Integrated Molecular and Microscopic Scale Insight into Morphology and Ion Dynamics in Ca2+-Mediated Natural Organic Matter Floccs

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• 作者：Geoffrey M. Bowers ; Haley E. Argersinger ; U. Venkataswara Reddy ; Timothy A. Johnson ; Bruce Arey ; Mark Bowden ; R. James Kirkpatrick
• 刊名：Journal of Physical Chemistry C
• 出版年：2015
• 出版时间：August 6, 2015
• 年：2015
• 卷：119
• 期：31
• 页码：17773-17783
• 全文大小：488K
• ISSN：1932-7455

文摘

Combined X-ray diffraction (XRD), helium ion microscopy (HeIM), and ⁴³Ca nuclear magnetic resonance (NMR) results provide novel insight into the nano- and microstructure of flocculated NOM; the molecular-scale interaction among natural organic matter (NOM), dissolved Ca²⁺ ions, and water in NOM floccs; and the effects of pH and ionic strength on these characteristics. Suwannee River humic acid (HA), fulvic acid (FA), and NOM flocculated from Ca²⁺ bearing solutions share similar morphological characteristics on the 100 nm to micron scales, including micron-sized equant fragments and rounded, rough areas with features on the 100 nm scale. HeIM suggests that the NOM floccs are built from a fundamental spheroidal structure that is 鈭?0 nm in diameter, in agreement with published AFM and small-angle X-ray scattering results. Calcium is incorporated into these floccs at 100% relative humidity in a wide range of disordered structural environments, with basic pH leading to shorter mean Ca鈥揙 distances and lower mean coordination numbers with respect to floccs formed under acidic conditions. The NMR results show that dynamical processes involving water and Ca²⁺ occurring at frequencies >10⁴ Hz are important for hydrated OM floccs, in agreement with published molecular dynamics simulations of OM in solution. From the NMR results, we find evidence for two Ca²⁺ dynamic averaging mechanisms: one related to rapid exchange (>100 kHz) between surface proximity-restricted (those within 5 脜 of a surface) and bulk solution environments when excess Ca²⁺ is present in the pore solution when pore water is unfrozen and a second consisting of intermediate scale (tens of kHz) site exchange among strongly sorbed inner-sphere sites when excess Ca²⁺ is absent and the carboxylic and phenolic functional groups of the NOM are deprotonated.