Effect of common inorganic anions on iron-catalyzed secondary brown carbon formation from guaiacol

Molecular-Scale Investigation on the Formation of Brown Carbon Aerosol via Iron-Phenolic Compound Reactions in the Dark

“…Moreover, the absorption peaks near 400 (Figure S1k,l) and 470 nm (Figure S1f) showed a similar trend of first increasing and then decreasing, which were identified as the n−π* transition of semiquinone or quinone (i.e., the oligomer intermediates formed by Fe-organic complexes), , indicating that the intermediates disappeared and further transformed into the polymer with the progress of the reaction . Electron transfer induced the reduction of Fe 3+ ; therefore, we monitored the Fe 2+ and total Fe concentration in solution during the reaction and calculated the concentration of Fe 3+ by mass conservation (Text S8).…”

“…With time increasing, 80–94% of Fe 3+ was reduced to Fe 2+ within 24 h. Considering that the molar ratio of Fe 3+ to phenol in this study was 2:1, this indicated that Fe 3+ can be continuously reduced by phenols and the intermediate products. Because Fe binds to O in the phenolic hydroxyl group, the reduction of Fe 3+ is due to the electron transfers from O to Fe, resulting in the formation of phenoxy radicals (ArO • ). , Although this has been proposed by many previous studies, ,, the ArO • formed in the acidic condition has not been detected experimentally. This is because of the short lifetime of ArO • in acidic conditions making it difficult to capture and detect using conventional methods .…”

“…Because Fe binds to O in the phenolic hydroxyl group, the reduction of Fe 3+ is due to the electron transfers from O to Fe, resulting in the formation of phenoxy radicals (ArO • ). 59,60 Although this has been proposed by many previous studies, 14,15,58 the ArO • formed in the acidic condition has not been detected experimentally. This is because of the short lifetime of ArO • in acidic conditions making it difficult to capture and detect using conventional methods.…”