Activated carbon derived from Spathodea campanulata pods via H₃PO₄activation and low-temperature carbonization for efficient Rhodamine B removal: Mechanistic insights and spiking study

This study presents the synthesis and application of activated carbon (AC) derived from Spathodea campanulata seed pods, an abundant and underutilized lignocellulosic biomass, for the efficient removal of Rhodamine B (RhB) dye from aqueous systems. The AC was synthesized through H₃PO₄assisted chemical activation and low-temperature carbonization at 400 °C for 2 h, yielding a mesoporous material with an average pore size of 3.2 nm and a high Brunauer–Emmett–Teller (BET) surface area of 2854.27 m²/g. Field Emission Scanning Electron Microscopy (FESEM) revealed a heterogeneous and porous surface morphology, which appeared smoother and more compact after RhB adsorption, indicating partial pore blockage. Fourier Transform Infrared Spectroscopy (FTIR) spectra showed characteristic shifts in CH, –OH, CO and CC stretching vibration, emphasizing the contribution of van der Waals force, hydrogen bonding and π–π stacking interaction in RhB adsorption. X-ray Photoelectron Spectroscopy (XPS) indicated shifts in C1s, O1s, and P2p binding energies, supporting electronic interactions with RhB. Under optimized batch conditions (pH 4.0, 0.20 g/L, 40 mg/L RhB, 303 K), the adsorbent achieved a maximum capacity of 165.28 mg/g (Langmuir R² = 0.995). Adsorption followed a pseudo-second-order kinetic model (R² = 0.978), indicating chemisorption. Thermodynamic analysis confirmed an endothermic (ΔH^ο = 51.49 kJ/mol), spontaneous, and entropy-driven (ΔS^ο = 191.99 J/mol K) process due to water displacement and surface reorganization. Among seven tested water matrices, the highest removal (79.21 %) occurred in industrial groundwater. The material retained 78 % capacity after six methanol-based regeneration cycles. These results confirm the practical utility of SPAC as a cost-effective, sustainable, and high-efficiency adsorbent for dye remediation in real water systems.