Search

The application of mercury (Hg) stable isotopes as tracers of Hg biogeochemical cycling requires high accuracy and precision for Hg isotope analysis. A common method for sample introduction in Hg isotope analyses is cold vapor generation (CVG), which converts soluble Hg(II) in sample solutions into Hg(0) vapor that is then transported to the plasma. However, the efficiency of Hg CVG and its influence on Hg isotope analysis have not been fully quantified. In this study we systematically investigate the Hg CVG efficiency and the influence of non-quantitative CVG on Hg isotope analysis. Our results show that Hg CVG can be strongly affected by the sample uptake rate, sample matrix, and the design of the gas–liquid separator (GLS). Increases of the sample uptake rate shorten the reaction time of CVG, and thus can decrease the CVG efficiency and induce mass-dependent fractionation (MDF) due to incomplete volatilization of Hg(0). Different types and concentrations of sample matrices also result in different degrees of suppression on Hg CVG and Hg isotope fractionation via multiple mechanisms including partial volatilization of Hg(0), incomplete Hg(II) reduction and/or reoxidation of Hg(0), but matrix separation significantly alleviates or even eliminates the suppression on Hg CVG. The design of GLS has a particularly important impact on Hg CVG and thus the accuracy of Hg isotope analysis. Generally, GLSs that have larger reaction surfaces that promote more complete Hg(II) reduction and Hg(0) volatilization tend to show higher CVG efficiency. In addition, the degrees of suppression on Hg CVG can be variable even for samples with the same matrix. Thus, the Hg isotope fractionation caused by non-quantitative Hg CVG cannot always be fully corrected by matrix matching and standard-sample bracketing. We conclude that ensuring high, near 100% CVG efficiency by performing matrix separation, adopting an optimal design of GLS, and tuning the sample uptake rate is the key to obtaining the highest accuracy and precision in Hg isotope analysis. [ABSTRACT FROM AUTHOR]

Mercury (Hg) bound to fine aerosols (PM 2.5 -Hg) may undergo photochemical reaction that causes isotopic fractionation and obscures the initial isotopic signatures. In this study, we quantified Hg isotopic compositions for 56 PM 2.5 samples collected between 15 September and 16 October 2015 from Beijing, China, among which 26 were collected during daytime (between 08:00 and 18:30 LT) and 30 during night (between 19:00 and 07:30 LT). The results show that diel variation was statistically significant (p<0.05) for Hg content, Δ199Hg and Δ200Hg , with Hg content during daytime (0.32±0.14 µ g g -1) lower than at night (0.48±0.24 µ g g -1) and Δ199Hg and Δ200Hg values during daytime (mean of 0.26‰±0.40‰ and 0.09‰±0.06‰ , respectively) higher than during nighttime (0.04‰±0.22‰ and 0.06‰±0.05‰ , respectively), whereas PM 2.5 concentrations and δ202Hg values showed insignificant (p>0.05) diel variation. Geochemical characteristics of the samples and the air mass backward trajectories (PM 2.5 source related) suggest that diel variation in Δ199Hg values resulted primarily from the photochemical reduction of divalent PM 2.5 -Hg, rather than variations in emission sources. The importance of photoreduction is supported by the strong correlations between Δ199Hg and (i) Δ201Hg (positive, slope = 1.1), (ii) δ202Hg (positive, slope = 1.15), (iii) content of Hg in PM 2.5 (negative), (iv) sunshine durations (positive) and (v) ozone concentration (positive) observed for consecutive day–night paired samples. Our results provide isotopic evidence that local, daily photochemical reduction of divalent Hg is of critical importance to the fate of PM 2.5 -Hg in urban atmospheres and that, in addition to variation in sources, photochemical reduction appears to be an important process that affects both the particle mass-specific abundance and isotopic composition of PM 2.5 -Hg. [ABSTRACT FROM AUTHOR]

