*3.2. 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 1O2 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−<sup>1</sup> to 0.0080 h−<sup>1</sup> and 0.0265 h−<sup>1</sup> (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 <sup>h</sup>−1, *k*obs decreased to only 0.0753 h−<sup>1</sup> and 0.0457 h−<sup>1</sup> (Figure S3), respectively. Thus, the above results showed that both 1O2 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 1O2 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]:

$$\text{GO} + \text{hv} \rightarrow \text{GO}^\* \left( \text{e}\_{\text{CB}}^- - \text{h}\_{\text{VB}}^+ \right) , \tag{1}$$

$$\rm{CO}^\* + \rm{O}\_2 \rightarrow \rm{O}\_2^{-1} + \rm{GO}\_{\prime} \tag{2}$$

$$\text{Co}\_{\text{CB}}^{-} + \text{O}\_{2} \rightarrow \text{O}\_{2}\text{-}^{-}, \tag{3}$$

$$\rm{H}\_{\rm{VB}}^{+} + \rm{H}\_{2}\rm{O} \rightarrow \rm{\cdot OH} + \rm{H}^{+}, \tag{4}$$

$$\cdot \text{h}\_{\text{VB}}^{+} + \text{OH}^{-} \rightarrow \cdot \text{OH}\_{\text{'}} \tag{5}$$

$$\text{O}\_2\text{\textdegree\text{\textdegree}}^{\text{\textdegree}} + \text{h}\_{\text{VB}}^{\text{\textdegree}} \rightarrow \text{O}\_2\text{\textdegree\text{\textdegree}}^{\text{1}},\tag{6}$$

$$\rm O\_2\cdot^- + e^-\_{VB} + 2H^+ \to H\_2O\_{2'} \tag{7}$$

$$\text{H}\_2\text{O}\_2 + \text{e}\_\text{CB}^- \rightarrow \cdot \text{OH} + \text{OH}^-,\tag{8}$$

*3.3. Effects of Different Conditions on SMZ Degradation*
