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Mercury isotope signatures as tracers for Hg cycling at the New Idria Hg mine Jan Georg Wiederhold, Robin Sue Smith, Hagar Siebner, Adam Jew, Gordon E. Brown, Bernard Bourdon, and Ruben Kretzschmar Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es305245z • Publication Date (Web): 10 May 2013 Downloaded from http://pubs.acs.org on May 18, 2013

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Environmental Science & Technology

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Mercury isotope signatures as tracers for Hg cycling at the

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New Idria Hg mine

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Jan G. Wiederhold1,2*, Robin S. Smith1,2, Hagar Siebner3, Adam D. Jew3, Gordon E. Brown, Jr.3,

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Bernard Bourdon2,4, Ruben Kretzschmar1

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1

Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Switzerland

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Isotope Geochemistry Group, Institute of Geochemistry and Petrology, ETH Zurich, Switzerland

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Department of Geological and Environmental Sciences, Stanford University, Stanford, CA, USA

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Laboratoire de Géologie de Lyon, ENS Lyon, CNRS and UCBL, Lyon, France

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*

Corresponding author e-mail: [email protected]; phone: +41-44-6336008; fax: +41-44-6331118

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Abstract

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Mass-dependent (MDF) and mass-independent (MIF) fractionation of Hg isotopes provides a new tool

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for tracing Hg in contaminated environments such as mining sites, which represent major point sources

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of Hg pollution into surrounding ecosystems. Here, we present Hg isotope ratios of unroasted ore waste,

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calcine (roasted ore), and poplar leaves collected at a closed Hg mine (New Idria, CA, USA). Unroasted

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ore waste was isotopically uniform with δ202Hg values of -0.09 to 0.16‰ (±0.10‰, 2SD) close to the

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estimated initial composition of the HgS ore (-0.26‰). In contrast, calcine samples exhibited variable

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δ202Hg values ranging from -1.91‰ to +2.10‰. Small MIF signatures in the calcine were consistent

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with nuclear volume fractionation of Hg isotopes during or after the roasting process. The poplar leaves

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exhibited negative MDF (-3.18 to -1.22‰) and small positive MIF values (∆199Hg of 0.02 to 0.21‰).

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Sequential extractions combined with Hg isotope analysis revealed higher δ202Hg values for the more

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soluble Hg pools in calcines compared with residual HgS phases. Our data provide novel insights into

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possible in-situ transformations of Hg phases and suggest that isotopically heavy secondary Hg phases

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were formed in the calcine, which will influence the isotope composition of Hg leached from the site.

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ACS Paragon Plus Environment


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Introduction

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Mercury (Hg) has been listed by the World Health Organization as one of the top ten chemicals of

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public health concern1 and a global mercury treaty coordinated by the United Nations Environmental

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Programme will be launched in 2013.2 Mercury poses a serious threat for human and ecosystem health

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at local and global scales, especially in its methylated form, which bioaccumulates in natural food webs.

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However, all Hg species are toxic and can be transformed by various abiotic and biotic processes and

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transported in particulate, dissolved, or gaseous forms in the environment.3,4 The modern global Hg

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cycle is dominated by anthropogenic influences, and current research is focused on understanding the

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complex interplay of different natural and anthropogenic sources and processes controlling

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biogeochemical Hg cycling and its environmental impact on ecosystems worldwide.5

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Despite its toxicity, Hg has a long history of commercial use for various applications and has been

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mined since Roman times.6 Mercury is a very rare element in the Earth’s crust (< 0.1 µg g-1), but occurs

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locally in ore deposits at much higher concentrations mainly in the form of cinnabar (α-HgS).7 These

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deposits are globally distributed with the two largest being located in Spain (Almaden) and Slovenia

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(Idrija). The two largest and historically most productive Hg mines in North America are located in the

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California Coast Range mercury belt8 in the New Almaden and New Idria deposits, which were named

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after the European mines. All of these Hg mines are closed today, but the legacy of the intense mining

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activities is still present at the former mining sites and in the surrounding ecosystems. Abandoned Hg

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mines are major point sources for Hg emissions, mainly due to the presence of large volumes of calcine,

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the residue of the on-site roasting process, which was deposited in tailings at the mine sites.9 Emissions

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occur in gaseous form into the atmosphere10 and in particle-bound or dissolved form in the mine

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drainage11 and leaching of tailing piles12 into aqueous environments13. Despite intense research on Hg

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emissions from Hg mines worldwide9,14-18, many questions concerning the long-term fate of Hg and its

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environmental impact still remain unanswered.

