The behaviour of technetium during microbial reduction in amended soils from Dounreay, UK

Plutonium mobilization from contaminated estuarine sediments, Esk Estuary (UK)

Since 1952, liquid radioactive effluent containing^238-242Pu, ²⁴¹Am, ²³⁷Np, ¹³⁷Cs, and ⁹⁹Tc has been released with authorization from the Sellafield nuclear complex (UK) into the Irish Sea. This represents the largest source of plutonium (Pu) discharged in all western Europe, with 276 kg having been released. In the Eastern Irish Sea, the majority of the transuranic activity has settled into an area of sediments (Mudpatch) located off the Cumbrian coast. Radionuclides from the Mudpatch have been re-dispersed via particulate transport in fine-grained estuarine and intertidal sediments to the North-East Irish Sea, including the intertidal saltmarsh located at the mouth of the Esk Estuary. Saltmarshes are highly dynamic systems which are vulnerable to external agents (sea level change, erosion, sediment supply, and freshwater inputs), and their stability remains uncertain under current sea level rise projections and possible increases in storm activity. In this work, we examined factors affecting Pu mobility in contaminated sediments collected from the Esk Estuary by conducting leaching experiments under both anoxic and oxic conditions. Leaching experiments were conducted over a 9-month period and were periodically sampled to determine solution phase Pu via multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS), and to measure redox indicators (Eh, pH and extractable Fe(II)). Microbial community composition was also characterized in the sediments, and at the beginning and end of the anoxic/oxic experiments. Results show that: 1) Pu leaching is about three times greater in solutions leached under anoxic conditions compared to oxic conditions, 2) the sediment slurry microbial communities shift as conditions change from anoxic to oxic, 3) Pu leaching is enhanced in the shallow sediments (0-10 cm depth), and 4) the magnitude of Pu leached from sediments is not correlated with total Pu, indicating that the biogeochemistry of sediment-associated Pu is spatially heterogeneous. These findings provide constraints on the stability of redox sensitive Pu in biogeochemically dynamic/transient environments on a timescale of months and suggests that anoxic conditions can enhance Pu mobility in estuarine systems.

Biogeochemical Cycling of 99Tc in Alkaline Sediments

⁹⁹Tc will be present in significant quantities in radioactive wastes including intermediate-level waste (ILW). The internationally favored concept for disposing of higher activity radioactive wastes including ILW is via deep geological disposal in an underground engineered facility located ∼200-1000 m deep. Typically, in the deep geological disposal environment, the subsurface will be saturated, cement will be used extensively as an engineering material, and iron will be ubiquitous. This means that understanding Tc biogeochemistry in high pH, cementitious environments is important to underpin safety case development. Here, alkaline sediment microcosms (pH 10) were incubated under anoxic conditions under "no added Fe(III)" and "with added Fe(III)" conditions (added as ferrihydrite) at three Tc concentrations (10^-11, 10^-6, and 10^-4 mol L^-1). In the 10^-6 mol L^-1 Tc experiments with no added Fe(III), ∼35% Tc(VII) removal occurred during bioreduction. Solvent extraction of the residual solution phase indicated that ∼75% of Tc was present as Tc(IV), potentially as colloids. In both biologically active and sterile control experiments with added Fe(III), Fe(II) formed during bioreduction and >90% Tc was removed from the solution, most likely due to abiotic reduction mediated by Fe(II). X-ray absorption spectroscopy (XAS) showed that in bioreduced sediments, Tc was present as hydrous TcO₂-like phases, with some evidence for an Fe association. When reduced sediments with added Fe(III) were air oxidized, there was a significant loss of Fe(II) over 1 month (∼50%), yet this was coupled to only modest Tc remobilization (∼25%). Here, XAS analysis suggested that with air oxidation, partial incorporation of Tc(IV) into newly forming Fe oxyhydr(oxide) minerals may be occurring. These data suggest that in Fe-rich, alkaline environments, biologically mediated processes may limit Tc mobility.

