Miljøprojekt,1024 – Metode til analyse af reducerende stoffer i sedimenter

Metode til analyse af reducerende stoffer i sedimenter

Summary and conclusions

A review of relevant literature identified a chemical oxidation process as the most appropriate for the measurement of reduced compounds in sediments and a number of oxidizing agents were listed. An examine of these oxidizing agents eliminated potassium-dichromate (K₂Cr₂O₇), potassium-permanganate (KMnO₄), and a cerium compound with Ce(IV) as the most useful in the development of a new methods.

Each of these three oxidizing agents was studied in lab experiments under different conditions, e.g., temperatures, concentrations of oxidizing agent, concentrations of acid, and reaction times to optimize the analysis methods. Sediment samples from localities within Denmark of different geological and redox properties were analyzed using the three optimized methods.

The results obtained for these sediments showed for all three oxidizing agents greater contents of reduced compounds in sediments samples from the reduced zones than for samples from the oxidized geochemical environments. Also the content of reduced compounds was greater in clayey sediments than in sandy sediments of comparable redox status, except for the sandy samples very rich in organic matter which had total amounts of reduced compounds like, or even higher than, than for the reduced clayey sediments.

In general, the potassium-permanganate (KMnO₄) method gave only low amounts of reduced compounds comparable to the amount that was calculated based on the measured contents of total organic matter, pyrite and ferrous iron. These low contents exclude the potassium-permanganate method as a useful method in this study.

The concentrations of reduced compounds obtained by the use of potassium-dichromate (K₂Cr₂O₇) were lower than the amounts obtained by the cerium sulfate Ce(SO₄)₂ method. The latter method was identified as the one that came up with contents of reduced compound in the sediments close to the total amount calculated on the basis of the estimated available contents of organic matter, pyrite, and ferrous. The total content of reduced compounds measured with the Ce(SO₄)₂-method was always lower than the calculated total amount based on the total amounts of organic matter, pyrite, and ferrous iron.

The method to measure the amount of reduced compounds in sediments by Ce(SO₄)₂ is described (in Danish) in section 7 in the present report.