Cd Isotopic Compositions Research Articles

Impact of Rhizosphere Biostimulation on Cd Transport and Isotope Fractionation in Cd-Tolerant and Hyperaccumulating Plants Based on MC-ICP-MS and NanoSIMS.

Phytoremediation efficiency can be enhanced by regulating rhizosphere processes, and the Cd isotope is a useful approach for deciphering Cd transport processes in soil-plant systems. However, the effects of adsorption and complexation on Cd isotope fractionation during the rhizosphere processes remain unclear. Here, we cultivated the Cd hyperaccumulator Sedum alfredii and Cd-tolerance Sedum spectabile in three different soils with citric acid applied as a degradable rhizosphere biostimulant. Cellular elemental distributions in the tissues and Cd isotope compositions were determined through NanoSIMS and MC-ICP-MS, respectively. Cd precipitation/adsorption on cell walls and intracellular regional distribution were the main mechanisms of Cd tolerance in S. spectabile. Plant roots became enriched with heavier Cd isotopes relative to the surrounding soils upon increasing secretion of rhizosphere organic acids. This indicates that organic matter with O and N functional groups preferentially chelates heavy Cd isotopes. In addition, Cd isotope fractionation between roots and shoots varies within the three soils, which could be due to the influence of protein and metallothionein contents in roots and leaves. The finding indicates that sulfur-containing ligands preferentially chelate light Cd isotopes. This study suggests that organic ligands play a vital role in Cd isotope fractionation and consequent hyperaccumulation of soil-plant systems.

Phytoavailability, translocation, and accompanying isotopic fractionation of cadmium in soil and rice plants in paddy fields

Rice consumption is a major pathway for human cadmium (Cd) exposure. Understanding Cd behavior in the soil-rice system, especially under field conditions, is pivotal for controlling Cd accumulation. This study analyzed Cd concentrations and isotope compositions (δ114/110Cd) in rice plants and surface soil sampled at different times, along with urinary Cd of residents from typical Cd-contaminated paddy fields in Youxian, Hunan, China. Soil water-soluble Cd concentrations varied across sampling times, with δ114/110Cdwater lighter under drained than flooded conditions, suggesting supplementation of water-soluble Cd by isotopically lighter Cd pools, increasing Cd phytoavailability. Both water-soluble Cd and atmospheric deposition contributed to rice Cd accumulation. Water-soluble Cd’s contribution increased from 28‐52% under flooded to 58‐87% under drained conditions due to increased soil Cd phytoavailability. Atmospheric deposition’s contribution (12‐72%) increased with potential atmospheric deposition flux among sampling areas. The enrichment of heavy Cd isotopes occurred from root-stem-grain to prevent rice Cd accumulation. The different extent of enrichment of heavy isotopes in urine indicated different Cd exposure sources. These findings provide valuable insights into the speciation and phytoavailability changes of Cd in the soil-rice system and highlight the potential application of Cd isotopic fingerprinting in understanding the environmental fate of Cd.

Cadmium isotope evidence for near-modern bio-productivity in the early Cambrian ocean

The Ediacaran–Cambrian transition was one of the major transitions in Earth's history, with mass extinctions and radiation of animals. This event has been linked with perturbations in ocean geochemistry, including redox conditions and the oxygenation of Earth's surface environments. Here we present a highly resolved cadmium isotope record (δ114Cd) from shales deposited in a deep-water environment covering the Late Ediacaran to Cambrian Stage 4. Our δ114Cd data vary from −0.70 to +0.35 ‰ and demonstrate the importance of local redox-dependent burial phases on bulk sediment Cd isotope compositions, which are superimposed on global-scale changes in the Cd cycle. The heaviest Cd isotope composition recorded in the early Cambrian Liuchapo Formation is similar to the modern deep ocean, implying a broad similarity in global Cd cycling, perhaps involving similar organic-Cd burial rates. The provision of bio-limiting nutrients by continental weathering with rising oxygen concentrations in Earth's surface environments could have expanded the habitable area of the ocean and contributed to animal diversification.

