Jiubin Chen

Chromatographic pre-concentration of Hg from dilute aqueous solutions for isotopic measurement by MC-ICP-MS

Previous studies have demonstrated both mass-dependent fractionation (MDF) and mass-independent fractionation (MIF) of Hg isotopes in the environment and the potential for their application in biochemistry and geochemistry. However, little has been reported for Hg isotope geochemistry in natural aqueous environment due to the very low concentration (1–20 ng/L). We report here, for the first time, a new protocol for directly pre-concentrating Hg using AG 1 × 4 ion-exchange resin, which allows the measurement of Hg isotope composition in freshwater samples. The protocol is validated by testing both synthetic and natural solutions of varying concentrations, and by assessing the effect of Hg recovery and organic complexants on the reproducibility and the accuracy of measured isotopic ratios. The method results in acceptable procedural blanks and quantitative yields (101% ± 6%) in the final Hg eluates. The instrumental mass bias was corrected using modified empirical external normalization (MEEN) with Tl as internal dopant. The sensitivity of the method to the solution matrix (Hg/Tl ratio, organic matter concentration) was also evaluated. The pre-concentration and isotopic measurement protocol was applied to 16 natural water samples with Hg concentrations ranging from 0.9 ng/L to 15600 ng/L. The result showed a total variation of 2.4‰ for δ202Hg and evident MDF and MIF of Hg isotopes. Our study demonstrates that more research is required to fully understand the behavior of Hg isotopes in the aqueous environment.

Mercury Stable Isotopic Compositions in Coals from Major Coal Producing Fields in China and Their Geochemical and Environmental Implications

Total mercury (Hg) concentrations (THg) and stable mercury isotopic compositions were measured in coal samples (n = 61) from major coal producing fields in China. The THg concentrations in coals ranged from 0.05 to 0.78 μg g(-1), with a geometric mean of 0.22 μg g(-1). Hg isotopic compositions in coals showed large variations both in mass-dependent fractionation (MDF, δ(202)Hg: -2.36 to -0.14‰) and mass-independent fractionation (MIF, Δ(199)Hg: -0.44 to +0.38‰). The MIF signatures in coals may reveal important information on the coal-forming conditions (e.g., humic and sapropelic). The Δ(199)Hg/Δ(201)Hg of ∼1 determined in coals indicated that a portion of Hg has been subjected to photoreduction process prior to being incorporated to coals. On the basis of THg, Hg isotopic signatures, and other geological factors (e.g., total ash content and total sulfur content), the potential sources of Hg in coals from different coal producing regions were estimated. The main source of Hg in coals from southwestern China and eastern part of northern China is likely geogenic Hg, whereas the source of Hg in coals from other parts of northern China is mainly biogenic Hg. Finally, we estimated that Hg emission from coal combustion in China is characterized by diagnostic Hg isotopic signatures (δ(202)Hg: ∼-0.70‰ and Δ(199)Hg: ∼-0.05‰). The present study demonstrates that Hg isotopes can serve as a tool in understanding the sources and transformation of Hg in coals and may also be used as a tracer to quantify Hg emissions from coal combustion.

An improved dual-stage protocol to pre-concentrate mercury from airborne particles for precise isotopic measurement

The Hg isotopic signature may provide insight into tracking the sources and pathways of both airborne particulate matter (PM) and particle-bound Hg (PHg) in the atmosphere. However, separating and concentrating trace levels of PHg and accurately analyzing its isotope ratios remain a technical challenge. Here, we optimized a combustion-trapping dual-stage protocol specifically for the pre-concentration of Hg from a PM sample collected on a quartz fiber membrane (QFM) for high-precision Hg isotopic analysis. The protocol was validated by testing synthetic samples of varying concentrations and trapping solutions of different volumes, and by comparison with two conventional methods (acid digestion and column purification). Using the dual-stage protocol, an individual sample containing up to 570 ng of Hg can be combusted at programmed temperatures in an Hg-free O2 stream, and the volatilized gaseous Hg was trapped in a 5 mL acid (4 M HNO3 and 1.3 M HCl) mixture. The method results in a relatively lower procedural blank and quantitative recovery (99 ± 6%, 2SD, n = 90). Long-term measurements of three certified reference materials (CRM021, CRM024, and GBW07405) with complex matrices using the optimized protocol gave identical Hg isotopic ratios of both mass-dependent fractionation (MDF) and mass-independent fractionation (MIF), in agreement with the results obtained from the standard addition method. The protocol was applied to two PM2.5 samples collected on a 20.3 × 12.5 cm QFM. The result showed evident variations of both MDF and MIF, highlighting the importance of studying Hg isotopic compositions in PM of variable environments in order to fully understand the behaviors of Hg and its isotopes in the atmosphere.

