Multi-scale mechanisms of Zn²⁺ coordination by humic substances from different bio-stabilized sludge: insights from...

The structural features of humic substances (HS) critically determine their interactions with heavy metal ions, playing a significant role in heavy metal immobilization during sludge bio-stabilization processes and subsequent land applications. However, the specific effects of chain structure and acidic functional group distribution within HS molecules on metal coordination remain insufficiently understood. Here, representative molecular models of HS derived from vermicomposted sludge (VCS), aerobically composted sludge (ACS) and anaerobically digested sludge (ADS) were constructed based on structural characterization. The effects of structural variations on coordination kinetics, geometries and stability between HS molecules and Zn²⁺ ions were systematically investigated by integrating experimental analysis with molecular dynamics (MD) simulations. The results demonstrated that carboxyl groups dominated Zn²⁺ coordination by providing stronger electrostatic binding energy and a greater number of coordination sites. Multidentate coordination promoted Zn²⁺ complexation and immobilization by enhancing binding strength and inducing entropy gain from component aggregation and hydration water release. Compared to VCS and ACS, ADS HS showed the highest Zn²⁺ coordination capacity due to its higher abundance of carboxyl groups. Short chain HS molecules exhibited higher multidentate coordination efficiency resulting from reduced steric hindrance and enhanced diffusion, which increased the coordination number and binding stability of Zn²⁺. Reduced water content induced rapid HS aggregation, limiting Zn²⁺ access to internal acidic groups and reducing coordination numbers, whereas weakened hydration promoted multidentate coordination and enhanced Zn²⁺ binding stability. This study combined structure-based model construction with MD simulation to provide an effective framework for investigating the coordination behavior between complex organic components and heavy metal ions, enriching the theoretical basis for metal immobilization in sludge stabilization.