2005 Progress Report: Natural Mercury Isotopes as Tracers of Sources, Cycling, and Deposition of Atmospheric MercuryEPA Grant Number: R830603
Title: Natural Mercury Isotopes as Tracers of Sources, Cycling, and Deposition of Atmospheric Mercury
Investigators: Odom, A. Leroy , Landing, William , Salters, Vincent
Institution: Florida State University
EPA Project Officer: Chung, Serena
Project Period: October 2, 2002 through December 31, 2006
Project Period Covered by this Report: October 2, 2004 through December 31, 2005
Project Amount: $827,147
RFA: Mercury: Transport, Transportation, and Fate in the Atmosphere (2001) RFA Text | Recipients Lists
Research Category: Mercury , Air Quality and Air Toxics , Safer Chemicals , Air
Previously, we demonstrated that the isotopic composition of mercury in ore minerals differs from refined mercury and that isotopic changes are induced by the evaporation of mercury. For this project, we proposed to use variations in the isotopic composition of mercury as a new way of investigating natural and anthropogenic emissions of mercury into the atmosphere and of the atmospheric processes that affect transportation and deposition. We also proposed to determine the isotopic composition of atmospheric mercury: thought to be dominated by far-field sources (Olympic Peninsula, WA; Barbados; New Zealand); (2) influenced by both far-field and regional or local sources (Florida “Supersite”); and (3) influenced by known point sources (Southern Company’s coal burning generators as well as waste incinerators).
These studies include both elemental and oxidized mercury, collected total gaseous mercury (TGM), and rain. Near the Everglades, a Florida Supersite established by the U.S. Environmental Protection Agency, Florida Department of Environmental Protection (DEP), and Broward County receives mercury from both far-field and local sources. This site is also heavily instrumented for studying ambient atmospheric mercury speciation, transport, and deposition. We will collect for isotopic analyses rain samples and time integrated TGM samples for a 4-8 week interval during the summer “wet.” We shall perform stack sampling of those stacks that influence the “Supersite” and plan to include the use of Spanish moss. We shall conduct a detailed study of mercury isotope systematics of coal burning generators and local atmosphere. We will sample at mercury monitoring “Supersites” in collaboration with the Southern Company. These sites will be heavily instrumented for a variety of high-frequency measurements, which can be used as a context for the interpretation of mercury isotopes. Southern Company will also provide samples of the fuel burned at the time of the stack sampling, as well as flyash. A variety of coal types and samples in addition to those used by Southern will be analyzed for mercury isotopic composition. In addition, we will determine the mercury isotope systematics around several waste incinerators to determine their contribution to the isotopic composition of atmospheric mercury. Another important area of study will be the measurement of the isotopic ratios of mercury emissions from naturally enriched soils in collaboration with the flux chamber measurements of Dr. Mae Gustin of the University of Nevada at Reno (a letter of collaboration is on file). Ore, soil, and plant also will be isotopically measured. We are joined in this effort by Frontier Geosciences Inc. Collaborators include Southern Company, Florida DEP, Mae Gustin (University of Nevada at Reno), and Keith Hunter (University of Otago, New Zealand).
Rain samples were collected on October 14-16, 2002 at Yorkville, Georgia, in collaboration with the Southeastern Aerosol Research and Characterization mercury emission experiment at the nearby Bowen Plant (Georgia Power) in Cartersville, Georgia. Nearly 7 liters of rain were collected on 2 consecutive days. These samples are intended to be analyzed for total mercury and mercury isotopes. Subsamples were obtained from the stack emission samples collected using the Ontario-Hydro sampling scheme, also intended to be analyzed for mercury isotopes. Samples of the fuel coal have also been collected for analysis of total mercury and mercury isotopes.
