Iron isotopes were used to investigate iron transformation processes during an in situ field experiment for removal of dissolved Fe from reduced groundwater. This experiment provided a unique setting ...for exploring Fe isotope fractionation in a natural system. Oxygen-containing water was injected at a test well into an aquifer containing Fe(II)-rich reduced water, leading to oxidation of Fe(II) and precipitation of Fe(III)(hydr)oxides. Subsequently, groundwater was extracted from the same well over a time period much longer than the injection time. Since the surrounding water is rich in Fe(II), the Fe(II) concentration in the extracted water increased over time. The increase was strongly retarded in comparison to a conservative tracer added to the injected solution, indicating that adsorption of Fe(II) onto the newly formed Fe(III)(hydr)oxides occurred. A series of injection-extraction (push-pull) cycles were performed at the same well. The δ
57Fe/
54Fe of pre-experiment background groundwater (−0.57 ± 0.17 ‰) was lighter than the sediment leach of Fe(III) (−0.24 ± 0.08 ‰), probably due to slight fractionation (only ∼0.3 ‰) during microbial mediated reductive dissolution of Fe(III)(hydr)oxides present in the aquifer. During the experiment, Fe(II) was adsorbed from native groundwater drawn into the oxidized zone and onto Fe(III)(hydr)oxides producing a very light groundwater component with δ
57Fe/
54Fe as low as −4 ‰, indicating that heavier Fe(II) is preferentially adsorbed to the newly formed Fe(III)(hydr)oxides surfaces. Iron concentrations increased with time of extraction, and δ
57Fe/
54Fe linearly correlated with Fe concentrations (R
2 = 0.95). This pattern was reproducible over five individual cycles, indicating that the same process occurs during repeated injection/extraction cycles. We present a reactive transport model to explain the observed abiotic fractionation due to adsorption of Fe(II) on Fe(III)(hydr)oxides. The fractionation is probably caused by isotopic differences in the equilibrium sorption constants of the various isotopes (K
ads) and not by sorption kinetics. A fractionation factor α
57/54 of 1.001 fits the observed fractionation.
Coal fly ash (CFA) disposal is controversial because constituents of potential concern such as heavy metals in the ash can be released to the ground and reach aquifers, which poses risks to ...communities and the environment. Whereas thoughtful utilization of CFA may result in economic and environmental benefits. Geotechnical use of CFA as fill material in embankments could be economically competitive while reducing environmental impact. However, CFA must compete with low-cost alternatives like sand, which do not need to adhere to stringent environmental regulations and have not been evaluated against CFA previously in life cycle assessment (LCA) studies. Therefore, this work aims to evaluate the environmental effects of using CFA versus sand through LCA. Leaching experiments were carried out to create mass balance emission profiles of heavy metals from the embankment. Those emissions were integrated into a complete life cycle inventory for use of the material. Results show a net reduction in the impact categories for CFA re-used in embankments compared to landfilled. The effects of sand impacts were mostly attributed to the sand mining process. The ultimate environmental impacts from utilizing CFA as fill material in an embankment were a result of truck emissions from transporting CFA from power stations to point of utilization, diesel production and its consumption during truck transport, and leachate emissions. The breakeven distance for transporting CFA as opposed to sand was evaluated as 115 km.
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•Comparison of coal fly ash and soil fill use in embankments with LCA.•Net reduction in impact categories for higher lime content coal fly ash.•Leaching contributes significantly to freshwater ecotoxicity life cycle impact.•Arsenic contributes to freshwater ecotoxicity.•Chromium contributes to cancer and non-cancer human toxicity impacts.
In the context of geological disposal of radioactive waste, one of the controlling mechanisms for radionuclide migration through subsurface strata is sorption to mobile colloidal bentonite particles. ...Such particles may erode from the repository backfill or bentonite buffer and yield measurable (0.01–0.1 g/L) concentrations in natural groundwater. The extent of sorption is influenced by colloid concentration, ionic strength, radionuclide concentration, and the presence of competing metals. Uranium (VI) and cesium sorption to bentonite colloids was investigated both separately and together in low ionic strength (2.20 mM) artificial rainwater (ARW) and high ionic strength (169 mM) artificial groundwater (AGW; representative of a fractured carbonate rock aquitard). Sorption experiments were conducted as a factor of colloid concentration, initial metal concentration and opposing metal presence. It was shown that both U(VI) and Cs sorption were significantly reduced in AGW in comparison to ARW. Additionally, the sorption coefficient Kd of both metals was found to decrease with increasing colloid concentration. Competitive sorption experiments indicated that at high colloid concentration (1–2 g/L), Cs sorption was reduced in the presence of U(VI), and at low colloid concentration (0.01–0.5 g/L), both Cs and U(VI) Kds were reduced when they were present together due to competition for similar sorption sites. The results from this study imply that in brackish carbonate rock aquifers, typical of the Israeli northern Negev Desert, both U(VI) and Cs are more likely to be mobile as dissolved species rather than as colloid-associated solids.
