Association of dissolved aluminum with silica: Connecting molecular structure to surface reactivity using NMR☆
Introduction
Aluminum plays a key role in mineral reactivity in a wide range of environments that include the formation of hydrous aluminosilicates from natural waters, preservation of deep sea sediments, and precipitation of geothermal scales. Doucet et al. (2001) found that hydroxyaluminosilicate (HAS) phases form by competitive condensation of silica on a hydroxy-aluminum template and that these HAS phases have similar 27Al MAS NMR spectra to HAS found in soil horizons (Barron et al., 1982). The structural similarities between precipitated HAS and those found in soils suggest that the precipitation of HAS may regulate the bioavailability and thus toxicity of aqueous Al in natural waters (Doucet et al., 2001). As another example, Van Cappellen et al. (2002) examined the solubility of diatom cultures and siliceous sediments containing different amounts of Al and found that bulk solubilities typically decreased by an order of magnitude when the Al/Si increased by a factor of two. It was shown using X-ray Absorption Near Edge Spectroscopy (XANES) that Al in diatoms had been incorporated into 4-fold coordination (Gehlen et al., 2002). This suggests that incorporation of tetrahedral Al into the diatom framework during synthesis is most likely the reason that dissolution rates of biogenic opal in deep-sea sediments are reduced by over an order of magnitude relative to surface waters. Lastly, Carroll et al. (1998) measured silica precipitation rates for amorphous silica in complex geothermal brines at the Wairakei geothermal energy plant (New Zealand) and found that rates were much faster than in laboratory studies (pH 7–8, reaction time 48 h). They suggested that the accelerated silica precipitation rates found in the field were due to precipitation of Al-containing phases. Gallup (1997) reported an 27Al MAS NMR spectrum for an Al-rich silica scale from Awibengkok geothermal field which showed only tetrahedrally coordinated Al (27Al MAS NMR, [4]Al = 52.5 ppm; uncorrected for second-order quadrupolar shift), most likely due to aluminosilicate precipitation in which the Al had been supplied by the geothermal brine.
These examples illustrate the importance of understanding the connection between structure and reactivity. Due to the complex structural arrangement of natural systems, however, spectroscopic studies are often difficult to perform and interpret. Much of our understanding of metal cation sorption and precipitation comes from changes in bulk, aqueous measurements. While extremely useful, bulk-chemistry studies lack molecular-scale information which allows for the identification of reaction mechanisms that control solubility.
The motivation behind our study is to explain silicate surface reactivity in terms of molecular structure using NMR spectroscopy. We use NMR to identify the structural form of Al associated with amorphous silica over a pH range similar to that of natural waters. Because amorphous silicas are often used as a proxy for natural silicas, we anticipate the surface chemistry to be analogous to that of natural phases. The speciation information we obtain from NMR combined with the water chemistry data allow us to identify three reaction mechanisms that describe the solubility of Al and surface reactivity of amorphous silica.
Section snippets
Sample preparation
Amorphous silica was chosen because it serves as a high-surface area proxy for naturally occurring silica. Silica gel (Mallinckrodt silicar: 306 m2/g surface area by BET, 75–100 μm particle size) was washed three times ultrasonically with distilled water and then dried at 50 °C overnight. Batch-sorption experiments were performed by equilibrating the silica gel in a background electrolyte of 0.1 M NaCl at 25 °C for ∼22 h with constant stirring (solid/solution = 5 g/L). The pH was adjusted with 100 mM
Silica gel dissolution
The pH and time dependence of silica gel dissolution kinetics were first investigated in the absence of Al to understand how dissolved Al affects the structure and reactivity of amorphous silica. Fig. 1 shows the change in aqueous silica over time for gel suspended in a 0.1 M NaCl solution at pH 5.4. Initially, dissolution kinetics are fast but decrease at longer reaction times as silica levels approach constant concentration (>3 days). The data show that during t = 22–24 h, which is the reaction
Discussion
Reaction of dissolved Al with amorphous silica consists of at least three reactions. These reactions include sorption of [4]Al as an inner-sphere complex to silanol sites,surface-enhanced precipitation of an aluminum hydroxide,and bulk precipitation of an aluminosilicate phase.The pH-dependence of all three reactions is shown in Fig. 11 and mass balance calculations are shown in
Acknowledgments
We thank Kevin Knauss, Sarah Roberts, Bill Bourcier, and Carol Bruton for helpful discussions throughout the course of this study. We also thank three anonymous reviewers for their insightful comments. This work was funded by the Department of Energy, Office of Basic Energy Science and performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract W-7405-Eng-48 and Contract DE-AC52-07NA27344.
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