Renewable N-doped biochars for H2S removal at room temperature: Characterization, performance and mechanism

It is crucial for the environment, the economy, and society to create low-cost adsorbents from renewable resources to remove H₂S from biogas. Herein, a series of nitrogen-doped biochars derived from mixtures (70 % sewage sludge and 30 % pine sawdust) were prepared at different temperature (i.e., 600–800 ℃) using urea as the nitrogen source. In this study, the H₂S adsorption capacities of various biochars were investigated at room temperature. The textual properties and N content of the biochars have been significantly enhanced after N-doping. Higher N content was detected on the N-doped bicohars (9.68–10.97 wt%) rather than the pristine biochars (0.53–1.08 wt%). Furthermore, the N-doped biochars exhibited higher H₂S adsorption capacities (140.2–365.5 mg/g) compared with the pristine biochars (35.1–72.9 mg/g). Particularly, N-doped biochars pyrolyzed at 800℃ (BC800U) showed the highest H₂S adsorption capacity (365.5 mg/g), due to the large specific surface area (1065 m²/g), the greatest pore volume (0.78 cm³/g) and the highest N content (10.97 %). The H₂S adsorption mechanism was investigated by FTIR, XRD, quasi in-situ XPS, suggesting that the abundant nitrogen-containing functional groups owing to urea modification could significantly enhance the H₂S adsorption. Moreover, quasi-in-situ XPS and DFT analysis revealed that physical adsorption played a crucial role in H₂S adsorption and higher pyridine nitrogen (N-6) content could grant the N-doped biochars for higher H₂S adsorption performance, due to lower adsorption energy between H₂S and N-6 compared with pyrrole nitrogen (N-5) or pristine graphene. This study provides a comprehensive insight into N-doped biochars derived from feedstock mixtures for H₂S adsorption.