Ultrasound-treated citrus peels as functional biomaterial for heavy metals adsorption: structural, kinetic and isotherm studies

Biosorption is an ecofriendly process for removing metals from aqueous solutions, offering advantages such as high efficiency, low cost and reduced generation of chemical sludge. The adsorption of three metals, iron, copper and cadmium, was evaluated using ultrasound (US)-treated orange and lemon peels as natural adsorbent materials. The used biosorbents were named T-OP and T-LP for US treated-orange and lemon peels, and UT-OP and UT-LP for untreated orange and lemon peels. Experiments of batch adsorption were done, as a function of solute concentration, adsorbent dose and time contact. The adsorption of these heavy metals was selective in the order of Cu 2+ > Cd 2+ > Fe 2+ . The results indicated that optimum conditions for copper adsorption were achieved with a sorbent dose of 0.5 g and a contact time of 60 min, resulting in a maximum Cu 2+ sorption capacity of 15.28 mg/g (76%) for T-LP compared to 12.52 mg/g (59.6%) for the untreated one (UT-LP). However, in the case of Cd 2+ , T-OP gave the best sorption capacity of 12.20 mg/g (61%) at a contact time of 30 min and a sorbent dose of 0.5 g, outperforming the untreated orange peel (UT-OP), which achieved 10.9 mg/g (54.5%). Structural modifications were observed in the infrared spectra and electron microscopy images after the sorption of each metal using US-treated samples. The results of the adsorption models showed that the calculated equilibrium data fitted well to Langmuir with correlation coefficient (R 2 ) of 0.994 for copper and to Freundlich with coefficient (R 2 ) of 0.93 for cadmium. The pseudo-second order (R 2 = 1 and R 2 = 0.999) fitted well the both metals Cu 2+ and Cd 2+ , respectively, suggesting a chemisorption process with chemical bands interactions. According to these data, the orange and lemon peel powders could be effective and environmentally absorbents to remove Cu 2+ and Cd 2+ metal ions from wastewater.