Cd Isotope Ratios Research Articles

Precise Determination of Cd Isotope Ratios at 3-10 ng Level by Thermal Ionization Mass Spectrometry Using a Molybdenum Silicide Emitter.

Thermal ionization mass spectrometry (TIMS) combined with the double spike technique has excellent analytical precision for Cd isotopic ratio analysis. However, because of the low ionization efficiency of Cd by TIMS, it is still not possible to obtain high precision Cd isotope ratios for small sample size (<100 ng) due to the lack of a highly sensitive emitter for Cd. A new loading method using molybdenum silicide (MoSi2) emitter has been developed for Cd isotope ratio measurements. This emitter produces a significant enhancement in the ionization efficiency of Cd and thus significantly reduces the required sample size to the 3-10 ng level. Analyses of δ114/110Cd for the NIST SRM 3108 using 108Cd-116Cd double spike method show excellent reproducibility (2 SD) that reaches ±0.032‰, ±0.042‰, and ±0.051‰ for 10, 5, and 3 ng of Cd, respectively. This method was further verified with a suite of geological reference materials. Replicate digestions and analyses (n = 8, 2 SD) of δ114/110Cd for NIST SRM 2711a, NOD A-1, and GBW08401 demonstrated good external reproducibility with results of 0.596 ± 0.024‰ for NIST SRM 2711a, 0.150 ± 0.036‰ for NOD A-1, and -0.665 ± 0.084‰ for GBW08401. These data clearly indicate that MoSi2 is an excellent alternative for traditional silica gel to Cd isotopic measurements, especially for samples with a low content of Cd.

Roles of Chloride and Sulfate Ions in Controlling Cadmium Transport in a Soil-Rice System as Evidenced by the Cd Isotope Fingerprint.

Anions accompanying inorganic fertilizers, such as chloride and sulfate ions, potentially affect the solubility, uptake, and transport of Cd to rice grains. However, the role of anions in controlling Cd transport in the soil-soil solution-Fe plaque-rice plant continuum remains poorly understood. Cd isotope ratios were applied to Cd-contaminated soil pots, hydroponic rice, and adsorption experiments with or without KCl and K2SO4 treatments to decipher transport processes in the complex soil-rice system. The chloride and sulfate ions increased the Cd concentrations in the soil solution, Fe plaque, and rice plants. Accordingly, the magnitude of positive fractionation from soil to the soil solution was less pronounced, but that between soil and Fe plaque or rice plant is barely varied. The similar isotope composition of Fe plaque and soil, and the similar fractionation magnitude between Fe plaque and the solution and between goethite and the solution, suggested that desorption-sorption between iron oxides and the solution could be important at the soil-soil solution-Fe plaque continuum. This study reveals the roles of chloride and sulfate ions: (i) induce the mobility of light Cd isotopes from soil to the soil solution, (ii) chloro-Cd and sulfato-Cd complexes contribute to Cd immobilization in the Fe plaque and uptake into roots, and (iii) facilitate second leaves/node II-to-grain Cd transport within shoots. These results provide insights into the anion-induced Cd isotope effect in the soil-rice system and the roles of anions in facilitating Cd migration and transformation.

Cadmium isotope fractionation and gene expression evidence for tracking sources of Cd in grains during grain filling in a soil-rice system