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Stable Hg isotope analysis offers a new tool for addressing some of these questions by providing

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characteristic “fingerprint” signatures of sources and processes in biogeochemical Hg cycling. Due to

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recent analytical advances, especially in multicollector inductively-coupled plasma mass spectrometry

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(MC-ICPMS), high-precision analysis of many “non-traditional” stable isotope systems19,20 including

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Hg21,22 have become feasible. The rapid development of stable Hg isotope geochemistry during the last

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few years was additionally sparked by the discovery of mass-independent fractionation (MIF)23

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affecting mainly the two odd-mass Hg isotopes

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fractionation (MDF) influencing all seven stable Hg isotopes (196Hg, 198Hg, 199Hg, 200Hg, 201Hg, 202Hg,

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and

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associated with kinetic radical-pair mechanisms in photochemical processes in which the two odd-mass

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Hg isotopes possessing nuclear spin and magnetic moment can be strongly fractionated. Smaller MIF of

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Hg isotopes can also originate from nuclear volume effects25 caused by the non-linear increase of

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nuclear charge radii with mass, again affecting mainly the two odd-mass Hg isotopes. The two effects

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can be distinguished by their different extent of MIF for

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MDF signature obtained from the

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samples. Experimental work and an increasing number of field studies have revealed that Hg isotopes

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are fractionated by kinetic and equilibrium effects during abiotic and biotic processes and that

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significant variations exist in the Hg isotope composition of samples from various environments (see

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recent reviews26-28). These variations form the basis for the application of Hg isotopes as source and

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process tracers of biogeochemical Hg cycling in nature. Despite the impressive progress over the last

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years, many processes and mechanisms controlling the distribution of Hg isotopes in natural samples are

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still poorly understood, and case studies from field systems with well-defined Hg sources coupled with

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previously studied transformation processes are needed to further develop this novel isotopic tool.

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Mining environments offer an ideal field system for this purpose because the dominant Hg species and

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pathways are relatively well-known, in addition to the high Hg concentrations and the environmental

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199

Hg and

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Hg, in addition to mass-dependent

Hg). Large MIF of Hg isotopes can be caused by magnetic isotope effects,24 probably mainly

202

199

Hg and

201

Hg,26 which, together with the

Hg/198Hg ratio, provides an “isotopic fingerprint” of Hg in natural

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relevance of these sites as detailed above. The pioneering study of Stetson et al.29 investigating Hg

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isotope ratios of samples from Hg mines in Nevada and Texas (USA) has already revealed that

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significant differences exist between different materials, which were at least partly caused by the ore

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processing. Calcine samples, although exhibiting a relatively wide range of Hg isotope compositions,

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were found to be mainly enriched in heavy isotopes relative to the respective cinnabar ore samples.

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Interesting isotopic variations were also reported for leachates (single step at pH 5) of calcine samples

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and for museum specimens of several Hg-containing minerals, which could occur as trace constituents

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in calcine. Recently, Yin et al.30 reported isotopic differences between unprocessed Hg ores and calcine

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materials from the Wanshan Hg mine (SW China) with an enrichment in heavy Hg isotope in the

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calcine samples of on average +0.8‰ in δ202Hg, which was interpreted as being caused by the roasting

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process. Our study builds upon the data reported in these earlier studies by presenting Hg isotope data of

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different materials collected at the New Idria Hg mine, California (USA). The specific objectives of the

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study were (1) to investigate the Hg isotope composition of unroasted ore waste and roasted calcine, (2)

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to apply sequential extraction methods to assess the isotope signatures of individual Hg pools, thereby

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allowing us to gain insights into in-situ transformations of Hg species in calcine during or after the

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roasting process, and (3) to report Hg isotope ratios of poplar leaf samples collected at the site, which

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are presumably influenced by the isotope composition of Hg emitted to the atmosphere.

