Investigation of the role of the production process on the luminescence of sea salt products

Common salt produced in Australia is sourced predominantly from sea water, saline lakes or saline ground water from salt reclamation schemes, which aim to remove salt leached from the soil from irrigation runoff water before being returned to the environment. Cheetham & Co have been producing salt using solar evaporation pans for over 100 years and operate in most states of Australia, producing a wide range of products.

The prerequisite for solar production of salt from seawater is a clean source of seawater, sufficient flat land, preferably located on the seashore, and a suitable climate viz. warm to hot temperatures with long hours of sunshine and low rainfall (resulting in a high annual evaporation rate). Suitable geographic locations on the South Australian coast and the mid to lower coast of West Australia are blessed with a Mediterranean climate with abundant sunshine, warm to hot temperatures for much of the year and rain predominantly in the cooler months.

The production facility essentially consists of a series of shallow, inter-connected ponds through which the seawater transits. As water evaporates and the salt concentration increases the brine is transferred to the next in the series of ponds before the seawater is replenished. The concentration of the brine in each pan is maintained within specified limits depending on the location in the process and, as it increases stepwise, various salts preferentially precipitate out of the brine. Relatively insoluble salts precipitate in the first pans (carbonates and calcium sulphate), followed by NaCl (the generally desirable main product) followed by more soluble salts (potassic and magnesium salts) in the later pans. The waste water left at the end of the process (“bitterns” so named from the bitter taste imparted by MgCl) is separately harvested for MgCl or returned to the sea. The water temperature is maintained at 20°C throughout the entire process with the end result being a product which is ~98% NaCl with minor impurity elements derived from the source seawater, which sometimes playing a significant role in luminescence. For example, manganese in its various aggregations and ionisation states is the predominant source of the 590 nm emission (López et al., 1977, Aguilar et al., 1978) despite only just being detectable in the samples tested here (Appendix A – salt assays).

When sufficient salt has been precipitated on the bottom of the pans the brine is drained and the salt is scraped into mounds (from which we obtained sample #45) where it is further rinsed with brine to remove surface impurities (from which we obtained sample #6) and then centrifuge dried to a moisture content of ~2%. It may be used as is, where a coarse grain size is acceptable, or further rinsed, kiln dried to a moisture content of ~0.5%, ground, graded by size, packed and sent to market (from which we obtained samples #2, #4 and #5).

Samples were purchased locally as a commercial product after which up to 10 g of the salt sample was placed in a closed vial stored within a desiccator to prevent deterioration of the salt crystals due to absorption of atmospheric moisture. All samples were produced in white light conditions with no signal of formation found in samples #2, #4, #5 and #6 as previously reported (Hunter et al., 2012).

Salt samples #2, #4, #5, and #6 were amongst samples previously reported by (Hunter et al., 2012) where the 3D TL spectra were presented as well as the dose response, and were sourced from commercially marketed salt (bulk packaged table, cooking salt) and sachets from a restaurant. Sample #45 was subsequently obtained from the manufacturer in 2013 and is the input feedstock used in the manufacturing process. The “as received” individual sample details are as follows:

Sample #45 (feedstock): was supplied directly from the crystallisation pan and as such was in the concentrated brine, the crystal size ~5–6 mm. Over the course of a year the salt was air dried at laboratory temperature with the crystals being “welded” together by the brine as it crystallised. It was not subjected to heating above ambient room temperature.

Sample #6 (Diamond) was marketed as water softener salt and was supplied as coarse crystals of ~5–6 mm diameter and subject to some amalgamation in the packaging but with crystal size of the order of the feedstock.

Samples #2, #4, #5 were all processed by the manufacturer in a similar manner, being double washed, kiln dried for ~5 minutes @135°C to give a moisture content of ~0.5 %, ground, graded by size and packed for distribution.

Sample # 2 (Saxa)- cooking salt which had a received grain size ~150 µm with sodium ferrocyanide added as an anti-caking agent.

Sample #4 (Coles) - home brand table salt marketed by Coles supermarkets in 1 kg packages. It has received grain size ~100 µm in size and has sodium aluminate added as an anti-caking agent and fortified with potassium iodate to help ensure adequate iodine in the diet.

Sample #5 (ISM)- sachet salt sold to local cafes for use as a condiment and since it is sold in single serve packs is ground to a smaller size, ~100µm, ensuring it will dissolve in the food when used. No additives were listed on the salt packaging.

Spectral measurement was carried out using the 3D TL spectrometer (Prescott et al., 1988) which was developed in a series of research projects and is a manually operated TL system with automated software modules that ensure a consistent approach to apparatus operation and record taking. It is a single aliquot apparatus where each step of the process (sample loading, chamber evacuation and TL testing) is initiated by the operator, requiring their constant operational presence. To minimize operator time, it is therefore preferable that only one aliquot of each sample be processed for each step in a project under the assumption that multiple aliquots of the same sample will give essentially identical results. The test parameters used were 5 Gy beta dose using a Daybreak irradiator with 40 mCi ⁹⁰Sr/⁹⁰Y plaque β source followed by immediate glow to 400 °C @ 5 °C/s heating rate, reheat for background subtraction and PMT quantum efficiency correction. The results for Saxa, Coles, ISM and Diamond salt samples were previously presented as contour plots of the TL emissions by Hunter et al., 2012 as a part of a larger sample study of NaCl luminescence. An order of magnitude of the luminescence intensity (z axis) is represented by the displayed contour lines, referenced to the peak emission intensity (Hunter et al., 2012). The 3D TL spectrum of the feedstock is presented here for the first time.

Comparative testing of the separate TL emission bands (single aliquot only per sample) and OSL response (two aliquots per sample) was carried out for the TL signals at 360 nm, 420 nm and 590 nm and the UV OSL using a Riso TL/OSL DA-20 reader. Beta irradiations were administered using a 1.5 GBq ⁹⁰Sr/⁹⁰Y plaque, delivering 0.105 Gy/s, and luminescence was detected using an electron tube 9235QB bi-alkali PMT. Optical stimulation was performed using LEDs emitting at 490 nm. A 20 second dose (at 0.105 Gy/s) was applied from the internal beta irradiator to each sample aliquot in turn and luminescence read out almost immediately. All TL measurements were made after evacuation of the chamber to 7 Pa and then flushing of the chamber with dry nitrogen gas, to prevent spurious TL emissions.

All TL measurements were made to a maximum of at least 450°C, at a heating rate of @1°C/s for 590 nm and 360 nm TL and 5°C/sec for 420 nm TL. The signal was summed over the temperature range ~200–400°C, with the lower temperature set to ensure the low temperature emission peaks were as completely as possible excluded without impacting on the higher temperature peaks. Each aliquot reheat was subtracted to eliminate the background incandescence, and intensity correction was made to reflect PMT quantum efficiency response. This is important for quantitative sensitivity comparison, as there is a lower quantum efficiency for the 590 nm signal (15%) and the 420 nm signal (85%) compared to the 360 nm response, which was taken as unity.

OSL measurements were made at 100°C for 100 sec. after preheating to 160°C for 10 s to remove the short lived luminescence signal originating from shallow traps. The total light sum was used with the background (average of the 90–100 s) subtracted.

For all OSL and TL testing a fused quartz window was used in addition to 7.5 mm HOYA U340 filter for OSL, 3 mm Schott glass OG530 for 590 nm TL (passing wavelengths greater than 530 nm), Corning 4 mm 7–59 band-pass filter and 3 mm Schott glass GG420 filters for 420 nm TL (giving a pass band of 400–450 nm) and 4 mm Schott glass UG11 filter for 360 nm TL (passing wavelengths less than 380 nm).