Integrated Molecular and Microscopic Scale Insight into Morphology

Jul 17, 2015 - Integrated Molecular and Microscopic Scale Insight into Morphology and Ion Dynamics in Ca2+-Mediated Natural Organic Matter Floccs. Geo...
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Integrated Molecular and Microscopic Scale Insight into Morphology and Ion Dynamics in Ca2+-Mediated Natural Organic Matter Floccs Geoffrey M. Bowers,*,† Haley E. Argersinger,† U. Venkataswara Reddy,‡ Timothy A. Johnson,† Bruce Arey,∥ Mark Bowden,∥ and R. James Kirkpatrick§ †

Division of Chemistry, Alfred University, 1 Saxon Drive, Alfred, New York 14802, United States Department of Chemistry and §College of Natural Science, Michigan State University, East Lansing, Michigan 48824, United States ∥ William R. Wiley Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354, United States ‡

ABSTRACT: Combined X-ray diffraction (XRD), helium ion microscopy (HeIM), and 43 Ca 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 ∼10 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−O 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 Å 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.

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INTRODUCTION

and the transport of inorganic ions, pharmaceuticals, and other organic contaminants.7,8,14,30,32−56 Despite recent progress in studying NOM in solution, numerous molecular-scale questions remain about its fundamental structure; the mechanisms of its interactions with surfaces, ions, other organic material, and water; and the role of dissolved ions, water, pH, ionic strength, and other factors in its aggregation or flocculation. For this paper, we consider aggregation to be the physical association of individual molecules to form low mass colloids stable in solution and flocculation to be the process by which these colloids adhere to make large particles visible to the eye and heavy enough to settle out of solution under centrifugation. Several recent laboratory experiments and computational chemistry investigations have suggested that large NOM macromolecules are in fact aggregates of smaller units held together by weak noncovalent interactions such as hydrogen bonding and electrostatic attraction among functional groups, cations, and

Natural organic matter (NOM) and its interaction with mineral surfaces have received considerable attention in the energy science, wastewater treatment, materials, environmental chemistry, and geoscience communities.1−29 NOM is a complicated mixture of organic molecules derived naturally from the decay of principally plant matter. Because of the multitude of starting materials and complex (bio)chemical pathways involved in NOM formation, it contains a wide variety of chemical structures and functional groups. It also has a range of molecular weights, and the mechanisms of its aggregation to form larger units remain controversial.1−3,11,30−32 The chemical and structural complexity of NOM and its ubiquitous occurrence in the environment make it an important, chemically active component of surface water, soils, and groundwater. Thus, understanding the behavior and chemistry of hydrated NOM across several length and time scales is important to understanding NOM−mineral interactions, the global carbon balance, soil biogeochemistry, plant nutrition, industrial fouling, and a variety of fundamental chemical processes in the environment, including weathering reactions © 2015 American Chemical Society

Received: June 9, 2015 Revised: July 16, 2015 Published: July 17, 2015 17773

DOI: 10.1021/acs.jpcc.5b05509 J. Phys. Chem. C 2015, 119, 17773−17783

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The Journal of Physical Chemistry C

introduced. The findings are in agreement with those of molecular dynamics modeling, atomic force microscopy, and small-angle X-ray scattering studies and what is known about NOM flocculation in water and wastewater treatment. They also provide an important reference to interpret studies of Ca− NOM−clay mineral composite materials representative of shale nanopores and surface soils; such studies are in progress in our laboratory.