One of our goals is to use Spanish moss as a passive collector and integrator of atmospheric mercury. So, the cold-vapor atomic fluorescence analytical system (CVAFS) was used to quantify the mercury content of samples of Spanish moss collected from the Tallahassee area. New growth was found to contain lower total Hg concentrations (0.01-0.04 ng/g dry weight), but the sample replicate precision (samples from the same tree) was much better than for older growth (0.03-0.09 ng/g dry). The CVAFS system was also optimized for the analysis of total mercury in various solids including sediment, soils, fish tissue, and other organic materials. The analysis is calibrated with aqueous mercury standards and an elemental gaseous mercury standard, and the accuracy is assured via analysis of standard reference materials.
Samples of atmospheric mercury have been collected on Mt. Bachelor in central Oregon. The samples were collected using sorbent traps with flow rates of about 10 liters/minute over 24-48 hours to collect from 25-50 ng/Hg/trap. Using sorbent traps spiked with known amounts of mercury, experiments were conducted to determine the best way to digest the sorbent traps for mercury isotope analyses. It is considered necessary to obtain close to 100 percent recovery to limit the possibility of inducing a mass fractionation of the isotopes in the atmospheric samples.
Samples of U.S. and international coal were selected to contain between 20 and 100 ng of mercury. These have been digested using a high-pressure asher and ultra-pure nitric acid. Samples will be measured for the isotopic composition of mercury.
As reported earlier, unanticipated technical problems were only recognized after attempting to measure the isotopic composition of the low levels of atmospheric mercury collected by standard techniques. Our high precision isotopic analyses of mercury by low-temperature secondary ion mass spectrometry (SIMS) analyses were found to be complicated by uncontrolled, nonreproducible, molecular interferences when we attempted measurements of very small samples such as atmospheric mercury. Considerable time and experiments were spent (without success) trying to identify and overcome the source of such interferences. As a consequence, we have turned to analyses by inductively coupled plasma mass spectroscopy (ICPMS). There is an inherent mass bias associated with measurements made by ICPM, so techniques have to found to reduce, correct for, or consistently reproduce this mass bias.
We began utilizing a standard-sample-standard bracketing technique, so that isotope ratios in samples could be measured as deviation from ratios in a standard rather than in absolute values. This is an effective technique as long as the instrumental mass bias can be kept constant over bracketing durations.
We have made a mercury isotopic standard solution from a sample of the Almaden cinnabar. Multiple analyses of a selected mineral specimen by multicollector SIMS has shown the specimen to be isotopically (Hg) homogeneous. A mercury sulfide precipitate from a solution of that specimen also has been repeatedly analyzed by SIMS. Isotope ratios of our standard are:
|± 0.00008||± 0.00010||± 0.00008||± 0.00002||± 0.00004||± 0.0005|
As a second isotopic standard, replicate analyses of a sulfide precipitate from the National Institute of Standards and Technology mercury elemental standard solution has also been repeatedly analyzed by SIMS. Values are:
|± 0.00008||± 0.00007||± 0.00013||± 0.00017||± 0.00005||± 0.0006|
In addition to standard sample bracketing, we have also begun to employ a double spiking technique. In this method known amounts of both mercury 198Hg (93%) and 204Hg (98%) are added to samples as tracers prior to analyses. The measured 198Hg/204Hg can then be used to correct for much of the analytically induced mass fractionation.
Mercury is introduced to the mass spectrometer as a vapor phase in the reduced state of Hg(0). The reduction is done by ultrapure 2 percent SnCl2 in Cetac HGX-200 Hydride Generation and Cold Vapor System. The HGX-200 system features a specialized gas liquid separator (GLS), reagent vessels, clearly labeled tubing and connections, solution mixing blocks and coils, and a built-in gas flow meter. The special U-shaped GLS incorporates a “frosted” glass post that provides a high surface area for liquid film evaporation and release of Hg(0). This feature helps to enhance analyte sensitivity by evolving the analytes into the gaseous form without loading the plasma with water. The GLS also features a porous polytetrafluoroethylene membrane and droplet separator to achieve complete gas/liquid separation and reduced signal noise. The sample mercury solution is typically mixed with SnCl2. The cold vapor generated from the reduction is carried on by Teflon tubes to the plasma ion source.
No future activities were reported by the investigators.