Mechanism Of Isotope Fractionation Research Articles

Applications of zinc stable isotope analysis in environmental and biological systems: a review

Zinc (Zn) is an essential trace element that is involved in both biotic and abiotic processes in Earth's surface environments. Over the last 20 years, advances in mass spectrometry instrumentation have enabled unprecedented high-precision Zn isotope abundance ratio determinations in natural and anthropogenic samples. Supported by a solid theoretical background inherited from the traditional non-metal stable isotopes (C, N, H, O, S), the understanding of the controlling factors of Zn isotope fractionation has rapidly evolved. In this article, we review the main applications and findings of Zn isotopes in a variety of scientific domains, including metal contamination, magmatic differentiation, plant uptake, weathering, global climate change, dietary and trophic chains, and biomedicine. The analytical aspects of Zn isotope determination in solid and water samples and the principles and mechanisms of Zn isotope fractionation are also reviewed. This work aims to provide a general yet in-depth panorama of Zn isotope chemistry and demonstrate the versatility and potential of this isotope system for studying the biological, geological, and chemical cycling of Zn. Supplementary material: https://doi.org/10.6084/m9.figshare.c.7240146

Antimony isotopic fractionation induced by Sb(V) adsorption on β-MnO2

Antimony (Sb) isotopes hold immense promise for unraveling Sb biogeochemical cycling in environmental systems. Mn oxides help control the fate of Sb via adsorption reactions, yet the behavior and mechanisms of Sb isotopic fractionation on Mn oxides are poorly understood. In this study, we examine the Sb isotopic fractionation induced by adsorption on β-MnO2 in different experiments (kinetic, isothermal, effect of pH). We observe that adsorption on β-MnO2 surfaces preferentially enriches lighter Sb isotopes through equilibrium fractionation, with Δ123Sbaqueous-adsorbed of 0.55–0.79 ‰. Neither the pH or surface coverage affects the fractionation magnitude. The analysis of extended X-ray absorption fine structure (EXAFS) demonstrates that the enrichment of light isotope results from the adsorption of inner-sphere complexation on solids. Our finding of this study enhances our comprehension of the impact of β-MnO2 on Sb isotopic fractionation behavior and mechanism and facilitate the applicability of Sb isotopes as effective tracers to elucidate the origins and pathways of Sb contamination in environmental systems, as well as provide a new insight into forecasting the isotopic fractionation of other similar metals adsorbed by manganese oxides.

Geographical origin traceability of sweet cherry (Prunus avium (L.) Moench) in China using stable isotope and multi-element analysis with multivariate modeling

The deliberately origin mislabeling of sweet cherry causes significantly disruptions to market integrity and consumers' trust. In this study, 153 cherry samples from five provinces in China and the corresponding irrigation water and soil samples were collected. 5 stable isotope ratios (δ13C, δ15N, δ2H, δ18O, 87Sr/86Sr) and 8 multi-element contents (Na, Mg, P, K, Ca, Fe, Zn, Se) of cherry were determined by EA-IRMS and ICP-MS to study isotopic fractionation and elemental enrichment mechanisms for origin traceability. The results show the δ2H and δ18O of cherry exhibit a strong correlated with its irrigation water (r2 > 0.85), while δ15N, 87Sr/86Sr, Fe, Zn and Se contents are related to its cultivated soil (r2 > 0.75), and the δ13C is related to the local microclimate. ANOVA reveals that the regional differences of δ13C, δ2H, δ18O, 87Sr/86Sr as well as Na, Mg, Ca contents of cherry are significant (P < 0.05), and are important geographical indicators. Various multivariate modeling methods, HCA, PLS-DA, and LDA, were employed with the overall accuracy exceeding 90%. This strategy provides an effective mean to verify the label authenticity of cherry origin in Chinese market.

