Coexisting Ions and Humic Acid Regulate Cd²⁺ Adsorption on Biotite: Mechanistic Insights

Understanding the adsorption behavior of Cd²⁺ onto mineral surfaces is pivotal for addressing severe Cd contamination in mining areas. However, most existing studies have focused exclusively on single-Cd²⁺ systems, which are far removed from real-world environmental scenarios. In fact, Cd²⁺ typically coexists with other metal ions (e.g., Pb²⁺, Zn²⁺) and humic acid in heavy metal-contaminated sites. Yet, the adsorption mechanisms of Cd²⁺ onto minerals under such environmentally relevant complex conditions remain largely unexplored. To tackle this critical issue, this study systematically investigated Cd²⁺ adsorption onto biotite in the presence of coexisting ions (i.e., Pb²⁺, Zn²⁺) and humic acid (HA) using batch experiments combined with spectroscopic techniques. Batch results revealed that pH and ionic strength strongly regulated Cd²⁺ adsorption: in 0.001 M NaNO₃, adsorption efficiency increased sharply from 7.18% to 92.14% as pH rose from 2.3 to 9.5; at pH 7, increasing ionic strength (0.001–0.1 M NaNO₃) induced a 40% reduction in Cd²⁺ uptake. Supported by SEM-EDS, XRD, and XPS characterizations, these findings confirmed that inner-sphere complexation and ion exchange/outer-sphere complexation were the dominant adsorption mechanisms. Coexisting ions exerted distinct effects: Pb²⁺ significantly inhibited Cd²⁺ adsorption via competitive binding to identical sites, whereas Zn²⁺ showed no notable impact—attributed to its specific adsorption onto biotite surface defects (validated by FT-IR). HA exhibited a pH-dependent dual role: it enhanced Cd²⁺ adsorption at low pH (3.6–6.6) through formation of biotite-HA-Cd²⁺ ternary complexes, but inhibited adsorption at high pH (>6.6) by forming water-soluble HA-Cd²⁺ complexes.