Development of mechanistic models for the description of radionuclide retention onto calcareous gravels.

The retention capacity of natural calcareous gravels used as a filling material between the packages containing radioactive waste materials in the disposal cells was evaluated for Sr, U, Ni, Ag, 14C, 99Tc, 239Pu, 228Th and 152Eu. The thermodynamic calculations performed considering repository conditions indicated that the chemistry of most of these elements was dominated by neutral or cationic species, this is the case of 238Pu 152Eu, 228Th, Ni and Ag. The studied gravels presented high sorption capacities for these cationic or neutral species, but they are not efficient on retaining anionic species as in the case of 99Tc or Sr, which is already present in the composition of the studied gravels. For those elements where the predominant species are carbonated (14C and U) low distribution coefficients were obtained. A preliminary mechanistic sorption model was developed for each radionuclide considering carbonate (>CO3H) and calcium hydroxide (>CaOH) sites. Our preliminary model allowed to successfully reproduce the experimental trend of the data obtained in this work.

Redox Interaction between Selenite and Mackinawite in Cement Pore Water.

In cement-rich radioactive waste repositories, mackinawite (FeS) forms at the steel corrosion interface within reinforced concrete and potentially retards the transport of redox-sensitive radionuclides (e.g., 79Se) in porous cement media. Redox interactions between selenite and mackinawite under hyperalkaline conditions remain unclear and require further investigations. Here, using comprehensive characterization on both aqueous and solid speciation, we successfully monitored the whole interaction process between selenite and mackinawite under hyperalkaline conditions. The results show similar chemical environments for SeO32- and S2-/Sn2- at the mackinawite-water interface, verifying an immediate reduction. After 192 h of reaction, SeO32- was reduced to solid Se0 and SeS2 species, accompanied by the oxidation of S2-/Sn2- to S2O32- and Fe(II) to Fe(III) in mackinawite. Aqueous speciation results showed that ∼99% of aqueous selenium was present as Se4S nanoparticles due to the dissolution of Se from the solid. In parallel, ∼62% of S2-/Sn2- was released into the solution, with mackinawite transforming into magnetite, Fe(OH)3 and FeS2O3+ complexed to Cl- or OH- species, and magnetite subsequently dispersed in the solution. This study provides valuable data about the retardation mechanisms of redox-sensitive radionuclides by soluble iron sulfides, which is critical to advance our understanding of reactive concrete barriers used in nuclear waste disposal systems.

Selenite Sorption on Hydrated CEM-V/A Cement in the Presence of Steel Corrosion Products: Redox vs Nonredox Sorption.

Reinforced cementitious structures in nuclear waste repositories will act as barriers that limit the mobility of radionuclides (RNs) in case of eventual leakage. CEM-V/A cement, a ternary blended cement with blast furnace slag (BFS) and fly ash (FA), could be qualified and used in nuclear waste disposal. Chemical interactions between the cement and RNs are critical but not completely understood. Here, we combined wet chemistry methods, synchrotron-based X-ray techniques, and thermodynamic modeling to explore redox interactions and nonredox sorption processes in simulated steel-reinforced CEM-V/A hydration systems using selenite as a molecular probe. Among all of the steel corrosion products analyzed, only the addition of Fe0 can obviously enhance the reducing ability of cement toward selenite. In comparison, steel corrosion products showed stronger reducing power in the absence of cement hydrates. Selenium K-edge X-ray absorption spectroscopy (XAS) revealed that selenite immobilization mechanisms included nonredox inner-/outer-sphere complexations and reductive precipitations of FeSe and/or Se(0). Importantly, the hydrated pristine cement showed a good reducing ability, driven by ferrous phases and (bi)sulfides (as shown by sulfur K-edge XAS) originated from BFS and FA. The overall redox potential imposed by hydrated CEM-V/A was determined, hinting to a redox shift in underground cementitious structures.

Influence of soil redox state on mercury sorption and reduction capacity.