Presence of non-solar derived krypton and xenon unveiled by Chang'e-5 lunar soils

The extent of volatile elements on the surface and interior of the Moon remains a highly debated topic. Previous studies conducted on bulk lunar soil samples and solar wind samples collected by the Genesis mission indicate a discernible isotope mass- or non-mass-dependent fractionation of krypton and xenon. However, a detailed investigation of these processes is missing, particularly in determining the possible incorporation of cometary volatiles in the lunar regolith. New lunar soil samples returned by the Chang'e-5 mission provide a chance to answer these key questions. In this study, noble gas isotopes of nine subsamples from a Chang'e-5 scooped sample were analysed through stepwise-heating and total fusion laser extraction. The results reveal that a simple binary mixture of solar wind and cosmogenic components did not explain alone the isotopic composition of these samples. The Xe data shows insignificant amounts of atmospheric Xe and presents clear evidence of cometary contributions to the lunar regolith, with a significant depletion of 134,136Xe compared to that in the solar wind. Additionally, a meteoritic component is identified. Compared to the Apollo results, our findings further validate the theory of Earth's atmospheric escape, substantiate the plausibility of these exogenous admixtures to elucidate the isotopic fractionation mechanisms of Kr and Xe within the lunar regolith, and provide novel insights into long-term constancy in the solar wind composition.

Diagenesis Decreasing the Mo Isotopic Composition in Estuarine Systems: Implications for Constraining Its Riverine Input to Ocean

AbstractUnderstanding the geochemical behavior of the Mo isotopes in estuarine systems is essential for determining the isotopic composition of riverine inputs to the ocean and for assessing the historical oxidation state of Earth’s ancient oceans. However, the extent and mechanisms of Mo isotope fractionation during estuarine processes are not yet fully understood. This study systematically investigated the seasonal and spatial variations in aqueous and particulate δ98Mo values within the Pearl River Estuary (PRE). Our research found that aqueous δ98Moaque. values in both summer and winter deviated significantly from the theoretical mixing line for the PRE. Based on the geochemical characteristics of the water column, particulate matter, and pore water in the PRE, we first propose that diagenesis release from sediments is the predominant factor resulting in lower than anticipated aqueous δ98Moaque. values. Given the prevalence of suboxic and anoxic sediments in estuarine and coastal areas, such diagenetic release may substantially decrease the global riverine influx of aqueous Mo isotopes to the ocean. Additionally, particulate δ98MoSPM values exhibit an increasing trend (from 0.02 to 1.62‰) with increasing salinity in both seasons, suggesting that the terrestrial input particulate δ98MoSPM value would be heavier than the mean value for UCC. We hypothesize that the adsorption and desorption processes involving Fe (hydro) oxides predominantly influence this trend. This study advances our understanding of the mechanisms of aqueous and particulate Mo isotopic fractionation in estuarine systems and would be helpful in constraining Mo isotopic compositions of rivers and oceans.

Mechanisms of nitrogen isotope fractionation at an ancient black smoker in the 2.7 Ga Abitibi greenstone belt, Canada

Abstract The biological nitrogen (N) cycle on early Earth is enigmatic because of limited data from Archean (meta-)sediments and the potential alteration of primary biotic signatures. Here we further investigate unusual 15N enrichments reported in 2.7 Ga meta-sediments from the Abitibi greenstone belt, Canada, purportedly related to a 15N-enriched Archean atmosphere. Given that sediments from this region are contemporaneous with large-scale volcanogenic massive sulfide deposits, we utilize Cu and Zn contents to trace the effects of hydrothermal circulation on N isotope fractionation. We show that high δ15Nbulk values as high as +23‰ are associated with Cu-Zn mineralization, whereas unmineralized organic-rich shales exhibit much lower δ15Nbulk and δ15Nkerogen values. Moreover, we find a large offset between δ15Nbulk and δ15Nkerogen of as much as 17‰ and relate this to the addition of organic-bound N during the late-stage emplacement of organic-rich veins. We conclude that the previously reported high δ15N values are most parsimoniously explained by biotic and abiotic mechanisms rather than a 15N-enriched atmosphere. Crucially, both mechanisms require the presence of NH4+ in hydrothermal fluids, supporting the hypothesis that hydrothermal discharge was an important nutrient source for Neoarchean marine life.