Using chemical fractionation and speciation to describe uptake of technetium, iodine and selenium by Agrostis capillaris and Lolium perenne

To understand the dynamic mechanisms governing soil-to-plant transfer of selenium (Se), technetium-99 (⁹⁹Tc) and iodine (I), a pot experiment was undertaken using 30 contrasting soils after spiking with ⁷⁷Se, ⁹⁹Tc and ¹²⁹I, and incubating for 2.5 years. Two grass species (Agrostis capillaris and Lolium perenne) were grown under controlled conditions for 4 months with 3 cuts at approximately monthly intervals. Native (soil-derived) ⁷⁸Se and¹²⁷I, as well as spiked ⁷⁷Se, ⁹⁹Tc and ¹²⁹I, were assayed in soil and plants by ICP-MS. The grasses exhibited similar behaviour with respect to uptake of all three elements. The greatest uptake observed was for ⁹⁹Tc, followed by ⁷⁷Se, with least uptake of ¹²⁹I, reflecting the transformations and interactions with soil of the three isotopes. Unlike soil-derived Se and I, the available pools of ⁷⁷Se, ⁹⁹Tc and ¹²⁹I were substantially depleted by plant uptake across the three cuts with lower concentrations observed in plant tissues in each subsequent cut. Comparison between total plant offtake and various soil species suggested that ⁷⁷SeO₄^2-, ⁹⁹TcO₄^- and ¹²⁹IO₃^-, in soluble and adsorbed fractions were the most likely plant-available species. A greater ratio of ¹²⁷I/¹²⁹I in the soil solid phase compared to the solution phase confirmed incomplete mixing of spiked ¹²⁹I with native ¹²⁷I in the soil, despite the extended incubation period, leading to poor buffering of the spiked available pools. Compared to traditional expressions of soil-plant transfer factor (TF_total), a transfer factor (TF_available) expressed using volumetric concentrations of speciated 'available' fractions of each element showed little variation with soil properties.

Incorporation and retention of 99-Tc(IV) in magnetite under high pH conditions

Technetium incorporation into magnetite and its behavior during subsequent oxidation has been investigated at high pH to determine the technetium retention mechanism(s) on formation and oxidative perturbation of magnetite in systems relevant to radioactive waste disposal. Ferrihydrite was exposed to Tc(VII)(aq) containing cement leachates (pH 10.5-13.1), and crystallization of magnetite was induced via addition of Fe(II)aq. A combination of X-ray diffraction (XRD), chemical extraction, and X-ray absorption spectroscopy (XAS) techniques provided direct evidence that Tc(VII) was reduced and incorporated into the magnetite structure. Subsequent air oxidation of the magnetite particles for up to 152 days resulted in only limited remobilization of the incorporated Tc(IV). Analysis of both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data indicated that the Tc(IV) was predominantly incorporated into the magnetite octahedral site in all systems studied. On reoxidation in air, the incorporated Tc(IV) was recalcitrant to oxidative dissolution with less than 40% remobilization to solution despite significant oxidation of the magnetite to maghemite/goethite: All solid associated Tc remained as Tc(IV). The results of this study provide the first direct evidence for significant Tc(IV) incorporation into the magnetite structure and confirm that magnetite incorporated Tc(IV) is recalcitrant to oxidative dissolution. Immobilization of Tc(VII) by reduction and incorporation into magnetite at high pH and with significant stability upon reoxidation has clear and important implications for limiting technetium migration under conditions where magnetite is formed including in geological disposal of radioactive wastes.

