Marc Chénier

Influence of Matrix Composition on the Bioaccessibility of Copper, Zinc, and Nickel in Urban Residential Dust and Soil

ABSTRACT This study examines factors affecting oral bioaccessibility of metals in household dust, in particular metal speciation, organic carbon content, and particle size, with the goal of addressing risk assessment information requirements. Investigation of copper (Cu) and zinc (Zn) speciation in two size fractions of dust (< 36 μ m and 80–150 μ m) using synchrotron X-ray absorption spectroscopy (XAS) indicates that the two metals are bound to different components of the dust: Cu is predominately associated with the organic phase of the dust, while Zn is predominately associated with the mineral fraction. Total and bioaccessible Cu, nickel (Ni), and Zn were determined (on dry weight basis) in the < 150 μ m size fraction of a set of archived indoor dust samples (n = 63) and corresponding garden soil samples (n = 66) from the City of Ottawa, Canada. The median bioaccessible Cu content is 66 μ g g−1 in dust compared to 5 μ g g−1 in soil; the median bioaccessible Ni content is 16 μ g g−1 in dust compared to 2 μ g g−1 in soil; and the median bioaccessible Zn content is 410 μ g g−1 in dust compared to 18 μ g g−1 in soil. For the same data set, the median total Cu content is 152 μ g g−1 in dust compared to 17 μ g g−1 in soil; the median total Ni content is 41 μ g g−1 in dust compared to 13 μ g g−1 in soil; and the median total Zn content is 626 μ g g−1 in dust compared to 84 μ g g−1 in soil. Organic carbon is elevated in indoor dust (median 28%) compared to soil (median 5%), and is a key factor controlling metal partitioning and therefore bioaccessibility. The results show that house dust and soil have distinct geochemical signatures and should not be treated as identical media in exposure and risk assessments. Separate measurements of the indoor and outdoor environment are essential to improve the accuracy of residential risk assessments.

Metal impurities provide useful tracers for identifying exposures to airborne single-wall carbon nanotubes released from work-related processes

This study investigated the use of metal impurities in single-wall carbon nanotubes (SWCNT) as potential tracers to distinguish engineered nanomaterials from background aerosols. TEM and SEM were used to characterize parent material and aerosolized agglomerates collected on PTFE filters using a cascade impactor. SEM image analysis indicated that the SWCNT agglomerates contained about 45% amorphous carbon and backscatter electron analysis indicated that metal impurities were concentrated within the amorphous carbon component. Two elements present as impurities (Y and Ni) were selected as appropriate tracers in this case as their concentrations were found to be highly elevated in the SWCNT parent material (% range) compared to ambient air particles (μg/g range), and background air concentrations were below detection limits for both elements. Bioaccessibility was also determined using physiologically-based extractions at pH conditions relevant to both ingestion and inhalation pathways. A portable wet electrostatic precipitation system effectively captured airborne Y and Ni released during sieving processes, in proportions similar to the bulk sample. These observations support the potential for catalysts and other metal impurities in carbon nanotubes to serve as tracers that uniquely identify emissions at source, after an initial analysis to select appropriate tracers.

Comparison of Two In Vitro Extraction Protocols for Assessing Metals’ Bioaccessibility Using Dust and Soil Reference Materials

ABSTRACT The bioaccessibility of arsenic, cadmium, chromium, copper, lead, nickel, and zinc in four National Institute of Standards and Technology (NIST) standard reference materials and two Canadian dust samples as determined using the Solubility/Bioavailability Research Consortium (SBRC) in vitro procedure ranged from a low of 1.8% for chromium in standard reference material NIST 2711 to a high of 95.2% for cadmium in NIST 2584. The SBRC data were compared to data generated using a modified EN-71 Toy Safety protocol conducted at two different laboratories. Results for the two extraction methods compared well with differences between the means (SBRC vs. modified EN-71) generally less than 10% for the majority of the metals. These differences between the two extraction methods were negligible compared to variability caused by (a) the inherent heterogeneity of typical house dust samples and (b) differences in ICP-MS analytical approaches employed in the different laboratories. Results indicate that the modified EN-71 method is useful and appropriate as a relatively simple, rapid, and reproducible screening test for estimating metals’ bioaccessibility in soil and dust samples.

