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Geological disposal is the globally preferred long-term solution for higher activity radioactive wastes (HAW) including Intermediate Level Waste (ILW). In a cementitious disposal system, cellulosic waste items present in ILW may undergo alkaline hydrolysis, producing significant quantities of isosaccharinic acid (ISA), a chelating agent for radionuclides. Although microbial degradation of ISA has been demonstrated, its impact upon the fate of radionuclides in a geological disposal facility (GDF) is a topic of ongoing research. This study investigates the fate of U(VI) in pH-neutral, anoxic, microbial enrichment cultures, approaching conditions similar to the far field of a GDF, containing ISA as the sole carbon source, and elevated phosphate concentrations, incubated both (i) under fermentation and (ii) Fe(III)- reducing conditions. In the ISA-fermentation experiment, U(VI) was precipitated as insoluble U(VI)-phosphates, whereas under Fe(III)-reducing conditions, the majority of the uranium was precipitated as reduced U(IV)-phosphates, presumably formed via enzymatic reduction mediated by metal-reducing bacteria, including Geobacter species. Overall, this suggests the establishment of a microbially-mediated "bio-barrier" extending into the far field geosphere surrounding a GDF is possible and this bio-barrier has the potential to evolve in response to GDF evolution and can have a controlling impact on the fate of radionuclides.

Understanding interactions between iron (oxyhydr)oxide nanoparticles and plutonium is essential to underpin technology to treat radioactive effluents, in cleanup of land contaminated with radionuclides, and to ensure the safe disposal of radioactive wastes. These interactions include a range of adsorption, precipitation, and incorporation processes. Here, we explore the mechanisms of plutonium sequestration during ferrihydrite precipitation from an acidic solution. The initial 1 M HNO3 solution with Fe(III)((aq)) and Pu-242(IV)((aq)) underwent controlled hydrolysis via the addition of NaOH to pH 9. The majority of Fe(III)((aq)) and Pu(IV)((aq)) was removed from solution between pH 2 and 3 during ferrihydrite formation. Analysis of Pu-ferrihydrite by extended X-ray absorption fine structure (EXAFS) spectroscopy showed that Pu(IV) formed an inner-sphere tetradentate complex on the ferrihydrite surface, with minor amounts of PuO2 present. Best fits to the EXAFS data collected from Pu-ferrihydrite samples aged for 2 and 6 months showed no statistically significant change in the Pu(IV)-Fe oxyhydroxide surface complex despite the ferrihydrite undergoing extensive recrystallization to hematite. This suggests the Pu remains strongly sorbed to the iron (oxyhydr)oxide surface and could be retained over extended time periods.

One of the most challenging components of the UK nuclear legacy is Magnox sludge, arising from the corrosion of Mg alloy-clad irradiated metallic U fuel that has been stored in high pH ponds. The sludges mainly comprise Mg hydroxide and carbonate phases, contaminated with fission products and actinides, including Pu. Cementation and deep geological disposal is one option for the long-term management of this material, but there is a need to understand how Pu may be leached from the waste, if it is exposed to groundwater. Here, we show that cemented Mg(OH)2 powder prepared with Pu(IV)aq is altered on contact with water to produce a visibly altered &lsquo, leached zone&rsquo, which penetrates several hundred microns into the sample. In turn, this zone shows slow leaching of Pu, with long-term leaching rates between 1.8&ndash, 4.4 &times, 10&minus, 5% of total Pu per day. Synchrotron micro-focus X-ray fluorescence mapping identified decreased Pu concentration within the &lsquo, A comparison of micro-focus X-ray absorption spectroscopy (&micro, XAS) spectra collected across both leached and unleached samples showed little variation, and indicated that Pu was present in a similar oxidation state and coordination environment. Fitting of the XANES spectra between single oxidation state standards and EXAFS modeling showed that Pu was present as a mixture of Pu(IV) and Pu(V). The change in Pu oxidation from the stock solution suggests that partial Pu oxidation occurred during sample ageing. Similarity in the XAS spectra from all samples, with different local chemistries, indicated that the Pu oxidation state was not perturbed by macro-scale variations in cement chemistry, surface oxidation, sample aging, or the leaching treatment. These experiments have demonstrated the potential for leaching of Pu from cementitious waste forms, and its underlying significance requires further investigation.

Mud Hills clinoptilolite has been used in an effluent treatment plant (SIXEP) at the Sellafield nuclear reprocessing site. This material has been used to remove 134/137Cs and 90Sr successfully from effluents for 3 decades. Samples of the zeolite have been tested in column experiments to determine their ability to remove radioactive Cs+ and Sr2+ ions under increasing concentrations of competing ions, Ca2+, Mg2+, Na+ and K+. These ions caused increased elution of Cs+ and Sr2+. Ca2+, Mg2+ and K+ were more effective competitors than Na+. For Na+, it was found that if concentration was reduced, then column performance recovered rapidly. Electronic supplementary material The online version of this article (10.1007/s10967-018-6329-8) contains supplementary material, which is available to authorized users.

One of the nuclear fuel storage ponds at Sellafield (United Kingdom) is open to the air, and has containeda significant inventory of corroded magnox fuel and sludge for several decades. As a result, some fissionproducts have also been released into solution. 90Sr is known to constitute a small mass of theradionuclides present in the pond, but due to its solubility and activity, it is at risk of challenging effluentdischarge limits. The sludge is predominantly composed of brucite (Mg(OH)2), and organic molecules areknown to be present in the pond liquor with occasional algal blooms restricting visibility. Understandingthe chemical interactions of these components is important to inform ongoing sludge retrievals andeffluent management. Additionally, interactions of radionuclides with organics at high pH will be animportant consideration for the evolution of cementitious backfilled disposal sites in the UK. Batchsorption–desorption experiments were performed with brucite, 90Sr and natural organic matter (NOM) (humic acid (HA) and Pseudanabaena catenata cyanobacterial growth supernatant) in both binary and ternary systems at high pH. Ionic strength, pH and order of addition of components were varied. 90Sr was shown not to interact strongly with the bulk brucite surface in binary systems under pH conditionsrelevant to the pond. HA in both binary and ternary systems demonstrated a strong affinity for the brucite surface. Ternary systems containing HA demonstrated enhanced sorption of 90Sr at pH 11.5 and vice versa, likely via formation of strontium–humate complexes regardless of the order of addition of components. The distribution coefficients show HA sorption to be reversible at all pH values studied, andit appeared to control 90Sr behaviour at pH 11.5. Ternary systems containing cyanobacterial supernatant demonstrated a difference in 90Sr behaviour when the culture had been subjected to irradiation in the first stages of its growth.

