Mössbauer spectroscopy for optimising systems for environmental remediation

Mössbauer spectroscopy was used to examine Fe-containing microporous materials zeolite X and AlPO-5 designed for the reduction of Cr(VI) to Cr(III) and Cr(III) uptake. XRD, XRF, EPR and XPS were used to provide supporting information. AlPO-5 was found to only incorporate inactive Fe(III), whilst Fe(II) and Fe(III) were identified in zeolite X across a distribution of sites. Fe coordinates to oxygen atoms belonging to both water molecules and framework positions. EPR confirmed these findings and showed how the site distribution narrows upon dehydration. XPS showed that a distribution of Fe sites exists at the sample surface. Mössbauer spectroscopy was also used to track oxidation and reduction processes in Fe-zeolite X. Over time, gradual Fe oxidation takes place in air-exposed samples. Reduction under hydrogen flow increases the populations of low oxidation state Fe; as the reaction time increases, Fe(II) populations first increase, then decrease as Fe(0) is evolved.