Grain filling is the key period that causes excess cadmium (Cd) accumulation in rice grains. Nevertheless, uncertainties remain in distinguishing the multiple sources of Cd enrichment in grains. To better understand the transport and redistribution of Cd to grains upon drainage and flooding during grain filling, Cd isotope ratios and Cd-related gene expression were investigated in pot experiments. The results showed that the Cd isotopes in rice plants were much lighter than those in soil solutions (∆114/110Cdrice-soil solution = −0.36 to −0.63 ‰) but moderately heavier than those in Fe plaques (∆114/110Cdrice-Fe plaque = 0.13 to 0.24 ‰). Calculations revealed that Fe plaque might serve as the source of Cd in rice (69.2 % to 82.6 %), particularly upon flooding at the grain filling stage (82.6 %). Drainage at the grain filling stage yielded a larger extent of negative fractionation from node I to the flag leaves (∆114/110Cdflag leaves-node I = -0.82 ± 0.03 ‰), rachises (∆114/110Cdrachises-node I = -0.41 ± 0.04 ‰) and husks (∆114/110Cdrachises-node I = -0.30 ± 0.02 ‰), and significantly upregulated the OsLCT1 (phloem loading) and CAL1 (Cd-binding and xylem loading) genes in node I relative to that upon flooding. These results suggest that phloem loading of Cd into grains and transport of Cd-CAL1 complexes to flag leaves, rachises and husks were simultaneously facilitated. Upon flooding of grain filling, the positive fractionation from the leaves, rachises and husks to the grains (∆114/110Cdflag leaves/rachises/husks-node I = 0.21 to 0.29 ‰) is less pronounced than those upon drainage (∆114/110Cdflag leaves/rachises/husks-node I = 0.27 to 0.80 ‰). The CAL1 gene in flag leaves is down-regulated relative to that upon drainage. Thus, the supply of Cd from the leaves, rachises and husks to the grains is facilitated during flooding. These findings demonstrate that the excess Cd was purposefully transported to grain via xylem-to-phloem within nodes I upon the drainage during grain filling, and the expression of genes responsible for encoding ligands and transporters together with isotope fractionation could be used to tracking the source of Cd transported to rice grain.

Biological, Physical, and Atmospheric Controls on the Distribution of Cadmium and Its Isotopes in the Pacific Ocean

AbstractDespite the Pacific being the location of the earliest seawater Cd studies, the processes which control Cd distributions in this region remain incompletely understood, largely due to the sparsity of data. Here, we present dissolved Cd and δ114Cd data from the US GEOTRACES GP15 meridional transect along 152°W from the Alaskan margin to the equatorial Pacific. Our examination of this region's surface ocean Cd isotope systematics is consistent with previous observations, showing a stark disparity between northern Cd‐rich high‐nutrient low‐chlorophyll waters and Cd‐depleted waters of the subtropical and equatorial Pacific. Away from the margin, an open system model ably describes data in Cd‐depleted surface waters, but atmospheric inputs of isotopically light Cd likely play an important role in setting surface Cd isotope ratios (δ114Cd) at the lowest Cd concentrations. Below the surface, Southern Ocean processes and water mass mixing are the dominant control on Pacific Cd and δ114Cd distributions. Cd‐depleted Antarctic Intermediate Water has a far‐reaching effect on North Pacific intermediate waters as far as 47°N, contrasting with northern‐sourced Cd signatures in North Pacific Intermediate Water. Finally, we show that the previously identified negative Cd* signal at depth in the North Pacific is associated with the PO4 maximum and is thus a consequence of an integrated regeneration signal of Cd and PO4 at a slightly lower Cd:P ratio than the deep ocean ratio (0.35 mmol mol−1), rather than being related to in situ removal processes in low‐oxygen waters.

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.

High-precision measurement of Cd isotopes in ultra-trace Cd samples using double spike-standard addition MC-ICP-MS

A flowchart of obtaining Cd isotope ratios using double spike-standard addition MC-ICP-MS.

Lithologic controls on the mobility of Cd in mining-impacted watersheds revealed by stable Cd isotopes

Cd-rich wastes from open-pit mining can be transported into rivers, which are often followed by deposition in river sediments and/or further transfer into agricultural soils. The lithology of bedrock exerts a huge effect on physicochemical properties (e.g., buffering capacities, metal species, mineral phases, etc.) of the river system, thereby potentially impacting the Cd mobility in watersheds. However, to date, little is known about the microscopic processes (e.g., dissolution, adsorption, and precipitation) controlling the migration of Cd from mines to varied watersheds. This study, therefore, aims to determine the controlling factors on Cd mobilization in two mining-impacted watersheds with contrasting bedrock lithology using both Cd and Pb isotopes. The Pb isotope ratios of sediments and soils in both watersheds fall into a binary mixing model with two isotopically distinct sources, i.e., mining wastes and bedrock. These results indicate that mining activities are the main sources of Cd in sediments and soils. However, the Cd isotope ratios reveal different Cd migration processes between the two watersheds. In the siliceous watershed, the δ114/110Cd values of sediments decrease from -0.116‰ in the upper reach to -0.712‰ in the lower reach, with a concomitant increase in Cd concentration, which may result from Cd adsorption by goethite due to the increased pH. In contrast, in the calcareous watershed, the Cd isotope compositions of sediments (-0.345 to -0.276‰) and the pH of river water are nearly invariable, suggesting that the adsorption and release of Cd in sediments are limited. This may result from the strong pH buffering effect due to the presence of carbonate rocks. This study highlights the different fates of Cd in siliceous and calcareous watersheds and suggests that the development of Cd pollution control policies must consider regional lithology.

