Formation of brominated and nitrated byproducts during unactivated peroxymonosulfate oxidation of phenol

Brominated and nitrated byproducts generated from bromide (Br^-) and nitrite (NO₂^-), respectively, by sulfate radical (SO₄^•-) oxidation have raised increasing concern. However, little is known about the concurrent generation of brominated and nitrated byproducts in the unactivated peroxymonosulfate (PMS) oxidation process. This study revealed that Br^- can facilitate the transformation of NO₂^- to nitrated byproducts during unactivated PMS oxidation of phenol. In the co-existence of 0.1 mM Br^- and 0.5 mM NO₂^-, the total yield of identified nitrated byproducts reached 2.316 μM in 20 min, while none was found with NO₂^- alone. Nitryl bromide (BrNO₂) as the primary nitrating agent was formed via the reaction of NO₂^- with free bromine in situ generated through the oxidation of Br^- by PMS. BrNO₂ rapidly reacted with phenol or bromophenols, generating highly toxic nitrophenols or nitrated bromophenols, respectively. Increasing NO₂^- concentration led to more nitrated byproducts but less brominated byproducts. This study advances our understanding of the transformation of Br^- and NO₂^- in the unactivated PMS oxidation process. It also provides important insights into the potentially underestimated environmental risks when PMS is applied to degrade organic contaminants under realistic environments, particularly when Br^- and NO₂^- co-exist.