Iron within the erionite cavity and its potential role in inducing its toxicity: Evidence of Fe (III) segregation as extra-framework cation

In this work we report the results of the crystal chemical, structural and surface characterization of erionite-K fibres from Rome (Oregon, USA) after interaction with Fe (III) chloride solutions at different concentrations. In addition, Fe (III) loaded samples were investigated after incubation in ascorbic acid in order to monitor the mobility of reduced Fe (II) and to highlight its possible incorporation as EF cation through ion exchange. Comparison between released and acquired charges under the form of Fe confirms, in perfect agreement with previous studies that Fe (III) is mainly fixed at the fibre surface. Nevertheless, in very diluted Fe (III) solutions (below 50 μM FeCl3) a significant fraction of Fe (III) is segregated by an ion-exchange mechanism in the erionite cavity at the Ca3 site, albeit with a significantly lower efficiency with respect to Fe (II). It is worth mentioning that, as a result of the catalytic properties of zeolites, the location of iron in well-defined crystallographic positions is the prerequisite for behaving as a very active site in the generation of reactive oxygen species. Incubation in ascorbic acid revealed that only Fe (III) residing at the fibres surface and characterized by low nuclearity is significantly reduced, whereas this reaction does not occur (or possibly occurs very marginally) in the case of the ion-exchanged metal. Considering that the total iron in lung fluids occurs at very low concentration (ca. 0.21 μM), our results strongly suggest that the physiological environment unfortunately represents the optimum condition for iron to behave as a very active site