Unraveling Cd2+ Adsorption Mechanisms on Biotite: Effects of Coexisting Pb2+, Zn2+ and Humic Acid via Batch and Spectroscopic Studies

Understanding the molecular mechanisms of cadmium (Cd) adsorption at the soil-water interface is critical for addressing severe mining areas Cd pollution. This study systematically investigated Cd2+ adsorption onto biotite in the presence of coexisting ions (e.g., Pb2+, Zn2+) and humic acid (HA) via batch experiments and spectroscopic techniques. Batch experiments showed that pH and ionic strength significantly affected Cd2+ adsorption onto biotite. Adsorption efficiency increased from 7.18% to 92.14% as pH increased from 2.3 to 9.5 in a 0.001 M NaNO3 solution, while at pH 7, a 40% decreased in adsorption was observed with increasing ionic strength from 0.001 M NaNO3 to 0.1 M NaNO3. These results, supported by SEM-EDS, XRD and XPS, confirmed that inner-sphere complexation and ion exchange/outer-sphere complexation were the important mechanisms for Cd2+ adsorption onto biotite. Coexisting ions (i.e., Pb2+ and Zn2+) exerted distinct effects on the adsorption of Cd2+ by biotite. Pb2+ significantly inhibited Cd2+ adsorption onto biotite by competing for the same sites, while Zn2+ had no notable impact on Cd2+ adsorption, attributed to its specific binding to defect sites on the biotite surface (verified by FT-IR). HA influenced Cd2+ adsorption onto biotite by playing a pH-dependent dual role. It promoted Cd2+ adsorption at low pH (3.6–6.6) through forming biotite-HA-Cd2+ ternary complexes, but inhibited adsorption at high pH (>6.6) by forming water-soluble HA-Cd2+ complexes.