Grantee Research Project Results
2000 Progress Report: Photo Induced Reduction of Mercury in Lakes, Wetlands, and Soils
EPA Grant Number: R827632Title: Photo Induced Reduction of Mercury in Lakes, Wetlands, and Soils
Investigators: Nriagu, Jerome O. , Keeler, Gerald J. , Zhang, Hong , Lindberg, Steve
Current Investigators: Nriagu, Jerome O. , Keeler, Gerald J. , Zhang, Hong , Lehrnan, John , Lindberg, Steve , Qang, Xia-Qin
Institution: University of Michigan , Oak Ridge National Laboratory
EPA Project Officer: Chung, Serena
Project Period: September 1, 1999 through August 31, 2002 (Extended to September 30, 2003)
Project Period Covered by this Report: September 1, 1999 through August 31, 2000
Project Amount: $865,771
RFA: Mercury: Transport and Fate through a Watershed (1999) RFA Text | Recipients Lists
Research Category: Watersheds , Heavy Metal Contamination of Soil/Water , Water , Safer Chemicals
Objective:
Objective(s) of the Research Project:This project is designed to provide a comprehensive evaluation of photoinduced formation of elemental mercury, Hg(0), in a watershed. Planned studies include field measurements, in situ incubation experiments, and simulations in lab microcosms. The experimental sites include a lake, wetlands and soils. Focus of the investigations include photo-dependent biotic, abiotic, physical or chemical processes while the reaction mechanisms of interest may be homogeneous or heterogeneous. An over-aching aim is to compare the similarities and differences in pathways and rates of Hg(0) formation in different segments of a watershed. Saginaw Bay to Lake Huron system has well defined gradients in chemical parameters (including chlorophyll-a, DOC, phosphorus, nitrogen, chloride and suspended particulates), trophic conditions, taxonomic composition and biomass and provides a unique study site. Diurnal and seasonal variations in cross-gradient generation of Hg(0) will be monitored, and the release of Hg(0) from surface waters of Lake Huron will be measured. Laboratory experiments will be conducted to test a number of working hypotheses on the oxidation of Hg(0) under dark conditions, activation of the reduction of Hg(II) and Hg(I) in soils and water by various photo-active compounds (such as ferric ion), formation of polymeric Hg(I) species in aquatic environments, the formation of free Hg+ and Hg2+ as photoproducts during solar irradiation of dissolved organic matter (DOM) in water, and release of Hg during degradation of organic matter under light and dark laboratory conditions.
Progress Summary:
The first year of the project was devoted to establishing background levels of mercury in various parts of the Saginaw Bay (SB) ecosystem. We measured the ambient airborne Hg levels and fluxes of Hg at 26 sites covering forest soils, agricultural soils, wetlands, city park soils, beach sands, and industrial/municipal waste dumps. We also measured the Hg levels in soil samples from 12 of the stations and in water samples from the rivers and various parts of the Saginaw Bay. From the survey data, we have selected a number of sites for intensive monitoring in subsequent years.Ambient atmospheric concentrations of mercury in the terrestrial parts of SB varied from <1.0 ng/m3 to over 10 ng/ m3. The fluxes of Hg from the sites ranged from about 1.2 µg/ (m2/hr) to over 12 ng/(m2/hr). The low ambient air levels observed are surprising considering that some of the industrial sites have been listed by Michigan Department of Environmental Quality (MDEQ) as being contaminated with mercury. Concentrations in soils varied from 11 ng/g to over 1200 ng/g found at a metal recovery plant site. Significant enrichment (up to 10-fold) of Hg in the forest litter compared to the soil was observed. Mercury concentrations in all the water samples analyzed were <3 ng/L. P>
Photochemical Reduction of Hg(II) in Freshwater. Diel changes in levels of dissolved gaseous mercury (DGM) in response to solar radiation have been observed in freshwater environments, implicating photo-induced processes as being important in the aquatic dynamics of DGM. Recent field studies involving incubation of lake water samples in Teflon bottles under sunlight have also reported increased production of DGM. Sunlight-induced reduction of Hg(II) in freshwater ecosystems can exercise a moderating influence on the aquatic toxicity of mercury-the reductive reactions leading to evasional loss of Hg(0) can reduce the amount of Hg(II) available for methylation and food chain transfer. A variety of abiotic photochemical and perhaps biotic processes can hypothetically effect the production of DGM in natural waters. Our knowledge of the mechanisms of DGM production and the chemodynamics of this species is lacking or inconclusive. Photochemical reduction of heavy metal ions by free radicals of various organic ligands, produced through photolysis of Fe(III)-organic acid coordination, has been documented in a number of studies and a similar reaction pathway may be involved in the Hg cycle.