Ultrasonic dissolution for ICP-MS determination of trace elements in lightly loaded airborne PM filters

A simple single-step ultrasonic dissolution procedure for low mass (<1 mg) particulate matter (PM) filter samples using HNO3–HF acid solution is proposed for multi-element determination using ICP-MS. The PM-loaded PTFE filter samples are inserted directly into disposable centrifuge tubes for acid extraction using ultrasonic digestion (UD). Potential interferences owing to contamination and element loss are minimized. Key factors influencing element recoveries are investigated, including digestion solution composition, acid concentration, temperature, and matrix interferences. Optimized conditions for UD include an acid mixture consisting of 4.0 mL HNO3 and 0.1 mL HF with ultrasonication proceeding at 90°C for one hour, followed by 10-fold dilution. Recoveries of 80–120% are achieved for almost all of the 20 elements tested (Be, Al, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Sr, Mo, Ag, Cd, Sb, Ba, Pb and U) in four standard reference materials with contrasting matrices: NIST 1648 (urban PM), NIST 2584 (indoor dust), NIST 2710 (soil), and NIST 1633b (coal fly ash). The exception is Cr in NIST 1648 for which recovery is low (30%) using this method. Element concentrations obtained for PM-loaded filter samples using the proposed UD + ICP-MS method agree with results obtained using energy dispersive X-ray fluorescence (paired t-test p > 0.2; 95% CI).

Impact of humidity on speciation and bioaccessibility of Pb, Zn, Co and Se in house dust

Understanding bioaccessibility, defined as the fraction of an element that is soluble under gastrointestinal conditions, has become an important component of human health risk assessments. This study investigated the impact of humidity on the bioaccessibility of lead (Pb), zinc (Zn), cobalt (Co) and selenium (Se) in house dust. Three dust samples (two vacuum samples and NIST 2584 Indoor Dust) were weathered in an oxygenated humidity chamber for four months. Bioaccessibility was determined in original and weathered samples using a simulated gastric extraction followed by Inductively-Coupled Plasma Optical Emission Spectrometry (ICP-OES) and/or Mass Spectrometry (ICP-MS) as required. Exposure to humid conditions significantly increased bioaccessibility in all samples by 45–93% for Pb and by 21–65% for Zn. In contrast, bioaccessibility of Se was significantly decreased in all three weathered samples (by 75–93%), whereas bioaccessibility of Co displayed a variable response (16% increase; no significant change; 35% decrease respectively). Synchrotron XAS analysis of Pb speciation in the original and weathered dust samples determined that Pb transformed from less bioaccessible inorganic species towards more bioaccessible organic species under humid conditions. These results indicated that chemical transformations in damp microenvironments where house dust accumulates, such as window troughs or the base of carpets, may increase the bioaccessibility of particle-bound elements. Changes in bioaccessibility caused by weathering were shown to be significant in the context of the nationally representative Canadian House Dust Study (n = 1025). The national datasets indicated that bioaccessibility is element-dependent (median 59% for Pb; 67% for Zn; 33% for Co; and 13% for Se). Variations in the frequency distributions were attributed to site-specific differences in speciation and the varied bioaccessibility of individual species. The importance of speciation in determining overall bioaccessibility was further demonstrated by measurements of dozens of Pb, Zn and Co compounds that could feasibly occur in house dust. It was concluded that interactions between these elements and the organic matter content of house dust are key to understanding element bioaccessibility and accumulation behaviour in the indoor environment.

Zinc in house dust: speciation, bioaccessibility, and impact of humidity.

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.

