Linking transformation products of tannic acid to DBPs during UV/Chlorine and UV/H2O2

Characterization of disinfection by-product (DBP) formation following advanced oxidation processes (AOPs) remains critical for their implementation in drinking water treatment. AOPs partially oxidize natural organic matter, producing transformation products (TPs) whose role in DBP formation is not fully understood. Tannic acid (TA) was used due to its polyphenolic structure and relevance to NOM to investigate the formation pathways of DBPs from TPs under hydroxyl and chlorine radicals. Five new TPs were identified during UV/chlorine - trigallic acid (C 21 H 14 O 13 ), benzoic acid 3,4,5–trihydroxy-2-oxo-1,3-propanediyl ester (C 17 H 14 O 11 ) and beta-Glucogallin (C 13 H 16 O 10 ) for UV/H 2 O 2 and 2-chloro-3,4,5-trihydroxybenzoic acid (C 7 H 5 ClO 5 ) and 2,6-dichloro-3,4,5-trihydroxybenzoic acid (C 7 H 4 Cl 2 O 5 ). In the UV/H 2 O 2 process, • OH addition was proposed as a pathway for degradation of TA and its TPs, whereas in UV/chlorine, hydrogen abstraction followed by Cl addition were hypothesized. Of the ten TPs detected under UV/chlorine, only one was observed during dark chlorination (1,3,3-trichloroprop-1-ene), indicating reactive chlorine species (RCS)-mediated pathways for the remaining nine. UV/H 2 O 2 tended to reduce the formation of unregulated DBP with lower H/C ratios as the UV dose increased (up to 1200 mJ cm −2 ), while with UV/chlorine there were no clear trends. DBP formation was strongly influenced by the distinct TPs generated under each AOP. For example, correlative analysis would point out at 1,6-digalloylglucose (DGG) as a key THM precursor with increasing UV dose through both • OH and RCS routes but only increased HAA formation via • OH pathways, although authentic standard validation would be required to confirm it. When investigating the formation of unregulated DBPs, the molecular fingerprint of UV/chlorine differed from that of UV/H 2 O 2 , further highlighting their molecular diversity, although formation yields or health risk should be assessed in future. Although further testing is needed to fully confirm the proposed pathways, these findings underpin the distinct TP fingerprint and DBPs derived from UV/chlorine and UV/H 2 O 2 and formed from polyphenolic structures common to aromatic-rich surface waters, providing insights for DBP risk mitigation in drinking water treatment.