The ‘Implementing Geological Disposal – Technology Platform’ (IGD-TP) is composed of European waste-management organizations and has a vision that “by 2025 the first geological disposal facilities for spent fuel, high-level waste and other long-lived radioactive waste will be operating safely in Europe”. The first IGD-TP Geodisposal Conference was held at The University of Manchester, UK from June 24th – 26th 2014 and provided a new forum for researchers in this area to present their work. The Conference brought together >270 delegates from across 23 countries and an important aspect of the conference was to showcase and publish research relevant to geological disposal of radioactive waste. The conference was also an opportunity for more advanced geological programmes to share >30 years of Research and Development and Demonstration (RD & D) and implementation experience in geological disposal with less advanced programmes. The event received funding from the European Union's European Atomic Energy Community's (EURATOM) Seventh Framework programme FP7 (2007–2013) under grant agreements in no 249396, Sec IGD and no 323260, Sec IGD2. Additional support for the conference proceedings was received from: Radioactive Waste Management Limited; The University of Manchester Dalton Nuclear Institute Research Centre for Radwaste Disposal; The Royal Society of Chemistry; The Geological Society, UK; The Mineralogical Society, UK; the National Nuclear Laboratory; the STFC EnvRadNet Network; and the NERC BIGRAD Consortium. The Conference organising committee comprised: Raymond Kowe, Radioactive Waste Management Ltd., UK; Nic Bilham, Geological Society, UK; Nick Bryan, National Nuclear Laboratory, UK; Gunnar Buckau, Joint Research Centre, Germany; Jacques Delay, ANDRA, France; Daniela Diaconu, Institute for Nuclear Research, Romania; Nick Evans, Loughborough University, UK; Hans Forsstrom, SKB, Sweden; Erik Kremer, NWMO, Canada; Katherine Morris, The University of Manchester, …

Previously, it has been suggested that metal ions complexed to humic acid in the environment might show slower dissociation than those added to humic substances in the laboratory, which has serious implications for the transport of radionuclides in the environment. The dissociation of lanthanide and anthropogenic actinide ions from humic substance complexes has been studied as a function of humic concentration and metal ion:humic concentration ratio. The results suggest that the apparently slower kinetics observed for metal ions complexed in the environment are probably due to the large humic concentrations that are used in those studies. Further, there is no evidence that the dissociation rate constant varies at very low metal ion concentrations. Although humic samples size-fractionated by ultrafiltration showed that more metal may be bound non-exchangeably, there was no evidence for different rate constants. Ultrafiltration of Eu(III)/humic acid mixtures did show a shift in Eu from smaller to larger fractions over a period of two days. Therefore, the results suggest that dissociation rate constants determined in the laboratory at metal ion concentrations higher than those expected in the environment may be used in predicting radionuclide mobility, provided that the humic acid concentration is in the range expected at the site.

Ternary systems of 152Eu(III), bulk bentonite and ethylenediaminetetraacetic acid (EDTA) ([Eu] = 7.9 × 10–10 M; pH = 6.0–7.0) have been studied. Without EDTA, there was slow uptake in a two-stage process, with initial rapid sorption of Eu(III) (96%), followed by slower uptake of a much smaller fraction (3.0% over a period of one month). The reversibility of Eu(III) binding was tested by allowing Eu(III) to sorb to bentonite for 1–322 days. EDTA was added to the pre-equilibrated Eu bentonite systems at 0.01 M, a concentration that was sufficient to suppress sorption in a system where EDTA was present prior to the contact of Eu(III) with bentonite. A fraction of the Eu was released instantaneously (30–50%), but a significant amount remained bound. With time, the amount of Eu(III) retained by the bentonite reduced, with a slow fraction dissociation rate constant of approximately 4.3 × 10–8 s–1 (values in the range 2.2 × 10–8 – 1.0 × 10–7 s–1) for pre-equilibration times ≥7 days. Eventually, the amount of Eu(III) remaining bound to the bentonite was within error of that when EDTA was present prior to contact (4.5% ± 0.6), although in systems with pre-equilibration times >100 days, full release took up to 500 days. Europium interactions with colloidal bentonite were also studied, and the dissociation rate constant measured by a resin competition method. For the colloids, more Eu was found in the slowly dissociating fraction (60–70%), but the first-order dissociation rate constant was faster, with an average rate constant of 8.8 × 10–7 s–1 and a range of 7.7 × 10–7 –9.5 × 10–7 s–1. For both bulk and colloidal bentonite, although slow dissociation was observed for Eu(III), there was no convincing evidence for 'irreversible' binding.

It is UK Government policy to dispose of higher activity radioactive waste through geological disposal into an engineered deep underground geological disposal facility (GDF; DECC, 2014). Those wastes include low-level (LLW) and intermediate-level (ILW) radioactive wastes that are very heterogeneous, containing a range of inorganic and organic materials, the latter including cellulosic items. After closure of the GDF, eventual resaturation with groundwater is expected, resulting in the development of a hyperalkaline environment due to the proposed use of a cementitious backfill. Under these high-pH conditions, cellulose is unstable and will be degraded chemically, forming a range of water-soluble, low molecular weight compounds, of which the most abundant is isosaccharinic acid (ISA). As ISA is known to form stable soluble complexes with a range of radionuclides, thereby increasing the chance of radionuclide transport, the impact of microbial metabolism on this organic substrate was investigated to help determine the role of microorganisms in moderating the transport of radionuclides from a cementitious GDF. Anaerobic biodegradation of ISA has been studied recently in high-pH cementitious ILW systems, but less work has been done under anaerobic conditions at circumneutral conditions, more representative of the geosphere surrounding a GDF. Here we report the fate of ISA in circumneutral microcosms poised under aerobic and anaerobic conditions; the latter with nitrate, Fe(III) or sulfate added as electron acceptors. Data are presented confirming the metabolism of ISA under these conditions, including the direct oxidation of ISA under aerobic and nitrate-reducing conditions and the fermentation of ISA to acetate, propionate and butyrate prior to utilization of these acids during Fe(III) and sulfate reduction. The microbial communities associated with these processes were characterized using 16S rRNA gene pyrosequencing. Methane production was also quantified in these experiments, and the added electron acceptors were shown to play a significant role in minimizing methanogenesis from ISA and its breakdown products.

Sediment samples were collected from the vicinity of the abandoned South Terras uranium mine in south-west UK and analysed for uranium and (226)Ra to explore their geochemical dispersion. The radioactivity concentrations in the sediment samples were measured using alpha spectrometry for uranium, and gamma spectrometry for radium. Sequential chemical extraction was applied to selected sediments in order to investigate the speciation of the radionuclides and their association with stable elements. The activity ratio of the uranium isotopes was used to explore the mobility of uranium, and scanning electron microscopy (SEM) and electron microprobe analysis (EMPA) were used to characterise the sediments. The radiochemical results identified two locations with enhanced radioactivity, so two samples from these locations were further investigated. The geochemical distribution of the radionuclides in these two samples varies within the five operationally-defined fractions. In one sample, the majority of the uranium was released from the 'carbonate' fraction, followed by the organic fractions. Similarly, in the second sample, the uranium was mainly resealed from the carbonate fraction, although a considerable percentage associated with the resistant fraction. The fractionation trend of radium noticed to show some similarities to that of barium, as expected from the similarity in their chemistries. Geochemical distributions of the stable elements, such as Mn, Ti and As, were different in the enhanced radioactivity samples. The activity ratio of (234)U/(238)U shows different trends in the two sediments, signifying the impact of organic matter and/or the exchange between water and sediment. SEM and EMPA analysis identified uranium-bearing phases in association with potassium, calcium, iron, manganese and arsenic.

