Upcycling medical waste into a magnetic adsorbent for amoxicillin removal: a circular economy approach

Abstract Pharmaceutical pollution, particularly contamination by antibiotics such as amoxicillin (AMX), represents an increasing threat to aquatic ecosystems. Conventional wastewater treatment technologies often exhibit limited removal efficiency and may involve high operational costs. In this study, a waste-to-resource strategy was developed by converting discarded Thermacare Heat Patches (THPs), a common form of medical waste, into a magnetic carbon-based adsorbent for AMX removal from water. The novelty of this work lies in valorizing an underutilized medical waste into a reusable adsorbent, thereby addressing both medical waste management and water remediation challenges. The THP-derived material was characterized using FESEM, EDS, XRD, ATR-FTIR, VSM, BET, and zeta potential analyses. The material exhibited a mixed micro/mesoporous structure, with a specific surface area of 60.34 m²/g, an average pore diameter of 3.36 nm, superparamagnetic behavior with a saturation magnetization of 27.232 emu/g, and a point of zero charge at pH 5.6. Batch adsorption experiments were performed to evaluate the effects of contact time, solution pH, adsorbent dosage, and initial AMX concentration. Under optimum conditions of pH 5, 50 min contact time, and 2 g/L adsorbent dosage, the THP-derived adsorbent achieved 99.6% AMX removal from a 50 mg/L solution, with a maximum adsorption capacity of 24.9 mg/g. Kinetic analysis indicated that the pseudo-first-order model provided the best fit for the adsorption process, with an R² value of 0.97 as it exhibited a lower residual sum of squares (RSS = 34.50) and Akaike information criterion value (AIC = 22.89) than the PSO model (RSS = 41.70; AIC = 24.60). The equilibrium data were best described by the Freundlich and Temkin isotherm models, while the Dubinin–Radushkevich adsorption energy of 1.85 kJ/mol supporting a possible contribution from physical adsorption, although electrostatic interactions, hydrogen bonding, and other surface interactions may also contribute. Thermodynamic analysis confirmed that the adsorption process was spontaneous and endothermic (ΔH° = 42.38 kJ/mol), while the positive entropy change (ΔS° = 238 J/mol·K) indicated increased randomness at the solid–solution interface during adsorption. The adsorbent also demonstrated good reusability, maintaining 87% removal efficiency after five regeneration cycles. These findings demonstrate that discarded THPs can be successfully upcycled into a sustainable, reusable, and cost-effective magnetic adsorbent for removing antibiotics from contaminated water, thereby supporting circular economy principles and environmental remediation.