Heterogeneous formation of ferric oxide nanoparticles on chlorite surfaces studied by x-ray absorption spectromicroscopy (STXM)

Abstract : The homogeneous and heterogeneous formation of ferric iron oxides from acidic solution (pH 2.06-2.50) has been studied in the presence of chlorite under constant pH conditions using titration experiments. Significantly shorter induction times for the ferrihydrite formation have been macroscopically observed in the presence of chlorite particles, thus indicating heterogeneous nucleation as a major precipitation mechanism. A combination of Scanning Electron Microscopy (SEM) and Scanning Transmission X-Ray microscopy (STXM) on the OK-edge and Fe L-edge was used to clearly distinguish ferric iron precipitates from chlorite and to obtain the spatial distribution of iron precipitates on the chlorite surface under conditions of homogenous and heterogeneous formation. Oxygen K-edge results show the precipitation of a highly hydrolyzed chlorite surface associated ferric iron phase under heterogeneous conditions. Precipitates with different oxygen K-edge features are observed on the chlorite basal faces compared to the edge surfaces which can be attributed to cluster size effects and differences in the O/OH ratio. The results obtained from iron L-edge XANES corroborate the observations of the oxygen Kedge, indicating a thin surface layer of ferric iron precipitates and preferential precipitation at the chlorite edge surfaces. Under conditions where homogeneous ferrihydrite formation dominates, beside surface coatings separate iron colloids/nanoparticles are observed. Iron L-edge analysis and the pre-edge splitting on the oxygen K-edge indicate a higher long-range order/lower hydration comparable to ferrihydrite reference spectra. The results show that the Fe( Ill) secondary phase formation as surface coating or nanoparticles and their spatial distribution depends on the degree of supersaturation. These observations may have direct consequences for the retention and mobility of various contaminants in aquifer systems and potential remediation strategies.