrometer (Renishaw inVia Reflex, New Mills, UK) before and after UV illumination.

SMZ Degradation
The effects of different concentrations of GO on SMZ degradation were first studied. GO did not adsorb SMZ much in the dark, and little degradation of SMZ occurred under UV illumination without GO ( Figure S1). However, the degradation of SMZ was accelerated in the presence of GO. The degradation rates of SMZ were 33.32 ± 2.54%, 34.90 ± 2.69%, and 37.44 ± 2.12% in the presence of 10 mg/L, 30 mg/L, and 50 mg/L GO, respectively ( Figure 1a). According to the first-order kinetic fitting of the reaction in the first two hours ( Figure S2), the observed reaction rate constants (k obs ) were 0.0732 h −1 , 0.0964 h −1 , and 0.1129 h −1 (Figure 1b).

ROS Generation
Generally, nanoparticles can generate ROS under UV light irradiation, which can participate in the degradation of chemicals. ROS generation by GO is similar to that of semiconductors. A large number of oxygen-containing functional groups attached to the GO surface play an important role in electron transfer and promote ROS generation [21]. To further explore the mechanism of GO on SMZ transformation, free radical scavengers, including L-histidine and KI, were added to the reaction solution to identify the role of 1 O2 and •OH. As shown in Figure 2a,b, 5 mM/10 mM L-histidine significantly inhibited SMZ degradation, reducing its degradation rate from 32.52 ± 4.34% to 6.57 ± 3.24% and 4.18 ± 1.63%, with kobs decreasing from 0.1004 h −1 to 0.0080 h −1 and 0.0265 h −1 ( Figure S3), respectively. Similar results were also observed in the presence of KI, where the decomposition of SMZ reduced to 26.36% for 10 mM KI and 18.59% for 50 mM KI. Compared with the initial kobs of 0.1004 h −1 , kobs decreased to only 0.0753 h −1 and 0.0457 h −1 ( Figure S3), respectively. Thus, the above results showed that both 1 O2 and •OH participated in SMZ degradation.
ROS quantification was performed during the photochemical experiments. Figure  2c,d shows that the free radical production of GO was proportional to the illumination time; 75.70 μM 1 O2 and 0.35 μM •OH could be produced after 6 h of illumination in the presence of 30 mg/L GO. Based on the above experimental results, possible ROS generation pathways were further proposed, as shown in the following reaction formulas [23,41,42]: H O + e →• OH + OH ,

ROS Generation
Generally, nanoparticles can generate ROS under UV light irradiation, which can participate in the degradation of chemicals. ROS generation by GO is similar to that of semiconductors. A large number of oxygen-containing functional groups attached to the GO surface play an important role in electron transfer and promote ROS generation [21]. To further explore the mechanism of GO on SMZ transformation, free radical scavengers, including L-histidine and KI, were added to the reaction solution to identify the role of 1 O 2 and ·OH. As shown in Figure 2a,b, 5 mM/10 mM L-histidine significantly inhibited SMZ degradation, reducing its degradation rate from 32.52 ± 4.34% to 6.57 ± 3.24% and 4.18 ± 1.63%, with k obs decreasing from 0.1004 h −1 to 0.0080 h −1 and 0.0265 h −1 ( Figure S3), respectively. Similar results were also observed in the presence of KI, where the decomposition of SMZ reduced to 26.36% for 10 mM KI and 18.59% for 50 mM KI. Compared with the initial k obs of 0.1004 h −1 , k obs decreased to only 0.0753 h −1 and 0.0457 h −1 ( Figure S3), respectively. Thus, the above results showed that both 1 O 2 and ·OH participated in SMZ degradation.
ROS quantification was performed during the photochemical experiments. Figure 2c,d shows that the free radical production of GO was proportional to the illumination time; 75.70 µM 1 O 2 and 0.35 µM ·OH could be produced after 6 h of illumination in the presence of 30 mg/L GO. Based on the above experimental results, possible ROS generation pathways were further proposed, as shown in the following reaction formulas [23,41,42]:

