Geochemistry of alkaline environments

 

Geochemistry of Trace Elements in Alkaline Systems

Alkaline environments are common in cements, cement based waste solidification by-products, fly ashes, and evaporites. The alkaline materials from these systems are highly reactive when exposed to natural waters and are found to modify the soil and sediment mineralogy and solution chemistry. Knowledge of the major and trace element geochemistry of these systems is very important to predict the stability of cements and concrete, contaminant migration and reactions at waste disposal sites and in natural alkaline systems, and water quality in their vicinity. While the majority of previous geochemical investigations have focused at near neutral pH environments, trace element behavior in alkaline systems is poorly understood. Although information obtained at near neutral pH values represent geochemical conditions common to natural systems, several of the alkaline environments mentioned above often have pH values greater than 9.0. The mineralogy and geochemistry of these systems is highly complex, and trace element behavior in these systems cannot be deduced from those studies conducted at or below neutral pH conditions.

Ettringite (Ca6Al2(SO4)3(OH)1226H2O) and other minerals in its group (e.g. thaumasite, sturmanite, charlesite) are common in alkaline environments, and this mineral can be used as a surrogate to study trace element behavior in alkaline environments. Several researchers have shown that the formation of this mineral control the activity of several contaminants, both cations and oxoanions, in cement based waste solidification byproducts and alkaline flyashes. In addition, the stability and weathering reactions of ettringite helps in understanding the fate and transport of some of the major and trace elements in alkaline materials. Under ambient conditions, certain ions, such as PO43- and AsO43-, are found to form inner-sphere complexes, and certain other ions such as Cl -, ClO4-, and NO3- are thought to form outer-sphere complexes. Some other ions such as SO42-, SeO42-, and CrO42-are found to form both types of complexes, though these studies are highly controversial. In addition, it is not understood why certain ions form inner-sphere complexes while others form outer-sphere complexes, and the geochemical conditions that control their formation. Our research is focused on the following:

  • Stability and weathering reactions of ettringite.
  • Oxoanion interactions with ettringite surfaces and their substitution for sulfate in tunnels.

The structure of ettringite consists of columns and channels. The columns are made up of Ca and Al polyhedra, which coordinate to H2O (Ca only), and OH (Al and Ca). The sulfate ion occupies channels as an outer-sphere complex and forms H-bonds with Ca-coordinated water molecules.

 

Figure 1
Figure 1: Crystal structure of ettringite. Left: View perpendicular to the C-axis. Right: Section of a column. The water molecules coordinated to Ca atoms in the columns are removed for clarity.

 

Figure 2
Figure 2: Scanning electron microscopic images of ettringite needles.

 

The solubility and weathering reactions of ettringite were used to study the geochemical equilibria of the Ca(OH)2-Al2(SO4)3-H2O system at environmental pH conditions. Ettringite is a stable mineral above a pH of 10.7 and dissolved congruently with a log Ksp of -111.6. Between pH 10.7 and 9.5 ettringite underwent incongruent dissolution to gypsum and Al-hydroxide. The Al-hydroxy sulfate phases exhibited prismatic and anhedral shapes and had variable Al/S ratios. In addition, some new poorly crystalline Ca-Al-hydroxysulfate phases were identified in microscope studies when pH was acidic (~ 5). The reaction products are shown below.

 

Figure 3
Figure 3: Scanning electron microscope images of ettringite weathering products. a1 and a2: Gypsum crystals coated with Al-oxyhydroxides and sulfates; b: Al-hydroxy sulfate; c: poorly crystalline Ca-Al hydroxy sulfates.

 

The activities of Ca2+, Al3+, and SO42- suggest that the geochemistry of Ca(OH)2-Al2(SO4)3-H2O system in the pH range of 7 to 10 is simple and its component Ca(OH)2-SO3-H2O and Al2(SO4)3-H2O systems behave independently of each other. The precipitation of Al-hydroxysulfates below pH 7.0 significantly influenced Ca2+ and SO42- activities. This effect was pronounced when Ca-Al-hydroxy sulfate phases started precipitating below pH 5.0. Reaction path calculations using the EQ6 computer code predicted ion activities close to the experimental values above pH 5.0. The observed differences between thermodynamic modeling and experimental data below this pH can be explained by the formation of Ca/Al-hydroxy sulfate phases in the system, as detected by electron microscopy and x-ray elemental analyses. These results are useful in the prediction of Al and Ca geochemistry in natural systems.

More information on this research can be found in the following article:

Ettringite solubility and geochemistry of the Ca(OH)2-Al2(SO4)3-H2O system at 1 atm pressure and 298 K. Chem. Geol.148, 1-19.

Figure 4
Figure 4: Stability diagram for the Ca-Al-SO4-H2O system. Open circles are experimental data points, and the solid line with the arrow represents the reaction path predicted using EQ6. Estimated Al3+ activity in acid mine drainage and fly ashes are also plotted (filled points, collected from the literature).

For getting an improved understanding of the oxoanion reactions in alkaline systems, we focused our investigation on AsO43- and CrO42-, since they were reported to form different complexes on soil mineral surfaces under ambient pH conditions. One of our research objectives is to determine whether AsO43-, an ion that forms inner-sphere complexes at near neutral pH, can substitute for sulfate as an outer-sphere complex. Our studies with these oxoanions showed interesting results and the nature of these oxoanion-ettringite interactions are different from those reported for other mineral surfaces at neutral pH conditions. We examined the macroscale sorption and desorption of these anions and their molecular structures on ettringite surfaces using the Fourier transform infrared, Raman, and X-ray spectroscopy methods. The structure and grain sizes of ettringite were examined using X-ray diffraction and scanning electron microscopy, respectively. Our study shows that AsO43- can form both outer and inner-sphere complexes, and chromate can form inner-sphere complexes with ettringite. Further experimental details and the results obtained may be found in the following articles:

  • Arsenate sorption on ettringite: Macroscopic resultsEnviron. Sci. Technol. 31, 1761-1768
  • Arsenate interactions with ettringite: Vibrational spectroscopic resultsGeochim. Cosmochim. Acta, 62, 3499-3514
  • Arsenate interactions with ettringite: X-ray absorption spectroscopy results.
  • Arsenate interactions with CaO and formation of Johnbaumite.
  • Chromate sorption in ettringite: Macroscopic & spectroscopic results.

This research was carried out at The Ohio State University.