*2.3. 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 Na2SO4. 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.
