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

Abstract

X-ray Photoelectron Spectroscopy (XPS) was used to investigate oxidation of aqueous Cr(III) at the surface of 7 {angstrom}-birnessite [MnO{sub 1.75}(OH){sub 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{sub 3/2}), Cr(2p{sub 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){sub aq} is adsorbed onto the MnO{sub 2} 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 thatmore » 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){sub 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.« less

Authors:
;  [1]
  1. Univ. of Western Ontario, London, Ontario (Canada). Dept. of Earth Sciences
Publication Date:
OSTI Identifier:
687699
Resource Type:
Journal Article
Journal Name:
Geochimica et Cosmochimica Acta
Additional Journal Information:
Journal Volume: 63; Journal Issue: 11-12; Other Information: PBD: Jun 1999
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; CHROMIUM COMPOUNDS; OXIDATION; MANGANESE OXIDES; SURFACE PROPERTIES; REACTION INTERMEDIATES; REDOX REACTIONS; SOILS; AQUATIC ECOSYSTEMS; ENVIRONMENTAL TRANSPORT

Citation Formats

Banerjee, D, and Nesbitt, H W. Oxidation of aqueous Cr(III) at birnessite surfaces: Constraints on reaction mechanism. United States: N. p., 1999. Web. doi:10.1016/S0016-7037(99)00003-4.
Banerjee, D, & Nesbitt, H W. Oxidation of aqueous Cr(III) at birnessite surfaces: Constraints on reaction mechanism. United States. https://doi.org/10.1016/S0016-7037(99)00003-4
Banerjee, D, and Nesbitt, H W. 1999. "Oxidation of aqueous Cr(III) at birnessite surfaces: Constraints on reaction mechanism". United States. https://doi.org/10.1016/S0016-7037(99)00003-4.
@article{osti_687699,
title = {Oxidation of aqueous Cr(III) at birnessite surfaces: Constraints on reaction mechanism},
author = {Banerjee, D and Nesbitt, H W},
abstractNote = {X-ray Photoelectron Spectroscopy (XPS) was used to investigate oxidation of aqueous Cr(III) at the surface of 7 {angstrom}-birnessite [MnO{sub 1.75}(OH){sub 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{sub 3/2}), Cr(2p{sub 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){sub aq} is adsorbed onto the MnO{sub 2} 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){sub 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.},
doi = {10.1016/S0016-7037(99)00003-4},
url = {https://www.osti.gov/biblio/687699}, journal = {Geochimica et Cosmochimica Acta},
number = 11-12,
volume = 63,
place = {United States},
year = {Tue Jun 01 00:00:00 EDT 1999},
month = {Tue Jun 01 00:00:00 EDT 1999}
}