Influence of alkaline co-contaminants on technetium mobility in vadose zone sediments

Pertechnetate was slowly reduced in a natural, untreated arid sediment under anaerobic conditions (0.02 nmolg(-1)h(-1)), which could occur in low permeability zones in the field, most of which was quickly oxidized. A small portion of the surface Tc may be incorporated into slowly dissolving surface phases, so was not readily oxidized/remobilized into pore water. In contrast, pertechnetate reduction in an anaerobic sediment containing adsorbed ferrous iron as the reductant was rapid (15-600 nmolg(-1)h(-1)), and nearly all (96-98%) was rapidly oxidized/remobilized (2.6-6.8 nmolg(-1)h(-1)) within hours. Tc reduction in an anaerobic sediment containing 0.5-10mM sulfide showed a relatively slow reduction rate (0.01-0.03 nmolg(-1)h(-1)) that was similar to observations in the natural sediment. Pertechnetate infiltration into sediment with a highly alkaline water resulted in rapid reduction (0.07-0.2 nmolg(-1)h(-1)) from ferrous iron released during biotite or magnetite dissolution. Oxidation of NaOH-treated sediments resulted in slow Tc oxidation (∼0.05 nmolg(-1)h(-1)) of a small fraction of the surface Tc (13-23%). The Tc remaining on the surface was Tc(IV) (by XANES), and autoradiography and elemental maps of Tc (by electron microprobe) showed Tc was present associated with specific minerals, rather than being evenly distributed on the surface. Dissolution of quartz, montmorillonite, muscovite, and kaolinite also occurred in the alkaline water, resulting in significant aqueous silica and aluminum. Over time, aluminosilicates, cancrinite, zeolite and sodalite were precipitating. These precipitates may be coating surface Tc(IV) phases, limiting reoxidation.

Interactions of Tc(IV) with citrate in NaCl media

This paper presents the experimental determination of the stability constant for the citrate complexes with TcO(OH)+ and TcO(OH)2 0 at different ionic strengths (NaCl), using a solvent extraction method. Data show that the stability constants for the formation of TcO(OH)Cit2− and TcO(OH)2Cit3− are 107.5 ± 0.2 and 102.8 ± 0.2, respectively, at zero ionic strength, with an average of 106.5 ± 0.3 and 102.8 ± 0.2, respectively, in the 1.0–3.0 M ionic strength range. PHREEQC calculations based on these stability constants show that the TcO(OH)Cit2− is the predominant species between pH 3 and 8 and that 50 mM citrate leads to a Tc(IV) solubility of 2 × 10−5 M at pH 5.

Complexation of rhenium with ethylenediaminetetraacetic acid: studies of technetium analogue using Raman spectroscopy

The complexation of rhenium, to the polyamino-polycarboxylate EDTA was investigated using Raman spectroscopy. Complexation occurred when ReVIIO4 - was placed in the presence of a Sn(II) reducing agent and EDTA. Rhenium complexation was confirmed by the Raman and UV/Vis spectra of the solution samples. Binding modes were assigned using the Raman spectra of a solid-phase sample. Three complexes are proposed, a six-coordinate dimer H4[Re2(μO)2(EDTA)2], a seven-coordinate monomer H2[ReO(EDTA)(OH2)2] and a mono-oxorhenium [ReO(EDTA)] 2- . The solubility of Re(IV)-EDTA complexes is greater than that of ReO4. If this behaviour is analogous to Tc, it is proposed that complexation to EDTA by Tc(IV) may increase the solubility and enhance the environmental transport of 99Tc from geological repositories.

Incorporation of pertechnetate and perrhenate into corroded steel surfaces studied by X-ray absorption fine structure spectroscopy

