The influence of montmorillonite colloids on the mobility of 238Pu, 233U and 137Cs through a chalk fracture was investigated to assess the transport potential for radioactive waste. Radioisotopes of ...each element, along with the conservative tracer tritium, were injected in the presence and absence of montmorillonite colloids into a naturally fractured chalk core. In parallel, batch experiments were conducted to obtain experimental sorption coefficients (Kd, mL/g) for both montmorillonite colloids and the chalk fracture material. Breakthrough curves were modelled to determine diffusivity and sorption of each radionuclide to the chalk and the colloids under advective conditions. Uranium sorbed sparingly to chalk (log Kd = 0.7 ± 0.2) in batch sorption experiments. 233U(VI) breakthrough was controlled primarily by the matrix diffusion and sorption to chalk (15 and 25% recovery with and without colloids, respectively). Cesium, in contrast, sorbed strongly to both the montmorillonite colloids and chalk (batch log Kd = 3.2 ± 0.01 and 3.9 ± 0.01, respectively). The high affinity to chalk and low colloid concentrations overwhelmed any colloidal Cs transport, resulting in very low 137Cs breakthrough (1.1–5.5% mass recovery). Batch and fracture transport results, and the associated modelling revealed that Pu migrates both as Pu (IV) sorbed to montmorillonite colloids and as dissolved Pu(V) (7% recovery). Transport experiments revealed differences in Pu(IV) and Pu(V) transport behavior that could not be quantified in simple batch experiments but are critical to effectively predict transport behavior of redox-sensitive radionuclides. Finally, a brackish groundwater solution was injected after completion of the fracture flow experiments and resulted in remobilization and recovery of 2.2% of the total sorbed radionuclides which remained in the core from previous experiments. In general, our study demonstrates consistency in sorption behavior between batch and advective fracture transport. Finally, the results suggest that colloid-facilitated radionuclide transport will enhance radionuclide migration in fractured chalk for those radionuclides with exceedingly high affinity for colloids.
The effect of aerosols on cloud properties, such as its droplet sizes and its glaciation temperatures, depends on their compositions and concentrations. In order to examine these effects, we ...collected rain samples in northern Israel during five winters (2008–2011 and 2013) and determined their chemical composition, which was later used to identify the aerosols' sources. By combining the chemical data with satellite‐retrieved cloud properties, we linked the aerosol types, sources, and concentrations with the cloud glaciation temperatures (Tg). The presence of dust increased Tg from −26°C to −12°C already at relatively low dust concentrations. This result is in agreement with the conventional wisdom that desert dust serves as good ice nuclei (INs). With higher dust concentrations, Tg saturated at −12°C, even though cloud droplet sizes decreased as a result of the cloud condensation nucleating (CCN) activity of the dust. Marine air masses also encouraged freezing, but in this case, freezing was enhanced by the larger cloud droplet sizes in the air masses (caused by low CCN concentrations) and not by IN concentrations or by aerosol type. An increased fraction of anthropogenic aerosols in marine air masses caused a decrease in Tg, indicating that these aerosols served as poor IN. Anthropogenic aerosols reduced cloud droplet sizes, which further decreased Tg. Our results could be useful in climate models for aerosol‐cloud interactions, as we investigated the effects of aerosols of different sources on cloud properties. Such parameterization can simplify these models substantially.
Key Points
Chemical composition of rain is used to identify aerosol types in air masses
Aerosols' types and concentrations affect glaciation temperature
Clouds droplets interacting with dust start glaciating at −12°C
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.
