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Biogeochemical Cycling of Mercury

Overview of the various processes and systems involved in the global mercury cycle
by

Pinder Dhindsa

on 14 April 2011

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Transcript of Biogeochemical Cycling of Mercury

The Biogeochemistry of Also known as 'quicksilver' or hydrargyrum Found in the following forms in marine systems:
Hg(0)
Hg(II)
Methylmercury
Dimethylmercury
Particulate & colloidal mercury

Hg(II) can be deposited by wet & dry deposition, & Hg(0) by dry deposition

Oxidation States:
elemental (Hg)- water, soil, atmosphere
inorganic (Hg+)- forms salts
organic (Hg2+)- organomercury and salts http://www.theodoregray.com/PeriodicTable/Samples/080.14/s14s.JPG Cinnabar (HgS) is one of the most common forms of mercury found on the planet http://www.soulpix.com/download/desktop_pics/Sat_MediterraneanSea.jpg http://lh5.ggpht.com/jaana.heikkila/R_Dl5mMKw6I/AAAAAAAACK4/6sct1DhLHcQ/Augusta-Avola-Noto-16.JPG.jpg http://www.dartmouth.edu/~rpsmith/cinnabar.jpg http://www.newportplumbing.com/images/sulfbac.jpg Methylation of mercury is a biologically mediated process facilitated by some strains of sulfate- & iron-reducing bacteria
HgS -----> Ch3Hg + H2S http://www.geo.mtu.edu/volcanoes/hazards/primer/images/volc-images/puuoo.jpg Where Does the Hg Go?
Hg(0) in soils is not a problem until disturbed and released into water.
Adsorbed mercury remains in the soil
Once in water:
Can settle into sediment (HgS)
Can be converted into organic (methylated) and inorganic forms.
Can run off into a watershed
Factors controlling adsorption & deposition of mercury:
Type of soil & dissolved species (S-, Cl-)
Dissolved organic carbon Why is Hg Not a Problem in Soil?
No bioaccumulation/magnification in plants
Only mushrooms uptake Hg to any significant amount http://www.arl.noaa.gov/images/AirQuality/mercury_cycle50.jpg Atmospheric Mercury Cycling Emitted from geological sources and from land and ocean in elemental form [Hg(0)] - most abundant form in atmosphere
Main sink of Hg(0) is oxidation to Hg(II) Anthropogenic emissions occur in 2 different forms - divalent mercury [Hg(II)] & particulate mercury [Hg(P)] Global Biogeochemical Cycling of Mercury: A Review
Noelle E. Selin

This paper focuses on our current understanding of the
biogeochemical cycle of Hg and how it travels through the
atmosphere, ocean, and land. Health Concerns: Case Studies
From the Past 1950s Japan:
in Minamata people who were eating contaminated
fish experienced neurological problems including visual,
auditory, and sensory disturbances.
In utero exposure lead to mental retardation, cerebral palsy,
deafness, and blindness.


1970s Iraq:
Grain was sprayed with a Hg containing fungicide
People eating this Hg treated bread experienced problems
with vision, speech, and hearing.
Death occured in older adults and young children. http://www.mikeputnamphoto.com/wp-content/uploads/2008/11/mt-bachelor-in-winter.jpg RGM measurements increase with altitude and Hg(0) decreases
Reflects a conversion of Hg(0) to Hg(II) because total mercury remains constant
High altitude RGM may be an important reservoir of mercury Behavioral, neurochemical,
hormonal, and reproductive
effects have also been observed
in wildlife that was exposed to Hg. Ultimate sink:
Deep ocean sediment burial
Can take thousands of years http://wallpaper-s.org/11__Arctic_Seaside.htm http://4.bp.blogspot.com/_copRHv93JEI/TABUWm6rveI/AAAAAAAAEMk/xjLM8XwjBE0/s1600/5772519.jpg Mercuriferous Belts:
Western NA, central Europe,
and southern Asia.
Cinnabar ore (HgS)
Lots of mining in these areas releases
Hg(0) into atmosphere. Br also thought to be responsible for Arctic mercury depletion events (AMDEs)
During springtime, HG(0) decreases rapidly while RGM spikes
Correlate strongly with tropospheric ozone depletion events, which result from rapid oxidation by halogens such as Br & Cl Br Controlled by photochemistry: Hg(2+) peaks at midday Prompt Recycling:
Newly deosited Hg shows preferrential revolatization
Up to 5-60% of deposited Hg
Mechanism is relatively unknown. Hg from soils returns to atm by reduction to Hg(0).
reductants: Fe2+, humic and fulvic compounds http://upload.wikimedia.org/wikipedia/commons/7/75/Gerard_de_Jode_1593_Map_Northern_hemisphere.jpg Total Gaseous Mercury (TGM) Relatively well mixed in global atmosphere, with concentrations generally higher in the Northern Hemisphere - most emission sources are in Northern Hemisphere Consists of Hg(0) plus a small contribution of Hg(II) in the gaseous phase Sulfate reducing bacteria thrive on the
interface between noxic and anoxic conditions. Cyanobacterial blooms
seasonal blooms
creates large source of DOC
Policy
Chemical species that acts as primary oxidant of Hg(0) in the atmosphere remains unknown Samantha Sinclair & Pinder Dhindsa Policies to date have been focused on controlling anthropogenic emissions.

