A. Reagents and instruments

NH₄TcO₄ was purchased from Oak Ridge National Laboratory (ORNL) and dissolved in distilled water. 2,4,6-trimethylpyridine was bought from Fluca chemical reagent company. Liquid scintillation spectrometer manufactured by Beckman was used for the measurement of technetium. The cocktail solution used in the liquid scintillation measurement was prepared by dissolving 1 g triphosphoxane, 16 g 2,5-diphenyloxazole and 300 g naphthalene in 2 L dioxane. All the other chemicals used were of analytical grade.

B. Analysis

The concentrations of MMH and HNO₃ were determined by titration method using a glass electrode to monitor the pH value. Tc(VII) concentration was detected using liquid scintillation method after it was extracted by 2,4,6-trimethylpyridine in alkaline solution [7].

C. Procedure

All the experiments were carried out in a 20 mL test tube, which was set in a constant temperature bath pot. All the reactants were preheated to keep the reaction temperature. After the reactants were mixed together, a tiny volume of solution was sampled immediately and at regular intervals in the whole experiment course. The sampled solution was kept in a sealed tube and put in ice water bath to freeze the reaction for latter analysis.

To predict technetium behavior in the real reprocessing process, the initial concentrations of all the reactants in the experiments are similar as their concentrations in the U/Pu splitting stage of the advanced PUREX process. The concentrations of MMH and HNO₃ are hundreds times as high as that of technetium and only a little MMH and HNO₃ is consumed in reaction process, so the concentrations of MMH and HNO₃ can be treated as constants in data processing.

A. Mechanism consideration and technetium effect on the reaction

The effect of initial Tc(VII) on the reaction is shown in Fig. 1. The curves present a typical sigmoid shape. Tc(VII) is reduced relatively slow at the beginning, which is called induction period. Then Tc(VII) concentrations decline sharply, the reaction turns into fast reaction period. At the end of the reaction, the reduction of Tc(VII) becomes slow again. A remarkable phenomenon is that Tc(VII) is consumed much faster in the condition of higher initial Tc(VII) concentration at the experiment conditions, which suggests that the reduced product of Tc(VII) may accelerate the consume of Tc(VII). In all experiments Tc(VII) could not be reduced completely, about 10% remained in the solution.

Fig. 1.Full-screen View

(Color online) Tc(VII) concentration versus time under different initial Tc(VII) concentrations 40 ℃, c₀(HNO₃)=1.5 M, c₀(MMH. HNO₃)=0.15 M, c₀(Tc(VII)), 1: 3.48×10^-4M, 2: 6.55×10^-4M, 3: 1.39×10^-3M, 4: 2.69×10^-3M.

The dissociation of CH₃N₂H₄⁺ in HNO₃ solution is expressed as

where the k_a is dissociation constant.

It was reported that pka value is 7.87 [8], which means that MMH exists in the form of CH₃N₂H₄⁺ in this solution.

The reduction of Tc(VII) by MMH in HNO₃ aqueous solution is very complicated. Tc(VII) is reduced in two ways, as shown in reaction (3) and reaction (5), the stepwise reduced by MMH and catalytic reduced by Tc(IV).

Technetium is a polyvalent nuclide, there may be tetravalent, pentavalent, hexavalent and heptavalent technetium involved in this reaction. Other reactions such as the disproportionation of Tc(VI) and Tc(V), the oxidation of Tc(VI) and Tc(V), and the redox reactions between MMH and HNO₃ are not presented here. The reaction (3) and (5) mentioned above are dominant when MMH is excess in situation. Other reactions will have little effect in reduction of Tc(VII). The interim-valence technetium, Tc(VI) and Tc(V), are not stable in this system and tend to transform to stable valence, Tc(IV) or Tc(VII), very quickly after their emergence [9]. So little Tc(VI) and Tc(V) will exist in the solution. To simplify the data processing, we neglecte the interim-valence technetium and suppose that only Tc(VII) and Tc(IV) exist in this system. So the reduction of Tc(VII) can be expressed as

where k₁ and k₂ are the rate constants of reaction (3) and (5) respectively. Substituting [Tc(IV)] with [Tc(VII)], this equation can be transformed as follows

where c₀ represents the initial Tc(VII) concentration.

Let $k=k_1[{\text{CH}}_3{\text{N}}_2{\text{H}}_4^{+}][{\text{H}}^{+}]/k_2+c_0$, so the above equation can be written as

From above equation, we can easily get the integral form

where $k/k_2=k_1{\text{[CH}}_{\text{3}}{\text{N}}_{\text{2}}{\text{H}}_4^{+}{\text{][H}}^{\text{+}}\text{]}/k_2+c_0>[\text{Tc(VII)]}$, so the modulus can be taken away and the equation can be expressed as an exponential form

So, [Tc(VII)] can be expressed as

It is obvious that the mechanism equation has the same format as logic function. The logic function used in fitting is y = a/{1 + [-b(x–x_c)]}. Relationships between the logic function and mechanism function are displayed in Table 1. The fitting job was done by using the software Origin 8.