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Materials and Methods

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Description of Sampling Site. The New Idria Hg mine, which is a silica-carbonate type Hg deposit in

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San Benito County, California, USA, was in operation between 1854 and 1972 and was the second

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largest Hg mine in North America producing a total of ~20,000 tons of elemental Hg.31-35 The HgS-rich

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rock was mined in underground tunnels, crushed to smaller pieces (generally < 5 cm), and roasted

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on-site in rotary furnaces or retorts at ~600-700°C, producing gaseous elemental Hg (HgS(s) + O2(g) →

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Hg0(g) + SO2(g)) which was trapped in condenser coils and bottled for sale (e.g., used in gold mining and 4


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various industrial applications). The Hg removal process was incomplete and the roasted calcine, which turned reddish due to the formation of ferric iron oxide minerals such as hematite, was piled on-site in tailings that still contained high Hg concentrations. As a result of the mining activities, an estimated volume of 0.38 to 1.5 million m3 of waste rock and calcine materials is spread today over an area of ~162,000 m2 at the New Idria site.35 The site was placed on the National Priority List of the US Environmental Protection Agency (“Superfund list”) in September 2011 and remediation actions with the goal to minimize the environmental impact (e.g., acid mine drainage from open adit) are currently underway.36 However, at the time of sampling for this study, the site was still in a state similar to the situation after the mine closure in 1972. Previous studies have investigated various aspects of Hg cycling at New Idria12,13,37,38, and Hg isotope ratios from a limited number of samples from New Idria were already reported in compilations of signatures for cinnabar ores,22 rocks from the California coast ranges,39 and potential Hg sources for San Francisco Bay. 40 Sampling and Sample Preparation. Unroasted ore waste from seven locations (O1 to O7) and calcine from nine locations (C1 to C9) were collected as grab samples at New Idria between March 2009 and April 2011 (Supporting Information, SI Fig. S1). At some locations, multiple samples were collected, resulting in a total number of ten unroasted ore waste (grey) and fourteen calcine (red) samples. Six leaf samples of poplar trees (Populus spec.) were collected 1 to 1.75 m above ground from different locations at New Idria (SI Fig. S1) in April 2008 and May and October 2009, washed thoroughly using deionized water, freeze-dried, ground, homogenized, and stored in a desiccator before further treatment. A sample of the original HgS ore from the New Idria mine was obtained from a Stanford University Research Mineral Collection. Many of the larger calcine samples exhibited a pronounced internal layer structure with a black core, a grey rim, and a red rind at the surface that must have formed during the roasting process. A detailed small-scale investigation on these layered calcine materials, including Hg isotope variations between the individual layers will be presented in a separate paper. In the present study, all calcine samples represent bulk materials that were homogenized by grinding. Elemental 5



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concentrations in the ore waste and calcine samples were determined by energy-dispersive X-ray fluorescence spectrometry (Spectro-X-Lab 2000). For the ore waste and calcine samples, ~500 mg of powdered material was digested with aqua regia and BrCl. For the poplar leaf samples, 1 g of freezedried, ground material was digested with H2SO4/HNO3 (see SI for details). For the sequential extractions, ~1 g of powdered ore waste or calcine material was leached first with 25 mL of a 0.1 M acetic acid / 0.01 M HCl solution (F1: simulated “stomach acid”-step targeting more soluble Hg forms) and placed on an end-over-end shaker overnight before centrifugation and filtration with 0.45 µm filters. The second extraction step was performed in an analogous manner using 12 M HNO3 (F2: targeting all remaining non-sulfidic Hg species), before the residual material was digested with aqua regia and BrCl as described above (F3: containing the remaining stable HgS phases). This three-step sequential extraction procedure corresponds to the extraction steps F2, F4, and F5 of the established five-step method published by Bloom et al..41 Step F1 (water extract) was omitted to yield sufficiently high Hg concentrations for Hg isotope analysis in the first extraction step (thus containing F1+F2 after Bloom et al.). Step F3 of Bloom’s procedure (1 M KOH) was omitted as we assumed there was no significant organically bound Hg pool in the investigated ore waste and calcine samples. Concentration and Isotope Analyses. Total Hg concentrations in all digest and extract solutions were measured by cold vapor atomic fluorescence spectrometry (CV-AFS, Millennium Merlin, PS Analytical). All sample and standard solutions were diluted to 20 µg L-1 Hg prior to isotope analysis, except for the poplar leaf digests which were diluted to 10 µg L-1 Hg due to their lower Hg contents. Stable Hg isotope analysis was performed by MC-ICPMS (Nu Plasma, Nu instruments) with cold-vapor introduction (HGX-200, Cetac) and online Tl addition with a desolvating nebulizer for mass-bias correction in addition to standard-bracketing with NIST-3133 following our published methods42,43 which are based on previous method developments44,45. The results are reported in permil (‰) as δ202Hg = [(202Hg/198Hg)sample / (202Hg/198Hg)NIST-3133] - 1 for mass-dependent fractionation (MDF) and