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•Batch experiments determined sorption of U(VI) and Cs under environmental conditions.•Experiments show sorption in high salinity groundwater and low salinity rainwater.•Kd for both elements increased with decreasing colloid concentration.•Presence of U(VI) reduced Cs sorption to bentonite surfaces, and vice versa.•Transport models should account for colloid concentration and competitive sorption.
Sapropels S5 and S7 formed in the semi-enclosed Eastern Mediterranean Sea (EMS) during peak interglacial periods MIS5e and MIS7a, respectively, are considered among the most strongly developed ...Quaternary sapropels. This study investigates the redox dynamics of the water column during their formation, via Fe isotope and Fe speciation studies of cores taken at 2550 m depth at site ODP-967. Both sapropels show an inverse correlation between δ56Fe and FeT/Al, with slopes mostly matching that found for the Black Sea, pointing to a benthic shelf to basin shuttle of Fe and subsequent precipitation of Fe sulphides in euxinic bottom waters. An exception to these Black Sea-type trends occurs during the later, peak, stages of S7, where the negative δ56Fe - FeT/Al slope shallows. Fe speciation studies reveal that the dominant highly reactive Fe phase (FeHR) in the sapropels is pyrite, with Fe (oxyhydr)oxides forming the second major mineral component. Correspondingly, FeHR/FeT plots show increased strengthening of anoxic water conditions during the passage from pre-sapropel sediment into the sapropel. Nevertheless, despite the evidence for euxinic conditions from both Fe isotopes and high Mo concentrations in the sapropel, Fepy/FeHR ratios remain below values commonly used to identify water column euxinia. This apparent contradiction is ascribed to the sedimentary preservation of a high flux of crystalline Fe (oxyhydr)oxide minerals to the basin, which resulted in a relatively low degree of sulphidation, despite the presence of euxinic bottom waters. Thus, the operationally defined ferruginous/euxinic boundary for EMS sapropels is better placed at Fepy/FeHR = 0.6, which is somewhat below the usually ascribed lower limit of 0.7. Consistent with the significant presence of crystalline Fe (oxyhydr)oxides, the change in the δ56Fe - FeT/Al slope during peak S7 is ascribed to an enhanced monsoon-driven flux of detrital Fe(III) oxides from the River Nile into the EMS basin and comcomitant diagenetic sulphidation. Euxinic water column conditions in sapropel S5 and S7 are interpreted here to reflect the positive balance between dissolved sulphide formation and rates of reductive dissolution of Fe (oxyhydr)oxide minerals. Both of these parameters in turn depend on the extent to which water overturn times are reduced and export productivity increased during sapropel formation.
•Inverse FeT/Al-δ56Fe correlations in sapropels S5 and S7 indicate benthic Fe shuttle.•Euxinic water column reflects positive balance between dissolved sulphide formation and Fe (oxyhydr)oxide dissolution rate•Ferruginous/euxinic boundary for EM sapropels is best placed at Fepy/FeHR = 0.6•Change in Fe vs. δ56Fe trend at peak S7 is related to increased Fe oxide flux•Water overturn rate and export productivity control the strength of euxinic water conditions
The hominin-bearing Middle Palaeolithic MP Layer C of Tabun Cave, Mount Carmel, Israel, has been extensively studied for over 90 years, but many questions about its chronology, the identity of its ...hominin remains and the nature of its formation remain open. Tabun Layer C, part of the 13-m thick MP deposits at the cave, presents a complex sedimentary sequence composed of multiple beds and thinner laminae within them (<5 cm thick), varying in colour, texture, and the amount of associated remains from human activity. In this study, we re-sampled, in higher resolution than any previous study, the sediments exposed along the full stratigraphic sequence of Layer C (⁓2.5 m thick), including the deposits immediately below and above it. The studied sequence largely conforms to the three Major Sedimentological Units (MSU I-III, themselves analogous to Garrod's layers B-D) previously defined by Jelinek et al. (1973). Using state-of-the-art micro-stratigraphic and micro-geoarchaeological approach and methods we examined the deposits within MSU II and those associated with the transitions between MSU III–II and between MSU II–I. The results demonstrate that the beds and laminae of MSU II (Garrod's Layer C) are primarily composed of bat and bird guano at various stages of preservation, occurring with varying amounts of allochthonous terra rossa and human activity remains, and generally not showing severe diagenetic changes. Significantly, this study allowed us to identify four depositional sub-units in MSU II, each composed of tens to hundreds of cm-to mm-thick laminae. An abrupt change from slow to rapid deposition of guano occurs between MSU III and MSU IId, followed by intensive use of fire in MSU IIc, then intermittent hominin use of the cave in MSU IIb. Lowered anthropogenic use of the cave characterises the upper part of the section, where deposits become primarily biogenic (MSU IIa) and geogenic (MSU I). Our identification of large amounts of guano further contributes towards more informed research of the Levantine MP in cave sites. We demonstrate that attention to components within guano deposits can allow distinguishing among different guano producers (e.g., bats and birds), and that it may form a sounder basis for reconstructing changing environments at both regional and local scales, as well as human–environment relations.
