Removal of zearalenone mycotoxin with kaolin group-based photocatalysts: exploration of mechanisms and photodegradation pathways

Water pollution arises not only from anthropogenic sources but also from natural contaminants, which can cause equally significant harm. Among these natural pollutants, mycotoxins – secondary metabolites produced by fungi – stand out due to their widespread occurrence and the serious health risks they pose to both humans and animals. Zearalenone (ZEN), one of the most prevalent mycotoxins, is known to induce oxidative stress, DNA and mitochondrial damage, apoptosis, and alterations in gene expression, emphasizing its toxicological relevance. To address ZEN contamination efficiently, economically, and without producing secondary pollutants, a UV-driven photodegradation approach was used.

Our research focused on the impregnation of the mineral supports with 20 wt% of semiconductors, including TiO₂, GCN, and their 1:1 mixture. The selected mineral supports included natural kaolinite, purified halloysite, and synthetic kaolinite nanotubes. The resulting materials were evaluated for their efficiency in removing ZEN under both UV and visible light. Their performance was further assessed in the presence of a co-occurring mycotoxin, deoxynivalenol (DON), as well as under varying pH levels and ionic strengths of the solution. Most importantly, extensive electrochemical studies were conducted to elucidate the mechanisms underlying their functionality, with particular attention to their photocatalytic properties.

The most effective materials – kaolinite nanotubes combined with GCN and TiO₂/GCN – achieved ZEN removal efficiencies of 98.8% and 97.7%, respectively, from an initial concentration of 10 ppm after just 25 min of irradiation. While the kinetics of ZEN removal under visible light were noticeably slower than under UV light, the results remain promising when compared to the literature. Experiments conducted under varying pH conditions highlighted the role of ZEN protonation in the removal process. The results revealed that DON was not removed under the tested conditions, and its presence slightly reduced the efficiency of ZEN degradation. Scavenger experiments, supported by electron paramagnetic resonance (EPR) with spin-trapping measurements, identified O₂^•- and •OH radicals as the key species involved in the photodegradation process. Time-resolved photoluminescence (TRPL) lifetime measurements demonstrated a prolonged carrier lifetime in materials containing kaolinite nanotubes and GCN. This finding is consistent with the chopped light voltammetry (CLV), which indicated the presence of traps within the structure of the photocatalyst. These results provide strong evidence for the beneficial role of mineral supports, particularly kaolinite nanotubes. Furthermore, electrochemical impedance spectroscopy (EIS) suggested enhanced mobility of photogenerated holes at the interface between the mineral support and GCN, further reinforcing the positive impact of the mineral supports.

The photodegradation pathways of ZEN, proposed based on identified radical formation and UHPLC-ESI-MS/MS analysis, involved a series of reactions, including hydrolysis and, most notably, oxidation and cleavage. These processes lead to the formation of several intermediate products with both lower and higher molecular masses compared to ZEN. Their chronic and acute toxicity was evaluated using dedicated ECOSAR software.

This project was supported by the National Science Centre Poland, under a research project awarded by Decision No. 2021/43/B/ST10/00868.