(

)

(

)

∆199 Hg = δ 199 Hg − δ 202 Hg × 0.2520 and ∆201 Hg = δ 201 Hg − δ 202 Hg × 0.7520 for mass-independent 6


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fractionation (MIF) of Hg isotopes, following recent recommendations for data reporting.45,46 The reproducibility based on replicate measurements of our in-house standard (ETH Fluka Hg) in the same analytical sessions was ±0.10‰ for δ202Hg and ±0.04‰ for ∆199Hg and ∆201Hg (2SD, n=11). The accuracy of our data was verified by analyses of the UM-Almaden standard and digests of the reference material NIST-2711 (Montana soil) yielding results that were in excellent agreement with published values (SI Table S3). Further information about method details and data quality control measures is provided in the SI.

Results and Discussion. Hg Concentrations. The samples exhibited a wide range of Hg concentrations in the unroasted ore waste (17 to 943 µg g-1) and in the calcine (1.7 to 665 µg g-1) (Fig. 1), but all values were highly elevated relative to background values for Hg in uncontaminated rocks and soils (< 0.1 µg g-1).7 The unroasted ore waste samples, collected from the large grey waste pile close to the mine adit, likely represented host rock material from the underground tunnels which was considered not to be rich enough in Hg for ore processing. The Hg contents of the calcine samples represented the residual amounts of Hg after incomplete removal during thermal processing of the high-grade HgS-rich ore materials. Further geochemical differences between the unroasted ore waste and the calcine samples, in addition to the striking difference in color, can be derived from the element concentrations measured by XRF (SI Fig. S7). As expected, the calcine samples exhibited generally lower S and higher Fe contents compared with the unroasted ore waste samples. The Hg concentrations in all samples did not correlate with the concentrations of major elements but exhibited significant correlations with selected trace elements (e.g., Br, Tl, Se), which suggests that these elements were hosted together in similar mineral phases. The initial Hg concentration of the processed ore material before roasting is unknown and was likely heterogeneous. The New Idria HgS ore sample from the mineral collection exhibited a Hg content of 49% which can be taken as a maximum estimate of the initial ore concentration. According to historic 7



