TY - JOUR
T1 - Photoreductive dissolution of colloidal iron oxide
T2 - Effect of citrate
AU - Waite, T. David
AU - Morel, Francois M. M.
N1 - Funding Information:
The authors thank V. Burns, M. Frongillo, G. Pa-paefthymiou, G. Wiseman, T. Ring, and D. Dzombak for help with various aspects of the work. The useful input of D. Sherman and R. Frankel is also acknowledged. This research was supported by NSF Grant OCE-8119103 and NOAA Grant NA79AA-D-00077.
PY - 1984/11
Y1 - 1984/11
N2 - The light-induced dissolution of the iron oxide, lepidocrocite (γ-FeOOH), has been investigated and found to be greatly enhanced in the presence of citrate. A conceptual model of the dissolution process is presented and validated through studies of citrate adsorption to lepidocrocite, net iron oxide dissolution under a variety of conditions, and solution phase redox reactions. The initial dissolution rate is directly related to the concentration of the surface bound ferric citrate and the first order rate constant for the photodissolution process is very similar to that found for the photodissociation of soluble ferric citrate. Dissolution most likely occurs through direct excitation of charge transfer bands of the surface bound ferric citrate. At low pH (pH 4.0), a constant rate of dissolution is observed while at higher pH (pH 6.5 and 8.2), the dissolution rate decreases on continued photolysis. This decrease is due to (1) an oxygen-dependent deactivation process occurring at the surface and (2) loss of photo-produced iron from solution by "ligand-like" adsorption of soluble iron citrate complexes by the colloidal iron oxide. Superimposed on the dissolution process at these higher pH is a rapid oxidation-reduction cycle involving solution phase iron species with the reduction step induced by photodissociation of ferric citrate complexes and the oxidation step controlled by the formation of ferrous citrate.
AB - The light-induced dissolution of the iron oxide, lepidocrocite (γ-FeOOH), has been investigated and found to be greatly enhanced in the presence of citrate. A conceptual model of the dissolution process is presented and validated through studies of citrate adsorption to lepidocrocite, net iron oxide dissolution under a variety of conditions, and solution phase redox reactions. The initial dissolution rate is directly related to the concentration of the surface bound ferric citrate and the first order rate constant for the photodissolution process is very similar to that found for the photodissociation of soluble ferric citrate. Dissolution most likely occurs through direct excitation of charge transfer bands of the surface bound ferric citrate. At low pH (pH 4.0), a constant rate of dissolution is observed while at higher pH (pH 6.5 and 8.2), the dissolution rate decreases on continued photolysis. This decrease is due to (1) an oxygen-dependent deactivation process occurring at the surface and (2) loss of photo-produced iron from solution by "ligand-like" adsorption of soluble iron citrate complexes by the colloidal iron oxide. Superimposed on the dissolution process at these higher pH is a rapid oxidation-reduction cycle involving solution phase iron species with the reduction step induced by photodissociation of ferric citrate complexes and the oxidation step controlled by the formation of ferrous citrate.
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U2 - 10.1016/0021-9797(84)90206-6
DO - 10.1016/0021-9797(84)90206-6
M3 - Article
AN - SCOPUS:0021529151
SN - 0021-9797
VL - 102
SP - 121
EP - 137
JO - Journal of Colloid And Interface Science
JF - Journal of Colloid And Interface Science
IS - 1
ER -