Methods of analysis

Tab. 23 summarises the position and temperature reached at the bentonite/metal interface samples in the FEBEX gallery. Position and temperature data presented in this table are approximate values. Temperature values were extrapolated from data provided by thermocouples installed in the FEBEX experiment (Martínez et al. 2016).

Tab. 23: Position and temperature of the metal/bentonite interfaces studied by CIEMAT for the characterisation of the metal/bentonite interface.

Section Reference Temperature

(°C)

Distance from the axis of the gallery

(cm)

37 B-S-37-1 30 45

42 B-B-42-5 85 10

42 BM-C-42-1 85 10

42 BM-C-42-2,3 85 10

45 BM-D-45-2 90 48.5

45 ML-45-3* 90 48.5

47 B-C-47-10 45 103

52 BM-D-52-2 90 48.5

52 ML-52-4^a 90 48.5

54 BM-S-54-7 105 20

* Samples sent by Tecnalia: bentonite scrapped from the external surface of the liner

Pore size distribution

The pore size distribution of each subsample was determined by mercury intrusion porosimetry (MIP). This technique allows the determination of porosity and pore size distribution by injecting mercury into the sample at different pressures while controlling the volume intruded.

The porosimeter used was a Micromeritics AutoPore Series IV 9500, allowing the exploration of pore diameter sizes between 0.006 and 600 μm. Consequently, the mercury does not intrude the microporosity (pores size less than 0.002 μm, according to the classification of Sing et al.

1985). The mercury intrusion method allows access to be gained only to the macroporosity and to part of the mesopores. Before the samples were inserted in the porosimeter, the water was removed from the pores by freeze-drying. Samples smaller than 3 cm³ were lyophilised to eliminate the water in the pores.

Specific surface area

Classical nitrogen adsorption/desorption isotherms were obtained by Ciemat on a discontinuous volumetry sorptometer, Micromeritics ASAP 2020. Approximately 2 – 4 grams of total sample were ground in an agate mortar. The samples were dried at 90 °C during at least 24 h before the tests. Prior to the nitrogen adsorption, the samples were out-gassed by heating at 90 °C for 18 hours using a mixture of helium and nitrogen under a residual vacuum between 500 and 6 – 10 mmHg. The tests were performed at the boiling point of liquid nitrogen (77 K) considering a molecular cross section area of 0.162 nm for the nitrogen molecule. External specific surface areas were calculated using the standard N2-BET method, using a series of data points over the P/P0 range from 0.02 to 0.25 on the nitrogen adsorption isotherm (Gregg & Sing 1982). An average value was determined by the measurement of two or three aliquots of each subsample.

Mineralogy XRD

XRD diffraction patterns were obtained from random powders and oriented aggregates in order to identify the mineralogical species in the samples.

The grinding of the bulk sample to produce the powders was made in a RETSCH RM 100 mortar grinder with an agate pestle after drying the samples at room temperature. Next, the size fraction of less than 63 μm was obtained after sieving the powder through a nylon ASTM sieve.

The preparation of oriented clay specimens (air-dried, ethylene-glycol solvated and heated at 550 °C) for quantitative analysis was made using the suction-onto-ceramic technique (Shaw 1972). The size fractionation was made in deionized water by settling (Stokes Law). The final clay suspension was ultrasonically dispersed using 1 g in 5 mL of deionized water. The oriented mounts were prepared by suction of the dispersion through 3 mm thick ceramic tiles.

The XRD patterns registered from bulk, randomly-oriented powders were recorded in an angular range (°2θ) of 3 – 70° a θ/2θ X-PERT Panalytical instrument with an X-CELERATOR detector. This set-up allowed taking measurements equivalent to 0.016° angular steps during 100 s each step. Voltage and Intensity of the operated X-ray Cu tube were 45 kV and 40 mA, respectively. The equipment uses monochromatic radiation provided by a Ge 111 monochromator. The slit settings were: soller slit (0.04 rad); divergence and antiscatter slits both of 0.5°. The XRD database used for mineral identification was the Power Diffraction Files from the International Center for Diffraction Data (ICDD).

SEM and TEM

Electron microscopy was used to study morphology, mineralogy and microstructure of some samples. Prior to scanning electron microscope (SEM) imaging samples were dried at 60 °C during 48h and sputter coated with gold to reduce sample charging and improve secondary electron emission. A SEM JEOL JM-6400 coupled to a dispersive X-ray energy spectrometer X LINK LZ_5 was used at the National Center for Electron Microscopy of the Complutense University of Madrid.

For TEM examination a JEOL 2100FX with 200 kV acceleration voltage (3.4 Å point- to-point resolution) coupled to an OXFORD ISIS X-ray energy dispersive spectrometer giving a resolution of 136 eV at 5.39 keV was used at the National Center for Electron Microscopy of the Complutense University of Madrid. Bentonite samples were dispersed in acetone and dropped on a carbon coated copper grid.

