Co-pyrolysis tailored biochar from biowaste: Synergistic sites for ammonia adsorption

As a hazardous pollutant, ammonia (NH₃) poses significant environmental and health risks. This study synthesized multifunctional NH₃-adsorption biochar through co-pyrolysis of food waste digestate (FWD) and wood waste (WW), focusing on the physicochemical properties of derived biochar and their contributions to NH₃ adsorption. The optimized conditions enabled biochar with tunable production yields (25.8-67.9 wt%) and enhanced porosity (specific surface area reaching up to 331 m²/g). Thermogravimetric mass spectrometry combined with multiple spectral analyses uncovered a three-stage devolatilization process and tracked the structural transformations of the resultant biochar. Additionally, acid washing was applied to selectively remove surface-bound metallic salts from high-ash-content FWD-WW co-pyrolysis biochar, thereby liberating more active sites. Langmuir and Freundlich adsorption equilibrium models were systematically applied to evaluate the NH₃ adsorption performance of the biochar with/without modification, revealing a maximum adsorption capacity of 124.2 mg/g at 25 °C (1 bar). Adsorption kinetics and thermodynamics indicated that NH₃ adsorption occurred predominantly via spontaneous physisorption, enhanced by chemisorption at Brønsted acid sites (-COOH/Si-OH) and Lewis acid sites (Fe³⁺/Al³⁺). This study demonstrates a waste-to-adsorbent strategy through controlled co-pyrolysis and surface engineering for sustainable air pollution mitigation.