The complexation of Tc(IV) with EDTA and picolinic acid at high pH

The currently preferred UK option for the management of intermediate-level radioactive waste (ILW) is to store it in a deep underground repository. This may then be backfilled with a cementitious material. Once closed, the repository will become saturated with groundwater and highly alkaline porewater will develop with an initial pH of around 13.4. This will decrease to 12.5 as the groundwater flow dissolves and removes any NaOH and KOH present. The mineral phases in the cement will act as a buffer and maintain the pH at 12.5 for ~10⁵ years. Corrosion of waste-containing steel canisters will lead to the gradual formation of reducing conditions. The behaviour of the radionuclides present in the waste must be understood in the context of this chemistry (Warwick et al., 2003).

Technetium-99 is an important species in the performance assessment of any proposed geological disposal facility (GDF), due to its high yield and long half-life. The aqueous chemistry of technetium is likely to be dominated by the highly mobile pertechnetate anion (⁠$TcO4−$⁠) in aerobic waters, and by Tc(IV), as solid TcO_2(am), in anaerobic waters (Cui and Eriksen, 1996). The solubility of Tc(IV) in anoxic conditions at high pH above TcO_2(am), the phase that will most probably be present in a GDF, is independent of pH from circum-neutral to pH 13.5 (Warwick et al., 2007). The species $TcO(OH)3−$ is only likely to be formed in significant quantities above the highest pH likely to be found in a cementitious GDF, and hence is of little interest to performance assessment.

The small amounts of waste technetium that have been produced in the UK have in the past been discharged into the Irish Sea. It was originally thought that the waste technetium dispersed widely, but was found to accumulate in seaweed (Copplestone et al., 2004). Treatment with tetraphenylphosphonium bromide (TPPB) is now used to precipitate out the technetium as TPPTc, to reduce marine discharges to an acceptable level. This leads to the possibility that the floc may be sent to a cementitious repository for disposal. However, TPPB degrades by alkaline hydrolysis at high pH. It is also prone to radiolytic degradation.

Organic complexing agents will be present as inherent components of the waste, especially EDTA and picolinic acid, which are heavily used as decontamination agents. These are highly complexing and can cause significant increases in radionuclide solubility at high pH. The GDF will not be homogenous and there are likely to be areas of reducing and oxidizing potential. The possibility of increased solubility when such organics are in contact with reduced technetium (TcO_2(s)) is of relevance to technetium mobility. Therefore, studies were undertaken in which $TcO4−$ was reduced electrochemically, and by use of Sn(II) and Fe(II), to determine whether there was an increase in technetium solubility when TcO₂ was contacted with the organic ligands. It can assumed that these ligands will have been washed out by the time the pH of the GDF decreases to 12.5.

All experiments were conducted in a Unilab MBraun nitrogen glove box with O₂ levels kept below 1 ppm. All solutions were boiled and N₂ sparged. Solid sodium EDTA and sodium picolinate were added to carbonate-free NaOH_(aq) (2 mol dm⁻³) to give concentrations between 0.3 and 0.01 mol dm⁻³ at pH 13.3, with sodium dithionite added as a holding reductant. Ammonium pertechnetate was added and reduction was achieved using a potential difference of 5 V across the solution for at least 12 h. The activity in solution was measured by liquid scintillation counting using Canberra Packard TRI-Carb 2750TR/LL, indicating an aqueous concentration of Tc(IV) of ~4 × 10⁻⁹ mol dm⁻³. Five replicates were used. Control experiments without ligands showed no increase in aqueous technetium concentration, indicating that reducing conditions were maintained for the requisite periods of time. To measure the stability constants for the reaction of Tc(IV) with EDTA and picolinic acid, the solubility product approach was used (Warwick et al., 2004). The solids were aged for 14 days before ligand addition and the solutions left for at least 10 days to reach a steady state, following the procedure reported by Maes et al. (1988). The pH was measured using a high-pH solution Orion 720A glass electrode. The electrode was calibrated using Aldrich volumetric standard sodium hydroxide solutions. Experiments were performed at ambient laboratory temperature.

The solubility product for the TcO_2(am) phase formed in these experiments was determined by Warwick et al. (2004) to be log K_sp = −33.6±0.32.

Figure 1 shows the effect of increasing the concentration of EDTA on technetium(IV) solubility, the aqueous concentration of Tc(IV) rising from ~10⁻⁹ mol dm⁻³ to ~10⁻⁷ mol dm⁻³, albeit at high EDTA concentrations. The slope of close to unity in the log–log plot (1.18) indicates that the increase in solubility of Tc is being controlled by the formation of a 1:1 Tc(IV)–EDTA complex. This relationship allows the calculation of a conditional stability constant for this complex using the solubility product approach described in Warwick et al. (2004).

Following the derivation in Warwick et al. (2004), the conditional stability constant of the complex is given by expression 5 below.

The conditional stability constant for the Tc(IV)–EDTA complex was calculated using equation 5, and determined to be β_{Tc(IV)−EDTA} = 1.6 × 10²⁶ which corresponds to log β_{Tc(IV)−EDTA} = 25.2±0.6 (1 SD).

Figure 2 shows the effect of increasing the concentration of picolinic acid on technetium(IV) solubility, the aqueous concentration of Tc(IV) rising from ~10⁻⁹ mol dm⁻³ to ~10⁻⁶ mol dm⁻³ at high picolinate concentrations. The slope of close to unity in the log–log plot (1.05) indicates that the increase in solubility of technetium is being controlled by the formation of a 1:1 Tc(IV)–PA complex. This relationship allows the calculation of a conditional stability constant for this complex using the solubility product approach.

The conditional stability constant for the Tc(IV)–picolinic acid complex was calculated using equation 5, and was determined to be β_Tc(IV)−PA = 8.65 × 10²⁶, which corresponds to log β_Tc(IV)-PA = 26.9±0.1.