Rapid degradation of chloroquine derivatives in a novel advanced reduction process: Role of intramolecular hydrogen bond in the formation of excited triplet state compound

The role of triplet excited state compounds (TESC) in contaminants degradation by advanced reduction processes (ARPs) is rarely reported. In this study, Salicylic acid derivatives (SAD, e.g. Salicylic Acid (SA) and Sulfosalicylic acid (SSA)) were applied in ultraviolet activated sulfite process (UV/Sulfite) to generate TESC and enhance the decomposition of chloroquine derivatives (CQD, antiviral drugs being used for treating COVID-19 infection, e.g. chloroquine phosphate (CP)). The results indicated that the pseudo 1st order rate constant of CP (k(obs-CP)) increased by 2.76 and 6.26 fold, which increased from 9.10 x 10(-4) s(-1) in UV/Sulfite process to 2.51 x 10(-3) s(-1), 5.72 x 10(-3) s(-1) in UV/Sulfite/SA and UV/Sulfite/SSA process respectively. Similar phenomena were observed in other CQDs. Transient absorption spectra demonstrated that both triplet excited state SAD ((3)SAD*) and hydrated electron (e(aq)(-)) were the reactive intermediates in UV/Sulfite/SAD process, and contribute to the abatement of CP. The enhancement of SAD on contaminants degradation in UV/Sulfite/SAD process could attribute to the strong acceleration of e(aq)(-) toward the formation of (3)SAD* in its photosensitive process, where SAD with intramoleuclar hydrogen bond would not undergo a directly photoionization process after absorbing a photon but undergo intersystem crossing to transform into (3)SAD* which was then ionized to e(aq)(-) by biphotonic process, and constitute a hydrated electrons mediated photosensitization cycle with the supplement of e(aq)(-) from UV/Sulfite process. The higher CP degradation rate in UV/Sulfite/SSA process than that in UV/Sulfite/SA process was ascribed to a higher second order rate constant of (3)SSA* toward CP and a higher yield of (3)SSA*, which resulted from its higher intramolecular hydrogen bond energy enhancing photosensitization and photoionization.