Antimony isotopic fractionation during intensive chemical weathering of basalt in the tropics

This study explored the mechanism of antimony (Sb) isotopic fractionation during chemical weathering and soil biogeochemical processes, which have important implications for Sb isotopic composition in the hydrosphere. We analyzed the major and trace element contents and Sb isotopic composition along an 8 m deep basalt weathering profile in the Huguangyan area of Guangdong Province, southern China. Mineral adsorption experiments were conducted to determine the mechanism of Sb isotopic fractionation in the weathering profile. The following minerals, α-FeOOH, γ-Fe2O3, β-MnO2, γ-Al2O3, montmorillonite-K10, and kaolinite-1A, preferentially adsorbed light Sb isotopes, and increased pH may lead to a lighter isotopic composition of adsorbed Sb. The positive values of τSbTiO2 in the upper profile reflect exogenous Sb inputs, such as rainfall and litterfall. Accordingly, Sb isotopic fractionation is controlled by Sb adsorption onto organic matter and Fe (hydr)oxides in surface soil. At a depth of 1.8 m, Sb and other redox-sensitive elements, such as Fe, Mn, Ce, Co, Cr, and Mo, were distinctly enriched. Changes in the redox conditions result in strong Sb isotopic fractionation. In the middle layer (2–4 m), the combined effects of changes in organic matter content, soil pH, and weathering intensity explains the enrichment of light Sb isotopes. The bottom layer of the profile did not receive exogenous Sb and exhibited an Sb isotopic fractionation pattern similar to that of the basaltic bedrock. In the bottom layer, secondary Fe (hydr)oxides (e.g., goethite and hematite) preferentially adsorb the light Sb isotope (121Sb), whereas the heavy Sb isotope (123Sb) is released into the aquatic environment during intensive chemical weathering.

Antimony Isotope Fractionation during Adsorption on Iron (Oxyhydr)oxides.

The fate of antimony (Sb) is strongly affected by adsorption, yet Sb isotope fractionation and the associated mechanism have not been widely reported. Here we experimentally investigated the process of Sb(V) adsorption on iron (oxyhydr)oxides and the associated isotope effects. Sb isotope fractionation occurs during adsorption (Δ123Sbsolution-mineral = 1.20 ± 0.02‰ for ferrihydrite and 2.35 ± 0.04‰ for goethite). Extended X-ray absorption fine structure (EXAFS) analysis shows that Sb(V) adsorption on iron (oxyhydr)oxides occurs via inner-sphere surface complexation, including mononuclear bidentate edge-sharing (2E) and binuclear bidentate corner-sharing (2C) complexes. A longer atom distance of Sb-Fe in ferrihydrite leads to less Sb isotope fractionation during Sb adsorption than in goethite. The Gibbs free energy and Mayer bond order were calculated based on density functional theory (DFT) and suggested that the strength of the bonding environment can be summarized as Sb(OH)6- > 2E > 2C. In turn, the bonding environment indicates the mechanism of Sb isotope fractionation during the process. This study reveals that Sb isotope fractionation occurs during Sb(V) adsorption onto iron (oxyhydr)oxides, providing a basis for the future study of Sb isotopes and further understanding of the fractionation mechanism.

Large sulfur isotope fractionation during abiotic consumption of hydrogen sulfide after cessation of bacterial growth in batch culture experiments of sulfate reducing bacteria

Microbial sulfate reduction is a key process in the oceanic sulfur cycle and imparts a large sulfur isotope fractionation. The mechanism of sulfur isotope fractionation during microbial sulfate reduction has been studied from the geochemical and biochemical aspects since the 1950s. Recently, however, the large sulfur isotope fractionation, exceeding 47‰, has been observed in some pure culture experiments of sulfate reducing bacteria and the tentative understanding of microbial sulfur isotope fractionation has not well explained the mechanism. Here we quantified growth phase dependent sulfur isotope fractionation of a type sulfate reducer Desulfovibrio desulfuricans (DSM642) and revealed that the magnitude of isotope fractionation increased from -13.4 ± 3.6‰ in early exponential phase to -65.9 ± 21.0‰ in later exponential phase having clear negative correlation with cell specific sulfate reduction rate. Our results show the importance of cells growth phase and states that control cell specific sulfate reduction rate and sulfur isotope fractionation. Although microbial sulfate reduction likely continued to stationary phase of cells, microbial sulfur isotope fractionation could not be quantified because of the decreasing total dissolved sulfide (ΣH2S: H2S, HS−, and S2−) concentration after stationary phase. A non-microbial ΣH2S-consuming reaction is interpreted to have occurred during the stationary phase and this had large sulfur isotope fractionations, -10.5 ± 1.1‰ and -45.6 ± 12.4‰. Besides, the non-microbial ΣH2S-consuming reaction decreased Δ33S’ value of the ΣH2S. While the end-product of the non-microbial ΣH2S-consuming reaction in the stationary phase remains unidentified, we observed precipitates of some sulfide minerals and organic sulfur in the stationary phase media. Regardless of the end-product, the reaction increases the sulfur isotopic composition of dissolved ΣH2S, which can account for the high sulfur isotopic compositions of ΣH2S compared to pyrite and organic sulfur observed in some modern marine sediments.

