Speciation of Mercury

The usual form of mercury in aqueous solution is the Hg + ion. Mercury has two oxidation states, Hg(I) and Hg(II), but the first of these, that contains the unusual ion +Hg - Hg+, is stable only as insoluble salts such as Hg2Cl2. It disproportionates in solution as follow 

We can see from the reaction that reduction of Hg + under anaerobic conditions, for example, in bottom sediments, gives the metal in liquid form. 

Mercury(II) is a very soft Lewis acid, which forms stable complexes preferentially with soft Lewis bases such as sulfur ligands. You should remember here that the major natural form of mercury is sulfides. Increasing the pH of the aqueous solution due to 

The major complicating factor in environmental biogeochemistry of mercury and its speciation is the biological methylation of Hg to CHjHg and (CH3)2Hg. This process converts inorganic mercury to organo-mercury, which is both more lipophilic and toxic (see below). 

Mercury is methylated in nature by the attack of methylcobalamin(vitamin B12) upon Hg . Methycobalamin contains a methyl group bonded to a central cobalt atom, making a methyl group somewhat carbanion-like. Representing methylcobalamin as L5C0 — CH3, the simplified equation is 

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The following issues are here discussed in depth ecosystem-dependent depositions, speciation of mercury depositions, mercury concentrations in precipitation. One of the... 

Because MIPs are formed at low temperatures, liquid samples cannot be introduced because they extinguish the plasma, even small amounts of organic vapour. However, the on-line coupling of HPEC to MIP-OES has been described for the speciation of mercury and arsenic compounds. Continuous cold vapour (CV) or hydride generation (HG) techniques were used as interfaces between the exit of the HPEC column and the MIP, held in a surfatron at reduced pressure [24]. 

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C.F. Harrington, The speciation of mercury and organomercury compounds by using high-performance liquid chromatography, Trends Anal. Chem., 12 (2000) 167-178. 

Figure 4.5 Separation of two trialkyl lead and three organomercury species. Column 1.5mm i.d. x 5cm long flow rate lOOpimirT1 mobile phase 5mM ammonium pentanesulfonate in 20 80 v/v ACN-H20 (pH 3.4) injection volume 2jul, sample size, 40 pg (as Pb) for (Me)3Pb+, 80 pg (as Pb) for (Et)3Pb+ and 2 ng (as Hg) for each of the organomercury species. Taken from Speciation of mercury and lead compounds by microbore column liquid chromatography inductively coupled plasma chromatography mass spectrometry (Shum ef a/. 1992).

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Grinberg, P., R.C. Campos, Z. Mester, and R.E. Sturgeon. 2003. A comparison of alkyl derivatization methods for speciation of mercury based on solid phase microextraction gas chromatography with furnace atomization plasma emission spectrometry detection. J. Anal. At. Spectrom. 18 902-909. 

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Tseng, C.M., Amouroux, D., Abril, G, Tessier, E., Etcheber, H., and Donard, O.F.X. (2001) Speciation of mercury in a fluid mud profile of a highly turbid macrotidal estuary (Gironde, France). Environ. Sci. Technol. 35, 2627-2633. 

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Kim, C.S., Brown Jr., G.E., and Rytuba, J.J., Characterization and speciation of mercury-bearing mine wastes using x-ray absorption spectroscopy, Sci. Total Envi-ron., 261, 157, 2000. 

Assuming all y s = 1, calculate the speciation of mercury in typical seawater (35 psu at 25 °C) given the following values for stepwise stability constants for successive chlorocomplexes = 10 ", K2 — 10 -, Kj = 10 - K4 = lO ). Note that [Cl ] is 0.559 mmol L and that it is not necessary to know the mercury concentration in seawater. The total mercury concentration is given as the sum of all contributing species. Thus... 

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Akagi H, Malm 0, Branches F JP, et al. 1995. Human exposure to mercury due to gold mining in the Tapajos river basin, Amazon, Brazil Speciation of mercury in human hair, blood and urine. Water Air and Soil Pollution 80(l-4) 85-94. 

Bulska E, Emteborg H, Baxter DC, et al. 1992. Speciation of mercury in human whole blood by capillary gas chromatography with a microwave-induced plasma emission detector system following complexometric extraction and butylation. Analyst 117(3) 657-663. 

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