Sphalerite Records Cd Isotopic Signatures of the Parent Rocks in Hydrothermal Systems: A Case Study From the Nayongzhi Zn–Pb Deposit, Southwest China

AbstractMetal stable isotopes (e.g., Zn, Cd, and Cu) have been used to track metal sources in different types of hydrothermal systems. However, metal isotopic variations in sulphides could be triggered by various factors such as mineral precipitation and fluid mixing. Thus, tracking the metal sources of hydrothermal systems is still a big challenge for metal isotopes. In this study, we investigated the Cd isotopic systematics of sphalerite from the Nayongzhi Zn–Pb deposit, which is a Mississippi Valley‐type (MVT) deposit in the Sichuan–Yunnan–Guizhou mineralization province (SYGMP). We reinterpreted the published S isotope data for the SYGMP and found that the large S isotopic variations were controlled by Rayleigh fractionation between sulphide and reduced S. As such, a model that involves mixing of a metal‐rich fluid with a reduced S pool formed by thermochemical sulfate reduction (TSR) can explain the ore formation in the Nayongzhi deposit. Based on this model, no Cd isotopic fractionation was observed due to its low solubility in fluids during mixing, and thus the Cd isotopic variations of sphalerite were inherited from the source rocks. The large range of Zn/Cd ratios and uniform Cd isotopic compositions of the sulphides are similar to those of igneous rocks but different from those of sedimentary rocks, indicating that Zn and Cd were derived mainly from basement rocks (e.g., migmatite, gneiss, and granulite). Our results reaffirm that metal stable isotopes, particularly Cd isotope compositions of sphalerite, are powerful geochemical tracers for investigating the formation mechanisms of ore deposits.

Cadmium isotope fractionation in a S-type granite related large magmatic–hydrothermal system

Granite-associated deposits are one of the important ore types in the Earth’s crust and can dominantly be subdivided into S-, I- and A-type granite-related deposits with different mineralization elements and isotopic signatures (e.g., sulphur and boron). The object of this study is to investigate whether Cd isotopic difference exists between I- and S-type granite-related deposits, and to provide new insights on geochemical behavior of Cd in such systems that have not been well constrained. The Dulong deposit in southwest China is a world-class skarn-type polymetallic Zn–Sn–In deposit that is thought to be genetically related to the emplacement of the highly fractionated S-type Laojunshan granite. The extremely negative δ114/110Cd values of sulfides (−0.82 ‰ to −0.42 ‰) within the Dulong deposit are all lower than those of sulfides from other Zn–Pb ore deposits globally. In comparison, representative sphalerites from I-type granite-related deposits yield heavier Cd isotopic compositions relative to the samples from the Dulong deposit. The Laojunshan granite samples are enriched in light Cd isotopes and have δ114/110Cd values that range from −0.73 ‰ to −0.40 ‰, overlapping with the δ114/110Cd values of sphalerite within the Dulong deposit and suggesting that the Cd in the deposit was derived from the Laojunshan granite. Similarly, I-type granite-related deposits elsewhere have Cd isotopic compositions that are similar to mafic–intermediate igneous rocks, suggesting little or no Cd isotopic fractionation took place during the magmatic–hydrothermal processes that formed these deposits. These results suggest that geochemical signatures of metal sources are the most likely control on Zn/Cd ratios and δ114/110Cd values of sulfides in these deposits, with different mineral deposit types having different Zn/Cd ratios and δ114/110Cd values. In summary, Cd isotopes provide a potentially effective tool for discriminating between different types of hydrothermal system and may also be useful in the investigation of granite petrogenesis.

Cd–Pb isotopic characteristics within the Hehuashan carbonate-hosted Pb–Zn deposit, Middle-Lower Yangtze River Metallogenic Belt: Constraints for ore-forming metal sources and implications for exploration

