An advanced magnetic biochar (MBC) was facilely prepared via one-pot FeCl3-activation of lotus seedpod. Simultaneous carbonization, activation, and magnetization formed magnetic Fe3O4 nanoparticles and nanowires over the biochar base. The specific surface area (SBET) and the total pore volume (Vtotal) of MBC were 349 m2/g and 0.31 cm3/g, which were 2.0-fold and 3.9-fold higher than those of biochar, respectively. In addition, the saturation magnetization of MBC reached 6.94 emu/g, which could facilitate its magnetic separation and recovery. In heterogeneous Fenton-like catalytic oxidation, 0.40 g/L MBC decolorized 100% Orange G and reduced 58% COD by 350 ppm H2O2 within 120 min. The degradation kinetics were calculated with different MBC samples and reactions followed pseudo-first-order kinetics with the highest rate constant of 0.034 min-1. Moreover, catalytic activity dropped by only 6.4% after four reuse cycles, with negligible iron leaching. Based on these results, MBC could be a low-cost, highly effective, and relatively stable catalyst for treating Orange G in wastewater.

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, SBET and Vtotal 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.