Jerzy A. Sawicki

PRECIPITATION OF IRON MINERALS BY A NATURAL MICROBIAL CONSORTIUM

Abstract A microbial biofilm consortium enriched from Shield surface water is able to mediate geochemical cycling of iron within a biofilm. Iron can be leached from Fe(II) containing minerals such as magnetite, biotite and ilmenite to generate a colloidal Fe(III) suspension. The Fe(III) can then be reduced back to Fe(II) by iron-reducing bacteria that utilize it as an electron acceptor. On precipitation, different iron compounds are formed depending on the ratio of iron to carbon in the media and upon the local environment. Mossbauer and X-ray diffraction spectroscopy show these compounds to include ferrous hydroxide, vivianite, ferrihydrite and hematite. These minerals may then become incorporated into stratifer iron deposits such as Banded Iron Formations.

MICROBIAL TRANSFORMATION OF MAGNETITE TO HEMATITE

Abstract A microbial consortium enriched from surface water draining the Lac du Bonnet granitic batholith in Manitoba, Canada, was able to mediate the transformation of 11 % of the Fe in a sample of magnetite into hematite within three weeks. Holes, up to 100 μm in diameter, were formed in grains of magnetite in a polished section of granite that had been exposed to a microbial biofilm in the laboratory. Bacteria were seen by scanning electron microscopy to be attached to magnetite after three days incubation in a flask, while a covering of biofilm was nearly complete after six days. Some hexagonal grains of an iron mineral were visible after incubation for eight weeks. Dissolution of the magnetite and the presence of hematite in the incubated flasks was confirmed by Mossbauer spectra and powder X-ray diffraction patterns. No hematite was found either in the original sample of magnetite or in duplicate sterile controls.

IRON-BEARING PRECIPITATES IN ZIRCALOYS: A MOSSBAUER SPECTROSCOPY STUDY

Abstract Mossbauer spectroscopy of 57Fe was used to study the chemical form and distribution of iron in Zircaloy-2 and Zircaloy-4. The spectra of Zircaloy-4 consist of two overlapping quadrupole-split doublets, which can be ascribed to precipitates of the pseudobinary Laves-phase intermetallic compound Zr(Cr,Fe)2, which has a hexagonal MgZn2 (C14) type structure and cubic MgCu2 (C15) type structure. In Zircaloy-2, a large fraction of iron was also found in the form of Zr2(Fe,Ni) precipitates, having a tetragonal Al2Cu (C16) type structure. Characteristic room-temperature values of Mossbauer spectral parameters were determined for the three types of intermetallic precipitates. At liquid helium temperature, both alloys exhibit a poorly resolved magnetic hyperfine splitting, attributed to the low-temperature ferromagnetism of cubic Zr(Cr,Fe)2 Laves-phase particles. This effect is more pronounced in Zircaloy-4 than in Zircaloy-2. The possibility of determining volume fractions of different phases from their relative spectral areas is discussed.

Evidence of zinc ferrite formation on carbon steel in primary-side coolant with added soluble zinc

Abstract Conversion electron and X-ray backscattering Mossbauer spectroscopy was used to identify iron compounds that were formed on carbon steel in simulated CANDU® reactor coolant containing 15 to 60 ppb (μg/L H 2 O) soluble zinc. Analyses of the coupons exposed to coolant in the absence of zinc indicated the formation of magnetite, whereas, in the presence of zinc, the formation of a layer of zinc-bearing ferrite Zn x Fe 3− x O 4 (with x > 0.8 was observed. The role of the high electrical resistivity of zinc ferrite in reducing corrosion rates, corrosion-product release rates, and 60 Co activity buildup on carbon steel is briefly discussed.

197AU,57Fe and121Sb Mössbauer study of gold minerals and ores

A survey of the197Au Mössbauer spectra of naturally occurring gold species is given. Gold minerals have been studied as natural specimens or as synthetic analogues. Gold impurities have been identified in pyrites and arsenopyrites. An example of the use of121Sb, and57Fe Mössbauer spectroscopy in characterizing gold-bearing ore minerals is given.

Mössbauer spectroscopy of tin in unirradiated and neutron irradiated Zircaloys

Abstract Mossbauer spectroscopy of the 23.9 keV γ-rays in 119Sn nuclei was applied to study Zircaloy-2, Zircaloy-4, and other tin-bearing zirconium-based alloys of interest to nuclear power technology. Zircaloys are extensively used in nuclear reactors as fuel cladding. In CANDU reactors, Zircaloys are also used as major structural components such as calandria tubes, and were used until the late 1970s as pressure tubes (now replaced by Zr–2.5Nb alloy). Unirradiated specimens of these alloys, as well as radioactive specimens, both neutron-irradiated in high-flux test reactors and extracted from nuclear power-reactor components after many years of service, were examined. The obtained spectra consistently showed tin in substitutional solid solution in α-Zr, whereas no evidence was found of metallic Sn or intermetallic Zr4Sn precipitates. In oxide scrapes removed from Zircaloy-2 pressure tube of one of CANDU reactors, where the alloy was exposed for about 10 years to pressurized heavy water coolant at temperatures of ∼280°C, a considerable fraction of tin was found in the Sn(IV) state, in the form that coincides with the state of tin in stannic oxide, SnO2. The same form of tin was identified in filterable deposits in the primary heavy water coolant of CANDU reactors. For comparison, in Zircaloy heated in air, SnO2 was formed only at temperatures above 500°C.

Conversion electron Mössbauer spectroscopy of the 73.0 keV 193Ir resonance

Abstract Conversion electron Mossbauer spectra of 73.0 keV γ-rays from iridium 193Ir were obtained at liquid helium temperature by detecting low-energy electrons emitted from the resonant iridium absorber with a channel electron multiplier. This new technique analyzes iridium within a depth of about 1 μm from the surface of materials, with a sensitivity several times greater than in conventional Mossbauer transmission measurements. A resonant effect of up to 10% has been observed for thin metallic iridium and IrO2 films having natural isotopic composition.

Mössbauer study of corrosion products from a CANDU secondary system

Mossbauer spectroscopy was used to identify corrosion products in the secondary loop of CANDU reactors at the Darlington nuclear generating station. Analysis of the filtered deposits shows that during the early operational phase iron is transported mostly as suspended particles of Fe3O4, α-Fe2O3 and λ-FeOOH. The Fe3O4/ α-Fe2O3 ratio decreases when going from the condensate extraction pump, through the boiler feed pump to the high-pressure heater outlet. α-FeOOH has been found in the condenser hot-well and deaerator. Fe1−x and metallic Fe have also been observed in several samples.

Annealing behaviour of iron implanted zirconia

A zirconia film was implanted at room temperature with 100 keV57Fe+ to a fluence of 8×1016 ions/cm2. The analysis of the Conversion Electron Mössbauer spectra shows that iron is distributed among different charge states: Fe0 (in a form of small and large metallic iron aggregates) Fe2+ and Fe4+. The evolution of the iron depth profile deduced from Rutherford backscattering measurements as well as the change in the charge states of iron as a function of annealing under argon atmosphere are presented.

193Ir Mössbauer study of bimetallic platinum-iridium catalysts

Using the 73.0 keV Mössbauer resonance in193Ir, the chemical form of iridium in bimetallic Pt−Ir catalysts supported on amorphous silica has been determined after ionic co-exchange, calcination and reduction in hydrogen. The compositions of the highly dispersed bimetallic Pt1−xIrx clusters as determined from the measured isomer shifts reveal a strong tendency for segregation of iridium and platinum.