Synergistic adsorption of NH3 and H2S over layered hydrochar-derived pyrochars: mechanistic divergence and cooperative acid-base activation

NH₃ and H₂S are major hazardous odorants emitted from intensive livestock operations, presenting distinct challenges for simultaneous removal. This study evaluates a novel layered filtration system utilizing swine-manure-derived pyrochars produced via a two-stage hydrothermal carbonization (HTC) and pyrolysis process. Chars produced at 350 °C (PMB350) and 550 °C (PMB550) exhibited divergent physicochemical properties. Increasing pyrolysis temperature increased BET surface area (27.94 to 90.00 m2 g− 1) and alkalinity (pH 8.59 to 9.80), while reducing oxygenated surface functionalities. In single-gas fixed-bed tests (100 ppm; 100 mL min− 1), PMB350 exhibited higher NH3 capacity (7.10 mg g− 1) than PMB550 (4.86 mg g− 1), consistent with Brønsted acid–base capture to form surface NH4+. In contrast, PMB550 showed substantially higher H2S capacity (9.70 mg g− 1) than PMB350 (2.54 mg g− 1), reflecting the combined influence of pore development and mineral/alkaline sites. Layered beds (0.25 g + 0.25 g) provided mixed-gas performance (NH3 + H2S, each 100 ppm) with H₂S capacities of 7.15–7.60 mg g− 1 depending on bed order. X-ray photoelectron spectroscopy (XPS) analysis revealed that co-adsorbed NH3 functioned as a surface activator, enhancing the oxidative conversion of H2S to sulfate in the acidic upstream layer (PMB350) and amplifying sulfur mineralization in the alkaline downstream layer (PMB550). The synergy is attributed to an in-situ surface alkalinization mechanism where adsorbed NH3 facilitates H2S dissociation. These findings demonstrate that spatially arranging sorbents with complementary acid-base properties can exploit cooperative chemical interactions, offering a sustainable, high-efficiency strategy for complex odor mitigation.