| Summary: | Humic acids (HAs) have important environmental and geochemical effects on soil, water environments and sediment. HAs strongly complex some metal ions, which affects the migration of metal ions and the colloidal aggregation of HA. Here, the complexation of Ca<sup>2+</sup> and Mg<sup>2+</sup> with HA in aqueous solution under neutral conditions has been systematically studied by molecular dynamics (MD) simulation. The results show that the aggregation of HA is caused by the complexation of HA and metal ions, mainly due to the intermolecular bridging between Ca<sup>2+</sup>, Mg<sup>2+</sup> and COO<sup>−</sup> groups. Monodentate and bidentate coordinations have been found between Ca<sup>2+</sup> and COO<sup>−</sup> groups of different HA molecules in the same simulation system. Mg<sup>2+</sup> only has a monodentate coordination with COO<sup>−</sup> group.
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