3.3.3. 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 1O2 in the presence of HA. As shown in Figure 6b, compared with the control, HA slightly influenced the generation of 1O2, indicating that 1O2 contributed little to SMZ degradation. Therefore, it could be speculated that HA mainly quenched ·OH to decrease the decomposition of SMZ.