Travertine samples deposited in Earth's surface environments can be used as an effective archive for paleo-climatic reconstruction. As a common element in carbonates, magnesium (Mg) and its isotopic composition in travertine could provide useful information for evaluating paleo-environment changes. In this study, we investigate the Mg isotope systematics in both endogenic travertines (mainly calcite) and spring/stream waters at Baishuitai, Yunnan, SW China. Our results show a systematic increase in δ26Mg value from −1.37 to −1.26‰ for water samples downstream, but varied δ26Mg values between −4.12 and −3.95‰ (average −4.02‰) for solid carbonates, thus a corresponding fractionation Δ26Mg calcite-water between −2.76 to −2.59‰ (mean value of −2.69‰). Therefore, the solid carbonates preferentially incorporate light Mg isotopes during travertine formation. More interestingly, the Mg distribution coefficient (K Mg/Ca) between travertine and water exhibits two variation trends with the calcite deposition rate (R p) along the canal, which can be explained by the change of calcite formation mechanism from direct nucleation to precipitation via amorphous calcium carbonate (ACC) intermediate. In the upper-stream, the direct nucleation of calcite results in the rapid incorporation of Mg ions into crystal lattice, while a relatively slow precipitation of calcite downstream would incorporate Mg via ACC formation pathway in a quasi-equilibrium pattern. This is consistent with the grain size distribution and crystal morphology observed under SEM. Our results show the important control of water Mg/Ca ratios on the calcite precipitation during travertine formation, and imply the potential and complexity of using Mg isotopes of travertine deposits to reconstruct paleo-environments. • Mg isotope fractionation was investigated on naturally-occurring travertine samples from Baishuitai, China. • The Mg/Ca ratios of spring/stream water have important controls on the Mg-Ca exchange and Mg isotope fractionation. • Our results can be explained by changing precipitation mechanism from direct nucleation to the precipitation via ACC. • This study suggests both temperature and Mg/Ca ratio in solution may have significant effects on Mg isotope fractionation. • This study reveals the potential and complexity of using δ26Mg values of travertine deposits as a paleo-proxy. [ABSTRACT FROM AUTHOR]

Gallium (Ga) isotopes are potentially advantageous for characterizing the surficial biogeochemical cycles of Ga and tracking the geochemical behavior of the monoisotope element aluminum (Al) during chemical weathering. To test the potential of Ga and its isotopes to trace continental weathering, we studied Ga isotope compositions of a strongly-weathered latosol profile developed from basalt on the Leizhou Peninsula, Guangdong Province, South China. In the profile, δ71Ga SRM-994 values of latosol samples decrease from ∼0.69‰ in unweathered basalts at the bottom to ∼0.51‰ in shallow sections, while the Ga concentration increases from ∼19.9 µg/g in the bedrock to ∼42.7 µg/g in soil, indicating significant enrichment of Ga and its isotope fractionation triggered by weathering processes. Bulk samples analysis and sequential leaching of selected weathering products suggest that the majority of Ga in these samples occurs in the crystal lattice of Al-rich and Fe-rich minerals, with the light Ga isotope (69Ga) enriched in latosol samples relative to the coexisting solution. Interestingly, our results show δ71Ga values of latosols display a bigger variation (about 3–4 times of its analytical uncertainty) than their Ga/Al ratios (about 1.5 times of its analytical uncertainty) throughout the profile, indicating δ71Ga values may be a more sensitive proxy to track the loss of Al3+ and involvement of Fe3+ during weathering process. Based on a simple Rayleigh model, the upper limit of Ga isotope fractionation between solution and weathered basalt (Δ71Ga solution-weathered rocks) is estimated to be ∼1.50‰, implying that heavy Ga isotopes may be enriched in surface river systems. This study highlights the potential of Ga elemental and isotope geochemistry to trace continental weathering and global Ga cycling. [ABSTRACT FROM AUTHOR]

