Organic photoredox catalysts for wastewater remediation: Beyond the established advanced oxidation processes

Contaminants of emerging concern (CECs) constitute a wide class of organic molecules, which are found dissolved in wastewaters. Chemical properties of CECs are as wide as their chemical structures; nevertheless, most of the CECs can be removed from wastewaters using Advanced Oxidation Processes (AOPs). However, there are examples of CECs that have proven recalcitrant to AOPs. In such cases, Advanced Reduction Processes (ARPs) include those techniques able to achieve degradation of contaminants by producing highly reactive reducing radicals.Organic photocatalysts have demonstrated to be an efficient alternative to other types of photocatalysts in AOPs. Moreover, several organic photocatalysts, according to their redox properties, could also act as reductants under appropriate scenarios, a process that has been much less explored than oxidation. As an example, Riboflavin (RF) is a natural water-soluble molecule, well-known as vitamin B2. The photophysical and redox properties of RF have been extensively applied for the oxidative photodegradation of contaminants in water. More recently, the potential of RF has been expanded to the photoreduction of more recalcitrant pollutants. For instance, benzotriazoles UV-stabilizers (BUVSs) constitute a vast family of recalcitrant contaminants, which have been successfully degraded in the presence of a commercially available derivative of RF (2′,3′,4′,5′-tetraacetylated riboflavin, RFTA) and visible light, assisted by an amine, which acts as electron donor to the excited states of RFTA, generating the RFTA.−. This species ultimately produces the reduction of the selected BUVSs. Photophysical experiments demonstrated the key role of the radical anion RFTA.−, and allowed determining the rate constants that were correlated to the observed photodegradations.