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Particle Atlas of World Trade Center Dust

By Heather A. Lowers and Gregory P. Meeker

U.S. Department of the Interior
U.S. Geological Survey

Any use of trade, product, or firm names in this report is for descriptive purposes only and does not imply endorsement by the U.S. Government.


Table of Contents


Introduction

The United States Environmental Protection Agency (EPA) has begun a reassessment of the presence of World Trade Center (WTC) dust in residences, public buildings, and office spaces in New York City, New York. Meeker and others (2005a) have identified slag wool (a man-made vitreous fiber, MMVF), gypsum (CaSO4_2H2O) (or anhydrite (CaSO4)), and phases compatible with concrete as signature components of the WTC dust. In addition to these phases, other MMVF, metal or metal oxides, mineral material, and asbestos are present in trace to minor amounts. Background dust samples collected from residences, public buildings, and office spaces will be analyzed by multiple laboratories for the presence of WTC dust. Other laboratories are currently studying WTC dust for other purposes, such as health effects studies. To assist in inter-laboratory consistency for identification of WTC dust components, this particle atlas of phases in WTC dust has been compiled.

This particle atlas contains energy dispersive x-ray spectra (EDS) of the common phases found in WTC dust. In addition, scanning electron photomicrographs showing typical morphology of selected particles are included. The dust is a product of the collapse of WTC buildings and contents. While the list of spectra provided is comprehensive, it is by no means complete. Therefore, it is likely phases and compounds will be identified in the future that are not listed in this atlas.

It is recognized that different laboratories will have different equipment, analytical conditions, and capabilities which may result in differences in the energy dispersive x-ray spectrum for a given phase. Particle size and shape can also affect relative x-ray peak heights. In order to facilitate inter-laboratory comparison, energy dispersive x-ray spectra at three different accelerating voltages (10, 15, and 20 keV) of a basalt glass standard (BIR-1G) acquired on a polished mount are included (Meeker and others, 1998). These spectra will allow laboratory personnel to evaluate possible differences between the spectra included in this report and spectra from similar phases obtained in their own laboratory. Samples of BIR-1G are available from the U.S. Geological Survey1.

Methods

A JEOL JSM-5800LV scanning electron microscope (SEM) equipped with an Oxford ISIS energy dispersive spectrometry system was used to acquire the spectra. A silicon detector with resolution better than 133 eV full width half maximum (FWHM) for Mn was used. The display resolution was set to 10 eV per channel. The operating conditions for the dust were 15 keV, approximately 0.1 nA beam current (cup), 100 second acquisition time (livetime), with 20-30% detector deadtime. Typical spectra range from 10 to 20 thousand counts full scale. The samples were analyzed at a 10 millimeter working distance (instrument specific) with zero degree tilt. The samples were mounted on carbon conductive tabs and coated with carbon for conductivity.

Results

The collected spectra in Table 1 are categorized into MMVF and glass fragments, gypsum/anhydrite, phases compatible with concrete, asbestos, metal or metal oxides, and mineral material groups (discussed below). To view the spectrum for each phase, click on the particle label. An image of the spectrum will appear in a new window. A column labeled “image” will open a photomicrograph of selected particles from which spectra were collected. A list of elements detected in each spectrum is also given which allows the user to search by element for an unidentified particle.

All spectra contain a peak for carbon because of the conductive carbon coat. This has been omitted from the table unless is it reasonable for carbon to be in the particle as in the case of calcite (CaCO3), dolomite (CaMg(CO3)2), or organic matter. The carbon peak for the concrete phases has been labeled although its presence is questionable. It is not possible to distinguish between calcite (CaCO3), Portlandite (Ca(OH)2), and lime (CaO) using a detector that does not allow for detection of light elements, such as carbon and oxygen.

Gypsum/anhydrite

Gypsum (CaSO4_2H2O) and anhydrite (CaSO4), along with a variety of other hydrated Ca sulfates are the primary components of wall board (drywall). Other minor and trace components of drywall such as quartz (SiO2), barite (BaSO4), and calcite (CaCO3) are included with mineral material below.

Gypsum and anhydrite are indistinguishable using qualitative EDS methods. The spectrum for each is dominated by Ca and S. The peak height ratios of these two elements will vary depending on particle geometry, orientation to the detector, and other adjacent or adhering phases. For instance, Gypsum-01 is a typical spectrum for gypsum whereas Gypsum-03 occurs with a small amount of carbonate which increases the Ca and C peaks relative to S.

MMVF and glass fragments

Man-made vitreous fibers (MMVF) are abundant in WTC dust. Glass fibers range in diameter from < 1 μm to > 50 μm with lengths up to several hundred micrometers. The best compositional match for the majority (>85%) of WTC glass fibers is slag wool, a by-product of pig iron production (TIMA, 1991). Pieces of yellow thermal insulation found in bulk samples are composed of soda-lime glass fibers (Meeker and others, 2005b). Very few fibers of this composition exist in the fine (<150 μm) microscopic portion of the dust. Rock wool is also present as a trace constituent of the fine dust portion.