The installation rate of denitrification devices is accelerating in Chinese urban boilers. Previous studies on pulverized coal-fired boilers without denitrification devices showed that combustion products containing mainly oxidized mercury (Hg) preferably enriched lighter Hg isotopes than feed coals. However, the magnitude of this enrichment becomes less pronounced if denitrification devices are installed. The underlying Hg isotope fractionation mechanisms are still unclear. In this study, three types of urban boilers (two pulverized coal-fired boilers, one circulating fluidized bed boiler and one municipal waste incinerator boiler) all installed with denitrification devices were measured for Hg isotope compositions of their feed fuels and corresponding combustion products. We observed little mass independent fractionation but very significant mass dependent fractionation (MDF) between feed fuels and combustion products. The fly ash and desulfurization products both enriched heavier Hg isotopes than feed coals in three coal-fired boilers, and the enrichment of heavy Hg isotopes increased with sequential removal of combustion products in all boilers. Different from previously suggested kinetic MDF for gaseous Hg0(g)→HgII(g) and gaseous HgII(g)→particulate HgII(p) in coal combustion flue gases, we propose an equilibrium MDF for Hg0(g)↔HgII(g) followed by a kinetic MDF for HgII(g)→HgII(p). This equilibrium MDF most likely occurs during Hg0(g) oxidation in denitrification devices, which enriches heavy Hg isotopes in oxidized products (HgII(g) and HgII(p)) that are then sequestrated in fly ash and desulfurization products. The paradigm shift of MDF in boilers with denitrification devices was further verified by parallel Hg isotope measurement in urban atmosphere particulates. Our study clearly demonstrates that modern coal-fired boilers with denitrification devices have a quite different MDF compared to traditional boilers without denitrification devices. This has important implications for estimating isotope signatures of urban boiler Hg emissions, and for isotope tracing of anthropogenic Hg emissions. Image 1 • Hg isotope fractionation in urban boiler denitrification devices is first addressed. • Hg(0) oxidation in denitrification devices enriches heavy Hg isotopes in products. • Heavy Hg isotope enrichment increases with sequential removal of combustion products. • Particulate matter registers Hg isotope signatures of fly ash from urban boilers. Mercury vapor oxidation in denitrification devices enriches heavier mercury isotopes in oxidized products, and the enrichment of heavy mercury isotopes increases with sequential removal of combustion products. [ABSTRACT FROM AUTHOR]

Gallium (Ga) isotopic fractionation during its adsorption on calcite and goethite was investigated at 20 °C as a function of the solution pH, Ga aqueous concentration and speciation, and the solid to solution ratio. In all experiments Ga was found to be enriched in light isotopes at the solid surface with isotope fractionation △ 71 Ga solid-solution up to −1.27‰ and −0.89‰ for calcite and goethite, respectively. Comparison of Ga isotopic data of this study with predictions for ‘closed system’ equilibrium and ‘Rayleigh fractionation’ models indicates that the experimental data are consistent with a ‘closed system’ equilibrium exchange between the fluid and the solid. The results of this study can be interpreted based on Ga aqueous speciation and the structure of Ga complexes formed at the solid surfaces. For calcite, Ga isotope fractionation is mainly triggered by increased Ga coordination and Ga–O bond length, which vary respectively from 4 and 1.84 Å in Ga(OH) 4 − to 6 and 1.94 Å in the >Ca–O–GaOH(OH 2 ) 4 + surface complex. For goethite, despite the formation of Ga hexa-coordinated >FeOGa(OH) 2 0 surface complexes (Ga–O distances of 1.96–1.98 Å) both at acid and alkaline pH, a similar extent of isotope fractionation was found at acid and alkaline pH, suggesting that Ga(OH) 4 − is preferentially adsorbed on goethite for all investigated pH conditions. In addition, the observed decrease of Ga isotope fractionation magnitude observed with increasing Ga surface coverage for both calcite and goethite is likely related to the formation of Ga surface polymers and/or hydroxides with reduced Ga–O distances. This first study of Ga isotope fractionation during solid-fluid interactions suggests that the adsorption of Ga by oxides, carbonates or clay minerals could yield significant Ga isotope fractionation between secondary minerals and surficial fluids including seawater. Ga isotopes thus should help to better characterize the surficial biogeochemical cycles of gallium and its geochemical analog aluminum. [ABSTRACT FROM AUTHOR]

