Secondary Mineral Products Research Articles

Aggregate-scale spatial heterogeneity in reductive transformation of ferrihydrite resulting from coupled biogeochemical and physical processes

Iron (hydr)oxides are ubiquitous in soils and sediments and play a dominant role in the geochemistry of surface and subsurface environments. Their fate depends on local environmental conditions, which in structured soils may vary significantly over short distances due to mass-transfer limitations on solute delivery and metabolite removal. In the present study, artificial soil aggregates were used to investigate the coupling of physical and biogeochemical processes affecting the spatial distribution of iron (Fe) phases resulting from reductive transformation of ferrihydrite. Spherical aggregates made of ferrihydrite-coated sand were inoculated with the dissimilatory Fe-reducing bacterium Shewanella putrefaciens strain CN-32, and placed into a flow reactor, the reaction cell simulates a diffusion-dominated soil aggregate surrounded by an advective flow domain. The spatial and temporal evolution of secondary mineralization products resulting from dissimilatory Fe reduction of ferrihydrite were followed within the aggregates in response to a range of flow rates and lactate concentrations. Strong radial variations in the distribution of secondary phases were observed owing to diffusively controlled delivery of lactate and efflux of Fe(II) and bicarbonate. In the aggregate cortex, only limited formation of secondary Fe phases were observed over 30 d of reaction, despite high rates of ferrihydrite reduction. Under all flow conditions tested, ferrihydrite transformation was limited in the cortex (70–85 mol.% Fe remained as ferrihydrite) because metabolites such as Fe(II) and bicarbonate were efficiently removed in outflow solutes. In contrast, within the inner fractions of the aggregate, limited mass-transfer results in metabolite (Fe(II) and bicarbonate) build-up and the consummate transformation of ferrihydrite – only 15–40 mol.% Fe remained as ferrihydrite after 30 d of reaction. Goethite/lepidocrocite, and minor amounts of magnetite, formed in the aggregate mid-section and interior at low lactate concentration (0.3 mM) after 30 d of reaction. Under high lactate (3 mM) concentration, magnetite was observed only as a transitory phase, and rather goethite/lepidocrocite and siderite were the dominant secondary mineralization products. Our results illustrate the importance of slow diffusive transport of both electron donor and metabolites concentrations and concomitant biogeochemical reactions within soils and sediments, giving rise to heterogeneous products over small spatial (μm) scale.

XAFS Investigation of the Interactions of UVI with Secondary Mineralization Products from the Bioreduction of FeIII Oxides

Biogenic Fe(II) phases (magnetite, green rust, siderite, vivianite, etc.) provide a reservoir of reducing capacity in many subsurface environments that may contribute to the reduction of contaminants such as U(VI). We have examined the uptake and reduction of U(VI) in the presence of biogenic green rust (BioGR), magnetite (BioMAG), and siderite (BioSID) formed during the reduction of Fe(III) oxides by Shewanella putrefaciens CN32. Within 48 h, total solution-phase U(VI) concentrations decreased from 500 microM to 1.5 microM, 392 microM, and 472 microM in the U-BioGR, U-BioMAG, and U-BioSID systems, respectively. Analysis of the samples by U L(III) extended X-ray absorption fine structure spectroscopy (EXAFS) indicated that despite a stoichiometric excess of Fe(II), no more than 6% of U(VI) was reduced to U(IV) in the U-BioSID system, and no more than 22% of U(VI) was reduced in the U-BioMAG system. For comparison, in the U-BioGR system, >99% of U(VI) was reduced to U(IV). Uptake of U(VI) by BioGR and BioMAG was accompanied by formation of nanoparticulate uraninite. The U EXAFS data for the U-BioSID system were consistent with partial U(VI)/U(IV) substitution for Fe(II) in the surface layer of siderite particles and adsorption of U(IV).

Iron phase transformations resulting from the respiration of Shewanella putrefaciens on a mixed mineral phase

The initial Fe(III) minerals and the secondary mineralization products of Shewanella putrefaciens CN32 grown in the presence of dissolved phosphate and a commercial Fe(III) oxide, nominally nanoparticulate lepidocrocite, were determined using XRD and XAFS. The starting material was transformed by the bacteria from a reddish brown, rust colour mineral to a dark green phase over 90 days. Acid extraction of the bioreduced solids with 0.75 M HCl recovered 83% of the total iron as Fe(II), leaving a solid, acid-resistant phase. The latter was identified as nanoparticulate hematite by EXAFS. Subsequently, the starting Fe(III) phase was determined to be a mixture of 60% lepidocrocite, 26% ferrihydrite, and 14% hematite, using linear combination EXAFS analysis. For the acid-extractable phase, XANES and EXAFS indicated a predominantly Fe(II) valence state and a spectrum consistent with a mixture of brucite-type minerals(e.g., green rust or ferrous hydroxide) and siderite. The observed transformations suggest that in this mixed-mineral system, lepidocrocite and ferrihydrite are readily reducible to green rust and siderite, whereas hematite is less amenable to bacterial reduction. This study also demonstrates the utility of XAFS spectroscopy in the quantitative characterization of dissimilatory metal transformations, particularly in complex systems such as nanoparticulate minerals in hydrated mineral-bacteria assemblages.

