Molly M. McGuire

Geomicrobiology of Pyrite (FeS2) Dissolution: Case Study at Iron Mountain, California

Geomicrobiology of pyrite weathering at Iron Mountain, CA, was investigated by molecular biological, surface chemical, surface structural, and solution chemical methods in both laboratory and field-based studies. Research focused at sites both within and peripheral to the ore-body. The acid-generating areas we have examined thus far at Iron Mountain (solution pH 35 C) were populated by species other than Thiobacillus ferrooxidans . 16S rDNA bacterial sequence analysis and domain- and specieslevel oligonucleotide probe-based investigations confirmed the presence of planktonic Leptospirillum ferrooxidans and indicated the existence of other species apparently related to other newly described acidophilic chemolithotrophs. T. ferrooxidans was confined to relatively moderate environments (pH 2-3, 20-30 C) that were peripheral to the orebody. Dissolution rate measurements indicated that, per cell, attached and planktonic species contributed comparably in acid release. Surface colonizati...

Geochemical and biological aspects of sulfide mineral dissolution: lessons from Iron Mountain, California

Abstract The oxidative dissolution of sulfide minerals leading to acid mine drainage (AMD) involves a complex interplay between microorganisms, solutions, and mineral surfaces. Consequently, models that link molecular level reactions and the microbial communities that mediate them to field scale processes are few. Here we provide a mini-review of laboratory and field-based studies concerning the chemical, microbial, and kinetic aspects of sulfide mineral dissolution and generation of AMD at the Richmond ore body at Iron Mountain, California.

Kinetics, surface chemistry, and structural evolution of microbially mediated sulfide mineral dissolution

The effects of different microbial populations on the oxidative dissolution of sulfide minerals at 37°C and pH 1.5 were examined over a period of 22 days. Samples of pyrite, marcasite, and arsenopyrite were exposed to a sulfur-oxidizing isolate (Thiobacillus caldus), an iron-oxidizing isolate (Ferroplasma acidarmanus), and a mixed enrichment culture containing T. caldus, F. acidarmanus, and Leptospirillum ferrooxidans. Changes in chemical speciation of the mineral surface products were monitored by Raman spectroscopy over the course of the experiment, structural evolution was examined with scanning electron microscopy, and the total soluble iron was used as a measure of the dissolution rate. In the case of all three minerals, an increase in dissolution rate was observed only in the presence of iron-oxidizing microorganisms (i.e., F. acidarmanus or the enrichment culture). The chemical speciation at the mineral surface in the presence of these iron-oxidizing species is indistinguishable from that of abiotic control reactions under the same conditions; both are dominated by elemental sulfur. In contrast, experiments with T. caldus indicate that the quantity of elemental sulfur on the mineral surface is <1% of the amount observed on samples exposed to the F. acidarmanus culture. It is surprising that removal of the elemental sulfur from the mineral surface by the sulfur-oxidizing species is not accompanied by an increase in the dissolution rate of the mineral. This finding suggests that although elemental sulfur forms on the surface during oxidative dissolution, it does not passivate the mineral surface.

Chemical mapping of elemental sulfur on pyrite and arsenopyrite surfaces using near-infrared Raman imaging microscopy

Abstract Near-infrared Raman imaging microscopy (NIRIM) was used to produce chemical images of the distribution of elemental sulfur on oxidized pyrite and arsenopyrite surfaces. Analysis using Savitsky–Golay filtering permits an unambiguous identification of surface products even in the presence of broad background signals. Rather than forming a continuous, passivating layer at the mineral surface, the NIRIM images reveal that elemental sulfur forms in isolated patches on the order of tens of microns in diameter. The potential implications of this strongly heterogeneous distribution of chemical products for geochemical modeling of acid mine drainage (AMD) are discussed.

Quantitative determination of elemental sulfur at the arsenopyrite surface after oxidation by ferric iron: mechanistic implications

The elemental sulfur formed at the arsenopyrite surface after oxidation by ferric iron was quantitatively measured by extraction in perchloroethylene and subsequent quantitative analysis by HPLC. Reactions with ferric iron in perchloric acid solutions or in sulfuric acid solutions (both at pH = 1 and 42°C, which approximate extreme acid mine drainage conditions) produced elemental sulfur in quantities greater than 50% of the total reacted sulfur. The controversy surrounding the mechanism of the oxidative dissolution of arsenopyrite is discussed in light of these measurements. Based on the observation of greater than 50% production of elemental sulfur, a mechanism by which all the sulfur from the mineral proceeds through thiosulfate can be eliminated as a possible description of the dissolution of arsenopyrite. Instead, it is likely the other constituents of the mineral lattice, Fe and As, are leached out, leaving behind a S0 lattice. Nucleation reactions will then result in the formation of stable S8 rings.