Precise/small sample size determination of stable Cd isotope ratios of geological samples with double spike MC-ICP-MS

The modified chemical purification protocol and double spike Cd isotope method avoid using the toxic HBr to isolate Cd from Zn and show very high tolerance to the presence of Zn in the sample solutions.

Redox dependence of manganese controls cadmium isotope fractionation in a paddy soil-rice system under unsteady pe + pH conditions

Flooding in paddy soils alters the soil redox of manganese (Mn) and produces elevated concentrations of soluble Mn that can reduce cadmium (Cd) uptake by rice. To better understand the fates of Mn and Cd, along with changes in soil redox conditions, we conducted microcosm incubations in paddy soil covering the reduction to oxidation to re-reduction phases. The extractable Cd concentration decreased rapidly during the reduction phases but increased upon oxidation, and Cd availability largely depended on soil pH, Eh, pe + pH, and the extractable Mn concentration. Exogenous Mn can promote Cd binding with Fe–Mn(oxyhydro)oxides. A trade-off effect between the soil-extractable Cd and Mn concentrations across changes in pH, Eh, pe + pH was identified, and attaining an optimal pe + pH value of 6.8 was targeted. Furthermore, to provide insights into how the redox status of Mn changes to alter Cd mobilization in a paddy soil-rice system, Cd isotope ratios across the paddy soil-rice tissue continuum were investigated using planted rhizobox experiments under different irrigation regimes. The heavy Cd isotopes from the soil to liquid-phase (Δ114/110Cdextract-soil = 0.40–0.82‰) and from the soil to rice grain (Δ114/110Cdgrain-soil = 0.84–0.89‰) were preferentially enriched. Light isotopes were likely to be enriched in Cd bound to Fe/Mn-oxides, a process that was promoted by increased Mn availability. These results suggest that Cd isotopes are systematically fractionated within the paddy soil–rice system, which is caused by the unsteady soil redox, and the stabilization of Cd in the bound soil pool such as Fe–Mn(oxyhydro)oxides-Cd under reducing conditions could be developed as a Cd retention mechanism in paddy soils.

Water Management Alters Cadmium Isotope Fractionation between Shoots and Nodes/Leaves in a Soil-Rice System.

The drainage of rice soils increases Cd solubility and results in high Cd concentrations in rice grains. However, plant Cd uptake is limited by sorption to iron plaques, and Cd redistribution in the plant is regulated by the nodes. To better understand the interplay of Cd uptake and redistribution in rice under drained and flooded conditions, we determined stable Cd isotope ratios and the expression of genes coding transporters that can transport Cd into the plant cells in a pot experiment. In soil, both water management practices showed similar patterns of isotope variation: the soil solution was enriched in heavy isotopes, and the root Fe plaque was enriched in light isotopes. In rice, the leaves were heavier (Δ114/110Cdleaf-shoot = 0.17 to 0.96‰) and the nodes were moderately lighter (Δ114/110Cdnode-shoot = -0.26 to 0.00‰) relative to the shoots under flooded conditions, indicating preferential retention of light isotopes in nodes and export of heavy isotopes toward leaves. This is generally reversed under drained conditions (Δ114/110Cdleaf-shoot = -0.25 to -0.04‰, Δ114/110Cdnode-shoot = 0.10 to 0.19‰). The drained treatment resulted in significantly higher expression of OsHMA2 and OsLCT1 (phloem loading) but lower expression of OsHMA3 (vacuolar sequestration) in nodes and flag leaves relative to the flooded treatment. It appeared that OsHMA2 and OsLCT1 might preferentially transport isotopically heavier Cd, and the excess Cd was purposefully retranslocated via the phloem under drained conditions when the vacuoles could not retain more Cd. Cd in seeds was isotopically heavier than that in stems under both water management practices, indicating that heavy isotopes were preferentially transferred toward seeds via the phloem, leaving light isotopes retained in stems. These findings demonstrate that the Fe plaque preferentially adsorbs and occludes light Cd isotopes on the root surface, and distinct water management practices alter the gene expression of key transporters in the nodes, which corresponds to a change in isotope fractionation between shoots and nodes/leaves.

