Magnetic graphene aerogel derived from lotus seedpod-based activated carbon: A promising adsorbent for malachite green removal in wastewater treatment

Abstract

Water-soluble dyes, including malachite green (MG), pose a significant challenge in wastewater treatment, necessitating the development of efficient removal strategies. In this study, we report for the first time the synthesis of a novel magnetic composite adsorbent composed of lotus seedpod based activated carbon (LSAC) and hollow CuFe₂O₄ nanospheres co-modified graphene aerogel (MLGA). This adsorbent material is synthesized using an efficient alkali-activated pyrolysis method followed by one-step solvothermal electrostatic co-assembly for the effective removal of MG. In this adsorbent, LSAC serves as a reinforcing and bridging agent, effectively preventing the aggregation of graphene nanosheets by promoting the formation of a three-dimensional hierarchical porous network structure. Consequently, LSAC imparts MLGA with several desirable properties, including superhydrophilicity characterized by a water contact angle of 0°, a high specific surface area of 406.31 m²/g, abundant oxygen-containing functional groups, and strong magnetic properties with a magnetic saturation (Ms) value of 45.83 emu/g. The effects of pH (2 ∼ 6), initial dye concentration (25 ∼ 250 mg/L), adsorbent dosage (0.1–0.6 g/L), temperature (25–45 °C) and contact time (15–420 min) on the adsorption efficiencies are investigated by batch experiments. The adsorption process is consistent with the Langmuir isotherm and the pseudo-second-order kinetic models. The MG removal mechanism of MLGA is primarily driven by π-π interactions, hydrogen bonding, electrostatic attraction and pore filling, providing a maximum adsorption capacity of 1023.04 mg/g at 25 °C, surpassing that of most reported biomass-based graphene adsorbents. The adsorption performance of MLGA is stable, retaining 85.2 % of its initial adsorption capacity after 12 consecutive adsorption–desorption cycles. Moreover, directional adsorption can be achieved using magnetron technology, allowing for the easy recovery of adsorbents even in extreme environments. In summary, the low-cost MLGA adsorbent exhibits high adsorption capacity and excellent recyclability, providing a promising approach for the high-value utilization of agricultural waste and demonstrating significant potential for dye wastewater treatment.