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Formation of Lepidocrocite from Iron(II) Solutions: Stabilization by Citrate
Author(s) -
Krishnamurti G. S. R.,
Huang P. M.
Publication year - 1993
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1993.03615995005700030037x
Subject(s) - lepidocrocite , maghemite , goethite , chemistry , crystallization , precipitation , hydrous ferric oxides , inorganic chemistry , nuclear chemistry , oxide , magnetite , mineralogy , materials science , adsorption , metallurgy , organic chemistry , hematite , physics , sorption , meteorology
Goethite and maghemite are the stable species of Fe oxyhydroxides‐oxides formed in acidic and alkaline terrestrial environments, respectively. However, the mechanism of formation and stabilization of lepidocrocite in natural soil environments remains obscure. The influence of citrate on the freshly precipitated products formed from the oxidation of 0.01 M Fe(ClO 4 ) 2 ·6 H 2 O at pH 7.5 and a constant rate of O 2 supply at 5 mL/min −1 of air was studied by x‐ray diffraction, infrared, and transmission electron microscope analyses. The nature of precipitation products formed at pHs of 6.0 and 7.5 after aging for 9 mo in suspension was also studied. Presence of citrate promoted the formation of lepidocrocite at the expense of goethite and maghemite, which were the stable species of oxyhydroxide‐oxide of Fe formed at pHs of 6.0 and 7.5, respectively, in the absence of citrate. The promotion effect was especially pronounced at citrate/Fe molar ratios of 0.001 and 0.01 at pHs of 6.0 and 7.5, respectively. Further increase in the amounts of citrate to citrate/Fe molar ratios of 0.01 at pH 6.0 and 0.1 at pH 7.5 perturbed the crystallization processes and resulted in the formation of x‐ray noncrystalline Fe oxide. The influence was catalytic in nature and lasted at least the 9‐mo aging period. Even small amounts of citrate with concentrations of 10 −5 and 10 −4 M citrate at pHs of 6.0 and 7.5, respectively, promoted and stabilized the formation of lepidocrocite. Lepidocrocite formation as promoted by citrate would contribute, at least in part, to lepidocrocite formation in natural terrestrial environments.