jaffer ppt

7/28/2019 Jaffer Ppt

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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

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