The element-release mechanisms of two pyrite-bearing siliciclastic rocks from the North German Basin at temperatures up to 90 °C under oxic and anoxic conditions

Leaching tests with synthetic brines (25 g/L NaCl) between 25 °C and 90 °C were performed under oxic and anoxic conditions over 7 days on two pyrite-bearing siliciclastic rocks from the Lower Jurassic Hettangian and Sinemurian stages in the North German Basin. The release mechanisms of the mobile elements Al, As, Ba, Ca, Cu, Fe, Mn, Ni, Si, and Pb were studied and explained by means of numerical simulations of the leaching tests. The study was performed in the context of aquifer thermal energy storage (ATES) to improve the understanding of water–rock interactions during heat storage. Results showed that release patterns of Ba, Ca, Cu, Fe, Ni, and Pb were predominantly controlled by the dissolution of pyrite under oxic conditions and iron hydroxides under anoxic conditions. The mobilizations of Al and Mn could be explained by a combination of desorption and the dissolution of hydroxides. Si was mainly released from amorphous silica. The mobilization of Ca was governed by pH-sensitive desorption and calcite dissolution in one of the samples. Arsenic was immobile in both studied rocks. In general, elemental release was augmented by the presence of oxygen and the subsequent dissolution of pyrite and reduction of pH, which should therefore be avoided in ATES systems.