Mechanistic insights into rhodamine B removal through alkali-activated Camellia sinensis biosorbent via multi-criterion error analysis

Rhodamine B (RhB)-bearing effluents are difficult to treat because the dye is water-soluble, photostable, and resistant to conventional processes, while spent tea leaves are abundantly generated lignocellulosic waste. This study converts Camellia sinensis waste into a 2.0 M NaOH-activated biosorbent (BT-STL) and establishes its RhB removal performance using experimentally grounded mechanistic modelling. Alkali activation produced an anionic mesoporous adsorbent (SBET = 28.86 m 2 g −1 , pore volume = 0.2464 cm 3 g −1 , average pore diameter = 36.67 nm, pHpzc = 5.0, ζ = −28.4 mV at pH 6.0), as verified by Fourier-transform infrared spectroscopy (FTIR), Brunauer–Emmett–Teller/Barrett–Joyner–Halenda/non-local density functional theory (BET/BJH/NLDFT), field-emission scanning electron microscopy/energy-dispersive X-ray spectroscopy (FE-SEM/EDS), X-ray diffraction (XRD), thermogravimetric analysis–differential scanning calorimetry (TGA–DSC), zeta potential, and point of zero charge (pHpzc) analyses. Under optimized conditions (pH 6.0, 50 mg/50 mL), BT-STL removed 98.0, 95.3, and 83.7% of RhB at 5, 10, and 20 mg L −1 , respectively. Non-linear kinetic fitting supported Bangham pore diffusion as the dominant rate-controlling step, while equilibrium adsorption followed the Temkin model (SSE = 16.66, χ 2 = 0.745, R 2 = 0.9879), with a Sips maximum capacity of 40.78 mg g −1 . Regeneration with 0.1 M NaOH/50% ethanol retained 76.4% capacity after 10 cycles, and > 79% RhB removal was maintained at 100 mg L −1 Ca 2+ , demonstrating practical stability under competitive-ion conditions. The novelty lies in integrating alkali-activated tea-waste valorization with non-linear, multi-criterion error-based kinetic/isotherm validation, regeneration assessment, and hard-water ion interference testing to deliver a reliable mechanistic framework for low-cost cationic dye remediation.