The Hehuashan carbonate-hosted Pb–Zn deposit is a large-scale deposit situated in the Middle-Lower Yangtze-River Metallogenic Belt (MLYRMB) in China. It contains 0.99 million tons of Zn at an average grade of 1.5 % and 0.41 million tons of Pb at an average grade at 1.9 %, accompanied by critical metals of Cd and Ga. The ores are mainly composed of breccia-type and vein-type in the deposit. Though previous chemical compositions of sphalerite showed that Cd is enriched in both two types of mineralization and the sulfide sulfur isotopes research showed that the reduced sulfur is derived from the marine sulfate reduction, the source of ore-forming metals is still unclear. In this research, we studied the non-traditional stable isotope of Cd and the radioactive isotope Pb of the Hehuashan deposit to discuss the ore-forming metal sources and provide genesis constraints to the Hehuashan Pb–Zn deposit. The sphalerite Cd isotopic compositions are relatively concentrated but the Zn/Cd ratios are scattered in the Hehuashan deposit, of which the early-stage sphalerite δ114/110Cd values ranged from 0.21 ‰ to 0.30 ‰ with an outlier of 0.09 ‰, mean of 0.22 ‰ and a wide range of Zn/Cd ratios (78 to 430), and the late-stage sphalerite δ114/110Cd values ranged from 0.09 ‰ to 0.15 ‰ with two outliers of −0.05 ‰ and 0.52 ‰, mean of 0.17 ‰ with also a wide range of Zn/Cd ratios (50 to 286), both stages of Cd isotopic compositions are overlapped with the ranges of diorite and OIB & MORB, implying the Cd is might be derived from the dioritic intrusions of the Hehuashan deposit district, and the varied Zn/Cd ratios imply that the limestone also provided some of ore-forming metals to the both two ore-forming stages. Additionally, the Pb isotopic compositions of galena in the two mineralization stages exhibit a concentrated range (206Pb/204Pb = 18.2806 – 18.4190, 207Pb/204Pb = 15.6262 – 15.6484, and 208Pb/204Pb = 38.5710 – 38.6529) and all the Pb isotopic data points of the Hehuashan deposit are plotted in the right-hand side of the geochron and the overlapped field of marine sediments, Tongling dioritic intrusions, and Lower Triassic limestones in the Tongling area in the diagrams of (207Pb/204Pb)i vs. (206/204Pb)i and in the field (208Pb/204Pb)i vs. (206/204Pb)i, implying that the Pb is mainly derived from the magma of dioritic intrusions and marine sediments. Subsequently, the sulfide Cd–Pb isotopic compositions show that the ore-forming metals in the Hehuashan deposit are mainly originated from the dioritic intrusions and marine sediments. The plots of deposit genesis classification show that the Cd isotopic compositions in the Hehuashan deposit are unsuitable to the plots of δ114/110Cd vs. Zn/Cd ratios, but imply the metals were derived from multi sources. Finally, the Cd-Pb studies of the Hehaushan Pb–Zn deposit show that the earlier Lower Triassic carbonate formations and its contact with dioritic rocks should be ideal exploration targets for Pb–Zn and associated Cd sources in the Tongling mining area.

Refining the roles of productivity, redox, and remineralization on the cadmium isotope composition of marine sediments

Cadmium (Cd) has a nutrient-like distribution in the ocean, similar to the macronutrient phosphate. Significant isotope fractionation induced by the biological cycling of Cd makes it a potential tracer for nutrients and productivity. However, the Cd flux and Cd isotope composition of marine sediments may also be influenced by local redox conditions and partial remineralization of organically hosted Cd. These confounding factors are under-constrained and render it challenging to use Cd as a reliable paleoproxy. To understand the relative importance of each of these processes, we examined the Cd isotope systematics of 69 modern sediments deposited across a wide range of environments. We complement these data with four profiles of particulate Cd isotope compositions from the Southern Ocean. We report three main results. First, we show that the sedimentary flux of Cd is tightly coupled to that of organic matter. Second, most Cd burial occurs in regions with some bottom-water oxygen, and the flux of CdS to anoxic regions is, globally, minor. Finally, we find that remineralization can substantially modify sedimentary Cd isotope compositions, though it is challenging to relate pelagic and sedimentary processes. For example, we find that the relationship between sedimentary Cd isotope compositions and surface seawater [Cd] is the reverse of that predicted by isotope reactor models. Likewise, sedimentary Cd isotope compositions are anti-correlated with bottom-water oxygen. While this pattern is consistent with preferential remineralization of isotopically heavy Cd, profiles of marine particulate matter reveal the reverse, whereby the Cd isotope composition of large particles, which are most likely to reach the seafloor, becomes increasingly ‘heavy’ with depth. These results highlight how productivity, redox, and remineralization all influence the flux and isotope composition of Cd to marine sediments. While our study suggests that there is no simple way to relate sedimentary Cd isotopes to surface nutrient utilization, our data point toward several potential controls that could form the basis of novel proxies for local redox conditions and remineralization.

