3.3.1. Effect of pH

The pH value of the solution has a grea<sup>t</sup> 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 <sup>h</sup>−1, 0.0814 <sup>h</sup>−1, 0.1214 <sup>h</sup>−1, and 0.1193 h−<sup>1</sup> (Figure S4), respectively. The relative high degradation of SMZ at higher pH conditions was probably due to the following two reasons: firstly, SMZ (*p*Ka1 = 2.6, *p*Ka2 = 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.

**Figure 2.** Degradation kinetics of SMZ (**a**) and *k*obs (**b**) with free radical scavengers: L-histidine and KI; generation kinetics of ·OH (**c**) and 1O2 (**d**).

3.3.2. 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.

**Figure 3.** Effect of pH on SMZ degradation kinetics (**a**) and *k*obs of SMZ degradation (**b**).

**Figure 4.** Effect of Cl− (**a**) and SO4<sup>2</sup>− (**b**) on SMZ degradation kinetics.

To further explore the effect of Cl− and SO4<sup>2</sup>− on the photolysis of SMZ, quantitative analysis of 1O2 and ·OH was carried out (Figure 5). It was evident that Cl− showed a negative influence on the production of 1O2, 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.

$$\text{Cl}^- + \text{hv} \rightarrow \text{Cl} + \text{e}\_{\text{aq}}^-,\tag{9}$$

Similar to Cl<sup>−</sup>, the presence of SO4<sup>2</sup>− also inhibited the generation of 1O2 but prompted the production of ·OH. The concentration of 1O2 decreased from 75.70 μM to 30.28 μM with increasing SO4<sup>2</sup>− from 0 to 30 mM, but the ·OH concentration gradually increased from 0.35 μM to 0.51 μM. Therefore, the introduction of SO4<sup>2</sup>− into the solution promoted SMZ degradation by increasing the steady-state concentration of ·OH. On the other

hand, SO4<sup>2</sup>− existing on the GO surface would form reactive sulfate radicals by holes (Equation (10)) [46], which may also accelerate the transformation of SMZ [50].

$$\text{SO}\_4^{2-} + \text{h}^+ \rightarrow \text{SO}\_4^- \tag{10}$$

**Figure 5.** Effects of Cl− and SO4<sup>2</sup>− on 1O2 production (**<sup>a</sup>**,**<sup>c</sup>**) and ·OH production (**b**,**d**).