Effect of pH
The pH value of the solution has a great influence on the photolysis of SMZ ( Figure 3). The degradation rates were 26.02 ± 3.05%, 41.06 ± 4.23%, 49.33 ± 5.11%, and 51.14 ± 5.63% as the pH increased from 3.0 to 9.0, and k obs were 0.0641  Figure S4), respectively. The relative high degradation of SMZ at higher pH conditions was probably due to the following two reasons: firstly, SMZ (pK a1 = 2.6, pK a2 = 8.0) can be degraded more easily in its ionic forms compared with the neutral form [43]. Secondly, similar to semiconductors [44], GO produces holes after UV illumination, which can further react with OH − to produce ·OH [45,46]. The generation of ·OH increased with increasing pH, resulting in the promotion of SMZ conversion. In addition, the dispersion of GO was higher at higher pH because of the deprotonation of oxygencontaining functional groups on the GO surface [47], which might result in an increase in the steady-state concentration of ROS. Therefore, SMZ degradation by GO was higher at high pH than at low pH.

Effect of pH
The pH value of the solution has a great influence on the photolysis of SMZ ( Figure  3). The degradation rates were 26.02 ± 3.05%, 41.06 ± 4.23%, 49.33 ± 5.11%, and 51.14 ± 5.63% as the pH increased from 3.0 to 9.0, and kobs were 0.0641 h −1 , 0.0814 h −1 , 0.1214 h −1 , and 0.1193 h −1 (Figure S4), respectively. The relative high degradation of SMZ at higher pH conditions was probably due to the following two reasons: firstly, SMZ (pKa1 = 2.6, pKa2 = 8.0) can be degraded more easily in its ionic forms compared with the neutral form [43]. Secondly, similar to semiconductors [44], GO produces holes after UV illumination, which can further react with OH − to produce •OH [45,46]. The generation of •OH increased with increasing pH, resulting in the promotion of SMZ conversion. In addition, the dispersion of GO was higher at higher pH because of the deprotonation of oxygen-containing functional groups on the GO surface [47], which might result in an in-

Effect of Coexisting Anions
The effects of ionic strength and species on SMZ degradation are presented in Figure 4. NaCl improved the degradation of SMZ, with the degradation rate rising from 35.36 ± 1.69% to 43.83 ± 2.21%, 45.18 ± 2.88%, and 47.9 ± 2.79% in the presence of NaCl from 100 mM to 600 mM, respectively. Similarly, when 10 mM, 20 mM, and 30 mM Na 2 SO 4 were added to the solution, the SMZ decomposition rate increased to 37.92 ± 2.38%, 41.94 ± 2.57%, and 46.52± 2.78%, respectively.

Effect of Coexisting Anions
The effects of ionic strength and species on SMZ degradation are presented in Figure  4. NaCl improved the degradation of SMZ, with the degradation rate rising from 35.36 ± 1.69% to 43.83 ± 2.21%, 45.18 ± 2.88%, and 47.9 ± 2.79% in the presence of NaCl from 100 mM to 600 mM, respectively. Similarly, when 10 mM, 20 mM, and 30 mM Na2SO4 were added to the solution, the SMZ decomposition rate increased to 37.92 ± 2.38%, 41.94 ± 2.57%, and 46.52± 2.78%, respectively.  To further explore the effect of Cl − and SO4 2− on the photolysis of SMZ, quantitative analysis of 1 O2 and •OH was carried out ( Figure 5). It was evident that Cl − showed a negative influence on the production of 1 O2, whose level was reduced to 21.63 μM with increasing Cl − concentration, compared with that of the control 75.70 μM. However, the presence of Cl − accelerated the generation of •OH, especially 100 mM NaCl, which increased the amount of •OH by 1.6 times compared with the control. This could be explained by the fact that Cl − generated hydrated electrons under UV irradiation, which were then transferred to nanomaterials to generate more ROS (Equation (9)) [48]. It should be noted that excessive Cl − would agglomerate GO under high ionic strength [48,49], which would reduce the surface area of GO and the concentration of ROS. Thus, the steady-state concentration of •OH first increased and then decreased with increasing NaCl concentration. The presence of Cl − promoted the decomposition of SMZ, which was

