Artificial humic acids as model systems for resolving key structural controls on humic acid-metal complexation

Abstract The thermodynamic stability of metal–humic complexes governs metal mobility and persistence in natural environments, yet remains difficult to predict because of the molecular heterogeneity of humic matter. Existing frameworks mainly emphasize functional group abundance, while overlooking how precursor chemistry is encoded into supramolecular network topology during humification. Here, using artificial humic acids as a controllable model system, we show that precursor-inherited supramolecular topology governs the thermodynamic stability of metal–humic complexes. By integrating ultrahigh-resolution mass spectrometry, synchrotron X-ray absorption spectroscopy, and molecular dynamics simulations, we identify two contrasting thermodynamic trajectories determined by network architecture. Rigid, aromatic-dominated networks undergo thermodynamic hardening, generating stable, multidentate, and interference-resistant coordination domains, whereas flexible, nitrogen-rich networks exhibit thermodynamic softening, favoring kinetically accessible but metastable metal binding. Using cadmium as a probe metal, we further show that supramolecular rigidity enhances coordination saturation and entropic stabilization, increasing resistance to competitive ion displacement, while flexible topologies favor adsorption capacity at the cost of long-term stability. These results establish supramolecular topology as a governing principle linking precursor chemistry to the thermodynamic fate of metals in humic matter.