Intrinsic Ion Properties Regulate Metal Binding and Supramolecular Assembly in Humic Acid Systems

Abstract Humic acid (HA), a chemically active fraction of natural organic matter (NOM), regulates metal ion fate; however, the intrinsic ionic properties governing the transition from specific binding to supramolecular HA assembly remain poorly understood. Using purified HA, we combined adsorption experiments and molecular dynamics simulations for Ca2+, Cd2+, and Pb2+, NICA–Donnan parameter transfer for Mg2+, Cu2+, and Hg2+, and quantum calculations for all six cations to develop a descriptor-based six-metal framework. Metal electronegativity (χ) was associated with overall binding affinity, while its product with a period number (nshell) was strongly associated with site selectivity (R2 = 0.997). At low loading, metals preferentially occupied high-affinity, preorganized bidentate phenolic sites, whereas at higher loading, binding shifted toward carboxylate-mediated intermolecular coordination that facilitated HA association. Within the simulated HA model, metal ions acted as triggers and stabilizers rather than the dominant cohesive force. By reducing electrostatic repulsion, metals promoted the HA–HA approach, allowing weak intermolecular forces and aliphatic carboxylate coordination to stabilize supramolecular assembly. Across monovalent, divalent, and trivalent ion datasets, log-transformed critical coagulation concentration correlated with log (ionic charge2 × nshell) (R2 = 0.951). These findings provide testable hypotheses for metal mobility, contaminant sequestration, and carbon stability in HA-like or humic-rich NOM environments.