Optimized composition of banana peel-derived activated carbon modified with aluminum oxide and chitosan for enhanced adsorption of Pb(II), Cu(II), and Cd(II) in wastewater treatment

This study highlights the importance of utilizing banana peel, an abundant and low-cost agricultural waste, as a sustainable precursor material for producing activated carbon for removing heavy metals in wastewater treatment. Adsorption experiments were conducted with single-metal (Pb²⁺) and mixed-metal (Pb²⁺, Cd²⁺, and Cu²⁺) solutions to evaluate the effects of synthetic sequences, weight ratios, and competitive adsorption. The results demonstrated that the activation sequence played a critical role in adsorption performance, with banana peel-derived activated carbon (BPAC) activated with ZnCl₂ after pyrolysis (BPAC(I)) achieving around 5% higher Pb²⁺ adsorption compared to activation before pyrolysis (BPAC(II)). BPAC(I) was modified with Al₂O₃ and chitosan to significantly enhance its adsorption capacity for heavy metal ions. Composite adsorbents with varying weight ratios of BPAC, Al₂O₃ and chitosan (4:2:1 (denoted as 421), 3:2:1 (321), 3:1:2 (312) and 2:1:1 (211)) were synthesized and evaluated. Among all samples, 321 showed the highest adsorption performance, with a maximum adsorption capacity of 39.0 mg/g, reaching almost 100% Pb²⁺ removal after 24 h, suggesting that the increased weight ratio of hydrophilic Al₂O₃ with abundant surface –OH groups enhances the adsorption amount of Pb²⁺. A similar trend was also observed for other heavy metals in mixed solutions, with the adsorption percentage Pb²⁺, Cu²⁺ and Cd²⁺ were 85.1%, 88.9% and 26.5%, respectively. The effects of different experimental parameters (including adsorbent mass, pH level, and initial concentration of the solution) on the adsorption of Pb²⁺ ions were studied. The adsorption isotherms revealed that BPAC(I) and 312 fitted both the Langmuir and Freundlich isotherm models, with the latter providing a slightly better fit, suggesting heterogeneous surface adsorption. Regeneration tests found that the adsorption capacity of the adsorbent could be reduced to approximately one-third with four repeated adsorption–desorption cycles owing to irreversible adsorption and detachment of surface modifiers.