Environ. Sci. Technol. 1997, 31, 2898-2902
Precipitation Equilibria of the Chromium(VI)/Iron(III) System and Spectrospcopic Characterization of the Precipitates M . A . O L A Z A B A L , † N . P . N I K O L A I D I S , * ,‡ S. A. SUIB,§ AND J. M. MADARIAGA† Kimika Analitikoaren Departmentua, Euskal Herriko Unibertsitatea, 644 P. K., E-48080 Bilbao, Spain, Environmental Engineering Program, Department of Civil and Environmental Engineering, University of Connecticut, Storrs, Connecticut 06269, and Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269
Precipitation equilibria and spectroscopic studies of the Cr(VI)/Fe(III)/H2O system have been performed to identify the aqueous and solid speciation of the system in the pH range 1-9. The results suggest that three different precipitates FeOHCrO4, FeOHCrO4‚2Fe(OH)3, and Fe(OH)3 exist depending on the pH range. The last two precipitates have been reported in the literature before, and they are confirmed in this study. The formation of a new precipitate, FeOHCrO4, that predominates in the pH range 1.5-2.5 is being suggested, and its thermodynamic solubility constant was estimated to be pKso ) 22.50 ( 0.07. The mixed precipitate predominated in the 2.5-3.5 pH range and the pure ferric iron hydroxide predominated in the pH range grater than 3.5. All three precipitates were analyzed using X-ray photoelectron spectroscopy (XPS). The spectroscopic results suggest that the first and second precipitates contain one type of chromium species which corresponds to Cr(VI) type and one type of Fe species. The third precipitate exhibited two peaks with width greater than 5 eV, indicating the presence of two chromium species, one of which is closely related to CrOOH type of binding and the other may be attributed to metal-chromate binding. We postulate that in this case chromate was adsorbed onto the iron hydroxide forming two surface complexes. Since iron is abundant in nature, the results of this study are important for the understanding of the mobility and reactivity of hexavalent chromium in contaminated groundwater sites.
Introduction Contamination of soils and groundwaters with chromium and other heavy metal resulting from direct spillage and past disposal practices (such as disposal of sludges in laggons) can frequently be found near the properties of metal-finishing industries. Nikolaidis and co-workers (1) showed that, in a chromium contaminated glaciofluvial aquifer, most of the chromium mass (more than 99%) is bound to the soil. An analysis of the processes that control the binding of chromium with the soil conducted by Asikainen and Nikolaidis (2) and * To whom correspondence should be addressed. Telephone: (860) 486-5648; fax: (860) 486-2298; e-mail:
[email protected]. † Euskal Herriko Unibertsitatea. ‡ Department of Civil and Environmental Engineering, University of Connecticut. § Department of Chemistry, University of Connecticut.
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Nikolaidis and co-workers (1) indicated that surface processes such as binding of chromium to soil organic matter and adsorption to iron oxide coatings can explain only a small fraction of the apparent partitioning of chromium in the aquifer. Both studies postulated the existence of slow, kinetically controlled processes or diffusive mechanisms that are predominant in that aquifer. In addition, the strength of the extractants used in the Asikainen and Nikolaidis (2) study indicated that chromium was tightly bound to the soil matrix. These conclusions were confirmed with results of adsoption experiments using uncontaminated soil (3) and a modeling study conducted by Shen (4). Baron and co-workers (5) identified two iron-chromate precipitates [KFe3(CrO4)2(OH)6 and KFe(CrO4)2‚2H2O] in a soil contaminated with chrome plating solutions using electron microscopy and powder X-ray diffraction techniques. They reported that the solutions were highly acidic, and the lowest groundwater pH measured at the site was 2.3. With the occurrence of iron-chromate precipitates in the environment and the fact that ferric iron is an abundant soil constituent in glaciated soils and aquifers, a study of its interaction with Cr(VI) is warranted. Olazabal and co-workers (6) studied the complexation and precipitation equilibria of the Cr(VI)/Fe(III)/H2O system in the pH range between 2.5 and 3.5. Their results suggest the formation of a soluble complex, FeCrO4+ as well as a solid with stoichiometry of Fe3OH7CrO4. They also postulated that the solid was a mixed precipitate of the form FeOHCrO4‚ 2Fe(OH)3. Considering the similar behavior of CrO42- and other anions like SO42- (7), which forms a FeOHSO4 precipitate (8), the possible formation of a FeOHCrO4 precipitate in a more acidic pH range can be expected. On the basis of this, the suggestion of the formation of a mixed precipitate, though not confirmed, was reasonable. The objective of this study was to conduct an equilibrium study of the Cr(VI)/Fe(III)/H2O system that would define the stoichiometries and solubility constants of all precipitates formed in the pH range between 1 and 9 and characterize the precipitates using spectroscopic techniques. In particular, our aim was to examine the possible occurrence of a FeOHCrO4 precipitate and show whether the Fe3OH7CrO4 precipitate identified by Olazabal et al. was a mixed precipitate of the form FeOHCrO4‚2Fe(OH)3 or not.
Experimental Section Apparatus and Experimental Technique. The precipitation experiments were performed using the same concentration of metals (50 mM) and changing the pH of the solution by adding different volumes of a KOH solution. Since the initial chomium and iron solution (before the addition of KOH) was very acidic (pH
