Deep geological repository is considered the internationally accepted method for spent fuel (SF) disposal. In countries where salt, clay, tuff and granite are unavailable at geologically suitable ...area, other rock types may come into consideration. In Israel, carbonate rocks make up a significant portion of the surface and subsurface lithologies, thus, low permeability carbonates were evaluated as possible host rocks for a repository, and for an interim storage facility.
Sorption and retardation capacity of SF components to low permeability carbonate rocks were evaluated using their chemical simulants. Strontium and Cs represent components that may leach during interim storage, while U and Ce (as a simulant for redox-active actinides) represent components that may leach under repository conditions.
Rocks from the Upper Cretaceous Mount Scopus Group were sampled from boreholes at the Yamin Plateau, Israel. Single point batch experiments were conducted with synthetic rainwater spiked with tracers and interacted with five rock types of various particle sizes at 25 °C. Results were evaluated using the LeachXS™-ORCHESTRA geochemical speciation and data management program.
Cerium removal was found to be related to the HCO3– concentration in solution, where Ce precipitated as Ce2(CO3)3·XH2O and as an amorphous carbonate phase. Removal of Cs and Sr was controlled by clays. No Sr co-precipitation as carbonate species was observed. Uranium was removed mainly by sorption onto solid organic matter, whereas clays had no significant role in U sorption. Iron-(hydr) oxides may have also played a role in U removal. Calculated partition coefficients for U, Cs, and Sr were in the order of 101–102 mL/g. Grain size had no significant effect on the retention capacity of the studied rocks due to similar effective surface area.
The current study indicates that a repository or an interim storage facility within carbonate rocks, would provide only partial isolation of radionuclides from the environment, hence, additional engineered barriers may be required.
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•Low permeability carbonates evaluated for Spent Fuel repository & interim storage•Batch experiments of synthetic rainwater & 5 rock types doped with Ce, Cs, Sr & U•Cerium precipitated as Ce2(CO3)3·XH2O and as an amorphous carbonate•Cs and Sr removed by clays, U sorbed onto organic matter and Fe-(hydr)oxides•Spent Fuel storage within carbonate rocks may require accessory engineered barriers.
Iron isotopic compositions potentially provide a powerful new tracer of planetary formation and differentiation processes and of secular and spatial changes in mantle oxidation state. However, the ...processes governing iron isotope fractionation in igneous rocks remain poorly understood. Here we show that there are significant variations in the iron isotope compositions (
δ
57/54Fe) of mantle rocks (0.9‰) and minerals (olivines 0.6‰, clinopyroxenes 0.9‰ and orthopyroxenes 0.8‰), with spinels showing the greatest total variation of 1.7‰. Positive linear functional relationships with slopes that are, within error, equal to unity are found between the
δ
57/54Fe values of coexisting orthopyroxene, clinopyroxene and olivine, strongly suggesting that the
δ
57/54Fe values of these minerals reflect intra-sample mineral–mineral isotopic equilibrium. Positive correlations between the
δ
57/54Fe values of silicate minerals and spinels also exist, although they are more scattered, which could be caused by late disturbance of mineral-spinel isotopic equilibrium. Bulk-rock, clinopyroxene and spinel
δ
57/54Fe values correlate with chemical indices of both melt extraction and oxidation. Iron isotope fractionation during spinel-facies partial melting is investigated using simple models, which demonstrate that the maximum expected fractionation between melt and residue will be ∼0.5‰, with the residue becoming isotopically light relative to the melt and to the initial source region. Hence melt extraction, in combination with significant changes in mantle oxidation state, may be an explanation for Fe isotopic variations in mantle peridotites. Metasomatism of the sub-arc mantle by iron-rich silicate melts originating from the subducting slab may also explain the light bulk-sample
δ
57/54Fe values of some arc peridotites (−
0.2‰ to −
0.6‰), but mass-balance calculations require these metasomatic agents to have extreme
δ
57/54Fe values (e.g. −
3.0‰). The large differences in the
δ
57/54Fe values of garnet and spinel facies rocks are likely to be caused by the contrasting behaviour of Fe
3+
during melting in the spinel and garnet facies. However, there is little difference in the
δ
57/54Fe values of MORB and OIB, despite the fact that OIB are considered, on the basis of incompatible element abundances, to arise dominantly by melting in the garnet stability field. Given that iron is a relatively compatible element, the similarities in the
δ
57/54Fe values of MORB and OIB provide strong evidence that MORB and OIB are both dominated by melting in the spinel facies.