Molybdenum isotope heterogeneity of metal sulfides from magmatic hydrothermal systems

The sensitivity of molybdenum (Mo) stable isotopes to redox changes enables innovative geochemical means of tracing the evolution of medium- to high-temperature magmatic hydrothermal processes. However, the Mo isotope geochemical behaviors and fractionation mechanisms in magmatic-hydrothermal systems are still unclear. This study performed high-precision Mo isotope research on well-characterized hydrothermal metal sulfides (pyrite, chalcopyrite, molybdenite and pyrrhotite) from different metallogenic stages and the ore-forming porphyries and surrounding rocks from the giant Pulang porphyry copper deposit in Yunnan, Southwest China, to better understand Mo isotope behaviors in magmatic hydrothermal systems. The results show that compared to the high-temperature magmatic system with homogeneous Mo concentrations (1.44 ppm to 11.3 ppm) and δ98Mo values (−0.41‰ ± 0.05 to −0.08‰ ± 0.03), medium- to high-temperature hydrothermal stage molybdenite displays slightly lighter δ98Mo values (−0.90‰ ± 0.04 to 0.08‰ ± 0.04), and pyrite, chalcopyrite and pyrrhotite, which have low Mo concentrations (0.13 ppm to 28.5 ppm), feature significant δ98Mo variations from −0.34 ± 0.04‰ to 3.01 ± 0.03‰. The absence of fractionated but low δ98Mo values in the magmatic system, combined with the lack of any correlation between the δ98Mo and geochemical index values (e.g., Co/Ni value) of the metal sulfides, implies that the Mo isotope signatures of sulfides in the hydrothermal stage were not inherited from component signatures of the magmatic phase or contamination of surrounding rocks. Our study proposes that heterogeneous partitioning and transportation of heavy/light Mo isotopes into progressively precipitated metal sulfides as a result of changes in the redox-driven metallogenic setting and differential geochemical behaviors of Mo species contribute to the observed δ98Mo variations and that the heavier and strongly variable Mo isotopic signatures correspond to the dominant metallogenic stage of the deposit. Such fractionation may be explained by the Rayleigh fractionation model, whereby 95Mo is preferentially partitioned into isotopically light molybdenite, and the subsequent precipitation of pyrite, chalcopyrite and pyrrhotite from the residual 98Mo-rich ore-forming fluids manifests heavier isotopic compositions. We therefore highlight that the Mo isotopes of metal sulfides provide valuable insights into the precipitation of metal-rich fluids, the identification of favorable mineralization zones, and the ability to trace the evolution of sophisticated magmatic-hydrothermal systems.

Barium isotope fractionation during granitic magmatism and potential of δ138/134Ba for distinguishing magmatic-hydrothermal transition in granitic magma systems

