Molecular dynamics simulation of cationic complexation with natural organic matter

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 molec-ular mechanisms of metal-NOM complexation. This paper presents the results of molecular dynamics (MD) computations of the interaction of NOM with dissolved Naþ, Csþ, Mg2þ and Ca2þ. They show that Naþ forms only very weak outer-sphere complexes with NOM, whereas Csþ interacts somewhat more strongly, but also mainly via outer-sphere association. Mg2þ interacts little with NOM due to its strongly held hydration shell. Ca2þ 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-Naþ outer-sphere complex) to;95 % (NOM-Ca2þ 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