An Infrared and X-ray Absorption Study of the Structure and Equilibrium of Chromate, Bichromate, and Dichromate in High-Temperature Aqueous Solutions

Abstract

The structure and speciation of bichromate, dichromate, and chromate was studied to temperatures up to 400 °C using the combined information obtained from infrared (IR) and X-ray absorption fine structure (XAFS) spectroscopy. Actual species concentrations were obtained from the IR measurements, which were then used as fixed input parameters for the XAFS analysis. With this procedure a complete description of the molecular structure of each chromate species to high temperatures was obtained. In agreement with previous high temperature studies on the chromate system, the bichromate species was found to become thermodynamically favored at high temperatures under acidic or near-neutral conditions. For example, only the bichromate species was found to be present at 350 °C in a 0.145 m aqueous chromate solution having an initial solution pH of 3.3. Because of this, the bichromate molecular structure could be fully characterized from the measured XAFS data. The first-shell structure around the central chrome atom is extremely similar in the bichromate and dichromate structures with virtually identical Cr−O bond distances for the three terminal oxygen atoms and with only a slightly different Cr−O bond distance for the bridging oxygen. As a result of this structural similarity, the νas(CrO3) asymmetric stretching frequency of the bichromate and dichromate species nearly overlap in the IR, as is evident from the quantitative analysis of this band. Furthermore, the chromate structure in the ion-paired species, (Na+)(CrO4 2-), at high temperature is nearly identical to that in the free chromate ion.

An X-ray absorption spectroscopy study of argutite solubility and aqueous Ge(IV) speciation in hydrothermal fluids to 500 °C and 400 bar

“…The Ge-O distances change only very little with increasing temperature (in the limit of 0.01 Å from 20 to 450°C). This behavior is similar to that of other strongly-bound (OH)/O aqueous complexes like arsenite (As(OH) 3°, Pokrovski et al, 2002a;Testemale et al, 2004) Hoffmann et al, 2001) which also show only minor changes in the metal-oxygen distances (within 0.01 Å) and DW factors in a wide temperature range. Hydrated mono-and divalent and some large trivalent cations (e.g., Rb + , Ag + , Sr 2+ , Yb 3+ , Fulton et al, 1996;Seward et al, 1996;1999;Mayanovic et al, 2002), and aqueous neutral or charged complexes with a more ionic character (e.g., SbCl 3 , SbCl 4 -, Oelkers et al, 1998 , Mayanovic et al, 2001;Simonet et al, 2002) demonstrate, in contrast, partial loss of the 1 st water hydration shell and significant shortening of the hydration shell radius and metal-ligand bond length with increasing temperature, typically about 0.05-0.10 Å in the range 25-300°C.…”

Ab-initio structure, energy and stable Cr isotopes equilibrium fractionation of some geochemically relevant H-O-Cr-Cl complexes

“…According to Hoffmann et al (2001aHoffmann et al ( , 2001b) the -CrO 3 structural unit is nearly identical in bichromate and dichromate structures, particularly in terms of Cr to terminal O bond distances, and this results in similar vibrational attitudes in relation to the asymmetric stretching of the structural unit (Tables 7 and 8). Due to the importance of this complex as a ligand, a different set of computational data are available in the literature, obtained by different ab initio procedures and with more or less extended basis sets.…”

“…Table 9). According to Hoffmann et al (2001b) the two -CrO 3 units in the aqueous cluster are not coaxial but entail a Cr-O-Cr angle of 123.5°. Results of our gas-phase calculations indicate a more symmetric gaseous molecule with three identical bond lengths for chromium linked to terminal oxygens (O T ) and two identical (and longer) lengths for chromium linked to the central oxygen (O C ).…”

Molecular-scale study of Cr(vi) adsorption onto lepidocrocite facets by EXAFS, in situ ATR-FTIR, theoretical frequency calculations and DFT+U techniques

“…The region of Cr-O stretching vibrations was distributed between Environmental Science: Nano Paper 1000 and 800 cm −1 , while peak positions below 800 cm −1 may be ascribed to Cr-OH stretching, Cr-O-Fe bridge vibrations, or OH torsional modes. 52 The deconvoluted spectra of Cr(VI)adsorbing P-LEP and R-LEP are shown in Fig. S7.…”