Place a 10g sample in a 250ml beaker, add 5 ml HNO3 and 15ml HCI, cover with a watch glass, and gently boil for 45 minutes. Dilute to 50ml with 3N HCI and determine total Hg by cold vapor atomic absorption spectrometry (CV AAS).
Place two 10g samples in two 250ml beakers in an oven for 48 hours at 150°C [the work of Biester (1994) clearly indicates that 150°C is required to liberate elemental Hg from some soils]. Place petri dishes containing activated charcoal over the uncovered beakers, covering approximately 1/ 2 of each beaker's opening. This minimizes transfer of Hg vapor from one sample to another during heating and captures Hg vapor that would otherwise be released to the laboratory environment (this charcoal is subsequently disposed of with other hazardous laboratory wastes). Determine the Hg content of one of the heated samples and compare this concentration to the total Hg content of the sample; the difference is elemental Hg. The second heated sample is saved for the determination of other Hg species in the sample, starting at step 3.
Using the second, heated sample from step 2, add 50ml 0.5M MgCl2, mix at room temperature for 2 hours, let stand for 60 minutes, decant 40ml, centrifuge, and determine exchangeable Hg by CV AAS. Wash residue with 50ml of deionized water, let stand for 60 minutes or until clear and pour off wash solution; save residue for step 4.
Using the sample residue from step 3, add 50ml 0.5N HCI, mix at room temperature for 2 hours, let stand for 60 minutes, decant 40ml, centrifuge, and determine strongly bound Hg by CV AAS. Wash residue with 50ml of deionized water, let stand for 60 minutes or until clear and pour off wash solution; save residue for step 5.
Using sample residue from step 4, add 25ml 0.2N NaOH, mix at room temperature for 2 hours. Add 25ml 4% (v/v) CH3COOH (glacial acetic acid), mix at room temperature for 2 hours, let stand for 60 minutes, decant 40ml, centrifuge, and determine organic Hg by CV AAS. Wash residue with 50ml of deionized water, let stand for 60 minutes or until clear and pour off wash solution; save residue for step 6.
Using sample residue from step 5, add 25ml saturated sodium sulfide (Na2S) solution, mix at room temperature for 24 hours; centrifuge and decant clear supernatant solution, add 25ml DI water, mix, centrifuge, and decant, twice. Determine residual Hg on residue (by running total Hg on this portion). Subtract elemental, exchangeable, strongly bound, organic, and residual Hg from total Hg; difference is Hg sulfide.
Elemental Hg is that fraction of the total Hg which is volatile upon heating, as described above. Mercuric chloride will contribute to the elemental Hg number if it is present in significant quantities and should first be removed by leaching with deionized water, and quantified if desired.
Exchangeable Hg is that portion of the total Hg which can be exchanged off of active sites in soils or sediments (e.g., clays, Fe and Mn oxyhydroxides, and organic matter).
Strongly-bound Hg reflects Hg from several soil/sediment components such as Fe and Mn oxyhydroxides, a portion of the organically bound Hg which is exchanged to solution through protenation of organic sites, and exchange from mineral surface sites where Hg is strongly sorbed. This makes interpretation of this species or chemical form particularly ambiguous and difficult.
Organic Hg is that Hg which is released for analysis upon digestion of organic acids with NaOH and organic bases with CH 3COOH.
Hg sulfide is that portion of the total Hg soluble in saturated Na 2S solution and may be cinnabar, metacinnabar, or amorphous HgS.
Residual Hg is that fraction of the total Hg which is not soluble and available for analysis in solution with other than hot aqua regia.
Although methyl Hg is important to environmental investigations of Hg pollution, it is not determined in the method described here.