Comparative tetracycline removal by Acacia mearnsii-derived carbons and a cellulose-derived iron oxide/oxyhydroxide...

Tetracycline (TC) contamination of aquatic systems poses severe ecological and public health challenges. This study compares four bio-derived carbon adsorbents utilizing targeted iron-functionalization strategies to optimize antibiotic decontamination: (A) thermally modified Acacia mearnsii bark extract, (B) A. mearnsii carbon loaded with 25% Fe(OH)₃ (acidic precipitation), (C) A. mearnsii carbon loaded with 25% Fe(OH)₃ (NH₄OH-adjusted), and (D) a delignified cellulose-derived carbon-fibre composite bearing surface iron oxide/oxyhydroxide phases. Nitrogen physisorption revealed a transition from mesoporous architectures (Type IV(a), Sample A) to highly microporous frameworks (Type I(a), Sample D), with t-plot micropore volumes increasing from 0.032 to 0.102 cm³ g^-1. High-resolution Fe 2p XPS confirmed that the surface iron species are predominantly Fe(iii) (hydr)oxides. Non-linear kinetic modelling of the 20-120 min uptake data showed that the pseudo-first-order model provided the best statistical description for all samples (R ² = 0.950-0.995). The fitted equilibrium capacities ranged from 6.57 to 9.48 mg g^-1, remaining within the 10 mg g^-1 mass-balance limit imposed by the kinetic conditions (C ₀ = 10 mg L^-1; dosage = 1.0 g L^-1). Intraparticle-diffusion analysis indicated that diffusion contributed to TC uptake but was not the sole rate-controlling process. Under optimized batch parameters (1.0 g L^-1, C ₀ = 10 mg L^-1, 25 °C), maximum TC removal efficiencies were 92.7% at pH 7.0 (A), 88.7% at pH 5.0 (B), 84.6% at pH 5.0 (C), and 90.0% at pH 7.0 (D). Equilibrium data were best described by the Freundlich model for Sample A (R ² = 0.997) and by the Langmuir model for Samples B-D. The fitted Langmuir capacities of 20.3-20.8 mg g^-1 were obtained from the isotherm dataset collected at initial TC concentrations of 10-40 mg L^-1 and should not be interpreted as capacities measured under the separate fixed-concentration kinetic condition of C ₀ = 10 mg L^-1. Sample D achieved 77.5 ± 0.5% TC removal efficiency in the fourth regeneration cycle, corresponding to 86.5% relative retention of its first-cycle removal efficiency. This four-cycle result provides preliminary laboratory-scale evidence of reusability under the tested conditions.