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NANOMINERALS AND ULTRAFINE PARTICLES IN SUBLIMATES FROM THE RUTH MULLINS COAL FIRE, PERRY COUNTY, EASTERN KENTUCKY, USA Luis F. O. Silva, Marcos L. S. Oliveira, Carlos H. Sampaio Universidade Federal do Rio Grande do Sul, Escola de Engenharia, Departamento de Metalurgia, Centro de Tecnologia, Av. Bento Gonçalves, 9500. Bairro Agronomia. CEP: 91501-970. Porto Alegre – RS, Brazil, Tel. (0xx) 51 33087067. E-mail: [email protected], [email protected], [email protected] James C. Hower University of Kentucky, Center for Applied Energy Research, 2540 Research Park Drive, Lexington, KY 40511, USA E-mail: [email protected] ABSTRACT Coal fires typically have a variety of mineral and organic deposits associated with the venting emission gases. In addition to the tars typically found at the Ruth Mullins coal fire, Perry County, Kentucky, a sooty carbon, superficially similar to a carbon from a university-based stoker-fired power plant, was sampled in an August 2010 visit. Carbons in the soot include complex carbon particles, nanotubes encapsulating Hg, onion- like structures with polyhedral and quasi-spherical morphology with hollow centers, and metal-bearing multiwalled nanotubes. Mineral sublimates from the Ruth Mullins fire in abandoned underground and surface mines in the high volatile A bituminous Middle Pennsylvanian Hazard No. 7 coalbed, Perry County, Kentucky, were examined by optical mineralogy, X-ray diffraction (XRD), and high-resolution–transmission electron microscopy (HR-TEM). Optical examination revealed the presence of salammoniac and a fine, unidentified fibrous mineral. XRD also showed the presence of salammoniac, along with trace amounts of quartz, kaolinite, and, possibly, phengite. Both cubic and dendritic salammoniac forms were observed with HR-TEM. Gypsum, jarosite, with cubic pseudomorphs after pyrite, and Fe-minerals, including Cr-bearing hematite in association with jarosite, were observed with HR-TEM. Dehydration of jarosite can lead to the formation of less hydrous Fe-sulfates and hematite. Mineral and amorphous inorganic phases included glassy Al-Si spheres with associated Pb and Se; nanopyrite grains with trace As and Se; nanohematite with V3+; salammoniac; quartz; Cr- and Pb-bearing jarosite; fibrous pickeringite with surficial natrojarosite; and Cd-, Co-, Mo-, Ni, V-, W-, and Zr-bearing nanospheres. KEYWORDS: hazardous volatile elements; carbon nanotubes; fullerenes. 1. INTRODUCTION Humans Coal fires can be the consequence of spontaneous combustion; incidental natural sparks (lightning strikes, forest fires); or malicious, negligent, or accidental human interventions. A number of coal properties, often interrelated, affect spontaneous combustion (e.g., Güney, 1968; Kaymakçi and Didari, 2002), including: 1) Moisture content and volatile matter content 2) Particle size and available surface area 1 3) Mineral matter type and pyrite content in particular 4) Coal rank, and 5) Petrographic composition (coal type). Oxidation of coal, an exothermic reaction, at ambient temperature is considered to be a major cause of spontaneous combustion (Banerjee et al., 1990; Goodarzi and Gentzis, 1990). Minerals sublimating from the coal fire vent gases are among the most obvious products deposited at the vents (Stracher et al., 2005; Pone et al., 2007; Ribeiro et al., 2010; O'Keefe et al., 2010). Coal fires can be significant contributors to greenhouse gases and can damage or destroy valuable resources and infrastructure. Along with the work by Ribeiro et al. (2010), this study is among the first to investigate nanominerals and associated trace elements in the sublimate minerals resulting from coal fires. The Ruth Mullins fire, the site of this study, is developed in the underground- and surface-mined high volatile A bituminous Middle Pennsylvanian Hazard No. 7 coalbed on the northeast side Lost Mountain in Perry County, Kentucky. Coal fires in eastern Kentucky generally occur in abandoned mines. The coal was last mined in the late 1950s. Anecdotal evidence, based on interviews with local residents, suggests that the fire, possibly started by a forest fire, has been burning for nearly 50 years. The source of samples in this study, the fire developed in the remnants of a succession of underground, contour surface, and auger mines active intermittently from the 1930’s to late 1950’s. Based on anecdotal evidence from conversations with local residents, the fire has been burning since about 1960. The fire was previously studied by O’Keefe et al. (2010) and Silva et al. (2011a) and continue to investigate aspects of the fire. Carbon nanotubes, anthropogenic amorphous and/or crystalline mixed complex nanominerals, and fullerenes have been detected in various geological materials in trace concentration (Buseck, 2002; Jehlička et al., 2005; Hower et al., 2008; Silva et al., 2009; Vítek et al., 2009; Silva et al., 2010). These include hard coals, coal ashes, solid bitumen, clays from the Cretaceous-Tertiary boundary, and rocks from the Permian-Triassic boundary (Buseck et al., 1992; Jehlička et al., 2000; Jehlička