Low-cost and Sustainable Bioadsorbent from Banana Peel Waste for Crystal Violet Dye Removal

Abstract

A direct comparison was made between raw banana peel waste (RBPW) and acid-treated banana peel waste (ABPW), under identical conditions, for adsorption of crystal violet (CV). Sorption kinetics, isotherms, and thermodynamics were considered to reveal the underlying mechanisms. The effects of contact time, pH, initial CV concentration, temperature, and adsorbent dosage were evaluated. The sorption process obeyed a pseudo-second-order kinetic model, while the Langmuir isotherm model best explained the equilibrium data with maximum adsorption capacities. The Dubinin–Radushkevich model supported the potential of ion-exchange mechanisms for the acidified sample. Adsorption was spontaneous and endothermic, as revealed by negative Gibbs free energy, positive enthalpy (+16.4 kJ/mol for RBPW and +53.5 kJ/mol for ABPW) and positive entropy (RBPW = 6.79 J/mol·K and ABPW = 14.65 J/mol·K) values. The lower ΔH for the raw peel is more consistent with physisorption, while higher ΔH of the acid-treated peel suggests stronger interactions consistent with chemisorption/ion-exchange. The FT-IR analysis confirmed that functional groups such as –OH, –COOH, C=O, C-O, and possibly aromatic moieties on banana peel waste are involved in the sorption of CV. The enhanced performance of ABPW is attributed to acid-induced surface modifications that increased porosity, making the functional groups available for sorption process.