Sustainable Adsorption of Antibiotics in Water: The Role of Biochar from Leather Tannery Waste and Sargassum Algae in Removing Ciprofloxacin and Sulfamethoxazole

A comparative study on the adsorption of ciprofloxacin (CIP) and sulfamethoxazole (SMX) onto CO 2 -activated biochars derived from leather tannery waste (ABT) and Sargassum brown macroalgae (ABS) is presented. N 2 physisorption revealed that ABS possesses a higher Langmuir surface area (1305 m 2 /g) and a hierarchical micro–mesoporous structure, whereas ABT exhibits a lower surface area (412 m 2 /g) and a predominantly microporous texture. CHNS and FTIR analyses confirmed the presence of N-, O-, and S-containing heteroatoms and functional groups on both adsorbents, enhancing surface reactivity. Adsorption isotherms fitted well to the Langmuir model, with ABS showing superior maximum capacities of 256.41 mg/g (CIP) and 256.46 mg/g (SMX) compared to ABT (210.13 and 213.00 mg/g, respectively). Kinetic data followed a pseudo-second-order model (R 2 > 0.998), with ABS exhibiting faster uptake due to its mesoporosity. Over eight reuse cycles, ABS retained >75% removal efficiency for both antibiotics, while ABT declined to 60–70%. pH-dependent adsorption behavior was governed by the point of zero charge (pH PZC ≈ 9.0 for ABT; ≈7.2 for ABS), influencing electrostatic and non-electrostatic interactions. These findings demonstrate that ABS is a highly effective, sustainable adsorbent for antibiotic removal in water treatment applications.