Valorization of lemon peels wastes into a potential adsorbent for simultaneous removal of copper ion (Cu²⁺) and Congo red from wastewater

This research focused on the development of an adsorbent material from the Tahiti lemon peels (LP) using thermochemical treatment with ZnCl₂ at 550 °C, and its application for simultaneous removal of copper ion (Cu²⁺), and Congo red (CR) from contaminated water resources. Results showed a mesoporous activated carbon with a high area (S_BET = 945 m²g⁻¹) and a wide pore distribution (2.0–25 nm), with carboxylate, phenolic and ether as the main functional surface groups. The adsorption experiments showed that LP activated with ZnCl₂ (LPZn) exhibited a higher effectiveness than LP without any treatment and LP calcined at 550 °C (CLP). The pH at the point of zero charge (pH_PZC) of LP, CLP, and LPZn was 2.78, 8.04, and 4.72, respectively, indicating a wide diversity and proportion of functional groups in the materials. The maximum adsorption capacities of LPZn were 1.78 meq g⁻¹ (618.35 mg g⁻¹) for CR and 0.91 meq g⁻¹ (28.85 mg g⁻¹) for Cu²⁺, which are comparable with other adsorbents previously reported. In multicomponent systems, at the highest concentration of Cu²⁺ (90.0 mg L^-1 ≈2.9 m_eq L^-1), a favoring of the adsorbed amount of Cu²⁺ was observed, suggesting the occurrence of a synergistic effect for the presence of Cu²⁺ under these conditions. Therefore, the material derived from Tahiti lemon peels can be considered a promising adsorbent with good physicochemical properties for the treatment of contaminated wastewater by dyes and heavy metal ions.