Batch reaction experiments and solid-phase characterization analyses were completed to examine the uptake of dissolved perrhenate [Re(VII)] or pertechnetate [Tc(VII)] by A-516 steel coupons that corroded in simulated groundwater solutions or dilute water. The goal was to identify the mechanism(s) that control the uptake of 99Tc by corrosion products on carbon steel in the presence of dilute solutions. X-ray absorption fine structure spectroscopy (XAFS) was used to study the oxidation states of Re and Tc incorporated into the corroded steel coupon surfaces. X-ray fluorescence maps showed that the corroded coupons contain localized regions enriched in Re or Tc. The Re L 3 near edge XAFS results for the coupons reacted with Re-spiked waters were consistent with nearly all of the sorbed Re being present as perrhenate and not significantly reduced to Re(IV). Linear combination fits of the extended XAFS signals for the perrhenate and ReIVO2 standards indicate that Re sorbed to the steel coupons corroded in simulated J-13 (a relatively dilute Na-HCO3-CO3 groundwater) and even more dilute waters consists of a maximum of 5 and 10% Re(IV), respectively. The fluorescence results also showed that the Re concentrations increased with increasing time of exposure to the X-ray beam, which suggests that the perrhenate ions are only weakly bonded to the matrix of the corrosion product. In contrast to the Re results, the Tc K edge XAFS results for the coupons reacted in 99Tc-spiked waters indicate that most of the sorbed Tc had been reduced to Tc(IV). The shape of the near edge and extended fine structure is similar to the Tc(IV)-hydrous ferric oxide (HFO) and not the TcO2·nH2O standard. Differences were noted in the XAFS results for steel coupons reacted with waters spiked with 0.001 vs. 0.1 mmol/L 99Tc in that much more of the sorbed Tc from 0.001 mmol/L 99Tc experiments was in the form of pertechnetate. Comparison of the XAFS results for coupons reacted with 0.001 mmol/L 99Tc-spiked dilute simulated Na-HCO3-CO3 groundwater vs. 0.001 mmol/L 99Tc-spiked dilute water also suggest that there are likely differences in the sorption mechanism for the pertechnetate fraction in the corrosion product which formed in these two test solutions. The cause for these differences is not known, but is likely due to differences in the compositions of the dilute simulated Na-HCO3-CO3 groundwater and more dilute waters, such as the dissolved carbonate concentrations.

Dissolution of technetium(IV) oxide by natural and synthetic organic ligands under both reducing and oxidizing conditions

Technetium-99 (Tc) in nuclear waste is a significant environmental concern due to its long half-life and high mobility in the subsurface. Reductive precipitation of technetium(IV) oxides [TcO(2)(s)] is an effective means of immobilizing Tc, thereby impeding its migration in groundwater. However, technetium(IV) oxides are subject to dissolution by oxidants and/or complexing agents. In this study we ascertain the effects of a synthetic organic ligand, ethylenediaminetetraacetate (EDTA), and two natural humic isolates on the dissolution and solubility of technetium(IV) oxides. Pure synthetic technetium(IV) oxide (0.23 mM) was used in batch experiments to determine dissolution kinetics at pH ∼6 under both reducing and oxidizing conditions. All organic ligands were found to enhance the dissolution of technetium(IV) oxides, increasing their solubility from ∼10(-8) M (without ligands) to 4 × 10(-7) M under strictly anoxic conditions. Reduced Tc(IV) was also found to reoxidize rapidly under oxic conditions, with an observed oxidative dissolution rate approximately an order of magnitude higher than that of ligand-promoted dissolution under reducing conditions. Significantly, oxidative dissolution was inhibited by EDTA but enhanced by humic acid compared to experiments without any complexing agents. The redox functional properties of humics, capable of facilitating intramolecular electron transfer, may account for this increased oxidation rate under oxic conditions. Our results highlight the importance of complex interactions for the stability and mobility of Tc and thus for the long-term fate of Tc in contaminated environments.

Interactions of Tc(IV) with humic substances

To understand the key processes affecting 99Tc mobility in the subsurface and help with the remediation of contaminated sites, the binding constants of several humic substances (humic and fulvic acids) with Tc(IV) were determined, using a solvent extraction technique. The novelty of this paper lies in the determination of the binding constants of the complexes formed with the individual species TcO(OH)+ and TcO(OH)2(0). Binding constants were found to be 6.8 and between 3.9 and 4.3, for logβ1,-1,1 and logβ1,-2,1, respectively; these values were little modified by a change of ionic strength, in most cases, between 0.1 and 1.0 M, nor were they by the nature and origin of the humic substances. Modeling calculations based on these show TcO(OH)-HA to be the predominant complex in a system containing 20 ppm HA and in the 4-6 pH range, whereas TcO(OH)2(0) and TcO(OH)2-HA are the major species, in the pH 6-8 range.

