Degradation of refractory organic compounds from dinitrodiazophenol containing industrial wastewater through UV/H2O2 and UV/PS processes

In this study, refractory organic compounds from dinitrodiazophenol (DDNP) containing industrial wastewater were degraded through two ultraviolet (UV)-based advanced oxidation processes: UV/hydrogen peroxide (UV/H₂O₂) and UV/potassium persulfate (UV/PS) processes. In both processes, the synergistic effects, operational parameters (i.e., oxidant dosage and initial pH value), and pseudo first-order constant k were systematically studied. Moreover, the reactive oxygen species formed in the UV/H₂O₂ and UV/PS processes were identified, and the degradation of refractory organic compounds was characterized through UV-visible spectra analysis. The improvement in biodegradability of DDNP industrial wastewater after treatment by different processes was compared. Both the UV/H₂O₂ (synergistic coefficient F = 61.34) and UV/PS (synergistic coefficient F = 54.85) processes showed significant, highly synergistic effects. The increase in oxidant dosage was beneficial in organic compound removal in both the UV/H₂O₂ and UV/PS processes, but excessive H₂O₂ showed a stronger inhibition of the increase in organic compound removal than that in the UV/PS process. In addition, an acidic environment was more conducive to organic compound degradation in the UV/H₂O₂ process, whereas the initial pH value had less of an influence on the UV/PS process. Under optimal conditions for the UV/H₂O₂ and UV/PS processes, the CN and COD removal efficiencies were 99.71%, 66.35%, 99.69%, and 70.81%, respectively, and the k values for COD removal were 0.0804 and 0.0824 min⁻¹. Tests to identify reactive oxygen species showed that the hydroxyl radical was the predominant oxidizing species in the UV/H₂O₂ process, whereas the hydroxyl and sulfate radicals were both identified in the UV/PS process, and the sulfate radical contributed the most to the degradation of organic compounds. In addition, spectrum analysis revealed that the complex structure (e.g., benzene ring, nitro group, and diazo group) of refractory organic compounds from DDNP industrial wastewater was effectively destroyed by the UV/H₂O₂ and UV/PS processes, and both processes improved the biodegradability (biochemical oxygen demand for 5 days/chemical oxygen demand (BOD₅/COD)) of DDNP industrial wastewater from 0.052 to 0.665 and 0.717, respectively. Overall, both the UV/H₂O₂ and UV/PS processes effectively degraded the refractory organic compounds from DDNP industrial wastewater, and the UV/PS process exhibited a higher organic compound removal efficiency and better applicability.