FTIR

Fourier transform infrared spectrometry (FTIR) was used to further characterise the bentonite at the interface. Bentonite samples were homogenized in a Retsch RM200 mortar grinder with an agate pestle and mixed with KBr (2 mg of clay or shotcrete sample and 100 mg of KBr). FTIR spectra were obtained using a Nicolet 6700 FTIR spectrometer in transmission mode with a DTGS KBr detector and recording over the middle-IR region spectral range (4'000 – 400 cm^-1) with a resolution of 2 cm^-1 in an atmosphere continuously purged from water and atmospheric CO2. Measured FTIR spectra were processed by Omnic (Version 3.1) software (Nicolet Instruments Co., Madison, USA).

Soluble ions by aqueous leaching

Aqueous extracts were obtained from the bentonite samples. Sample powders were obtained by grinding the samples in a Retsch RM200 mortar grinder with an agate pestle to a size of less than 63 μm after drying them overnight in an oven at 105 °C. Next, they were placed in contact with deionized and degassed water at a solid to liquid ration of 1:4 (2 mg of clay in 8 mL of water), shaken end-over-end and allowed to react for 24 hours. Separation was made by centrifugation (30 minutes at 12'500 rpm) and the supernatant was filtered by a 0.45-μm pore size filter and analysed. pH and Eh were not determined in the extracts because they do not represent reliable environmental conditions regarding the actual cell system, but the method is assumed to give a useful indicator for the content of soluble ions. Duplicates were made for all samples.

Cations in supernatants were analysed by Inductive Coupled Plasma - Optical Emission Spectrometry (ICP-OES) in a Spectro ARCOS spectrometer after acidification of the samples to pH < 2 with HNO3 (8 mL/L). Anions were analysed using ion chromatography (Dionex DX-4500i).

Determination of exchangeable cations

For the analysis of the exchangeable cations, the CsNO3 0.5N displacement method was used (Sawhney 1970). FEBEX bentonite was equilibrated with CsNO3 0.5 N using a S:L ratio of 1:8 in a glove box. Samples were shaken end-over-end for 1 day. After phase separation by centrifuging, the supernatant solutions were decanted.

For the chemical analysis of the supernatants, an IC Metrohm (Switzerland) with Metrosep C3-250 and A SUPP 4-250 columns was used.

Determination of the Cation Exchange Capacity (CEC)

Cation Exchange Capacity (CEC) of bentonite was determined by using the photometric method with the Copper(lI)-trien complex (Meier & Kahr 1999). 200 mg of clay sample were added to 35 ml of distilled water and dispersed by ultrasonic treatment (20 kHz, 400 W, 5 min). The suspension was diluted in a 50 ml volumetric flask to 50 ml, then completely transferred into a 100 ml beaker. While stirring the suspension, 10 ml of the solution of the Cu complex was added. After 3 min reaction time, the suspension was centrifuged at 13'000 rpm for 3 min. The supernatant solution was carefully removed and the extinction was measured at 620 nm in a 10 mm cuvette against water as a blank.

Fe measurements

One of the aims of this work was to identify and quantify the different contributions to total Fe content measured in the analysed bentonite samples.

Therefore, during this work, three major contributions were considered:

• Fe in exchange positions: determined with the CsNO3 0.5N displacement method.

• Fe sorbed in edge surfaces: citric acid 0.3M was used as extractant

• Fe from iron oxides and (oxy)hydroxides precipitated in the bentonite samples: analysed according to the dithionite-citrate-bicarbonate (CBD) method

Total Fe content was measured by means of Total Reflection X-Ray Fluorescence Spectroscopy (TR-XRF).

Sample preparation for Fe measurements

Samples for Fe determination were prepared under anoxic conditions inside a glove box.

Interface samples were sampled and grounded in an agathe mortar under an N2 inert atmosphere. Water content corrections were made from gravimetric determination after heating at 105 ºC.

Sorbed iron

A solution of citric acid 0.3M was used as extractant to quantify Fe sorbed at edge surfaces of bentonite. For the extraction, FEBEX bentonite was equilibrated with citric acid 0.3M using a S:L ratio of 1:10 in a glove box purged with N2. Samples were shaken end-over-end for 1 day.

After centrifugation, supernatants were decanted and analysed by ICP-OES after acidification of the samples to pH < 2 with HNO3 (8 ml/l).

Amorphous iron oxides and (oxy)hydroxides

The determination of free iron oxides (amorphous Fe oxides) in bentonite was made according to the dithionite-citrate-bicarbonate (CBD) method proposed by Mehra and Jackson (1960). For this, 1 g of clay was placed in a 100 mL centrifuge tube and 40 mL of 0.3 M Na-citrate solution

and 5 mL of 1 M NaHCO3 solution was added. Temperature was brought to 80 °C in a water bath, then 1 g of solid Na2SO4 was added and the mixture was stirred for a total of 15 minutes.

At the end of the digestion period, 10 mL of saturated NaCl solution and 10 mL of acetone were used to promote flocculation. The suspension was warmed in a water bath and centrifuged for 5 min. at 2500 rpm. The supernatant was decanted and kept for Fe determinations by ICP-OES.

Total Fe content

Total Fe content was measured by means of Total Reflection X-Ray Fluorescence Spectroscopy (TR-XRF). For the analysis of Fe in the bentonite samples, a TXRF S2 PicoFox spectrometer was used.