BACKGROUD: Nitrite is ubiquitous in natural water and wastewater. It is generally accepted that nitrite is an ozone consumer and has a negative influence on contaminants degradation during ozonation. Recently, nitrite was found to react with ozone to produce .OH. However, the potential positive effect of nitrite on the degradation of ozone refractory compounds has never been recognized previously during ozonation. Herein, the influence of nitrite on the degradation of atrazine by ozonation is evaluated. RESULTS: The presence of nitrite was found to enhance atrazine degradation during ozonation. The enhancing effect of nitrite was attributed to . OH formation with a yield of 8.4% in the reaction of ozone with nitrite, which was demonstrated in scavenging experiments with tert-butyl alcohol. The maximum atrazine degradation was observed at the nitrite dose of 10 mu mol L-1, when the ozone dose was 20 mu mol L-1. The enhancing effect of nitrite on atrazine degradation by ozonation was more pronounced at lower pHs. The enhancing effect of nitrite on atrazine degradation by ozonation was still observed in surface water, especially at acidic pH. Six transformation products of atrazine were identified in the ozone/nitrite system, and the degradation mechanism is discussed. CONCLUSIONS: This study is the first to demonstrate that nitrite not only acts as an ozone consumer, but may also have a positive effect on the degradation of ozone refractory contaminants by activating ozone to produce .OH. It provides a more comprehensive understanding to the role of nitrite in ozonation. (C) 2016 Society of Chemical Industry