Ammonium changes the pathway of tetrabromobisphenol S degradation by sulfate radical oxidation

Abstract

Sulfate radical (SO4•-)-based advanced oxidation processes (SR-AOPs) show great potential in the elimination of organic pollutants in wastewater treatment and groundwater remediation. Tetrabromobisphenol S (TBBPS) as an emerging contaminant can be effectively degraded in SR-AOPs. However, the present study found that SO4•- also reacted with ammonium (NH4+), a ubiquitous inorganic nitrogen species in aquatic environments, thus markedly affected the rates and pathways of TBBPS degradation in SR-AOPs. TBBPS underwent β-scission and debromination upon SO4•- attack, leading to various transformation products. The released bromide (Br-) underwent further oxidation by SO4•- to form free bromine which reacted with certain TBBPS intermediates, generating disinfection byproducts (DBPs). When NH4+ was present in such system, the abatement of TBBPS was apparently suppressed. Meanwhile, NH4+ was oxidized by SO4•- to nitrogen dioxide radical (NO2•) which coupled with certain radical intermediates of TBBPS oxidation, yielding nitrophenolic products including 2,6-dibromo-4-nitrophenol (DBNP) and mono-nitro substituted TBBPS. When 10 μM TBBPS was treated by heat activated peroxydisulfate with 1 mM NH4+, the molar yield of DBNP reached 22% in 2 h. More importantly, NH4+ scavenged the in situ formed free bromine to form monobromamine (NH2Br), decreasing regulated DBPs formation but leading to highly toxic dibromoacetonitrile (DBAN). This study broadens the understanding of interactions between the transformation of halogens and NH4+ during SO4•- oxidation. It also reveals the potential environmental risks when SR-AOPs are employed for the organohalogen pollutants degradation in realistic environments where NH4+ is present.