Redox exerts a critical control on organic carbon-rich sedimentation. This is particularly true for Eastern Mediterranean sapropels where seawater stratification is regarded as a major driving force ...for oxygen depletion, but in which sulphidic (euxinic) bottom waters occur only sporadically. Here we apply a powerful array of geochemical proxies (Fe and Mo stable isotopes together with Mo/U ratios and redox sensitive trace elements (RSTE)) to the determination of water redox evolution during the deposition of Holocene S1 sapropel and its underlying and overlying sediments (ODP core 967D; 2550 m depth).
RSTE are asymmetrically distributed within the sapropel, with peak enrichments occurring in its lower (early) part. Negative correlations are found between δFe57 and both Fe/Al and S wt% in the lower sapropel, and are consistent with the benthic Fe shuttle model Fe enrichment in euxinic basins. MoEF/UEF enrichment factor variations show well defined trends identical to those proposed for open marine settings, in which sub-oxic conditions in the background sediments give way to sulphidic waters at the RSTE peak in the lower sapropel. The most notable features of the Mo isotope profile are ‘atypically’ light values (δMo98/95<−0.7‰) in the lower sapropel. Such light Mo isotope values (relative to sea water δMo98/95=2.3‰) have been related to oxic remobilisation. However, negative correlations between δMo98/95 and Fe/Al, Ba/Al, Mo/Al and S imply that the lowest Mo isotopic compositions are associated with peak reducing conditions. Taken in conjunction with the evidence from the other proxies for a sulphidic water column, the light Mo isotope values in the lower sapropel are best explained by a large isotopic fractionation between sea water molybdate and thiomolybdate species in mildly euxinic bottom waters (H2Saq <10 μM). The data from this study thus show that hitherto unrecognised euxinic conditions occur during the early stages of deposition of the Holocene sapropel S1. Molybdenum isotopes and Ba/Al ratios identify a short-lived sapropel re-ventilation event timed to coincide with the 8.2 ka cold climatic Event.
•Geochemical depth profiles early peak reducing conditions in sapropel S1.•Fe isotope compositions consistent with benthic shelf to basin Fe-shuttle.•Mo/U ratios indicate sulfidic bottom waters during peak sapropel development.•Light Mo isotope compositions compatible with mild euxinic conditions.•Mo isotopes detect Holocene cold Event.
Pollution history of Pb and other trace metals was reconstructed for the first time for the Eastern Mediterranean, from a small urban winter pond (Dora, Netanya), located at the densely populated ...coastal plain of Israel. An integrated approach including geochemical, sedimentological, and historical analyses was employed to study sediments from the center of the pond. Profiles of metal concentrations (Pb, Zn, V, Ni, Cu, Cr, Co, Cd, and Hg) and Pb isotopic composition denote two main eras of pre- and post-19th century. The deeper sediment is characterized by low concentrations and relatively constant 206Pb/207Pb (around 1.20), similar to natural Pb sources, with slight indications of ancient anthropogenic activity. The upper sediment displays an upward increase in trace metal concentrations, with the highest enrichment factor for Pb (18.4). Lead fluxes and isotopic composition point to national/regional petrol-Pb emissions as the major contributor to Pb contamination, overwhelming other potential local and transboundary sources. Traffic-related metals are correlated with Pb, emphasizing the polluting inputs of traffic. The Hg profile, however, implies global pollution rather than local sources.
