Removal of methylene blue from aqueous solutions by using cold plasma- and formaldehyde-treated onion skins

In this paper, the use of cold plasma-treated and formaldehyde-treated onion skins as a biosorbent has been investigated to remove methylene blue dye from aqueous solutions. The surface characteristics of the treated onion skins were investigated using Fourier Transform-infrared spectroscopy. The influence of process variables such as adsorbent dosage, initial dye concentration and pH were studied. Equilibrium isotherms were analysed by Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherm models. The results indicated that the data for adsorption of methylene blue onto onion skins fitted well with the Langmuir isotherm model. The sorption capacities for cold plasma-treated and formaldehyde-treated onion skins by Langmuir isotherm were found to be 250 and 166.67 mg/g, respectively. The equilibrium time was found to be 150 min for 50 mg/l dye concentrations. The maximum removals for cold plasma-treated and formaldehyde-treated onion skins obtained were 90.94 and 95.54% at natural pH 10.0 for adsorbent doses of 0.15 g/200 ml, respectively. The rates of sorption were found to conform to pseudo-first-order kinetics. Results indicated that onion skins could be used as a biosorbent to remove methylene blue dye from contaminated waters.