Submarine hydrothermal fluids facilitated the accumulation of cadmium in Lower Cambrian black shales, South China

The effects of Cd-rich venting fluids on the Cd contents and isotopic compositions of continental margin sediments remain poorly understood. Characterization of the Cd isotopic composition of sediment overprinted by venting fluids is critical, as this may provide insights into how mixing between the venting fluids and bottom waters affects the Cd isotopic composition. Our study determined the Cd isotopic compositions of Lower Cambrian metalliferous sediment deposited on the passive continental margin of the southeastern Yangtze Block, South China. The Cd is predominantly hosted in sphalerite of sulfide-bearing barite sediments, V deposits, and Ni-Mo−platinum group element (PGE) sulfide layers. The Cd isotopic compositions of samples from these three units indicate that the distribution of Cd at this type of continental margin was controlled by submarine hydrothermal activity. Following the venting of hydrothermal fluids onto the seafloor, Cd2+ reacted with a limited amount of H2S generated mainly by thermochemical sulfate reduction near the vents. This resulted in isotopically lighter Cd in the sulfide-bearing barite sediments and V deposits, which have δ114/110Cd values of 0.00‰ ± 0.13‰ and 0.00‰ ± 0.14‰, respectively. The residual dissolved Cd in the fluids migrated with the upwelling seawater and ultimately precipitated almost entirely as sphalerite in the sulfidic wedge, which resulted in Ni-Mo−PGE sulfide layers with a narrow range of high δ114/110Cd values (0.35‰ ± 0.03‰). The presence of Cd-rich venting fluids can alter the Cd contents and isotopic compositions of passive margin sediments, particularly those located within long-term stratified seawater and fault depressions. Previous studies may have underestimated the migration distance and affected areas of hydrothermal activity involving Cd-rich fluids.

Revealing the Sources of Cadmium in Rice Plants under Pot and Field Conditions from Its Isotopic Fractionation.

The highly excessive uptake of cadmium (Cd) by rice plants is well known, but the transfer pathway and mechanism of Cd in the paddy system remain poorly understood. Herein, pot experiments and field investigation were systematically carried out for the first time to assess the phytoavailability of Cd and fingerprint its transfer pathway in the paddy system under different treatments (slaked lime and biochar amendments), with the aid of a pioneering Cd isotopic technique. Results unveiled that no obvious differences were displayed in the δ114/110Cd of Ca(NO3)2-extractable and acid-soluble fractions among different treatments in pot experiments, while the δ114/110Cd of the water-soluble fraction varied considerably from -0.88 to -0.27%, similar to those observed in whole rice plant [Δ114/110Cdplant-water ≈ 0 (-0.06 to -0.03%)]. It indicates that the water-soluble fraction is likely the main source of phytoavailable Cd, which further contributes to its bioaccumulation in paddy systems. However, Δ114/110Cdplant-water found in field conditions (-0.39 ± 0.05%) was quite different from those observed in pot experiments, mostly owing to additional contribution derived from atmospheric deposition. All these findings demonstrate that the precise Cd isotopic compositions can provide robust and reliable evidence to reveal different transfer pathways of Cd and its phytoavailability in paddy systems.

Combining Cd and Pb isotope analyses for heavy metal source apportionment in facility agricultural soils around typical urban and industrial areas

Facility agriculture enhances food production capabilities. However, concerns persist regarding heavy metal accumulation resulting from extensive operation of this type of farming. This study integrated the total content, five fractions, and isotope composition of Cd and Pb in intensively farmed soils in regions characterized by industrialization (Shaoguan, SG) and urbanization (Guangzhou, GZ), to assess the sources and mechanisms causing metals accumulation. We found significantly more severe Cd/Pb accumulation and potential mobility in SG than GZ. Cd displayed higher accumulation levels and potential mobility than Pb. The distinct isotopic signals in SG (−0.54 to 0.47‰ for δ114/110Cd and 1.1755 to 1.1867 for 206Pb/207Pb) and GZ (−0.86 to 0.12‰ for δ114/110Cd and 1.1914 to 1.2012 for 206Pb/207Pb) indicated significant differences in Cd/Pb sources. The Bayesian model revealed that industrial activities and related transportation accounted for over 40% and approximately 30%, respectively, of the average contributions of Cd/Pb in SG. While urban-related (26.6%) and agricultural-related (26.3%) activities primarily contributed to Cd in GZ. The integration of δ114/110Cd and 208Pb/206Pb has further enhanced the regional contrast in sources. The present study established a comprehensive tracing system for Cd-Pb, providing crucial insights into the accumulation and distribution of these metals in facility agricultural soils.