Intermediate-level radioactive waste (ILW), which dominates the radioactive waste inventory in the United Kingdom on a volumetric basis, is proposed to be disposed of via a multibarrier deep geological disposal facility (GDF). ILW is a heterogeneous wasteform that contains substantial amounts of cellulosic material encased in concrete. Upon resaturation of the facility with groundwater, alkali conditions will dominate and will lead to the chemical degradation of cellulose, producing a substantial amount of organic co-contaminants, particularly isosaccharinic acid (ISA). ISA can form soluble complexes with radionuclides, thereby mobilising them and posing a potential threat to the surrounding environment or 'far field'. Alkaliphilic microorganisms sampled from a legacy lime working site, which is an analogue for an ILW-GDF, were able to degrade ISA and couple this degradation to the reduction of electron acceptors that will dominate as the GDF progresses from an aerobic 'open phase' through nitrate- and Fe(III)-reducing conditions post closure. Furthermore, pyrosequencing analyses showed that bacterial diversity declined as the reduction potential of the electron acceptor decreased and that more specialised organisms dominated under anaerobic conditions. These results imply that the microbial attenuation of ISA and comparable organic complexants, initially present or formed in situ, may play a role in reducing the mobility of radionuclides from an ILW-GDF, facilitating the reduction of undue pessimism in the long-term performance assessment of such facilities.

At nuclear contaminated sites, microbially-mediated Fe(III) reduction under alkaline conditions opens up the potential for co-treatment of the groundwater contaminants Tc-99, though reduction to less mobile Tc(IV) phases, and Sr-90, through increased sorption and/or precipitation promoted at higher pH. In the experiments described here, microbial enrichment cultures derived from representative Sellafield sediments were used to probe the effect of microbially-mediated Fe(III) reduction on the mobility of Tc-99 and Sr (as stable Sr2+ at elevated concentrations and Sr-90(2+) at ultra-trace concentrations) under both neutral and alkaline conditions. The reduction of Fe(III) in enrichment culture experiments at an initial pH of 7 or 9 resulted in the precipitation of an Fe(II) bearing biomineral comprised of siderite and vivianite. Results showed that TcO4- added at 1.6 x 10 (6) M was removed (>80%) from solution concurrent with Fe(III) reduction at both pH 7 and pH 9. Furthermore, X-ray absorption spectroscopy of the reduced biominerals confirmed reduction of Tc(VII) to Tc(IV). To understand Sr behaviour in these systems, Sr2+ was added to enrichment cultures at ultra-trace concentrations (2.2 x 10 (10) M (as Sr-90(2+))) and at higher concentrations (1.15 x 10 (3) M (as stable Sr2+)). In ultra-trace experiments at pH 7, microbially active systems showed enhanced removal of Sr-90 compared to the sterile control. This was likely due to sorption of Sr-90(2+) to the Fe(II)-bearing biominerals that formed in situ. By contrast, at pH 9, the sterile control showed comparable removal of Sr-90 to the microbially active experiment even though the Fe-minerals formed were of very different character in the active (vivianite, siderite) versus sterile (an amorphous Fe(III)-phase) systems. Overall, Sr-90 bioreduction experiments showed 60-70% removal of the added Sr-90 across the different systems: this suggests that treatment strategies involving bioreduction and the promotion of Fe(III)-reducing conditions to scavenge Tc(IV) are not incompatible with treatment of groundwater Sr-90 contamination. In systems with elevated Sr2+ concentrations and an initial pH of 7, microbially active systems showed

Isosaccharinic acid (ISA) forms through alkaline hydrolysis of cellulose in intermediate level nuclear waste and is a strong complexant of metals with the potential to mobilize priority radionuclides in an underground geological disposal facility (GDF). In this study, microbial ISA degradation was studied under anaerobic conditions with Fe(III) oxyhydroxide as the terminal electron acceptor at pH 7 to 10, representative of conditions in the geosphere surrounding a geological disposal facility. A multidisciplinary approach was used to study the fate and limits of ISA biodegradation, including mineralogical analyses (XRD, ESEM and TEM), alongside geochemical profiling of ISA and its biodegradation products, and microbial community analysis by 16S rRNA gene sequencing. Under the conditions imposed, ISA degradation was constrained to a pH limit of ≤9 and was degraded via Fe(III) reduction and fermentation. Biominerals, resulting from ISA degradation, were analyzed. These revealed two Fe(II) minerals, siderite [FeCO3] and vivianite [(Fe3PO4)2·8H2O], not normally associated with the microbial reduction of ferrihydrite in laboratory cultures, possibly as a result of chelation of ISA with Fe(III) that made the electron acceptor more soluble. After the enrichment at pH 7, a complex microbial consortium, including Clostridia and close relatives to known Fe(III)-reducing bacteria Geobacter sp. and Rhodoferax ferrireducens, was associated with these activities. The results are discussed in the context of geological disposal safety case development, including the potential impact on priority radionuclides including U(VI), Np(V), and Tc(VII), which are highly susceptible to reductive immobilization when in contact with Fe(II)-bearing biominerals.

A number of geodisposal concepts for intermediate level radioactive waste involve geological emplacement within cementitious repositories. Such facilities, once rehydrated with groundwater, will create high pH environments due to aqueous phase reaction of the cements. This work focuses on the interactions of several important long-lived radionuclide cations with dissolved organic matter (DOM) constituents (humic and fulvic acids) under high pH conditions. We also sought to test the comprehensive speciation model WHAM/Humic Ion Binding Model VII for these specific conditions. Results for Th demonstrate high fractions present as organic complexes at all pH values. Binding of neptunyl to DOM shows a maximum over the pH range expected within an evolving repository. Uranyl exhibits decreasing binding with pH, however, the majority of metal in solution is present as organic complexes under the lower pH conditions investigated (10–10.5). We have updated the WHAM/Model VII binding values for UO 2 2 + , and have for the first time added NpO 2 + values to the database. These updates now allow application of the model for more complex mixtures across the entire repository pH range. Calculations for three simulated cement interstitial waters (representing different degradation phases) suggest U(VI) and Np(V) are not likely to be significantly bound to DOM under these conditions.

Batch experiments have been used to assess the sorption properties of a potential cementitious repository backfill, NRVB (Nirex reference vault backfill), using Eu3+as a model trivalent radionuclide and ethylenediaminetetraacetic acid (EDTA) as a competing ligand. The NRVB is an effective scavenger of Eu from solution, with most sorbed within minutes onto the crushed material and less than 1.5% remaining in solution after one day (Rdvalues in the range 0.3–2.4 × 104l kg−1). Ultrafiltration showed that nearly all of this remaining Eu (>94%) is attached to NRVB derived colloids or particulates that are mainly retained by a 100 kDa ultrafilter. High concentrations of EDTA (>0.01 M) reduced the extent of sorption at apparent equilibrium. The addition of EDTA to a pre-equilibrated system of Eu3+and NRVB resulted in a temporary suspension of some Eu, but this very quickly returned to the solid phase. There is some irreversibility in these systems, with EDTA able to prevent removal of Eu(III) from solution, but unable to bring it back into solution under the same conditions.

