Resourcization of Argillaceous Limestone with Mn₃O₄ Modification for Efficient Adsorption of Lead, Copper, and Nickel

Argillaceous limestone (AL) is comprised of carbonate minerals and clay minerals and is widely distributed throughout the Earth's crust. However, owing to its low surface area and poorly active sites, AL has been largely neglected. Herein, manganic manganous oxide (Mn₃O₄) was used to modify AL by an in-situ deposition strategy through manganese chloride and alkali stepwise treatment to improve the surface area of AL and enable its utilization as an efficient adsorbent for heavy metals removal. The surface area and cation exchange capacity (CEC) were enhanced from 3.49 to 24.5 m²/g and 5.87 to 31.5 cmoL(+)/kg with modification, respectively. The maximum adsorption capacities of lead (Pb²⁺), copper (Cu²⁺), and nickel (Ni²⁺) ions on Mn₃O₄-modified argillaceous limestone (Mn₃O₄-AL) in mono-metal systems were 148.73, 41.30, and 60.87 mg/g, respectively. In addition, the adsorption selectivity in multi-metal systems was Pb²⁺ > Cu²⁺ > Ni²⁺ in order. The adsorption process conforms to the pseudo-second-order model. In the multi-metal system, the adsorption reaches equilibrium at about 360 min. The adsorption mechanisms may involve ion exchange, precipitation, electrostatic interaction, and complexation by hydroxyl groups. These results demonstrate that Mn₃O₄ modification realized argillaceous limestone resourcization as an ideal adsorbent. Mn₃O₄-modified argillaceous limestone was promising for heavy metal-polluted water and soil treatment.