Strontium can be found in low/intermediate level radioactive waste streams, often solidified by Ordinary Portland Cement (OPC) matrices. Amorphous silica is added to improve wasteform performance, ...leading to low-pH systems. Our aim is to identify the phases controlling strontium immobilization in OPC matrices. The novelty of this work resides in combination between two approaches: 1) Comparing OPC systems to model systems based on alite, yielding matrices containing only major phases, emphasizing the relative role of minor phases; 2) Comparing high- & low-pH pastes. Pastes were characterized using calorimetry, XRD, DTG, SEM/EDS and porosimetry. Leaching experiments were conducted to study Sr retention and diffusivity. Strontium diffusivities in low-pH pastes are lower with respect to high-pH equivalents, despite their higher porosities, due to preferred Sr sorption to low Ca/Si C-S-H. OPC pastes show better Sr retention relative to model systems, suggesting that minor phases (like ettringite and AFm) improve Sr retention.
•Sr is better retained in cement-based systems compared with alite-based systems.•Minor cement hydrated phases, AFt and AFm, increase Sr retention.•Sr is better retained in low-pH systems, rich in amorphous silica.•The low-pH effect on leachability is stronger than the minor hydrates effect.•Superposition of these two effects allows better insight into strontium retention.
The influence of montmorillonite colloids on the mobility of 238Pu, 233U and 137Cs through a chalk fracture was investigated to assess the transport potential for radioactive waste. Radioisotopes of ...each element, along with the conservative tracer tritium, were injected in the presence and absence of montmorillonite colloids into a naturally fractured chalk core. In parallel, batch experiments were conducted to obtain experimental sorption coefficients (Kd, mL/g) for both montmorillonite colloids and the chalk fracture material. Breakthrough curves were modelled to determine diffusivity and sorption of each radionuclide to the chalk and the colloids under advective conditions. Uranium sorbed sparingly to chalk (log Kd = 0.7 ± 0.2) in batch sorption experiments. 233U(VI) breakthrough was controlled primarily by the matrix diffusion and sorption to chalk (15 and 25% recovery with and without colloids, respectively). Cesium, in contrast, sorbed strongly to both the montmorillonite colloids and chalk (batch log Kd = 3.2 ± 0.01 and 3.9 ± 0.01, respectively). The high affinity to chalk and low colloid concentrations overwhelmed any colloidal Cs transport, resulting in very low 137Cs breakthrough (1.1–5.5% mass recovery). Batch and fracture transport results, and the associated modelling revealed that Pu migrates both as Pu (IV) sorbed to montmorillonite colloids and as dissolved Pu(V) (7% recovery). Transport experiments revealed differences in Pu(IV) and Pu(V) transport behavior that could not be quantified in simple batch experiments but are critical to effectively predict transport behavior of redox-sensitive radionuclides. Finally, a brackish groundwater solution was injected after completion of the fracture flow experiments and resulted in remobilization and recovery of 2.2% of the total sorbed radionuclides which remained in the core from previous experiments. In general, our study demonstrates consistency in sorption behavior between batch and advective fracture transport. The results suggest that colloid-facilitated radionuclide transport will enhance radionuclide migration in fractured chalk for those radionuclides with exceedingly high affinity for colloids.
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•Transport experiments were performed in a fractured chalk core•Montmorillonite colloids did not influence 137Cs and 233U but did affect 238Pu•Migration of 238Pu, 233U and 137Cs was successfully modelled using CrunchFlow•Batch experiment Kds agreed well with modelled Kds•Saline water flow induced desorption of previously sorbed radionuclides
Internationally, it has been agreed that geologic repositories for spent fuel and radioactive waste are considered the internationally agreed upon solution for intermediate and long-term disposal. In ...countries where traditional nuclear waste repository host rocks (e.g., clay, salt, granite) are not available, other low permeability lithologies must be studied. Here, chalk is considered to determine its viability for disposal. Despite chalk's low bulk permeability, it may contain fracture networks that can facilitate radionuclide transport. In arid areas, groundwater salinity may change seasonally due to the mixing between brackish groundwater and fresh meteoric water. Such salinity changes may impact the radionuclides' mobility. In this study, radioactive U(VI) and radionuclide simulant tracers (Sr, Ce and Re) were injected into a naturally fractured chalk core. The mobility of tracers was investigated under abrupt salinity variations. Two solutions were used: a low ionic strength (IS) artificial rainwater (ARW; IS ∼0.002) and a high IS artificial groundwater (AGW; IS ∼0.2). During the experiments, the tracers were added to ARW, then the carrier was changed to AGW, and vice versa. Ce was mobile only in colloidal form, while Re was transported as a conservative tracer. Both Re and Ce demonstrated no change in mobility due to salinity changes. In contrast, U and Sr showed increased mobility when AGW was introduced and decreased mobility when ARW was introduced into the core. These experimental results, supported by reactive transport modeling, suggest that saline groundwater solutions promote U and Sr release via ion-exchange and enhance their migration in fractured chalk. The study emphasizes the impact of salinity variations near spent fuel repositories and their possible impact on radionuclide mobility.