The firing of depleted uranium (DU) weapons during conflicts and military testing has resulted in the deposition of DU in a variety of sand-rich environments. In this study, DU-amended dune sand microcosm and column experiments were carried out to investigate the corrosion of DU and the transport of corrosion products. Under field-moist conditions, DU corroded to metaschoepite ((UO(2))(8)O(2)(OH)(12).(H(2)O)(10)) at a rate of 0.10+/-0.012 g cm(-2)y(-1). This loosely bound corrosion product detached easily from the coupon and became distributed heterogeneously within the sand. The corrosion of DU caused significant changes in the geochemical environment, with NO(3)(-) and Fe(III) reduction observed. Column experiments showed that transport of metaschoepite was mainly dependent on its dissolution and the subsequent interaction of the resulting dissolved uranyl (UO(2)(2+)) species with sand particles. The modelling results predict that the transport of U released from metaschoepite dissolution is retarded, due to a slowly desorbing surface species (first order desorption rate constant=5.0 (+/-1.0)x10(-8)s(-1)). The concentrations of U eluting from the metaschoepite column were orders of magnitude higher than the World Health Organisation's recommended maximum admissible concentration for U in drinking water of 15 microg L(-1). Therefore, a relatively high level of mobile U contamination would be expected in the immediate proximity of a corroding penetrator in a sand-rich environment.

Safety assessments for radioactive waste disposal require estimations of the migration of radionuclides in soils. The influence of humic acid (HA) on the sorption of uranium in its +4 oxidation state to kaolin has been examined from pH 4 to 8, with HA concentrations of 15–200 ppm. In the absence of HA, 20–40% of the U(IV) was in solution, with more sorption occurring at higher pH. The presence of HA solubilised up to 90% of the uranium with higher solubilities at higher HA concentrations and higher pH values. Uranium sorption was mapped against HA sorption, and it was found that there was a lower level of U(IV) sorption than can be accounted for by just measuring HA sorption. However, this effect got less marked as the pH rose. Sensitivity analysis indicated that the fraction of surface-bound HA is the controlling parameter for modelling in these systems.

Feldspar minerals are thermodynamically unstable in the near-surface environment and their surfaces are well known to react readily with aqueous solutions, leading to incongruent dissolution at low pH values, but congruent dissolution at neutral and high pH values. Interactions with mineral surfaces are an important control on the environmental transport of trace elements and detrital feldspars are abundant in soils and sediments. However, the interactions of metal ions in solution with the reacting feldspar surface have not been widely explored. The interactions of Pb(II), U(VI) and Np(V) ions with the feldspar surface have therefore been studied. Lead is taken up by the microcline surface at pH 6 and 10, but no uptake could be measured at pH 2. There was measurable uptake of Pb(II) on the plagioclase surface at pH 2, 6 and 10. Uptake always increased with pH. In most conditions, Pb(II) reacts through cation exchange process although, at high pH, a discrete phase, probably hydrocerrusite, is observed by atomic force microscopy (AFM) to precipitate on the plagioclase surface. Supersaturation with hydrocerrusite in these conditions is expected from thermodynamic calculations. Uptake of uranyl ion (UO22+), generally through surface complex formation, could only be measured at pH 6 and 10. At pH 6 and an initial U(VI) concentration above 21.0 mu M, precipitation of becquerelite (Ca[(UO2)(3)O-2(OH)(3)](2)center dot 8H(2)0), identified by AFM and characterised by grazing incidence X-ray diffraction and X-ray photoelectron spectroscopy, is observed on plagioclase. The U(VI) concentration range in which becquerelite precipitation begins (dissolved U(VI) 1-5 mu M.) is consistent with that predicted from thermodynamic modelling. On plagioclase feldspar, secondary ion mass spectrometry showed diffusion of uranium into the altered surface layer. Uptake of the neptunyl ion (Np(V)) was found at pH 6 and 10 for microcline and at pH 2, 6 and 10 for plagioclase, and was generally lower than uptake of U(VI). By combining batch sorption experiments with imaging and surface analysis, and thermodynamic modelling, it has been possible to gain a mechanistic insight into the reactions of the feldspar surface with metal ions in solution. (C) 2007 Elsevier Ltd. All rights reserved.

Dulas Bay in Anglesey, North Wales, is an area of scientific interest not only because it receives radioactive contaminants from Sellafield, but also it receives high concentrations of contaminant metals from the abandoned Parys Mountain mine via the Afon Goch. This system, therefore, provides an ideal case study to investigate the transport and the accumulation of radionuclides and stable metals and any connections between them. Samples were collected and U and Pu were separated prior to electrodeposition and alpha spectrometry. ICP-MS, XRF and gamma spectrometry were also carried out.

Nuclear facilities in coastal locations often discharge low-level liquid wastes into the sea and the radioisotopes in these discharges are of interest both in assessing possible environmental impacts and as tracers for coastal processes. The distributions of a range of artificial radionuclides, derived from the authorized discharges from British Nuclear Fuels (BNFL) Sellafield, have been determined in the sediments of an intertidal salt marsh in the Esk Estuary, Cumbria, UK. Where published discharge histories exist (for 137Cs, 238Pu, 239,240Pu and 241Am), the sediment core-profile distributions of these radionuclides have been compared with the releases from Sellafield, and consistent values of the accumulation rate (0·226 ± 0·007 g cm−2 yr−1) are obtained. A quantitative model has been developed, describing association of radionuclides with suspended particulate material, which is then accumulated and mixed in an offshore mud patch before resuspension and deposition in the salt marsh. The model has been used to describe radionuclide distributions observed in both the mud patch and the salt marsh, and to identify isotopes for which post-depositional remobilization or solution transport from the discharge point are important. The behaviour of the commonly studied isotopes (137Cs, 238Pu, 239,240Pu and 241Am) is similar to that observed at this and other nearby locations. The activation product isotope 236U is enhanced in these sediments over the natural baseline by four to eight orders of magnitude, and the results suggest that Sellafield-derived uranium is comparably mobile to 137Cs in these sediments although the processes governing the behaviour of these two elements may be different. In situ production of 241Am by decay of its 241Pu parent has generated only 17% of the current sediment inventory of this isotope, insufficient to account for the increase over the last 20–25 years, and suggesting that the input material for these sediments is preferentially enriched in Am relative to Pu during transport from the offshore mud patch. The discharge history of 244Cm, which is unknown, has also been reconstructed from the sediment profile and the model.

