Molecular dynamics simulation of cationic complexation with natural organic matter

Abstract

Molecular computer modelling of natural organic matter (NOM) and its interactions with metal cations in aqueous solution is a highly effective tool for helping to understand and quantitatively predict the molecular mechanisms of metal-NOM complexation. This paper presents the results of molecular dynamics (MD) computations of the interaction of NOM with dissolved Na+, Cs+, Mg2+ and Ca2+. They show that Na + forms only very weak outer-sphere complexes with NOM, whereas Cs+ interacts somewhat more strongly, but also mainly via outer-sphere association. Mg2+ interacts little with NOM due to its strongly held hydration shell. Ca2+ has the strongest association with NOM and forms inner-sphere complexes with NOM carboxylate groups. This last result supports the idea of supramolecular, Ca-mediated NOM aggregation. Cation-NOM binding occurs principally with carboxylate groups, and to a lesser extent with phenolic and other -R-OH groups. The contributions of other NOM functional groups are minimal. The diffusional mobility of NOM-bound cations is ∼20% (NOM-Na+ outer-sphere complex) to ∼95% (NOM-Ca 2+ inner-sphere complex) less than in aqueous solutions without NOM. The MD simulation results are in good agreement with NMR spectroscopic measurements for Cs-NOM solutions. © 2007 The Authors.