CRYSTALLOGRAPHIC SITE DISTRIBUTION AND REDOX ACTIVITY OF Fe IN NONTRONITES DETERMINED BY OPTICAL SPECTROSCOPY

Optical absorption spectroscopy has the potential to uncover many characteristics of Fe-bearing, redox-active smectites that have heretofore been hidden. The purpose of this study was to exploit this technique to reveal the temperature dependence of the spectra and to characterize the behavior of octahedral and tetrahedral Fe(III) under various stages of reduction. The Uley nontronites, NAu-1 and NAu-2, were compared using optical spectroscopy, which probed the crystallographic-site distribution of Fe in the clay structures as well as the resulting differences in the reduction process in the two minerals. All of the major differences in the spectra of the two minerals in the wavelength range 450–950 nm are due to the presence of a significant amount of tetrahedral Fe(III) in NAu-2. In situ observation of the optical spectra of NAu-1 suspensions as a function of the degree of reduction reveals a steady increase in the dominant intervalence charge transfer (IVCT) band and the resulting blue-green color as the Fe(II) content of the octahedral sheet increases. Although the spectrum of NAu-2 at ∼50% reduction looks nearly identical to the spectrum of NAu-1 at a similar state of reduction, the spectra corresponding to the initial stages of reduction are quite different. Stepwise reduction of NAu-2 causes a rapid decrease in the absorbance features due to crystal-field transitions of tetrahedral Fe(III) before the IVCT band appears, suggesting that tetrahedral Fe(III) is preferentially reduced before the octahedral Fe(III). The intensity of the absorbance features due to tetrahedral Fe(III) also exhibit an inverse temperature dependence, suggesting that they are enhanced due to exchange-coupling with Fe(III) ions in neighboring sites. Spectra of NAu-1 at liquid nitrogen temperature, therefore, allowed the identification of a small amount of tetrahedral Fe(III) in NAu-1 that had not been noted previously.

POLARIZED ATR-FTIR INVESTIGATION OF Fe REDUCTION IN THE ULEY NONTRONITES

Reduction of structural Fe in smectites affects the surface chemical behavior of the clay, but the underlying mechanism and changes in clay structure are still in need of investigation, particularly with respect to changes in the tetrahedral sheet. The purpose of this study was to probe changes in the tetrahedral sheet that occur when structural Fe is reduced in the Uley nontronites, NAu-1 and NAu-2, using polarized attenuated total internal reflection Fourier-transform infrared spectroscopy. Despite the differences in their structures — NAu-2 has tetrahedral Fe3+ while NAu-1 does not — the changes observed in the Si-O stretching region were quite similar. Reduction results in a shift of the in-plane Si-O stretching modes to lower frequencies, while the out-of-plane Si-O stretch shifts to higher frequencies. The magnitude of these shifts is greater in NAu-2 than in NAu-1, but the crystallinity of the tetrahedral silicate sheet of NAu-2 is preserved upon reduction. In both nontronites, the orientation of the out-of-plane Si-O bond changes and becomes completely perpendicular to the basal (001) surface of the clay, indicating the formation of trioctahedral domains wherein the individual tetrahedra reorient relative to the plane of the clay layer.

Treatment of Vapor-Phase Organohalides with Zero-Valent Iron and Ni/Fe Reductants

The reduction of vapor-phase cis-1,2-dichloroethylene (cis-DCE) by zero-valent iron [Fe(0)] and nickel-plated iron (Ni/Fe) was assessed as a possible treatment strategy for organohalides in landfill gas and soil vapor extraction system offgas. The rate of cis-DCE removal and the consequent formation of ethylene, ethane, and vinyl chloride were measured in both batch and column reactors. Auger Electron Spectroscopy was used to determine nickel surface loading before and a fter reaction. In batch experiments, nickel surface loadings greater than 30% increased the pseudo-first-order rate constant for cis-DCE reduction by a factor of 10–20 over that of unamended Fe(0). In column studies using Ni/Fe particles (40% nickel surface loading), 100% removal of gaseous cis-DCE was initially achieved from both anaerobic and aerobic gas streams, compared to only partial removal of cis-DCE in columns containing Fe(0). The enhanced reactivity of Ni/Fe r eductants relative to Fe(0) diminished, however, over time in both a...

MEASUREMENT OF SWELLING OF INDIVIDUAL SMECTITE TACTOIDS IN SITU USING ATOMIC FORCE MICROSCOPY

Atomic force microscopy (AFM) is a novel method for measuring changes in clay swelling in situ at the tactoid level in an aqueous environment. While the swelling process has been directly observed at the mesoscale level for multi-tactoid aggregates and the associated pores, no method to date has allowed the direct observation of swelling dynamics at the nanoscale. In initial proof-of-concept studies, individual tactoids of a Na-exchanged nontronite (NAu-1) were imaged in a solution of 5 mM NaCl. When multiple line profiles were examined on the same tactoid, the changes in height varied and depended on which layers of the profile were transected, and demonstrated that AFM analyses can be used to directly probe intratactoid heterogeneity in the swelling process. To better visualize this heterogeneity, a method was developed to restrict AFM images to include only the portions of a tactoid above a threshold height. A comparison of the changes in these images for multiple threshold values revealed that swelling in one part of a tactoid may occur simultaneously with compression in another portion, which suggests that the encroachment of layers into intra-tactoid micropores can partially compensate for the overall volume change. Finally, to demonstrate the ability of this technique to monitor in situ swelling changes as the surrounding aqueous environment is modified, a tactoid of K-montmorillonite (SWy-2) was monitored during cation exchange as a KCl solution was replaced with NaCl. After exchange, a transition from the crystalline swelling regime to the osmotic regime was observed. Subsequent height profiles were unchanged for a period of several hours and indicated that the AFM measurements were stable in the absence of changes to the aqueous phase composition. Because this technique is the first method that allows the swelling of a single tactoid to be monitored in an aqueous solution, it complements the ensemble-averaged data obtained from diffraction and scattering techniques.