Surface chemistry of hydrous ferric oxide and hematite as based on their reactions with Fe(II) and U(VI)
Open Access
- Author:
- Jang, Je-Hun
- Graduate Program:
- Environmental Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- January 07, 2004
- Committee Members:
- Brian Dempsey, Committee Chair/Co-Chair
William D Burgos, Committee Member
Gary Lee Catchen, Committee Member
David Lawrence Allara, Committee Member
Paul J Tikalsky, Committee Member - Keywords:
- reduction
sorption
surface chemistry
hematite
hydrous ferric oxide
transformation of iron oxides
Mossbauer spectroscopy - Abstract:
- Reactions of hydrous ferric oxide (HFO) and hematite (¥á-Fe2O3) were studied. Both HFO and hematite are of environmental importance in immobilizing contaminants through sorption and subsequent reduction. In anoxic environments, ferric (hydr)oxides with Fe(II) are important redox buffers. The hypothesis of this thesis is that the surface of hematite (the most thermodynamically stable ferric oxide at ambient temperature and pressure) was energetically similar to the surface of HFO. The reactions of interest are: 1) characterization of sub-mm hematite particles; 2) transformation of HFO in the presence of Fe(II), other metals, and various ligands; 3) sorption of U(VI) and Fe(II) onto HFO or hematite; 4) precipitation of schoepite; 5) reduction of U(VI) by Fe(II) in the presence of HFO or hematite plus natural organic matter (NOM); and 6) transformation of HFO or hematite induced by reduction of U(VI) by Fe(II). The experimental goals are: 1) synthesize sub-mm hematite by thermal hydrolysis, measure the solubility of Fe(III), and analyze solid phase(s) using Transmission 57Fe-Mössbauer spectroscopy, 2) analyze the transformation products of HFO and hematite using Transmission 57Fe-Mössbauer spectroscopy, 3) measure sorption and solubility of U(VI) in the presence of HFO or hematite using isotherm technique, and 4) measure the reduction of U(VI) by Fe(II) in the presence of HFO or hematite. Hematite (0.1 mm) was synthesized by thermal hydrolysis of acidic FeCl3 at 100 oC (Chapter II). The [Fe(III)]dissolved of the suspension was initially close to equilibrium with hematite at room temperature. As pH increases above ~3, [Fe(III)]dissolved was in equilibrium with HFO. However, Mössbauer spectroscopy detected only hematite. This finding indicates that the hydrated surface of hematite might be similar to HFO. The objective of Chapter III was to determine the effects of divalent metals or of ligands on transformation of HFO into more thermodynamically stable ferric oxides. Transformation of HFO to more stable phases can result in decreased surface area and lower redox potential. In some experiments, HFO was precipitated in the presence of chloride salts of Cu(II), Zn(II), Mn(II) and/or Fe(II). In other experiments, Fe(II) and Cl-, NO3-, or SO42- were added to pre-formed HFO. Transmission 57Fe-Mössbauer spectroscopy was used to monitor the phase changes. At pH 6.5 and 65 ¡ÆC, HFO was transformed into hematite in the presence of Zn(II) or Mn(II). Both metals were significantly adsorbed for these conditions, occupying about 1.2 sorption sites per nm2 of HFO surface. Transformations were not observed at pH 6.5 in the presence of Cu(II), which was weakly adsorbed (0.06 sites per nm2). No transformation occurred in the absence of the divalent metals. At pH 6.5 and room temperature, HFO plus Fe(II) transformed into poorly crystalline goethite in the presence of chloride, into goethite and lepidocrocite in the presence of sulfate, and into goethite and magnetite in the presence of nitrate. The reaction in the presence of nitrate was of particular interest, since the result leads to the inference that oxidation of Fe(II) led to the formation a fresh HFO that was more reactive than aged HFO. At pH 8.5 and room temperature, HFO that was formed with 0.033 or 0.33 mM Zn(II) and then aged with Fe(II) was transformed into magnetite that was depleted in octahedral Fe, i.e. non-stoichiometric or possibly mixed metal spinel, (Fe3+)IV(MexFe2+1-xFe3+)VIO4. HFO that was aged with Cu(II) and Fe(II) was transformed into goethite and into magnetite that was also depleted in octahedral Fe. The transformations at pH 8.5 were completely inhibited by 3.3 mM Zn(II) and transformations were significantly decreased by 3.3 mM Cu(II). These results have extended observations of the transformation of HFO to neutral pH ranges and to lower concentrations of metals than previously reported. In Chapter IV, the incorporation of U(VI) into solid phases (sorption and/or precipitation) in the presence of HFO or hematite was measured versus time (up to 48 days) for pH 5.9, 6.8, and 7.8. Experiments were run at room temperature, under ambient partial pressure of CO2 (10-3.5 atm), and with 0.01 M NaNO3. U(VI)T ranged from 0.1 ¥ìM to 7 mM. HFO or hematite concentration was 0.1 mM as Fe(III)T. At 0.01 day, plots of log [U(VI)solid-associated] vs. log [U(VI)dissolved] were linear with slopes less than one for all 3 pH values. A previously reported sorption model for U(VI) on HFO (based on sorption fronts) was modified to be consistent with the sorption isotherms that were reported in this chapter. Sorption reactions for (UO2)2(OH)2+2, (UO2)3(OH)5+, and UO2+2 were required in order to match sorption isotherms at all three pH values. The new model successfully modeled sorption on hematite after accounting for lower surface area and a slightly higher pHzpc. On and after 3 days, several data points that were initially super-saturated approached equilibrium with schoepite, except the most super-saturated initial conditions resulted in a more soluble UO3(s) precipitate. There are significant differences in reported thermodynamic constants for the formation of hydrolysis species of U(VI); the solubility data was used to justify selection and use of thermodynamic constants that were reported by Langmuir. In Chapter V, the abiotic reduction of U(VI) by Fe(II) was measured versus time in the presence of HFO or hematite, plus natural organic matter (NOM) in some experiments. Solid products were identified with Mössbauer spectroscopy. Although U(VI) reduction by Fe(II) is spontaneous, it has been reported that the abiotic reaction does not occur in the absence of ferric (hydr)oxides. About 80% of initial U(VI) was reduced by Fe(II) in the presence of hematite or HFO for low U(VI) concentrations such that sorption sites of the ferric oxides were sparsely occupied. The initial rate of reduction was slower in the presence of HFO compared to hematite, but reduction after three days was independent of the ferric oxide phase despite thermodynamic predictions that final U(VI) should have been three orders of magnitude lower in the presence of hematite compared to HFO. This indicated that the thermodynamic characteristics of hydrated hematite and HFO were similar, consistent with similar energy of sorption for the ferric oxides. The final condition indicated that the solubility of ferric (hydr)oxide consistent with goethite, for both HFO and hematite. NOM slowed the rate of reduction of U(VI) and decreased the eventual extent of reduction of U(VI) by HFO. Goethite was the major ferric (hydr)oxide phase identified by Mössbauer spectroscopy for experiments that started with HFO or hematite (1.8% 57-Fe) and Fe(II) (50% 57-Fe). Magnetite was also present. Mössbauer spectroscopy indicated that both hematite and HFO were partially converted to goethite, i.e. more goethite was present than could have been formed due to oxidation of Fe(II) that is stoichiometric with U(VI) reduction. Reduction did not occur when a higher concentration of U(VI) was used, that resulted in greater saturation of sorption sites, in spite of the larger thermodynamic driving force for reduction of U(VI).