Should maybe focus on prevention of mercury conversion to methylmercury.
Need to improve knowledge on land-atmosphere cycling (prompt recycling) and the methylation process. Safe Water Drinking Act (1992)
Hg(total) set to 2ppb
Drinking reservoirs tested every 3 months
Removal: coagulation/filtration, Granular Activated Carbon (GAC), lime softening, and reverse osmosis. Coagulation/filtration:
use AlSO4 to react with Hg, which forms a solid
Precipitate is collected and disposed in a hazardous waste facility.
Cheap and reliable Measurements of Hg(II) in atmosphere can help constrain uncertainties in redox reactions
Collect Hg(II) on potassium chloride (KCl) coated denuders and reduce to Hg(0) - species measured referred to as reactive gaseous mercury (RGM) GAC:
uses a porous charcoal material, which water passes through.
Contaminants are absorbed.
Effectiveness depends on [Hg] in water Lime softening:
Uses excess CaOH to raise pH level
Hg then precipitates out as HgOH
Cheap and very reliable Reverse Osmosis:
H20 pushed through a semi-permeable membrane (polyamide film)
Traps the Hg
Leads to high quality H2O, but very expensive Oxidation is thought to be photochemically controlled RGM peaks around noon rather than early afternoon (OH peak), so Br thought to be responsible for Hg(0) oxidation (Br rises earlier in day than OH) Future Issues 1. Predominant oxidant of mercury (Hg0 to Hg2+) remains unknown
Br, ozone, and OH radicals are possible contenders 2. Improved deposition monitoring
dry deposition not monitored
wet deposition only monitored in NA and Europe 3. Negotiations for a global agreement on Hg regulation
financial aid for developing nations 4. Future efforts to control Hg need to address the different form of Hg outputs, and their effects on both global and regional scales. 5. More research is needed on the timescales of Hg reponses to changes in anthropogenic emissions.
[MeHg] do not respond linearly to changes in anthropogenic emissions. Terrestrial Mercury Cycling Wet and dry deposition brings mercury to terrestrial surface
Outside geologically enriched areas, most mercury in global land surface is deopsited as Hg(II) from atmosphere A portion of this mercury will rapidly revolatilize to atmosphere, and remainder will be incorporated into a long-lived soil pool Prompt Recycling - newly deposited mercury has been shown to preferentially revolatilize
More available for reduction and subsequent emission as Hg(0) than mercury already present in the system So now consider these factors... 2. Climate Change