Rock wool and slag wool can have similar EDS spectra. The two can be distinguished based on the presence of iron. Slag wool will generally have less than 2 weight percent FeO whereas rock wool contains from 3 to 12 weight percent FeO (TIMA, 1991). Soda-lime glass has a distinct EDS spectrum from both slag wool and rock wool. The Na peak is higher and Ca, Mg, and Al peaks are smaller in the soda-lime glass spectrum than the slag wool and rock wool spectra.

Glass shards, fragments, and spheres are also present in the dust samples. The microscopic glass shards and fragments are less abundant than the ubiquitous slag wool fibers in the fine dust (<150 μm). Most of the glass fragments fall within the compositional range for soda lime glass, a common type used as window glass (TIMA, 1991). Other glass fragments are present which contain mostly Si with trace amounts of Na, K, and/or Al. The majority (> 90%) of glass spheres, generally less than 500 μm in diameter, are of slag wool composition.

Concrete

Concrete is composed of aggregate, sand, and Portland cement (Chandra and Berntsson, 2003). The aggregate material in WTC concrete sample appears to be expanded shale. The sand is primarily quartz, but can contain feldspar, iron and titanium oxides, micas, and other rock-forming minerals. Portlandite (Ca(OH)2) and magnesium hydroxide are also present in minor to trace amounts. Portland cement hydrates to form a large variety of Ca-rich phases including calcium silicate hydrate, calcium aluminum hydrate, calcium aluminum iron hydrate, and Portlandite (Ca(OH)2) (Chandra and Berntsson, 2003). Portlandite is indistinguishable from lime (CaO) using qualitative EDS. Minor gypsum is added to control the set of the concrete.

Particles identifiable as concrete in WTC dust are those constituting the Portland cement component. Portland cement particles will usually have a high Ca peak accompanied by Si and/or Al, Mg, Fe. Most particles of Portland cement will be composed of several Ca-rich phases. These phases are generally extremely fine grained and often occur without distinct grain boundaries. These phases can usually be differentiated using backscattered electron imaging combined with EDS analysis.

A large proportion of the quartz in WTC dust is likely from disaggregated concrete, but has been grouped below with mineral material. Calcite (CaCO3) and dolomite (CaMg(CO3)2) are also components of concrete, but as with quartz, have been grouped with mineral material because they can be present from a variety of sources.

Asbestos

Chrysotile (Mg3Si2O5(OH)4) is present in most bulk WTC samples at levels of approximately 0.1 to 1.0 wt. percent (Meeker and others, 2005a,b; Clark and others, 2001). Chatfield and others (2002) observed amosite ((Mg,Fe)7Si8O22(OH)2) and richterite (Na(CaNa)Mg5Si8O22(OH)2) or winchite ((CaNa)Mg4(Al,Fe3+)Si8O22(OH)2) fibers in one sample collected north of the WTC site. Spectra of richterite/winchite and amosite have been included in this data set for completeness.

Chrysotile (Mg3Si2O5(OH)4) can be differentiated from talc (Mg3Si4O10(OH)2) based on the relative peak heights of Mg and Si. The Mg peak is higher than the Si peak in the chrysotile spectrum but lower than the Si in the talc spectrum.

Metal and Metal-Oxide Phases

The primary metal and metal-oxide phases in WTC dust are Fe-rich and Zn-rich particles (Meeker and others, 2005b). Many other metal and metal oxide phases have been identified including phases rich in Al, Ti, Pb, Bi, Mo, Zr, Sn, Cu, and others. It is often difficult to distinguish between metals and metal oxides with qualitative EDS because of adsorbed surface oxygen or thin coatings of oxide phases such as rust. It is impossible to distinguish metals and metal-oxides with qualitative EDS analysis using a Be window x-ray detector.

In order to distinguish Mo-, Pb-, and Bi- rich phases it is necessary to look for additional M, L, and K series peaks. This may require higher accelerating voltages to excite these x-ray energies. If additional M, L, or K series peaks are not observed, these elements are probably not present and the peak occurring near ~2.3 keV can be attributed primarily to S.

Mineral material

Mineral material includes all particles that generally occur as rock-forming minerals. The primary components in this group include quartz (SiO2), feldspars ((Ca,Na,K)1(Si,Al)4O8), micas (including vermiculite), talc (Mg3Si4O10(OH)2), calcite (CaCO3), dolomite (CaMg(CO3)2), sulfide minerals, barite (BaSO4), and others. Quartz is distinguished from other Si-rich phases, such as glass shards, based on the absence of other trace to minor elements such as Na, K, and Al. Compare Si-03 and Si-01, respectively.

Summary

This particle atlas has been compiled to serve as a guide to identify common phases in WTC dust. It is not a complete guide to all phases that may be found in WTC dust. The particles have been identified by stoichiometric criteria using data acquired by SEM and x-ray microanalysis. Identification is based on extensive experience gained by previous research on WTC dust using electron probe microanalysis, x-ray diffraction, infrared spectroscopy, and other techniques (Meeker and others, 2005a; Meeker and others, 2005b; Lowers and others, 2005; Clark and others, 2001).

>>Follow this link to Table 1.

References


1Stephen Wilson, U.S.G.S., DFC MS973, Denver, CO 80225-0046 or swilson@usgs.gov

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