Effect of Coexisting Anions
The effects of ionic strength and species on SMZ degradation are presented in Figure  4. NaCl improved the degradation of SMZ, with the degradation rate rising from 35.36 ± 1.69% to 43.83 ± 2.21%, 45.18 ± 2.88%, and 47.9 ± 2.79% in the presence of NaCl from 100 mM to 600 mM, respectively. Similarly, when 10 mM, 20 mM, and 30 mM Na2SO4 were added to the solution, the SMZ decomposition rate increased to 37.92 ± 2.38%, 41.94 ± 2.57%, and 46.52± 2.78%, respectively.  To further explore the effect of Cl − and SO4 2− on the photolysis of SMZ, quantitative analysis of 1 O2 and •OH was carried out ( Figure 5). It was evident that Cl − showed a negative influence on the production of 1 O2, whose level was reduced to 21.63 μM with increasing Cl − concentration, compared with that of the control 75.70 μM. However, the presence of Cl − accelerated the generation of •OH, especially 100 mM NaCl, which increased the amount of •OH by 1.6 times compared with the control. This could be explained by the fact that Cl − generated hydrated electrons under UV irradiation, which were then transferred to nanomaterials to generate more ROS (Equation (9)) [48]. It should be noted that excessive Cl − would agglomerate GO under high ionic strength [48,49], which would reduce the surface area of GO and the concentration of ROS. Thus, the steady-state concentration of •OH first increased and then decreased with increasing NaCl concentration. The presence of Cl − promoted the decomposition of SMZ, which was To further explore the effect of Cl − and SO 4 2− on the photolysis of SMZ, quantitative analysis of 1 O 2 and ·OH was carried out ( Figure 5). It was evident that Cl − showed a negative influence on the production of 1 O 2 , whose level was reduced to 21.63 µM with increasing Cl − concentration, compared with that of the control 75.70 µM. However, the presence of Cl − accelerated the generation of ·OH, especially 100 mM NaCl, which increased the amount of ·OH by 1.6 times compared with the control. This could be explained by the fact that Cl − generated hydrated electrons under UV irradiation, which were then transferred to nanomaterials to generate more ROS (Equation (9)) [48]. It should be noted that excessive Cl − would agglomerate GO under high ionic strength [48,49], which would reduce the surface area of GO and the concentration of ROS. Thus, the steady-state concentration of ·OH first increased and then decreased with increasing NaCl concentration. The presence of Cl − promoted the decomposition of SMZ, which was in accordance with the role of ·OH. Therefore, ·OH was expected to be the main ROS species that regulated SMZ degradation.
Similar to Cl − , the presence of SO 4 2− also inhibited the generation of 1 O 2 but prompted the production of ·OH. The concentration of 1 O 2 decreased from 75.70 µM to 30.28 µM with increasing SO 4 2− from 0 to 30 mM, but the ·OH concentration gradually increased from 0.35 µM to 0.51 µM. Therefore, the introduction of SO 4 2− into the solution promoted SMZ degradation by increasing the steady-state concentration of ·OH. On the other Nanomaterials 2021, 11, 2134 7 of 11 hand, SO 4 2− existing on the GO surface would form reactive sulfate radicals by holes (Equation (10)) [46], which may also accelerate the transformation of SMZ [50].
Nanomaterials 2021, 11, x FOR PEER REVIEW 7 of 11 in accordance with the role of •OH. Therefore, •OH was expected to be the main ROS species that regulated SMZ degradation.
Cl + hv → Cl + e , Similar to Cl − , the presence of SO4 2− also inhibited the generation of 1 O2 but prompted the production of •OH. The concentration of 1 O2 decreased from 75.70 μM to 30.28 μM with increasing SO4 2− from 0 to 30 mM, but the •OH concentration gradually increased from 0.35 μM to 0.51 μM. Therefore, the introduction of SO4 2− into the solution promoted SMZ degradation by increasing the steady-state concentration of •OH. On the other hand, SO4 2− existing on the GO surface would form reactive sulfate radicals by holes (Equation (10)) [46], which may also accelerate the transformation of SMZ [50].