The transformations of 2-line ferrihydrite to hematite (pH 10.7) or goethite (pH 13.7), and of phosphate-doped 2-line ferrihydrite to goethite (pH 13.7), were studied at 60–137 °C using synchrotron-based, in-situ energy dispersive powder diffraction (EDPD). The time-resolved data for the growth of the diffraction peaks were fitted with a pseudo first-order kinetic model. As shown in previous studies, the conditional rate constant of goethite formation increases with increasing pH and is significantly lower than that for hematite crystallization. The activation energies of nucleation for hematite (pH 10.7), pure goethite (pH 13.7), and phosphate-doped goethite (pH 13.7) are 24, 7, and 21 kJ/mol, respectively, whereas the activation energies of crystallization are 69, 39, and 26 kJ/mol. The crystallization of phosphate-doped ferrihydrite produced large rectangular goethite crystals with dense ferrihydrite cores on which the goethite grew epitaxially. The rate of goethite formation is greatly reduced in the presence of phosphate due to an increase in the entropic component of the free energy of activation. This increase in entropy arises from adsorption of phosphate on to the (210) crystal faces, with an associated increase in relative growth rate on the (101) faces.

Hydrated zirconium molybdate (ZMh) is known to precipitate from solutions of dissolved spent nuclear fuel, particularly from the waste fission product solution after the uranium and plutonium have been extracted during reprocessing. Its precipitation can cause major problems during waste treatment, and therefore a complete understanding of its chemical behaviour, especially with regard to its role in the nuclear fuel cycle, is desirable. We have used a number of complementary analytical techniques to elucidate the hitherto incompletely understood chemistry of formation of ZMh in synthetic fuel reprocessing solutions. We have demonstrated that ZMh formation was governed by multi-step surface reactions and does not involve the formation of colloids or particulates in solution. The first step in the deposition of ZMh onto surfaces is the formation of an amorphous film with a Zr ∶ Mo ratio close to unity. It is followed by the formation, growth and nucleation of ZMh particles of varying degrees of crystallinity with a Zr ∶ Mo ratio close to 0.5. The X-ray diffraction pattern of deposited ZMh particles is in agreement with the reported crystallographic data. The structural features of the film and ZMh were also examined at the nanometer scale.

The spatial distribution of 238U-series radionuclides, specifically 238U, 234U, 230Th and 226Ra, has been determined in stream sediments from Edale, Derbyshire, United Kingdom, to explore the behaviour of U-series radionuclides during weathering. For uranium and thorium, two different extraction methods were used, total dissolution with HNO3/HF in a microwave and leaching with aqua regia. This was followed by radiochemical separation using extraction chromatography, then alpha spectrometry measurement. The total radium contents in the sediments were measured using gamma spectrometry, while the leached fraction was measured in the same way as for uranium and thorium. The total sediment content of uranium and thorium ranges from ∼10 up to ∼200 Bq kg-1, while the radium specific activity lies between ∼15 and 180 Bq kg-1. In the aqua regia extractions, the uranium and thorium contents are in the range of ∼5 to ∼100 Bq kg-1, while the radium specific activities are similar to those measured by total dissolution. All the radionuclides show no correlation with organic matter content. The activity ratios 234U/238U, 230Th/ 238U and 226Ra/238U were used to determine the degree of radioactive disequilibrium. The data show disequilibrium in most of the sediments, with activity ratios of 234U/238U, 230Th/238U and 226Ra/238U > 1, inconsistent with evolution through straightforward weathering processes. Multivariate cluster analysis based on five variables, the specific activities of 238U, 234U, 230Th, 226Ra and loss on ignition, was employed to group the data and identify five distinct clusters. There seems to be a link between high radionuclide concentrations and proximity to landslips. This journal is © the Partner Organisations 2014.

Zeta potential measurements have been performed on colloidal humate in the presence of differing concentrations of sodium ions at pH = 6.0. A series of calculations has been performed in which the radius of the humic colloid was varied until the predicted surface potential was equal to the experimentally determined zeta potential. The results of the calculations showed that as the ionic strength increases, the apparent radius of the colloid decreases. Similar calculations in which a model colloid was treated as a rigid sphere, i.e., the radius of the colloid was kept constant, failed to simulate the observed zeta potential measurements.

UV scanning analytical ultracentrifugation has been used to measure the aggregation of dissolved humic substances by metal ions. An adaptation of the Archibald approach to equilibrium technique has been used to monitor the change in average molecular weight, both with time, and also with mass of sample. The results have been analysed to give molecular weight distributions in the aggregated systems. For each metal studied, it was found that there was a threshold concentration below which no aggregation was observed, the magnitude of that concentration depending upon the metal. The range of concentrations from the onset of aggregation to the precipitation of the humic also depends upon the metal. It was found that the metal ions promoted aggregation in the order: La3+>Cu2+>Cd2+>Ag+. Tl4+, however, were found not to induce aggregation. In addition, the effects of ageing time, solution pH and ionic strength have been studied. It was also found that many of the aggregated humic populations were stable in solution for periods of weeks, and that the aggregation threshold concentration was independent of solution ionic strength. It was found that the aggregation was not the result of simple double layer compression as predicted by the Schulze-Hardy rule. Rather, the metal ions were reducing the intermolecular repulsion by reducing the humic charge.

UV scanning analytical ultracentrifugation has been used to measure the aggregation of dissolved humic substances by copper(II) ions. An adaptation of the Archibald approach to equilibrium technique has been used which monitors the charge in average molecular weight, both with time, and also with mass of sample. Also, a novel adaptation of the method was developed which, for the first time, allows molecular weight distributions to be determined for highly polydisperse populations. The method has been applied to the aggregation by copper(II) ions, of an aquatic humic acid isolated from surface water on Whitray Fell in the Forest of Bowland, UK. It is found that there is a threshold ion concentration of approximately 5×10 −5 M at which the copper(II) ions initiate aggregation of the humic substance. The aggregated systems contain aggregates with molecular masses up to 500,000, consisting of agglomerates of at least 10 species present in the metal-free samples.

The humic colloid borne Am(III) transport was investigated in column experiments for Gorleben groundwater/sand systems. It was found that the interaction of Am with humic colloids is kinetically controlled, which strongly influences the migration behavior of Am(III). These kinetic effects have to be taken into account for transport/speciation modeling. The kinetically controlled availability model (KICAM) was developed to describe actinide sorption and transport in laboratory batch and column experiments. Application of the KICAM requires a chemical transport/speciation code, which simultaneously models both kinetically controlled processes and equilibrium reactions. Therefore, the code K1D was developed as a flexible research code that allows the inclusion of kinetic data in addition to transport features and chemical equilibrium. This paper presents the verification of K1D and its application to model column experiments investigating unimpeded humic colloid borne Am migration. Parmeters for reactive transport simulations were determined for a Gorleben groundwater system of high humic colloid concentration (GoHy 2227). A single set of parameters was used to model a series of column experiments. Model results correspond well to experimental data for the unretarded humic borne Am breakthrough.

The dissociation of Eu(III)–HS (HS = fulvic or humic acid) complexes has been investigated using cation exchange resins to separate ‘free’ from ‘complexed ’ Eu(III). With relatively small amounts of resin, the amount of complexed Eu(III) remaining after any resin contact time is calculated using four components to describe the Eu(III) binding iz.: an easily exchangeable fraction of Eu (calculated using a single equilibrium constant) and three kinetically hindered fractions (calculated from three consecutive pseudo-first order rate expressions). With larger amounts of resin, the easily exchangeable Eu(III) becomes negligible. The slowest dissociation step, which probably is the most important with regard to radionuclide transport, has a half life of ≈200 h. The fraction of Eu(III) in the kinetically hindered sites increased with metal–HS contact time, pH, FA concentration and the degree of humification (HA>FA) but competing cations (Ca, Mg, Al or Fe) had little effect. Information on the effect of temperature on the rate constant for the slowest dissociation step was used to calculate the activation energy barrier.

