Kinetics and Thermodynamics of Ternary Dye System Adsorption on to Melon (Citrillus lanatus) Seed Husk

Abstract

The adsorption characteristics of ternary dye system (TDS), comprising of a mixture of congo red, methyl orange and methylene blue, on formaldehyde-treated melon husk (FMH) were investigated with respect to the changes in initial pH of TDS, contact time, and initial TDS concentration. The surface of FMH was studied by investigation using  Fourier Transform infrared (FTIR) and Scanning Electron Microscopy (SEM). The adsorption equilibrium of TDS was rapidly reached after 45 minutes of contact time and pH of 8.5. The equilibrium adsorption data were interpreted using the following isotherm models Langmuir, Freundlich, Temkin, Dubinin-Radushkevich (D-R), Harkin-Jura and Halsey and the goodness of fittings were monitored using linear regression analysis (R2), sum-of- square-error (SSE) and chi-square (χ2). The results indicated that the isotherm could be interpreted by Freundlich model as it was the best fitted of all the models used, (R2 = 0.998) suggesting multilayer adsorption. The kinetics of the adsorption process were analyzed using three kinetic models; pseudo first-order, pseudo second-order and intraparticle diffusion models and the adsorption data followed the pseudo second-order model more closely (R2 = 0.998 – 1). The calculated values of thermodynamic parameters such as ΔGº and ΔSº were positive, indicating the presence of an energy barrier in the adsorption process. The positive values of ΔHº for biosorbent indicated that the adsorption mechanism was endothermic.  The adsorption capacity of TDS was found to be 26.42 mg/g.