Metal Cation Complexation with Natural Organic Matter in Aqueous Solutions: Molecular Dynamics Simulations and Potentials of Mean Force

Abstract

Natural organic matter (NOM, or humic substance) has a known tendency to form colloidal aggregates in aqueous environments, with the composition and concentration of cationic species in solution, pH, temperature, and the composition of the NOM itself playing important roles. Strong interaction of carboxylic groups of NOM with dissolved metal cations is thought to be the leading chemical interaction in NOM supramolecular aggregation.Computational molecular dynamics (MD) study of the interactions of Na + , Mg 2+ , and Ca 2+ with the carboxylic groups of a model NOM fragment and acetate anions in aqueous solutions provides new quantitative insight into the structure, energetics, and dynamics of the interactions of carboxylic groups with metal cations, their association and the effects of cations on the colloidal aggregation of NOM molecules. Potentials of mean force and the equilibrium constants describing overall ion association and the distribution of metal cations between contact ion pairs and solvent separated ions pairs were computed from free MD simulations and restrained umbrella sampling calculations. The results provide insight into the local structural environments of metal-carboxylate association and the dynamics of exchange among these sites. All three cations prefer contact ion pair to solvent separated ion pair coordination, and Na + and Ca 2+ show a strong preference for bidentate contact ion pair formation. The average residence time of a Ca 2+ ion in a contact ion pair with the carboxylic groups is of the order of 0.5 ns, whereas the corresponding residence time of a Na + ion is only between 0.02 and 0.05 ns. The average residence times of a Ca 2+ ion in a bidentate coordinated contact ion pair vs. a monodentate coordinated contact ion pair are about 0.5 ns and about 0.08 ns, respectively. On the 10-ns time scale of our simulations, aggregation of the NOM molecules occurs in the presence of Ca 2+ but not Na + or Mg 2+ . These results agree with previous experimental observations and are explained 3 by both Ca 2+ ion-bridging between NOM molecules and decreased repulsion between the NOM molecules due to the reduced net charge of the NOM-metal complexes. Simulations on a larger scale are needed to further explore the relative importance of the different aggregation mechanisms and the stability of NOM aggregates.4

Effects of Ca2+ on supramolecular aggregation of natural organic matter in aqueous solutions: A comparison of molecular modeling approaches

“…The detailed analysis of Ca 2+ dynamics around the NOM carboxylic groups conducted for Run III shows that an average residence time of a bidentate coordinated CIP is about 0.5 ns, compared with the about 0.1 ps for a monodentate CIP coordination, and about 0.2 ns for a SSIP coordination (Iskrenova-Tchoukova et al, 2010). These values provide useful estimates for the time scale for the processes of Ca-NOM association in aqueous solutions and point out to the fact that our earlier simulation (Kalinichev and Kirkpatrick, 2007), presented here as Run I, was not long enough for reliable quantitative estimates of the structure and dynamics in this system (Table 2).…”

“…Ca 2+ cations can associate with these carboxylic groups via several typical and relatively stable coordination geometries (Iskrenova-Tchoukova et al, 2010). In a bidentate inner-sphere coordination (at the bottom of Figure 1), Ca 2+ is coordinated simultaneously with the two oxygen atoms of the carboxylic group and stays approximately equidistant from both of them predominately near the bisector plane orthogonal to the plane of the carboxylic group.…”

Interaction of Natural Organic Matter with Layered Minerals: Recent Developments in Computational Methods at the Nanoscale

“…In aqueous solution, the residence time of calcium ion coordination to NOM carboxylic groups was found to be 0.5 ns, more than an order of magnitude longer than for sodium ion coordination [107,108]. Aggregation of NOM particles occurred in calcium solutions due to ion bridging, but not in sodium solutions where ion bridging did not occur [107,108]. Ion bridging between NOM particles was also found to predominate near polyethersulfone membrane surfaces, while no evidence of NOM surface complexes was seen [101].…”

“…Kalinichev et al have used MD simulation to test ion bridging models of NOM aggregation in aqueous solution [107,108] and at polyethersulfone membrane surfaces [101]. In aqueous solution, the residence time of calcium ion coordination to NOM carboxylic groups was found to be 0.5 ns, more than an order of magnitude longer than for sodium ion coordination [107,108].…”

Effect of organic matter on phosphorus recovery from sewage sludge subjected to microwave hybrid pretreatment

“…(In this study, low concentrations of organic matter corresponded to low-MWt organics, such as organic acids). The complexation of metallic cations and organic acids is known to occur under certain conditions (Iskrenova-Tchoukova et al, 2010). This would lead to removal rates of Ca 2+ and Mg 2+ in the S 1K solution being much higher than in the S RAW , S MF , and S 10K solutions.…”