Eawag_06074
Sulphate and Chromate-AFt Solid Solutions; Characterization
and Thermodynamic Modeling
Sabine Leisinger 1,2, Barbara Lothenbach3, Gwenn Lesaout3, Ralph Kägi1, Bernhard Wehrli1, C.Annette
Johnson1
1
Swiss Federal Institute of Aquatic Science and Technology (Eawag), Ueberlandstrasse 133, CH-8600 Duebendorf,
Switzerland
2
Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich CHN, CH-8092 Zürich, Switzerland
Swiss Federal Laboratories for Materials Testing and Research (EMPA), Section Concrete and Construction Chemistry,
8600 Dübendorf, Switzerland
3
Abstract
The aim of the research project is to determine the processes controlling the solubility of chromate in cementitious
matrices in order to be able to predict porewater concentrations from solid-phase composition. In the absence of
thermodynamic data concerning possible CrO4 limiting phases we are at present not in a position to predict
solubility of CrO42- in cementitious systems. In a first step solid solutions of CrO4- and SO4-AFt have been
prepared, characterized and thermodynamic stability ranges were determined. Solubility calculations of the solid
solution series were carried out using the geochemical modelling code GEMS (Gibbs Energy Minimization
Selektor). Solubility data are presented in a Lippmann diagram.
Keywords : chromate, solid solution, solubility calculation
1. Introduction
Heavy metals are increasingly found in association with cementitious materials that are exposed in the environment and that
will, in the long term, be susceptible to leaching. On the one hand, wastes are increasingly used in the production of
cements. They may be added as raw meal substitutes, fuels or as mineral additives. On the other hand cement is used to
stabilize hazardous wastes that contain high concentrations of inorganic contaminants through both physical and chemical
processes [1, 2] . Therefore it is important to be able to predict long-term leachability. Chromium is of particular interest
because it is present in two thermodynamically stable redox states in the environment as the trivalent Cr(III) and the
hexavalent Cr(VI) which appears as oxyanion CrO42- under ambient conditions. Cr(VI) is very toxic and mobile and poses
therefore a potential leaching problem. It is generally accepted that chromate incorporates in AFt and AFm phases in
hydrated cement pastes and that these phases have a substantial uptake capacity [3]. However published solubility data of
pure chromate-AFt and -AFm phases exist only from Perkins [4, 5]. To date there are no thermodynamic data published for
CrO4- /SO4-ettringite solid solutions. Solid solution formation can stabilize CrO4 by lowering the solubility product
compared to the system without solid solution.
The aim of this study is to investigate changes in solubilities and solid-phase characteristics resulting from partial SO4 and
CrO4 substitutions in the ettringite structure. Derived thermodynamic data will help to model chromate leachability out of
Cr(VI)-doped cements.
2. Experimental Methods
2.1 Materials
The used chemicals were all of pro analysis (p.a.) grade. Calcium Chromate (CaCrO4), Calcium Carbonate (CaCO3),
Calcium Sulphate (CaSO4•2H2O) and Al2O3- powder were bought. The handling of materials, the sampling and the pH
measurements were carried out in a glovebox to prevent possible CO2 contamination. Ultrapure water (resistivity
>18MΩ.cm) was used for the solutions and rinsing processes.
2.2 Mineral synthesis
Ca6[Al(OH)6]2(CrxS1-xO4)•26H2O = 6Ca2++2Al(OH)4- + 3x CrO42- + (3-3x)SO42- + 4OH- +26H2O. Solid solution phases
were synthesized by adding stoichiometric amounts of freshly burnt C3A, CaCrO4 and CaSO4•2H2O to ultrapure water. The
amounts of reactants varied to obtain different chromate mole fractions (XCrO4 = XCrO4/(XCrO4+XSO4). Solids with
XCrO4 of 0, 0.1, 0.2, 0.4, 0.5, 0.8 and 1 were synthesized. The liquid/solid ratio was 20. The mixtures were stored n sealed
HDPE bottles of 50ml and shaken at a temperature of 25°C on a rotary shaker at 125rpm. Equilibration time was 90 days.
For each solid solution phase between 8 and 15 samples were analyzed to show the reproducibility.
2.2 Characterization of the solid and liquid phase
The solids were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and thermogravimetric
analysis (TEM). Solutions were analyzed using inductively couples plasma optical emission spectroscopy (ICP-OES) and
inductively couples plasma mass spectroscopy (ICP-MS). Charge balance errors of all samples lie between 1 and 20%. A
part of the liquid phase was used for pH-measurement which was carried out with a pH-electrode Aquatrode Plus, Pt1000.
First the pH electrode was calibrated by titration with fresh KOH-solutions (0.001-1M) to minimize the alkali error. Ionic
strength as well as the measured temperature of the solution was considered.
2.3 Thermodynamic modeling
Calculations of aqueous activities, solubility and stability ranges were done with the geochemical modeling code GEMS
(Gibbs Energy Minimization Selector). GEMS computes mass balances, based on equilibrium phase assemblages and
speciation in a complex chemical system from its total bulk elemental composition. Thermodynamic data were taken from
Hummel [6].
3. Results and discussion
3.1 Solid characterization
XRD analysis of the solid solution series show sharp peaks indicating well crystallized phases. The CrO4-endmember is a
little bit less crystalline observed in peak broadenings. Peak positions correspond to ettringite. Beside ettringite gypsum is
detected in some of the samples closer to the SO4-endmember. Other additional phases are not detected. Beside phase
identification unit cells are determined. The values for a- and c-axis as well as the unit cell volume are consistent with the
values of the powder diffraction files 041-0218 for CrO4-ettringite and 037-1476 for SO4-ettringite. A general peak shift to
lower 2Theta values starting at the pure chromate endmember is observed. This is expected since the CrO4-ion (2.4Å) has a
larger radius than the SO4-ion with 2.3Å. A second possible feature of solid solution investigations by XRD are peak
splittings. Peak splitting is shown in Figure 1 for the sample Cr0.6. This indicates that there are two phases present instead
of one and that there must be a miscibility gap in the solid solution series. According to the present samples the miscibility
gap exists for samples with CrO4 mole fractions between 0.4 and 0.6.
