Preparation of nitrogen-doped biocarbon from sewage sludge and pine sawdust for superior hydrogen sulfide removal: Experimental and DFT studies

Hydrogen sulfide (H₂S) is a major air pollutant posing a serious threat to both the environment and public health. In this study, a novel nitrogen-rich biocarbon that effectively removes H₂S was produced from a mixture of sewage sludge and pine sawdust using melamine as nitrogen source. Compared with pristine biocarbons, nitrogen (N)-doped biocarbons possessed an adjustable porosity, e.g., higher specific surface area of 786-1547 m²/g, richer nitrogen content of 9.02-10.22%, and larger pore volume of 0.31-0.88 cm³/g. In particular, when H₂S flow rate was 100 mL/min at 25 °C under dry conditions, a higher H₂S adsorption capacity (376.7 mg/g) was observed on nitrogen-rich biocarbon pyrolyzed at 800 °C (BC800N) due to the larger surface area (1547 m²/g), higher pore volume (0.88 cm³/g), and richer nitrogen content (10.22%). The presence of CO₂ in the gas reduced H₂S adsorption; however, this was partially overcome by the presence of water vapor. After ten consecutive adsorption/desorption cycles, BC800N retained 96.7% of its adsorption capacity. Scanning electron microscopy, X-ray diffraction analysis, and quasi-in-situ X-ray photoelectron spectroscopy were employed to identify the transformed composites of H₂S on the biocarbons. The higher nitrogen content resulting from melamine doping mainly increased the pyridinium N-6 and pyrrole N-5 levels, which serve as nitrogen-containing active sites for H₂S adsorption. Density functional theory analysis confirmed that the N-6 atoms affected the adsorption of H₂S molecules significantly and play an important role in gas adsorption. This innovative biocarbon material has the potential to facilitate optimized adsorption of H₂S and assist in effectively mitigating the environmental impact thereof.