Response surface modelling, equilibrium and kinetic evaluation of tetracycline adsorption by acid- and base-treated Tetrapleura tetraptera adsorbents

Tetracycline residues in aquatic environments remain a major concern because antibiotics are persistent, biologically active, and may contribute to antimicrobial resistance. This study prepared two modified adsorbents from Tetrapleura tetraptera fruit shells and evaluated their performance for tetracycline removal from aqueous solution. The powdered biomass was chemically modified with hydrochloric acid and sodium hydroxide to obtain acid-treated sample (ATS) and base-treated sample (BTS). The materials were characterized using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM), while batch adsorption behaviour was evaluated using response surface methodology with pH, adsorbent dosage, initial tetracycline concentration, temperature, and contact time as process variables. The RSM analysis identified initial tetracycline concentration as the dominant factor controlling removal efficiency for both adsorbents, while pH and selected quadratic terms also contributed to the adsorption response. Although the quadratic models were statistically significant, model-diagnostic outputs indicated that the RSM predictions should be interpreted primarily as design-space estimates rather than absolute endpoint predictions. Therefore, the adsorption behaviour was further supported using equilibrium, kinetic, and material characterisation results. Isotherm modelling showed favourable adsorption based on Langmuir separation factors below unity. BTS displayed the higher Langmuir maximum adsorption capacity (37.89 mg/g) than ATS (29.03 mg/g), whereas ATS showed stronger Temkin agreement, indicating stronger adsorbate–adsorbent interaction effects. Kinetic analysis showed that ATS followed pseudo-second-order behaviour, while BTS showed strong intraparticle-diffusion contribution together with pseudo-second-order interaction. Overall, the findings demonstrate that acid- and base-treated Tetrapleura tetraptera shell adsorbents have potential for tetracycline removal, with ATS showing a stronger percentage-removal response and BTS showing a higher equilibrium adsorption capacity.