Alkali feldspar dissolution and secondary mineral precipitation in batch systems: 3. Saturation states of product minerals and reaction paths

In order to evaluate the complex interplay between dissolution and precipitation reaction kinetics, we examined the hypothesis of partial equilibria between secondary mineral products and aqueous solutions in feldspar–water systems. Speciation and solubility geochemical modeling was used to compute the saturation indices (SI) for product minerals in batch feldspar dissolution experiments at elevated temperatures and pressures and to trace the reaction paths on activity–activity diagrams. The modeling results demonstrated: (1) the experimental aqueous solutions were supersaturated with respect to product minerals for almost the entire duration of the experiments; (2) the aqueous solution chemistry did not evolve along the phase boundaries but crossed the phase boundaries at oblique angles; and (3) the earlier precipitated product minerals did not dissolve but continued to precipitate even after the solution chemistry had evolved into the stability fields of minerals lower in the paragenesis sequence. These three lines of evidence signify that product mineral precipitation is a slow kinetic process and partial equilibria between aqueous solution and product minerals were not held. In contrast, the experimental evidences are consistent with the hypothesis of strong coupling of mineral dissolution/precipitation kinetics [e.g., Zhu C., Blum A. E. and Veblen D. R. (2004a) Feldspar dissolution rates and clay precipitation in the Navajo aquifer at Black Mesa, Arizona, USA. In Water–Rock Interaction (eds. R. B. Wanty and R. R. I. Seal). A.A. Balkema, Saratoga Springs, New York. pp. 895–899]. In all batch experiments examined, the time of congruent feldspar dissolution was short and supersaturation with respect to the product minerals was reached within a short period of time. The experimental system progressed from a dissolution driven regime to a precipitation limited regime in a short order. The results of this study suggest a complex feedback between dissolution and precipitation reaction kinetics, which needs to be considered in the interpretation of field based dissolution rates.

Trace lead impacts biomineralization pathways during bacterial iron reduction

Dissimilatory Iron Reduction (DIR) is often implicated in both the remobilization of trace metals from sediments, and in the sequestration of trace metals into secondary mineral products. However, the factors controlling the extent of dissolution versus solid-phase redistribution have been elusive. We have made the following observations after incubating pure and Pb-substituted Fe (hydr)oxides with Shewanella putrefaciens 200R over 1200 h: 1) Pb substitution at a molar ratio of Pb:Fe = 5.19 × 10 − 4 inhibited Fe reduction; 2) the presence of Pb enhanced Fe dissolution and inhibited biologically induced magnetite formation; 3) Pb dissolved incongruently with Fe and accumulated to 190 nmol l − 1 in the aqueous phase; and 4) the residual solid-phase Pb was partitioned between an unresolved poorly reducible phase and weak acid extractable (surface sorbed or carbonate) phases. The presence of trace metals and Pb in soils and sediments could partially explain the apparent absence of magnetite in many Fe-reducing environments and its paucity in the sedimentary record. Organic matter degradation rates in Pb contaminated ecosystems may be reduced because of the inhibition of DIR by Pb. We assert that Pb should be largely immobile under Fe-reducing conditions due to its incorporation into refractory secondary minerals.

Fungal transformations of uranium oxides

The biogeochemical activities of free-living and symbiotic fungi must be acknowledged in attempts to understand uranium cycling and dispersal in the environment. Although the near-surface geochemistry of uranium is very complex and a wide variety of mineral phases is known, uranium trioxide (UO3) and triuranium octaoxide (U(3)O(8)) can be used as well characterized models in the study of biotransformations. We have used a complex methodological approach involving advanced solid state speciation and scanning electron microscopy to study the ability of saprotrophic, ericoid and ectomycorrhizal fungi to transform these model oxides. This study has revealed that fungi exhibit a high uranium oxide tolerance, and possess the ability to solubilize UO3 and U(3)O(8) and to accumulate uranium within the mycelium to over 80 mg (g dry weight)(-1) biomass. X-ray absorption spectroscopy of uranium speciation within the biomass showed that in most of the fungi the uranyl ion was coordinated to phosphate ligands, but in ectomycorrhizal fungi mixed phosphate/carboxylate coordination was observed. Abundant uranium precipitates associated with phosphorus were found in the mycelium and encrusted the hyphae. Some of the fungi caused the biomineralization of well-crystallized uranyl phosphate minerals of the meta-autunite group. This is the first experimental evidence for fungal transformations of uranium solids and the production of secondary mycogenic uranium minerals.

