Dogan Paktunc

The physical and chemical evolution of aerosols in smelter and power plant plumes: an airborne study

National and international concern about the health effects and continued use of Pb, Cd, As and Hg as well as other metals has defined a need for improved estimates of the long-term risks to ecosystems and human health from metals released from mining, metallurgical and energy production activities. A research aircraft was used to determine the microphysical and chemical properties of airborne particulate metal emissions from the Nanticoke coal-fired power-generating station located on the north shore of Lake Erie, Ontario, and the Horne copper smelter at Rouyn-Noranda, Quebec. These properties are critical to the determination of the deposition rates of the metals emitted, and hence the potential for these species to have impacts on local or distant ecosystems. An overview of the measurements made during the study is given. The size distributions of particles emitted from the stacks and observed within 5 km of the point of emission are briefly described. After dilution by ambient air, the concentration of particles smaller than 0.135 μm in diameter in the plumes is tens of thousands per cubic centimetre, far exceeding the concentrations found in ambient air. However, in the size range 0.135 to 3 μm diameter the plumes generally contribute about one to four times more particles than present in ambient air.

Characterization of ferric arsenate-sulfate compounds: Implications for arsenic control in refractory gold processing residues

Abstract A combination of techniques, including powder X-ray diffraction (XRD), electron microprobe analysis (EPMA), transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAFS), is used to characterize the common ferric-arsenate-sulfate compounds, which could result from the pressure oxidation of refractory gold ores at elevated temperatures. Three general types of precipitate are identified; namely, arsenate-bearing basic ferric sulfate [FeSO4(OH) and designated as BFS], ferric arsenate-sulfate [an extensive solid solution Fe(AsO4)0.2-0.7(SO4)0.7-0.2(OH)0.7-0.2 and designated as FAS], and hydrated ferric orthoarsenate (FeAsO4·0.75H2O). The crystal structure of FAS is solved by precession electron-diffraction experiments. The structures of BFS and FAS are constructed from octahedral Fe3+ chains, which are cross-linked by sulfate and arsenate tetrahedra. Extensive substitution of arsenate for sulfate occurs in both types of compounds with charge neutrality being maintained by variations in the (OH) content. The XAFS spectra indicate that the local structures of both BFS and FAS are made of corner-linked single chains of FeO6 octahedra where the chains are linked by AsO4 or SO4 tetrahedra forming alternating layers of FeO6 octahedra and AsO4 or SO4 tetrahedra. Preliminary toxicity characteristics leaching procedure (TCLP) testing of the precipitates indicates that FAS with a molar ratio As/(As+S) ratio of ≤0.5 could be an acceptable material for disposal in a tailings impoundment, whereas more As-rich FAS and BFS may require further treatment. The results for the laboratory-prepared precipitates are compared with those obtained on three residues from the processing of refractory gold ores. The major As-carrier in one of the residues is FAS, whereas As-bearing goethite and hematite are the dominant As-carriers in the other two residues. Thus, the mineralogical characteristics of the residues dictate the appropriate arsenic management and disposal options in the processing of refractory gold ores.

Comment on "New clues to the local atomic structure of short-range ordered ferric arsenate from extended X-ray absorption fine structure spectroscopy".

Short-Range Ordered Ferric Arsenate from Extended X‐ray Absorption Fine Structure Spectroscopy” M Mandaliev, and Kretzschmar claimed that ferric arsenate, estimated to be ∼1 nm in size, has a scorodite-type local structure (framework model) instead of short chains of corner-linked FeO6 octahedra with arsenate bridging adjacent octahedral corners as proposed by Paktunc et al. and Paktunc (chain model). The argument of Mikutta et al., based on Asand Fe-EXAFS spectra collected from two precipitates, suffers from a number of serious shortcomings in the analysis and interpretation of the EXAFS spectra. As-EXAFS. The statement about the need for more than two Fe neighbors to more accurately reproduce the As-EXAFS spectra is unfounded because it contradicts with an earlier statement on the existence of high correlation (r = 0.98) between the coordination numbers (CN) and Debye−Waller parameters (σ) for the Fe shell. This correlation would not permit a reliable distinction between the two models with fixed Fe CNs (2 vs 4) which amounts to an R-factor difference of 0.001. This is augmented by the fact that neither the framework nor the chain model can reproduce the amplitude and shape of the EXAFS signal at low k. Furthermore, Fe CN of 4, nominal for bulk scorodite, is too high for a scorodite cluster measuring ∼1 nm. For instance, based on a cluster made of 8 AsO4 tetrahedra and 8 FeO6 octahedra (i.e., 2 As with Fe CN of 4, 4 As with Fe CN of 2 and 2 As with Fe CN of 1), a more realistic Fe CN at this size range would be ∼2.3. Fe-EXAFS. Fe-EXAFS results also suffer from the authors’ failure to recognize the loss of coordination due to nanoscale particle size. Use of CNs of 4As, 2O2, 3O3, and 3O4 nominal for scorodite, are unrealistic for clusters measuring ∼1 nm. The authors’ fitting strategy for Fe-EXAFS spectra involved the use of three multiple scattering (MS) paths, all within the Fe octahedron: “Fe−O−O (triangular)”, “Fe−O−Fe-O (collinear)”, and “Fe−O−O (near-collinear)”, noted, respectively, as DS-T, TS-C, and DS-NC in Figure 1a. These MS paths based on Voegelin et al. are for an undistorted FeO6 octahedron with an Fe−O distance of 1.98 Å, yet Mikutta et al. arbitrarily constrained the CNs of these MS paths to their degeneracy. The validity of this approach is questionable because the FeO6 octahedron in scorodite is highly

