Eco-friendly biomass aerogels from moxibustion waste for sustainable remediation of organic contaminants

Powder adsorbents present limited recyclability when used for wastewater treatment. Bulk materials prepared via eco-friendly approaches typically exhibit excellent reproducibility over powders in terms of removing pollutants from aquatic environments. Here, biomass-based aerogels were synthesized via ball milling a mixture of modified moxa ash (a type of moxibustion waste), carbon nanotubes and ammonia, combined with agarose as biomolecule. The obtained aerogels display low density, high porosity, remarkable mechanical properties, and exceptional repeatability and stability for removing organic pollutants. By employing a two-dimensional (2D) aerogel membrane, pollutants such as 10 mg/L mitoxantrone (MTX), 5 mg/L methylene blue, 5 mg/L malachite green, or 5 mg/L rhodamine B could be dynamically adsorbed and swiftly filtered out, achieving nearly 100 % of the removal rate. For three-dimensional (3D) aerogels, efficient removals of MTX (98.4 % at 135 min) and cationic dyes under various conditions were observed, in which the adsorption capacity reached 28.23 mg/g for low concentration of MTX at 120 min. After four cycles, the aerogel still retained 93 % of its original capacity and the removal rate was still 92.4 %. The adsorption process obeys second-order kinetics and adheres to the Freundlich and Temkin isotherm models. Furthermore, the primary mechanism by which the aerogels adsorb MTX includes pore filling, electrostatic interactions, π-π conjugation and hydrogen bonding. The adsorption mechanism was better explained by the results of density functional theory. Biomass-based 2D/3D aerogels are promising, low-cost, eco-friendly and highly efficient absorbents for mitigating wastewater contamination.