Figure 1: Solid solution phases of pure CrO4-ettringite (Cr1) and pure SO4-ettringite (Cr0) show peak shift and peak
splitting of sample Cr0.6.
2440
1 phase
2 phases
1 phase
3
unit cell volume (Å )
2420
2400
2380
2360
2340
2320
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
XCrO 4
Figure 2: Volume of unit cell parameters indicating miscibility gap for samples Cr0.4 and Cr0.6. The upper limit of the pure
CrO4-ettringite is unknown. Powder diffraction file values of both end members are indicated with a circle.
3.2 Liquid characterization and solubility calculation
Dissolved concentrations of Ca, Al, CrO4 and SO4 have been determined. The dissolution of solid phases proceeded
incongruently; the ratio of the determined elements is different in the solid and in solution. Incongruent dissolution indicates
the precipitation of a secondary mineral. In this case it was gypsum which was detected by XRD. Since CrO4-ettringite is
less stable than the SO4 counterpart, higher ion concentrations for the CrO4 end member were expected and occurred.
Using the modeling code GEMS, activities and solubility calculations were done based on the measured ion concentrations.
In this study a solubility product for pure CrO4-ettringite of -40.3 +/- 0.4 was determined for the reaction
(
Ca 6 [ Al (OH )6 ]2 CrO 42 −
)
3
⋅ 26 H 2 O = 6Ca 2 + + 2 Al (OH )4 + 3CrO 42 − + 4OH − + 26 H 2 O ; while for SO4−
ettringite a solubility product of -45.0 +/-0.9 resulted. From the data given in Perkins and Palmer [4] a slightly lower
solubility product of logKs0 = -41. +/-0.3 for CrO4-ettringite was calculated. Solubility data of a solid solution series can be
illustrated with a Lippmann diagram by calculating the total solubility product [7]
∑∏ ={Ca } ⋅ {Al(OH ) } ⋅ [{SO }+ {CrO }] ⋅ {OH } ⋅ {H O}
− 2
4
2+ 6
2−
4
2−
4
3
− 4
26
2
= K CrO4 ⋅ X CrO4 ⋅ λCrO4 + K SO4 ⋅ X SO4 ⋅ λSO4
Figure 3 shows that determined total solubility products of the solid phases match the model of ideal solid solutions (activity
coefficients = 1) best. However, small deviations in ion concentrations lead to a large increase in the solubility product
because of the high powers included in the calculation. Therefore large standard deviations partly exist. The model of a nonideal solid solution system including the observed miscibility gap for samples with CrO4 mole fractions of 0.4 and 0.6
cannot be excluded.
-39
CrO4 stabilization through
solid solution formation
-40
log ΣΠ
-41
-42
-43
-44
-45
-46
0.0
0.1
0.2
no solid solution
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
XCrO4 (solid)
ideal solid solution
non-ideal solid solution with miscibility gap 0.4<XCr>0.6
Figure 3: Lippmann diagram with total solubility products of CrO4- and SO4-ettringite solid solution series. White arrow
indicates the stabilization of CrO4 through solid solution formation by lowering the solubility product.
It can be clearly shown that the presence of a solid between CrO4- and SO4-ettringite stabilizes CrO4-ettringite and lowers
the concentrations of chromate (Figure 3 white arrow). These are the first thermodynamic data of CrO4- and SO4-ettringite
solid solutions. Beside ettringite also solid solutions of monosulphate and monochromate may play an important role
concerning the CrO4-solubilty limiting phase in a hydrated cement paste. This should be taken into account in CrO4 leaching
prediction calculations.
References
[1] Spence, R.D., Chemistry and microstructure of solidified waste forms. 1993, Boca Raton, Florida: Lewis Publishers.
276.
[2] Glasser, F., A. Johnson, B. Lothenbach, F. Winnefeld, E. Wieland and A. Wällisch, Mechanisms and modeling of
waste/cement interactions: International Workshop, May 8 to 12, 2005, Meiringen, Switzerland. Waste Management, 2006.
26(7): p. 687-688.
[3] Chrysochoou, M. and D. Dermatas, Evaluation of ettringite and hydrocalumite formation for heavy metal
immobilization: Literature review and experimental study. Journal of Hazardous Materials, 2006. 136(1): p. 20-33.
[4] Perkins, R.B. and C.D. Palmer, Solubility of Ca6[Al(OH)6]2(CrO4)3·26H2O, the chromate analog of ettringite; 5-75°C.
Applied Geochemistry, 2000. 15(8): p. 1203-1218.
[5] Perkins, R.B. and C.D. Palmer, Solubility of chromate hydrocalumite (3CaO·Al2O3·CaCrO4·nH2O) 5-75°C. Cement
and Concrete Research, 2001. 31(7): p. 983-992.
[6] Hummel, W., U. Berner, E.Curti, F.J Pearson and T.Thoenen, Nagra/PSI Chemical Thermodynamic Data Base 01/01.
2002, Parkland, Florida, USA: Universal Publishers/uPUBLISH.com.
[7] Lippmann, F., Phase diagrams depicting aqueous solubility of binary mineral systems. N.JB.Miner.Abh., 1980. 139(1):
p. 1-25.