²H and ²⁷Al Solid-State NMR Study of the Local Environments in Al-Doped 2-Line Ferrihydrite, Goethite, and Lepidocrocite

Abstract

Although substitution of aluminum into iron oxides and oxyhydroxides has been extensively studied, it is difficult to obtain accurate incorporation levels. Assessing the distribution of dopants within these materials has proven especially challenging because bulk analytical techniques cannot typically determine whether dopants are substituted directly into the bulk iron oxide or oxyhydroxide phase or if they form separate, minor phase impurities. These differences have important implications for the chemistry of these iron-containing materials, which are ubiquitous in the environment. In this work, 27Al and 2H NMR experiments are performed on series of Al-substituted goethite, lepidocrocite, and 2-line ferrihydrite in order to develop an NMR method to track Al substitution. The extent of Al substitution into the structural frameworks of each compound is quantified by comparing quantitative 27Al MAS NMR results with those from elemental analysis. Magnetic measurements are performed for the goethite series to compare with NMR measurements. Static 27Al spin–echo mapping experiments are used to probe the local environments around the Al substituents, providing clear evidence that they are incorporated into the bulk iron phases. Predictions of the 2H and 27Al NMR hyperfine contact shifts in Al-doped goethite and lepidocrocite, obtained from a combined first-principles and empirical magnetic scaling approach, give further insight into the distribution of the dopants within these phases.

Temperature dependent structure and dynamics in smectite interlayers: 23Na MAS NMR spectroscopy of Na-hectorite

“…Thus, there has been substantial effort devoted to understanding the structural, dynamical, and energetic aspects of the interaction of ions and water with geochemically important minerals 1-28 and materials of technological importance. [29][30][31][32] The temperature and pressure dependence of these interactions is complex and has been a long standing topic of discussion. [33][34][35][36][37][38] The smectite group of clay minerals has been of particular interest in geoscience, chemistry and materials science, because of their abundance and ability to intercalate a wide variety of organic and inorganic cations, along with water and other uid species.…”

Imaging of Al/Fe ratios in synthetic Al‐goethite revealed by nanoscale secondary ion mass spectrometry

“…X‐ray diffraction (XRD) is the most commonly used standard method to estimate the amount of Al substituted in the goethite structure . Other spectroscopic methods such as Fourier transform infrared spectroscopy (FTIR), X‐ray absorption near‐edge structure (XANES) spectroscopy, Mössbauer spectroscopy and 27 Al‐nuclear magnetic resonance (NMR) spectroscopy have also been used . However, these studies concentrated on the changes in the unit cell dimension as affected by Al‐substitution and the arrangements of the Al ions within the crystal structure.…”

“…4,6,9,13,20 Other spectroscopic methods such as Fourier transform infrared spectroscopy (FTIR), X-ray absorption near-edge structure (XANES) spectroscopy, Mössbauer spectroscopy and 27 Al-nuclear magnetic resonance (NMR) spectroscopy have also been used. 8,[21][22][23][24][25][26] However, these studies concentrated on the changes in the unit cell dimension as affected by Al-substitution and the arrangements of the Al ions within the crystal structure. In contrast, nanoscale secondary ion mass spectrometry (NanoSIMS) allows the measurement of elemental abundances in Fe-oxide minerals with high spatial resolution and a low detection limit.…”

Probing the local environment of substitutional Al $$^{3+}$$ 3 + in goethite using X-ray absorption spectroscopy and first-principles calculations

“…The local structure of aluminous goethites has been studied using various experimental and theoretical approaches (Ildefonse et al, 1998;Alvarez et al, 2007;Bazilevskaya et al, 2011;Blanchard et al, 2014;Kim et al, 2015). By performing solid-state nuclear magnetic resonance experiments (NMR), Kim et al (2015) show that the use of static 27 Al spin echo mapping provides clear evidence of Al incorporation into the goethite structural framework.…”

“…By performing solid-state nuclear magnetic resonance experiments (NMR), Kim et al (2015) show that the use of static 27 Al spin echo mapping provides clear evidence of Al incorporation into the goethite structural framework. Alvarez et al (2007) investigate the local structure of (Al,Mn)-substituted goethites using XANES at the Al, Fe and Mn K-edges and EXAFS (extended X-ray absorption fine structure) at the Mn and Fe K-edges.…”