Techno-Economic Evaluation of Doxycycline Recovery from Aqueous Media Using Agricultural Waste-Derived Biochar: Adsorption Performance and Regeneration Potential

Environmental contamination by pharmaceutical residues has become a major environmental and public health concern because of their persistence, toxicity, and resistance to conventional wastewater treatment processes. In this study, biochar produced from mandarin peels was prepared, characterized, and evaluated as a sustainable, low-cost adsorbent for the removal of doxycycline from aqueous solutions.

Adsorption kinetics were investigated at an initial doxycycline concentration of 50 mg/L. The adsorption process exhibited a rapid uptake during the first 60 min, followed by a slower stage until equilibrium was reached after approximately 180 min, with an equilibrium adsorption capacity of 44.8 mg/g. The pseudo-second-order kinetic model (R² = 0.999) provided a better fit to the experimental data than the pseudo-first-order model (R² = 0.987), indicating that doxycycline adsorption is better described by pseudo-second-order kinetics.

Equilibrium adsorption studies revealed an L-type adsorption isotherm, indicating a strong affinity of doxycycline molecules toward the biochar surface and progressive occupation of adsorption sites with increasing equilibrium concentration. Among the equilibrium models evaluated, the Dubinin–Radushkevich model provided the best fit to the experimental data (R² = 0.940), followed by the Langmuir (R² = 0.895), Temkin (R² = 0.878), and Freundlich (R² = 0.824) models. The Langmuir model predicted a theoretical monolayer adsorption capacity of 306.0 mg/g, while the maximum experimental adsorption capacity reached approximately 272 mg/g under the investigated conditions.

These findings demonstrate that mandarin peel biochar is an effective, renewable, and environmentally friendly adsorbent for doxycycline removal from wastewater. The valorization of mandarin peel waste into high-performance biochar provides a sustainable strategy for mitigating emerging pharmaceutical pollutants while supporting circular economy principles and green wastewater treatment technologies.