Rice husk pristine biochar for dynamic gas-phase NH3 adsorption: impact of pyrolysis temperature and mechanism

Ammonia (NH 3 ) is a key atmospheric pollutant emitted from livestock housing and manure management, that contributes to PM 2.5 formation, ecological eutrophication, and health risks. Developing economically feasible technologies based on low-cost adsorbents for NH 3 capture is therefore crucial. Rice husk (RH), an abundant agro-industrial residue, represents a sustainable carbon precursor. In this work, RH-biochars were synthesized at three pyrolysis temperatures (350, 550, and 750 °C) were systematically assessed for continuous NH 3 adsorption. Comprehensive characterization was performed to establish properties-adsorption relationships. Pyrolysis temperature exerted a dual influence: higher temperatures enhanced surface area and microporosity, while simultaneously reducing surface acidity due to the progressive loss of oxygenated groups·NH 3 adsorption capacities ranged from 3.8 ± 0.1 to 7. 5 ± 0.3 mg NH 3 g −1 at 1,000 ppm of NH 3 and 25 °C (conditions selected as representative of those encountered in pig manure storage and air stripping management), with RH-350 being the most efficient biochar, achieving capacities comparable to many activated carbons without requiring chemical activation or metal impregnation. Characterization and thermodynamic analyses revealed that this superior uptake derives from the prevalence of oxygen-containing functionalities which promote NH 3 binding through hydrogen-bonding, outweighing the influence of textural properties. The calculated adsorption enthalpy (−31 ± 4 kJ mol −1 ) using the Sips model confirmed this interaction, which enabled the complete regeneration of RH-350 sample at 150 °C across consecutive cycles. These findings provide mechanistic insight into gas–solid interactions on oxygen-rich carbon surfaces and support the use of pristine RH-biochars as competitive and regenerable adsorbents for NH 3 capture.