1. Biogeochemical Cycle of Mercury (Hg)
PB 饶子璇

2. Introduction Distribution Toxicity Mobility
Among the heavy metals, mercury is one of the most studied environmental pollutants.
The main pathway of contamination of the humans with mercury is the food chain, where the most toxic species, methylmercury, bioaccumulates.
Not only the total concentration of mercury in an environmental compartment but it's chemical forms determine the toxicity/essentiality and bioavailablity.
To understand the chemical species in the sample implies the use of a dedicated measurement approach called speciaton analysis.
Distribution
Toxicity
Mobility

3. Sources Anthropogenic: Natural:
Current use of mercury in products/processes.
Coal burning and to a lesser extent wood/petrol combustion and incineration proesses.
Erosion, rainfall and leaching processes.
Anthropogenic:
volcanoes
geothermal vents
degasification form soils and water surface
degradation of minerals
forest firing
Natural:

4. PROCESSES Oxidation Reduction Methylation Demethylation Precipitation
Dissolution
Sorption
Desorption

5. Mercury in the air TGM RGM TPM
(total gaseous mercury)
RGM
(reactive gaseous mercury)
TPM
(total particulate mercury)
Elemental mercury vapor (Hg0) with minor fractions of other volatile species such as Me2Hg, MeHg and to a less extent HgCl2
HgCl2 and possibly other Hg(II) speicies
Mercury bound or absorbed on atmospheric particulate matter (Hg0 and/or RGM, including MeHg)

6. Main Mechanisms of Deposition
Wet deposition:
Removal processes associated with precipitation. Among the atmospheric mercury fractions, RGM is easiest scavenged from the atmosphere during rain events because it is water-soluable.
RGM has an atmospheric lifetime of days or at maximum weeks.
Dry deposition:
Any physical removal processes that does not involve precipitation. Such physical deposition processes account mostly for TPM and include gravitational settling, impaction with objects on the ground and adsorption on liquid water or solid surfaces such as vegetation or soil.
TGM and particularly Hg0 have the longest environmental life, ranging between yrs.

7. Mercury in water and biota
It is estimated that over 90% of the mercury loading the watersheds results from atmospheric deposition.
Background concentrations of total mercury:
pg/g (open ocean waters), pg/g (coastal estuaries and rainwater)
Main processes(Hg2+ ) in aquatic environment:
Microbial methylation and biomagnification by biota
Microbial and/or photochemical reducing to Hg0 and volatilization to atmosphere
Conversion to insoluble HgS and precipitation in sediments
Conversion to MeHg, mainly under anaerobic conditions, via methylation.

8. Mercury in soils and sediments
Methylation of mercury
Methylation requires the transfer of a methyl group from a donor, and such reactions in nature can be either the result of photochemical processes or need to be catalyzed by microorganisms.
SRB are responsible for methylation in natural water.
The main methyl-donor in the environment is methylcobalamin.
Mercury in soils and sediments
Reduced to Hg0
Hg2+ incorporated in sediments
Methylation-demethylation cycles

9. Chemical Analysis of Species
Hyphenated techniques:
Chromatography
Element Specific Detectors
Inductively Coupled Plasma Mass Spectrometry
Liquid Chromatography - Atomic Fluorescence Spectrometry
On-site analysis of Hg2 in the air

10. Mercury levels in fish respond rapidly and directly to changes in atmospheric mercury.

11. References:
Krabbenhoft, David P.; Rickert, David A. Mercury contamination of aquatic ecosystems. US Geological Surveys Facts sheet. Available at
Daniel R. Engstrom. Fish respond when the mercury rises, 2007, Vol. 104, No.42 PNAS.
P. Jitaru, F. Adams. Toxicity, Sources and Biogeochemical Cycle of Mercury. 2004, Journal de Physique IV France(Proceedings)
Noelle E. Selin. Global Biogeochemical Cycling of Mercury: A Review. 2009, Annual Review of Environment and Resources, Vol. 34.

12. Thank You!
(September 28th , 2016)