Effects of dissolved Ca2+, Mg2+, and Na+ ions on the supramolecular aggregation of natural organic matter in aqueous solutions

The complexation of natural organic matter (NOM) with metal ions, minerals and organic species in soil and water allows NOM to form water-soluble and water-insoluble aggregates of widely differing chemical and biological stabilities. Metal-NOM interaction induces strong correlations between the concentration of natural organic matter and the speciation, solubility and toxicity of many metals in the environment. In water purification and desalination, NOM is also implicated in fouling of nanofiltration and reverse osmosis membranes, either as the primary foulant or as a conditioning layer for microbial attachment ("biofouling"). In this work we investigated the effects of various metal ions on NOM aggregation in aqueous solutions, by a combination of dynamic light scattering (DLS), small angle neutron scattering (SANS) and large-scale molecular dynamics (MD) computer simulations. This allows a detailed molecular-scale statistical analysis of the size and the structural topology of metal-NOM aggregates. The DLS measurements show that Ca2+ ions present in a Suwannee River NOM (SRNOM) solution lead to the formation of a wide range of supramolecular structures with sizes between 100 and 1,000 nm. In contrast, Mg2+ and Na+ do not affect the aggregation of SRNOM as strongly. SANS data are inconclusive but indicate the presence of quite large (>50 nm) fractal particles formed presumably through a cluster-cluster aggregation. MD simulations confirm these observations and show that NOM can aggregate in aqueous solutions by two different mechanisms. On the one hand, NOM molecules can spontaneously aggregate by hydrogen bonding between their functional groups when only Na+ and Mg2+ are present as background cations. This promotes the formation of uniformly shaped NOM clusters. On the other hand, if Ca2+ ions are present in solution, they can more strongly bind two different NOM molecules by co-complexing the carboxylate groups, thus promoting the formation of longer linear and branched aggregate structures. When Ca2+ is added to the MD simulated solution, the aggregation also equilibrates noticeably faster (within 5 ns) and the observed aggregates are more stable than those with Mg2+ or Na+.