Linking molecular heterogeneity of artificial humic acid to divergent cadmium coordination behaviors

Humic substances strongly influence the environmental behavior of toxic metals, but the molecular basis by which compositionally similar humic systems promote either metal immobilization or activation remains unclear. To resolve this, cadmium (Cd) was selected as a model divalent metal with well-constrained coordination chemistry, while artificial humic acid (A-HA) was used as a structurally tunable analogue of natural humic substances, enabling controlled interrogation of molecular-scale interactions. By integrating 2D-COS-FTIR, FT-ICR MS, molecular transformation network analysis, and interpretable machine learning, we systematically elucidate how molecular heterogeneity controls Cd complexation by A-HA. A-HA exhibited a Cd-binding capacity of up to 2.76 mg L⁻¹ at 5 mg L⁻¹ dissolved organic carbon. Spectroscopic analyses revealed an early response of N-rich, structurally accessible fractions, followed by progressive involvement of aromatic and polycarboxyl-rich domains. This sequence demonstrates that Cd complexation is not a uniform binding process, but is governed by the sequential and selective participation of distinct molecular fractions. FT-ICR MS further showed substantial molecular turnover, with formula diversity decreasing by 8.4%, CHON species declining from 1878 to 1507 and CHO species increasing from 2153 to 2221. These changes indicate preferential depletion of N-containing labile molecules and relative enrichment of O-rich molecular fractions during complexation. Network analysis further revealed that carboxyl-enriched molecules formed longer-range and more cooperative subnetworks than amino-enriched molecules, thereby providing a structural basis for stable coordination versus labile association. Interpretable machine learning further identified C-number, AI_mod, O/C, N/C and NOSC as the key descriptors governing molecular behavior. Overall, the environmental fate of Cd in humic systems is determined not simply by bulk humic properties, but by the selective recruitment, depletion, and reorganization of distinct molecular ensembles.