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•Sr sorbs to chalk under rainwater chemistry conditions and is mobilized by the introduction of saline groundwater.•Ion exchange induces the mobilization of Sr.•Ce is mobile in colloidal form in rainwater and is not mobilized by changes in salinity.•Re behaves conservatively and is an effective tracer for physical transport parameterization.•U(VI) may co-precipitate with carbonate phases in chalk-equilibrated saline groundwater conditions.
Lead (Pb) poses a significant risk to infants and children through exposure to contaminated soil and dust. However, there is a lack of information on Pb speciation and distribution at the ...neighborhood-scale. This work aimed to determine: (1) the distribution of acid-extractable (labile) Pb and other metals (M
AE
) in two neighborhoods in Akron, Ohio (USA) (Summit Lake and West Akron; n = 82 samples); and (2) Pb speciation and potential sources. Total metal concentration (M
T
) and M
AE
was strongly correlated for Pb and Zn (R
2
of 0.66 and 0.55, respectively), corresponding to 35% and 33% acid-extractability. Lead and Zn exhibited a strong positive correlation with each other (R
2
= 0.56 for M
T
and 0.68 for M
AE
). Three types of Pb-bearing phases were observed by electron microscopy: (1) galena (PbS)-like (5–10 μm); (2) paint chip residuals (10–20 μm); and (3) Pb-bearing Fe-oxides (20 μm). Isotope ratio values for Pb
AE
were 1.159 to 1.245 for
206
Pb/
207
Pb, and 1.999 to 2.098 for
208
Pb/
206
Pb, and there was a statistically significant difference between the two neighborhoods (
p
= 0.010 for
206
Pb/
207
Pb and
p
= 0.009 for
208
Pb/
206
Pb). Paint and petrol are the dominant sources of Pb, with some from coal and fly ash. Lead speciation and distribution is variable and reflects a complex relationship between the input of primary sources and post-deposition transformations. This work highlights the importance of community science collaborations to expand the reach of soil sampling and establish areas most at risk based on neighborhood-dependent Pb speciation and distribution for targeted remediation.
Cementitious materials are widely-applied for the solidification and stabilization of low and intermediate level radioactive waste streams, that often contain strontium-90. Low-pH cement pastes, ...based on ordinary Portland cement (OPC) supplemented with pozzolanic additives, were found to have better Sr retention capabilities than unblended OPC pastes. However, precise definition of the phases and processes that control Sr retention in unblended and low-pH OPC pastes is still lacking. The present study combines experimental work with thermodynamic modelling to investigate the retention of Sr in unblended pastes compared to low-pH pastes. The unblended and low-pH pastes were prepared each in two formulations: one based on reagent grade alite (tri-calcium silicate) and the other based on OPC. The chemical and mineralogical behavior of the paste were studied by pH-dependent leaching test. The pastes characteristics were compared in order to assess the impact of: (1) the addition of amorphous silica to the pastes and (2) the presence of minor OPC phases (which are absent in the alite-based pastes) on Sr binding. The low-pH systems showed one order of magnitude lower Sr leachability than the unblended systems at the natural pH. Moreover, the leaching curves indicated that the low-pH systems bind Sr effectively at pH > 7 while for the unblended systems binding is effective only at pH > 10. These results are attributed to the difference in Sr binding to low Ca/Si C–S–H, dominant in low-pH systems, versus the high Ca/Si C–S–H, dominant in unblended systems. Additionally, it was shown that Sr binds mainly to the C–S–H phase rather than to minor phases. The leaching results were thermodynamically modelled and correlated successfully with the experimental findings. A novel modelling approach is offered for the consideration of partial pozzolanic reaction and SiO2-gel formation. The effect of these processes on strontium retention has been successfully demonstrated.