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•High-resolution geoarchaeological study of Layer C (MSU II) at Tabun Cave.•Identification of tens to hundreds of mm-cm thick laminae in each sediment bed.•The major source of material is bat and bird guano alongside reworked terra rossa.•Division of MSU II to four sub-units and precise stratigraphic position of its beginning and end.•New insights into the site's formation processes and the interplay between human action and animal activity.
Karstic caves in the Eastern Mediterranean region hold significant archaeological importance as they served as an important dwelling place for prehistoric hominins. Yet, such caves were used at the ...same time as a habitat for birds and bats. As a result, bat guano and bird droppings are not only a major source of cave deposits, but also trigger significant chemical alterations (diagenesis) due to their decomposition that can greatly affect the preservation of archaeological materials as reported from many Palaeolithic cave sites. While the diagenetic processes associated with bat guano degradation are quite well understood, the identification of partially-preserved or burnt guano residues in archaeological cave sites has not received much attention. Here we present a micromorphological, chemical and mineralogical study of modern guano of insectivorous bats, frugivorous bats and pigeons, collected from karstic caves in Israel. Each of these guano types was studied in fresh, charred and ashed forms. Based on the patterns obtained we present guidelines that allow identification of partially-degraded and burnt bat and bird guano remains. These include (1) high phosphorous and calcium associated with high quantities of chitin fragments in insectivorous bat guano; (2) low phosphorous and calcium associated with high quantities of seed coats, fibrous organic matter and red oval organic particles in frugivorous bat and pigeon guano. We found that faecal spherulites are present in bat guano and absent in pigeon droppings. Blackening and rounding of the edges of organic components characterise guano upon charring, while ashed guano is characterised by a micritic calcite groundmass in the case of fruit bat and pigeon guano without any pseudomorphs after calcium oxalate, unlike wood ash. We further show that ashed insectivorous bat guano is characterised by a micritic groundmass that includes phosphatic particles and potassium sulphate. These data indicate that while the combustion of guano presents similarities to other organic-based burnt deposits found in archaeological sites, a careful micromorphological study, aided by certain elemental and mineralogical signatures, can reveal whether or not burnt deposits originate from guano. Last, we studied the micromorphology of in situ degraded frugivorous and insectivorous bat guano in caves, observing that a bedded and laminated structure characterises fresh deposits while, as degradation progresses the proportion of authigenic minerals, appearing as nodules and veins, increases, disrupting the laminated structure. Importantly, chitin fragments in insectivorous bat guano and seed coats and some organic fibers in frugivorous bat guano preserve within degraded bat guano deposits and can serve as indicators for the presence and feeding habits of bats. Applying the guidelines given here to archaeological studies at prehistoric cave sites may produce new insights regarding human behavior, human–bat interactions, as well as for the formation processes at these sites.
•Micromorphological, mineralogical and elemental study of bat and pigeon guano.•Characterising guano in fresh, charred and ashed forms.•Insectivorous bat guano shows high concentrations of P, Ca and chitin fragments.•Frugivorous bat and pigeon guano is characterised by seed coats and fibrous organics.•Degraded guano is laminated, disrupted by authigenic nodules and veins.
Fly ashes are produced in Israel via the combustion of bituminous coals and are defined as Class F (i.e., very basic because of the high Ca content). Most fly ashes produced in Israel are from South ...African and Colombian coals and, therefore, were chosen for the present study. It has been shown that fly ash can be used as a scrubber and fixation reagent for acidic wastes (from the phosphate or regeneration of used motor oil industries). Furthermore, the scrubbed product can serve as a partial substitute to sand and cement in concrete, while the produced bricks have proven to be strong enough for concrete standards. To explore the fixation mechanism, the fly ashes have been treated with acidic (0.1 M HCl) and neutral (ultrapure deionized water, denoted as UPDI) aqueous solutions. Chemical compositions and surface analysis before and after treatment were conducted for assessing changes in the coal fly ash particles. The treated fly ash structure has been changed appreciably. For example, dissolution of Ca resulted in exposure of the outer surface. Hence, the treatment may change the modes of interaction of trace elements with the surface of the fly ash particles. Three possible modes of interaction between the fly ash and waste are suggested: cation exchange, chemical bonding, and electrostatic adsorption of very fine precipitate at the fly ash surface. Probably, the silicate and aluminate groups (or aluminosilicates) at the surface of the fly ash particles are involved in these interactions.
