Construction of a Novel Closed-Loop Livestock Waste Valorization Paradigm: Bridging Manure and Ammonia Gas via Phosphate-Doped Hydrochar

Sustainable livestock production plays an important role in meeting the growing global demand for animal products. Livestock manure and odor emission are regarded as the major obstacles to the low-carbon livestock farming. In response, we propose a novel loop for the valorization of livestock wastes, via bridging manure and the typical odor NH3 with P-doped hydrochar. P-doped cow manure hydrochar was facilely prepared via hydrothermal carbonization followed by H3PO4 activation. The organic composition and crystallinity of hydrochar as the precursor, depending on hydrothermal severity, significantly impacted the subsequent H3PO4 activation behavior. The activated sample with modified surface exhibited favorable NH3 adsorption capacity in the dynamic adsorption experiment. Density functional theory calculation confirmed that the P-containing group improved the NH3 adsorption efficiency as the active site. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analyses demonstrated the oxidation of reduced phosphorus groups during NH3 adsorption in room temperature. Phosphorus species strengthened the immobilization of NH3 on the surface via Brønsted and Lewis-type acid-base interaction. This study provided a new strategy to develop a green NH3 adsorbent from the manure-based hydrochar, supporting the further investigation on the promising solution of odor control and waste valorization in livestock farms.