Enhanced elemental mercury removal using modified natural zeolites: a study on activation methods and adsorption stability

The removal of elemental mercury (Hg⁰) from industrial gas streams remains a pressing environmental challenge. This study investigates the performance of natural zeolites modified via two activation methods – spraying and ion exchange – to enhance mercury adsorption efficiency. Samples (B1–B12, C1–C12) were functionalized using silver and iron precursors in nitrate and chloride forms. Their physicochemical properties were analysed using XRD, SEM-EDS mapping, and FTIR. Mercury removal efficiency was evaluated over six consecutive adsorption cycles using a custom-built laboratory scale measurement system. Twelve months after exposure, long-term mercury retention and potential re-emission were assessed with a direct mercury analyser. Results show that ion exchange, particularly with silver nitrate (C1–C3) and iron nitrate (C4–C6), led to more homogeneous precursor incorporation and higher Hg⁰ retention. The C6 sample exhibited the best overall performance. In contrast, the spraying method resulted in less uniform precursor distribution, although silver chloride-modified samples (B10–B12) also demonstrated high adsorption capacity. Nitrate-based precursors outperformed chloride counterparts in both efficiency and long-term stability. No direct relationship was found between zeolite grain size and mercury removal efficiency, indicating that internal porosity and precursor diffusion are more influential than particle dimensions. These findings highlight the critical role of activation strategy and precursor chemistry in optimizing zeolite-based sorbents. Ion exchange with nitrate precursors appears most promising for effective, stable mercury capture in industrial applications.