Equilibrium, Kinetic and Thermodynamic Studies of Biosorption of Methylene Blue on Goethite Modified Baobab Fruit Pod (Adansonia Digitata L.)

Abstract

Methylene Blue (MB) was adsorbed from aqueous solution using Baobab (Adansonia digitata L.) fruit pod and its goethite modified form. Adsorbents were characterized using Fourier Transform-Infra Red (FTIR) spectroscopy and Scanning Electron Microscopy (SEM). Batch experiments were conducted at room temperature (26.8 °C) and the adsorption data were fitted using Langmuir, Freundlich, Temkin and Dubinin- Radushkevich isotherms. Also, kinetic data was fitted using Pseudo-first order, pseudo-second order, Elovich and intra-particle diffusion models. Goethite modified baobab (GMB) appeared to have a coarse microporous surface with smoother surface and larger pore volumes compared to unmodified baobab (UB). The –C=O band was observed at 1631 and 1636 cm^-1 for UB and GMB. The –OH band was observed at 3447.00 cm^-1 and 3442 cm^-1 for UB and
GMB respectively. Langmuir model was suitable for describing the adsorption data of UB with R² of 0.9293 while Temkin model was best for fitting adsorption data of MB on GMB with R² of 0.9691. However, maximum adsorption capacity was obtained with Freundlich adsorption isotherm (15.4253 and 43.1301 mg/g for UB and GMB respectively). The maximum biosorption were 8.98 mg/g and 9.86 mg/g for UB and GMB respectively at pH 10. Pseudo-second-order kinetic model best fitted the kinetic data with R² values of 0.9968 and 0.9993 for UB and GMB, ΔH^o values were 83.123 KJ/mol and 361.094 KJ/mol for UB and GMB, while ΔS^o values were 3.084 J/mol/ K and 1.765 J/mol/K for UB and GMB respectively. GMB adsorbed more of MB than UB and the process was endothermic.

Keywords: Biosorption, Goethite, Baobab, Isotherms, Methylene blue.