Identification of anthropogenic source of Pb and Cd within two tropical seagrass species in South China: Insight from Pb and Cd isotopes

Seagrass beds are susceptible to deterioration and heavy metals represent a crucial impact factor. The accumulation of heavy metal in two tropical seagrass species were studied in South China in this study and multiple methods were used to identify the heavy metal sources. E. acoroides (Enhalus acoroides) and T. hemperichii (Thalassia hemperichii) belong to the genus of Enhalus and Thalassia in the Hydrocharitaceae family, respectively. Heavy metal concentrations in the two seagrasses followed the order of Cr > Zn > Cu > Ni > As > Pb > Co > Cd based on the whole plant, and their bioconcentration factors were 31.8 ± 29.3 (Cr), 5.7 ± 1.3 (Zn), 7.0 ± 3.8 (Cu), 3.0 ± 1.9 (Ni), 1.2 ± 0.3 (As), 1.7 ± 0.9 (Pb), 9.1 ± 11.1 (Co) and 2.8 ± 0.6 (Cd), indicating the intense enrichment in Co and Cr within the two seagrasses. The two seagrasses were prone to accumulate all the listed heavy metals (except for As in E. acoroides), especially Co (BCFs of 1124) and Cr (BCFs of 2689) in the aboveground parts, and the belowground parts of both seagrasses also accumulated most metals (BCFs of 27) excluding Co and Pb. The Pb isotopic ratios (mean 208Pb/204Pb, 207Pb/204Pb and 206Pb/204Pb values of 38.2054, 15.5000 and 18.3240, respectively) and Cd isotopic compositions (δ114/110Cd values ranging from –0.09‰ to 0.58‰) within seagrasses indicated the anthropogenic sources of Pb and Cd including coal combustion, traffic emissions and agricultural activities. This study described the absorption characteristics of E. acoroides and T. hemperichii to some heavy metals, and further demonstrated the successful utilization of Pb and Cd isotopes as discerning markers to trace anthropogenic origins of heavy metals (mainly Pb and Cd) in seagrasses. Pb and Cd isotopes can mutually verify and be helpful to understand more information in pollution sources and improve the reliability of conclusion deduced from concentrations or a single isotope.

Cadmium isotopes analysis of environmental samples with high organic matter by dry ashing method under wet plasma conditions

The acquisition of accurate Cd isotopic compositions and high tolerance of interfering elements during measurements were achieved by means of quartz microfiber filter membrane-wrapped samples for dry ashing and wet plasma conditions.

An efficient Cd two-stage column system for high-precision determination of Cd isotopic compositions by double spike MC-ICP-MS

The modified two-stage column chromatography system circumvents the potential P problem associated with TRU Spec resin.

Cadmium isotopic evidence for reduced deep-water marine primary productivity during the end-Permian mass extinction

