Beyond Pollutant Removal: Solar-Driven Photo-Oxidation Pathways for Circular and Intelligent Wastewater Treatment

Although the photo-oxidation process is often portrayed as a technology for pollutants destruction,yet the future of it relies on its ability to remove pollutants from real wastewater, in a process that requires minimal external energy, is capable of recovering useful products, maintains controlled toxicity levels, and has measurable environmental benefits. This review critically assesses photolysis routes from photon absorption to the generation of reactive-oxygen-species, as well as from the reactor to the post-treatment, recovery of the resources and intelligent control. It discusses semiconductor photocatalysis, photo-Fenton chemistry, photoelectrocatalysis, heterogeneous oxidants, and hybrid solar–biological trains and the difference between removal, transformation, mineralization, detoxification and circularity. Special emphasis is placed on the occurrence of matrix effects, catalyst deactivation, photon utilization, mass transfer, energy demand, secondary pollution and reliability of reported kinetic parameters. The review reveals significant differences between well optimized laboratory experiments and working systems: most studies are conducted with synthetic water, short batch tests, single contaminant solutions, incomplete mass balances, and inconsistent energy accounting. The proposed circular-intelligence framework involves: (1) solar photo-oxidation, (2) online sensing, (3) mechanistic-datadriven modeling, (4) predictive control, (5) catalyst recovery, (6) product quality checking and (7) life cycle assessment. The review finds that future progress will neither be made by achieving maximum removal percentages nor will it be made by achieving a process that is easy to execute in the lab, but difficult or impossible in practice.