Atmospheric dust loads and chemical compositions serve as a key link between global climate patterns and marine biogeochemical cycles. The primary source of atmospheric dust in the world today is the ...Sahara-Arabian desert belt. Although this source was also active during the Quaternary, the interpretation of paleo-dust records and their effects on marine ecosystems is complicated by the scarcely reported atmospheric load patterns of bioavailable phases (i.e., water and acid leachable phases) and present-day contamination of anthropogenic components. This study reports a multi-annual time series of atmospheric dust loads (2006–2016) and their chemical compositions (2006–2010) collected in the north Gulf of Aqaba (north Red Sea) at a weekly to bi-weekly resolution. Major and trace element abundances in each sample are reported for three fractions: water-soluble salts, carbonates and oxides (weak acid leach), and Al-silicates.
Dust loads vary seasonally from low values in late summer (∼20–30μgm−3) to higher values in the fall, and highest values in late winter and early spring (∼150–250μgm−3). Major and trace element abundances allow to distinguish between the sources and chemical compositions that dominate high and low dust loads in each season. The water leachable fraction (L0) is relatively enriched in Na, Ca, K and Mg, the acid-leachable fraction (L1) is enriched in Ca as well as Na, Al, Mg, Zn, Cd and Pb, and the silicate residue (L2) in Al and Fe.
High dust loads occurring mainly during winter and spring months are characterized by low Mg/Ca (L1, L2), low K/Al and Na/Al (L1) and high Ca/Al (L1), high Mg/Al (L2) and relatively un-weathered (L2) contents. High dust load intervals during winter months are characterized by low passing air masses originating from the Sahara, while the ambient winter dust (low dust load) is associated with proximal source regions from the East Sahara and Arabian Peninsula. During late winter and spring months, high dust loads originate from central and west Sahara and to a lesser extent from north Sahara. Low dust loads characterize the summer with limited compositional variability relative to winter-spring months. Summer dust is generally characterized by high K/Al (L1) ratios relative to late winter and spring. It is also relatively high in anthropogenic trace elements in the L0 and L1 fractions (e.g., Zn/Al, Pb/Al, Cr/Al, Ni/Al and V/Al), whereby back trajectories indicate the source of these components is primarily from south and east Europe. The total load (ngm−3) of anthropogenic trace elements however, remains higher during winter and spring, stemming from the overall significantly higher dust loads characterizing this time window.
The temporal load patterns of important micronutrients such as Fe, Cd, Zn, Cu, Ni and others in the bio-available phases (L0, L1) are not correlated with major nutrients or Chlorophyll-a sea surface concentrations, suggesting that the atmospheric dust plays a limited role in driving primary productivity in the oligotrophic surface waters of the Gulf of Aqaba.
On a wider scale, the results provide unique chemical fingerprinting of Sahara-Arabian dust that can be applied to reconstruct past trends in dust loads recorded in deep-sea cores and other geological archives from this and other regions.
Colloid facilitated transport of radionuclides has been implicated as a major transport vector for leaked nuclear waste in the subsurface. Sorption of radionuclides onto mobile carrier colloids such ...as bentonite and humic acid often accelerates their transport through saturated rock fractures. Here, we employ column studies to investigate the impact of intrinsic, bentonite and humic acid colloids on the transport and recovery of Ce(III) through a fractured chalk core. Ce(III) recovery where either bentonite or humic colloids were added was 7.7–26.9% Ce for all experiments. Greater Ce(III) recovery was observed when both types of carrier colloids were present (25.4–37.4%). When only bentonite colloids were present, Ce(III) appeared to be fractionated between chemical sorption to the bentonite colloid surfaces and heteroaggregation of bentonite colloids with intrinsic carbonate colloids, precipitated naturally in solution. However, scanning electron microscope (SEM) images and colloid stability experiments reveal that in suspensions of humic acid colloids, colloid-facilitated Ce(III) migration results only from the latter attachment mechanism rather than from chemical sorption. This observed heteroaggregation of different colloid types may be an important factor to consider when predicting potential mobility of leaked radionuclides from geological repositories for spent fuel located in carbonate rocks.
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•Colloidal Ce migrates with or slightly faster than solutes.•Greater recovery results from the presence of bentonite and humic acid colloids.•Carbonate can precipitate with Ce to form intrinsic colloids.•Intrinsic carbonate colloids heteroaggregate with carrier colloids.•Heteroaggregates play an important role in Ce transport.
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. Here the study emphasizes the impact of salinity variations near spent fuel repositories and their possible impact on radionuclide mobility.
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