Biochar: from agricultural waste byproducts to novel adsorbents for ammonia and micro/nanoplastics (MNPs)

Abstract Biochar, a stable carbon material derived from biomass pyrolysis, shows promise in agriculture and environmental remediation due to its high porosity, surface area, and ion exchange capacity. Its effectiveness as a filter for emerging contaminants like ammonia and micro/nanoplastics (MNPs) depends on feedstock type and pyrolysis conditions. This study first evaluated biochars produced from corn cob, cocoa husk, walnut shell, and bamboo under varying pyrolysis temperatures and residence times. Comprehensive characterization included elemental composition, porosity, surface functional groups, and polycyclic aromatic hydrocarbons (PAHs). Woody feedstocks (bamboo, walnut) yielded biochars with higher carbon content, surface area, and pore volume, while corn cob biochars provided balanced properties that warranted detailed investigation. Increasing pyrolysis temperature and time enhanced these properties but reduced oxygen-containing functional groups and variably affected PAH levels. Subsequent adsorption studies focused on corn cob biochars. Filtration tests with ammonia (1–100 ppm) and polystyrene MNPs (0.10–2.10 μm, up to 2 × 10⁷ particles/mL) showed that high-temperature samples achieved the best performance. Corn cob biochar prepared at 700 °C for 2.5 h removed 63.95% of ammonia (10 ppm, 30 g loading) and 97.99% of MNPs, with removal efficiency influenced by pyrolysis conditions, biochar load, and contaminant concentration. Importantly, no detectable release of 16 EPA priority PAHs was observed in leaching tests, confirming environmental safety. Regeneration experiments further demonstrated that corn cob biochars could be re-pyrolyzed and reused for three cycles with only modest efficiency losses, with CCB700 maintaining > 55% ammonia removal after Cycle 3. These findings highlight biochar’s potential as a sustainable, low-cost, and reusable filtration material for water treatment. Produced from agricultural residues, biochar can mitigate emerging contaminants when engineered under optimized pyrolysis conditions while minimizing environmental risks, supporting its practical application in eco-friendly wastewater treatment systems. Graphical Abstract