View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by PubMed Central Environ. Sci. Technol. 2009, 43, 7331–7336 Cinnabar (hexagonal, HgS) is the dominant Hg-bearing Isotopic Variability of Mercury in ore mineral in Hg mines worldwide, although minor ore Ore, Mine-Waste Calcine, and minerals such as elemental Hg (Hg0), metacinnabar, (iso- metric HgS, metastable relative to cinnabar), calomel (Hg2Cl2), · Leachates of Mine-Waste Calcine kleinite (Hg2N(Cl,SO4) n(H2O)), montroydite (HgO), and terlinguaite (Hg+Hg2+ClO)) are found in some deposits from Areas Mined for Mercury (7-10). Extraction of Hg during mining is generally carried out in a retort or a rotary furnace where Hg ore is heated to 0 ,† ‡ 600-700 °C, which converts cinnabar to elemental Hg , the SARAH J. STETSON,* JOHN E. GRAY, - RICHARD B. WANTY,‡ AND final Hg product that is sold commercially (6, 9, 11 13). DONALD L. MACALADY† Retorting of Hg-bearing ore is known to be an incomplete process, and as a result, calcine found at most Hg mines Colorado School of Mines, Department of Chemistry and 0 Geochemistry, 1500 Illinois St, Golden, Colorado 80401, contains unconverted cinnabar, elemental Hg , metacinna- and U.S. Geological Survey. P.O. Box 25046, bar, and other ionic Hg byproduct compounds formed during Denver, Colorado 80225 ore retorting (5, 7). One consequence of Hg mining is that considerable calcine is generated during ore processing and this calcine is typically discarded on the surface at mined Received March 5, 2009. Revised manuscript received June sites. Even after many years of inactivity, Hg mining areas 17, 2009. Accepted July 21, 2009. continue to be characterized by highly elevated Hg con- centrations as a result of the incomplete Hg extraction process (3, 14). Microscopic fine-grained cinnabar and several The isotopic composition of mercury (Hg) was determined in byproduct Hg compounds found in calcine can adversely cinnabar ore, mine-waste calcine (retorted ore), and leachates affect surrounding environments due to sediment and water runoff containing high concentrations of Hg. Minor byprod- obtained from water leaching experiments of calcine from uct Hg compounds such as chlorides, oxychlorides, and two large Hg mining districts in the U.S. This study is the first sulfates, which are water-soluble are generally released to report significant mass-dependent Hg isotopic fractionation downstream in mine runoff, potentially contaminating between cinnabar ore and resultant calcine. Data indicate that nearby ecosystems (15-17). Unprocessed ore and tailings δ202Hg values relative to NIST 3133 of calcine (up to 1.52‰) containing cinnabar also remain at some mines, and can be in the Terlingua district, Texas, are as much as 3.24‰ heavier a significant source of Hg to surrounding environments. than cinnabar (-1.72‰) prior to retorting. In addition, δ202Hg New analytical methods for isotopic analysis have been values obtained from leachates of Terlingua district calcines are developed to help characterize environmental cycling of Hg. isotopically similar to, or as much as 1.17‰ heavier than Measurements of the isotopic composition of Hg hold associated calcines, most likely due to leaching of soluble, promise as a basis for tracing sources of Hg contamination in the environment and evaluation of isotopic fractionation byproduct Hg compounds formed during ore retorting that are during geochemical processes (18-22). Before Hg isotopes a minor component in the calcines. As a result of the large can be evaluated as a tracer of Hg in mined areas, the fractionation found between cinnabar and calcine, and because variability in Hg isotopic composition of the Hg sources calcine is the dominant source of Hg contamination from the involved must be measured. Therefore, the objective of this mines studied, δ202Hg values of calcine may be more study was to measure the isotopic compositions of Hg in (1) environmentally important in these mined areas than the cinnabar from two areas of past Hg mining to evaluate the primary cinnabar ore. Measurement of the Hg isotopic composition variability of Hg isotopic composition within Hg deposits; of calcine is necessary when using Hg isotopes for tracing (2) the minor Hg ore minerals metacinnabar, calomel, Hg sources from areas mined for Hg, especially mine water kleinite, montroydite, and terlinguaite to evaluate potential runoff. isotopic fractionation during formation of these Hg minerals; (3) calcine to evaluate potential fractionation of Hg resulting from retorting of ore during mining; and, (4) water leachates Introduction of calcine to evaluate the variability in isotopic composition of the Hg in simulated runoff from Hg mines. Two mining Mercury is a common contaminant in the environment due districts of significant past Hg production, McDermitt, to both natural and anthropogenic sources. It is of envi- Nevada, and Terlingua, Texas