Marine primary productivity is a key control on the stability of marine ecosystems. Decreasing marine primary productivity might have led to the end-Permian mass extinction, although this is debated. However, the changes in primary productivity at different seawater depths during the end-Permian mass extinction remain poorly constrained. We investigated the Cd isotopic systematics of shallow-water (Dajiang) and deep-water (Shangsi) sedimentary sections in South China. Combined with previously published data from an intermediate–deep-water section (Meishan), we were able to reconstruct the depth changes in marine primary productivity. In the Dajiang section, prior to the mass extinction, the Cd isotopic compositions of seawater (δ114/110CdSW) varied from +0.73‰ to +0.92‰, with an average of +0.83‰±0.13‰ (2SD; n=11). During and after the mass extinction, δ114/110CdSW varied from +0.73‰ to +0.91‰, with an average of +0.81‰±0.12‰ (2SD; n=12). In the Shangsi and Meishan sections, prior to the mass extinction, δ114/110CdSW varied from +0.47‰ to +0.85‰, with an average of +0.66‰±0.16‰ (2SD; n=35). During and after the mass extinction, δ114/110CdSW varied from −0.19‰ to +0.36‰, with an average of −0.02‰±0.27‰ (2SD; n=27). During the end-Permian mass extinction, the marine primary productivity recorded by Cd isotopes in the relatively deep-water sections was considerably reduced, which may have caused the destruction of relatively deep-water marine ecosystems. We suggest that upward expansion of sulfidic and anoxic deep water, possibly due to the volcanic eruption of the Siberian Traps large igneous province, was one of the causes of the decrease in marine primary productivity. However, the marine primary productivity did not change in the shallow-water section during the end-Permian mass extinction, although changes in the types of marine primary producers are evident from the fossil record. This change in the types of primary producers may have contributed to the mass extinction in shallow-water platform environments. Vertical variations in the changes in primary productivity indicate that the environmental factors leading to the extinction event mainly began in deep waters during the end-Permian mass extinction.

Tracing acid mine drainage and estuarine Zn attenuation using Cd and Zn isotopes

It has been estimated that the acid mine drainage (AMD) impacted Odiel river basin in southern Spain supplies 0.37% and 15% of the global riverine fluxes of Cd and Zn to the oceans, respectively (Sarmiento et al., 2009). However, the behaviour of Cd and Zn in the Ria of Huelva estuary, which connects the Odiel and Tinto watersheds with the Gulf of Cadiz, has yet to be fully investigated. Furthermore, very few studies have investigated Cd and Zn isotope behaviour in estuaries worldwide. This study presents Cd and Zn concentrations and isotopic compositions for the Ria of Huelva estuary and surrounding watersheds, sampled in 2017 and 2019. Sulfide-rich rock samples extracted from three mines yield Cd and Zn isotope compositions that range from –0.14‰ to +0.07‰ (n = 4) for δ114Cd and –0.01‰ to +0.29‰ (n = 4) for δ66Zn. However, a uniform riverine signal of about +0.02‰ for Cd and +0.17‰ for Zn indicates that tracing of individual mining regions using Cd and Zn isotopes is challenging. Limited variability was observed in dissolved δ114Cd values throughout the watershed, including AMD, the estuary, and the Gulf of Cadiz, with a mean value of ±0.00 ± 0.13‰ (n = 25, 2 SD; excludes one AMD outlier, at +0.48‰), including both 2017 and 2019 data. By contrast, δ66Zn values ranged from –0.12‰ to +0.35‰ (n = 28) for the same geographical and temporal scope. In May 2017, a large spill from an abandoned mine, La Zarza, resulted in a drastic increase in the concentrations of trace metals reaching the estuary compared to 2019, but no impact of this mine spill on Cd or Zn isotope compositions is observed. In 2019, an increase in δ66Zn values in the estuary coincided with high pH values (up to pH = 8.8) and chloride concentrations (2.73%), which may reflect an alkaline anthropogenic input from the active neighbouring industrial complex. Overall, Cd concentrations and isotope compositions in the estuary are largely consistent with conservative mixing behaviour. By contrast, Zn behaviour is non-conservative, with removal of 49–97% of dissolved riverine Zn in the estuary during the period 2017–2019, associated with a relatively small isotopic shift to lighter Zn isotope compositions. Removal of Zn to the particulate phase in the Ria of Huelva estuary therefore largely attenuates high riverine Zn fluxes from AMD, indicating that previously estimated Odiel river basin Zn fluxes were overestimated. Nevertheless, the variable but generally light AMD Cd and Zn isotope compositions, coupled with high dissolved metal concentrations, suggest that Cd and Zn isotopes may be useful tracers of regionally averaged AMD inputs to the Gulf of Cadiz and beyond.

