Chemical transformation of ferrihydrite coating into green rust followed by Raman and X-ray photoelectron spectroscopies

This study describes the mineralogical transformation of ferrihydrite (Fh) coated on grains of pozzolana into mixed Fe-II-Fe-III sulphated green rust {GR( [GRAPHICS] )}. The formed GR( [GRAPHICS] ) coating is intended to be used for the reduction of nitrate in batch and column experiments. Raman and X-ray photoelectron spectroscopies (XPS) are used to characterize the pozzolana substrate and the coatings and proved that in the presence of sulphated anions and Fe-II cations, Fh is transformed into chemically stable sulphated green rust. The Raman spectra exhibit the presence of characteristic bands of sulphated green rust, spectra on which we rely on to interpret the O 1s XPS high resolution spectra which are characterized by a major contribution located around 531.5eV, in good agreement with the formation of OH- coordinated to Fe-II and Fe-III ions in brucite-like layers of GR. The reduction of nitrate by GR( [GRAPHICS] ) coatings is then investigated from combined solution and solid phases analyses. In agreement with previous studies carried out on bulk mineral, GR( [GRAPHICS] ) coating reduces nitrate into ammonium and is mainly transformed into magnetite.