Covalent or ionic bonding of Eosin Y to silica: New visible-light photocatalysts for redox wastewater remediation

Eosin Y (EY) is an organic dye widely used as a photoredox catalyst in organic synthesis, but there are only few studies on its use in heterogeneous photocatalysis for the degradation of aqueous Contaminants of Emerging Concern (CECs). In this context, several synthetic strategies were used to prepare heterogeneous silica-based photocatalysts where EY is anchored to glass wool (GW) covalently or using ionic bonds for obtaining GW-C-EY and GW-I-EY, respectively. The percentage of EY linked to GW-C-EY (up to 0.0034% w/w) was always lower than the obtained in GW-I-EY (0.165% w/w). Photodegradation of CECs such as noscapine (NOS), sulfamethoxazole (SMX) and ofloxacin (OFX) was studied using the new heterogeneous photocatalysts in water under visible light irradiation and results showed that only NOS is photodegraded. It was also observed that EY photoreactivity in GW-C-EY and GW-I-EY is quite similar. Interestingly, when the photocatalysts were studied using mixtures of the three CECs, a synergistic effect that favors the photodegradation of SMX and OFX was observed. Specifically, GW-I-EY achieves 100% photodegradation for NOS and ca. 50% removal of SMX and OFX. Based on photophysical and photochemical experiments, redox processes initiated by an electron transfer from NOS to the EY triplet excited state and the subsequent reaction of the generated EY•− with SMX or OFX justify the observed synergistic photodegradation. This finding opens up the possibility of working with complex aqueous matrices and also of smart selecting the combination of different types of contaminated waters to favor the efficiency of the photocatalytic decontamination processes.