anions.1−3,5,30,35,57 Other recent laboratory experiments and computational molecular dynamics (MD) studies have shown that Ca2+ in particular plays a crucial role in the aggregation of NOM in solution and at solid−fluid interfaces.1−5,29,58−61 Kalinichev and colleagues have used MD methods to show that Ca2+ more readily forms cation bridges between deprotonated carboxylic groups in model NOM than Na+, Cs+, or Mg2+ and that Ca2+ prefers to form bidentate contact ion coordination (inner sphere) complexes with the carboxylic groups of NOM molecules at near-neutral pH.1−3,5 The molecular scale roles that Ca2+ and other cations play in determining the structure and range of dynamic behaviors possible in NOM aggregates and floccs, however, remain poorly understood. Experimental and computational quantum chemical studies of model Ca phases and organic−inorganic hybrid biomaterials have shown that 43Ca NMR spectroscopy is a promising molecular-scale probe of ion interactions with organic matter, especially for oxygen-rich organic molecules such as the fulvic acid (FA; NOM that is soluble at all pH) and humic acid (HA; NOM that is soluble only at basic pH) components of NOM. Wong and colleagues have shown strong correlations between the mean Ca−O internuclear distance and the 43Ca isotropic chemical shift in simple organic compounds and clusters representative of biomolecules such as albumins and calmodulins.62,63 For these materials, as is normally the case, the NMR chemical shifts become more negative (more shielded) with increasing mean nearest-neighbor Ca−O distance and increasing Ca coordination number, both of which correlate with decreasing covalency of the Ca−O coordination environment. Wong et al. also report that the 43Ca quadrupolar constants and asymmetry parameters are sensitive to the geometry of the local coordination environment. Prior to the work of Wong et al., Drakenberg and colleagues performed numerous studies demonstrating the feasibility of 43Ca NMR to examine Ca 2+ binding by proteins, DNA, and other biomolecules.64−67 Together, these studies add to the growing consensus that Ca2+ can play a crucial role in the formation and behavior of ion−organic complexes and, thus, should also play a critical role in NOM behavior in natural waters, soils, and sediments. They also demonstrate that 43Ca NMR spectroscopy is an effective tool for investigating these interactions. In this paper, we present the first 43Ca NMR study of cation binding and dynamics in flocculated NOM hydrates and link these results to those from X-ray diffraction (XRD) and heliumion microscopy (HeIM) to provide new understanding of the NOM−cation interactions and how they affect NOM flocc morphology. In particular, we examine the effect of solution pH, pH history, and ionic strength on the molecular-scale Ca2+ binding environments in floccs of the standard Suwanee River NOM and its HA and FA fractions. We observe no changes in the flocc morphologies with the type of NOM fraction, solution pH, or solution ionic strength, suggesting that the influences of Ca2+−NOM interactions on flocc morphology depend little on these properties. However, we do observe that certain pHs and ionic strengths are required to induce flocculation in Ca2+bearing solutions and that onset of flocculation differs for HA, FA, and their combination (NOM). The solution pH and ionic strength have a strong influence on the local Ca2+ binding environment in flocculated NOM and the type of dynamic processes that affect the 43Ca NMR spectrum. The molecularscale details of the local Ca2+ structural environment, dynamics, and how they change with temperature are discussed, and two mechanisms of potential cation dynamic averaging are

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EXPERIMENTAL METHODS NOM Flocculation. To explore the dynamic behavior of Ca2+ in NOM materials using 43Ca NMR in a temporally efficient manner, it was necessary to use 43Ca-enriched Ca2+. In all cases the fully protonated and cation-free organic matter (OM) solutions were initially brought to a basic pH (∼11.5 ± 0.2) using a 12% 43Ca-enriched saturated Ca(OH)2 solution (noted 43Ca(OH)2 in the rest of this paper). This solution was prepared by decarbonating 12% 43Ca enriched CaCO3 to CaO in a muffle furnace at 750 °C for several hours. Sample mass was monitored before and after heating and confirmed a nearly stoichiometric loss of CO2 plus a minor additional mass loss attributed to adsorbed H2O (0.1−10% depending on the extent of CaCO3 drying before heating to 750 °C). An appropriate amount of the resulting 43Ca-enriched CaO to exceed the Ca(OH)2 solubility limit was then added to 200 mL of deionized water in a polyethylene bottle, agitated, and allowed to equilibrate until the supernatant pH reached 12.5−12.8. Typically, this solution was used to prepare the NOM floccs within 24−48 h to minimize any carbonate-forming reactions in our stock solution. To prepare the OM floccs at pH ∼11.5, approximately 100 mg of Suwannee River standard FA, HA, or the full NOM fraction (obtained from the International Humic Substance Society) was brought into contact with the saturated 43Ca(OH) 2 solution and equilibrated for 24 h in a dark environment. The dry HA, FA, or NOM powder was added to a 400 mL glass beaker, covered with approximately 70 mL of deionized water, and stirred at 300−500 rpm using a stir plate and Teflon-coated stir bar. A standard glass pH electrode mated to a Vernier LabPro interface was used to monitor the suspension/solution pH. A Vernier drop counter and buret were then used to titrate the NOM suspension/solution to pH ∼11.5 using the saturated 43Ca(OH)2 solution. Typically, 37− 70 mL of the 43Ca(OH)2 solution was required to achieve the desired pH, leading to final solution OM concentrations between 1 and 0.7 mg/mL. Though these concentrations are high for most natural waters, such concentrations were necessary to obtain a suitable amount of flocc for the NMR experiments. The pH ∼11.5 samples were equilibrated overnight in a dark hood, and the subsequent floccs were isolated via centrifugation at ∼12000g for 10 min. The supernatant liquid was decanted from the centrifuge tubes, and the NOM flocc was freeze-dried. Samples were not rinsed following centrifugation to avoid potential dissolution/disaggregation of the flocculated NOM, and the floccs received no postprocessing (e.g., grinding) due to their small particle size and low flocc yields (tens of milligrams per tube). The dried floccs were stored in a desiccator over P2O5 (∼0% relative humidity) until they were shipped to Pacific Northwest National Laboratory (PNNL) for NMR, XRD, and HeIM analysis. For Ca-HA and Ca-FA synthesized at pH ∼11.5, it was necessary to add isotopically unenriched CaCl2 as a background 17774

