Comparative adsorption kinetics of malachite green using biochars from mulberry, Calotropis, castor, and tea leaves

Malachite green (MG), a toxic cationic dye pollutant, requires effective removal from wastewater. This study investigated biochars derived from four plant wastes, mulberry, Calotropis, castor, and tea leaves, as low-cost adsorbents for MG removal. Biochars were synthesised via slow pyrolysis at 400 °C and characterised by FTIR spectroscopy, SEM, N₂ physisorption (BET/BJH), XRD, Raman spectroscopy, XPS, and bulk CHNS analysis, revealing oxygen-functionalised turbostratic carbon matrices with distinct pore structures and mineral compositions that facilitate dye adsorption. Batch adsorption experiments were conducted at an initial MG concentration of 20 mg L^-1 with varying adsorbent dosages (2-100 mg) and contact times (10-70 min). A five-point calibration curve (4-20 mg L^-1, λ _max = 618 nm, and R ² = 0.9981) validated the analytical method. Tea biochar exhibited superior performance with the fastest kinetics, achieving 95% MG removal within 60 minutes and a maximum adsorption capacity of 90.57 mg g^-1 at 5 mg dosage, followed by mulberry (89.23 mg g^-1), castor (80.39 mg g^-1), and Calotropis (73.47 mg g^-1). Adsorption kinetics were best described by the pseudo-second-order model (R ² = 0.98-0.99), supported by favourable Elovich fits and multi-linear intraparticle diffusion behaviour, indicating that chemisorption-like surface processes coupled with pore diffusion dominate the rate-controlling steps, while pH-dependent uptake and spectroscopic changes (FTIR spectroscopy/XPS) corroborate the roles of electrostatic attraction, π-π interactions, hydrogen bonding and van der Waals pore-filling mechanisms.