We investigated the mechanisms of interactions between divalent aqueous Hg and rock samples originating from an outcropping rock formation, the Albian Tégulines Clay (France, Aube). Two solid samples collected at two different depths (7.7 and 21.2 m depth) in the rock formation were selected since, in situ, they had and were still experiencing contrasting redox conditions, and thus had different mineralogy with regards to the minerals containing redox-sensitive elements, in particular iron. The sample that was the closer to the surface was under oxidizing conditions and contained goethite and siderite, while the deeper one was under reducing conditions and had more siderite, together with pyrite and magnetite. The redox state of the samples was preserved throughout the present study by careful conditioning, preparation, and use them under O2-free conditions. The two samples had similar affinity for Hg, with a retention coefficient (RD) ranging between 102 and 106 mol·kg-1 when the aqueous Hg concentration ranged between 4.4 and 107 ng·L-1 with the lowest concentration for the highest RD. However, the mechanisms of interaction differed. In the oxidized sample, no change in Hg redox state was observed, and the retention was due to reversible adsorption on the mineral phases (including organic matter). In contrast, upon interaction with the deeper and reduced sample, Hg was not only adsorbed on the mineral phases, but part of it was also reduced to dissolve elemental Hg. This reduction was attributed to magnetite and siderite and highlights the influence of mineralogy on the geochemical cycle of Hg.

Passive sampler measurements of inorganic arsenic species in environmental waters: A comparison between 3-mercapto-silica, ferrihydrite, Metsorb®, zinc ferrite, and zirconium dioxide binding gels.

The performances of five Diffusive Gradients in Thin Films (DGT) binding gels, namely 3-mercapto-functionalized silica (3MP), ferrihydrite (Fh), Metsorb®, zinc ferrite (ZnFe2O4), and Zirconium oxide (ZrO2), were evaluated for in situ determination of As speciation in water and sediments. A combination of batch experiments at various pH (without addition of buffers) and in the presence of reduced species (Mn2+, Fe2+ and HS-),time-series experiments in oxic waters, and in situ deployment in anoxic river sediments has permitted to evaluate the potential interferences among the binding gels. Firstly, the efficiency of each DGT binding gel dedicated to total As (i.e., Fh, Metsorb®, ZnFe2O4 and ZrO2) or As(III) (i.e., 3MP) determination confirms that the determination of As species is possible in oxic freshwater and seawater over 96 h for a wide range of pH (5-9). Secondly, concerning the deployment in river sediment, high HCO3- concentrations have a little negative effect only on the DGT performances of the iron(III)-binding gels (i.e, Fh and ZnFe2O4). Thirdly, the presence of sulfides does not show any effect on the DGT uptake of As, but strongly affects the elution factor parameter. Discrepancies in elution between the different binding gels potentially result in precipitation of orpiment, especially in 1 mol L-1 HNO3. A correction of the classical elution factor derived from batch experiments was applied to provide more representative results. Finally, this study shows the difficulties to determine As speciation in anoxic sediments, and suggests that corrections of the elution factor may be required as a function of the species present in the deployment matrices.

XANES-Based Determination of Redox Potentials Imposed by Steel Corrosion Products in Cement-Based Media.

The redox potential (Eh) in a cementitious nuclear waste repository is critical to the retardation behavior of redox-sensitive radionuclides (RNs), and largely controlled by embedded steel corrosion but hard to be determined experimentally. Here, we propose an innovative Eh determination method based on chemical/spectroscopic measurements. Oxidized nuclides (UVI, SeIV, MoVI, and SbV) were employed as species probes to detect the Eh values imposed by steel (Fe0) and steel corrosion products (magnetite/hematite, and magnetite/goethite couples) in cement pore water. Nuclides showed good sorption affinity, especially toward Fe0, in decreasing Kd order for U > Sb > Se > Mo under both N2 and H2 atmospheres. The reduced nuclide species were identified as UO2, U4O9, FeSe, FeSe2, Se0, Sb0, and Sb2O3, but no redox transformation occurred for Mo. Eh values were obtained by using the Nernst equation. Remarkably, their values fell in a small range centered around -456 mV at pH ∼ 13.5 for both Fe0 and Fe-oxyhydroxides couples. This Eh value appears to be controlled by the nanocrystalline Fe(OH)2/Fe(OH)3 or (Fe1- x,Ca x)(OH)2/Fe(OH)3 couple, whose presence was confirmed by pair distribution function analyses. This approach could pave the way for describing the Eh gradient in reinforced concrete where traditional Eh measurements are not feasible.