Changes of Cadmium Storage Forms and Isotope Ratios in Rice During Grain Filling.

Rice poses a major source of the toxic contaminant cadmium (Cd) for humans. Here, we elucidated the role of Cd storage forms (i.e., the chemical Cd speciation) on the dynamics of Cd within rice. In a pot trial, we grew rice on a Cd-contaminated soil in upland conditions and sampled roots and shoots parts at flowering and maturity. Cd concentrations, isotope ratios, Cd speciation (X-ray absorption spectroscopy), and micronutrient concentrations were analyzed. During grain filling, Cd and preferentially light Cd isotopes were strongly retained in roots where the Cd storage form did not change (Cd bound to thiols, Cd–S = 100%). In the same period, no net change of Cd mass occurred in roots and shoots, and the shoots became enriched in heavy isotopes (Δ114/110Cdmaturity–flowering = 0.14 ± 0.04‰). These results are consistent with a sequestration of Cd in root vacuoles that includes strong binding of Cd to thiol containing ligands that favor light isotopes, with a small fraction of Cd strongly enriched in heavy isotopes being transferred to shoots during grain filling. The Cd speciation in the shoots changed from predominantly Cd–S (72%) to Cd bound to O ligands (Cd–O, 80%) during grain filling. Cd–O may represent Cd binding to organic acids in vacuoles and/or binding to cell walls in the apoplast. Despite this change of ligands, which was attributed to plant senescence, Cd was largely immobile in the shoots since only 0.77% of Cd in the shoots were transferred into the grains. Thus, both storage forms (Cd–S and Cd–O) contributed to the retention of Cd in the straw. Cd was mainly bound to S in nodes I and grains (Cd–S > 84%), and these organs were strongly enriched in heavy isotopes compared to straw (Δ114/110Cdgrains/nodes–straw = 0.66–0.72‰) and flag leaves (Δ114/110Cdgrains/nodes–flag leaves = 0.49–0.52‰). Hence, xylem to phloem transfer in the node favors heavy isotopes, and the Cd–S form may persist during the transfer of Cd from node to grain. This study highlights the importance of Cd storage forms during its journey to grain and potentially into the food chain.

Cadmium isotope fractionation during Cd-calcite coprecipitation: Insight from batch experiment

Cadmium (Cd) calcite coprecipitation experiments were conducted to constrain the possible Cd isotope fractionation mechanism under different conditions. The changes in Cd/Ca molar ratio, HCO3− concentration, addition of Mg2+, Na2EDTA, or ionic strength (INaNO3) did not affect the composition of the precipitation but resulted in substantial change in mineral morphology. Cd exhibited an apparent isotope fractionation in the process of coprecipitation with calcite, and the fractional coefficient αCaCO3-Cd(aq) was less than one, indicating that the solution is preferentially enriched in heavy isotopes. The Cd mainly existed in the form of Cd(H2O)62+ in the solution before the reaction and was dominated by Cd(NO3)2, CdNO3+, Cd(EDTA)2− at the end of the experiments. The different isotopic ratios of Cd between the liquid phase and the solid phase can be explained by changes in Cd speciation which have different bond lengths of CdO. Expect for the experiments with Na2EDTA and high ionic strength, the measured isotope ratios under different conditions plotted around the theoretical Rayleigh fractionation curve. This demonstrates that Cd isotopes almost reach equilibrium with Cd-calcite coprecipitation under the different experiment conditions and the Cd isotope fractionation mechanism is closely related to the thermodynamic isotope fractionation. In the experiment with Na2EDTA and high ionic strength, the formation of Cd(EDTA)2− and ion occlusion at surface sites inhibit Cd2+ entering the calcite and the isotopes of the solid and liquid phases do not reach equilibrium. As the result, obtained experiment data shift from the Rayleigh fractionation curve indicating the Cd isotope fractionation mechanism is closely related to the kinetic isotope effect. In general, the study results indicate that the Cd/Ca molar ratio, ionic strength, HCO3−, Na2EDTA, and Mg2+ concentrations of the solution can cause Cd isotope fractionation in different degrees during Cd-calcite coprecipitation. Cd isotope ratios might potentially provide important information on Cd mobilization and transportation in groundwater environment.

