Citrus fruit residues as alternative precursors to developing H2O and CO2 activated carbons and its application for Cu(II) adsorption.

Due to its toxicity, the presence of Cu(II) ions released in aquatic environments presents a serious threat to the environment and human health. In search of sustainable and low-cost alternatives, there are citrus fruit residues, which are generated in large quantities by the juice industries and can be used to produce activated carbons. Therefore, the physical route was investigated for producing activated carbons to reuse citrus wastes. In this work, eight activated carbons were developed, varying the precursor (orange peel-OP, mandarine peel-MP, rangpur lime peel-RLP, and sweet lime peel-SLP) and the activating agent (CO2 and H2O) to remove Cu(II) ions of the aqueous medium. Results revealed promising activated carbons with a micro-mesoporous structure, a specific surface area of around 400 m2 g-1, and a pore volume of around 0.25 cm3 g-1. In addition, Cu (II) adsorption was favored at pH 5.5. The kinetic study showed that the equilibrium was reached within 60 min removing about 80% of Cu(II) ions. The Sips model was the most suitable for the equilibrium data, providing maximum adsorption capacities (qmS) values of 69.69, 70.27, 88.04, 67.83 mg g-1 for activated carbons (AC-CO2) from OP, MP, RLP, and SLP, respectively. The thermodynamic behavior showed that the adsorption process of Cu(II) ions was spontaneous, favorable, and endothermic. It was suggested that the mechanism was controlled by surface complexation and Cu2+-π interaction. Desorption was possible with an HCl solution (0.5 mol L-1). From the results obtained in this work, it is possible to infer that citrus residues could be successfully converted into efficient adsorbents to remove Cu(II) ions from aqueous solutions.

Pyrolysis of citrus wastes for the simultaneous production of adsorbents for Cu(II), H2, and d-limonene.

A route based on pyrolysis and physical activation with H2O and CO2 was proposed to reuse citrus waste traditionally discarded. The citrus wastes were orange peel (OP), mandarine peel (MP), rangpur lime peel (RLP), and sweet lime peel (SLP). The main aim was to use the solid products of this new route as adsorbents for Cu(II) ions. Copper ions are among the most important water pollutants due to their non-degradability, toxicity, and bioaccumulation, facilitating their inclusion and long persistence in the food chain. Besides the solid products, the liquid and gaseous fractions were evaluated for possible applications. Results showed that the citrus waste composition favored the thermochemical treatment. In addition, the following yields were obtained from the pyrolysis process: approximately 30 % wt. of biochar, 40 % wt. of non-condensable gases, and 30 % wt. of bio-oil. The biochars did not present a high specific surface area. Nevertheless, activated carbons with CO2 and H2O presented specific surface areas of 212.4 m2/g and 399.4 m2/g, respectively, and reached Cu(II) adsorption capacities of 28.2 mg g-1 and 27.8 mg g-1. The adsorption kinetic study revealed that the equilibrium was attained at 60 min and the pseudo-second-order model presented a better fit to the experimental data. The main generated gases were CO2, which could be employed as an activating agent for activated carbon production. d-limonene, used for food and medicinal purposes, was the main constituent of the bio-oil.