Abstract
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 Ca2+ and Mg2+ 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 Ca2+, Mg2+ and COO− groups. Monodentate and bidentate coordinations have been found between Ca2+ and COO− groups of different HA molecules in the same simulation system. Mg2+ only has a monodentate coordination with COO− group.
Highlights
Humic acids (HAs) are a common chemical component in soil, groundwater and sediment.The acids are ubiquitous in the environment and are mainly formed by the weathering and decay of plants [1,2,3]
Important criteria determining whether the NPT simulation system was in equilibrium were as follows: (1) temperature balance—the standard deviation of temperature change was required to be less than 5%; (2) energy balance—the energy was required to be constant or to fluctuate along a constant value; and (3) density balance—the density change was required to be less than 5%
It was found that the temperature, energy and density deviation of the last 200 ps were less than 3%, and the system reached equilibrium
Summary
Humic acids (HAs) are a common chemical component in soil, groundwater and sediment. The acids are ubiquitous in the environment and are mainly formed by the weathering and decay of plants [1,2,3]. A comprehensive study of the binding behavior of metal ions with HA, especially different aggregations of HA, will increase our understanding of their impacts in petrochemical wastewater treatment. To characterize the aggregation process of HA metal complexes, the interaction energy, RDFs, solvent accessible surface areas (SASAs) and hydrogen bonds of different metal ion complexes were analyzed by using MD simulation. These analyses enhance our understanding the structure, kinetics and aggregation characteristics of HA, which is crucial to evaluate the application of HA in the removal process of metal ions from petrochemical wastewater and to reduce membrane pollution
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