DOI: 10.1021/acs.jpcc.5b05509 J. Phys. Chem. C 2015, 119, 17773−17783

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The Journal of Physical Chemistry C electrolyte to form sufficient flocc for the NMR and XRD experiments. In these cases, 1 M CaCl2(aq) was slowly added dropwise to the pH ∼11.5 OM solution until the background ionic strength increased from the ∼0.004 value it develops at this pH due to free OH− and charge-balancing Ca2+ to a final value of 0.04 ± 0.01. This ionic strength was selected to match the ionic strength of the pH ∼ 2.5 sample Ca-HA described below. Ionic strength calculations throughout this paper account for the free H+/OH−, Cl−, and free/liberated Ca2+ as needed based on the specific sample preparation conditions. The OM floccs generated at pH ∼2.5 were prepared using a similar procedure. These samples were produced by suspending ∼100 mg of the desired OM fraction in ∼70 mL of deionized water in opaque polyethylene bottles, stirring the suspension at 300−500 rpm, and raising the pH to ∼11.5 with the 43Ca(OH)2 solution. After 30 min of equilibration at this pH, the suspension pH was adjusted to 2.5−3.0 via titration using a 0.1 M HCl solution, leading to solution ionic strengths of 0.04 ± 0.02 due to displaced Ca2+, free H3O+, and the charge-balancing Cl−. Once the samples reached the desired pH, they were transferred to a dark hood to equilibrate overnight. After equilibration, the samples were centrifuged, the supernatant liquid was decanted, and the flocc was freeze-dried and stored using procedures identical to those used for the pH ∼11.5 samples. At pH ∼2.5, appreciable flocc formed only for Ca-HA over a range of ionic strengths (∼10−2 to 100). After arrival at PNNL, all the flocc samples were equilibrated over 99% 2H2O liquid, generating an atmospheric 2H2O activity equivalent to 100% relative humidity (RH). Equilibration times varied from 6 to 48 h before samples were loaded into watertight rotors for the 43Ca NMR experiments. These initially wet floccs were used to prepare the samples for XRD and HeIM as well. Table 1 provides an identification key for the samples described in this paper. Note that the labels “pH 12” refer to

