Sulfite-synergized VUV photolytic decomposition of metformin: A combined experimental and theoretical study

Metformin (MET), widely used in the treatment of type П diabetes, may pose a threat to environmental security when extensively used and improperly disposed of. This study aimed to provide a comprehensive and deeper understanding of MET decomposition under UV₁₈₅ (vacuum ultraviolet, VUV) photolytic treatment in the presence of sulfite (SO₃²⁻). A noticeable MET decomposition efficiency of 98.3 % was observed at pH 9, accompanied by a fast pseudo-first-order rate constant of 0.0459 min⁻¹ in the VUV/SO₃²⁻ treatment system. Radical scavenging experiments and electron paramagnetic resonance analysis confirmed that MET decomposition was primarily initiated by the attack of hydrated electron (e_aq⁻) generated from SO₃²⁻ under VUV irradiation. Time-of-flight mass spectrometry results, supported by density functional theory and wave function analysis, indicated four parallel pathways for the reductive decomposition of MET in the VUV/SO₃²⁻ treatment system. Demethylation and scission of C[sbnd]N bonds within the guanidine moiety were identified as the primary mechanisms underlying MET decomposition in this system. A substantial reduction in MET biotoxicity was observed following treatment using the VUV/SO₃²⁻ reduction system. The effects of treatment parameters and common water matrix components were meticulously investigated.