Post on 03-Apr-2018
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Specific characters of mercury
Boiling point is 629.88K (356.73C, 674.11F)
The melting point is 234.32K (-38.83C, -37.89F)
Exists with three valence states Hg0, Hg1+, and Hg2+
Only trace element which volatilizes at ambient temperature
Occurs in the environment in its metallic form as well as in variousinorganic and organic complexes
With an aqueous solubility that is comparable to that of oxygen,
elemental Hg (Hg0) is a ubiquitous component of natural waters
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Natural Sources: Volcanoes
Evaporation from soil and water surfaces
Degradation of minerals
Forest fires
Flooding
Sources Of Total Hg In Aquatic
Environment
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Anthropogenic Sources:
Atmospheric deposition
Urban discharges
Agricultural material runoff
Mining
Fossil fuel use and industrial discharges
Pharmaceutical production
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Fig. Anthropogenic sources of Hg
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Mercury Cycle
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Fate Of Mercury In Water
Exists in all the oxidation states, Hg0, Hg1+, and Hg2+
Hg0 is insoluble,HgSO4 most soluble, Hg(II) chloride is
readily soluble, Hg(I) chloride is much less soluble and
mercury sulfide has a very low solubility.
Chloro-complexes of Hg are most abundant in sea water at
pe + pH>12, especially in oxic waters
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Hg0 is found at all depths in the oceans,
supersaturated, especially in surface waters
Increase in salinity enhance HgCl42-
concentration while a decrease favors HgCl3-species
In anoxic sea water (pe + pH
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Effect Of Sulphur
Controls chemistry of mercury in anaerobic sites
Dominant mercury species in the anaerobic conditions are
mono- and bi-sulphide complexes such as HgS, HgS2H2, HgS2H
HgS (Cinnabar) is poorly water soluble, precipitates in
sediments and determines the solubility of Hg(II) compounds
Organic matter plays an important role in release of Hg from
HgS, inhibited by Ca2+
HgS is strongly bound with the sediments, it can be partly
dissolved by bacteria or under oxidizing conditions
(bioturbation)
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Diagram of hydroxo-, chloro- and sulfide complexes of MeHg at
pH= 7 as a function of chloride and total sulfide S (II)
concentration
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Methylation of Hg
Marine and estuary sediments, CH3Hg ~ 0.5% of totalHg whereas in fresh water sediments it usually
reaches 1-1.5%
Abiotic methylation, which generallyoccurs in anoxicsediments, is not considered very important in
freshwaterenvironments (Berman and Bartha 1986;
Miskimmin et al. 1992).
Biotic methylation occurs through the actionofsulphur reducing bacteria (SRBs) (Gilmouret al.
1992; Benoit et al. 2003)
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Group of SRB: Clostridium butyricum, Desulfobulbus propionicus,Desulfovibrio desulfuricans, Desulfococcus multivorans,Desulfobactersp.,Desulfobacterium sp
Methylation requires a suitable CH3 donor, and methylcobalamin(vitamin B12) is believed to be the only natural methylating agentcapable of transferring methyl groups
Factors affecting rate of Hg methylation includes
pH
Presence of sulphur
DOC
Microbial respiration,
Water temperature, and lake surface area
(Miskimmin et al 1992)
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Effect of pH
Acidic conditions favor mercurymethylation in thewater column and at the sediment-water interface
Microbial activity is enhanced in low pH waters
Low pH also favors the production ofmonomethylmercury overdimethylmercury(McMurtry et al. 1989)
At higher pH, the processes of mercurydemethylation (volatile) is favored
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Effect Of Redox Conditions
Surface layers of the bottom sediments (oxidizingconditions) make a geochemical barrier for diffusionof methylmercury from the deeper layers (reducingconditions) to the water column
Minimum conc. of Hg is expected around the suboxicto anoxic interface
Hg concentration increases as redox shifts to both
extremes
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Effects of D.O.C
Organic matter can bind up to 95% of the divalent Hgspecies
Serves as a carrier to transport Hg(II) into a water
body from catchment area
Grieb et al. (1990) found a positivecorrelationbetween Hg in fish tissue and DOC in drainage lakes,but a negativerelationship in seepage lakes
High DOC (10.5 mg/L) and low pH (
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Effect Of Salinity
Rate of mercury methylation decreases withincreasing concentration of salt
At higher salinity neutral species of Hg(II) such asHgCl2 or Hg(OH)2 are present instead of HgCl3 or
HgCl42-
The oxidation of elementary mercury ,Hg0 to Hg(II)
is significantly lower in fresh water than in marine
water
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Effects Of Temperature and Lake
Surface Area
Rate of mercury methylationwas greatest in theepilimnion during summer stratification anddemethylationrate was greatest in the hypolimnionduring winter stratification, Ramalal et al. (1993)
Increased temperature increases metabolic rate,
leading to increased uptake of food and water, leads
to an increased rate of MeHg uptake (Bodaly et al.1993)
Smaller lakes tend to be shallower and therefore
warmer in summer than larger, deeper lakes
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Factors Affecting Bioaccumulation
Of Hg In Fish
Among various species of Hg found in water
bodies, only MeHg bioaccumulated
Hg accumulation is affected by
Dissolved Hg conc.
Position of an organism in food web
Geographic location
Sea water salinity and
Temperature
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Average proportion of MeHg over total Hg is 10%
in water column ,15% in phytoplankton, 30% in
zooplankton, and 95% in fish
Hg conc. in fish species generally increase withincreasing age and body size (Lange et al. 1993;
Weiner and Spry 1996)
Piscivorous species such as lake trout acquire
more Hg than other species
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Desorption
pH:- Lower pH enhances desorption of Hg
Redox:- In anoxic sediment more Hg will be
complexed by sulphur ions (HS
-
,S2-
,S
2-
)
Salinity:- Increase in salinity forms mercury chlorocomplexes , brings more Hg to sea water ,
enhances desorption.
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Demethylation
CH3-Hg+ + H+ CH4 + Hg
2+
Catalysed by Pseudomonas group of bacteria
Hg demethylation are more effective in the marine
ecosystems with relatively high salinity than in thefresh waters
Methylmercury can be lost through microbial
demethylation and photodegradation
Iron and manganese hydroxides, catalysedemethylation of Hg in aerobic conditions