Competitive adsorption of dye species from aqueous solution onto melon husk in single and ternary dye systems

Polluted water may contain more than one dye species. Consequently, the behavior of a particular dye in a water system may be affected by the presence of the others. In this study, the adsorption of methylene blue (MB) in single dye system (SDS) and in ternary dye system (TDS) comprising of MB, congo red and methyl orange onto formaldehyde-treated melon husk (FMH) was investigated as a function of pH, contact time and species concentrations. Surface studies of FMH were investigated by Fourier transform infrared and scanning electron microscopy. The dye species adsorption equilibria were rapidly attained after 60 (SDS) and 90 min (TDS) of contact times. The adsorption kinetics were analyzed using pseudo first-order, pseudo second-order and intraparticle diffusion models and the adsorption data were well described by the pseudo second-order model. The equilibrium adsorption data were interpreted in terms of the Langmuir, Freundlich, Temkin, Dubinin–Radushkevich, Harkin–Jura and Halsey isotherm models and the goodness of fittings were inspected using linear regression analysis (R²). Our results indicated that the Langmuir model was best fitted, suggesting monolayer adsorption. Thermodynamic study showed that the adsorptions in SDS and TDS on FMH are favourable. The change in entropy (ΔS°) and heat of adsorption (ΔH°) of dye species on FMH in TDS were estimated as 82.2 J/ mol K and 17.95 kJ/mol. respectively while in SDS, they were respectively -43.76 J/mol K and -21.84 kJ/mol. The sorption process in both systems was thermodynamically feasible with negative ΔG° values.

Keywords
Melon husk; Single dye system (SDS); Ternary dye system (TDS); Kinetics; Thermodynamics; Isotherms