Abstract

The circular valorization of industrial residues as functional materials offers a promising strategy to reduce the environmental footprint of water treatment technologies. In this study, raw coal fly ash (FA), applied without pretreatment or structural modification, was evaluated as a waste-derived photocatalyst for the degradation of metamizole, a widely detected contaminant of emerging concern. A systematic assessment under five irradiation sources (visible light, UV-A, UV-B, UV-C, and V-UV) demonstrated that genuine photocatalytic activation occurred exclusively under UV-B irradiation, achieving 96.2% removal after 150 min and outperforming direct UV-B photolysis by 14.9 percentage points. In contrast, UV-C and V-UV induced near-complete photolysis independently of catalyst presence, while visible and UV-A irradiation produced negligible degradation, consistent with the measured band gap of FA (3.1 eV). Kinetic and scavenger experiments indicated a mechanism mainly governed by valence-band holes (h+) and hydroxyl radicals (circle OH), with minor contribution from superoxide species (O2 circle-). The photocatalytic performance remained stable in the presence of common inorganic salts, confirming robustness under realistic water matrix conditions. HPLC-MS/MS analysis enabled the detection of fourteen potential transformation products and the proposal of a degradation pathway distinguishing photocatalytic activity from direct photolysis. Under optimized UV-B/FA conditions, COD and TOC removals reached 67.11% and 56.73%, respectively, while metal leaching remained minimal. Overall, unmodified coal fly ash behaves as an intrinsic semiconductor photocatalyst under UV-B irradiation and represents a promising lowcost material for sustainable and circular water treatment applications.