Positive profile of natural small molecule organic matters on emerging antivirus pharmaceutical elimination in advance reduction process: A deep dive into the photosensitive mechanism of triplet excited state compounds

•NSOM strongly enhance emerging contaminants abatement in UV/Sulfite process.•eaq− triggers NSOM into its triplet excited state through photosensitization.•Triplet excited state NSOM instead of eaq− dominants pollutants abatement.•Substituent sites of NSOM affect its photosensitization and molar absorption coefficient.•eaq− and pHBA molar absorption coefficient show contrary changing trends with pH. Natural small molecular organic matter (NSOM), ubiquitous in natural waters and distinct from humic acid or fulvic acid, is a special type of dissolved organic matter (DOM) which is characterized as strong photosensitivity and simple molecular structure. However, little study had been directed on the role of NSOM in eliminating emerging contaminants in advanced reduction process (ARP). This study took three small molecular isomeric organic acids (p−hydroxybenzoic acid, pHBA; salicylic acid, SA; m−hydroxybenzoic acid, mHBA) as the representative substances of NSOM to explore these mechanisms on promoting Ribavirin (RBV, an anti COVID−19 medicine) degradation in ultraviolet activated sulfite (UV/Sulfite) process. The results demonstrated that the observed degradation rate constant of RBV (kobs-RBV) was 7.56 × 10−6 s−1 in UV/Sulfite process, indicating that hydrated electron (eaq−) from UV/Sulfite process could not effectively degrade RBV, while it increased by 178 and 38 times when pHBA and SA were introduced into UV/Sulfite process respectively, suggesting that pHBA and SA strongly promoted RBV degradation while mHBA had no promotion on RBV abatement in UV/Sulfite process. Transient absorption spectra and reactive intermediates scavenging experiment indicated that the triplet excited state pHBA and SA (3pHBA* and 3SA*) contributed to the degradation of RBV through non−radical process. Notably, eaq− played the role of key initiator in transforming pHBA and SA into their triplet states. The difference of kobs-RBV in UV/Sulfite/pHBA and UV/Sulfite/SA process was attributed to different generation pathways of 3pHBA* and 3SA* (high molar absorptivity at the wavelength of 254 nm and photosensitive cycle, respectively) and their second order rate constants towards RBV (kRBV-3pHBA* = 8.60 × 108 M−1 s−1 and kRBV-3SA* = 6.81 × 107 M−1 s−1). mHBA could not degrade RBV for its lack of intramolecular hydrogen bond and low molar absorptivity at 254 nm to abundantly transform into its triplet state. kobs-RBV increased as pH increased from 5.0 to 11.0 in UV/Sulfite/SA process,