Coagulation behavior of humic acid in aqueous solutions containing Cs⁺, Sr²⁺ and Eu³⁺: DLS, EEM and MD simulations

The coagulation behaviors of humic acid (HA) with Cs⁺ (10–500 mM), Sr²⁺ (0.8–10.0 mM) and Eu³⁺ (0.01–1.0 mM) at different pH values (2.8, 7.1 and 10.0) were acquired through a dynamic light scattering (DLS) technique combined with spectroscopic analysis and molecular dynamic (MD) simulations. The coagulation rate and the average hydrodynamic diameter (<D_h>) increased significantly as the concentration of nuclides increased. <D_h> could be scaled to time t as <D_h> ∝ t^a at higher Sr²⁺ concentrations, which shows that HA coagulation is consistent with the diffusion-limited colloid aggregation (DLCA) model. Trivalent Eu³⁺ induced HA coagulation at a much lower concentration than bivalent Sr²⁺ and monovalent Cs⁺. The coagulation value ratio of Sr²⁺ and Eu³⁺ to Cs⁺ is almost proportional to Z⁻⁶, indicating that the HA coagulation process is generally consistent with the Schulze-Hardy rule. Spectroscopic analysis indicated that the complexation between nuclides and carboxylic/phenolic groups of HA molecules played important roles in the coagulation of HA. MD modelling suggested that Sr²⁺ and Eu³⁺ ions increased the coagulation process through the formation of intra- or inter-molecular bridges between negatively charged HA molecules, whereas for Cs⁺, no inter-molecular bridges were formed. This work offers new insight into the interactions between HA and radionuclides and provides a prediction for the roles of HA in the transportation and elimination of radionuclides in severely polluted environments. The coagulation of HA in the presence of different metal ions was studied by batch, DLS, EEM and MD simulations, and the complexation between HA and metal ions strongly influenced the physicochemical behavior of HA in the natural environment.