Pyrite, iron sulfide, elemental sulfur

The sulfur contained in AVS and pyrite is determined by using an acid distillation followed by a Cr-II reduction to determine pyrite sulfur. Acid volatile sulfide (AVS) is an estimate of the non-pyrite, metal sulfide, component of sediments, and it is assumed to consist primarily of iron monosulfide. AVS techniques suffer to a large degree from non-specificity. Individual AVS methods either fail to solubilize all of the primary non-pyritic iron sulfide minerals (such as greigite, Fe₃S₄) while harsher methods attack pyrite (Cornwell and Morse, 1987). According to Cornwell and Morse (1987) cold 6N HCl will liberate H2S from mackinawite and amorphous FeS as well as 60-69% of the sulfur in greigite. Thus, AVS values reported could be underestimated by up to 40%. To sample more specifically for greigite, the three-step method of Cutter and Oates (1987) is recommended.
The sulfur contained in pyrite (FeS₂) is determined using the acidic Cr-II reduction method, originally described by Zhabina and Volkov (1978). In the sequential treatment described here, Cr-II reduction may overestimate pyrite sulfur concentration, because the 31-40% of greigite remaining after the HCl treatment, and the presence of elemental sulfur. Elemental sulfur can be separately determined (see below).

Reagents:

1 N HCl

Cr(II) Solution (see preparation in Sulfate Reduction Rate method

N,N Dimethylformamide (DMF)

References

Cornwell JC and Morse JW(1987). The characterization of iron sulfide minerals in marine sediments. Marine Chemistry 22, 193-206.

Cutter GA and Oatts TJ (1987). Determination of dissolved sulfide and sedimentary sulfur speciation using gas chromatography-photoionization detection. Anal. Chem. 59, 717-721.

Zhabina, NN and Volkov II (1978). A method of determination of various sulfur compounds in sea sediments and rocks. In: Krumbein WE (Ed). Environmental Biogeochemistry; Methods, Metals and Assessment. Vol. 3. Ann Arbor Science Publishers. 735-745.

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