Pre-industrial and recent (1970-2010) atmospheric deposition of sulfate and mercury in snow on southern Baffin Island, Arctic Canada.

Sulfate (SO4(2-)) and mercury (Hg) are airborne pollutants transported to the Arctic where they can affect properties of the atmosphere and the health of marine or terrestrial ecosystems. Detecting trends in Arctic Hg pollution is challenging because of the short period of direct observations, particularly of actual deposition. Here, we present an updated proxy record of atmospheric SO4(2-) and a new 40-year record of total Hg (THg) and monomethyl Hg (MeHg) deposition developed from a firn core (P2010) drilled from Penny Ice Cap, Baffin Island, Canada. The updated P2010 record shows stable mean SO4(2-) levels over the past 40 years, which is inconsistent with observations of declining atmospheric SO4(2-) or snow acidity in the Arctic during the same period. A sharp THg enhancement in the P2010 core ca 1991 is tentatively attributed to the fallout from the eruption of the Icelandic volcano Hekla. Although MeHg accumulation on Penny Ice Cap had remained constant since 1970, THg accumulation increased after the 1980s. This increase is not easily explained by changes in snow accumulation, marine aerosol inputs or air mass trajectories; however, a causal link may exist with the declining sea-ice cover conditions in the Baffin Bay sector. The ratio of THg accumulation between pre-industrial times (reconstructed from archived ice cores) and the modern industrial era is estimated at between 4- and 16-fold, which is consistent with estimates from Arctic lake sediment cores. The new P2010 THg record is the first of its kind developed from the Baffin Island region of the eastern Canadian Arctic and one of very few such records presently available in the Arctic. As such, it may help to bridge the knowledge gap linking direct observation of gaseous Hg in the Arctic atmosphere and actual net deposition and accumulation in various terrestrial media.

Historical variations of mercury stable isotope ratios in arctic glacier firn and ice cores.

The concentration and isotopic composition of mercury (Hg) were determined in glacier core samples from Canadian Arctic ice caps dating from pre-industrial to recent time (early 21st century). Mean Hg levels increased from ≤ 0.2 ng L-1 in pre-industrial time to ~0.8-1.2 ng L-1 in the modern industrial era (last ~200 years). Hg accumulated on Arctic ice caps has Δ199Hg and Δ201Hg that are higher (~-1 to 2.9 ‰) than previously reported for Arctic snow (mostly < -1 ‰) impacted by atmospheric Hg depletion events, suggesting that these events contribute little to Hg accumulation on ice caps. The range of δ202Hg, Δ199Hg and Δ201Hg in glacier cores overlaps with that of Arctic Hg0(g) and of seawater in Baffin Bay, but also with that of mid-latitude precipitation and industrial Hg sources, including coal and Hg ores. A core from Agassiz ice cap (80.7 °N) shows a ~ +1 ‰ shift in δ202Hg over the 19th-20th centuries that could reflect changes in the isotopic composition of the atmospheric Hg pool in the High Arctic in response to growing industrial emissions at lower latitudes. This study is the first ever to report on historical variations of Hg stable isotope ratios in Arctic ice cores. Results could help constrain future modeling efforts of the global Hg biogeochemical cycle and the atmospheres response to changing Hg emissions, past and future.

Are boron isotopes a reliable tracer of anthropogenic inputs to rivers over time

This study aims at determining how the boron signal of the Seine River evolved in terms of concentration and isotopic signatures over eighteen years (1994-95 and 2006-12) and if boron isotopes can reliably trace anthropogenic inputs over time. In the anthropised Seine River watershed, boron is widely released by human activities, and even if boron concentrations ([B]) are below the potability limit, our study confirms the potential of boron isotopes (δ11B) to trace urban anthropogenic contaminations. Between 1994 and 2012, [B] have decreased across the anthropised part of the Seine River basin (and by a factor of two in Paris) while δ11B has increased. This means either that urban inputs have been reduced or that the boron signature of urban inputs has changed over time. Both hypotheses are in agreement with the decrease of perborate consumption in Europe over 15years and are not mutually exclusive. Results of a thorough analysis of urban effluents from the sewage network of Paris conurbation that are in fine released to the Seine River suggest a shift of the urban δ11B from -10‰ in 1994 to 1.5±2.0‰ in 2012, in agreement with our second hypothesis. We attribute this change to the removal of perborates from detergents rather than to the modernisation of wastewater treatment network, because it does not significantly impact the wastewater boron signatures. Eighteen years after the first assessment and despite the decreased use of perborates, geochemical and isotopic mass budgets confirm, that boron in the Seine River basin is mainly released from urban activities (60-100%), especially in Paris and the downstream part of the basin. Contrastingly, in headwaters and/or tributaries with low urbanisation, the relative boron input to river from agricultural practices and rains increased, up to 10% and by 10 to 30%, respectively.