window of adhesive Kapton tape comprised most of the curved surface, which was ca. 40 mm from the sample surface and therefore did not contribute significant intensity in this parafocusing geometry. The powder was mounted in a shallow well in an off-axis quartz plate, further reducing background intensity. Patterns were acquired from 2° to 100° 2θ using a step size of 0.04° with a 16 s dwell time at each data point. The resulting diffraction patterns were compared to the ICDD database. Some samples exhibited evidence of minor crystallized CaCl2 or CaCO3 amounting to less than 1% of the total sample mass, as expected given the lack of rinsing during flocc isolation. Helium Ion Microscopy. Helium ion microscopy (HeIM) images were acquired for several of the floccs using the Zeiss Orion HeIM housed in the William R. Wiley Environmental and Molecular Sciences Laboratory (EMSL) at Pacific Northwest National Laboratory. The instrument was operated at an optimal imaging voltage of 30 kV using an aperture size of 10 μm and a beam blanking current of ∼0.5 pA. An Everhart− Thornley (E−T) detector was used to collect the signal from the sample. In all instances, the working distance and tilt were varied slightly between images and samples to produce optimal imaging conditions of electron detection, depth of field, and charge compensation. Typically, images at a minimum of three magnifications were acquired at several regions on each sample. 43 Ca NMR Experiments. 43Ca NMR spectra were acquired for all the floccs at a Larmor frequency of 57.185 MHz using the 850 MHz Agilent DDR2 spectrometer and a 4 mm TR probe in the NMR user facility at Pacific Northwest National Laboratory. All chemical shifts are referenced with respect to 0.1 M CaCl2(aq) using a secondary standard of 15% 43Caenriched CaO at 136.1 ppm. Bloch-decay magic angle spinning (MAS) spectra were acquired using a spectral width of 40.332 kHz, a spinning frequency of 10 kHz, and a 0.5 s pulse delay. For the OM floccs, 43Ca NMR spectra were obtained at temperatures of 173, 223, and 293 K. Temperature control was maintained by passing dry nitrogen gas through a liquid nitrogen tank and the standard Agilent VT stack. Spectra are summations of between 40 000 and 200 000 transients depending on the sample temperature and mass. A short 3 μs pulse width (∼π/6 with respect to CaCl2 solution) was used to acquire 2016 complex data points during each transient with the transmitter positioned a few kilohertz from 0 ppm. The sample moisture content was maintained by placing a Teflon screw in the rotor end-cap. Monitoring sample mass before and after the NMR experiments revealed mass changes from 0.0000 to 0.0004 g (2 ± 2% total mass), relatively small fractions of the moisture initially present. Data for the Bloch-decay experiments were processed using VNMRJ software (Varian) or iNMR (MestReC). All spectra were zero-filled to a Fourier number of 32 768 points. The first data point in each data set for the 850 MHz data was removed to eliminate probe-ringing artifacts. The free induction decay signals received the equivalent of 50 Hz of exponential apodization before Fourier transformation. Peak positions and line widths were obtained by fitting the data to Lorentzian functions using the program Abscissa, a free application written by Rudiger Bruhl available for MacOSX. The uncertainty in each parameter is dominated by potential uncertainty in the position of the peak maximum due to phasing and noise, which amount to ∼104 Hz (>an order of magnitude larger than the static line width). MD simulations show that the site hopping frequencies of Ca coordinated to −COO− sites on NOM are ∼109−1010 Hz, orders of magnitude greater than needed to achieve fastmotion-limit narrowing of the NMR resonances, supporting the model proposed here.1,3 Once again, we note that the absence of such line narrowing for the Ca-NOM-pH 12 sample with no background electrolyte demonstrates the necessity of the excess Ca2+ in the bulk solution to exchange with the surface sites, in agreement with the strong Ca2+−OOM associations observed by MD. This result also suggests that the presence of excess Ca2+ from background electrolyte may play a role in the uptake of water by the OM floccs, but this hypothesis must be investigated more thoroughly. When the excess electrolyte is absent at pH ∼11.5 (CaNOM-pH 12), the 43Ca dynamic averaging is dominated by the libration/reorientation of H2O coordinating Ca2+ combined with exchange between deprotonated surface sites within the proximity-restricted Ca2+ population. The S/N reduction with increasing temperature observed for Ca-NOM-pH 12 is quite similar to the 2H behavior of ice-1h75 in which the rate of 2H tetrahedral jump motion increases from ∼103 to 105 Hz as the temperature increases, causing lifetime broadening and signal loss that increases with increasing temperature. As in other hydrous materials where water is intimately involved in the dynamic processes, libration of water molecules and dynamic effects similar to those affecting the 2H signal of ice-1h are expected in the NOM floccs due to the importance of hydrogen bonding. Focusing on the ion motion itself, without a significant population of excess Ca2+ and no background Cl−, freezing of the pore H2O will not force extra cations and anions into the proximity-restricted domains. This means that pore ice formation will not change the overall 43Ca environment much when excess Ca2+ is absent and the OM functional groups are deprotonated, consistent with the observed lack of variability in the 43Ca line shape/position for the Ca-NOM-pH 12 sample. Likewise, the absence of excess Ca2+ in the pore H2O at the higher temperatures will dramatically slow Ca2+ exchange between proximity-restricted and solution-like environments, particularly with the need to maintain charge balance of the deprotonated NOM sites, increasing the Ca2+ residence times in proximity-restricted environments (inner and outer sphere). Together, these results suggest that exchange of Ca2+ between proximity restricted and bulk solution-like domains in hydrated pores is less important for the Ca-NOM-pH 12 sample than the other samples, consistent with the idea that more Ca2+ than needed to charge balance deprotonated OM functional groups is required to produce the narrow resonances observed at 223 K and higher in Ca-HA-pH 2.

