Insights into the degradation of diphenhydramine – An emerging SARS-CoV-2 medicine by UV/Sulfite

As Diphenhydramine (DPH) has been considered as a drug to treat SARS-CoV-2, the degradation of DPH from water was investigated and evaluated in this study by adopting an advanced oxidation/advanced reduction process – the UV/sulfite process. The UV/sulfite system was able to eliminate DPH within 6 mins under UV_254nm and 1.0 mM sulfite. It was observed that the presence of NO₃^-, NO₂^-, Cl^-, HCO₃^-, and SO₄^2- anions in water can affect the performance of UV/Sulfite degradation system. The mechanism of UV/sulfite/anions was evaluated which the presence of NO₃^- in UV/sulfite process has revealed faster initial decay rate but lower final DPH removal. It was observed that the UV/Sulfite process was extremely sensitive to pH as the dissociation of ion species varied among pH. The reaction became sluggish in acidic solution due to the dissociation of less reactive species such as HSO₃^-. In alkaline solution, SO₃^2- was the dominant species, producing powerful SO₃^∙- and e_aq^- when activated by UV at 254 nm. By conducting LC/MS analysis, the degradation pathway was proposed and can be summarized into four main pathways: hydroxylation, side chain cleavage, losing aromatic ring or ring opening. Scavenging tests were also carried out and validated the presence of various radicals contributing to the reaction, including e_aq^-, H^̇, OH^̇, SO₃^̇-, O₂^•- and SO₄^̇-.