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sources, the initial high-grade ore material processed at New Idria contained up to 10% Hg, but later averages range from 0.5 to 1%, and it was reported that the New Idria furnaces even allowed profitable mining of ore materials containing only 0.1% Hg.32 In addition, the relative fraction of Hg removal varied during different periods of the mining history at New Idria. Retort furnaces with lower extraction efficiencies were replaced in 1918 by more efficient rotary kilns and some of the previously deposited tailing materials were re-processed for further Hg extraction35. It was impossible, however, to relate the sampled calcine materials to specific processing times or methods. Another rough estimate of the average initial Hg concentrations in the roasted materials can be obtained by comparing the total amount of produced Hg in New Idria during the operation of the mine with the estimated volumes of deposited waste rock and calcine materials at the site35 yielding values of approximately 1-5% Hg (or 10,000 to 50,000 µg g-1), consistent with the other estimates above. In any case, the extraction of Hg during roasting was incomplete, leaving behind significant amounts of residual Hg in calcines with contents ranging between 1 to 1000 µg g-1. Thus, although highly variable, the residual fraction in the calcines likely corresponded to 90% of the total Hg pool from the ore into the elemental form (despite being not fully quantitative), the produced elemental Hg cannot be expected to have been strongly fractionated due to mass balance considerations and presumably exhibited an average Hg isotope composition which was similar to the initial ore value. This scenario does not exclude the possibility that significant Hg isotope fractionation occurred during and after the roasting process, impacting the signatures of the smaller Hg pools which were emitted to the atmosphere or remained in primary or secondary phases in the calcine materials. The variability of Hg isotope signatures in the calcine samples clearly suggests that the roasting process caused significant Hg isotope fractionation. It is important to note, however, that the potential to create an isotopically different residual pool only by preferential removal of light Hg isotopes during roasting is rather limited. This is because the Hg extraction process probably progressed along a moving reaction front into the roasted ore fragments during the calcination process (the ore fragments stayed about 30 min. in the rotary furnaces), where a quantitative conversion to elemental Hg on the outside contrasted with the less-affected inner parts of the ore fragments in which some HgS phases presumably remained more or less intact (“nugget effect”, as also proposed in Stetson et al.29). Thus, a kinetic Rayleigh-type fractionation model, which requires a homogenous, well-mixed substrate, does not adequately describe the evolution of isotope ratios during the calcination process, as also described in other isotopic studies investigating the evaporation of minerals.59 In addition, as already noted above, the observed Hg isotope ratios in the calcine samples did not correlate with the Hg concentrations and exhibited both enrichments and depletions in heavy Hg isotopes (Fig. 1). We propose, based on the data reported here and in agreement with the study by 13



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Stetson et al.,29 that the Hg isotope variations observed in the calcine samples were dominantly caused by the formation of isotopically fractionated secondary Hg phases during and/or after the roasting process. This inference is corroborated by the results of the sequential extraction analyses. If the preferential removal of light Hg isotopes from HgS phases during the roasting process were primarily responsible for the observed isotopically heavy calcine signatures, then positive δ202Hg values should be found mainly in the residual HgS phases probed by the F3 step of the sequential extraction. However, this fraction exhibited the smallest fractionations relative to the initial ore composition and was always isotopically lighter than the bulk calcine samples. The isotopically heavy calcine values are thus dominantly caused by the signatures of the more soluble Hg phases which dissolved in the first two extraction steps and most likely represent secondary mineral phases formed during or after the calcination process. Secondary “by-product” minerals (e.g., montroydite, HgO; eglestonite, Hg6Cl3O2H; corderoite, Hg3S2Cl2, and elemental Hg as found by Kim et al.37 and Jew et al.38) exhibit a much higher solubility compared with HgS phases and contain Hg in the mercuric (HgII) or mercurous (HgI) oxidation state, which would be expected to be isotopically heavy relative to elemental Hg vapor.25,42 Thus, the isotopically heavy signatures of the first extracts (especially F1) are consistent with the concept that secondary Hg phases formed in reactions with the large elemental Hg pool (unfractionated relative to the initial ore due to mass balance constraints) potentially during the cooling of the calcine pieces after leaving the furnaces. The exact geochemical conditions and pathways of Hg in the furnaces are impossible to reconstruct in detail, but it is likely that small Hg pools, which were not extracted from the calcine pieces and transported in gaseous elemental form to the condenser coils, were strongly affected by physical transport limitations within the cm-sized calcine fragments (e.g., diffusion controlled transport). In addition, re-condensation and re-oxidation reactions during and after passage through the rotary furnaces (~30 min) and the subsequent cooling of the fragments, which occurred from the outside to the inside, might have influenced the system. Thus, it is not surprising that these 14


