Influence of Nitrite on Ultraviolet-Activated Peroxydisulfate Degradation of 2,4-Dichlorophenol

Ultraviolet-activated peroxydisulfate (UV/PDS) oxidation has been considered as a promising technology for removing organic pollutants in wastewater. In this study, we investigated the abatement of 2,4-dichlorophenol (DCP) by UV/PDS, finding that the degradation can be attributed to both direct photolysis and sulfate radical (SO4 center dot-) oxidation. Intriguingly, the presence of trace levels of nitrite (NO2-) increased the degradation rate and dramatically changed the degradation pathways. We propose that an in situ-generated reactive species, nitryl chloride (ClNO2), was responsible for this effect. UV irradiation played a critical role in the generation of ClNO2 because direct photolysis caused dechlorination of DCP. The released chloride (Cl-) was further oxidized by SO4 center dot- to a chlorine radical (Cl-center dot). At the same time, NO2- was oxidized by SO4 center dot- to nitrogen dioxide radicals (NO2 center dot). Combination of Cl-center dot and NO2 center dot gave rise to ClNO2. This mechanism was not found in the heat-activated PDS system since SO4 center dot- oxidation resulted in little dechlorination. As a strong nitrating agent, ClNO2 reacted rapidly with DCP to form 2,4-dichloro-6-dinitrophenol (DCNP) as the primary transformation product. The yield reached 60.1% in 12 min in the presence of 50 mu M NO2-. The bioluminescence inhibition assay revealed that DCNP is more toxic than DCP. This contribution indicates that the activation approach can significantly influence the degradation behavior of halogenated pollutants in PDS oxidation systems.