Abstract
Reactions between aqueous Al and amorphous SiO2 and crystalline quartz were investigated in order to understand the speciation of dissolved Al in natural waters. Although these solids have the same chemical makeup, their physical structures differ in that the amorphous mineral is disordered while quartz is highly crystalline. These minerals were examined over several weeks with [Al] ranging from 0.05 mM – 86 mM over a pH range of 2 – 8.2 to determine which reactions occur. Possible reactions include 1) precipitation of aluminosilicates; 2) precipitation of Al(OH)3; 3) ion exchange between dissolved Al and surface Si; 4) sorption of Al onto the Si mineral surface; and 5) desorption/dissolution of Al from the mineral surface. Graphite Furnace Atomic Absorption (GFAA) was used to measure aqueous Al and Si concentrations before and after each reaction and 27Al Magic Angle Spinning Nuclear Magnetic Resonance (27Al MAS NMR at 11.7 T and 21.1 T) was used to analyze the coordination geometry of the solid Al species that formed. Experiments involving amorphous SiO2 (reaction time = 2 h) revealed that two reactions took place: Al sorption to the mineral surface occurred at pH > 3.7 while precipitation of an aluminosilicate occurred at pH > 5. Al geometry for both of these sites was found to be tetrahedral. Aqueous Si and Al data from the time dependent experiments (7 days, pH = 4.3, 5.4, and 8.2) showed that Al was not released into solution, either via dissolution or desorption, even after 7 days of reacting. Dissolved Si increased after 7 days of continuous mixing, especially at pH 4.3, in which the amount of Si in solution nearly doubled. This additional Si in solution was due to dissolution of the solid phase over time. Experiments with higher [Al] (24 mM and 86 mM, pH = 4.7) forced aluminum hydroxide precipitation with both 5-coordinate and 6-coordinate Al geometry. Investigation of the quartz mineral phase showed that dissolution did not occur over the pH range 4.3 – 8.2 at room temperature. With no dissolved Si available, an aluminosilicate precipitate did not form. At pH 6.4, there was some Al sorption to the surface but the majority of aqueous Al precipitated as aluminum hydroxide with 5-coordinate and 6-coordinate Al geometry. These results were very similar to the aluminum hydroxide precipitate formed in the amorphous SiO2 experiments at high Al concentrations. From these experiments, crystalline quartz appears to be stable at room temperature and does not react with aqueous Al. On the other hand, amorphous SiO2 has a high surface area and many surface sites, which allows for Al sorption as well as dissolution which leads to formation of an aluminosilicate precipitate.