pH. For both the Ca-FA-pH 12 and Ca-HA-pH 12 with background electrolyte, there is a wide range of possible Ca2+ NN environments at all temperatures (as for the Ca-HA-pH 2), including Ocarboxyl, Ophenol, Cl−, Namine, and H2O. There is no clear evidence for two distinct types of Ca2+ sites at 173 K such as observed for the Ca-HA-pH 2 sample, suggesting that greater overall disorder in the Ca2+ binding environments is produced at basic pH. Mechanisms of Ca2+ Dynamic Averaging in Ca-NOM Floccs. Together, the results above support the interpretation that all floccs with excess Ca2+ present experience Ca2+ dynamic averaging dominated by rapid exchange between proximityrestricted and bulk liquid-like environments in H2O trapped within NOM pores (Figure 5) and suggest that excess Ca2+ is critical to the dominance of a bulk liquid exchange-like dynamic averaging mechanism at 293 K. The narrow resonances and chemical shifts near 0 ppm at 293 K for Ca-HA-pH 2, Ca-HApH 12, and Ca-FA-pH 12 and relatively broad resonance at 293 K for Ca-NOM-pH 12 suggest that excess Ca2+ and Cl− from the background electrolyte are necessary for substantial dynamical peak narrowing at 293 K. In these samples the Ca2+ probably experiences a combination of exchange among proximity-restricted Ca2+ outer-sphere environments near OM surfaces, more tightly bound environments coordinated to deprotonated O-bearing OM functional groups, and bulk hydrous domains located in H2O in pores or surface films, with most Ca2+ spending a majority of the time rapidly tumbling in the liquid-like domains. At the lowest temperatures (173 K), bulk H2O domains within the NOM pore structure are probably frozen, forcing the ions to the NOM surface and leaving mobile H2O only in regions very close to the surface (∼5 Å) (Figure 5). The existence of such a nonfrozen surface layer at low temperatures has been well-documented experimentally and computationally (see refs 76 and 77 for a comprehensive listing of references). Exclusion of the cations and anions from the ice crystal structure is thermodynamically favored at cooling rates where ice-1h forms. Restricting the Ca2+ to the 5 Å mobile 2H2O region near the NOM surface (socalled proximity-restricted Ca2+) will lead to 43Ca peak broadening due to the absence of isotropic reorientation that occurs in bulk solutions and to some degree an increase in quadrupolar line broadening due to a non-time-averaged electric field gradient developing at the Ca2+ nucleus when near the surface.76−78 In the system with the most protonated functional groups (the Ca-HA-pH 2 sample), the two observed peaks may reflect either differing numbers of NN OOM in the Ca2+ coordination shell or sites with varying numbers of NN Cl−, since the anions will also be forced into the 5 Å surface region as the ice freezes, increasing the likelihood of Ca2+−Cl− ion pairing. When the OM is mostly deprotonated (the pH ∼11.5 samples), the more continuous distribution of local proximity-restricted Ca2+ coordination environments suggests that Ca2+−OOM− associations dominate in the near-surface mobile fluid layer. With increasing temperature, melting of the pore ice permits the excess Ca2+ and Cl− to migrate away from the OM surface to the bulk water environment, allowing rapid ion tumbling and facilitating rapid exchange between bulk liquid-like and proximity-restricted Ca2+ environments. This rapid exchange and large residence time in the bulk liquid-like regions leads to the observed 43Ca line narrowing at temperatures above 173 K (Figures 3 and 4) and homogenization of the time-averaged mean Ca−O bond distance toward the value in bulk aqueous