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residual Hg phases in calcine exhibit large variations in their Hg isotope composition considering that multiple redox changes, changes in the coordination environment, and incomplete physical separations of Hg pools by transport processes can all be expected to exert an influence on the Hg isotope distribution. Furthermore, our calcine data indicate that the Hg isotope fractionation related to the roasting process can shift δ202Hg values in both directions. A recent study reported only enrichments of heavy Hg isotopes in calcine relative to unroasted ore.30 However, Stetson et al.29 also found light Hg isotope ratios in some calcine samples presumably influenced by different fractionation mechanisms and Hg transport pathways, which are, as in our study, likely to vary between samples considering the complexity and heterogeneity of the geochemical conditions during and after the roasting processes as discussed above.The discovery of small MIF signatures in calcine relative to the unfractionated ore and unroasted ore waste suggests that at least some of the fractionating processes were accompanied by a small mass-independent component. Considering the magnitude of the observed effects and the “opposite trend” of MDF and MIF signatures (Fig. 2 for bulk samples, Fig. 4 and SI Fig. S5 for extracts), it is likely that nuclear volume fractionation (NVF), for instance during the reaction between elemental Hg and secondary oxidized Hg phases, was involved, which would be consistent with theoretical predictions.25,42 An accurate determination of the slope in the ∆199Hg/∆201Hg plot (SI Fig. S6) allowing further insight into the underlying MIF mechanisms is impeded by the small magnitude of the effects and the relatively large scatter in the data. The data points were in general agreement with a slope of 1.65 characteristic for NVF,42,58 but a potential influence of magnetic isotope effects causing slopes between 1.0 and 1.323 cannot be completely excluded, although there is no evidence for the involvement of radical-pair reactions or photochemical processes in the history of the studied calcine samples. Hg Isotope Signatures in Poplar Leaves. The Hg concentrations of the poplar leaves collected in New Idria (sampling locations see SI Fig. S1) were about an order of magnitude higher than the control sample of poplar leaves collected several kilometers away from the mine site. It was not possible with 15



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our available methods to analyze the isotopic composition of the control sample due to its low Hg concentration. However, the concentration difference indicated that the poplar leaves collected at the mine site were strongly influenced by the presence of the Hg mine. The observed strong enrichment of light Hg isotopes in poplar leaves from New Idria (-3.18 to -1.22‰ in δ202Hg) can be explained by different potential pathways. Fractionation during uptake through the root system and/or preferential translocation of light isotopes to the leaves has been demonstrated for other metal isotope systems60 such as Ca61, Fe62, Cu and Zn63. However, plant leaves absorb Hg under field conditions predominantly in gaseous form from the atmosphere, whereas upward transport from the root system to the leaves is generally believed to be of minor importance.48 The dominance of atmospheric uptake pathways for Hg in plant leaves has been clearly demonstrated in many studies.64-68 Poplars are deciduous trees and thus, the Hg in our leaf samples originated from recent emissions during the last growing season. Gaseous elemental Hg, which has been generated by reduction and/or volatilization processes, can be expected to be enriched in light Hg isotopes based on experimental studies.26-28 Thus, our poplar leaf data could be explained by the uptake of elemental Hg from the near-surface atmosphere at New Idria, likely enriched in light Hg isotopes by evasion processes from the mine tailings. However, Hg isotope fractionation could occur during different steps of the Hg transport from the mine tailings through the atmosphere into the poplar leaves. The observed mass-dependent enrichment of light Hg isotopes in the poplar leaves appeared to correlate with decreasing Hg concentrations in the leaves (R2 = 0.69) (Fig. 1). Such a relationship could be caused for instance by changes in the isotopic composition of the atmospheric Hg pool, fractionation during uptake of atmospheric Hg, or mixing of different Hg pools in the leaves. In addition, we cannot exclude the possibility that re-emission processes further influenced the Hg isotope composition of the poplar leaves. The first Hg isotope data from samples of higher plants were published by Ghosh et al.69 who reported large negative MIF effects for Tillandsia samples (“Spanish moss”), but their analytical method involving an internal normalization did not allow the extraction of the MDF signature. Recently, 16