Seven bulk chondrites, with
δ
57Fe/
54Fe values between −0.1‰ and 0‰ relative to IRMM-14, tend to be slightly lighter than 11 bulk iron meteorites, which have
δ
57Fe/
54Fe values ranging from 0.04‰ ...to 0.2‰. At the mineral scale, taenite from two iron meteorites, Cranbourne and Toluca, shows
δ
57Fe/
54Fe values heavier by up to 0.3‰ than their kamacite counterpart, thus calling into question the significance of bulk iron meteorite data. On three pallasites (Esquel, Marjalahti and Springwater) we measured a heavier iron isotope composition for the metal fractions compared to the coexisting olivines as previously observed on two other pallasites (Eagle Station and Imilac), but the range of
δ
57Fe/
54Fe differences (from 0.32‰ to 0.07‰) is larger than that originally found. Troilite from two pallasites appears to be even heavier than the metal fraction, whereas schreibersite is lighter than its olivine counterpart. There is thus a general tendency for minerals within a given rock to show a heavier Fe isotope composition as the coordination number of Fe increases, although troilite is an exception to this rule. Iron meteorites are classically considered as remnants of asteroid cores and pallasites as core–mantle interfaces. The simultaneous finding that the metal fractions of pallasites have a higher
δ
57Fe/
54Fe signature than the coexisting olivines, and that the iron meteorites are slightly heavier than chondrites could be taken as an indication that planetary core–mantle differentiation is accompanied by sizeable iron isotope fractionation. In this hypothesis, mass balance constraints imply that resultant planetary mantles should be isotopically lighter than the chondritic starting material. That is not observed, however, since all planetary mantles analyzed so far have
δ
57Fe/
54Fe values equivalent to or heavier than those of chondrites. It thus appears that the moderate temperature and pressure metal–silicate fractionation that occurred in pallasite and iron parent bodies is not readily transposable to planets as far as Fe isotopes are concerned. Instead, these mantle signatures could reflect equilibrium fractionation at a higher temperature, or the lack of a global core–mantle equilibration at the planetary scale. Overall, these new results show that the mass-dependent isotopic scatter observed among inner solar system bodies from the bulk-rock to the planetary scale (∼0.3‰
δ
57Fe/
54Fe) is more restricted than previously thought. This likely confirms a homogenization process that occurred in the protoplanetary accretion disk, between refractory inclusion condensation and chondrule formation.
Iron partitioning data and whole soil
δ
57Fe values were combined to calculate the isotopic composition of Fe mixing end-members in profiles of a Czech forest soil and an Israeli semi-arid soil. A ...least-squares method was used to estimate the Fe isotopic composition of the end-members representing the three main Fe reservoirs in the Czech soil: (1) silicates (
δ
57Fe
=
−
0.02
±
0.17‰), (2) organically bound Fe (
δ
57Fe
=
−
0.48
±
0.27‰), and (3) pedogenic Fe-oxides (
δ
57Fe
=
−
1.07
±
1.02‰). A lack of variation in the isotopic and chemical partitioning patterns in the Israeli soil prevented the application of the least-squares technique, although an Fe-oxide end-member is proposed using a similar mixing model (
δ
57Fe
=
−
1.72
±
1.16‰). Combination of the isotopic values for the different reservoirs with published fractionation data from previous studies suggests that the isotopic signature of the silicate fraction in the Israeli soil is dominated by lithogenic sources, while the Fe-oxide pool is influenced mainly by pedogenic precipitation/dissolution processes. The results demonstrate the potential for Fe isotopes as a tool to quantify Fe cycling in soils.