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CONCLUSIONS Ca NMR spectroscopy, X-ray diffraction, and He ion microscopy of flocculated Suwannee River NOM, FA, and HA as a function of solution pH and ionic strength provide significant new insight into the formation and morphology of 43

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The Journal of Physical Chemistry C Ca-mediated NOM floccs and the associated structural environments and molecular scale dynamics of Ca2+ in these materials. Though the flocculation behavior of HA, FA, and NOM differ in the presence of Ca2+(aq), the formation of flocculated OM depends significantly on the protonation state of the OM and the background electrolyte concentration, but not significantly on the overall oxygen density of the OM fraction (HA, FA, or NOM). Of particular note, Suwannee River NOM flocculates at basic pH when a nearly stoichiometric number of Ca2+ ions are present to balance the charge of deprotonated NOM functional groups, while pure HA and FA require excess CaCl2(aq) to form floccs at basic pH, suggesting that cooperative effects among the HA, FA, and Ca2+ may play an important role in many natural waters where HA, FA, and Ca2+ are present. All the Ca-OM floccs studied here are amorphous and have observable morphologic features at multiple scales, including micron-scale fragments with relatively flat surfaces, smaller, faceted globules with dimensions of hundreds of nanometers, and ∼10 nm globular building blocks visible in thin “string of pearl” structures. These results are in general agreement with AFM and small-angle X-ray scattering data in the literature, suggesting that the ∼101 nm scale globules represent the fundamental NOM molecular entities in solution. Variable temperature 43Ca NMR spectroscopy suggests that freezing behavior of pore water plays a critical role in Ca2+ dynamic behavior in Ca-OM floccs when background CaCl2(aq) is present. Above the temperature at which pore H2O freezes, we propose a mechanism involving rapid exchange of Ca2+ among surface and bulk-solution-like sites at rates in excess of 104 Hz. Below this temperature, all Ca2+ become proximity-restricted and dynamically averaged via slower 103 Hz scale exchange between inner- and outer-sphere sites combined with librations and reorientation of hydrating H2O molecules. In NOM flocc formed without excess CaCl2(aq) at pH ∼11.5, the Ca2+ remains tightly associated with the deprotonated functional groups and experiences incomplete dynamic averaging at all temperatures via mechanisms and rates similar to those for Ca-HA-pH 12 and Ca-FA-pH 12 floccs when the pore water is frozen. The mean Ca2+ coordination number and Ca−O internuclear distances are greater in floccs formed at pH ∼2.5 than in those formed at pH ∼11.5, perhaps reflecting the need for inner-sphere charge balance of deprotonated OM functional groups by Ca2+ at basic pHs.

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Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the United States Department of Energy Office of Science, Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. We thank Sarah Burton for assistance with setting up and accessing the NMR spectrometers. Helium ion microscopy images and the XRD patterns of the flocculated NOM were also obtained using instrumentation at EMSL. Thanks also to Ms. Arielle Polakos and Dr. Andrew Eklund for assistance in devising the 43Ca-enriched solution preparation procedure.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], tel 607-871-2822 (G.M.B.). Present Address

T.A.J.: Department of Environmental Engineering and Earth Sciences, Clemson University. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based on work supported by the United States Department of Energy, Office of Science, Office of Basic Energy Science, under Awards DE-FG02-10ER16128 (G.M.B., P.I.) and DE-FG02-08ER15929 (R.J.K., P.I.). The 43Ca NMR spectra were obtained using the High Field Magnetic Resonance User Facility housed at the Environmental 17781

DOI: 10.1021/acs.jpcc.5b05509 J. Phys. Chem. C 2015, 119, 17773−17783

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DOI: 10.1021/acs.jpcc.5b05509 J. Phys. Chem. C 2015, 119, 17773−17783