were selected for study. ronmental concern because elevated concentrations can be toxic to living organisms (1). Mercury has no known metabolic function in animals and is not easily eliminated by organisms, Study Areas including humans (2). Thus, exposure to Hg is considered Terlingua, Texas. Mines of the Terlingua district are found undesirable and potentially hazardous (1). in southwest Texas, in and around the town of Terlingua Areas around mined and unmined deposits of Hg contain (29° 19.170′, 103° 37.000′). Mercury was mined in this area some of the highest concentrations of Hg worldwide (3-5). from 1888 until 1973 (23, 24). Total production from this Abandoned and inactive Hg mines are of environmental region was >5 000 000 kg of Hg (∼150 000 flasks of 34.5 kg concern because of the high concentration of Hg present in each) (23). Consequently, of the Hg mines in the U.S., the discarded wastes at these sites. There are presently no Hg Terlingua district produced the third largest amount of Hg mines operating in the United States, primarily because of and only mines in the California Coast Ranges (120 000 000 low demand for Hg and environmental concerns (6). kg of Hg) and McDermitt, Nevada (10 000 000 kg of Hg) produced more (4, 25). The Terlingua district includes over * Corresponding author e-mail: [email protected]. 30 separate mines (10), but our work focused on the Mariscal, † Colorado School of Mines. Study Butte, Mariposa, and Terlingua (also known as the ‡ U.S. Geological Survey. Chisos) mines. As is true for most Hg mines worldwide, the 10.1021/es9006993 CCC: $40.75 2009 American Chemical Society VOL. 43, NO. 19, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 7331 Published on Web 08/26/2009 Hg ore in the Terlingua district is dominantly cinnabar, but which was generated from Hg2+ in solution using stannous minor Hg minerals, including metacinnabar, elemental Hg0, chloride in a commercially available cold vapor (CV) genera- calomel, montroydite, terlinguaite and kleinite were identified tion system (HGX-200, Cetac, Omaha, NE). Argon carrier gas at some mines (10, 23). The district has been estimated to (sample gas) was introduced into the CV generator at a flow contain over 2 000 000 m3 of calcine (15). This calcine contains rate of 50-60 mL/min and stannous chloride reductant and Hg concentrations as high as 19 000 µg/g (15), some of which sample solution, each at a rate of 1.5 mL/min were mixed are water-soluble (5, 7). Leaching, runoff, and downstream with the Ar gas. A makeup Ar gas flow of 0.9-1.0 L/min was transport of Hg from calcine potentially deliver significant added prior to introduction into the plasma. Background Hg to surroundings streams, which eventually flow into the signal (3% (v/v) HCl blank) was measured on each isotope Rio Grande (15). peak for 60 s and subtracted from the peak integrated data. McDermitt, Nevada. Mercury was mined at McDermitt Signal data were measured via time-resolved analysis software (also known as Cordero, 41° 54.970′, 117° 48.600′) from 1935 for approximately eight minutes and data were integrated to 1992, when the mine closed (6). McDermitt was the largest for approximately five minutes. The sensitivity of the MC- Hg mine in Nevada, and production of Hg was about ICP-MS for 202Hg was 700-1000 mV (µg/L) and data were 10 000 000 kg (25). Ore was dominantly cinnabar, but minor collected using solution THg concentrations of 1-2 µg/L. 0 elemental Hg and oxychlorides (e.g., Hg2ClO and Hg4Cl2O) Each sample was bracketed by NIST 3133 Hg standard at the were found locally (9). Numerous piles of calcine are scattered same THg as the sample and sample data were corrected for throughout the McDermitt mine site, but have been estimated mass bias using standard sample bracketing correction. The as >1 000 000 m3 (8). Similar to the Terlingua mines, high Hg standard sample bracket method was optimized to ensure concentrations (up to 1400 µg/g) in calcine at McDermitt that the precision was comparable to previously published potentially lead to Hg leaching and transport of Hg down- studies that used either standard-sample bracketing or Tl stream from the mined areas (8). correction for mass bias (18, 28, 29). Sensitivity for 202Hg was comparable to or greater than that of other published Experimental Section methods (28-30). Studies were performed to determine how Study Materials. All of the calcine and most of the cinnabar closely standard and sample concentrations and matrix samples from the McDermitt mine and Terlingua district should to be matched to obtain optimal precision. Common analyzed in this study were collected during previous studies matrix constituents did not cause mass bias in the measure- (8, 15, 26). All calcine was collected as grab samples about ments when no matrix matching between standards and 25-50 cm below the surface to avoid the highly oxidized samples was performed.