In a previous study, we exposed Fe(III)-spiked water samples from Whitefish Bay (Lake Superior, MI) in Teflon bottles to sunlight and found increased production of DGM. We repeated the experiments using water samples from a local pond in Oak Ridge, TN in quartz bottles. We found that production of DGM increased significantly in the Fe(III)-spiked water exposed to sunlight compared to the control without Fe(III) addition. The rate constants of the Fe(III)-induced photochemical production of DGM were estimated to be 0.05-0.2 h-1, assuming first-order kinetics. The results agreed with those obtained using freshwater samples from Whitefish Bay. We investigated the oxidation of DGM in water samples stored in the dark after they were exposed to sunlight. We found a time-dependent decrease in DGM concentration in the dark with decay rate constants of k = ~0.2-0.3 h-1 with Fe(III) spike and k = ~0.3 h-1 when no Fe(III) was added. These kinetic results generally agree with those reported by other researchers. The observation suggests that the redox cycling of Hg in water involves Hg(II) reduction in sunlight and Hg(0) oxidation in the dark, although the two processes may occur simultaneously under some light conditions. Dark oxidation of DGM may well be an important but unrecognized factor in the cycling of Hg in freshwater environments. The tests were conducted in exposure bottles, which represent a closed system. In an open lake or pond, the reduction step may predominate, especially under windy conditions because of evasional loss of the DGM produced. In an iron limited aquatic ecosystem, the input of Fe, for instance through atmospheric deposition of Fe-containing dusts, may change the local Hg cycle. We venture the suggestion that artificial introduction of Fe may be a potential means of reducing local aquatic Hg burden through the photoreduction-evasion pathways. The local 'solution' may contribute to global atmospheric Hg 'problem', however.
Photochemistry of Hg in Saginaw Bay of Lake Huron, MI. We conducted a preliminary study of changes in DGM levels in waters of Saginaw Bay during the summer of 2000. Strong diel changes in concentrations of DGM were observed in surface nearshore waters at the two stations investigated, namely Tawas Point and Tawas City, MI (both located at the northwestern corner of Saginaw Bay). The DGM concentration reached ~70 pg L-1 around noontime but declined to only ~10 pg L-1 before sunrise and around sunset at Tawas Point. Average DGM concentration was ~15 pg L-1 around noontime and only ~5 pg L-1 in the early morning and in the evening at Tawas City. These results point to significant spatial variations in DGM levels in surface waters of the SB which were strongly dependent on the sunlight. When the samples were spiked with Fe(III) and exposed to sunlight, the DGM concentrations increased moderately (by <25%) at the two stations. The effect of Fe(III) spike was much lower than what was observed using water samples from Whitefish Bay and a local pond in Oak Ridge mentioned previously. The subdued production of DGM may be due to a higher rate of oxidation of the DGM. Measurement of DGM levels in nearshore surface waters (from Saginaw Bay) in the dark after being pre-exposed to sunlight, indeed, showed a rapid disappearance of this species from the samples.
In some of the experiments, a suppression of photochemical DGM production was observed in Fe(III)-spiked samples compared to those with no Fe(III) additions. This phenomenon was also observed previously using samples from a local creek at Oak Ridge, TN, and in the water samples from the Everglades, FL. A possible explanation for this observation is that in the presence of Fe(III), the rate of formation of oxidants may exceed that of reductant species under some conditions (still to be defined). Other mechanisms can easily be invoked to explain the observation. The point that needs to be made here is that the preliminary studies have generated some fascinating results which may well be the clues we need to better understand the Hg cycle in aquatic environments. Nearshore waters of Saginaw Bay are very productive (in summer time, Secchi disk readings are typically < 2 meters) and the effects of catenated biological processes in the generation of DGM in this ecosystems needs to be carefully evaluated. This issue will be the focus of our continuing investigations.
Future Activities:
The effort in coming months will focus on: (1) monthly monitoring of Hg concentrations and fluxes at four to six selected sites in Saginaw Bay ecosystem; (2) assessment of tributary loadings of Hg into the Saginaw Bay; (3) development of a thermal desorption system for investigating the forms of Hg in soils, sediments and organic matter from the watershed; (4) investigation of kinetics and mechanisms of photo-induced reduction of Hg(II) and oxidation of DGM with and without spiking with photoactive material (including Fe(III) and humic acid) under field conditions and in controlled laboratory microcosms; and (5) evaluation of air/water exchange of Hg in aquatic environments in the watershed.Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 4 publications | 3 publications in selected types | All 2 journal articles |
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Zhang H, Lindberg S. Sunlight and iron(III)-induced photochemical production of dissolved gaseous mercury in freshwater. Environmental Science and Technology 2001; 35: 928-935. |
R827632 (2000) |
not available |
Supplemental Keywords:
heavy metals, Great Lakes, environmental chemistry, dissolved gaseous mercury, watershed, biogeochemical cycling, air/water exchange, humic acid, photochemical reactions, soils, wetland., RFA, Scientific Discipline, Air, Water, Waste, Geographic Area, Ecosystem Protection/Environmental Exposure & Risk, Nutrients, Hydrology, Ecology, particulate matter, Ecosystem/Assessment/Indicators, Ecosystem Protection, Chemistry, Fate & Transport, Ecological Effects - Environmental Exposure & Risk, Ecological Indicators, Great Lakes, Mercury, anthropogenic stress, aquatic ecosystem, fate and transport, hydrological stability, nutrient supply, ecological exposure, wetlands, anthropogenic disturbances, aquatic, re-emission, bioavailability, mercury loading, photo induced reduction, mercury cycling, soils, lakes, soil, photo induced reduction of mercury, methylation, suspended particulates, aquatic ecosystems, ecosystem, mercury in lakes, ecosystem stress, Lake Huron, lake ecosystem, wetland, heavy metalsRelevant Websites:
ORNL ESD atmospheric chemistry group Web page Exit Synthesis Report of Research from EPA’s Science to Achieve Results (STAR) Grant Program: Mercury Transport and Fate Through a Watershed (PDF) (42 pp, 760 K)
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.