Comparative isotherm and adsorption analysis of activated and non-activated tamarind wood-derived coke for near complete methyl orange removal

In the present work, Activated Tamarind Coke (ATC) was prepared by chemical activation of finely ground Tamarind Coke (TC) using nitric acid (25%). Both TC and ATC were employed as adsorbents for the removal of methyl orange (MO) dye from aqueous media. Adsorption efficiency, evaluated through UV-vis spectroscopy, was found to be 72% for TC which increased significantly to 99.3% for ATC. Structural characterisation using BET and BJH methods revealed a notable decrease in surface area for activated tamarind coke (ATC, 2.1 m²/g) compared to its non-activated counterpart (TC, 19.2 m²/g). Interestingly, this reduction contrasts with the significantly enhanced adsorption efficiency (99.3%) observed for ATC, suggesting that factors beyond surface area such as surface chemistry or pore structure play a critical role in dye removal performance. Despite the reduced surface area, the activation process led to a substantial expansion in pore structure, as evidenced by the increase in median pore diameter from 10 nm (TC) to 80 nm (ATC). The average pore diameter also increased from 6.2 nm to 8.1 nm, suggesting that enlarged mesopores in ATC facilitate improved dye molecule diffusion and adsorption. This structural modification likely underpins the observed high removal efficiency (99.3%). Batch adsorption data was fitted using Langmuir and Freundlich isotherm models. For ATC, the Langmuir model exhibited superior fit (R² >0.99) and a higher Langmuir constant ( $$\:{K}_{L}$$ ), indicating favorable monolayer adsorption. Freundlich parameters, including elevated $$\:{K}_{f}$$ values, further supported the enhanced adsorption capacity of ATC, while the lower ‘n’ values imply a more complex mechanism, potentially involving multilayer adsorption and pore-filling effects.