REACTIVITY OF VARIOUS TYPES OF IRON(III) (HYDR)OXIDES TOWARDS LIGHT-INDUCED DISSOLUTION

Laboratory experiments were conducted on the light-induced dissolution of three well defined Fe(III) (hydr)oxide phases (gamma-FeOOH, alpha-FeOOH, and alpha-Fe2O3) with oxalate as reductant/ligand. Upon irradiation of an aerated gamma-FeOOH suspension of pH 3, photooxidation of oxalate and photochemical formation of dissolved Fe(II) occurred according to a 1:1 stoichiometry. This was not observed with aerated alpha-FeOOH and alpha-Fe2O3 suspensions of pH 3, where photooxidation of oxalate was not accompanied by formation of appreciable concentrations of dissolved Fe(II). We hypothesize that in aerated alpha-FeOOH and alpha-Fe2O3 suspensions, oxidation of surface Fe(II) outcompetes its detachment from the crystal lattice. Also in deaerated suspensions, alpha-FeOOH and alpha-Fe2O3 behaved differently from gamma-FeOOH with regard to light-induced dissolution. We interpret our results by assuming that light-induced dissolution of alpha-FeOOH and alpha-Fe2O3 in deaerated suspensions of pH 3 occurred mainly through Fe(II)-catalyzed thermal dissolution of the solid phases, where Fe(II) was initially formed by photoreductive dissolution and then predominantly via photolysis of dissolved Fe(III) oxalate complexes. With gamma-FeOOH, on the other hand, dissolved Fe(II) formation occurred probably mainly through photochemical reductive dissolution under photooxidation of adsorbed oxalate. From our results we conclude that the efficiency of detachment of reduced surface iron is a key parameter of the overall kinetics of photoreductive dissolution of Fe(III) (hydr)oxides in aquatic systems, and that thermodynamically stable phases such as alpha-FeOOH and alpha-Fe2O3 are not readily dissolved in the presence of O-2, even at low pH-values and in the presence of light and reductants like oxalate. We propose that redox cycling of iron at the surface of crystalline Fe(III) (hydr)oxide phases, i.e. reduction and oxidation of surface iron without transfer into solution, may be an important pathway of transformation of thermodynamically stable atmospheric Fe(III) (hydr)oxides into less stable and thus more soluble phases.