Study by Verta et al. (2010)
lowered thermocline to see effects of climate change on Hg cycling.
Oxic conditions increased
Temperatures increased
Epilimnion/hypolimnion boundary decreased
result: less Hg accumulation in fish Also preferentially available for conversion to methylmercury http://static.guim.co.uk/sys-images/Travel/Pix/pictures/2008/11/24/EtnaGetty4.jpg http://www.gpb.org/files/news/images/body/power_plant_smoke_stack_nick_humphries_flickr_o.jpg http://www.worldinterestingfacts.com/wp-content/uploads/2010/01/most-amazing-hole-in-the-world-The-Diavik-Diamond-Mine-Canada.jpg Natural Sources:
Volcanoes & related geological activities
forest fires
Land emissions from areas naturally enriched in mercury - global mercuriferous belts along plate tectonic boundaries that are geologically enriched in mercury Anthropogenic sources:
Major source is the combustion of fossil fuels, especially coal (up to 60% of annual emissions)
Cement production
Nonferrous metal production
Pig iron & steel production
Caustic soda production
Mining of gold and silver
Direct mercury production/mining
Chlorine factories
Waste/sewage (disposal of batteries, thermometers, electrical switches, etc). Mercury in terrestrial aboveground biomass primarily originates from the atmosphere - Hg(II) deposits on leaves through precipitation (wet deposition) & dry deposition Uptake of Hg(0) thought to occur at leaf interior through gas exchange at stomata http://argosy.mta.ca/wp-content/uploads/2011/03/Rain_Forest_Tropic.jpg http://theinspirationroom.com/daily/print/2007/3/survival_bushman.jpg Mercury in the roots comes from the soil, which contains >90% of terrestrial mercury http://www.google.ca/imgres?imgurl=http://www.manywallpapers.com/d/18129/-/eye-in-the-sky_1680_x_1050.jpg&imgrefurl=http://www.manywallpapers.com/nature-wallpapers/scenery/eye-in-the-sky.html&usg=__oxsXwZ3eK2vccdvU-VBE9wHW7Bc=&h=1050&w=1680&sz=351&hl=en&start=60&zoom=1&tbnid=URP7HxmriEE65M:&tbnh=164&tbnw=225&ei=KgKaTdjtBIvQsAPS0KD3Ag&prev=/images%3Fq%3Dsky%26um%3D1%26hl%3Den%26client%3Dfirefox-a%26sa%3DN%26rls%3Dorg.mozilla:en-US:official%26biw%3D1280%26bih%3D837%26tbs%3Disch:10%2C2052&um=1&itbs=1&iact=hc&vpx=956&vpy=247&dur=547&hovh=177&hovw=284&tx=134&ty=113&oei=CgKaTciwHoTeiAKO7p3gCA&page=4&ndsp=20&ved=1t:429,r:4,s:60&biw=1280&bih=837 Mercury returns to atmosphere from soils by reduction to Hg(0) and subsequent diffusion or mass transport through soil and into the atmosphere Terrestrial emissions are a substantial, yet poorly understood, component of the global total mercury emission http://www.620ckrm.com/blogs/willycole/wp-content/uploads/fall-leaves.jpg Terrestrial uptake can in part explain the seasonal variation of Hg(0) atmospheric concentrations observed in the northern midlatitudes - display a seasonal variablilty similar to that of carbon dioxide Reduction & subsequent emission considered predominantly a physical (abiotic) process - although some recent research has shown that mercury speciation & emission from soil can be partially controlled by biotic processes http://www.aglabs.com/newletters/images/humus.jpg Reductants in soil can be species such as Fe2+, & humic & fulvic compounds http://www.bccdc.ca/NR/rdonlyres/E2AF0877-6926-46E4-A409-2FBB13079F05/0/solar_storm.jpg Reduction & volatilization processes can be enhanced by:
Temperature
Solar radiation
Increased soil moisture (in dry ecosystems) http://science.larc.nasa.gov/biomass_burn/images/bor_sib/385.gif Changes in burning & other climate-related changes, such as a loss of peatlands, could mobilize substantial amounts of mercury from soils into the atmosphere Summary http://thundafunda.com/33/underwater-animals-fish/School%20of%20Tropical%20Fish,%20Tahiti%20pictures%20underwater%20photos.jpg Aquatic Mercury Cycling Dominant pathway of human methylmercury exposure is through eating contaminated fish http://www.otonabee.com/images/watershed_labeled_hor.jpg Atmospheric mercury reaches freshwater ecosystems by direct deposition to lake surfaces & through runoff from watersheds Wet & dry deposition to watersheds & lake surfaces is predominantly as Hg(II) Wetlands & lake sediments are important environments where methylation occurs http://fr.academic.ru/pictures/frwiki/87/Wetlands_Cape_May_New_Jersey.jpg Estimated that 5%-60% of deposited mercury is promptly recycled to atmosphere - higher values for water & surface snow Also preferentially available for conversion into methylmercury http://media.photobucket.com/image/lake%20bottom/maryb1961/100_1691.jpg Microbes convert a small fraction of inorganic mercury [Hg(II)] to methylmercury (MeHg) over time in aquatic sediments A number of environmental factors affect the rate of MeHg formation by influencing the supply of bioavailable Hg(II) and/or the activity of methylating microbes http://www.rocbike.com/wp-content/uploads/2008/08/rain.jpg Ecosystems rspond to changes in deposition on varying timescales, depending on ecosystem type and the influence of watersheds Concentrations in most ocean basins are not at steady state with respect to atmospheric inputs & will likely continue to increase over the next several decades http://www.bestesoft.com/imgbest/5/7/57885-ocean-waves-free-screensaver.jpg Exchange of mercury at surface of ocean is thought to be rapid and extends atmospheric lifetime of mercury http://www.visions05.washington.edu/documents/VISION/sully2004.jpg Mercury methylation can occur in sediments of continental shelf regions and estuaries, within the water column, or at deep-ocean hydrothermal vents http://www.google.ca/imgres?imgurl=http://www.eco-tec.com/images/product_reverse_osmosis.jpg http://www.google.ca/imgres?imgurl=http://queenmadigan.files.wordpress.com/2011/01/mercury-pollution.jpg 1. Salinity

Farmer et al. (2010)
inverse relationship between salinity and [Hg] in fish tissue
diets and growth rates vary across salinity gradient Questions? 3. Human Impacts

Through coal burning, mining, & industrial activities, humans have brought mercury from long-term sedimentary storage into the atmosphere
An increased amount of mercury is circulating and will continue to cycle for centuries to millenia
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