The geochemistry of Ba is commonly used to study igneous processes and fluid-melt interaction in subduction zone and granitic environments. Recent advances in analytical techniques have also indicated that Ba isotope geochemistry has the potential to significantly augment these studies. Understanding the Ba isotope fractionation mechanisms is thus critical if this potential is to be fully realized. Hence, we have used the first-principles calculations based on the density functional theory (DFT) and long-time ab initio molecular dynamics (AIMD) simulation to calculate Ba isotope fractionation during felsic magma evolution. In fluids, the reduced partition function ratio (β factor) of aqueous Ba2+ is highly variable, depending on the coordination number (n) of H2O in the first hydration shell. At ambient surface conditions on the Earth, n = 8 best represents the aqueous Ba2+ species in natural fluid, and the temperature-dependent equilibrium Ba isotope fractionation between barite/witherite and fluids can be described by the relationships: 103lnαwitherite-fluid = −0.00009×(106/T2)2 + 0.0603×(106/T2) + 0.0003 and 103lnαbarite-fluid = −0.0001×(106/T2)2 + 0.0648×(106/T2) + 0.0004, which is consistent with previous theoretical and experimental studies. In hydrothermal fluids, the hydration number of Ba2+ varies from 4 to 6 at high P-T, and the cumulative average of 103lnβ is derived to be 0.0798 ± 0.005‰ in 923.15 K and 0.2 GPa. In silicate minerals, the enrichment of heavy Ba isotopes decreases in the sequence of muscovite > microcline ≈ celsian ≈ sanbornite > barylite > phlogopite. Variations in Ba concentrations and the degree of Al-Si disordering only induce limited Ba isotope fractionation, which would be hard to distinguish analytically. At high temperatures (> 600 °C), the melt phase is enriched in heavy Ba isotopes relative to any coexisting aqueous fluid and crystallizing minerals. During the early stages of felsic magma differentiation involving plagioclase crystallization as the main mineral, the δ138/134Ba and concentration in the residual melt remain close to their initial values, but any fluid exsolved from the melt will have light δ138/134Ba values. The high Ba distribution coefficient of K-feldspar means that the resultant K-feldspar granite inherits > 90% of the Ba contained in the initial magma reservoir. Hence, the δ138/134Ba of K-feldspar is close to that of the initial melt. During the final stage of felsic magma evolution, mixing of exsolved fluids from deeper reservoirs may significantly reduce the δ138/134Ba of the now Ba-depleted melt, and can account for the low δ138/134Ba values measured in highly differentiated leucogranites and pegmatites.

Molecular-scale insight into selenium isotope fractionation caused by adsorption on Fe (oxyhydr)oxides

Selenium (Se) isotopes have recently emerged as a potential proxy for tracing biogeochemical processes and reconstructing the evolution of global Se cycle in the oceans. However, little is known about Se isotope fractionation mechanism during the sequestration of dissolved Se oxyanions by marine ferromanganese oxides that are mainly composed of Fe and Mn (oxyhydr)oxides. Here we elucidate the molecular mechanism governing equilibrium Se isotope fractionation during adsorption on 2-line ferrihydrite, by combining isotope ratio measurements, extended X-ray absorption fine structure (EXAFS) analyses, and high level quantum chemical calculations. Results show that Se isotopes can be fractionated 0.89‰ (Δ82/76Seaqueous-adsorbed) with enrichment of lighter isotopes in the solid phase during the adsorption of Se(IV) on 2-line ferrihydrite, which is primarily driven by the formation of bidentate-binuclear inner-sphere complexes. By contrast, little or no Se isotope fractionation (<0.2‰) was observed during Se(VI) adsorption due to the outer-sphere complexation. In combination with previous results, our findings would provide molecular-scale insights into Se isotope compositions in marine ferromanganese oxides and lead to an improved understanding of Se biogeochemical cycle in the ocean. Our study also has an implication for the systematical understanding of mechanisms governing isotope fractionations of other metal oxyanions like Mo, which highlights the controls of their proton dissociation constants, electronic configurations, and radius ratios. This would improve our understanding for isotope signatures of metals in the dissolved species and the adsorbed sinks in modern and ancient environments.

Anaerobic biotransformation of two novel brominated flame retardants: Kinetics, isotope fractionation and reaction mechanisms.