The porewater concentrations of the transuranium elements Np, Pu, and Am have been measured over time at a salt marsh in west Cumbria, U.K., and all three show seasonal variations. Pu follows the pattern previously observed at this site very closely, with concentrations reaching a maximum of 3.2 mBq L -1 and a minimum of 0.8 mBq L -1 in 1996, compared with 3.5 mBq L -1 and 1.1 mBq L -1 , respectively, in 1994. However, no relationship with dissolved Fe and Mn concentrations was observed in this study. Plutonium and Am concentrations follow similar patterns from April to September, but there are fluctuations in the Am concentration in February which are not observed for Pu. Neptunium concentrations, measured by accelerator mass spectrometry, follow a less clear pattern, although they are at a marked maximum of 0.56 mBq L -1 in February. Changes in Pu and Am concentrations between April and September can be related to changes in the microbial community and suggest that these elements are taken up in biosorption processes. When biomass is low, no relationship is observed between Pu and Am concentrations. There is no correlation between the microbiological data and Np concentrations at any time of year.

A series of column experiments have been performed, which demonstrate the effect of humic substances on the transport of europium through a sand column. Two approaches have been used to model the results: an approach based on a assumption of local chemical equilibrium, and one which accounts for kinetically hindered reactions. A significant improvement in fit is observed upon the inclusion of kinetics. The model explains the observed behaviour of the europium in terms of two distinct humic bound fractions. One which is exchangeably bound, and interacts with the surface of the column packing, and another, kinetically hindered, which does not.

A new physicochemical model of metal complexation by humic substances is described. The model takes into account the effects of double layer relaxation, chemical bond formation, and the increase in entropy associated with metal dehydration. In the case of the double layer thermodynamics, mathematical models, which may be used to calculate the enthalpic, entropic and total free energy contributions to the complexation reaction are formulated. The contribution of chemical bond formation to the enthalpy of reaction is investigated by comparison with simple ligand reaction enthalpies. In the case of the entropic contribution of metal dehydration, partial molar entropies were found to give the best measure. Binding enthalpies and entropies for Eu3+ and UO22+ with Aldrich humic acid have been determined: ΔH(Eu)=+36 kJ mol−1; ΔS(Eu)=+276 J K−1 mol−1; ΔH(UO22+)=+62 kJ mol−1; ΔS(UO22+)=+62 J K−1 mol−1. In addition, the effect of ionic strength on the binding of Cu and Cd has been studied electrochemically. Using these data, combined with data from the literature, the model has been tested, both qualitatively and quantitatively. The suitability of the model for the prediction of unknown humic/metal complexation enthalpies, entropies and total binding strengths is discussed.

The uptake of Cu and Cd in aqueous solution by interaction with the surfaces of carefully synthesized finely sized particles of goethite, lepidocrocite, mackinawite, and pyrite was measured as a function of initial metal concentration in solution. The results show how reactions between metal ions in solution and mineral surfaces depend in a subtle way on the nature of the surfaces and, in certain cases, on the initial concentration of metal in solution. The uptake curves fall into two groups; type I in which the efficiency of uptake decreases with increasing concentration (Cu and Cd on goethite and lepidocrocite, Cd on pyrite), and type II in which it remains constant (Cu on mackinawite and pyrite, Cd on mackinawite). The total uptake is an order of magnitude greater for the latter group. X-ray absorption spectroscopies (XANES and EXAFS) were used to define the local environments of the metals taken up at the mineral surfaces. All examples showing the type I behavior yield information on local environments consistent with their being bound to the surfaces by an inner sphere complex formation mechanism. Thus, Cu on goethite appeared to form a Jahn-Teller distorted octahedral complex (four O atoms at 1.94 Aa; two O atoms at 2.41 Aa) but with evidence for interaction with two further Fe (or Cu) at 2.92 Aa. On lepidocrocite, the first coordination sphere was essentially identical to that on goethite but with a second Cu or Fe shell at 3.04 Aa and, for the highest Cu loadings, a third shell (2 Cu or Fe) at 3.67 Aa. The Cd on goethite showed best fits for sixfold coordination to O (at 2.26 Aa) but with evidence for a second shell of Fe atoms at 3.75 Aa. On lepidocrocite, the first shell was essentially the same as for goethite, but a second shell of Fe atoms appeared to occur at the shorter distance of 3.31 Aa. For Cd-loaded pyrite, best fits were given by a single shell of six O atoms at 2.27 to 2.28 Aa and with no evidence for a second shell of metal atoms. The systems exhibiting the type II behavior yielded spectra consistent with the formation of new phases on the surfaces, either by precipitation or replacement reactions. In the case of Cu interaction with mackinawite, a chalcopyrite phase appeared to form, whereas interaction with pyrite seemed to produce binary Cu sulfides--covellite at lower loadings and chalcocite at higher loadings. Cd interaction with mackinawite seemed to produce a CdS phase.

Humic substances are structurally complex large to macromolecules which occur in soils and natural waters as a consequence of the breakdown of plant and animal residues by microbial activity. A major portion (approx. 50%) of the earth's carbon is in the form of humic materials (fulvic and humic acids). The characterisation of humic substances is a major problem due to their heterogeneity both in terms of structure and size and to their tendency to associate in solutions as their concentration increases. Methods of characterisation of humic materials are reviewed and their interactions with substances such as clay particles, herbicides, pesticides and metallic ions which occur in the natural environment considered. The experimental methods of investigating the binding of metallic ions are also reviewed. There have been several approaches to the molecular modelling of metal ion binding based on the representation of humic molecules as colloidal particles with an associated electrical double layer. The various theoretical models have been described and the relative merits of the approaches compared.

The volumes of humic substances isolated from surface water and peat soil and characterised in terms of partial specific volume ( υ 2 ) measurements by digital densimetry, have been obtained over a range of solution conditions. Specifically, the effects of ionic strength, pH, temperature and several metal ion concentrations for both aquatic fulvic, humic acids and two peat humic acids have been investigated. The volumes have been found to contract at high salt concentrations and low pH for the aquatic materials due to a decrease in electrostatic charge repulsions. The volumes of peat humics are only slightly affected by pH changes. The volumes of all humics go through maxima in the temperature range 20–30°C possibly as a consequence of changes in hydrophobic interactions. Addition of cadmium ions leads to molecular contractions due to ion binding and charge neutralisation whereas both copper and lanthanum ions which more easily flocculate humics, result in expansion of the molecules and give rise to increases in partial specific volume. This effect is attributed to the entrapment of water in the flocs. The results show that the volumes of humic substances in the aquatic environment and in peat soils are sensitive to media conditions. The results have implications in the characterisation of these materials by techniques such as low angle X-ray scattering and analytical ultracentrifugation where a knowledge of partial specific volumes and their dependence on solution conditions are required.