Magmatic iron meteorites are considered to be remnants of the metallic cores of differentiated asteroids, and may be used as analogues of planetary core formation. The Fe isotope compositions (δ
...57/54Fe) of metal fractions separated from magmatic and non-magmatic iron meteorites span a total range of 0.39‰, with the δ
57/54Fe values of metal fractions separated from the IIAB irons (δ
57/54Fe 0.12 to 0.32‰) being significantly heavier than those from the IIIAB (
δ
57/54Fe 0.01 to 0.15‰), IVA (δ
57/54Fe −
0.07 to 0.17‰) and IVB groups (δ
57/54Fe 0.06 to 0.14‰). The δ
57/54Fe values of troilites (FeS) separated from magmatic and non-magmatic irons range from −
0.60 to −
0.12‰, and are isotopically lighter than coexisting metal phases. No systematic relationships exist between metal-sulphide fractionation factor (Δ
57/54Fe
M-FeS
=
δ
57/54Fe
metal
−
δ
57/54Fe
FeS) metal composition or meteorite group, however the greatest Δ
57/54Fe
M-FeS values recorded for each group are strikingly similar: 0.79, 0.63, 0.76 and 0.74‰ for the IIAB, IIIAB, IAB and IIICD irons, respectively. Δ
57/54Fe
M-FeS values display a positive correlation with kamacite bandwidth, i.e. the most slowly-cooled meteorites, which should be closest to diffusive equilibrium, have the greatest Δ
57/54Fe
M-FeS values. These observations provide suggestive evidence that Fe isotopic fractionation between metal and troilite is dominated by equilibrium processes and that the maximum Δ
57/54Fe
M-FeS value recorded (0.79
±
0.09‰) is the best estimate of the equilibrium metal-sulphide Fe isotope fractionation factor. Mass balance models using this fractionation factor in conjunction with metal δ
57/54Fe values and published Fe isotope data for pallasites can explain the relatively heavy δ
57/54Fe values of IIAB metals as a function of large amounts of S in the core of the IIAB parent body, in agreement with published experimental work. However, sequestering of isotopically light Fe into the S-bearing parts of planetary cores cannot explain published differences in the average δ
57/54Fe values of mafic rocks and meteorites derived from the Earth, Moon and Mars and 4-Vesta. The heavy δ
57/54Fe value of the Earth's mantle relative to that of Mars and 4-Vesta may reflect isotopic fractionation due to disproportionation of ferrous iron present in the proto-Earth mantle into isotopically heavy ferric iron hosted in perovskite, which is released into the magma ocean, and isotopically light native iron, which partitions into the core. This process cannot take place at significant levels on smaller planets, such as Mars, as perovskite is only stable at pressures >
23 GPa. Interestingly, the average δ
57/54Fe values of mafic terrestrial and lunar samples are very similar if the High-Ti mare basalts are excluded from the latter. If the Moon's mantle is largely derived from the impactor planet then the isotopically heavy signature of the Moon's mantle requires that the impacting planet also had a mantle with a δ
57/54Fe value heavier than that of Mars or 4-Vesta, which then implies that the impactor planet must have been greater in size than Mars.
Mobility of radionuclides originating from geological repositories in the subsurface has been shown to be facilitated by clay colloids. In brackish water, however, colloids may flocculate and act to ...immobilize radionuclides associated with them. Furthermore, little research has been conducted on radionuclide interactions with carbonate rocks. Here, the impact of bentonite colloid presence on the transport of a cocktail of U(VI), Cs, Ce and Re through fractured chalk was investigated. Flow-through experiments were conducted with and without bentonite colloids, present as a mixture of bentonite and Ni-altered montmorillonite colloids. Ce was used as an analogue for reactive actinides in the (III) and (VI) redox states, and Re was considered an analogue for Tc. Filtered brackish groundwater (ionic strength = 170 mM) pumped from a fractured chalk aquitard in the northern Negev Desert of Israel, was used as a solution matrix.
