An improved sequential extraction method to determine element mobility in pyrite-bearing siliciclastic rocks

ABSTRACT

A sequential extraction method was developed for pyrite-bearing (FeS₂) siliciclastic rocks. The focus of this study was to enhance the procedure by an improved oxidation step to completely dissolve not only organic matter but also microcrystalline pyrite. In the first experiment, four oxidation procedures were compared for pure pyrite at extraction temperatures of 25°C and 85°C with hydrogen peroxide (H₂O₂) as the main oxidant. It was found that pyrite dissolution was most effective by using a mixture of H₂O₂, ammonium acetate (NH₄OAc) and nitric acid (HNO₃) at 25°C. This procedure dissolved >90% pyrite, and detected >75% using solute iron measurements. The difference between these two results was explained by reprecipitation of secondary iron minerals. The procedure worked best at 25°C, since solvent evaporation at 85°C amplified iron oversaturation and precipitation. For the pyrite-bearing siliciclastic rocks, two sequential extraction schemes were compared to optimise solid–solvent ratio, extraction step order and type of solvent. Eventually, the most effective step order identified for siliciclastic rocks containing pyrite and little organic matter was to first (1) remove the exchangeable fraction, followed by (2) dissolution with acid and afterwards (3) with a reducing agent. The (4) oxidation step was performed last.

KEYWORDS:

• Sequential extraction
• siliciclastic rocks
• pyrite
• element mobilisation
• solid phase quantification

Acknowledgements

This work was performed in the framework of the project ‘Efficiency and reliability of energy systems in urban districts with seasonal energy storage in aquifers (Aquifer Thermal Energy Storage ATES Berlin)’ and was funded by the Federal Ministry for Economic Affairs and Energy (BMWi, 03ESP409A). The authors would like to thank Traugott Scheytt of TU Berlin and Michael Kühn of University Potsdam for fruitful discussions. We thank Fiorenza Deon and Elvira Feldbusch for contributing their scientific expertise, and Tanja Ballerstedt, Sabine Tonn, Andrea Gottsche, Anja Schneider, Andrea Vieth-Hillebrand, Ilona Schäpan, Helga Kemnitz and Oona Appelt for offering help with experimental issues and laboratory equipment. The authors are grateful to Iris Pieper, Sabine Rautenberg and Petra Marsiske of the geochemical lab at TU Berlin for conducting chemical analyses.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the German Federal Ministry for Economic Affairs and Energy (BMWi, 03ESP409A).