Heavy metal adsorption isotherms on tailor-made brittle micas in water treatment applications

Abstract

The removal of heavy metals from water is a critical challenge because of their high toxicity, persistence, and bioaccumulation in biological systems. Adsorption is widely recognised as a cost-effective and efficient treatment method, and equilibrium isotherms are essential for evaluating the performance of adsorbents. Here, the equilibrium isotherms of Pb²⁺, Cd²⁺ and Hg²⁺ adsorbed onto synthetic brittle micas, specifically as-made (Na-M2) and thiol-functionalised (MEA-M2) one, have been systematically analysed. Na-M2 exhibited a remarkably high adsorption capacity, up to ten times higher than conventional adsorbents (1402, 1111 and 1717 mmol/kg for Pb²⁺, Cd²⁺ and Hg²⁺). Despite its lower overall capacity, MEA-M2 demonstrated superior Hg²⁺ uptake (955 mmol/kg) compared to other thiol-functionalised materials. However, the high layer charge of MEA-M2 limited the access to thiol groups. Finally, the adsorption mechanism and the adsorption energy depended on both the nature of the heavy metal and the grafting, providing information on the structure-performance relationships for water treatment applications.