•Unblended and low-pH cement pastes were prepared based on alite or OPC.•pH-dependent leaching tests were performed and modelled thermodynamically.•Low Ca/Si C–S–H in low-pH pastes improves Sr retention compared to unblended pastes.•Modeling supports that Sr binds mainly to C–S–H rather than minor phases.•Observed partial reaction of pozzolanic additives was successfully modelled.
Migration of colloids may facilitate the transport of radionuclides leaked from near surface waste sites and geological repositories. Intrinsic colloids are favorably formed by precipitation with ...carbonates in bicarbonate-rich environments, and their migration may be enhanced through fractured bedrock. The mobility of Ce(III) as an intrinsic colloid was studied in an artificial rainwater solution through a natural discrete chalk fracture. The results indicate that at variable injection concentrations (between 1 and 30 mg/L), nearly all of the recovered Ce takes the form of an intrinsic colloid of >0.45 μm diameter, including in those experiments in which the inlet solution was first filtered via 0.45 μm. In all experiments, these intrinsic colloids reached their maximum relative concentrations prior to that of the Br conservative tracer. Total Ce recovery from experiments using 0.45 μm filtered inlet solutions was only about 0.1%, and colloids of >0.45 μm constituted the majority of recovered Ce. About 1% of Ce was recovered when colloids of >0.45 μm were injected, indicating the enhanced mobility and recovery of Ce in the presence of bicarbonate.
Altered crystalline rocks occur at the peneplain exposed in southern Israel and in other localities across North Africa and Arabia where they underlie an extensive blanket of Cambro–Ordovician ...sandstones. This study focuses on the petrography, mineralogy and geochemistry of top basement rocks of the northern Arabian‐Nubian Shield. The altered rocks are shown to be weathering profiles that can be subdivided into three horizons interpreted as apparently unweathered granite, or saprock, which grades upwards to a saprolite, topped by a thin clayey plasmic zone. The plasmic zone is enriched in iron and aluminium and is depleted in silicon, calcium, magnesium and potassium relative to the underlying saprolite. The chemical index of alteration increases upward, but does not exceed 90 and, therefore, lags behind values observed in strongly leached present‐day tropical soils. Petrographic examinations reveal iron mobility under local fluctuating redox conditions, similar to modern and Proterozoic soils. A variety of birefringence fabrics induced by shrinkage and expansion of clays during wetting and drying cycles and clay illuviation strongly indicate pedogenic processes rather than a post‐depositional alteration. Illite and ordered illite‐smectite phases coexist with smectitic illite‐smectite in the lower part of the saprolite and with kaolinite in the plasmic zone, in line with increasing chemical index of alteration. Observations are in accordance with the current profile being a remnant of a thick weathering profile whose top was truncated by fluvial incision just prior to deposition of the overlying Early Cambrian sequence. A previously documented Devonian thermal event reaching temperatures of at least 200°C overprinted the studied rocks. During burial diagenesis, illitization affected original smectite rather than kaolinite. However, in spite of the elevated temperatures, illitization was incomplete implying restricted potassium addition. The sub‐Cambrian weathering reflects warm and humid conditions in a tropical or sub‐tropical climate, in line with several plate reconstructions placing Israel at low latitudes during Cambrian time.
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