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

The comparison of the efficiency of the commercially available photocatalysts, TiO 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 TiO than ZnO, while for 1,4-benzoquinone in O-free suspensions, the higher contribution of the photogenerated electrons to the conversion was observed for ZnO than TiO, especially at 398 nm irradiation. An extremely fast transformation and high quantum yield of SMP in the TiO/LED process were observed. The transformation was fast in both O containing and O-free suspensions and takes place via desulfonation, while in other cases, mainly hydroxylated products form. The effect of reaction parameters (methanol, dissolved O content, HCO 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 TiO photocatalyst, and the effect of wavelength is also worth investigating.