Effect of NOM
As the representative NOM, HA is a macromolecular polymer containing carboxyl, phenolic, and keto groups, which is widely distributed in natural waters. Previous studies showed that NOM might play different roles in the transformation of organic pollutants. For example, Chen et al. reported that HA could consume a large amount of •OH under UV light [51], which decreased the degradation of diethyl phthalate. However, Niu et al. proposed that NOM could be transformed into excited-state substances or free radicals under UV irradiation, which enhanced the degradation of norfloxacin [38]. In the present study, as shown in Figure 6, the degradation of SMZ significantly decreased from 41.06 ± 2.34%, to 29.80 ± 2.64%, 24.72 ± 2.56%, and 23.95 ± 2.59% in the presence of 1 mg/L, 5 mg/L and 10 mg/L HA, respectively. HA had an inhibitory effect on the degradation of

Effect of NOM
As the representative NOM, HA is a macromolecular polymer containing carboxyl, phenolic, and keto groups, which is widely distributed in natural waters. Previous studies showed that NOM might play different roles in the transformation of organic pollutants. For example, Chen et al. reported that HA could consume a large amount of ·OH under UV light [51], which decreased the degradation of diethyl phthalate. However, Niu et al. proposed that NOM could be transformed into excited-state substances or free radicals under UV irradiation, which enhanced the degradation of norfloxacin [38]. In the present study, as shown in Figure 6, the degradation of SMZ significantly decreased from 41.06 ± 2.34%, to 29.80 ± 2.64%, 24.72 ± 2.56%, and 23.95 ± 2.59% in the presence of 1 mg/L, 5 mg/L and 10 mg/L HA, respectively. HA had an inhibitory effect on the degradation of SMZ. As HA might react with ·OH, we only measured the production of 1 O 2 in the presence of HA. As shown in Figure 6b, compared with the control, HA slightly influenced the generation of 1 O 2 , indicating that 1 O 2 contributed little to SMZ degradation. Therefore, it could be speculated that HA mainly quenched ·OH to decrease the decomposition of SMZ.

GO Transformation
Under UV illumination, the color of the GO suspension changed from light yellow to dark brown ( Figure S5), indicating that some oxygen-containing functional groups attached to the GO surface might be removed [19]. The variation of the solution absorbance with time was further determined by UV-vis spectrophotometry ( Figure S6). The peak at 225 nm was attributed to the π -π* transition of unsaturated C-C bonds of GO. After 6 h of UV irradiation, the absorbance at 225 nm increased, indicating that the sp 3 structure of GO was reduced and the sp 2 structure had been recovered [21]. It should be noted that our previous study demonstrated that UV light intensity greatly affected the absorbance of GO, and the absorption peak at 225 nm could be redshifted to 255 nm at a high light intensity of 54 mW cm −2 in 4 h [23]. In the present study, the light intensity was only 10 mW cm −2 ; thus, the absorption peak did not shift significantly. Raman spectra were further used to analyze the GO samples before and after UV illumination. The D band at approximately 1350 cm −1 and the G band at approximately 1580 cm −1 are the two characteristic peaks of GO. Peak D represents the vibration of sp 3 carbon atoms, and peak G is the characteristic peak of carbon sp 2 . The ratio of I D /I G is usually used as a qualitative measurement of the disorder degree caused by nonaromatic sp 3 carbon defects. After 6 h of illumination, I D /I G decreased only from 0.8481 to 0.8438 ( Figure S7), indicating that the graphitization structure of GO was somewhat improved and that the sp 2 region was expanded. However, the insignificant decrease in I D /I G suggested that the UV light intensity was not high enough in the current study, which was in accordance with the changes in UV-vis absorbance.
Nanomaterials 2021, 11, x FOR PEER REVIEW 8 of 11 SMZ. As HA might react with •OH, we only measured the production of 1 O2 in the presence of HA. As shown in Figure 6b, compared with the control, HA slightly influenced the generation of 1 O2, indicating that 1 O2 contributed little to SMZ degradation. Therefore, it could be speculated that HA mainly quenched •OH to decrease the decomposition of SMZ.