Sulfide Oxidation and Mobilization of Arsenic in the Ketza River Mine Tailings

The Ketza River mine is a former gold mine in Yukon, Canada operated from 1988 to 1990 producing over 2.8 tons of gold. There are ~310,000 tons of tailings containing on average 4 wt% As. As reported earlier, arsenic mineralogy of the tailings is complex with a wide range of arsenical minerals including iron(III)-oxyhydroxides, amorphous ferric arsenate, scorodite, arseniosiderite, yukonite, pharmacosiderite, jarosite and arsenopyrite (Paktunc et al., 2003, 2004). The tailings were re-sampled in 2006 at two locations with the aim to examine the extent of sulfide oxidation and determine the mineralogical changes occurred with depth over two decades. Mineralogical compositions of the dry tailings and those under water cover are similar with quartz, goethite, calcite, dolomite, muscovite, clinochlore, scorodite, ferric arsenate and lepidocrocite being the dominant minerals. Arsenopyrite, pyrite, Ca–Fe arsenates and jarosite are present in minor abundances. Arsenopyrite and pyrite occur as relict particles embedded in goethite and arsenate minerals indicating that both are oxidized. Powder X-ray diffraction results point to very little variation with depth of the dominant arsenate minerals. Bulk X-ray absorption spectroscopy (XAFS) resolved the changes and variation with depth of the arsenic-bearing minerals in both the exposed and water-covered tailings. Speciation of As in arsenopyrite and pyrite and their oxidation products were examined by collecting micro-XAFS spectra from about 2 × 2 µm spots along several traverses intersecting the sulfide-oxide boundaries in an exposed tailings sample. These spectra show that As originally occurring as As−1 in arsenopyrite and pyrite is transformed to As+5 in the reaction rims. No reduced As species were detected at the particle rims suggesting that oxidation of As+3 to As+5 species in the solution was rapid and irreversible.

Chemical Forms of Mercury in Pyrite: Implications for Predicting Mercury Releases in Acid Mine Drainage Settings

Pyrite (cubic FeS2) is the most abundant metal sulfide in nature and also the main host mineral of toxic mercury (Hg). Release of mercury in acid mine drainage resulting from the oxidative dissolution of pyrite in coal and ore and rock resulting from mining, processing, waste management, reclamation, and large construction activities is an ongoing environmental challenge. The fate of mercury depends on its chemical forms at the point source, which in turn depends on how it occurs in pyrite. Here, we show that pyrite in coal, sedimentary rocks, and hydrothermal ore deposits can host varying structural forms of Hg which can be identified with high energy-resolution XANES (HR-XANES) spectroscopy. Nominally divalent Hg is incorporated at the Fe site in pyrite from coal and at a marcasite-type Fe site in pyrite from sedimentary rocks. Distinction of the two Hg bonding environments offers a mean to detect microscopic marcasite inclusions (orthorhombic FeS2) in bulk pyrite. In epigenetic pyrite from Carlin-type Au deposit, up to 55 ± 6 at. % of the total Hg occurs as metacinnabar nanoparticles (β-HgSNP), with the remainder being substitutional at the Fe site. Pyritic mercury from Idrija-type Hg deposit (α-HgS ore) is partly divalent and substitutional and partly reduced into elemental form (liquid). Divalent mercury ions, mercury sulfide nanoparticles, and elemental mercury released by the oxidation of pyrite in acid mine drainage settings would have different environmental pathways. Our results could find important applications for designing control strategies of mercury released to land and water in mine-impacted watersheds.

METALS IN THE ENVIRONMENT AND MINE WASTES

The thematic issue on metals in the environment and mine wastes originated from the MAC-sponsored Metals in the Environment symposium and the Environmental Studies of Mine Wastes session held at the GAC–MAC–SEG Annual Meeting, May 25–28, 2003 in Vancouver, British Columbia. The symposium and