et al., 2003; Chen et al., 2004; Kovalevski et al., 2005; Zhao et al., 2009; Silva and DaBoit; 2011). Human exposure to ultrafine particulate matter (UFPs) has increased markedly over the past century due to anthropogenic activities, including coal spontaneous combustion (Ribeiro et al., 2010; Silva et al., 2011a,b). UFPs are usually unintentionally produced byproducts of coal processes involving power plants, spontaneous combustion, and coal mining activities and, in most cases, encapsulate hazardous elements (Chen et al., 2005; Giere et al., 2006; Hower et al., 2008). Consequently, UFPs may possess a wide range of physicochemical properties (e.g. surface metal contaminants and aromatic compounds) owing to different emission sources and geographical locations. Airborne UFPs produced from these sources contains particles in three size categories, collectively referred to as PM10. Overall, the metric PM10 is defined as particulate matter less than 10 µm in aerodynamic diameter, where particles less than 0.1 µm are regarded as being UFP, those between 0.1 and 2.5 mm are ‘fine’ in size, and particles that are 2.5–10 µm are referred to as ‘coarse’. 2. PROCEDURE The aim of our work was to compare analytical methods, e.g., (1) Time of flight secondary ion mass spectrometry (TOF-SIMS); (2) Field Emission Scanning Electron Microscope (FE-SEM); (3) High-resolution transmission electron microscope (HR-TEM) with Energy-dispersive X-ray Spectrometer (EDS), Selected Area Electron Diffraction (SAED) and/or Microbeam Diffraction (MBD), and Scanning Transmission Electron Microscopy (STEM); and (4) Inductively Coupled Plasma Mass Spectrometry (ICP-MS). FE-SEM, HR-TEM, EDS, SAED, MRB, and STEM do not require previous solvent sequential extraction, therefore, they are applicable directly to the coal fire soot sample. We focus, in particular, on the detection and geochemistry characterization of carbon 2 nanotube assemblages, UFPs and complex nanominerals present in soot from Ruth Mullins coal fire. 2.1 Samples collection Mineral samples were collected in February 2010 from three locations within the vent 5 complex at the Ruth Mullins coal fire, Perry County, Kentucky. As the cliff face fractures and separates from the side of the vegetated hill, minerals form on the roots of trees and both surrounding and within the small vents on the rock face (Fig. 1). The Ruth Mullins fire emissions are exhausted through multiple vents. Not every vent shows signs of mineralization, but vent 5 had particularly good shows of minerals (and tars) through late 2009 and early 2010. Two of the samples, Ruth 1 and Ruth 4, are represented in the figures. The soot was acquired at vent 4 of the fire on 19 August 2010. The coal fire soot occurs as a fine, powdery deposit on the rocks surrounding the vent Figure 1: Location of Kentucky in the continental USA (A). Minerals sublimated on tree roots (B), and surrounding and within vent openings in sandstone cliff face (C). 2.2 Optical characterization of minerals Samples from five different nucleation areas in and near vent 5 were separated from the roots, twigs, and rock they coated using a Nikon® SMZ645® dissecting microscope. The minerals were crushed and sized to 230 mesh. Grain mounts for transmitted light microscopy were made with Cargille® Meltmount®. Slides were examined with a Nikon® Labophot2-Pol® microscope using 10×, 20×, and 40×objectives. Photomicrographs of minerals were obtained using a Nikon® Digital Sight DS-SM® camera and interface. Refractive indices were determined using Cargille® Certified Refractive Index Liquids Series A and B for non-fibrous mineral forms. 2.3 X-ray diffraction 3 The mineral composition of the soot sample was determined by means of a Siemens D5005 X-ray diffraction (XRD). The samples were ground by hand in a ceramic mortar and pestle, dry mounted in aluminum holders, and scanned at 8-60o 2θ with Cu K-α radiation. 2.4 Electron beam methods Electron beam methods included Field Emission Scanning Electron Microscope (FE-SEM) with energy-dispersive X-ray spectrometer (EDS) capabilities and high-resolution transmission electron microscope (HR-TEM) with SAED (selected area electron diffraction) or MBD (microbeam diffraction), and scanning transmission electron microscopy (STEM). Time of flight secondary ion mass spectrometry (TOF-SIMS) was used to investigate the elemental and molecular structure of the samples. Surface composition was determined by X-ray photoelectron spectroscopy (XPS). Procedures for the electron beam methods are outlined in ours previous published works (Silva et al., 2009; Silva et al., 2010a,b; Silva et al., 2011a,b,c). 2.5 Mineral extractions FE-SEM and HR-TEM mineralogical analyses of sublimate and included minerals from a Kentucky coal fire were performed following sequential extraction (SE). A brief explanation of the selected experimental conditions and of the expected information offered by each step of the SE are presented below. 1) For the water-soluble fraction, a 1-mg mineral sample (five replicates) was mixed with Millipore-system water (1 mL) with a conductivity of 0.1–0.5 μS/cm.