1,2,5,6-tetrabromocyclooctane (TBCO) and 2,3-dibromopropyl-2,4,6-tribromophenyl ether (DPTE), as safer alternatives to traditional brominated flame retardants, have been extensively detected in various environmental media and pose emerging risks. However, much less is known about their fate in the environment. Anaerobic microbial transformation is a key pathway for the natural attenuation of contaminants. This study investigated, for the first time, the microbial transformation behaviors of β-TBCO and DPTE by Dehalococcoides mccartyi strain CG1. The results indicated that both β-TBCO and DPTE could be easily transformed by D. mccartyi CG1 with kobs values of 0.0218±0.0015 h-1 and 0.0089±0.0003 h-1, respectively. In particular, β-TBCO seemed to undergo dibromo-elimination and then epoxidation to form 4,5-dibromo-9-oxabicyclo[6.1.0]nonane, while DPTE experienced debromination at the benzene ring (ortho-bromine being removed prior to para-bromine) rather than at the carbon chain. Additionally, pronounced carbon and bromine isotope fractionations were observed during biotransformation of β-TBCO and DPTE, suggesting that C-Br bond breaking is the rate-limiting step of their biotransformation. Finally, coupled with identified products and isotope fractionation patterns, β-elimination (E2) and Sn2-nucleophilic substitution were considered the most likely microbial transformation mechanisms for β-TBCO and DPTE, respectively. This work provides important information for assessing the potential of natural attenuation and environmental risks of β-TBCO and DPTE.

Identification of alkaline amendment sources (slash and burn versus marling) for cereal crops grown in the North of France: A multiple isotope approach (87Sr/86Sr, δ44/40Ca, δ88/86Sr)

Early farmers used at least two types of agrarian amendments that could raise pH and base saturation levels to allow the cultivation of cereals: marling and plant ash. Ash can be input in many different ways: felling and burning in place, transferring plant material from wastelands and spreading the ash, charring sod or peat blocks, burning stubble after harvest. Marling includes all the practices of amending limestone, from marl to chalky limestone composed of 99% CaCO3. In order to understand the evolution of these agricultural practices, it is important to identify which of the two techniques was used to amend cereal crops in the past. In order to test the potential of δ44/40Ca-δ88/86Sr-87Sr/86Sr multiple isotope approach for archaeological samples, we first applied the technique to currently grown crops, amended either with marl or with ash from freshly-cut and burned trees. We found that this approach makes it possible to discriminate cereal grains amended either by marling (less radiogenic Sr) or with tree ash (more radiogenic Sr). We also identified a positive correlation between stable Ca and Sr isotope values, suggesting that the Ca and Sr came from similar sources and had undergone similar mass-dependent isotopic fractionation mechanisms. Consequently, we later on mainly focused on stable and radiogenic Sr isotopes. Stable Sr isotope fractionation was also observed between different locations, different organs of a given cereal species and between different cereal types, but also within the same cereal species or the number of grains studied for a given locality, pointing to biological fractionation combined with source variation.

Fe-Cu-Zn Isotopic Compositions in Polymetallic Sulfides from Hydrothermal Fields in the Ultraslow-Spreading Southwest Indian Ridge and Geological Inferences

Submarine hydrothermal sulfides from the ultraslow-spreading Southwest Indian Ridge (SWIR) were sampled from three hydrothermal fields, and the Fe-Cu-Zn isotopic compositions were analyzed in this study. The Fe isotopes ranged from −0.011‰ to −1.333‰. We believe the processes controlling the Fe isotope variability in the hydrothermal systems include the sulfide precipitation process, the initial isotopic composition of the hydrothermal fluid, and the temperature during precipitation. Among these factors, the sulfide precipitation process is the dominant one. The Cu isotope compositions of the sulfides varied from −0.364‰ to 0.892‰, indicating that the hydrothermal fluid preferentially leached 65Cu in the early stages and that hydrothermal reworking led to decreases in the Cu isotopes in the later stages. In addition, because mass fractionation occurred during sulfide precipitation, the Zn isotope variations ranged from −0.060‰ to 0.422‰. Combined with the S isotopic compositions, these results also implied that different Fe-Cu-Zn isotopic fractionation mechanisms prevailed for the different sample types. Based on these results, we are sure that the metallic elements, including Fe, Cu, and Zn, were derived from the mantle in the SWIR hydrothermal field, and the Fe-Cu isotope results indicated that these metallic elements were provided by fluid leaching processes. Using the isotopic fractionation and sulfide results, we calculated that the Fe-Cu-Zn isotopic compositions of the hydrothermal fluid in this field were δ56Fe(fluid): −0.8~0.0‰; δ65Cu(fluid): 0.3~1.3‰; and δ66Zn(fluid): 0~0.48‰.

