Bicarbonate enhances phenol degradation and nitro(so)phenol formation during nitrate photolysis: Importance of carbonate radical-mediated electron transfer process

Nitrate (NO 3 − ) photolysis in water generates hydroxyl radicals (HO • ) and reactive nitrogen species (RNS), driving a complex transformation of organic pollutants. This study investigates the degradation pathways of phenol during NO 3 − photolysis in the presence of bicarbonate (HCO 3 − ), which is inevitably present in practical waters that scavenges HO • to form carbonate radical (CO 3 •− ). As the HCO 3 − concentration increased from 0 to 10 mM, the concentrations of both HO • and • NO decreased during NO 3 − photolysis. But in contrast, the pseudo-first-order rate constant ( k ′) of phenol increased by 66.7%, which was attributed to a 4.8-fold increase in the relative contribution of RNS and CO 3 •− to the overall degradation rate. Laser flash photolysis confirmed that CO 3 •− reacted with phenol via electron transfer, which increased the yield of phenoxyl radicals. The elevated concentrations of phenoxyl radicals subsequently promoted the formation of nitro(so)phenols during NO 3 − photolysis with HCO 3 − , via the rapid combination of phenoxyl radicals with • NO and • NO 2 . Specifically, the conversion yield of phenol to nitrophenols significantly increased from 2.2% to 16.7% at 60 min as HCO 3 − increased from 0 to 10 mM. Furthermore, the formation of nitrated polymers was also enhanced during NO 3 − photolysis in the presence of HCO 3 − , due to the increase of CO 3 •− and • NO 2 . This study elucidates the critical role of HCO 3 − in transformation kinetics and pathways of organic compounds during NO 3 − photolysis in water treatment.