Molecular-level EDTA-2Na modification of zeolite–biochar composites for efficient heavy metal adsorption: Preparation, mechanism and adsorption kinetics analysis

Aquatic ecosystems and public health are persistently threatened by heavy metal contamination, emphasizing the urgent need for efficient and sustainable adsorbent materials. In this study, a zeolite–biochar composite was functionalized with disodium ethylenediaminetetraacetate (EDTA-2Na) to enhance its affinity for Cu(II) and Zn(II) ions. The molecular-level incorporation of carboxyl and amino groups derived from EDTA-2Na significantly enhanced surface reactivity and porosity, as supported by spectroscopic and morphological analyses. Under optimized conditions (pH = 5, contact time < 60 min), the modified composite exhibited superior adsorption capacities of 65.67 mg·g⁻ 1 for Cu(II) and 22.61 mg·g⁻ 1 for Zn(II), outperforming traditional acid-modified materials (59.43 mg·g⁻ 1 for Cu(II) and 21.31 mg·g⁻ 1 for Zn(II)) and pristine biochar. The adsorption followed both chemisorption and physisorption processes on heterogeneous active sites, consistent with the pseudo-second-order kinetic model and the Freundlich isotherm, indicating the presence of heterogeneous adsorption sites on the composite surface and that the process is predominantly multilayer adsorption. This work demonstrates that a molecular-level functionalization strategy is applied to biochar–zeolite composites, enhancing metal chelation and active-site accessibility, and offering a cost-effective and scalable route for heavy-metal removal from contaminated water.