Sequestration of Sulphide from Biogas by thermal-treated iron nanoparticles synthesized using tea polyphenols

Dark tea-iron nanoparticles (DT-Fe NPs) were prepared using extracts of dark tea leaves as a reducing agent, and further underwent thermal treatment in air. The H₂S removal performances of thermal-treated DT-Fe NPs for biogas were further evaluated using a custom-designed fixed-bed reactor (reaction temperature of 250°C, H₂S content of 1%). Significant morphology and chemical composition differences were observed when DT-Fe NPs were treated at different temperatures (300–800^oC). X-ray diffractometer analysis revealed that a phase transition from γ-Fe₂O₃ to α-Fe₂O₃ occurred under heat treatment. When the thermal treatment temperature was 300°C, only α-Fe₂O₃ was detected. Both α-Fe₂O₃ and γ-Fe₂O₃ were present in the sample treated at 400°C. When the thermal treatment temperature was 500–800°C, γ-Fe₂O₃ in the sample was completely converted to α-Fe₂O₃. The H₂S removal capacity is 14.72 mg H₂S/g for DT-Fe NPs without treatment. However, the value increased significantly to 408.30 mg H₂S/g after 400°C thermal treatment, which can be explained by the formation of highly active γ-Fe₂O₃. The reaction product of thermal-treated DT-Fe NPs at 400°C and H₂S were further characterized by X-ray diffractometer and X-ray photoelectron spectroscopy. The results showed that it is composed of FeS₂ and FeS, in which 72.6% of the sulphur existed as disulphide and 27.4% as monosulphide.