GO Transformation
Under UV illumination, the color of the GO suspension changed from light yellow to dark brown ( Figure S5), indicating that some oxygen-containing functional groups attached to the GO surface might be removed [19]. The variation of the solution absorbance with time was further determined by UV-vis spectrophotometry ( Figure S6). The peak at 225 nm was attributed to the π -π* transition of unsaturated C-C bonds of GO. After 6 h of UV irradiation, the absorbance at 225 nm increased, indicating that the sp 3 structure of GO was reduced and the sp 2 structure had been recovered [21]. It should be noted that our previous study demonstrated that UV light intensity greatly affected the absorbance of GO, and the absorption peak at 225 nm could be redshifted to 255 nm at a high light intensity of 54 mW cm −2 in 4 h [23]. In the present study, the light intensity was only 10 mW cm −2 ; thus, the absorption peak did not shift significantly. Raman spectra were further used to analyze the GO samples before and after UV illumination. The D band at approximately 1350 cm −1 and the G band at approximately 1580 cm −1 are the two characteristic peaks of GO. Peak D represents the vibration of sp 3 carbon atoms, and peak G is the characteristic peak of carbon sp 2 . The ratio of ID/IG is usually used as a qualitative measurement of the disorder degree caused by nonaromatic sp 3 carbon defects. After 6 h of illumination, ID/IG decreased only from 0.8481 to 0.8438 ( Figure S7), indicating that the graphitization structure of GO was somewhat improved and that the sp 2 region was expanded. However, the insignificant decrease in ID/IG suggested that the UV light intensity was not high enough in the current study, which was in accordance with the changes in UV-vis absorbance.
Based on the above results, we proposed the possible cotransformation pathways of GO and SMZ (Figure 7). Similar to semiconductors, GO generated electrons and holes − Figure 6. Effects of HA on SMZ degradation kinetics (a) and 1 O 2 production (b). Based on the above results, we proposed the possible cotransformation pathways of GO and SMZ (Figure 7). Similar to semiconductors, GO generated electrons and holes under illumination. Electrons could be captured by O 2 to generate O 2 · − , which was further converted into 1 O 2 and ·OH. Meanwhile, GO could form excited-state GO* under illumination, and then O 2 accepted excess energy and generated 1 O 2 . Therefore, ROS generated in the above ways promoted SMZ degradation. At the same time, GO could capture electrons to reduce its surface oxygen-containing functional groups.

Conclusions
In this study, the photochemical behaviors of GO and the degradation of SMZ were

Conclusions
In this study, the photochemical behaviors of GO and the degradation of SMZ were quantitatively analyzed. GO could promote the degradation of SMZ under UV light. ·OH and 1 O 2 were the main free radicals participating in the cotransformation between GO and SMZ. High pH, Cl − , and SO 4 2− improved the degradation of SMZ by affecting the formation of ·OH. However, the presence of HA consumed ·OH, leading to less degradation of SMZ. As for GO, its color changed from light yellow to dark brown under UV illumination. However, the absorption peak did not shift significantly, and the ratio of I D /I G was slightly smaller, which indicated that GO was somewhat reduced. The findings of this work may have significant implications for predicting the fate and assessing the potential risks of environmental pollutants and nanoparticles. However, to better understand the environmental behaviors of nanoparticles, long-term experiments under natural solar radiation are still needed.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/nano11082134/s1, Figure S1: Figure S1. Photolysis kinetics of SMZ (5 µM) under UV light without GO (a) and the adsorption of SMZ (5 µM) by GO (10 and 100 mg/L) in the dark within 6 h (b), Figure S2: Pseudo first-order fitting results for SMZ degradation kinetics under various GO concentrations (10-50 mg/L), Figure S3: Pseudo first-order kinetics fitting for kinetics of SMZ degradation with L-histidine and KI, Figure S4: Pseudo first-order kinetics fitting for kinetics of SMZ degradation at pH 3.0-9.0, Figure S5: Changes in the color of GO under UV light as a function of irradiation time, Figure S6: Variation of GO absorbance with time under light, Figure S7: Raman spectra of GO before and after UV illumination.  Data Availability Statement: The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.