Cadmium isotope fractionation during sorption to soil minerals: Lab evidence and field implication

Stable isotope is a potent geochemical tracer unveiling the dynamic sources of cadmium (Cd) in the environment. However, the sorption process of aqueous Cd onto solid surfaces (e.g., soil) fractionates isotopic signatures, which makes it difficult to interpret its sources and migration processes. Understanding how Cd fractionates during sorption to soil minerals, therefore, provides necessary data to open the isotope fractionation black box of liquid-solid phase partition of Cd. In this study, we explored isotopic fractionation characteristics during Cd sorption to typical soil minerals. Distinct Cd isotope fractionation was observed during sorption, including goethite (∆114/110Cdsolid−solution=−0.51∼−0.47‰), montmorillonite (−0.34∼−0.32‰), quartz (−0.34∼−0.30‰), gibbsite (−0.29∼−0.25‰), and kaolinite (−0.09∼−0.07‰), where lighter isotopes tended to be preferentially sorbed to the solid phase in a reversible manner. We've also conducted a field survey in a historical flooding-affected region to explore Cd isotopic compositions of soil samples adjacent to a polluted creek. Distinct isotopic signatures for surficial soils and deep soils were observed, which was depicted by a binary mixing mode. Leaching of Cd from mine tailings to the creek, as well as its re-adsorption to soil during historical flooding events both led to preferential enrichment of light isotopes in the solid phase, which was consistent with findings from lab sorption experiments. Mineralogical analyses including EPMA, TOF-SIMS, Mössbauer spectra, XRD, and chemical extraction tests, were applied to unveil how Cd was bound to soil minerals at field. Possible linkage between lab and field data were discussed on the basis of these findings. Results from this study are encouraging for future application of Cd isotopes for source tracing.

Effects of lithology and acid mine drainage on Cd concentration and isotope distribution in a large riverine system, Guangxi Province, South China

Cadmium is highly toxic to plants, animals, and humans. When assessing its risk, it is critical to distinguish anthropogenic activities from natural weathering processes. Specific Cd isotopes have been used to track the source and fate of Cd in the environment. However, Cd isotopic signatures in soils and sediments and its fractionation in large river systems, especially in high-population areas and within complex Cd source regions, remain poorly understood. Using chemical and isotope analyses, this study identifies Cd sources in the Xijiang riverine system, China, as being the result of natural weathering and acid mine drainage. Cadmium isotopic fractionation between stream sediment and topsoil was uniform, with Δ114/110Cdstream sediment–topsoil values averaging 0.50‰ ± 0.04‰ through natural weathering processes and fractionation being controlled mainly by the Cd isotopic composition of acid mine drainage. During the natural weathering of carbonate sediments, Ca2+ and HCO3− in soils tend to drain into the Xun River with leachate, resulting in a weakly alkaline water column with increasing Cd contents in stream sediments. In mining regions, H+, Cd2+, Zn2+, and other metal ions are prone to release from sphalerite into solution under supergene conditions, ultimately resulting in high Cd contents in sediments of the Meng River. This study highlights the use of Δ114/110Cdstream sediment–topsoil values as a tool for distinguishing the effects of anthropogenic activities from those of natural weathering processes in river systems at risk of Cd pollution.

Cd isotope constraints on metal sources of the Zhugongtang Zn–Pb deposit, NW Guizhou, China

The Sichuan–Yunnan–Guizhou metallogenic province (SYGMP) includes > 400 Zn − Pb deposits and prospects, eight of which are large-scale deposits with large reserves of critical metals such as Cd and Ge, including the Huize Zn − Pb − Cd − Ge and Daliangzi Zn–Pb–Ge–Cd deposits. The newly discovered Zhugongtang Zn − Pb deposit is a super-large deposit with Zn–Pb reserves of > 3 Mt. Its geochemical features are similar to those of the Huize deposit, with similar sulfide δ34S values and concentrations of critical elements in sphalerite (e.g., Cd and Ge). However, the two deposits have different host strata, and it remains unclear as to whether they have similar oregenesis. In this study, δ114/110Cd values and major- and trace-element compositions of sphalerites collected from a drill-core and tunnels of the Zhugongtang deposit were determined in an investigation of metal sources. Drill-core samples were impure and exhibited strong correlation (R2 = 0.89) between Zn and Cd contents. For samples from mining tunnels, the Cd and Fe contents of selected sphalerites were positively correlated, especially yellow sphalerites (R2 = 0.76). Cadmium is likely hosted in sphalerite by the substitution mechanism of (Fe2+, Cd2+) ↔ Zn2+. The δ114/110Cd values of all samples ranged from − 0.43 ‰ to 0.06 ‰. Based on Zn/Cd ratios, and excluding geochemical processes that may have caused the variable Cd isotopic compositions, we suggest that the metal sources of the deposit were derived from the mixing of sedimentary and basement rocks. This model is supported by the strong relationship between the δ114/110Cd and 1/Cd values of sphalerites from 11 typical Zn–Pb deposits in the SYGMP (R2 = 0.81). The quantification of metal contributions of source rocks indicates that deposits derived mainly from sedimentary rocks generally have relatively low sphalerite Ge contents and small Ge reserves, whereas those derived mainly from basement rocks have higher Ge contents and larger Ge reserves. This study provides a new model for explaining the enrichment of critical metals in Zn–Pb deposits of the SYGMP, thus extending the applications of Cd isotopes in hydrothermal systems.

