Solar-driven purification of highly polluted saline wastewater into clean water by carbonized lotus seedpod

Magnetic Activated Carbon from ZnCl2 and FeCl3 Coactivation of Lotus Seedpod: One-Pot Preparation, Characterization, and Catalytic Activity towards Robust Degradation of Acid Orange 10

Lotus seedpods (LSPs) are an abundant and underutilized agricultural residue discarded from lotus seed production. In this study, ZnCl₂ and FeCl₃ coactivation of LSP for one-pot preparation of magnetic activated carbon (MAC) was explored for the first time. X-ray diffraction (XRD) results showed that Fe₃O₄, Fe⁰, and ZnO crystals were formed in the LSP-derived carbon matrix. Notably, transmission electron microscopy (TEM) images showed that the shapes of these components consisted of not only nanoparticles but also nanowires. Fe and Zn contents in MAC determined by atomic absorption spectroscopy (AAS) were 6.89 and 3.94 wt%, respectively. Moreover, S_BET and V_total of MAC prepared by coactivation with ZnCl₂ and FeCl₃ were 1080 m²/g and 0.51 cm³/g, which were much higher than those prepared by single activation with FeCl₃ (274 m²/g and 0.14 cm³/g) or ZnCl₂ (369 m²/g and 0.21 cm³/g). MAC was subsequently applied as an oxidation catalyst for Fenton-like degradation of acid orange 10 (AO10). As a result, 0.20 g/L MAC could partially remove AO10 (100 ppm) with an adsorption capacity of 78.4 mg/g at pH 3.0. When 350 ppm H₂O₂ was further added, AO10 was decolorized rapidly, nearly complete within 30 min, and 66% of the COD was removed in 120 min. The potent catalytic performance of MAC might come from the synergistic effect of Fe⁰ and Fe₃O₄ nanocrystals in the porous carbon support. MAC also demonstrated effective stability and reusability after five consecutive cycles, when total AO10 removal at 20 min of H₂O₂ addition slightly decreased from 93.9 ± 0.9% to 86.3 ± 0.8% and minimal iron leaching of 1.14 to 1.19 mg/L was detected. Interestingly, the MAC catalyst with a saturation magnetization of 3.6 emu/g was easily separated from the treated mixture for the next cycle. Overall, these findings demonstrate that magnetic activated carbon prepared from ZnCl₂ and FeCl₃ coactivation of lotus seedpod waste can be a low-cost catalyst for rapid degradation of acid orange 10.