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Demers et al.70 reported a δ202Hg range of -2.53 to -1.89‰ for leaves of aspen trees (belonging to the same genus as poplar) from a forest ecosystem experiment in Wisconsin (USA), which lies within the range we observed for poplar leaves in this study. The authors were able to analyze the Hg isotope composition of total gaseous Hg in the atmosphere (0.48 to 0.93‰) and concluded that the uptake step of Hg from the atmosphere into the aspen leaves resulted in a large shift in δ202Hg of about -2.9‰. Another recent study by Yin et al.71 presented Hg isotope data of rice plants from the Wanshan Hg mining district in China. The rice leaves exhibited δ202Hg values of -2.88 to -3.38‰ compared with atmospheric total gaseous Hg (-1.85 to -2.32‰) and lichens (-1.83 to -2.32‰), suggesting a fractionation of about -1.0‰ during uptake of atmospheric Hg into the rice leaves, but not during uptake into the lichens. In previous studies, Hg isotope data of lichen samples exhibiting negative MDF and MIF signatures have been reported and used as an estimate of the atmospheric composition72 and for air pollution tracing73,74. Our poplar leaf data are generally consistent with the concept that the local atmospheric Hg in New Idria is enriched in light Hg isotopes, but we are not able to provide a definite answer to this issue, due to the lack of direct atmospheric Hg isotope data and the high probability of superimposed fractionation effects during uptake into the leaves.70,71 Considering the potential pathways of Hg into the poplar leaves at the New Idria site, it is very likely though that the evaporation of gaseous elemental Hg from tailing piles represented the most important source. However, this interpretation should be confirmed in future studies which will have to investigate in more detail the Hg isotope systematics of uptake into plant leaves and potential re-emission processes. The relatively small magnitude of the MIF signatures (∆199Hg of 0.02 to 0.21‰) measured in our poplar leaf samples does not indicate a major influence of photochemical processes involving magnetic isotope effects on the Hg evasion and atmospheric transformation processes at New Idria. No correlation with Hg concentrations was found for the MIF signatures (SI Fig. S4). Interestingly, all our poplar leaf samples exhibited positive ∆199Hg values, which constrasts to the mostly negative MIF values reported for leaf and lichen samples in previous studies69-74. This difference could be related to atmospheric 17



1 2 3 4 424 5 6 425 7 8 426 9 10 11427 12 13428 14 15429 16 17 18430 19 20431 21 22432 23 24 433 25 26 27434 28 29435 30 31 436 32 33 34437 35 36438 37 38439 39 40 41440 42 43441 44 45442 46 47 443 48 49 50444 51 52445 53 54 446 55 56 57447 58 59448 60

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processes or other site-specific parameters in our field site, but we are not able to provide a clear explanation based on the available data. Environmental Implications. The Hg isotope data reported in this study represent an important step towards understanding Hg isotope variations in mining environments. The variable Hg isotope signatures of calcines were shown to be caused mainly by secondary Hg phases that formed during or after the roasting process. In contrast, the preferential removal of light isotopes during the incomplete extraction of the HgS ore likely played only a minor role in causing the calcine isotope composition. Overall, our data indicate that Hg isotopes can indeed provide important new insights about Hg cycling in contaminated environments such as mining sites. The analysis of Hg pools separated by sequential extractions, performed in parallel with bulk analyses to verify the isotopic mass balance, allowed, for the first time, the resolution of in-situ differences between different components of the Hg isotope signature of a bulk sample. We suggest that this is a promising approach for future Hg isotope studies in other environmental systems. Obviously, sufficiently high Hg concentrations are required to allow the separation into different pools, but current analytical advances and the development of pre-enrichment steps will broaden the range of sequential extractions in future Hg isotope studies. The finding that the more soluble Hg pools of the calcine samples were enriched in heavy Hg isotopes compared with the bulk composition suggests that Hg emissions in dissolved forms caused by drainage of calcine tailings will most likely be strongly enriched in heavy Hg isotopes, consistent with the conclusion of Stetson et al.29 that the signature of leached Hg from mining environment can be expected to differ significantly from the initial ore signature. Finally, our poplar leaf data suggest that vegetation taking up atmospheric Hg at contaminated sites is characterized by strong mass-dependent enrichments in lighter Hg isotopes and slightly positive MIF values, which is probably related to the isotope composition of gaseous elemental Hg evading from tailing piles as well as potential fractionation effects during uptake into the leaves, but this interpretation requires confirmation and mechanistic clarification by future studies.

