Valorisation of <i>Ceratonia siliqua</i> leaves as an efficient green biosorbent for dye removal

Water pollution by synthetic dyes such as methylene blue (MB) poses serious risks to aquatic ecosystems and human health, making the development of cost-effective and eco-friendly adsorbents essential. In this study, Ceratonia siliqua leaves (CSL), an abundant agricultural by-product, were valorised as a sustainable biosorbent for MB removal from aqueous solutions. CSL material was characterised using Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction, and thermogravimetric/differential thermal analysis to elucidate its physicochemical and structural features. Batch adsorption experiments were conducted under varying pH levels (2–12), adsorbent doses (2–100 mg), contact times (2–60 min), dye concentrations (20–1000 mg/l), and temperatures (10°C–50°C) to determine the optimal conditions. Kinetic studies revealed that the pseudo-second-order model provided the best fit (R2 = 0.9992), while equilibrium data were described by both Langmuir (qmax = 1891.81 mg/g, R2 = 0.98) and Freundlich (R2 = 0.99) models. However, the superior correlation of Freundlich indicates that adsorption mainly occurs on a heterogeneous surface with sites of varying affinities, consistent with the lignocellulosic structure of CSL. Thermodynamic analysis confirmed that the adsorption was spontaneous and exothermic. Overall, CSL demonstrated excellent adsorption performance, highlighting its potential as a cheap, renewable, and sustainable adsorbent for large-scale wastewater treatment applications.