The fitting result and the calculated k₁ and k₂ of reaction (3) and (5) are presented in Table 2. When calculating k₁ and k₂, we make the hypothesis that the concentration of CH₃N₂H₄⁺ is equal to the initial MMH concentration.

B. The effect of MMH on the reaction

The effect of initial MMH concentration on the reduction of Tc(VII) is presented in Fig. 2. MMH concentration affects the induction period more distinctly than the fast reaction period. The length of the induction periods decreases with the increment of MMH concentration, but the fast reaction periods under different conditions present little difference of each other. It can be supported by the proposed mechanism. The stepwise reduction is dominant in the induction period, and MMH is first order for the stepwise reduction, so a higher MMH concentration is favorable for reduction of Tc(VII). Once enough Tc(IV) is produced, the fast reaction will take charge of the reaction, the reduction will be influenced mainly by the concentration of technetium but MMH. Meanwhile the MMH concentrations influence the extent of the reaction and less Tc(VII) remained in a higher initial MMH concentration condition.

Fig. 2.Full-screen View

(Color online) Tc(VII) concentration versus time under different initial MMH concentrations. 40 ℃, c₀(Tc(VII))=6.57×10^-4 M, c₀(HNO₃)=1.5 M c₀(MMH.HNO₃), 1: 0.068M, 2: 0.15M, 3: 0.225M, 4: 0.34M.

C. The effects of acidity on the reaction

The effect of initial HNO₃ concentration on the Tc(VII) reduction is presented in Fig. 3. HNO₃ has a similar effect as MMH on the reaction because of their analogical role in the reduction of Tc(VII).

Fig. 3.Full-screen View

(Color online) Tc(VII) concentration versus time under different initial HNO₃ concentration. 40 ℃, c₀(Tc(VII))=6.57×10^-4 M, c₀(MMH.HNO₃)=0.15 M, c₀(HNO₃), 1: 0.40M, 2: 0.80M, 3: 1.50M, 4: 2.20M.

The fitting result and deducted parameters are presented in Table 4.

In the logic function, if x = xc, the second derivative of y equals 0. That means, the xc defines the time when the reaction of Tc(VII) reduction gets fast. When x = xc, y = a/2 and the caculated value of "a" is very close to the iniatial Tc(VII) concentration, it suggests that Tc(VII) decreases most quickly when Tc(VII) concentration is nearly half of total technetium concentration. It confirms our hypothesis that there are mainly Tc(VII) and Tc(IV) in the solution, only a little other valent technetium exists in this system.

From the rate contant k₁ and k₂ obtained in different experiments, we calculated the average of k₁ to be 0.179 /(M² h) with a standard deviation 0.02 and the average of k₂ to be 961 /(M² h) with a standard deviation 129. k₂ has a bigger relative error than k₁, because we assume that the activity of MMH and HNO₃ is equal to their intial concentration in data processing, but it has a distinct departure. Based on above disscusion, it can be concluded that this reaction is very similar to the reaction between TcO₄^- and hydrazine [4]. There is a long induction period while the Tc(VII) is reduced by CH₃N₂H₄⁺ stepwisely to Tc(IV). When there is enough Tc(IV) accumulated and the autocatalytic reaction becomes prominent. The reaction turns into the fast reaction period. With the decrement of Tc(VII), the reduction of Tc(VII) turns to slow again.

So the MMH concentration and the acidity all have effects on the induction period. A higher initial technetium concentration, higher CH₃N₂H₃ concentration, and acidity are all propitious to get through the inducing period. But the velocity of fast reaction is mainly determined by technetium concentration.

D. The effects of temperature on the reaction

Temperature is known to have a significant effect on reaction kinetics. This effect is often translated into a dependence of the kinetic parameters on temperature, which has been widely studied, and can be generally expressed with an Arrhenius type equation as below

where k is a kinetic parameter, E_a is activation energy, R represents the universal gas constant, and T is standard temperature.

The effect of temperature on this reaction is presented in Fig. 4. The fitting result and the deduced parameters at different temperatures are displayed in Table 5. Based on the deduced parameters, the apparent activation energy (E_a) is calculated to be 31.51 kJ/mol for reaction (3) and 65.68 kJ/mol for reaction (5), respectively.

Fig. 4.Full-screen View

(Color online) The effect of temperature on the reaction. c₀(Tc)=6.70×10^-4 M, c₀(HNO₃)=1.5 M, c₀(MMH.HNO₃)=0.15 M. 1: 40 ℃, 2: 45 ℃, 3: 50 ℃, 4:55 ℃.