Assessment of an ecofriendly process to remove Nickel and Zinc from wastewater using ultrasound-treated Citrus fruit peels

In the present study, Citrus peels treated or untreated with ultrasound (US), were used as bio-sorbents for the removal of heavy metal ions from wastewater. The tested bio-sorbents were labeled as follows: CO for control orange peel, USO for US-treated orange peel, CL for control lemon peel, and USL for US-treated lemon peel. A batch adsorption mode was employed to investigate the effects of varying operational parameters, including metal ion concentration, contact time, pH solution and adsorbent dose. The results revealed that the USL had an excellent Ni2+ sorption capacity (5.45 mg/g) as well as a good removal efficiency (54 %) compared to that of raw lemon (3.95 mg/g and 39 %). Using USO, the sorption capacity and removal efficiency of Zn2+ were about 14.64 mg/g and 73 %, respectively, while with USL, they were about 14.31 mg/g and 71 %, respectively. The interaction was confirmed through structural modifications identified via scanning electron microscopy and Infrared techniques. The kinetic data for studied metal ions Zn2+ and Ni2+ demonstrated an excellent fit to the pseudo-second-order model (R2>0.99) for both USO and USL, indicating that the sorption process was governed by chemical interactions between bio-sorbent and metal ion. Regarding the isotherm study, the Langmuir model (R2=0.98) provided the best fit for the sorption of Nickel onto USL, indicating monolayer adsorption. In contrast, the adsorption of Zn2+ was described by the Freundlich model (R2=0.797 and R2=0.875 for USO and USL, respectively), suggesting a heterogeneous surface and multilayer adsorption. Based on the Dubinin–Radushkevich isotherm model, the calculated free energies were below 8 kJ/mol for all samples, confirming that the adsorption was physical. Therefore, the interaction bio-sorbent-metal ion could be mainly governed by chemical interactions involving physical ones. Altogether, these findings demonstrated the role of Citrus peels, as low cost, efficient and environmentally sustainable biomaterial, in the elimination of Ni2+ and Zn2+ from wastewater.