Strontium, hydrothermal systems and steady-state chemical weathering in active mountain belts

Hydrothermal systems are a common feature of active collisional orogens. These systems can have a profound effect on the Sr fluxes and Sr isotopic composition of riverine waters through the direct input of Sr from hot springs and groundwaters and the chemical weathering of hydrothermal calcite. In an effort to quantify the role of calcite weathering we present an analytical model that examines the relationship between rock uplift and steady-state chemical weathering of granites and greywackes that contain trace amounts (< 3%) of calcite. We also tested the affect of different mineral dissolution rates and production of secondary minerals on Sr fluxes. This model predicts that in areas of high rock uplift such as collisional mountain belts as much as 60% of the Sr released during chemical weathering comes from the weathering of small amounts of hydrothermal calcite. Moreover, the Sr isotopic composition of weathered material remains constant over a wide range of tectonic environments and is insensitive to the proportion of calcite to silicate weathering. As such, the Sr isotopic composition of river waters will not be an adequate proxy for silicate weathering in tectonically active orogens if calcite is present, and it is these very orogens that have the highest chemical weathering rates. We compare model results with Sr fluxes and isotopic composition of riverine waters from the Southern Alps of New Zealand, an area that is frequently used as an example of a steady-state collisional orogen. As our model predicts, roughly 50% of Sr flux is from the weathering of calcite in the areas of highest uplift along the west-flank of the Southern Alps. This proportion decreases substantially (< 20% from calcite) in the areas of lower uplift east of the Southern Alps. Because chemical weathering rates are approximately a factor of two higher in the rapidly uplifting areas as compared to the more stable regions a significant portion of strontium flux from rivers on South Island of New Zealand is from the weathering of calcite.

Chromium-Removal Processes during Groundwater Remediation by a Zerovalent Iron Permeable Reactive Barrier

Solid-phase associations of chromium were examined in core materials collected from a full-scale, zerovalent iron permeable reactive barrier (PRB) at the U.S. Coast Guard Support Center located near Elizabeth City, NC. The PRB was installed in 1996 to treat groundwater contaminated with hexavalent chromium. After eight years of operation, the PRB remains effective at reducing concentrations of Cr from average values >1500 microg L(-1) in groundwater hydraulically upgradient of the PRB to values <1 microg L(-1) in groundwater within and hydraulically downgradient of the PRB. Chromium removal from groundwater occurs at the leading edge of the PRB and also within the aquifer immediately upgradient of the PRB. These regions also witness the greatest amount of secondary mineral formation due to steep geochemical gradients that result from the corrosion of zerovalent iron. X-ray absorption near-edge structure (XANES) spectroscopy indicated that chromium is predominantly in the trivalent oxidation state, confirming that reductive processes are responsible for Cr sequestration. XANES spectra and microscopy results suggest that Cr is, in part, associated with iron sulfide grains formed as a consequence of microbially mediated sulfate reduction in and around the PRB. Results of this study provide evidence that secondary iron-bearing mineral products may enhance the capacity of zerovalent iron systems to remediate Cr in groundwater, either through redox reactions at the mineral-water interface or by the release of Fe(II) to solution via mineral dissolution and/or metal corrosion.

Water - Compacted Na-Bentonite Interaction in Simulated Nuclear Fuel Disposal Conditions: the Role of Accessory Minerals

