Molecular Simulation of Copper Complexation by a Model of Dissolved Organic Matter

The coadsorption of cations and organic matter to mineral surfaces has been proposed as a mechanism to stabilize soil organic carbon and explain enhanced cation adsorption in soils. Spectroscopic investigations of coadsorption in copper (Cu)-organic ligand-mineral systems have provided conclusive evidence that ternary surface complexes do occur in instances where the simple organic ligand possesses a minimal number of homogeneous binding sites. Although the results of many experimental and spectroscopic investigations of ternary systems with dissolved organic matter (DOM) suggest that ternary surface complexes may be present, unambiguous conclusions cannot be drawn due to the nature of DOM and the heterogeneity of DOM binding sites. Molecular simulations of ternary surface complexes are powerful tools that can be used in the interpretation of spectroscopic data from these experimental systems. Molecular simulations of Cu-DOM coadsorption at mineral surfaces require a model of Cu-DOM that adequately reproduces the physical-chemical behavior of natural Cu-DOM. The Cu-saturated Schulten DOM molecule, whose charge from deprotonated carboxyl groups was balanced by Cu2+ ions, was modeled using the COMPASS forcefield and energy minimization and molecular dynamics algorithms. Compared to previous simulations of Na- and Ca-saturated Schulten DOM, the Cu-DOM was more compact and spherical, as demonstrated by the decreases in the surface area, surface area: volume ratio, porosity, and radius of gyration and by an increase in the volume. The Cu-DOM complex is more polar than Ca-DOM and less polar than Na-DOM, as indicated by the dipole moment and extent of hydrogen bonding. The Cu2+ ion coordination environment contains an average of four oxygen atoms at 2.04 Å from the metal center. These first shell atoms are primarily water and carboxylate oxygen, although oxygen atoms from carbonyl, phenolic, and esters functionalities as well as nitrogen groups are also present. The DOM model reproduces the physical properties and reactivity of natural organic matter with Cu and is therefore an appropriate model for further simulations of Cu-DOM-mineral ternary surface complexes.