Oxidation of aqueous Cr(III) at birnessite surfaces: Constraints on reaction mechanism

X-ray Photoelectron Spectroscopy (XPS) was used to investigate oxidation of aqueous Cr(III) at the surface of 7 Angstrom-birnessite [MnO1.75(OH)(0.25)]. Special emphasis was placed on detection of intermediate oxidation states of chromium due to their critical environmental significance. No previous studies have been able to identify these intermediate oxidation states of chromium (namely, Cr[IV] and Cr[V]) on mineral surfaces or in natural solutions. Mn(2p(3/2)), Cr(2p(3/2)) and O(1s) spectra of the reacted surfaces reveal that Mn(IV) of synthetic birnessite undergoes reductive dissolution in two steps. The first step involves Mn(IV) reduction to Mn(III), that forms at the oxide surface probably as an oxyhydroxide (MnOOH), and in the second step Mn(III) is reduced to Mn(II) that is subsequently taken into solution. Each reductive reaction step involves transfer of only one electron to the Mn ion. After Cr(III)(aq) is adsorbed onto the MnO2 surface, it undergoes oxidation in three separate steps, each involving the loss of one electron to Mn ions, so that Cr(IV), Cr(V) and Cr(VI) are produced. The intermediate reaction products, namely Mn(III), and Cr(V) were positively identified by XPS spectral analyses. Similarity in XPS binding energy values of Cr(III) and Cr(IV) as well as that of Cr(V) and Cr(VI), however, preclude separate identification of Cr(III) from Cr(IV) and Cr(VI) from Cr(V) multiplets on the near-surface of the solid. A parallel reaction scheme (exclusive of sorption reactions) best describes the birnessite-Cr(III)(aq) redox reactions. The two parallel reactions proceed by separate mechanisms with a monodentate complex formed in one mechanism and a bidentate complex in another. The bulk of Cr(IV) probably is formed via the monodentate complex and Cr(V) via the bidentate complex. The rate expressions associated with these reactions display near-perfect correlation with changing surface abundances of Cr(IV) and Cr(V) as a function of reaction time. Copyright (C) 1999 Elsevier Science Ltd.