Sequential samples reveal significant variation of mercury isotope ratios during single rainfall events

Although the investigation of mercury (Hg) isotopes in precipitation has largely improved our knowledge of the source and transformation of Hg in the atmosphere, rainwater investigated in previous studies were integrated samples collected over an event and could obscure key information about the physiochemical transformation and deposition dynamics of Hg (and its isotopes) in short precipitation events. In this study, we investigated Hg isotopic composition of filtered (HgF) and particulate Hg (HgPM) in sequential rain samples from three single rainfall events in Guiyang, China. All samples showed a decrease of total Hg concentration, as well as HgF and HgPM with time in each rainfall event, and large variation of both mass-dependent fractionation (MDF) and mass-independent fractionation of odd Hg isotopes (odd-MIF) for both phases. Isotopic data indicated variable contributions of different sources triggered by the instant change of meteorological conditions, rather than internal atmospheric processes. The rapid response of MDF and odd-MIF of precipitation samples to the incense burning on the Tomb Sweeping Day implied that Hg isotopic composition was very sensitive to the momentary anthropogenic emission, which could have at least a regional short-lived effect and should be taken into account in future studies. Hg isotopes are a powerful tool for investigating both atmospheric transformation and instant deposition dynamic of Hg, and like stable H and O isotopes, could provide useful information about local or regional meteorological changes.

Temporal transport of major and trace elements in the upper reaches of the Xijiang River, SW China

Abstract This study investigated the temporal variations of major and trace element contents and controlling factors in the upper reaches of the Xijiang River, SW China. The results showed the major ions principally derived from the weathering of carbonate and silicate rocks, with a seasonal variation impacted by the monsoonal climate. The contents of dissolved trace elements also varied seasonally in the two main tributaries: the Nanpan and Beipan rivers. Most dissolved trace elements such as Mn, Zn and Pb being affected by anthropogenic contributions displayed a large seasonal variability, from onefold to hundred-fold, while V, Rb and U show a relative stable trend uncorrelated with the different seasons, suggesting a natural origin by rock weathering. Suspended particle matter in the Nanpan and Beipan rivers was depleted in Ca, Mg, Na, K and Sr, but enriched in Al, Fe and Sb compared to the upper crust (UC) values. A binary mixture of silicate-rich and carbonate-rich sources dominates the behavior of elements such as Ca and Sr in riverine sediments, while a third end-member with an anthropogenic signature influenced the content of trace elements like Mn, Sb, Pb, Zn, Cd, Cr and Cu. The enrichments of Sb relative to the UC would be related to the mining activity in the studied area. The study highlights the importance of investigating temporal variations of major and trace element contents of rivers controlled by the monsoon climate and impacted by human activity.

Diel variation of mercury stable isotope ratios record photoreduction of PM 2.5 -bound mercury

Abstract. 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 Sept. 15th and Oct. 16th, 2015 from Beijing, China, among which 26 were collected during the daytime (between 8:00 a.m. and 6:30 p.m.) and 30 during night (between 7:00 p.m. and 7:30 a.m.). The results show that diel variation was statistically significant ( p 199 Hg and Δ 200 Hg, with Hg content during the daytime (0.32 ± 0.14 µg g −1 ) lower than at night (0.48 ± 0.24 µg g −1 ) and Δ 199 Hg and Δ 200 Hg values during the daytime (mean of 0.26 ‰ ± 0.40 ‰ and 0.09 ‰ ± 0.06 ‰, respectively) higher than during the nighttime (0.04 ‰ ± 0.22 ‰ and 0.06 ‰ ± 0.05 ‰, respectively), whereas PM 2.5 concentrations and δ 202 Hg 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 Δ 199 Hg 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 Δ 199 Hg and: (i) Δ 201 Hg (positive, slope = 1.1), (ii) δ 202 Hg (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.

Cu Isotopic Composition in Surface Environments and in Biological Systems: A Critical Review

Copper (Cu) is a transition metal and an essential micronutrient for organisms, but also one of the most widespread toxic inorganic contaminants at very high content. The research on Cu isotopes has grown rapidly in the last decade. Hitherto, a large number of studies have been published on the theoretical fractionation mechanisms, experimental data and natural variations of Cu isotopes in variable environments and ecosystems. These studies reported a large variation of δ65Cu (−16.49 to +20.04‰) in terrestrial samples and showed that Cu isotopes could be fractionated by various biogeochemical processes to different extent. Several papers have previously reviewed the coupling of Cu and Zn isotope systematics, and we give here a tentative review of the recent publications only on Cu isotopesin variable surface repositories, animals and human beings, with a goal to attract much attention to research on Cu (and other metals) behaviors in the environment and biological systems.