Scientific and technical objectives

Goal of Recawa subproject A is to quantify sorption of various environmentally relevant inorganic trace elements at calcite, to characterize the sorption mechanisms at a molecular level, and to develop suitable models to quantify adsorption and incorporation as a function of aqueous solution composition. Such models can e.g. be applied to optimize water purification processes.

So far the molecular structure of the calcite-water interface has been studied in-situ by means of surface diffraction (Figure 1).

Figure 1: (left) Scheme showing the principle of surface diffraction measurements; (right) example for surface diffraction data: The 20L “crystal truncation rod” (CTR) measured at calcite (104) in contact to solutions of various compositions.

Surface Diffraction measurements basically showed that above terrace planes at the calcite(104) face two full layers (100 ± 8%) of water molecules (or OH^-) in 2.4 and 3.2 Å distance from the surface persist in all  the contact solutions investigated. Ca²⁺ and CO₃^2- ions at the surface relax only slightly from their bulk positions.

The structural information of the surface diffraction study and numerous data from zetapotential measurements have been used to constrain the parameters of a Basic Stern surface complexation model (SCM) for calcite (Figure 2). This SCM will in the future be adapted to enable mathematic description of adsorption reactions at the calcite surface.

Figure 2: The Basic Stern SCM for Calcite considers, as indicated by surface diffraction results, only protonation and depotonation reactions in the 0-plane. The zeta potential is mainly determined by Ca²⁺ and CO₃^2- ions adsorbing as outer-sphere complexes (in the b-plane).

In batchtype-adsorption experiments the adsorption of SeO₃^2- and SeO₄^2- is investigated as examples for anionic trace elements. Fe, Mn and UO₂²⁺ will be investigated as examples for cationic species. Adsorption sites of Se species at the calcite surface will be characterized by mean of polarization dependent grazing incidence EXAFS and by resonant anomalous surface diffraction.

With Fe, Mn and UO₂²⁺ as examples it will be tested in how far results from experiments on synthetic “pure” calcite can be transferred to calcite products, produced from natural lime stone, as they are used in water purification plants. An especially designed “mini water purification plant” will be used to verify if the applicability of the calcite SCM to calculate the filtering capacity of calcite filter columns.

Figure 3: Specially designed “mini water purification plant” at Rheinkalk Akdolit.

Due to the reactivity of the calcite surface and the ability of the calcite structure to tolerate variations in the chemical composition of calcite, trace elements do not only adsorb at the calcite surface but can also be incorporated into the calcite structure, by subsequent dissolution – reprecipitation reactions. At calcite equilibrium and standard conditions such incorporation processes run extremely slow and are superimposed by adsorption reactions. Therefore incorporation reactions are investigated by specific crystal growth experiments in mixed flow reactors (MFR). In MFR experiments crystal growth is investigated at steady state conditions at low supersaturations (SI<1) close to equilibrium. A schematic sketch of a MFR is shown in Figure 3.

Figure 3: Calcium-, carbonate, and selenium containing solutions are pumped into the MFR. There Se doped calcite grows onto calcite seed crystals (5-20 µm). The MFR content is agitated by a suspended magnetic stirring bar.

Two parameters result from MFR experiments: 

1. A partition coefficient, D, indicating the tendency of a trace element to enrich in the solid phase (D>1) or in the liquid phase (D<1).Trace elements investigated so far in the frame of Recawa subproject A exhibit no tendency for enrichment in the sold phase: D(U(VI)) ≈ 0.02, D(Se(IV)) ≈ 0.01, and D(Se(VI)) ≈ 0.001. It could be verified by quantum chemical simulations that tetrahedral Se(VI)O₄^2- species are not compatible with the calcite structure.
2. A macroscopic crystal growth rate, R. The growth rate, R, as a function of supersaturation, SI, is shown in Figure 4. This plot illustrates how the trace elements interact with the calcite surface and thereby inhibit calcite growth.

Figure 4: Calcite growth rate (R) in the MFR as a function of supersaturation (SI). Compared to pure calcite (orange, Cc) calcite growth rates are reduced in the presence of Se(VI) and Se(IV)  (green and blue datapoints, respectively). U(VI) (red datapoint) seems to have no influence on the crystal growth rate.

Macroscopic growth rates from MFR experiments will be compared to microscopic growth rates as seen in in-situ atomic force microscopy (AFM) crystal growth experiments. The influence of the cationic trace element Eu(III) on the microscopic calcite growth processes is shown in Figure 5.

Figure 5: Growth spirals at the calcite (104) face at pH 8.3 and SI(Calcite) = 0.6. Left without europium, and right in presence of 1 µmol/L europium in solution. It’s clearly visible how the presence of Eu(III) in solution changes the morphology of the growth spirals. This can be explained by the anisotropy of the steps. Obtuse [48-1]₊ and [-441]₊ steps (lower right) and acute [48-1]_- and [-441]_- steps (upper left) of the polygonal growth spirals are influenced differently by the presence of Eu(III) in the contact solution.

Participating institutions and companies:

	Institute for Nuclear waste Disposal (KIT-INE) Institute for Mineralogy and Geochemistry (KIT-IMG)
	Institute of Geosciences (IfG)
	Rheinkalk Akdolit GmbH & Co. KG