Experimental program

Thermodynamic data of aqueous species and mineral phases are derived from referenced and internationally acknowledged data bases (Thermodynamic data). However, several critical gaps still exist. Hence, experimental studies are undertaken to fill in missing data such as surface site areas, surface site densities, protolyses and surface complexation constants.

To a large degree these gaps concern mica and feldspars, being ubiquitously present minerals in the sedimentary formations in Northern Germany. Consequently, orthoclase and muscovite are chosen as major representatives of these groups to develop and perform laboratory studies. Additional sorption experiments are performed with quartz. Experiments comprise a wide range of defined geochemical conditions and cover extensive laboratory analyses. Within the research projects four PhD theses (see Cooperations and Publications) are realized - all covering different geochemical and experimental systems. Furthermore bachelor, master, and diploma theses are supervized (Publications).

First, state-of-the-art titration experiments are carried out to describe surface charge properties of muscovite, orthoclase, and quartz under varying geochemical conditions. Detailed information may be obtained from Britz (2018), a brief summary is provided here. In SMILE, a new titration technique will be tested. This page will be kept updated as often as possible. For more information do not hesitate to contact us.

  • Mineral phases: muscovite (20 - 400 µm), orthoclase (< 63 µm), quartz (150 - 180 µm)
  • pH: 3 - 8 (titration continued from low to high pH values)
  • Ar-atmosphere
  • different ionic strengths
  • Titrant: 5 mM L-1 HClO4
  • Solid/liquid ratio: 70 - 200 g L-1
  • Background electrolyte: NaClO4, Na2SO4

 

Batch experiments (Fig. 1) with muscovite, orthoclase and quartz are carried out to examine the sorption behavior of Eu3+, Al3+, Np, UO22+, Nd3+ and Ni2+ under different geochemical conditions. Solid-liquid ratios (M/V), element concentrations, and the pH are varied in each batch set. Experimental data is used to determine sorption coefficients, surface site densities and e.g. surface complexation constants of different surface complexes. Therefore, the geochemical speciation code PHREEQC (Parkhurst and Appelo, 1999) and the parameter estimation code UCODE (Poeter et al., 2005) are used (further information on Codes and Modelling and fitting) .

 

  • Mineral phases: muscovite, orthoclase, hematite, quartz, synthetic sediment (10 wt% muscovite, 10 wt% orthoclase, 80 wt% quartz)
  • Solid/liquid ratio:1/20, 1/80, 1/320 g ml-1 (200 - 800 µm)
  • Elements: Eu3+, Ni2+, Al3+, Sr2+, Cs+, UO22+, Np5+, Nd3+
  • Element concentration: 10-5 mol L-1 - 10-9 mol L-1
  • Background electrolyte: NaCl, NaClO4, Na2SO4
  • Ligand: CO32-, SO42-
  • Competing cations: Ca2+, Al3+
  • Ionic strength: 0.01 mol L-1, 1 mol L-1 NaClO4
  • pH: 3 - 9

 

Fig. 1: Experimental set-up of batch experiments under ambient conditions.

 

Column experiments (Fig. 2) offer detailed information on transport characteristics; here, transport characteristics of long-term safety relevant elements such as e.g. americium, uranium and curium are of major interest. Therefore, U(VI) and the trivalent chemical homologue Eu(III) as well as Ni(II) are used. After preliminary experiments with glass columns, a new experimental set-up is developed exclusively using PFA columns and PTFE frits. Transport processes though muscovite, orthoclase, and quartz columns are investiaged. Twofold experiments are realized for each geochemical boundary condition. Variations of geochemical parameters such as the pH, ligand concentration, and the composition of the background solution show major influences of transport and retardation. Detailed information can be fround in Maurer (2011), Schulze (2014), and Britz (2018),

 

  • Mineral phases: synthetic sediment, mix of mineral (quartz, orthoclase), muscovite, orthoclase, quartz, natural Gorleben sediment
  • Element: Eu3+, UO22+, Ni2+
  • Element concentration: 10-5 mol L-1
  • Ligand: SO42-, HCO3-   
  • Competing cations: Ca2+, Al3+, Mg2+    
  • Ionic strength: 0.01 mol L-1 NaClO4, > 0.01 mol L-1 NaClO4, artificial groundwater
  • pH: 3.5 - 5.5
  • Porosity: 35 % (quartz) - 60 % (muscovite)
  • Pore water velocity: 3.5 ml min-1 pm 0.1 ml min-1     
  • Background electrolyte: NaClO4, Na2SO4, artificial groundwater

 

Fig. 2: Experimental set-up column experiments (after Britz, 2018).

 

In future, column and batch experiments with a synthetic sediment and a natural Gorleben sediment will be carried out. The experiments will show whether predictions of transport processes using SCPs from static batch experiments satisfyingly represent experimental data (bottom-up approach, column experiments). Detailed information on the bottom-up approach and the application of SCPs to simulate Eu transport processes is provided in Britz (2018).

 

 

References:

Britz, S.: Europium sorption experiments with muscovite, orthoclase, and quartz: Modeling of surface complexation and reactive transport, PhD thesis, DOI 10.24355/dbbs.084-201806051207-0, GRS Braunschweig, Theodor-Heuss-Sr. 4, Abt. 401, 38122 Braunschweig (2018).

Klein, C., The 22nd edition of the manual of mineral science, John Wiley and Sons Inc., ISBN 0-471-25177-1 (2002).

Parkhurst, D.L., and Appelo, C.A.J., User’s guide to PHREEQC (Version 2) - A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations, U.S. Geological Survey Water-Resources Investigations Report 99-4259 (1999) 312 p.

Poeter, E.P., Hill, M.C., Banta E.R., Mehl, S., Christensen, S.,UCODE_2005 and Six Other Computer Codes for Universal Sensitivity Analysis, Calibration, and Uncertainty Evaluation, U.S Geological Survey, Techniques and Methods 6-A11 (2005) 299 p.

Simunek, J., van Genuchten, M. Th., Sejna, M., Toride, N., Leij, F. J., The STANMOD computer software for evaluating solute transport in porous media using analytical solutions of convection-dispersion equation, U. S. Salinity laboratory, Agricultural Research Service, U. S. Department of Agriculture, Riverside, California (1999) 20 p.