43Ca nuclear magnetic resonance (NMR) spectroscopy and helium ion and scanning electron microscopy (HeIM, SEM) provide novel molecular-scale insight into the interaction of NOM with mineral (smectite) surfaces, other NOM molecules, dissolved ions, and water, including the effects of hydration state, pH history, and ionic strength. Both the ionic strength and the protonation state of the NOM affect the flocculation in Ca2+-bearing suspensions of Suwannee River humic acid (HA), fulvic acid (FA), and NOM. The specific conditions leading to flocculation in the presence of Ca2+ varies for each NOM fraction. 43Ca NMR shows that Ca2+ is incorporated into flocculants of HA, FA, and NOM at basic pH in disordered structural environments with smaller mean Ca-O internuclear distances than at acidic conditions, where only HA forms large flocs at all ionic strengths examined. It is likely that Ca2+-NOM bidentate contact-ion pairs with deprotonated NOM carboxylic acid groups dominate at basic pH, while solvent-separated ion pairing and monodentate coordination to NOM functional groups dominates under acidic conditions, in agreement with molecular dynamics computer simulations. In contrast, for composites formed from suspensions of Ca-hectorite and the three NOM fractions, 43Ca NMR suggests that Ca2+ is structurally and dynamically more like Ca2+ in the base smectites than Ca2+ in NOM flocculants independent of the NOM protonation state and system hydration state. The results also support the idea that a significant fraction of the Ca2+ is bridging between the clay and the NOM. The NMR results also suggest that H2O activity is the most important control of Ca2+ dynamics on the kHz to MHz rate scales. HeIM and SEM/EDS results for the composites suggest that at basic pH organic matter is distributed homogeneously on a &lt;~100 nm length scale between and on the surfaces of packets of clay platelets. In contrast, under acidic conditions there is a mixture of homogeneous composite and flocculated NOM with little porosity at scales &gt;10 nm. Together, the data suggest that the pure NOM domains observed under acidic conditions in the composites do not contain significant quantities of Ca2+ and form via a hydrophobic mechanism as the solution is acidified. For the homogenous composite material, it is likely that NOM adheres to the smectite surface via cation bridging by partially hydrated Ca2+ and we hypothesize that these bridging interactions form under basic conditions.

Molecular-scale Interactions of Ca2+, NOM, H2O, and smectites in Ca-NOM-H2O and Ca-Smectite-NOM-H2O systems: Roles of pH history, NOM activity, H2O activity, and ionic strength

249th National Meeting (2015)

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