3.5.1 Inductively Coupled Plasma Optical Emission Spectroscopy
Inductively coupled plasma optical emission spectroscopy or ICP-OES was the used for determining leachant composition, this was used to analyse the elements released into solution during MCC-2 tests.
ICP-OES uses an argon based plasma torch, maintained at temperatures upwards of 7000K to completely ionise any element placed into the plasma chamber [184]. Liquids analysed using this technique must first be passed through a nebuliser, via a peristaltic pump; this aerosols the liquid changing it into a mist. This is then injected into the plasma chamber and βswirledβ around by an electromagnetic field.
When the sample is passed through the plasma all compounds are broken down into their constituent elements, which are then further broken down to ions, however this de-ionisation process is continuous, with electrons joining the nucleus before being removed from it again. The plasma emits light based on the elements present due to recombination of electrons, the
wavelengths of which are unique to that element. The light is passed through a spectrometer to help separate each wavelength before it reaches the main detector. Most modern detectors are a solid state type, based on charge coupled devices, thus removing the limitations on previous detectors which worked on a single wavelength basis requiring several detectors and were fixed once installed thus limiting the number of elements that can be to analysed.
Once the detector analyses the various wavelengths, a program compares the intensity of these wavelengths with known standards, thus allowing a high degree of accuracy on the order of ppm for analysed solutions. Some issues can arise from this such as an overlap of certain elements
wavelengths; the intensities of certain elements can make it difficult for the computer software to accurately detect other elements with similar wavelengths. As each element produces more than one wavelength of emitted light, several wavelengths were selected to avoid these issues as far as possible.
Leachant samples were removed via a syringe and passed through a filter to remove any solid particulates which could damage the apparatus. These were then immediately moved to the instrument, an iCAP 6000 series ICP-OES and analysed. ICP multi-element standard solution VIII (24 elements in dilute nitric acid, 100ppm) was used to determine the presence of Al, B, Ba, Be, Bi, Ca, Cd, Co, Cr, Cu, Fe, Ga, K, Li, Mg, Mn, Na, Ni, Pb, Se, Sr, Te, Tl, Zn. A separate standard was used to determine the concentration of silicon (1000ppm in sodium hydroxide solution) due to
88 incompatibility between the two sets of standards. The standards were prepared at concentrations of: 5, 10, 20 and 40ppm. These were then double checked by placing a separately constructed sample of known concentration among the leachant samples analysed. Any deviation from the separately prepared standard would indicate a mistake in the reference standards preparation and it would be performed again with new standards.
3.5.2 pH tests
Long term pH tests were conducted using the MCC-2 test, after a fixed time a 10 ml (Β±2 ml based on SA/V ratio) portion of liquid was withdrawn and analysed. The vessels were removed from the oven (at 90 oC) and allowed to cool to room temperature without breaking the seal; this was to avoid any evaporation of the leachant. The vessels were then opened and the solution analysed using the Thermo Scientific Orion star A111 pH meter to an accuracy of 0.02. The results were run in triplicate and the vessel resealed and returned to the oven. Error was either the max difference from the average of 3 repeat results on each 10 ml of solution, totalling 9 results for one time, or the inherent error of pH (0.02) whichever was greater.
3.5.3 Normalised Leach Rates
Normalised elemental release rates (NLr) are commonly used to calculate the rate loss of a material subject to a corrosion test as a function of time; normalising these results allows for comparison with other materials subject to the same test. The equation used for calculating NLr of an element i present in the solid [185] is shown below (Equation 11):
ππΏπ = (πΆπβπΆ0)βπ
ππβππ΄βπ‘ (11)
Where βCiβ is the concentration of element βiβ in solution (g/l), C0 is the initial concentration of element i in solution however as doubly deionised water was used the value was zero (confirmed by ICP-OES). βVβ is the volume of leachant used (l), βSAβ is the reactive surface area of the sample (cm2), and βtβ is the time of the corrosion experiment (days).
In glass corrosion experiments, fi is the mass fraction of element βiβ in the material, typically calculated using the following formula:
ππ = π€π
π€0 (12)
Where wi is the mass of element i in the wasteform (g), and wo is the total mass of the sample used (g).
However, due to uneven partitioning of elements into different phases in a GCM, Equation 12 could not calculate the total mass fraction of element βiβ in the wasteform, instead Equation 13 was used to determine the mass fraction of element βiβ in the wasteform:
ππ = β π€π€π
πππ
π
π=1 (13)
Where βnβ is the total number of phases present in the wasteform, wi/wo is the mass fraction of element βiβ in phase βnβ, and mi is the volume fraction of phase βiβ in the wasteform.
Averaged EDX wt% results were used to calculated the mass fraction of element βiβ in each phase. The volume fraction of each phase was calculated from SEM images; a low magnification was used
89 to allow full view of the analysed sample (approximately 1 mm2) and the surface area occupied by each phase was calculated, this was preformed 4-6 times to get as accurate a value as possible. Error was calculated using the larger value from the maximum and minimum values determined from Equation 11, this involved maximising the values for Ci , whilst minimising the values for fi and SA for calculating the total maximum value, and vice versa for determining the total minimum value.
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