Antimony Isotope Fractionation Revealed from EXAFS during Adsorption on Fe (Oxyhydr)oxides.

A lack of knowledge about antimony (Sb) isotope fractionation mechanisms in key geochemical processes has limited its environmental applications as a tracer. Naturally widespread iron (Fe) (oxyhydr)oxides play a key role in Sb migration due to strong adsorption, but the behavior and mechanisms of Sb isotopic fractionation on Fe (oxyhydr)oxides are still unclear. Here, we investigate the adsorption mechanisms of Sb on ferrihydrite (Fh), goethite (Goe), and hematite (Hem) using extended X-ray absorption fine structure (EXAFS) and show that inner-sphere complexation of Sb species with Fe (oxyhydr)oxides occurs independent of pH and surface coverage. Lighter Sb isotopes are preferentially enriched on Fe (oxyhydr)oxides due to isotopic equilibrium fractionation, with neither surface coverage nor pH influencing the degree of fractionation (Δ123Sbaqueous-adsorbed). Limited Fe atoms are present in the second shell of Hem and Goe, resulting in weaker surface complexes and leading to greater Sb isotopic fractionation than with Fh (Δ123Sbaqueous-adsorbed of 0.49 ± 0.004, 1.12 ± 0.006, and 1.14 ± 0.05‰ for Fh, Hem, and Goe, respectively). These results improve the understanding of the mechanism of Sb adsorption by Fe (oxyhydr)oxides and further clarify the Sb isotope fractionation mechanism, providing an essential basis for future application of Sb isotopes in source and process tracing.

Antimony isotope fractionation and the key controls in the soil profiles of an antimony smelting area

The controlling factors of antimony migration and transformation in soil profiles are still unclear. Antimony isotopes might be a useful tool to trace it. In this paper, antimony isotopic compositions of plant and smelter-derived samples, and two soil profiles were measured for the first time. The δ123Sb values of the surface and bottom layers of the two soil profiles varied in 0.23‰−1.19‰ and 0.58‰−0.66‰, respectively, while δ123Sb of the smelter-derived samples varied in 0.29‰−0.38‰. The results show that the antimony isotopic compositions in the soil profiles are affected by post-depositional biogeochemical processes. The enrichment and loss of light isotopes at 0–10 cm and 10–40 cm layers of the contrasted soil profile may be controlled by plant uptake process. The loss and enrichment of heavy isotopes in the 0–10 cm and 10–25 cm layers of the antimony from smelting source in the polluted soil profile may be controlled by the adsorption process, while the enrichment of light isotopes in the 25–80 cm layer may be related to the reductive dissolution process. The conclusion emphasizes that the promotion of the Sb isotope fractionation mechanism will play a crucial role in understanding the migration and transformation behaviors of Sb in soil systems.

Effect of Moisture Sources on the Isotopic Composition of Precipitation in Northwest China

Stable isotopes (18O/16O and 2H/1H) are fingerprints of water molecules and thus can be used to gain insight on water circulation. Especially, the factors controlling the isotopic composition of precipitation should be identified because they act as baseline determinants of the isotopic variations of surface water and groundwater. Here, using the HYSPLIT model, we attribute observed isotope variations to different moisture sources and characterize the isotopic composition of meteoric precipitation in Northwest China. Results show that the westerlies play a dominant role across the region throughout the year, while other moisture sources only affect some parts of the region during a specific season, i.e., Arctic airflow only affects the Altay Mountains as far as the Middle Tianshan Mountains; the East Asia Monsoon only affects the region east of 100° E longitude during the summer; and summer rainfall of local origin may contribute to the precipitation budget of basin areas. Given the different moisture sources across Northwest China, a local meteoric water line (NWMWL) of δ2H = 6.8δ18O − 1.6 is observed. Our findings not only can provide valuable insights into the mechanism of precipitation isotope fractionation in Northwest China but also can contribute to a better understanding of regional climate and hydrological studies.

Zinc Stable Isotope Fractionation Mechanisms during Adsorption on and Substitution in Iron (Hydr)oxides.

