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

The stability of metal-humic associations controls metal mobility and persistence in natural environments, yet remains difficult to predict from functional-group abundance alone. Using artificial humic acids derived from rice straw and soybean straw as controlled models, we identify a precursor-encoded separation between site supply and site state during metal binding. Humification reorganized precursor chemistry into distinct molecular frameworks that regulated donor accessibility and the exchangeability of the resulting coordination state. Integrating 13 C NMR, XAFS, FT-ICR MS, and molecular dynamics simulations, we linked precursor carbon domains to molecular organization and local Cd coordination. Soybean-straw humic acid contained more accessible O- and N-bearing donors, resulting in greater Cd uptake and higher intrinsic affinity under single-metal conditions. Rice-straw humic acid showed lower initial affinity but greater resistance to competitive displacement. Molecular association networks revealed a continuous, highly connected framework in rice-straw humic acid, whereas soybean-straw humic acid formed several relatively independent modules. These findings show that metal capture and retention are distinct outcomes governed by donor accessibility and coordination-state exchangeability, respectively. Precursor chemistry, therefore, acts as an upstream control on humic molecular frameworks and offers a basis for designing humic materials for rapid capture or sustained retention.