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

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 (k obs-RBV ) was 7.56 x 10 -6 s - 1 in UV/Sulfite process, indicating that hydrated electron (e aq- ) 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 ( 3 pHBA* and 3 SA*) contributed to the degradation of RBV through non -radical process. Notably, e aq- played the role of key initiator in transforming pHBA and SA into their triplet states. The difference of k obs-RBV in UV/Sulfite/pHBA and UV/Sulfite/SA process was attributed to different generation pathways of 3 pHBA* and 3 SA* (high molar absorptivity at the wavelength of 254 nm and photosensitive cycle, respectively) and their second order rate constants towards RBV (k RBV-3pHBA* = 8.60 x 10 8 M -1 s - 1 and k RBV-3SA* = 6.81 x 10 7 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. k obs-RBV increased as pH increased from 5.0 to 11.0 in UV/Sulfite/SA process, due to the high yield of e aq- in alkaline condition which promoted the generation of 3 SA* and the stable of the absorbance of SA at 254 nm. By contrast, k obs-RBV underwent a process of first increasing and then decreasing in UV/Sulfite/pHBA process as the increase of pH, and its highest value achieved in a neutral condition. This lied in the exposure of e aq- increased as the increase of pH which promoted the generation of 3 pHBA*, while the molar absorptivity of pHBA at 254 nm decreased as the increase of pH in an alkaline condition which inhibited the yield of 3 pHBA*. The RBV degradation pathways and products toxicity assessment indicated that UV/Sulfite/pHBA had better detoxification performance on RBV than UV/Sulfite/SA process. This study disclosed a novel mechanism of emerging contaminants abatement through non -radical process in NSOM mediated ARP, and provide a wide insight into positive profile of DOM in water treatment process, instead of only taking DOM as a quencher of reactive intermediates.