The Zn isotope fingerprint is widely used as a proxy of various environmental geochemical processes, so it is crucial to determine which are the mechanisms responsible for isotopic fractionation. Iron (Fe) (hydr)oxides greatly control the cycling and fate and thus isotope fractionation factors of Zn in terrestrial environments. Here, Zn isotope fractionation and related mechanisms during adsorption on and substitution in three FeOOH polymorphs are explored. Results demonstrate that heavy Zn isotopes are preferentially enriched onto solids, with almost similar isotopic offsets (Δ66/64Znsolid-solution = 0.25-0.36‰) for goethite, lepidocrocite, and feroxyhyte. This is consistent with the same average Zn-O bond lengths for adsorbed Zn on these solids as revealed by Zn K-edge X-ray absorption fine structure spectroscopy. In contrast, at an initial Zn/Fe molar ratio of 0.02, incorporation of Zn into goethite and lepidocrocite by substituting for lattice Fe preferentially sequesters light Zn isotopes with Δ66/64Znsubstituted-stocksolution of -1.52 ± 0.09‰ and -1.18 ± 0.15‰, while Zn-substituted feroxyhyte (0.06 ± 0.11‰) indicates almost no isotope fractionation. This is closely related to the different crystal nucleation and growth rates during the Zn-doped FeOOH formation processes. These results provide direct experimental evidence of incorporation of isotopically light Zn into Fe (hydr)oxides and improve our understanding of Zn isotope fractionation mechanisms during mineral-solution interface processes.

In situ/operando XAFS investigation of the sorption/precipitation of Zn(II) on palygorskite surface at the molecular scale: Implications for Zn stable isotope fractionation

In aqueous geological environments, the fate and transport of Zn are controlled by geochemical reactions (e.g., sorption) occurring at the mineral/water interface. However, many of the underlying Zn fixation mechanisms, especially the kinetics of surface-induced precipitation, are still unclear. Additionally, Zn stable isotope fractionation during sorption has recently been shown to be an important process, but the relationship between Zn isotope fractionation and sorption mechanisms (e.g., surface complexation, precipitation) is also not well understood. To address these issues, we employed X-ray absorption fine structure (XAFS) spectroscopy, high-resolution transmission electron microscopy (HRTEM), and in situ/operando quick-scanning XAS (QXAS) spectroscopy to elucidate the sorption/precipitation mechanisms of Zn at palygorskite/solution interfaces. We also measured Zn isotope ratios during some sorption experiments to illustrate how Zn isotope fractionation behavior is affected by the evolution of coordination environments at different pH values, initial concentrations, and reaction times. Our results demonstrate that Zn sorption mechanisms vary as a function of pH, ionic strength, initial concentration, and reaction time. At low pH (pH < 7.0) and low ionic strength (I ≤ 0.01 M), Zn(II) predominantly forms an outer-sphere surface complex in octahedral coordination. At low pH (pH < 7.0) and high ionic strength (I = 0.1 M), Zn(II) is predominantly sorbed as an inner-sphere octahedral surface complex with insignificant isotopic fractionation (Δ66Znsorbed-aqueous = −0.02 ± 0.05‰). At high pH and high initial Zn concentration (pH 7.5, C0 ≥ 0.2 mM), the formation of octahedral Zn phyllosilicate precipitates is observed, yielding a small fractionation with an average Δ66Znsorbed-aqueous of 0.10 ± 0.07‰. In addition, in situ/operando QXAS of Zn sorption at pH 7.5 in a flow cell revealed that the predominant sorbed Zn species shifted from inner-sphere tetrahedral complexes during the initial stage (e.g., within minutes) to inner-sphere octahedral complexes during later stages (e.g., within hours), followed by the formation of Zn-phyllosilicate precipitates (e.g., within days). This molecular evidence coincides with the evolution of Zn isotope fractionation from a large Δ66Znsorbed-aqueous value (0.53 ± 0.05‰) to a small value of 0.05 ± 0.04‰ during the sorption process. The combination of QXAS and isotope fractionation results reveals a change in the Zn local environment from tetrahedral coordination to octahedral coordination. The findings in this study not only provide new insight into surface adsorption/precipitation mechanisms but also demonstrate that stable isotope fractionation is linked with the local molecular/bonding environments at mineral–water interfaces.