Arsenic and antimony in the environment : release and possible immobilization mechanisms This thesis is dedicated to the release and immobilization of As5+ and Sb5+ in contaminated geomaterials. Although the physical properties and chemical composition of the contaminated materials vary widely the mechanisms which control the fate of the contaminants are supposedly similar. In order to understand these mechanisms I carried out a mineralogical and geochemical investigation of a soil which was mixed with contaminated As rich ash waste. With techniques including powder PXRD ICP MS sequential extractions EMPA and synchrotron based µ XRD and XANES the carriers of As5+ in the fresh ash waste and soil ash mixtures were identified and characterized. Glass particles and unburned coal particles with veinlets of As rich calcite are the main carriers of As5+. In the soil ash mixtures the soluble As5+ containing phases of the ash waste almost completely disappeared and As5+ migrated into the soil components presumably consisting of iron oxides. In the following study the immobilization of As5+ by iron oxides in the context of As5+ incorporation in hematite was investigated. As5+ ferrihydrite was transformed to hematite and examined by ICP MS powder XRD SEM TEM and EXAFS spectroscopy. The results show that As5+ can be incorporated into hematite with concentrations of 1.9 wt%. This novel immobilization mechanism encouraged me to investigate the possibility of Sb5+ incorporation into iron oxides in a subsequent study. Ferrihydrite was doped with different concentrations of Sb5+ As5+ and P5+ and transformed to more well crystalline products. The transformation products were analyzed and characterized by ICP MS PXRD SEM and TEM. The results show that Sb5+ directly controls the outcome of the ferrihydrite transformation and therefore the transformation products. These transformation products contain Sb5+ with up to 14 wt.% exceeding an exclusive adsorption mechanism by far suggesting structural incorporation of Sb5+ into the iron oxides.