Optimisation of surface modification of industrial recycled carbon fibres for the adsorption of antibiotics from aquaculture

This study investigates the use of recycled carbon fibers as precursors for activated adsorbents and applies nitric acid modification to enhance their selectivity toward two antibiotics: ciprofloxacin and oxytetracycline. A design of experiments was employed to optimize key modification parameters: acid concentration (1 – 6 M), hold time (16 – 24 h), and temperature (20 – 60°C), and their influence on adsorption. The modified adsorbents were characterized using N2 adsorption isotherms, Boehm titrations, and potentiometric titrations to determine porosity, acidity/basicity, and point of zero charge. Optimal conditions were identified as 1 M HNO3, 16 h, and 28°C. Under these conditions, modified adsorbents achieved adsorption capacities of 9.63 mg/g for ciprofloxacin and 7.72 mg/g for oxytetracycline. While ciprofloxacin removal decreased by 7% compared with unmodified counterparts, this reduction was not statistically significant. By contrast, oxytetracycline removal improved by 21%, a statistically significant increase. The superior uptake of oxytetracycline was attributed to its greater hydrophilicity and enhanced affinity for the oxidized, hydrophilic surface. Overall, acidic surface modification represents a promising strategy to improve the performance of recycled carbon fiber-derived adsorbents, particularly for tetracycline antibiotics. Further investigation is required to assess whether oxidative treatment methods can improve the removal of fluoroquinolones.