Equilibrium, Kinetic and Thermodynamic Studies of Biosorption of Methylene Blue on Goethite Modified Baobab Fruit Pod (<i>Adansonia Digitata</i> 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.