Abstract

This work presents an electrochemical sensor designed as a process-monitoring tool for tracking sulfamethoxazole (SMX) during advanced oxidation processes (AOPs) under pilot-scale, realistic wastewater conditions. The sensor is based on eco-friendly aqueous inks combining carbon nanomaterials with nanostructured biopolymers, specifically carbon nanofibers with cellulose nanocrystals (CNF/CNC) or multi-walled carbon nanotubes with chitin nanocrystals (CNT/ChNC), which were deposited onto glassy carbon electrodes (GCEs) to enhance the electrochemical response toward SMX. Among the tested configurations, the CNF/CNC-based sensor exhibited the best performance for SMX monitoring in the mg L-1 concentration range, combining a wide linear response and a detection limit of 0.17 mg L-1 with robust, reproducible behavior. Sensor calibration and performance were evaluated in both ultrapure water and synthetic hospital wastewater, highlighting the impact of matrix effects while confirming reliable operation under complex conditions. Crucially, the sensor was validated during the monitoring of SMX degradation in a pilot-scale solar photo-Fenton process operated at circumneutral pH using Fe3+-EDDS as a catalyst, with electrochemical measurements showing excellent agreement with UHPLC-DAD reference analyses (Pearson's r > 0.99). Rather than targeting ultra-trace detection, this study demonstrates the potential of electrochemical sensing as a rapid, cost-effective, and near-real-time tool for process monitoring and control in high-load effluents, such as hospital and pharmaceutical wastewater. These results bridge the gap between laboratory-scale sensor development and operational wastewater treatment applications, highlighting the relevance of sustainable nanocarbon-biopolymer inks for real-world environmental monitoring.