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 und er aerobic conditions are complex and not fully understood. The objective w as to study the mineral formation and dissolution of silicates using ion ac tivity product (IAP) calculations and x-ray diffraction (XRD) on samples of different degrees of pyrite oxidation. Three sediment samples were obtaine d 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)) pyrit e contents. These samples were oxidized in the laboratory for 3, 20, 67, an d 130 days to obtain four different degrees of pyrite oxidation for each se diment. Sequential batch experiments were carried out for each sediment and oxidation status. Additionally, cation exchange capacities were determined . XRD showed the formation of gypsum tall sediments), jarosite (ZM, ZH), an d rozenite (ZH) with increasing pyrite oxidation. IAP calculations suggeste d 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 e xchange 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 t he sediments also changed the activities of Al, Fe, and SO4 in solution pha ses. The results obtained in this study suggested the usefulness of predict ive models to estimate sediment and water acidification due to pyrite oxida tion.