Retention of arsenic, chromium and boron on an outcropping clay-rich rock formation (the Tégulines Clay, eastern France).

The retention behavior of three toxic chemicals, As, Cr and B, was investigated for an outcropping rock formation, the Albian Tégulines Clay (France, Aube). At a shallow depth, Tégulines Clay is affected by weathering processes leading to contrasted geochemical conditions with depth. One of the main features of the weathering is the occurrence of a redox transition zone near the surface. Batch sorption experiments of As(V), As(III), Cr(VI) and B were performed on samples collected at two depths representative either of oxidized or reduced mineral assemblages. Batch sorption experiments highlighted a distinct behavior between As, Cr and B oxyanions. Cr(VI) retention behavior was dominated by redox phenomena, notably its reduction to Cr(III). The in-situ redox state of the Tégulines Clay samples has a significant effect on Cr retention. On the contrary, As(V) reduction into As(III) is moderate and its retention slightly affected by the in-situ redox state of the Tégulines Clay. As(V) retention is higher than As(III) retention in agreement with literature data. B retention is strongly related to its natural abundance in the Tégulines clay samples. Distribution coefficient of B corrected from its natural content is expected to be very low for in-situ conditions. Finally, the retention and mobility of these oxyanions were affected by clay mineralogy, natural abundance, and reducing capacity of the Tegulines Clay.

Determination of total arsenic using a novel Zn-ferrite binding gel for DGT techniques: Application to the redox speciation of arsenic in river sediments.

A new laboratory-made Zn-ferrite (ZnFe2O4) binding gel is fully tested using Diffusive Gradient in Thin films (DGT) probes to measure total As [including inorganic As(III) and As(V), as well as MonoMethyl Arsenic Acid (MMAA(V)) and DiMethyl Arsenic Acid (DMAA(V))] in river waters and sediment pore waters. The synthesis of the binding gel is easy, cheap and its insertion into the acrylamide gel is not problematic. An important series of triplicate tests have been carried out to validate the use of the Zn-ferrite binding gel in routine for several environmental matrixes studies, in order to test: (i) the effect of pH on the accumulation efficiency of inorganic As species; (ii) the reproducibility of the results; (iii) the accumulation efficiency of As species; (iv) the effects of the ionic strength and possible competitive anions; and (v) the uptake and the elution efficiency of As species after accumulation in the binding gel. All experimental conditions have been reproduced using two other existing binding gels for comparison: ferrihydrite and Metsorb® HMRP 50. We clearly demonstrate that the Zn-ferrite binding gel is at least as good as the two other binding gels, especially for pH values higher than 8. In addition, by taking into consideration the diffusion rates of As(III) and As(V) in the gel, combining the 3-mercaptopropyl [accumulating only As(III)] with the Zn-ferrite binding gels allows for performing speciation studies. An environmental study along the Marque River finally illustrates the ability of the new binding gel to be used for field studies.

Mineral precipitation-induced porosity reduction and its effect on transport parameters in diffusion-controlled porous media.

BACKGROUND: In geochemically perturbed systems where porewater and mineral assemblages are unequilibrated the processes of mineral precipitation and dissolution may change important transport properties such as porosity and pore diffusion coefficients. These reactions might alter the sealing capabilities of the rock by complete pore-scale precipitation (cementation) of the system or by opening new migration pathways through mineral dissolution. In actual 1D continuum reactive transport codes the coupling of transport and porosity is generally accomplished through the empirical Archie's law. There is very little reported data on systems with changing porosity under well controlled conditions to constrain model input parameters. In this study celestite (SrSO4) was precipitated in the pore space of a compacted sand column under diffusion controlled conditions and the effect on the fluid migration properties was investigated by means of three complementary experimental approaches: (1) tritiated water (HTO) tracer through diffusion, (2) computed micro-tomography (µ-CT) imaging and (3) post-mortem analysis of the precipitate (selective dissolution, SEM/EDX). RESULTS: The through-diffusion experiments reached steady state after 15 days, at which point celestite precipitation ceased and the non-reactive HTO flux became constant. The pore space in the precipitation zone remained fully connected using a 6 µm µ-CT spatial resolution with 25 % porosity reduction in the approx. 0.35 mm thick dense precipitation zone. The porosity and transport parameters prior to pore-scale precipitation were in good agreement with a porosity of 0.42 ± 0.09 (HTO) and 0.40 ± 0.03 (µ-CT), as was the mass of SrSO4 precipitate estimated by µ-CT at 25 ± 5 mg and selective dissolution 21.7 ± 0.4 mg, respectively. However, using this data as input parameters the 1D single continuum reactive transport model was not able to accurately reproduce both the celestite precipitation front and the remaining connected porosity. The model assumed there was a direct linkage of porosity to the effective diffusivity using only one cementation value over the whole porosity range of the system investigated. CONCLUSIONS: The 1D single continuous model either underestimated the remaining connected porosity in the precipitation zone, or overestimated the amount of precipitate. These findings support the need to implement a modified, extended Archie's law to the reactive transport model and show that pore-scale precipitation transforms a system (following Archie's simple power law with only micropores present) towards a system similar to clays with micro- and nanoporosity. Graphical abstract.