Peat and aquatic humic and fulvic acids have been isolated and characterized from a site at Whitray Fell in the area of the Forest of Bowland, UK. The substances cover a range of molecular masses from approximately 2800 (aquatic fulvic acid) to 40500 (peat humic acid). The enthalpies of interaction of these humic substances with a range of metal ions, including Cu2+, Cd2+, La3+ and Al3+, have been measured by microcalorimetry in aqueous solutions of ionic strengths ranging from ca. 50 mmol dm−3 to 5 mol dm−3 at 25 °C. Apart from the interactions of La3+ ions at low ionic strength, the enthalpies of interaction were endothermic, in marked contrast to what would be expected for the complexing of metal ions by chemical groupings known to be present in humic substances such as carboxylate, hydroxy and phenolic groups. By representing the humic molecules by a solvent and ion penetrable sphere surrounded by a diffuse electrical double layer, the thermodynamic parameters for the discharge of the double layer were estimated. The enthalpy change on discharge of the humic-associated double layer is endothermic and ionic strength dependent and of the same magnitude as the experimental enthalpies of ion interaction with the humic substances. In contrast the enthalpy change on discharge of the humic molecules is exothermic. These observations suggest that the discharge of the electrical double layers makes a significant contribution to the enthalpy changes on ion interaction with humic substances. However, the major contribution to the Gibbs energy change on discharge of the electrical double layer arises from a large increase in entropy.

Summary The determination of the structure of humic substances from peat soils and natural waters is a major problem in soil science. Humic substances are chemically heterogeneous and consist of molecules covering a wide distribution of molecular size. A quantity related to both molecular size and structure in the hydrated state is the volume of the humic molecules per unit mass or partial specific volume (ν2). The partial specific volumes of humic substances have been estimated from atom and chemical group volume contributions for 17 proposed structures of fulvic and humic acids taken from the literature. The results show that the partial specific volume reflects the composition of humic substances, decreasing with increase in the carbon: hydrogen (C:H) molar ratio. The more oxidized the humic material the smaller is its partial specific volume. These trends agree with our experimental measurements of partial specific volumes for a range of humic materials (aquatic fulvics, aquatic and peat humics). The predicted values of partial specific volumes for the proposed structures, however, are overestimated by approximately 15% compared to the experimental values. There is little difference between values of ν2 calculated from the detailed humic structures and those calculated from their empirical formulae; calculations of partial specific volumes using a random modelling approach leads to overestimation by approximately 37%. It seems that the estimates based on additivity of atom and chemical group volumes do not take into account an overall contraction of the structures because of hydrophobic interactions. In general terms the values of the partial specific volumes of humic substances suggest they are more compact molecules than globular proteins, and intrinsic viscometry studies suggest that aqueous salt solutions approach θ-solvent conditions for humic substances.

The binding of the metal cations copper and calcium to peat humics and ground water (aquatic) humic and fulvic acids has been studied using ion selective electrochemistry under a variety of experimental conditions (pH, ionic strength, etc.). Competition between the metals for sites on humic acid molecules was also studied. The Random molecular model developed by Murray and Linder (1983, 1984) has been adapted in an attempt to simulate humate-metal binding behaviour. The Murray and Linder model uses a random structural approach to identify the likely metal binding sites present in each humic sample. The model contains no adjustable parameters, but instead relies on experimentally derived variables, namely, percentage carbon and hydrogen plus carboxylate and phenol concentrations. The model constructs molecules of humic acids which conform to these measurements and then searches the structures for a series of predefined metal binding sites. After generating a large number of similar structures, all of which conform to the experimental data, average concentrations of the binding sites are calculated. In the present work, the model has been extended to include nitrogen donor atoms and to search for nitrogen containing metal binding sites. In order to predict metal speciation, a version of the geochemical speciation code PHREEQE was devised in which the electrostatic effects produced by the large humic charge were accounted for. Tests on the model showed that the binding strengths predicted by the computer were consistently less than those observed by experiment, probably due to underestimation of the binding site complexity.

A concept for the disposal of intermediate level radioactive wastes involves emplacement within a geological disposal facility, followed by backfilling of the facility with cement. When the closed facility is re-saturated with groundwater, this will create a high pH environment due to dissolution of the cement minerals. Dissolved organic matter (DOM; defined here as naturally occurring organic acids and humin) will be present in the groundwater at a concentration that reflects the host rock environment and the recharge source and pathway. Interactions between DOM and radionuclides may enhance transport away from the facility and are an important consideration in safety performance assessments. This review specifically focuses on studies of DOM–radionuclide interactions at the high pH range that is expected during a repository lifetime. Whilst the vast majority of available data cover binary (DOM–radionuclide) and batch ternary systems (mineral–radionuclide–DOM), this review also covers other potentially important areas, such as reversibility kinetics and redox processes that can be mediated by DOM.

The DGT Chelex binding phase has not been tested for binding efficiency over the extreme high pH range (i.e., 10 to 13). Here, we examined the uptake efficiency of the gel-encapsulated Chelex cation exchange resin binding phase when in direct contact with solutions of radionuclides of different oxidation states over the circumneutral to high pH range (∼7 to 13). Results show that the Chelex binding gel is suitable for Eu3+ for circumneutral pH, for UO22+ up to at least pH 10.7 and for NpO 2+ up to at least pH 11.7. Application may be appropriate at higher pH values but testing of complete solution deployment units will be required. This work provides the framework to use DGT as a tool for the study of high pH radionuclide systems. © The Royal Society of Chemistry 2013.

Partial specific volume is an important molecular parameter in the measurement of molecular masses by analytical ultracentrifugation and X-ray diffraction studies. Humic substances (aquatic fulvic and humic acids and peat humic acid) were isolated from a natural source (Whitray Fell, UK). The partial specific volumes ( ) have been studied by two independent methods, an analytical ultracentrifugation technique based on simultaneous sedimentation equilibrium measurements in water and deuterium oxide and by high precision digital densimetry. Measurements were made in a range of conditions from 0.01 M to 0.5 M salt concentration and in the humic substance concentration range 0.01 to 2 mg ml−1. Partial specific volumes in the range 0.43 to 0.63 cm3 g−1 were found depending on the samples and conditions. No evidence was found for a dependence of ( ) on humic concentration within the ranges studied. Both methods of measurements gave results in reasonable accord. The ultracentrifugation method was more precise for the aquatic fulvic acid than for the more polydisperse humic acids. For the fulvic sample the reproducibility of the ultracentrifugation method was ± 4.31%, comparable to the average reproducibility of the densimetry method which was ± 4.07% over a range of 12 different systems.

Few studies have sought to investigate the potential for dissolved organic matter (DOM) to bind (and thus potentially transport) radionuclides under the high pH regimes that are expected in cementitious disposal. We have used equilibrium dialysis to investigate uranyl binding to humic acid over a pH range of ∼10 to 13. The experimental results provide evidence that DOM can bind uranyl ions over this pH range, including in the presence of competing ions. There is a general decrease in binding with increasing pH, from ∼80% of total uranyl bound at pH 9.8 to ∼10% at pH 12.9. Modelling of the system withWHAM/Model VIIcan yield representative results up to pH ∼10.5.