Effect of the Dry Ashing Method on Cadmium Isotope Measurements in Soil and Plant Samples

A dry ashing method is commonly used to remove organic material from samples prior to geochemical analysis. In the course of this study, the Cd isotope ratios of a series of soil and plant reference materials and samples were studied to evaluate the effect of the dry ashing method on measurement results of Cd isotope ratios. The samples were pre‐treated using the dry ashing method and high‐pressure bomb for comparison. The results show that the digestion using high‐pressure bombs did not lead to Cd loss, but using the dry ashing method would cause different proportions of Cd loss. The whole range of Cd isotope difference between two methods was from −0.07‰ to 3.01‰. There was also an obvious difference in measured Cd isotope ratios from the same leaf sample pre‐treated independently by the dry ashing method, indicating that the amount of Cd loss and the effect on Cd isotope measurement during dry ashing is related to the properties of the samples. Therefore, dry ashing may not be appropriate for the removal of organic material in Cd isotope ratio measurement, especially for samples with high organic contents. The δ114/110Cd values of reference materials NIST SRM 1573a and GSD‐30 are reported for the first time in this study.

Source apportionment of cadmium pollution in agricultural soil based on cadmium isotope ratio analysis

Anthropogenic activities, especially industrial production and agricultural practices have led to Cd accumulation in the agricultural soil locating near to the industrial areas of China threatening soil environmental quality and food security. In this study, the sources of Cd pollution were accurately analyzed, which may lead to efficiently control the Cd content in the soil. The samples of agricultural topsoil, four possible pollution sources such as smelter dust, automobile exhaust, cement factory dust and agricultural fertilizer were collected, and the Cd concentrations and Cd isotope ratios were analyzed using the instrument of Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Multi-collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS). The results indicated that the Cd concentration of agricultural topsoil (11.72 ± 0.30 mg kg−1) was much higher than the recommended value (0.6 mg kg−1). The highest δ114/110CdNIST3108 value (−0.15 ± 0.14‰) was observed for smelter dust and the lowest δ114/110CdNIST3108 value (−0.64‰) was found in cement factory dust. Similarly, the highest Cd concentration was found in smelter dust and it was clearly distinguishable from the other samples based on δ114/110CdNIST108 values. Cd isotope analysis suggested that smelter along with agricultural fertilizer were the major sources of Cd accumulation in agricultural topsoil. In addition, based on the IsoSource software, the relative Cd contribution rates due to automobile exhaust, cement factory, agricultural fertilizer, and smelter dust were 14%, 7%, 20%, and 59%, respectively, demonstrating that smelter dust is the main pollution source of Cd in the study area. Cd isotope analysis is a promising tool to accurately identify the heavy metal pollution sources.

Comparison of acid extraction and total digestion methods for measuring Cd isotope ratios of environmental samples.

To evaluate the pretreatment processing for Cd isotope analysis of environmental samples, acid extractions and total digestions were examined with various environmental certified reference materials. Four certified reference material samples, including fly ash, polluted soil, domestic sludge, and industrial sludge, were digested by six different acid extraction and total digestion methods, and then Cd was separated to remove other matrix by anion-exchange column. The Cd recovery rates of the acid extraction methods were 2.6-89.1%, while those of the total digestion methods were 21.6-88.7%. In these results, the Cd recovery rates were dependent on the sample type. More than 80% of the Cd in the polluted soil and domestic sludge samples could be recovered regardless of the decomposition method, except one method. On the other hand, the Cd recovery rate from fly ash was low when total digestion was performed using a HF mixture, and the recovery rate by total digestion methods for industrial sludge was higher than that by acid extraction. In our results, Cd isotope ratios tended to be more positive by increasing the Cd recovery rates, suggesting that the light isotope of Cd was decomposed preferentially during the decomposition procedures. However, when more than 80% of the Cd in the samples was recovered, the Cd isotope ratios were determined to be similar. This indicated that at least 80% of the Cd should be recovered from environmental samples to accurately measure the Cd isotopic ratio of environmental samples.

