Breakthrough of Carbon–Ash Recalcitrance in Hydrochar via Molten Carbonate: Engineering Mineral-Rich Biowaste Toward Sustainable Platform Carbon Materials

Abstract

The function-led design of porous hydrochar from mineral-rich biowaste for environmental applications inevitably suffers from carbon–ash recalcitrance. However, a method to alter the original carbon skeleton with ash remains elusive and hinders the availability of hydrochar. Herein, we propose a facile strategy for breaking the rigid structure of carbon–ash coupled hydrochar using phase-tunable molten carbonates. A case system was designed in which livestock manure and NaHCO3 were used to prepare the activated hydrochar, and NH3 served as the target contaminant. Due to the redox effect, we found that organic fractions significantly advanced the melting temperature of Na2CO3 below 800 °C. The Na species steadily broke the carbon–ash interaction as the thermal intensity increased and transformed inorganic constituents to facilitate ash dissolution, rebuilding the hydrochar skeleton with abundant hierarchical channels and active defect edges. The surface polarity and mesopore distribution collectively governed the five cycles NH3 adsorption attenuation process. Manure hydrochar delivered favorable potential for application with a maximum overall adsorption capacity of 100.49 mg·g−1. Integrated spectroscopic characterization and theoretical computations revealed that incorporating NH3 on the carbon surface could transfer electrons to chemisorbed oxygen, which promoted the oxidation of pyridine-N during adsorption. This work offers deep insight into the structure function correlation of hydrochar and inspires a more rational design of engineered hydrochar from high-ash biowaste.

Keywords

• Hydrochar
• Livestock manure
• Ammonia adsorption
• Deashing
• Oxygen activation