ABSTRACTIn an earlier laboratory study1, ion exchange processes and smectite alteration were investigated through the interaction between compacted Na-bentonite (Volclay MX-80) and simulated granitic groundwater solutions. The change of montmorillonite from Na- to Ca-rich was found to be the major alteration process in the bentonite. In the water, a concentration decrease in Ca, Mg and K, and an increase in Na, HCO3 and SO4 were recorded. The total amount of Ca available in the water, however, was found insufficient to account for the recorded formation of Ca-smectite, and it is therefore assumed that the accessory Ca-bearing minerals in the bentonite provide the fundamental source of these cations.X-ray powder diffraction analyses and microscope observations of the bentonite samples were re-conducted. Quartz, feldspars, pyrite, calcite and traces of gypsum were revealed as the primary accessories. In reacted samples, goethite and siderite are found as the secondary mineral products in association with corroded pyrite grains, while calcite and gypsum tend to disappear. From these results it is assumed that the oxygen present in the water and bentonite pore space promotes the oxidation of pyrite (dissolved) and the precipitation of goethite. The pore water pH decreases and calcite is partly dissolved. Through the dissolution, the bulk amount of Ca ions in addition to those arising by diffusion from the water is provided. Some of the reaction-released bicarbonate and Fe2+ re-precipitate in the bentonite as siderite, while the rest (as also SO4 ions) diffuse into the water.Though the relative oxygen content in the experiment may be considered higher than that of the repository concept for nuclear fuel disposal (interaction in a semi-closed system with high water/bentonite ratio), the near field geochemistry predictions imply limited oxidizing conditions, which will be characterized by the above-described processes in sulphide-bearing bentonite and occur for some time after the sealing of the repository.

Stress corrosion cracking of biotite and feldspar

Pervasive intragranular cracking of biotite and feldspar has been observed in a naturally deformed granodiorite, where the leading edge of cracks are sites for specific reactions of parent grains to secondary mineral products. This textural feature provides evidence for an interrelationship of chemical reactions to slow crack propagation, and is considered to be a natural example of stress corrosion cracking. It is probable that the mineral reactions are catalysed by increase of the stress intensity at fracture tips, with reaction rates controlling propagation of the fractures.

On a Volcanic Series in the Malvern Hills, near the Herefordshire Beacon

The rocks of the Malvern Hills have afforded scope for much study and discussion. Dr. Holl, Mr. Rutley, and Dr. Callaway have elaborated papers on them. Mr. Rutley's relates mainly to the general character of the rocks; while Dr. Callaway devotes himself to an enquiry into the genesis of the crystalline schists, and the production of secondary minerals in shear-zones. I propose to deal with a small area of the hills and describe some of the rocks to be found there. The district lies east and south-east of the Herefordshire Beacon, which has three spurs. The northernmost is Tinker's Hill, south of that comes Broad Down, and south of that again Hangman's Hill. The latter is, in its turn, bounded on the south by the ‘Silurian Pass,’ so named in the Geological Survey memoir by the late Prof. Phillips. The Beacon Hill itself is of the same character as the main axis of the hills. Prof. Phillips called attention to these rocks, and remarked that they ‘appear partly in lines like dykes.’ Dr. Holl also mentioned them, and considered them to be sandstone altered by the intrusion of trap-dykes. He noticed that some of the dykes contain a large proportion of augite. In a note to p. 100 ( op. cit. ) Dr. Holl says that fragments of these rocks are found in the Gullet Wood conglomerate. This statement requires investigation, as it might form a clue to the age of the series. Dr. Callaway has expressed the opinion that these rocks

On the Production of Secondary Minerals at Shear-zones in the Crystalline Rocks of the Malvern Hills

In an introductory paper * read before this Society in April 1887, I gave a rough sketch of the mode in which certain of the Malvern schists † were produced ; but no attempt was made to offer a complete solution of the questions discussed. At the British Association in 1887 and 1888, and at the International Geological Congress in 1888, I briefly stated certain additional results, reserving particulars of the evidence for more detailed communications. One of the most important branches of the inquiry is the genesis of the respective minerals, especially of the micas. If, as I hope to prove, some of the most abundant minerals are of secondary origin, the ground will be cleared for working out the details of the problem in hand. Minute particulars of rock-composition will rarely be necessary in this paper. The principal varieties of the Malvern rocks have lately been carefully described by Mr. Rutley ‡ , and, furthermore, I believe that the question is to be settled rather by field-evidence than by the study of minute structure. Indeed, the chief results were decided in the mind of the writer before a single slide had been cut. Nevertheless more than 150 rock-sections have been prepared. Some typical examples of these were submitted to the skilled examination of Dr. H. B. Patton, assistant to Prof. Rosenbusch, of Heidelberg, and he has generously permitted me to incorporate his notes in this communication § . My thanks are also duen to Mr. J. J. H. Teall for several valuable suggestions, and his admirable monograph * has been of much use. It is satisfactory to note that Mr. Teall has recognized † at the Lizard certain "banded gneisses" to which he assigns an origin similar to that of the "injection-schists" of my last paper ; but I understand that he expresses no decision on the genesis of the granite-veins. Prof. Bonney has also favoured me with his opinion on certain points.