Lepidocrocite surface hydroxyls

It should be noted that minerals, and oxide minerals in particular, have different types of OH groups, depending on the coordination of the O atoms, as revealed by spectroscopic studies. Goethite (a-FeOOH) has four types of surface hydroxyls whose reactivities are a function of the coordination environment of the O in the FeOH group (Sposito 1984 Sparks 2002). The FeOH groups are A-, B-, or C-type sites, depending on whether the O is coordinated with 1,3, or 2 adjacent Fe(III) ions. The fourth type of site is a Lewis acid-type site, which results from chemisorption of a water molecule on a bare Fe(III) ion. Only A-type sites are basic (can bind H+), and, on the contrary, A-type and Lewis acid sites can release a proton. The B- and C-type sites are considered unreactive. Thus, A-type sites can be either a proton acceptor or a proton donor (i.e., they are amphoteric). Other spectroscopic studies have shown that boehmite (y-AlOOH) and lepidocrocite (y-FeOOH) have two types of OH, presumably associated with different crystal faces (Lewis and Farmer 1986). 

Lewis, D. G., and V. C. Farmer. 1986. Infiared absorption of surface hydroxyl groups and lattice vibrations in lepidocrocite (y-FeOOH) and boehmite (y-AlCX)H). Clay Minerals 21, no. 1 93-100. doi 10.1180/claymin.l986.021.1.08. 

Corrosion of iron or steel (anodic reaction Fe —> Fe + 2e and cathodic reaction 4Fe + O2 + tOH20 4Fe(OH)3 + 8H+) leads to formation of surface hydroxides. The main constituents of oxides formed during atmospheric corrosion of steels are y-FeOOH (lepidocrocite), a-FeOOH (goethite), y5-FeOOH (akaganite) and -FeOOH (feroxyhite). These hydroxyl groups in rast have a tendency to react with hydroxyl groups of hydrolyzed metal alkoxide from sol-gel coatings. 

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