ARTICLE pubs.acs.org/est
Application of Synchrotron Microprobe Methods to Solid-Phase Speciation of Metals and Metalloids in House Dust S. R. Walker,†,§ H. E. Jamieson,*,† and P. E. Rasmussen‡ † ‡
Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario K7L 3N6, Canada Healthy Environments and Consumer Safety Branch, Health Canada, Ottawa, K1A 0K9 Ontario, Canada
bS Supporting Information ABSTRACT: Determination of the source and form of metals in house dust is important to those working to understand human and particularly childhood exposure to metals in residential environments. We report the development of a synchrotron microprobe technique for characterization of multiple metal hosts in house dust. We have applied X-ray fluorescence for chemical characterization and X-ray diffraction for crystal structure identification using microfocused synchrotron X-rays at a less than 10 μm spot size. The technique has been evaluated by application to archived house dust samples containing elevated concentrations of Pb, Zn, and Ba in bedroom dust, and Pb and As in living room dust. The technique was also applied to a sample of soil from the corresponding garden to identify linkages between indoor and outdoor sources of metals. Paint pigments including white lead (hydrocerussite) and lithopone (wurtzite and barite) are the primary source of Pb, Zn, and Ba in bedroom dust, probably related to renovation activity in the home at the time of sampling. The much lower Pb content in the living room dust shows a relationship to the exterior soil and no specific evidence of Pb and Zn from the bedroom paint pigments. The technique was also successful at confirming the presence of chromated copper arsenate treated wood as a source of As in the living room dust. The results of the study have confirmed the utility of this approach in identifying specific metal forms within the dust.
’ INTRODUCTION This paper focuses on the development of synchrotron microanalysis methods for investigating the speciation of metal and metalloids (hereafter metals) in residential dust and soil using a combination of micro-X-ray fluorescence (μ-XRF) analysis and micro-X-ray diffraction (μ-XRD) approaches. There is a need for speciation techniques capable of identifying interior and exterior sources of various metals in residential environments.1 6 Synchrotron-based X-ray absorption techniques that employ a macroscopic approach (rather than the microscopic analysis reported here) have been used in an attempt to identify Cu and Zn-hosting particles in house dust,7 but different reference compounds in different proportions could be successfully fit to the same spectra, leading to ambiguity. Such bulk fitting approaches also require that reference spectra are available for all possible compounds that may be present. This may be a particular challenge for house dust given the wide range of possible metal sources that may be present. Scanning electron microscopy (SEM) provides characterization of individual particles but spot analysis is typically limited to metal concentrations >1000 ppm, excluding the identification of trace metal-bearing particles. Synchrotron-based μ-XRF is highly sensitive to ultratrace metal concentrations in particles. X-ray diffraction has the advantage that an unknown pattern can be compared with published compilations of thousands of reference patterns of mineral structures and industrial compounds. r 2011 American Chemical Society
The application of conventional XRD to study metal speciation in environmental media has been limited because the metal-bearing compounds of interest are normally rare and below the detection limit (1 5% by volume), or may be amorphous or poorly crystalline and difficult to discern among well crystallized material. This may be overcome using a microfocused X-ray beam on individual particles.8 11 In this study, synchrotron μ-XRF and μ-XRD approaches are combined to elucidate the metal speciation of residential dust and soil samples, using archived sample material. The goal of the investigation is 2-fold: (1) to determine the solid metal forms present in dust and soil media, and (2) to investigate possible relationships between indoor and outdoor metal-bearing compounds. The utility and limitations of the technique in its present state are discussed in the context of adapting it for lower concentration ranges which are more routinely encountered in residential environments.
’ EXPERIMENTAL SECTION Samples and Chemical Analyses. An archived suite of residential dust and soil samples collected in 2000 20017,12 Received: March 14, 2011 Accepted: August 15, 2011 Revised: August 1, 2011 Published: August 15, 2011 8233
dx.doi.org/10.1021/es2008227 | Environ. Sci. Technol. 2011, 45, 8233–8240
Environmental Science & Technology was selected for detailed method development, based on anomalous elemental signatures that suggest the potential for a variety of metal compounds (and sources) in the dust and soil. The sample suite was collected from a single urban home in the City of Ottawa, Canada, and included vacuum dust from the living room (LR), parent bedroom (PBR), child bedroom (CBR) and exterior garden soil (GS). The GS sample was collected from a depth of 0 5 cm away from any visible sources of metal contamination. Limited information recorded at the time of sampling indicated the presence of chromated copper arsenate (CCA) treated wood structures and recent renovation activity. Indoor samples were collected from floors only (including bare hardwood and carpeted surfaces) using the purpose-built HVS3 vacuum sampler (HVS3 = high volume small surface sampler) following ASTM method D 5438-00. The dust and soil samples were air-dried and sieved to
