New Insights into the Role of Nitrite in the Degradation of Tetrabromobisphenol S by Sulfate Radical Oxidation.

Tetrabromobisphenol S (TBBPS) is a brominated flame retardant and a contaminant of emerging concern. Several studies found that sulfate radical (SO) oxidation is effective to degrade TBBPS. Here, we demonstrate that the presence of nitrite (NO) at environmentally relevant levels causes dramatic changes in the kinetics and pathways of TBBPS degradation by SO. Initially, NO suppresses the reaction by competing with TBBPS for SO. At the same time, SO oxidizes NO to form nitrogen dioxide radicals (NO), which actively react with some key TBBPS degradation intermediates, thus greatly altering the transformation pathway. As a result, 2,6-dibromo-4-nitrophenol (DBNP) becomes the primary TBBPS product. As TBBPS undergoes degradation, the released bromide (Br) is oxidized by SO to form bromine radicals and free bromine. These reactive bromine species immediately combine with NO or NO to form nitryl bromide (BrNO) that in turn attacks the parent TBBPS, resulting in its accelerated degradation and increased formation of toxic nitrophenolic byproducts. These results show that nitryl halides (e.g., BrNO or ClNO) are likely formed yet inadequately recognized when SO is applied to remediate halogenated pollutants in the subsurface environment where NO is ubiquitously found. These insights further underscore the potential risks of the application of SO oxidation for the remediation of halogenated compounds in realistic environmental conditions.