Eriochrome black adsorption on yellow passion fruit peel (Passiflora edulis f. Flavicarpa) treated with sodium hydroxide and nitric acid: study of adsorption isotherms, kinetic models and thermodynamic parameters

The yellow passion fruit peel was used for Eriochrome black adsorption studies in aqueous solution in such a way as to propose a use for this by-product of the Brazilian fruit juice industry. The passion fruit peel was treated with nitric acid and sodium hydroxide to simulate pectin removal conditions and provide surface modifications that enhance adsorption. The kinetics and equilibrium experiments were completed in batch, at room temperature and contact times from 15 to 240 min. The thermodynamic studies were accomplished through a 298–343 K temperature variation. The adsorption capacities ranged from 196 to 303 mg/g, with the NaOH-treated sample having the highest adsorption capacity at all temperatures. This result points out that the passion fruit peel is a promising adsorbent compared with other low-cost adsorbents. The adsorption process is exothermic and occurs with decreasing entropy. The spontaneity of the process is low except for the sample treated with NaOH. Gibbs free energies were − 1.70 kJ/mol for the untreated sample, − 2.55 kJ/mol for the HNO3-treated sample, and − 6.03 kJ/mol for the NaOH-treated sample at room temperature. The thermodynamic studies have shown that Eriochrome black adsorption on passion fruit peel is a physical process. The process kinetics may be described by the pseudo-second-order model, with diffusion limitations in the solution. The results indicate that NaOH-treated passion fruit peel is a promising adsorbent for Eriochrome black and may be used to adsorb other acid dyes in aqueous medium.