Study of the Adsorption of Humic Acid with Zn²⁺ by Molecular Dynamic Simulation and Adsorption Experiments

The adsorption of humic acid with zinc ions was investigated with special emphasis on effects of the number of carboxyl groups dissociated by molecular dynamic simulations. The computational results were further confirmed by adsorption experiments. Molecular dynamic results demonstrated that the adsorption energies between humic acid and Zn²⁺ rose gradually with the increase of the number of carboxyl dissociated. Moreover, the adsorption energies between humic acid and zinc species in decreasing order were Zn\( \left( {{\text{H}}_{2} {\text{O}}} \right)_{6}^{2 + } \)> Zn(OH)\( \left( {{\text{H}}_{2} {\text{O}}} \right)_{5}^{ + } \)> Zn(OH)₂(H₂O)₄, which can be attributed to the hydroxylation of zinc ions, reducing the electrostatic attraction of humic acid with Zn²⁺. According to the adsorption experimental results, it was found that the pseudo-second-order kinetic model could be the best one to describe the adsorption process of Zn²⁺ onto humic acid surface. The pH-dependent experimental results indicated that the amount of Zn²⁺ adsorbed rose abruptly with the increase of pH at pH < 5, reaching the maximum at pH = 5, which were verified by means of zeta potential tests. This work can provide a better understanding of the adsorption between humic acid and Zn²⁺ at the microscopic scale.