Methylene Blue Sorption Characterisation onto Orange and Lemon Peels

Chemical properties of cationic biosorbents, the orange peel and the lemon peel, have been studied. ATR and Raman spectra indicated that the functional groups in the peels structure responsible for dye sorption are carboxyl groups (COOH and COOM, M=Na,K,Ca,Mg) and hydroxyl groups (OH). Increasing the mass of the peels in water suspensions a decrease in dissociation of both, the –COOH and –COOM groups was obtained. Sorption isotherms of methylene blue onto the biosorbents had two sorption plateaus. Hence the carboxyl groups interacted with methylene blue up to the first plateau, while sorption by also the hydroxyl groups occurred from the first to the second plateau. A very good agreement between the amounts of the carboxyl groups estimated in the peels and the amounts of the dye molecules sorbed onto the peels was obtained.