Article pubs.acs.org/est
Zinc in House Dust: Speciation, Bioaccessibility, and Impact of Humidity Suzanne Beauchemin,*,† Pat E. Rasmussen,‡,§ Ted MacKinnon,† Marc Chénier,‡ and Kristina Boros§ †
Natural Resources Canada, CanmetMINING, 555 Booth Street, Ottawa, Ontario Canada, K1A 0G1 Environmental Health Science and Research Bureau, HECSB, Health Canada, 50 Columbine Driveway, Tunney’s Pasture 0803C, Ottawa, Ontario K1A 0K9, Canada § University of Ottawa, Earth Sciences Department, Ottawa, Ontario K1N 6N5, Canada ‡
S Supporting Information *
ABSTRACT: Indoor exposures to metals arise from a wide variety of indoor and outdoor sources. This study investigates the impact of humid indoor conditions on the bioaccessibility of Zn in dust, and the transformation of Zn species during weathering. House dust samples were subjected to an oxygenated, highly humid atmosphere in a closed chamber for 4 to 5 months. Zinc bioaccessibility before and after the experiment was determined using a simulated gastric acid extraction. Bulk and micro X-ray absorption structure (XAS) spectroscopy was used to speciate Zn in dust. Exposure to humid conditions led to a significant increase in Zn bioaccessibility in all samples, which was due to a redistribution of Zn from inorganic forms toward the organic pools such as Zn adsorbed on humates. ZnO readily dissolved under humid conditions, whereas ZnS persisted in the dust. Elevated humidity in indoor microenvironments may sustain higher Zn bioaccessibility in settled dust compared to drier conditions, and part of this change may be related to fungal growth in humid dust. These results help to explain the greater bioaccessibility of certain metals in house dust compared to soils.
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soil (65 vs 29%).9 The significant difference in Zn bioaccessibility between indoor dust and outdoor soil/dust results in part from differences in Zn speciation among indoor and outdoor sources. Transformations of metal species may occur in indoor environments where local conditions favor particle interactions with other surfaces. A related study showed that Pb metal, a species with low bioaccessibility found in some house dust samples, could be converted into Pb hydroxyl carbonate, a species with high bioaccessibility, during weathering in a humid microenvironment.4,10 Damp indoor environments are also associated with health effects, notably the linkage between mold exposure and childhood asthma.11 Contrasting indoor/outdoor temperatures may lead to condensation inside buildings even under moderate indoor humidity levels.12 Examples of humid microenvironments include the base of carpets or the condensation (and mildew) which commonly builds up between the storm window and the inside window in cold winter climates. Thus, in addition to identifying Zn species in indoor dust, this study investigates the impact of humidity on the transformation of Zn species to more bioaccessible forms, using humidity
INTRODUCTION Zinc is an essential nutrient and is ubiquitous in the environment. Potential risks of Zn toxicity from oral or dermal exposure are generally low but health risks can arise through inhalation of Zn particles associated with specific occupational settings1 and chronic exposure to urban air pollution.2 In contrast to Pb,3−5 the behavior of Zn in house dust has received little attention to date, and knowledge is scarce about Zn sources and pathways in residential dust. Addressing this knowledge gap is especially important considering that Zn is one of the many key metals that selectively accumulate in residential dust compared to outdoor soils, which may contribute to increased human exposure. In urban residential areas, Zn concentrations are on average 6 to 100 times greater in household dust than in local soils.6,7 The extensive Canadian House Dust Study (CHDS), based on a stratified random sampling of 1025 urban homes across Canada, reported a wide range of Zn concentrations in house dust from 144 to 6630 mg kg−1, with a geometric mean concentration of 749 mg Zn kg−1, 22 times greater than that of the natural background (34 mg Zn kg−1).8 Enrichment of Zn in the indoor environment reflects the widespread use of Zn compounds, especially ZnO and ZnS, in the production of a variety of domestic products (Table 1). In addition, bioaccessibility of Zn (expressed as percent of total Zn) tends to be higher in indoor dust compared to corresponding garden Published 2014 by the American Chemical Society
Received: Revised: Accepted: Published: 9022
April 16, 2014 July 15, 2014 July 21, 2014 July 21, 2014 dx.doi.org/10.1021/es5018587 | Environ. Sci. Technol. 2014, 48, 9022−9029
Dietary and medical use to treat common cold and Zn deficiency;37,38 used as a wood preservative, glaze for painting on porcelain, mordant in dyeing and component in dental cement for temporary filling.18
Zn acetate; Zn(OOCCH3)2·2H2O; Alfa Aesar
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a
Representative of Zn−S bonding in organic compounds. Detected in biological tissues and soil organic matter (e.g., thiols in proteins).41,42
Also reported are their occurrence in the environment and uses in the industry.