Technetium reduction and reoxidation behaviour in Dounreay soils

Technetium is a radioactive contaminant found in groundwaters at sites where nuclear wastes have been processed or stored. The redox chemistry of technetium is a major control on its environmental mobility. Under oxic conditions, technetium exists as the pertechnetate ion, Tc(VII)O4 −, which is poorly sorbed by minerals across a wide range of environmentally relevant pH values. Under reducing conditions pertechnetate is converted to lower valency species, of which Tc(IV) tends to predominate. Typically, the Tc(IV) oxidation state readily precipitates as insoluble hydrous Tc(IV) oxides or, at low concentrations, is removed from solution by association with mineral surfaces. Here, we present novel X-ray absorption spectroscopy (XAS) data examining Tc associations with reduced Dounreay soils. In reduced unamended microcosms and in microcosms where we added the co-contaminants ethylenediaminetetraacetic acid (EDTA) or bicarbonate to investigate their effect on Tc biogeochemistry, Tc was removed from solution on exposure to the reduced sediments and was present on solids as hydrous Tc(IV)O2-like phases. Furthermore, to investigate the long term stability and remobilization of solid phase associated Tc in reduced soils, we reoxidized reduced, Tc(IV)-labeled soils, in the presence of air and nitrate. The extent of remobilization of Tc to solution was dependent on the oxidant used. After reoxidation with air for 60 d, (42±6)% of the initial soil bound Tc was resolubilized. In the presence of 25 or 100 mmol L−1 nitrate as an oxidant, negligible microcosm reoxidation or remobilization of Tc to solution occurred. XAS analysis of soils treated with the two oxidants showed that in both systems, the remaining soil associated Tc was present as hydrous TcO2-like phases. The recalcitrance of Tc remobilization under reoxidizing conditions has implications for the fate of Tc in contaminated environments.

Reduction of Tc(VII) by Fe(II) sorbed on Al (hydr)oxides

Under oxic conditions, Tc exists as the soluble, weakly sorbing pertechnetate [TcO4-] anion. The reduced form of technetium, Tc(IV), is stable in anoxic environments and is sparingly soluble as TcO2 x nH2O(s). Here we investigate the heterogeneous reduction of Tc(VII) by Fe(II) adsorbed on Al (hydr)oxides [diaspore (alpha-AlOOH) and corundum (alpha-Al2O3)]. Experiments were performed to study the kinetics of Tc(VII) reduction, examine changes in Fe surface speciation during Tc(VII) reduction (Mössbauer spectroscopy), and identify the nature of Tc(IV)-containing reaction products (X-ray absorption spectroscopy). We found that Tc(VII) was completely reduced by adsorbed Fe(II) within 11 (diaspore suspension) and 4 days (corundum suspension). Mössbauer measurements revealed thatthe Fe(II) signal became less intense with Tc(VII) reduction and was accompanied by an increase in the intensity of the Fe(III) doublet and magnetically ordered Fe(III) sextet signals. Tc-EXAFS spectroscopy revealed that the final heterogeneous redox product on corundum was similar to Tc(IV) oxyhydroxide, TcO2 x nH2O.

The speciation, stability, solubility and biodegradation of organic co-contaminant radionuclide complexes: a review

The potential migration of radionuclides is of concern at contaminated land sites and, in the long term, waste repositories. Pathways of migration need to be characterised on a predictive level so that management decisions can be made with confidence. A pathway that is relatively poorly understood at present is radionuclide solubilisation due to complexation by organic complexing agents that are present in mixed radioactive wastes, and at radioactively contaminated land sites. Interactions of the complexing agents with radionuclides and the host environment, and the response to changes in the physicochemical conditions make their role far from simple to elucidate. In addition, chemical and biodegradation of the organic materials may be important. In this paper, key co-contaminant organics are reviewed with emphasis on their environmental fate and impact on radionuclide migration.