Zinc Supply Affects Cadmium Uptake and Translocation in the Hyperaccumulator Sedum Plumbizincicola as Evidenced by Isotope Fractionation.

This study employs stable isotope analysis to investigate the mechanisms of cadmium (Cd) and zinc (Zn) interaction in the metal hyperaccumulating plant species Sedum plumbizincicola. To this end, the Cd and Zn isotope compositions of root, stem, leaf, and xylem sap samples were determined during metal uptake and translocation at different Cd and Zn concentrations. The enrichment of light isotopes of both elements in plants during uptake was less pronounced at low metal supply levels, likely reflecting the switch from a low-affinity to a high-affinity transport system at lower levels of external metal supply. The lower δ114/110Cd values of xylem sap when treated with a metabolic inhibitor decreasing the active Cd uptake further supports the preference of heavier Cd isotopes during high-affinity transport. The Δ66Znplant-initialsolution or Δ66Znplant-finalsolution values were similar at different Cd concentrations, indicating negligible interaction of Cd in the Zn uptake process. However, decreasing Zn supply levels significantly increased the enrichment of light Cd isotopes in plants (Δ114/110Cd = -0.08‰) in low-Cd treatments but reduced the enrichment of light Cd isotopes in plants (Δ114/110Cd = 0.08‰) under high Cd conditions. A systematic enrichment of heavy Cd and light Zn isotopes was found in root-to-shoot translocation of the metals. The Cd concentrations of the growth solutions thereby had no significant impact on Zn isotope fractionation during root-to-shoot translocation. However, the Δ114/110Cdtranslocation values hint at possible competition between Cd and Zn for transporters during root-to-shoot transfer and this may impact the transport pathway of Cd. The stable isotope data demonstrate that the interactions between the two metals influenced the uptake and transport mechanisms of Cd in S. plumbizincicola but had little effect on those of Zn.

Cadmium isotope fractionation during adsorption onto calcite

Adsorption on mineral surfaces is a crucial process controlling (bio)geochemical cycling of Cd, and Cd isotopic behaviour during adsorption is an important but little investigated aspect of Cd isotope systematics. This work looked at the fractionation of Cd isotopes during Cd adsorption onto calcite, which is an important sink of Cd but isotopic behaviour has only previously been investigated in co-precipitation experiments. The Cd isotope composition of calcite adsorbed components was determined to be lighter and consistent with equilibrium fractionation in all experiments. And degree of fractionation observed in our experiments is not influenced by the Cd/CaCO3 ratio, pH value or Cd/PO43− ratio in the studied range. In MQ water, the observed fractionation value αsorbed-aqueous is 0.99934 but this α value is affected by the presence of other compounds. Notably, αsorbed-aqueous = 0.99984, a reduced but resolvably lower value fractionation, was observed in the presence of PO43− which may be an important factor to be considered when interpreting Cd isotope signatures in nature. The enrichment of lighter Cd isotopes may result from highly distorted octahedral CdO6 sites of inner-sphere Cd complexes on calcite, indicated by extended X-ray absorption fine structure (EXAFS). However, near identical coordination environments of the aqueous and sorbed Cd species in the presence of PO43− likely led to the reduced Cd isotope fractionation. This work suggests adsorption of Cd onto carbonates minerals may produce significant Cd isotope fractionation between fluids and solid phase. Our observations offer an alternative process apart from co-precipitation when interpreting Cd isotope ratios in the presence of calcite in paleo-geological studies and could be helpful to better understand Cd (bio)geochemical cycling.