Zn sorbed on cysteine (Zn_cysteine) at pH 7.0; synthesized (SI Appendix A)
Representative species for Zn sorbed on organic matter (main complexing ligands: carboxyl and phenol).39,40
Found in biological cells and contaminated soils e.g., in lichen as a detoxification mechanism35 and extracellular precipitates associated with fungal mycelium in zinc-contaminated media36
Zn oxalate; ZnC2O4·2H2O; Alfa Aesar
Zn sorbed on humate (Zn_humate) at pH 6 or on lignin (Zn_lignin) at pH 7.5; synthesized (SI Appendix A)
Significant use in commercial products (fertilizers, herbicides, supplements for humans, animals and plants feed, electrolytic plating, paper bleaching, skin fresheners, glue, textile dyeing, preservatives for wood);18,25 detected in dust from recycling units of Pb/Zn plants.29
Zn sulfate (ZnSO4); ZnSO4·7H2O; Alfa Aesar
Representative compounds for Zn sorbed respectively on Fe-, Mn- and Al-oxyhydroxides; detected in smelter-contaminated soils as weathering products;22,23 Zn on Fe-oxyhydroxides detected in indoor dust,9 urban road dust,31 and as weathering species from ZnO- and ZnS-spiked soils.20
Pigment in anticorrosive paints and to passivate steel;25 main form in fungus cell wall,24 and in plant root and leaves (Phaseolus plant)34
Zn phosphate; Zn3(PO4)2; Alfa Aesar
Zn sorbed on 2-line ferrihydrite (Zn_Fh), on birnessite (Zn_birn) or on gibbsite (Zn_gibb) at pH 6.5; synthesized (SI Appendix A)
Primary intermediate in the hydrometallurgical production of ZnO;25 detected in indoor dust.9 ZnCO3 detected in dust from steel plants.30
Zn hydroxide carbonate (Zn hyd. carb.); 5ZnO·2CO3·4H2O; Alfa Aesar
Used in the manufacture of rubber, PVC, pharmaceutical products, plastics; used in powder metallurgy.25
Sphalerite is the primary mineral in mining areas;33 ZnS: detected in smelter dust (sphalerite and/or wurtzite);27−29 paint pigment (wurtzite);17 found in indoor dust;9,32 also used in inks, lacquers, cosmetics, paper, oilcloths, linoleum, leather, and dental rubber, plastics, dyes, fungicides, and fertilizers, semiconductors.18
Zn sulfide - sphalerite (ZnS); mesh powder, Alfa Aesar
Zn stearate Zn[CH3(CH2)16CO2]2; Alfa Aesar
ZnO (nano and micron size) widely used in the production of rubber, fire retardants, electronic parts, varistors, ceramic glazes, glass, cookwear, cosmetics, pharmaceutical products and paint pigment.17,25,26 Found in smelter dust,27−29 dust from steel plant,30 urban road dust,31 and indoor dust.32
occurrence/uses
Zn oxide−zincite (ZnO); mesh powder and 8−10 nm particle size; Alfa Aesar
compounds (label)/origin
Table 1. Subset of Zn Reference Compounds Retained after PCA and Target Analysis As the Most Likely Species in Our House Dust Data Set and Used for LCFa
Environmental Science & Technology Article
dx.doi.org/10.1021/es5018587 | Environ. Sci. Technol. 2014, 48, 9022−9029
Environmental Science & Technology
Article
previously described.14 The retained subset of Zn standards used in LCF is listed in Table 1 and the maximum number of standards in the data analysis was limited to 3 based on PCA and to minimize the number of free parameters in the fit. LCF was conducted first on the Zn extended X-ray absorption fine structure (EXAFS) χ data (k = 3), which contain more structural variation than the XANES spectra (SI Figure A1). LCF was done using Athena 8.061. The EXAFS fitting ranged from 2 to 10.5 Å−1. The best combinations obtained on the EXAFS chi data were then tested for reliability by fitting the normalized XANES spectra over the relative energy range of −30 to 100 eV. Bulk XAS characterization was complemented with micro Xray fluorescence (μXRF) and μXANES analyses performed at the PNC/XSD facilities with beamline 20-BM (Advanced Photon Source, Illinois). Dust samples were spread thinly on Kapton tape and covered with ultralene for data collection. The X-ray beam was focused to a spot size of 5 μm (vertical) by 4 μm (horizontal) using a pair of Kirkpatrick-Baez mirrors. The sample was then rastered through the focused X-ray beam at 10 μm steps with a count time of 2 s per step. The emitted fluorescence X-rays of 8 elements (Ca Kα, Ti Kα, Mn Kα, Fe Kα, Ni Kα, Cu Kα, Zn Kα, and As Kα/Pb Lα) were collected simultaneously with a solid-state germanium, multielement detector. Micro-XANES patterns at the Zn K-edge were collected on selected areas of the μXRF maps where Zn counts were medium to high.
chambers designed to simulate damp moldy residential microenvironments.
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EXPERIMENTAL SECTION Weathering Experiment I with CHDS Dust. A first experiment was conducted on two house dust samples (90 percentile).8 The two house dust samples (H_112 = 3350 mg Zn kg−1; C_043 = 1960 mg Zn kg−1) and NIST 2584 Indoor Dust with Nominal 1% Lead (2580 mg Zn kg−1) were subjected to aging under ambient light at room temperature in closed chambers for 4 months under an oxygenated, 100% humid atmosphere. In each chamber, the samples were thinly spread on individual Teflon discs and the experiment was run in four replicates. At the end of the experiment, the samples were air-dried in a clean laminar flow hood to avoid contamination, and stored in clean Mason jars for 4 weeks until spectroscopic analysis. Details on the humidity chambers have been reported in MacLean et al.10 Weathering Experiment II with Zn-Spiked Dust. A second experiment was carried out with powdered Zn compounds, namely ZnO and ZnS, to determine changes in their speciation and bioaccessibility when aged alone or mixed with a background dust in a highly humid environment. The background dust consisted of a homogenized composite sample of urban residential dust (Faraday dust = 929 mg Zn kg−1; pH 6.2) and was thoroughly mixed with 2000 mg Zn kg−1 of one of the following purchased Zn compounds: (i) ZnO (zincite) as mesh powder