On-off mobilization of contaminants in soils during redox oscillations.

Near-surface biogeochemical systems can oscillate between oxic and anoxic conditions. Under such periodic changes many redox-sensitive inorganic contaminants undergo speciation, mobility and toxicity changes. We investigated the changes to chromium (Cr), arsenic (As), selenium (Se), antimony (Sb) and uranium (U) mobility during a series of laboratory experiments where argillaceous substrates were subjected to successive cycles of oxidizing and reducing conditions. The EH oscillated between -320 and +470 mV, induced via both abiotic and microbial forcings. Chemically induced cycles of oxidation and reduction were achieved via a combination of gas (N2:CO2 vs compressed air) and carbon (ethanol) addition, to stimulate the metabolism of a natively present microbial community. The contaminants were added either alone or as contaminant mixtures. Results show clear on-off switch mobility behavior for both major elements such as carbon (C), iron (Fe) and manganese (Mn) and for contaminants. Mn, Fe, and As were mobilized under anoxic conditions, whereas Sb, Se, and U were mobilized under oxic conditions. While As, Sb, and U were reversibly sorbed, Se and Cr were irreversibly sequestered via reductive precipitation. When present in aqueous solutions at high concentrations, Cr(VI) prevented the reduction of Mn and Fe, and inhibited the mobilization of elements with lower EH(o). To improve remediation strategies for multiple contaminants in redox-dynamic environments, we propose a mixed kinetic-equilibrium biogeochemical model that can be forced by oscillating boundary conditions and that uses literature rates and constants to capture the key processes responsible for the mobilization of contaminants in soils.

Arsenic behavior in river sediments under redox gradient: a review.

The fate of arsenic - a redox sensitive metalloid - in surface sediments is closely linked to early diagenetic processes. The review presents the main redox mechanisms and final products of As that have been evidenced over the last years. Oxidation of organic matter and concomitant reduction of oxidants by bacterial activity result in redox transformations of As species. The evolution of the sediment reactivity will also induce secondary abiotic reactions like complexation/de-complexation, sorption, precipitation/dissolution and biotic reactions that could, for instance, lead to the detoxification of some As species. Overall, abiotic redox reactions that govern the speciation of As mostly involve manganese (hydr)-oxides and reduced sulfur species produced by the sulfate-reducing bacteria. Bacterial activity is also responsible for the inter-conversion between As(V) and As(III), as well as for the production of methylated arsenic species. In surficial sediments, sorption processes also control the fate of inorganic As(V), through the formation of inner sphere complexes with iron (hydr)-oxides, that are biologically reduced in buried sediment. Arsenic species can also be bound to organic matter, either directly to functional groups or indirectly through metal complexes. Finally, even if the role of reduced sulfur species in the cycling of arsenic in sediments has been evidenced, some of the transformations remain hypothetical and deserve further investigation.

TRLFS evidence for precipitation of uranyl phosphate on the surface of alumina: environmental implications.

