Investigation in the release kinetics

For the kinetic test, a piece with a 1 dm2 surface of a one-part aluminium tray was cut out and totally immersed in 5 g/L citric acid solution. The time-dependent aluminium release resembles the behaviour observed in lag time experiments with diffusion through barriers [24]. After an initial delay and a slow aluminium release, a linear release was observed as shown in Fig 1. This indicates that the release is determined by two different processes. A linear increase in dissolution of the plain metal was achieved after the slow initial dissolution of the oxide layer. The asymptote of that part of the function is used to determine the lag times of each experiment as shown in the inset to Fig 1. As linear behaviour could not be found at 60°C, the lag time could not be calculated due to the experimental observation time of 120 minutes being too short for reaching an equilibrium. With the lag times, the thickness of the oxide layer is calculated based on the released amount of aluminium from the known surface area, assuming a uniform layer and chemical composition of Al 2 O 3 with a density of 3.94 g/cm3. The oxide layers were found to be 4–11 nm thick, which complies well with the literature [25]. Karbouj et al. showed the decrease in the release of aluminium following a pre-treatment in hot water near boiling point for a period of five hours. The longer the pre-treatment, the lower the release of aluminium in the following experiment in citric acid solution [18]. These findings underline the speed-reducing contribution of the oxide layer onto aluminium release as shown by the lag times.

With those kinetic results the activation energy (E A ) can be calculated according to the Arrhenius equation [15, 16] by changing the equation as follows: where k = reaction rate constant, A = pre-expotential factor, R = universal gas constant and T = absolute temperature.

The activation energy can be derived from the slope of the Arrhenius plot to 62 kJ/mol. The effect of neglecting the influence of the initial oxide dissolution can be demonstrated by inserting the aluminium release values obtained after the complete 2-hour experiments for each temperature. As shown in Fig 2, this leads to a non-linear curve in the Arrhenius plot (grey circles). Fitted to a linear curve (grey line) this would result in a value of the activation energy of 128 kJ/mol. Comparable behaviour could be seen in the doctoral thesis of Karbouj [17]. For comparison, the reaction rates from Karbouj’s work were converted to mmol/dm2/s applying the details of that study i.e. 0.75 dm2 sample surface and 275 mL. Lag time and steady state linear releases are shown and taken into account for 84 and 51°C. Unnoted by the author, the equilibrium for 20°C was not reached in the experiments. The reported E A of 110 kJ/mol should therefore be corrected. Considering only the values measured at temperatures of 84 and 51°C in the calculation, an E A of 65 kJ/mol can be recalculated, confirming our findings.

Release experiments for calculating the E A are laborious and time consuming, especially if data points at lower temperatures are to be included. A new approach was introduced to overcome these difficulties and simplify the experimental work. Instead of conducting consecutive isothermal experiments, all necessary data were obtained in one single experiment. For this purpose, the release experiment was conducted by pouring boiling 5 g/L citric acid solution over the 1 dm2 aluminium piece and allowing the solution to cool down over a period of two hours. Samples were drawn every 5 minutes during the first 30 minutes and every 10 minutes thereafter while constantly monitoring the temperature. The temperature average of each sampling period was used for calculation when constructing the Arrhenius plot in Fig 3. The E A was calculated as in the previous experiments. To enable comparison, the data of the previous experiments are shown together with the data from the cooling-down experiment. The calculated value for the E A is 68 kJ/mol, which again matches up well with the previous findings.

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larger image TIFF original image Download: Fig 3. Arrhenius plot of aluminium releases for kinetic results from cooling down experiments between 90 and 40°C compared to the isothermal experiments (Inset: Course of temperature and aluminium release). https://doi.org/10.1371/journal.pone.0200778.g003

In contrast to the release of aluminium, the release of thallium from the aluminium tray did not show any lag time as shown in Fig 4. Instead, the concentration of thallium increased nearly linearly up to a constant level. For 100°C and 90°C, the constant level was achieved after 60 minutes. At 80°C the release was slower but reached nearly the same level after 90 minutes. At temperatures of 70°C and 60°C, stagnation was not reached after two hours, but a significant release of thallium with a linear behaviour was also measured.

Thallium release reaches its maximum of about 0.03 μg/dm2 after 50 minutes at 100°C. The absence of a lag time indicates that the release is independent of the oxide layer and that it is distributed homogeneously on the surface. The total release of almost 12 mg aluminium per dm2 after two hours corresponds to the removal of a layer of about 440 nm as shown in Fig 5. From the findings that after 50 minutes at 100°C the increase in the concentration of thallium almost ceased and 4.15 mg aluminium had been released by then, the thickness of the aluminium layer containing thallium can be calculated to be approximately 150 nm by using the volumetric formula of a cuboid with the surface of 1 dm2 and an unknown height. With an oxide layer of about 5–10 nm, thallium is distributed much deeper in the material than Al 2 O 3 but it still seems to be near the surface.

The release of further components like vanadium (V) and manganese (Mn) from the menu trays, a grill tray and aluminium foil has also been analysed. In contrast to thallium, these elements showed a behaviour comparable to aluminium (Fig 6).

Fig 6 also demonstrates that all samples showed a release of thallium. Compared to the grill tray and the foil, the slope in release of thallium from the tray was significantly greater and was followed by stagnation. The source of thallium is not known but experiments in material science have been described which showed thallium inclusions by ion implementation in the outer 150 nm in pure aluminium, as we determined, too [26, 27]. It is described that the process of annealing up to 452°C forms different crystal structures of thallium so that they remain in the aluminium after heating [27]. Thallium is also reported to be a possible impurity in aluminium [28]. Possible contamination through the rolling process, where excipients like oils and filters are used, should be considered.