Molecular orbital-guided design of laccase–mediator systems for antibiotic removal in the presence of humic acid

Antibiotic residues, as emerging environmental contaminants, have raised significant concerns, underscoring the urgent need for efficient laccase–mediator systems (LMS) to facilitate their removal. However, the screening and design of high-performance mediators remain largely empirical, lacking robust theoretical guidance. In this study, we systematically evaluated the kinetic performance of Trametes versicolor laccase coupled with 19 redox mediators for sulfamethoxazole (SMX) degradation in the presence of humic acid (HA). Among the mediators tested, syringaldehyde (SA) exhibited superior degradation efficiency. By promoting laccase-catalyzed generation of phenoxyl radicals, the kinetics of antibiotic removal was improved. Toxicity assessments revealed that the degradation products of SMX in the SA-mediated LMS with HA exhibited low ecological risk. The system maintained high SMX degradation performance in various real water matrices (with a removal rate of up to 95 % and a reaction rate of up to 0.11 min −1 ), demonstrating its applicability in environmentally relevant scenarios. Furthermore, we established a structure–activity relationship between the electronic properties of natural mediators and SMX removal efficiency, demonstrating that catalytic performance is closely correlated with frontier molecular orbital energy levels and HOMO (Highest Occupied Molecular Orbital)–LUMO (Lowest Unoccupied Molecular Orbital) gaps. High-efficiency mediators typically possess higher HOMO energies and narrower energy gaps. This study offers molecular-level insights and design principles for developing efficient mediators, advancing the targeted application of LMS technology for the remediation of emerging contaminants.