The Interaction of Prussian Blue and Dissolved Hexacyanoferrate Ions with Goethite (α-FeOOH) Studied to Assess the Chemical Stability and Physical Mobility of Prussian Blue in Soils

Colloidal Prussian blue and dissolved hexacyanoferrate ions strongly interact with the surface of goethite (α-FeOOH) particles. Whereas Prussian blue is deposited on the surface of goethite as a solid phase with all the properties typical of solid Prussian blue, the hexacyanoferrate ions form initially a layer of surface bound hexacyanoferrate ions, which exhibit properties intermediate between those of free hexacyanoferrate ions and hexacyanoferrate ions bound in Prussian blue. The products of interaction of goethite with Prussian blue and hexacyanoferrate ions were studied by voltammetry of immobilized microcrystals, infrared spectrometry, Vis diffuse reflection spectrometry, transmission electron microscopy, and ESR spectroscopy. The kinetics of Prussian blue destruction was measured by solution Vis spectrometry and also by voltammetry of solid microcrystals. The remarkable stability of Prussian blue in soils can be explained by its strong adherence as a solid phase to iron oxides and oxide hydroxides in soils. This prevents the formation of soluble colloids of Prussian blue. It does not prevent the hydrolysis of Prussian blue to iron oxide hydroxides and hexacyanoferrate(II) ions at elevated pH which proceeds at almost the same rate as that of pure Prussian blue colloid. Even when this hydrolysis of Prussian blue occurs, it is the hexacyanoferrate(II) that is effectively adsorbed on the surface of iron oxide hydroxides, again decreasing the physical mobility of hexacyanoferrate.