Unveiling the impacts of bromide and nitrite interaction on the degradation of bisphenol A during sulfate radical oxidation

Abstract

Sulfate radical (SO₄^•-) oxidation processes have the capacity to effectively degrade bisphenol A (BPA), an endocrine disruptor of great concern. However, potential environmental risks associated with generating harmful byproducts in this process remain largely unknown. This study revealed that nitrated byproducts were generated during the degradation of BPA by SO₄^•- oxidation with nitrite (NO₂⁻) present. This is due to the production of nitrogen dioxide radical (NO₂^•) through the reaction of NO₂⁻ and SO₄^•-, which further combines with phenoxy radicals originating from BPA oxidation, leading to nitrated byproducts. Moreover, bromide (Br⁻) facilitated the transformation of NO₂⁻ to nitrated byproducts in this process. When 150 μM Br⁻ and 100 μM NO₂⁻ were both in the solution, the total formation of identified nitrophenols (i.e., 2-nitrophenol, 4-nitrophenol, and 2,4-dinitrophenol) reached 0.67 ± 0.02 μM, compared to 0.55 ± 0.01 μM with NO₂⁻ alone. Nitryl bromide (NO₂Br) generated from the interaction of NO₂⁻ and free bromine in situ formed from Br⁻ oxidation was identified to play a pivotal role. It directly reacts with BPA and some intermediates via electrophilic substitution, leading to nitrated byproducts with Br⁻ releasing. Coherently with the enhanced nitrated byproducts formation, toxicity test showed that the BPA sample treated by SO₄^•- in the co-presence of Br⁻ and NO₂⁻ was more toxic than that with NO₂⁻ alone. Thus, it is required to keep a watchful eye on undesired byproducts formation when SO₄^•- oxidation technologies are applied in realistic environments where Br⁻ and NO₂⁻ are commonly co-existing.