ZrO2/carbon nitride (C3N4) composite photocatalysts were prepared by solid-state grinding and subsequent calcination. They were characterized by X-ray diffraction, scanning electron microscopy, UV–Vis diffuse reflectance spectroscopy, and photoluminescence spectroscopy. Their photocatalytic activity was evaluated by the degradation of methylene blue (MB). Results showed that the grain size of the C3N4/ZrO2 composite photocatalyst was relatively small at about 10 nm. HRTEM showed that a heterojunction was formed between ZrO2 and g-C3N4. Under the assistance of H2O2, the visible-light degradation activity of MB by the composites was much higher than that by pure ZrO2. The 9%g-C3N4/ZrO2 composite photocatalyst had the best photocatalytic performance, and it could degrade 99% of MB in 60 min. Adding a small amount of NaCl to the solution had a significant effect on the photocatalytic effect of the composite. Such addition could inhibit the photocatalytic activity of samples with high C3N4 content and significantly improve the performance of photocatalysts with low C3N4 content. The 1%g-C3N4/ZrO2 composite (assisted by H2O2) had the best photocatalytic effect on the MB solution containing salt. It could degrade MB solution containing 0.15, 0.3, and 0.6 g/L NaCl within 30, 40, and 45 min, respectively.