Metal stable isotopes in marine carbonates have been widely used as novel proxies for metal cycling through the geological past. Amongst these metals, copper (Cu) and zinc (Zn) have received great attention owing to their vital roles in metabolic processes. However, whether modern marine biogenic carbonates record the seawater isotope signatures of Cu (δ65Cu) and Zn (δ66Zn) and mechanisms and factors controlling isotope fractionation of these metals remain unclear, hindering applications of both isotope systems in paleoceanography. Here we present annually-resolved records of δ65Cu and δ66Zn in seven coral (Porites) cores sampled from different marine settings in the South China Sea and the Luzon Strait, western Pacific. We find that the aragonitic skeletons of corals are enriched in light Cu but heavy Zn isotopes relative to surface seawaters, with δ65Cu and δ66Zn in the range of −0.16 ± 0.06‰ to 0.40 ± 0.05‰ and 0.06 ± 0.04‰ to 0.46 ± 0.08‰ (2SD), respectively. The coral δ65Cu exhibits significant inter- and intra-colony variations, which are most likely controlled by Rayleigh-type fractionation in the calcifying fluids of corals rather than by changes in environmental factors or seawater δ65Cu. We thus suggest that δ65Cu in ancient carbonates may not be a direct record of Cu isotope compositions in coeval seawaters. In contrast, coral δ66Zn shows insignificant temporal variation, and the Zn isotope fractionation between individual corals and seawaters are relatively small and constant (0.10 ± 0.05‰, n = 33, 2SD). This limited Zn isotope fractionation is much lower than that determined during inorganic precipitation of calcite, which is likely due to pH up-regulation by the coral that changes aqueous Zn speciation, and preferential organic complexation of the heavy Zn isotopes in the calcifying fluids. Therefore, Porites corals are promising archives for tracking historical changes of surface seawater δ66Zn. Our new datasets of reef carbonates, particularly the coral δ65Cu values which are measured for the first time, could provide a better constrain on marine Cu and Zn geochemistry and their modern oceanic mass and isotope budgets. [ABSTRACT FROM AUTHOR]

Gallium (Ga) has two naturally-occurring stable isotopes: 69Ga (60.1%) and 71Ga (39.9%). Their isotopic ratios were employed as a potential proxy to trace the geochemical behavior of Ga and its analogue Al. Ga is also an important component of many semiconductors. When exposed to aqueous solutions, it may be dissolved and then precipitate as Ga-bearing crystals (e.g., GaOOH). Thus, the exploration of Ga isotope fractionation during α-GaOOH precipitation could be helpful for understanding geochemical behaviors of Ga and other metals (including Al and Fe) during oxyhydroxide precipitation. In this study, three series of precipitation experiments with variable initial aqueous Ga concentrations (about 9.2, 2.0, and 1.8 ppm) were carried out to determine Ga isotope fractionation factor during α-GaOOH precipitation at ∼20 °C and neutral pH as a function of time. The experimental results could be modeled using a Rayleigh precipitation process, in which fractionation factors between crystals and solution (Δ71Ga solid-solution) are between −0.28 and − 0.37‰, depending on the initial Ga concentration and type of matrix anion (Cl− vs. NO 3 −). This isotope fractionation is significantly smaller in magnitude than the values reported for Ga adsorption on goethite (−0.89‰) and calcite (−1.27‰). The isotope fractionation is mainly caused by the increase in Ga O bond length from ∼1.84 Å in aqueous Ga OH 4 — to ∼1.991 Å in α-GaOOH solid, with a simultaneous change of Ga coordination number from 4 to 6, although kinetic isotope fractionation may play an important role. Our study provides the first experimental evidence of significant Ga isotope fractionation during kinetic oxyhydroxide precipitation, and suggests that the measurable Ga isotope fractionation occurred at water-solid interface and during precipitations can be potentially used for tracing global geochemical cycling of Ga and its analogue Al and Fe. [Display omitted] • Significant Ga isotope fractionation was observed during α-GaOOH precipitation. • α-GaOOH is preferentially enriched in 69Ga compared with coexisting solution. • Δ71Ga solid-solution are −0.28 ∼ −0.37‰ as modeled by Rayleigh precipitation process. [ABSTRACT FROM AUTHOR]