18


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Acknowledgements We thank Martin Jiskra for help in the MC-ICPMS lab and various additional support, Patrick Früh and Kurt Barmettler for support in the soil chemistry lab, and Joel Blum and two anonymous reviewers for helpful comments. This study was supported in part by ETH Zurich (Grant No. ETH-15-09-2), by the Stanford-NSF Environmental Molecular Science Institute (NSF-CHE—0431425), and by the NSF Center for Environmental Implications of Nanotechnology (NSF Cooperative Agreement EF-0830093). Supporting Information The Supporting Information (SI) contains a map of the sampling site, additional method descriptions, figures, and data tables. This material is available free of charge via the Internet at http://pubs.acs.org.

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2.5 unroasted ore waste [μg g-1]

2.0

calcine [μg g-1]

1.5

poplar leaves [ng g-1]

1.0 0.5

δ202Hg [‰]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38641 39 40 41642 42 43 643 44 45 46644 47 48645 49 50 51 52 53 54 55 56 57 58 59 60

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0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0 -3.5 0

200

400

600 800 Hg concentration [µ µg g-1 or ng g-1]

1000

Figure 1: Total Hg concentrations vs. δ202Hg in unroasted ore waste (grey diamonds), calcine (red squares) and poplar leaves (green triangles) from New Idria. Please note that concentrations for unroasted ore waste and calcine are in µg g-1 and for poplar leaves in ng g-1. Isotope data are reported relative to NIST-3133 and error bars for δ202Hg of ±0.10‰ (2SD) are smaller than symbol sizes.

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0.25 HgS ore

0.20

unroasted ore waste calcine

0.15

poplar leaves

0.10

∆199Hg [‰]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33646 34 35 36647 37 38 648 39 40 41649 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.05 0.00 -0.05 -0.10 -0.15 -4

-3

-2

-1

0

1

2

3

δ202Hg [‰] Figure 2: Hg isotope signatures in samples from New Idria with mass-dependent fractionation (δ202Hg) and mass-independent fractionation (∆199Hg) relative to NIST-3133. Error bars plotted for HgS ore indicate 2SD reproducibility for all samples.

25



unroasted ore waste

calcine

1.0

relative fraction of total Hg

0.8

0.6

F3 F2 F1

0.4

0.2

C1 C2 C3 C4 C5a C5b C6

0.0

O1g O1r O2 O3 O4 O5a O5b O6 O7a O7b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38650 39 40 41651 42 43652 44 45 46653 47 48654 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 3: Relative distribution of Hg pools in unroasted ore waste and selected calcine samples from New Idria based on operationally defined sequential extraction: F1 (0.1 M HAc / 0.01 M HCl), F2 (12 M HNO3), and F3 (aqua regia). Total Hg concentrations of the samples are given in the SI (Table S1).

26


C1

C3

C4

C6

C4 F1 C4 F2 C4 F3 C4 total calc. total


O4


3.0 2.5 2.0

δ202Hg [‰]

1.5 1.0 0.5 0.0 -0.5 -1.0

O4 F1 O4 F2 O4 F3 O4 total calc. total

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32655 33 34 35656 36 37657 38 39658 40 41 42659 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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Figure 4: Hg isotope signatures (MDF) in sequential extracts of one unroasted ore waste sample (O4) and four calcine samples (C1, C3, C4, C6) from New Idria. Total digest data (total) are plotted for comparison. The isotope mass balance (calc. total) is based on the relative pool sizes of the three fractions. Error bars indicate 2SD reproducibility (for calc. total with error propagation).

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TOC/Abstract Art:

elemental Hg0

Hg isotopes: MDF + NVF secondary Hg(II) phases

Hg for sale

HgS ore

Hg0

calcination

HgS

~700 °C

incomplete Hg removal

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Mercury Isotope Signatures as Tracers for Hg ... - ACS Publications

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