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. 2019 Jan 14;4(1):1033-1044.
doi: 10.1021/acsomega.8b02705. eCollection 2019 Jan 31.

Hydration and Hydroxylation of MgO in Solution: NMR Identification of Proton-Containing Intermediate Phases

Jessica M Rimsza  1 , Eric G Sorte  1 , Todd M Alam  1
Affiliations

Hydration and Hydroxylation of MgO in Solution: NMR Identification of Proton-Containing Intermediate Phases

Jessica M Rimsza et al. ACS Omega. .

Abstract

Magnesium oxide (MgO)-engineered barriers used in subsurface applications will be exposed to high concentration brine environments and may form stable intermediate phases that can alter the effectiveness of the barrier. To explore the formation of these secondary intermediate phases, MgO was aged in water and three different brine solutions and characterized with X-ray diffraction (XRD) and 1H magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. After aging, there is ∼4% molar equivalent of a hydrogen-containing species formed. The 1H MAS NMR spectra resolved multiple minor phases not visible in XRD, indicating that diverse disordered proton-containing environments are present in addition to crystalline Mg(OH)2 brucite. Density functional theory (DFT) simulations for the proposed Mg-O-H-, Mg-Cl-O-H-, and Na-O-H-containing phases were performed to index resonances observed in the experimental 1H MAS NMR spectra. Although the intermediate crystal structures exhibited overlapping 1H NMR resonances in the spectra, Mg-O-H intermediates were attributed to the growth of resonances in the δ +1.0 to 0.0 ppm region, and Mg-Cl-O-H structures produced the increasing contributions of the δ = +2.5 to 5.0 ppm resonances in the chloride-containing brines. Overall, 1H NMR analysis of aged MgO indicates the formation of a wide range of possible intermediate structures that cannot be observed or resolved in the XRD analysis.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Visual appearance of the MgO samples (starting material) evaluated in this work.
Figure 2
Figure 2
SEM images of the MgO samples. (a) Marinco-MgO starting material. (b) Marinco-MgO following aqueous aging. (c) WIPP-MgO starting material. (d) WIPP-MgO following aqueous aging. Aged samples were exposed to 1 M NaCl at 80 °C for 16 weeks.
Figure 3
Figure 3
XRD analysis comparing starting material (bottom, black traces) for (a) Marinco-MgO and (b) WIPP-MgO with material aged in DI water (middle, blue traces) or 1 M NaCl (red, top traces) at 80 °C for 16 weeks. The (*) denotes an artifact from the sample holder.
Figure 4
Figure 4
1H MAS NMR of MgO samples aged at 80 °C for 16 weeks in DI water of the different brine solutions. (a) WIPP-MgO. (b) Marinco-MgO. Spectra deconvolutions of the (c) GWB-aged WIPP-MgO and (d) DI-aged Marinco-MgO.
Figure 5
Figure 5
DQ-filtered 1H MAS NMR and deconvolutions of the 16-week DI-aged Marinco-MgO sample as a function of the DQ excitation period, Nr. Multiple different 1H environments are observed.
Figure 6
Figure 6
2D DQ-SQ 1H MAS NMR correlation experiments for aged Marico-MgO at different DQ excitation periods a) Nr = 1 and b) Nr = 4. The multiple auto-correlation peaks between protons in the same environment (chemical shift) are identified by solid circles, the missing auto-correlation peaks by the dashed circles, and the colored boxes identify regions of weak correlation between different proton environments.
Figure 7
Figure 7
1H MAS NMR for (a) WIPP-MgO and (b) Marinco-MgO aged at 80 °C in DI water with varying exposure time. Deconvolutions of the (c) WIPP-MgO and (d) Marinco-MgO 1H MAS NMR spectra.
Figure 8
Figure 8
1H MAS NMR of (a,c) WIPP-MgO and (b) Marinco-MgO samples aged at 80 °C in 1 M NaCl with varying exposure time. Deconvolutions of the (c) WIPP-MgO and (d) Marinco-MgO 1H MAS NMR spectra.
Figure 9
Figure 9
Proton density of each MgO sample as a function of aging time. (a) Samples aged at 80 °C. (b) Samples aged at 20 °C.
See this image and copyright information in PMC

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