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 4 •- ) oxidation is effective to degrade TBBPS. Here, we demonstrate that the presence of nitrite (NO 2 - ) at environmentally relevant levels causes dramatic changes in the kinetics and pathways of TBBPS degradation by SO 4 •- . Initially, NO 2 - suppresses the reaction by competing with TBBPS for SO 4 •- . At the same time, SO 4 •- oxidizes NO 2 - to form nitrogen dioxide radicals (NO 2 • ), 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 4 •- to form bromine radicals and free bromine. These reactive bromine species immediately combine with NO 2 • or NO 2 - to form nitryl bromide (BrNO 2 ) 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 2 or ClNO 2 ) are likely formed yet inadequately recognized when SO 4 •- is applied to remediate halogenated pollutants in the subsurface environment where NO 2 - is ubiquitously found. These insights further underscore the potential risks of the application of SO 4 •- oxidation for the remediation of halogenated compounds in realistic environmental conditions.