Coagulation mechanisms of humic acid in metal ions solution under different pH conditions: A molecular dynamics simulation

Humic acid (HA) exerts a variety of significant environmental and geochemical influences on the soils, sediments and aqueous environments. The interaction with metal ions induces strong HA-metal complexation, thus effecting the transport of the toxic metals as well as the colloidal aggregation of HA. In the present work, we systematically report and analyze the aggregation mechanisms of HAs in solutions filled with different heavy cations (Ag+, Cd2+ and Cr3+) or common metal ions (Na+, Ca2+ and Al3+) under neutral and low pH conditions by using molecular dynamic simulations. We aim to explore the effects of pH, metal ions type and other possible weak interactions on the aggregation capabilities of HA. Scrutiny of the simulation results showed that the aggregation of HAs under neutral condition was driven by the HA-metal complexation which combined the effects of electrostatic attraction and inter-molecular bridging between cations and COO– groups. Larger extent of aggregation was found in heavy metal ions compared with the common ones. On the other hand, under low pH condition, due to the protonation states of carboxyl and phenolic group, the aggregation of HAs was stabilized mainly by weak forces, such as hydrogen bonds between different functional groups. In addition, other weak interactions such as the hydrophilic and hydrophobic effects, the cation-π interactions have also been proposed to be progressive effects on the coagulation behavior. Our computational studies give supplement to the experimental observation and provide insights into the intrinsic mechanisms of the aggregation behavior of HAs and their complexation with metal ions. Such computational modelling supplied a highly effective tool for qualitatively evaluating their roles in environmental remediation.