Scientific and technical objectives

Main purpose of this sub-project is a detailed clarification of the processes of phosphate fixation and phase transformation on calcite surfaces under varying hydrochemical conditions. Due to this, a basis will be formed to optimise the efficiency and sustainability of dissolved phosphate. This process is of importance in (i) reduction of environmental phosphate pollution of eutrophic waters and (ii) the recovery of phosphate as a resource from waste water and sewage sludge.

The scientific and technical objectives of the sub-project cover

1. the clarification of the mechanisms of phosphate fixation on calcite surfaces
2. the systematisation of the experimental results by molecular calculation (DFT)
3. the characterisation of the hydrochemical conditions and crystallographic parameters for optimised and effective fixation of dissolved phophate on calcite surfaces
4. the identification and development of calcite products for the technical application

Figure 1: Left - Scheme of a phosphate sorption isotherm on calcite after van Cappellen (1991); right - experimentally determined sorption isotherm of phosphate on limestone powder (1.75 m²/g) and precipitated calcium caronate (PCC; 37.45 m²/g).

As a novel approach of this sub-project methods like XAFS spectroscopy (in particular XANES) are used to identify the speciation of phosphorus on the calcite surface. By applying this method, a main goal is to understand the different phases of fixation and sorption and the transition zone in between (Fig. 1, left), respectively, on the calcite surface at the molecular scale. Here, section (A) marks a range of concentration of calcite (Cc) and PO₄^3- in which only adsorption on the calcite surface occurs (high Cc/PO₄^3-values). (B) shows the precipitatio of meta-stable Ca-P compounds, (C) the transformation into stable compounds and (D) the adsorption of phosphorus on the newly formed Ca-P surface (low Cc/PO₄^3-). Figure 1 (right) shows that the experimentally determined sorption isotherms can be divided in different sections. On the basis of these observations further analytical investigations will be carried out.

Figure 2: XANES Spectra at the P k-edge (ANKA SUL-X).

First results show that XANES spectroscopy is a suitable tool to identify the speciation of phosphorus on calcite (Fig. 2). Two characteristic features are observed at the P k-edge, the phosphate whiteline at 2152.0 eV and a shoulder at ~2155 eV. The latter is characteristic in apatites and decreases with phosphorus concentration on the calcite powder, which shows a change in the coordination of phosphate. The analysed calcite samples are representative for parts (A) to (C) of the schematic sorption isotherm (see Fig. 1). Additional EXAFS data will help to complete the characterisation of the atomic and molecular environment.

Participating institutions and companies: 

	Institute for Mineralogy und Geochemistry (KIT-IMG) Institute for Nuclear Waste Disposal (KIT-INE)
	Institute of Geosciences (UniFaM-IfG)
	Rheinkalk GmbH