Insights into the size effect of Fe₃O₄ in Fe/AC for Hg⁰ removal and the corresponding mechanism

Abstract

Elemental mercury (Hg⁰) in coal-fired and metallurgical flue gases remains a critical concern and technical challenge in air pollution control. In this study, FeCl₃-loaded activated carbon (AC) catalysts were prepared from walnut shells via chemical activation and calcination at 700 °C (Fe-1/AC) and 800 °C (Fe-2/AC) to investigate the removal of Hg⁰. Within a wide temperature range of 50–400 °C, Fe-1/AC achieved a Hg⁰ removal efficiency exceeding 95%. Characterization results indicate that the lower calcination temperature yielded Fe-1/AC with smaller Fe₃O₄ nanoparticles, improved dispersion, a preserved microporous structure, and abundant surface chloride ions and defect oxygen species, all of which contributed to its effective Hg⁰ removal performance resulting from coupled adsorption and oxidation processes. However, the higher calcination temperature led to pore collapse and Fe agglomeration in Fe-2/AC, thereby reducing its efficiency. These findings highlight the crucial role of temperature-controlled synthesis strategies in optimizing the pore structure, metal dispersion, and surface chemistry of AC for efficient mercury capture, laying the foundation for the design of high-performance environmental remediation materials.

Graphical abstract

Keywords

Activated carbon / Fe₃O₄ particle size effect / Mercury removal / Synergistic mechanism / Adsorption–oxidation

Highlight

	● Fe-1/AC removes > 95% Hg0 at 50–400 °C, overcomes conv. sorbents’ temp sensitivity.
	● 700 °C calcination preserves micropores, yields smaller Fe3O4 for better dispersion.
	● Cl– & Oβ defects with Fe3O4 enable dual ads-oxidation, convert Hg0 to HgO/HgCl2.
	● A low-cost mercury adsorbent suitable for a wide temperature range is proposed.

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