Zinc, cadmium and sulfur isotope fractionation in a supergiant MVT deposit with bacteria

Zinc (Zn) and cadmium (Cd) are two important biological-sensitive elements, and their isotopes are strongly fractionated during biological processes. To develop the potential application of Zn and Cd isotopes for tracing the contribution of bacteria to metal mineralization, in this study we investigate for the first time the ranges and causes of isotope fractionation of Zn and Cd in a large hydrothermal system with bacteria from the world-class Jinding Zn-Pb deposit, southwest China. Extremely negative δ34SCDT values of sulfides (−48.6‰ to +7.7‰) obtained in this study and literature reveal the dominant biogenic sulfur associated with bacterial sulfate reduction. Both Zn and Cd isotope ratios are highly variable and significantly lighter compared with most of Zn-Pb ore deposits worldwide. The extremely negative δ66ZnJMC 3-0749L (down to −0.69‰) exceeds the range of Rayleigh fractionation during inorganic precipitation of ZnS. Zinc isotope ratios display a well-defined positive correlation with Zn/Cd ratios and the samples with lightest δ66Zn have lowest Zn/Cd ratios. This reflects that sphalerites formed in the early stages of mineralization already have extremely light Zn isotopic compositions. There is a negative relationship between δ34SCDT and δ66Zn, indicating that lighter Zn isotope ratios are related to stronger bacterial sulfate reduction, which results in heavier sulfur isotope ratios. Since organic compounds (e.g., Zn-carboxylate) in fluids are strongly enriched in heavy isotopes relative to free Zn2+ and Zn-sulfide, sulfides formed in the bacteria-rich hydrothermal systems would acquire light δ66Zn. Therefore, the extremely negative δ34SCDT and δ66Zn ratios demonstrate for the first time that bacterial metabolisms not only induced sulfate reduction, but also were directly involved in the formation of metals (e.g., Zn) in a large hydrothermal system. A similar process may have also partly contributed to the substantial fractionation of cadmium isotopes in the hydrothermal system. Overall, our studies show that zinc and cadmium isotopes may act as an effective tool in deciphering the specific role of organisms in formation of sulfides in hydrothermal systems and other settings.

Cadmium isotope fractionation during coal combustion: Insights from two U.S. coal-fired power plants

Coal combustion, one of the principal energy sources of electricity in the United States, produces over 100 million tons of coal combustion products (CCPs) per year in the U.S. The reuse and disposal of CCPs has the potential to release toxic trace elements, including cadmium (Cd), into the environment. In this study, we investigated CCPs, including bottom ash (BA), economizer fly ash (EFA), and fly ash (FA), as well as feed coal (FC) and pulverized coal (PC) collected from two U.S. coal-fired power plants in New Mexico and Ohio with different coal supplies. The New Mexico plant uses high volatile C bituminous, low-sulfur coals mined from the San Juan Basin (Cretaceous Fruitland Formation) and the Ohio plant uses high volatile A bituminous, high-sulfur central Appalachian Basin coals (Upper Pennsylvanian Monongahela Formation). Mineralogical and elemental analysis showed that these CCP samples consist of ∼70% amorphous Al-Si-rich glasses and ∼30% mineral phases of quartz (SiO2) and mullite (Ai6Si2O13). The Cd isotope compositions (δ114Cd, normalized to NIST Cd standard 3108) of FA and EFA samples (ranging from −0.51 to +0.47‰) are distinctively heavier than those of BA samples (−0.75 to −0.52‰) in both power plants. We interpret this Cd isotope difference as a result of Cd condensation from the gas phase during flue gas cooling, instead of evaporation of Cd phase during coal combustion. Cd condensation is the main process to generate the isotopically heavy Cd signatures that preferentially partition on the fine FA particles. We also investigated Cd isotope compositions in different leachate products from a series of batch-leaching experiments with these CCPs, using diluted acetic acid, hydroxyl ammonium chloride, hydrogen peroxide followed by ammonium acetate, and 5% nitric acid, as a possible means to identify CCP-released Cd in the environment. Unusually and significantly heavier Cd isotope compositions were observed in each leachate of FA samples (+1.10 to +7.09‰), which fall far outside from the range of Cd isotope ratios observed in natural soils and rocks, but less so for the EFA samples (−0.43 to +1.18‰). Such an observation is consistent with the interpretation that isotopically heavy Cd preferentially partitions on the fine FA particles after coal combustion and is readily to be released during these leaching experiments. This study demonstrates that high-temperature coal combustion can lead to a very large degree of fractionation of Cd isotopes that can be used as a unique tracer for identifying anthropogenic metal inputs in the environment. The major Cd isotope fractionation process occurs as the Cd gas phase condenses on fine FA particles during the flue gas cooling stage after coal combustion.

