Wavelength Dependence of the Transformation Mechanism of Sulfonamides Using Different LED Light Sources and TiO2 and ZnO Photocatalysts

The comparison of the efficiency of the commercially available photocatalysts, TiO₂ and ZnO, irradiated with 365 nm and 398 nm light, is presented for the removal of two antibiotics, sulfamethazine (SMT) and sulfamethoxypyridazine (SMP). The ^•OH formation rate was compared using coumarin, and higher efficiency was proved for TiO₂ than ZnO, while for 1,4-benzoquinone in O₂-free suspensions, the higher contribution of the photogenerated electrons to the conversion was observed for ZnO than TiO₂, especially at 398 nm irradiation. An extremely fast transformation and high quantum yield of SMP in the TiO₂/LED_398nm process were observed. The transformation was fast in both O₂ containing and O₂-free suspensions and takes place via desulfonation, while in other cases, mainly hydroxylated products form. The effect of reaction parameters (methanol, dissolved O₂ content, HCO₃⁻ and Cl⁻) confirmed that a quite rarely observed energy transfer between the excited state P25 and SMP might be responsible for this unique behavior. In our opinion, these results highlight that “non-conventional” mechanisms could occur even in the case of the well-known TiO₂ photocatalyst, and the effect of wavelength is also worth investigating.