Microfibers influence on imidacloprid photodegradation: Contrasting mechanisms in UV and H2O2/UV systems

Microplastic and micropollutant co-contamination poses a growing environmental challenge, yet their interactions in advanced oxidation processes (AOPs) remain underexplored. This study investigated the influence of natural (cotton) and synthetic (polyester, polyamide) microfibers (MFs), prepared by microcutting (MC) and cryomilling (CRM), on the degradation of imidacloprid (IMI) in UV photolysis and H₂O₂/UV systems. Experiments were conducted at equal initial concentrations (10 ppm) to evaluate kinetic changes and toxicity evolution. The results revealed contrasting mechanisms depending on the process. In direct UV photolysis, the presence of MFs inhibited IMI degradation (efficiency dropped from 73% to 60–70%), primarily due to the light shielding effect and UV scattering by suspended fibers, rather than adsorption, which was found to be negligible. Conversely, in the H₂O₂/UV process, MFs enhanced degradation efficiency (reaching 79–89% compared to 79% for IMI alone). Notably, polyester fibers prepared by microcutting (PE_MC) significantly accelerated the reaction kinetics (k obs =0.447 ± 0.016 min⁻¹) compared to the control (0.363 ± 0.014 min⁻¹), suggesting a surface-mediated promotion of radical pathways. Vibrio fischeri toxicity tests showed that H₂O₂/UV reduced toxicity more effectively than UV alone, while MF-dependent TP formation (e.g., desnitro-imidacloprid) influenced biological responses; accordingly, UV alone did not inhibit Synechocystis sp. or Chlorella sp. , whereas IMIalone and with PE/Cott_CRM after H₂O₂/UV caused sustained inhibition in both organisms.These findings highlight the dual role of MFs acting as optical barriers in photolysis while potentially serving as catalysts in radical-driven oxidation and underscore the importance of material type and preparation method in assessing environmental risks. • MFs prepared by MC showed a more pronounced effect on kinetics than CRM MFs. • In H 2 O 2 /UV, MFs enhanced IMI degradation (79-89%), showing a radical-driven effect. • PE_MC significantly accelerated IMI degradation in H 2 O 2 /UV (0.447 ± 0.016 min -1 ).