Pyrite oxidation in sediment samples from the German open‐cut brown coal mine Zwenkau: mineral formation and dissolution of silicates

Changes in mineral composition occurring in pyrite‐containing sediments under aerobic conditions are complex and not fully understood. The objective was to study the mineral formation and dissolution of silicates using ion activity product (IAP) calculations and x‐ray diffraction (XRD) on samples of different degrees of pyrite oxidation. Three sediment samples were obtained from the open‐cut brown coal mine of Zwenkau (Saxony, Germany) with low (ZL: 28 g kg^—1), medium (ZM: 67 g kg^—1) and high (ZH: 95 g kg^—1) pyrite contents. These samples were oxidized in the laboratory for 3, 20, 67, and 130 days to obtain four different degrees of pyrite oxidation for each sediment. Sequential batch experiments were carried out for each sediment and oxidation status. Additionally, cation exchange capacities were determined. XRD showed the formation of gypsum (all sediments), jarosite (ZM, ZH), and rozenite (ZH) with increasing pyrite oxidation. IAP calculations suggested an occurrence of gypsum in all samples, of schwertmannite in slightly (ZH) and moderately oxidized (ZM, ZL) samples, and of alunite in a moderately oxidized sample (ZL). The contents of feldspar (ZL), mica/illite (ZL, ZH), and kaolinite (ZH) decreased with increasing pyrite oxidation. The cation exchange capacities of the sediments decreased by 20 (ZH) to 70 mmol_c kg^—1 (ZM). The change in mineral phases with increasing oxidation status of the sediments also changed the activities of Al, Fe, and SO₄ in solution phases. The results obtained in this study suggested the usefulness of predictive models to estimate sediment and water acidification due to pyrite oxidation.