The geometries, relative energies and spectroscopic properties of a range of α-isosaccharinate complexes of uranyl(VI) are studied computationally using ground state and time-dependent density functional theory. The effect of pH is accommodated by varying the number of water and hydroxide ligands accompanying isosaccharinate in the equatorial plane of the uranyl unit. For 1 : 1 complexes, the calculated uranyl ν(asym) stretching frequency decreases as pH increases, in agreement with previous experimental data. Three different isosaccharinate chelating modes are studied. Their relative energies are found to be pH dependent, although the energetic differences between them are not sufficient to exclude the possibility of multiple speciation. At higher pH, the uranyl-ligand interactions are dominated more by the equatorial OH(-) than by the organic ligands. Calculated electronic excitation energies support experiment in finding the lowest energy transitions to be ligand → metal charge transfer. (13)C NMR chemical shifts are calculated for the coordinated isosaccharinate in the high pH mimics, and show good agreement with experimental data, supporting the experimental conclusion that the five-membered chelate ring is favoured at high pH. The effect of increasing the isosaccharinate concentration is modelled by calculating 1 : 2 and 1 : 3 uranyl : α-isosaccharinate complexes. Comparison of the results of the present study with those from our closely related investigation of uranyl(VI)-D-gluconate complexes (Dalton Transactions 40 (2011) 11248) reveals strong similarities in structure, bonding, coordination geometry and electronic excitations, but also differences in ΔG for key ligand replacement reactions, suggesting that caution should be exercised when using gluconate as a thermodynamic model for isosaccharinate in uranyl(vi) chemistry.

Some recent developments made during the European Union 6th Framework Integrated Project FUNMIG in the understanding and prediction of behaviour in ternary systems of radionuclides, humic substances and mineral surfaces are described. These developments are placed in the context of the existing literature. The aim is to describe the current understanding of humic substance mediated radionuclide transport as it may be applied to calculations in support of Radiological Performance Assessment. Some improvements in experimental techniques that provide the raw data to calibrate metal ion binding models are explained. The various metal ion binding models that are available are described and contrasted, before the recent development of ternary system models, in particular the Ligand Charge Distribution model that can predict metal ion and humic substance behaviour in ternary systems. The kinetic effects in ternary systems are described along with the models that are used to describe them. Finally, the remaining challenges in making predictions of radionuclide transport for the Radiological Performance Assessment of radioactive waste repositories are discussed. © 2011 Elsevier Ltd. All rights reserved

Field observations suggest that plutonium and americium in the environment are present in very different chemical forms in the interstitial waters of an intertidal sediment. Themodynamic modelling using the PHREEQE code predicts that plutonium is present entirely in oxidation state (V) as the PuO2CO 3 − ion, whereas americium is present entirely in oxidation state (III), largely as the uncharged Am(OH)CO3 species, but with significant concentrations of the Am3+ and the AmSO 4 + ions. There are, however, differences between these predictions and others published for a very similar system which apparently arise from uncertainties in the thermodynamic data. Field data cannot resolve these differences unambiguously.

The effect of the anthropogenic complexing agent EDTA on thorium transport in groundwater has been studied using sand-packed columns and flow rates in the range of 20-100 m y⁻¹. The concentrations injected into the columns were in the range of 0.4-4 mM for Th and 4-40 mM for EDTA, and with EDTA:Th ratios in the range 1:1 to 10:1. The results show that EDTA can significantly increase Th transport, but two very different behaviours are observed at Th concentrations of 0.4 and 4 mM. At the lower concentration, Th breakthrough is retarded with respect to a conservative tracer, with a peak width that is consistent with a single K(d) value, followed by a longer tail, and the behaviour is very sensitive to the flow rate. However at 4 mM Th, the breakthrough peak appears near to that of the tracer, and the width of the peak is consistent with a distribution of K(d) values and/or a larger dispersivity than the tracer. Speciation and transport modelling have been used to interpret the data, and a model was developed that could explain the 0.4 mM behaviour. This suggests that ternary surface complexes are important in these systems, with at least two different species involved, although the complexity of Th speciation in these systems leads to significant uncertainty in the values of the equilibrium and kinetic parameters. For the 4 mM systems, the rapid transport observed could not be explained by a simple chemical model; instead it is likely that EDTA plays an important role in stabilising and transporting thorium colloids and clusters.

Using previously established procedures that utilise linear free energy relationships, we estimated binding constants for the Windermere Humic Aqueous Model VII (WHAM/Model VII) for several radionuclide cations (Pd 2+, Sn 2+, U 4+, NpO 2 2+, Pu 4+ and PuO 2 2+). This extends the number of cations that can be calculated with the model above the 40 included in the original Model VII work. When combined with equilibrium constants for inorganic species this allows the calculation of equilibrium distributions of chemical species under a wide range of conditions. © 2011 The Royal Society of Chemistry.

In the UK, there is a nuclear waste legacy associated with over 50 years of nuclear power generation that is currently stored at the Earth's surface. This is a global phenomenon in which many nations are now facing up to the radioactive waste legacy of several decades of nuclear power generation. As society considers new nuclear power as a low carbon, secure source of energy, it is apparent that geological disposal of higher activity radioactive wastes is now the favoured route for management of this highly radioactive legacy material. Timely implementation of geological disposal is therefore a current challenge facing the UK and other nuclear nations if we are to demonstrate safe management of these materials for future generations. In this chapter, we review the type and characteristics of the higher activity wastes that the UK needs to dispose of; examine the concept of a geological disposal facility in the context of UK and international experience; and discuss the proposed implementation pathway for UK higher activity waste geodisposal in the context of our large and complex nuclear legacy. Finally, we discuss the environmental chemistry research challenges that we see as vital to the safe management and disposal of these legacy radioactive wastes.

Dynamic gamma-camera imaging of radiotracer technetium ((99m)Tc) was used to assess the impact of biostimulation of metal-reducing bacteria on technetium mobility at 10(-12) mol L(-1) concentrations in sediments. Addition of the electron donor acetate was used to stimulate a redox profile in sediment columns, from oxic to Fe(III)-reducing conditions. When (99m)Tc was pumped through the columns, real-time gamma-camera imaging combined with geochemical analyses showed technetium was localized in regions containing biogenic Fe(II). In parallel experiments, electron microscopy with energy-dispersive X-ray (EDX) mapping confirmed sediment-bound Tc was associated with iron, while X-ray absorption spectroscopy (XAS) confirmed reduction of Tc(VII) to poorly soluble Tc(IV). Molecular analyses of microbial communities in these experiments supported a direct link between biogenic Fe(II) accumulation and Tc(VII) reductive precipitation, with Fe(III)-reducing bacteria more abundant in technetium immobilization zones. This offers a novel approach to assessing radionuclide mobility at ultratrace concentrations in real-time biogeochemical experiments, and confirms the effectiveness of biostimulation of Fe(III)-reducing bacteria in immobilizing technetium.