Innovative light-driven chemical/catalytic reactors towards contaminants of emerging concern mitigation: A review

Urban wastewater treatment plants (WWTPs) are among the major sources of contaminants of emerging concern (CECs) release to surface water. This is mainly associated with the low performance of conventional technologies (e.g. activated sludge, sand filtration and UV disinfection processes) for the removal of residual amounts (ng L−1–μg L−1) of CECs. In this review, a list of CECs commonly detected in the aquatic environments is presented. Light-driven chemical/catalytic processes for CECs removal, namely UV/oxidant and photocatalysis, are presented with discussion of their process, advantages, drawbacks, and the main attainments. In this context, process intensification has been gaining high relevance for wastewater treatment purposes due to its potential to overcome the main treatment limitations. Accordingly, an overview of advances to overcome the limitations in light-driven chemical/catalytic processes for CECs mitigation was carried out, focusing mainly on the design of novel reactors and devices. Some aspects related to reactor configuration, removal mechanism, devices applied, illumination system, comparison criteria, and catalyst immobilization through implementation of novel reactors designs are also discussed. Regarding the reactor design, several devices exhibit a satisfactory mass transfer due to a large surface-to-volume ratio or enhanced mixing conditions. However, improvements in photon transfer are still the biggest challenge to be overcome. The irradiation of the entire reaction solution and/or catalyst surface has shown to be challenging in many reactor configurations. Nevertheless, the reactor design and treatment process to be applied for CECs removal must be selected according to the site-specific conditions, in order to accomplish CECs mitigation and attain the best reactor performance.