Stable isotope fractionation during uptake and translocation of cadmium by tolerant Ricinus communis and hyperaccumulator Solanum nigrum as influenced by EDTA

The isotopic fractionation could contribute to understanding the Cd accumulation mechanisms in plant species. However, there are few of systematical investigations with regards to the Cd isotope fractionation in hyperaccumulator plants. The Cd tolerant Ricinus communis and hyperaccumulator Solanum nigrum were cultivated in nutrient solutions with varying Cd and EDTA concentrations. Cd isotope ratios were determined in the solution, root, stem and leaf. The two investigated plants were systematically enriched in light isotopes relative to their solutions (Δ114/110Cdplant-solution = -0.64‰ to-0.29‰ for R.communis and-0.84‰ to-0.31‰ for S.nigrum). Cd isotopes were markedly fractionated among the plant tissues. For both plant species, an enrichment in light Cd isotopes from solution to root was noted, followed by a slight depletion in light Cd isotopes from root to shoot. Noticeably, the chelation process has caused lighter Cd isotope enrichment in the root of R.communis and S.nigrum. Further, the good fits between △114/110Cdroot-plant and ln Froot (or between △114/110Cdshoot-plant and ln Fshoot) indicate that Cd isotopic signatures can be used to study Cd transportation during the metabolic process of plants. This study suggests that knowledge of the Cd isotope ratios could also provide new tool for identifying the Cd-avoiding crop cultivars.

Cadmium isotopic evidence for the evolution of marine primary productivity and the biological extinction event during the Permian-Triassic crisis from the Meishan section, South China

Abstract Cadmium (Cd) isotopic compositions in seawater are important parameters for tracing the biogeochemical cycling of Cd in modern oceans and may be used as a direct proxy for nutrient utilization by phytoplankton and an indirect proxy for primary productivity. In this study, we measured the stable Cd isotope ratios (δ114Cd) of marine sedimentary carbonates from the Meishan section in Changxing County of Zhejiang Province, South China, the Global Stratotype Section and Point (GSSP) of the Permian-Triassic Boundary (PTB). The results suggest that the inferred marine primary productivity recorded in the marine carbonates from the Late Permian (i.e., before the mass extinction) was high and relatively stable (the δ114/110Cd values of carbonates range from +0.31‰ to +0.60‰; after correction for salinity-controlled fractionation into inorganic calcite, the δ114/110Cd values of palaeo-seawater range from +0.47‰ to +0.79‰). During the mass extinctions, the δ114/110Cd values of carbonates abruptly decreased at the beginning of each episode of mass extinction, suggesting that the primary productivity of the palaeo-ocean declined before each episode of extinction events at the PTB. We suggest that the mass extinction events at the PTB in the Meishan section could be a ripple effect due to the destruction of primary producers at the base of the food chain in the palaeo-ocean ecosystem.

Biogeochemical cycling of Zn and Cd and their stable isotopes in the Eastern Tropical South Pacific

Abstract The distribution of Zn and Cd and their stable isotope ratios (δ66Zn and δ114Cd) were measured on samples from the Eastern Subtropical South Pacific Zonal Transect (EPZT), US GEOTRACES section GP16. The broad trends in both δ66Zn and δ114Cd are similar to those observed in other ocean basins, suggesting global similarities in the biogeochemical processes which cycle Zn and Cd. For example, average deep ocean δ66Zn along this transect (+ 0.46‰) are similar to the ~+0.5‰ values observed in the deep North Atlantic and North Pacific. Also similar to other locations, δ66Zn decreases towards the surface ocean. A plume of hydrothermal Zn emanating from the East Pacific Rise is used to calculate hydrothermal end-member δ66Zn as + 0.24‰, which is similar to crustal values and similar to other sources of Zn to the oceans. Average deep-ocean δ114Cd along this transect is + 0.28‰, which is similar to deep-ocean values between + 0.2 and + 0.3 measured elsewhere in the Atlantic, Pacific, and Southern Oceans. δ114Cd increases towards the surface ocean, as observed in other ocean basins, reflecting the preferential biological uptake of lighter Cd isotopes. Elsewhere it has been hypothesized that Cd can precipitate in sulfidic microenvironments on sinking particles, affecting seawater Cd concentrations and isotope ratios. However, we don't find evidence of particularly strong CdS precipitation along this transect, despite the fact that the Peru OMZ is one of the most reducing ocean environments worldwide.