Rhenium transport was identical to that of the conservative tracer, uranine. The sorption coefficient (Kd) of U(VI), Cs and Re, calculated from batch experiments with crushed chalk, proved to be a good predictor of mass recovery in transport experiments conducted without bentonite colloids. A meaningful Kd value for Ce could not be calculated due to its precipitation as a Ce-carbonate colloids. Transport of both U(VI) and Cs was indifferent to the presence of bentonite colloids. However, the addition of bentonite in the injection solution effectively immobilized Ce, decreasing its recovery from 17-41% to 0.8–1.4%. This indicates that radionuclides which interact with clay colloids that undergo flocculation and deposition may effectively be immobilized in brackish aquifers. The results of this study have implications for the prediction of potential mobility of radionuclides in safety assessments for future geological repositories to be located in fractured carbonate rocks in general and in brackish groundwater in particular.
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•Geochemical conditions impact mobility of colloid-bound radionuclides.•Mobility of U, Cs, Ce and Re was studied in brackish water in chalk fractures.•Re, an analogue for Tc, behaved conservatively in all cases.•Cs and U transport was not affected by bentonite colloid presence.•Ce was mobile without bentonite colloids, but immobilized when colloids were added.
Iron isotope fractionation during dissolution of goethite (α-FeOOH) was studied in laboratory batch experiments. Proton-promoted (HCl), ligand-controlled (oxalate dark), and reductive (oxalate light) ...dissolution mechanisms were compared in order to understand the behavior of iron isotopes during natural weathering reactions. Multicollector ICP-MS was used to measure iron isotope ratios of dissolved iron in solution. The influence of kinetic and equilibrium isotope fractionation during different time scales of dissolution was investigated. Proton-promoted dissolution did not cause iron isotope fractionation, concurrently demonstrating the isotopic homogeneity of the goethite substrate. In contrast, both ligand-controlled and reductive dissolution of goethite resulted in significant iron isotope fractionation. The kinetic isotope effect, which caused an enrichment of light isotopes in the early dissolved fractions, was modeled with an enrichment factor for the 57Fe/54Fe ratio of −2.6‰ between reactive surface sites and solution. Later dissolved fractions of the ligand-controlled experiments exhibit a reverse trend with a depletion of light isotopes of ∼0.5‰ in solution. We interpret this as an equilibrium isotope effect between Fe(III)−oxalate complexes in solution and the goethite surface. In conclusion, different dissolution mechanisms cause diverse iron isotope fractionation effects and likely influence the iron isotope signature of natural soil and weathering environments.
The iron isotope compositions of Shergotty–Nakhla–Chassigny (SNC) meteorites thought to come from Mars, eucrites and diogenites assumed to sample asteroid 4 Vesta, and rocks from the Moon and Earth ...have been measured using high precision plasma source mass spectrometry. The means of eight samples from Mars and nine samples from Vesta are within error identical despite a range of rock types. They are lighter by ∼0.1‰ in
δ
57Fe/
54Fe compared to the average of 13 terrestrial mantle-derived rocks. The latter value is identical within uncertainty with a previously published mean of 46 igneous rocks from the Earth. The average for 14 lunar basalts and highland plutonic rocks covering a broad spectrum of major element composition is heavier by ∼0.1‰ in
δ
57Fe/
54Fe relative to our estimate for the Earth's mantle, and therefore ∼0.2‰ heavier than the eucrites, diogenites and SNC meteorites. However, the data scatter somewhat and the Apollo 15 green glass and Apollo 17 orange glass are identical to samples from Mars and Vesta. There is no clear relationship between petrological characteristics and Fe isotope composition despite a wide spectrum of samples. Instead, contrasted planetary isotopic signatures are clearly resolved statistically. After evaluating alternative scenario, it appears that the most plausible explanation for the heavier Fe in the Earth and Moon is that both objects grew via processes that involved partial vaporisation leading to kinetic iron isotope fractionation followed by minor loss. This is consistent with the theory in which the Moon is thought to have originated from a giant impact between the proto-Earth and another planet. Combined with numerical simulations, Fe isotope data can offer the potential to provide constraints on the processes that occurred in planetary accretion.
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