New theoretical investigation of mechanism, kinetics, and toxicity in the degradation of dimetridazole and ornidazole by hydroxyl radicals in aqueous phase

Dimetridazole (DMZ) and ornidazole (ONZ) have been widely used to treat anaerobic and protozoal infections. The residues of DMZ/ONZ persist in the water environment. The mechanisms and kinetics of hydroxyl-initiated oxidation, the primary DMZ/ONZ degradation method, were evaluated by quantum chemical methods.·OH-induced degradation of DMZ and ONZ shared many mechanistic and kinetic characteristics. The most feasible degradation pathway involved forming OH-imidazole adducts and NO2. The OH-imidazole adducts were subsequently degraded into double·OH imidazole intermediates. The rate coefficients for·OH degradation of DMZ and ONZ were 4.32 × 109 M−1 s−1 and 4.42 × 109 M−1 s−1 at 298 K, respectively. The lifetimes of DMZ and ONZ treated with·OH at concentrations of 10−9–10−18 mol L−1 at 298 K were τDMZ = 0.231–2.31 × 108 s and τONZ = 0.226–2.26 × 108 s, respectively. Toxicity assessment showed that the first degradation products of DMZ and ONZ exhibited enhanced aquatic toxicity, whereas most of the secondary degradation products were not harmful to aquatic organisms. Some of transformation products were still developmental toxicant or mutagenicity positive.