We studied the ligand-enhanced sorption of uranyl ions (1-12 µM) on α-alumina colloids suspended in (and pre-equilibrated with) solutions at various concentrations of phosphate ions (P(T) = 0-900 µM). A highly sensitive technique, time resolved laser-induced fluorescence spectroscopy (TRLFS), was used to examine the chemical speciation of uranyl sorbed at trace concentrations (0.4-4 µmol U·g⁻¹). The suspensions with P(T) ≥ 100 µM exhibited high uranyl adsorption, and a very high intensity of fluorescence that increased with the sorbed amounts of phosphate and uranyl. These samples exhibited similar spectral and temporal characteristics of fluorescence emission, evidencing a uniform speciation pattern and a single coordination environment for sorbed U, despite large variation in parameters such as aqueous uranyl speciation, U loading, and extent of coverage of alumina by secondary Al phosphates precipitating on the surface. The results pointed formation of surface precipitates of uranyl phosphates, which are characterized by high quantum yield, peak maxima at positions similar to those of U(VI) phosphate minerals and four lifetimes indicating distortions, in-homogeneities or varying number of water molecules in the lattice. The findings have major implications for our understanding of the mechanisms of immobilization of U at trace levels on surfaces of oxides submitted to phosphated solutions in soils with low pH.

Mechanisms of uranyl and phosphate (co)sorption: complexation and precipitation at alpha-Al2O3 surfaces.

This study presents new in situ electrophoretic and ATR-FTIR data on the surface species controlling the cosorption of uranyl and phosphate ions in alpha-Al(2)O(3) suspensions at acidic pH (3.3). It was shown that the uranyl sorption (i) was promoted in the presence of phosphate, (ii) induced significant changes in zeta potential of P-loaded alumina, and (iii) was governed by two mechanisms, surface complexation and surface precipitation, with the predominant species being mainly dependent on phosphate surface coverage. Formation of surface precipitates of uranyl phosphate at high phosphate surface coverage was inferred from the high negative charges imparted to the surface by uranyl and phosphate (co)sorption, and from assignments of IR bands at 1107, 1024, and 971 cm(-1) to P-O-stretching vibrations for phosphate coordinated to uranyl, at the alumina surface. The ATR-FTIR study showed that the precipitates of uranyl phosphate formed at the surface of alpha-Al(2)O(3) for aqueous concentrations of uranyl at trace levels. It also evidenced that formation of surface precipitates of U(VI)-phosphate was occurring along with the transformation of alumina into secondary surface precipitates of Al-phosphate, at very high phosphate concentrations. These findings are relevant to the mechanisms of adsorption of trace uranyl on naturally occurring oxide surfaces, in soils with low pH where cosorption of phosphate and uranyl ions is known to play a crucial role in the long-term retention of U.

Surface reactivity of alpha-Al(2)O(3) and mechanisms of phosphate sorption: In situ ATR-FTIR spectroscopy and zeta potential studies.

We have investigated the effect of solution parameters on the adsorption of phosphate ions and on charges and structures, i.e., on the nature of species, at the alpha-Al(2)O(3) colloid/solution interface by using the batch method, zeta potential measurements, and in situ ATR-FTIR spectroscopy. The uptake of phosphate decreases with the extent of surface deprotonation (i.e., pH), imparts negative charges to the colloid surface, and induces IEP shifts showing chemical sorption. Use of complementary techniques provides evidence that phosphate is sorbed at low pH (3.3) by a combination of surface reactions of complexation and precipitation, whose relative contributions depend on phosphate loading. Surface complexation includes fast reactions of ligand exchange with single coordinated hydroxyls, and electrostatic attraction of H(2)PO(4)(-) ions at positively charged surface sites. This is supported by experiments at low coverage showing sharp and linear decrease of zeta potential (i.e., surface charge) with amount of phosphate sorbed. At high coverage, zeta potential values are low and independent of phosphate loading. Formation of surface precipitates of Al-phosphate is inferred from the assignment of the ATR-FTIR absorption band at 1137cm(-1), whose intensity increases with phosphate solution content and reaction time, to the P-O-stretching vibration mode for phosphate sorbed at high concentrations on alpha-Al(2)O(3). In situ ATR-FTIR spectroscopy reveals also structural reorganizations of surface hydroxyls with time, due to surface hydration and to surface precipitation continuing over extended periods along alumina dissolution.