Evaluating the capabilities of Aerosol-to-Liquid Particle Extraction System (ALPXS)/ICP-MS for monitoring trace metals in indoor air

This study investigates the application of the Aerosol-to-Liquid Particle Extraction System (ALPXS), which uses wet electrostatic precipitation to collect airborne particles, for multi-element indoor stationary monitoring. Optimum conditions are determined for capturing airborne particles for metal determination by inductively coupled plasma–mass spectrometry (ICP-MS), for measuring field blanks, and for calculating limits of detection (LOD) and quantification (LOQ). Due to the relatively high flow rate (300 L min−1), a sampling duration of 1 hr to 2 hr was adequate to capture airborne particle-bound metals under the investigated experimental conditions. The performance of the ALPXS during a building renovation demonstrated signal-to-noise ratios appropriate for sampling airborne particles in environments with elevated metal concentrations, such as workplace settings. The ALPXS shows promise as a research tool for providing useful information on short-term variations (transient signals) and for trapping particles into aqueous solutions where needed for subsequent characterization. As the ALPXS does not provide size-specific samples, and its efficiency at different flow rates has yet to be quantified, the ALPXS would not replace standard filter-based protocols accepted for regulatory applications (e.g., exposure measurements), but rather would provide additional information if used in conjunction with filter based methods. Implications This study investigates the capability of the Aerosol-to-Liquid Particle Extraction System (ALPXS) for stationary sampling of airborne metals in indoor workplace environments, with subsequent analysis by ICP-MS. The high flow rate (300 L/min) permits a short sampling duration (< 2 hr). Results indicated that the ALPXS was capable of monitoring short-term changes in metal emissions during a renovation activity. This portable instrument may prove to be advantageous in occupational settings as a qualitative indicator of elevated concentrations of airborne metals at short time scales.

Comparison of Gastric versus Gastrointestinal PBET Extractions for Estimating Oral Bioaccessibility of Metals in House Dust

Oral bioaccessibility estimates for six metals which are prevalent as contaminants in Canada (zinc, lead, cadmium, copper, nickel, and chromium) are investigated for house dust using the simple gastric phase versus the two-phase physiologically-based extraction technique (PBET). The purpose is to determine whether a complete gastrointestinal (GI) assay yields a more conservative (i.e., higher) estimate of metal bioaccessibility in house dust than the gastric phase alone (G-alone). The study samples include household vacuum dust collected from 33 homes in Montreal, Canada, plus four certified reference materials (NIST 2583, NIST 2584, NIST 2710 and NIST 2710a). Results indicate that percent bioaccessibilities obtained using G-alone are generally greater than or equivalent to those obtained using the complete GI simulation for the six studied metals in house dust. Median bioaccessibilities for G-alone/GI in household vacuum dust samples (n = 33) are 76.9%/19.5% for zinc, 50.4%/6.2% for lead, 70.0%/22.4% for cadmium, 33.9%/30.5% for copper and 28.5%/20.7% for nickel. Bioaccessible chromium is above the detection limit in only four out of 33 samples, for which G-alone results are not significantly different from GI results (p = 0.39). It is concluded that, for the six studied metals, a simple G-alone extraction provides a conservative and cost-effective approach for estimating oral bioaccessibility of metals in house dust.

An assessment of the inhalation bioaccessibility of platinum group elements in road dust using a simulated lung fluid

Metal enrichment of road dust is well characterized but available data on the bioaccessibility of metals in particle size fractions relevant to human respiratory health remain limited. The study goal was to investigate the bioaccessibility of platinum group elements (PGE), which are used as catalysts in automotive exhaust converters, in the inhalable fraction of road dust. Street sweepings were provided by the City of Toronto, Canada, collected as part of its Clean Roads to Clean Air program.The particle size relevance of road dust for inhalation exposures was confirmed using a laser diffraction particle size analyzer (mean Dx(50): 9.42 μm). Total PGE were determined in both bulk and inhalable fractions using nickel sulfide (NiS) fire-assay and instrumental neutron-activation analysis (INAA). PGE lung solubility was examined for the inhalable fraction using Gambles extraction. Sample digests were co-precipitated with Te-Sn, to pre-concentrate and isolate PGE, prior to their measurement using inductively coupled plasma mass spectrometry (ICP-MS).Total PGE concentrations were enriched in the inhalable fraction of road sweepings. Geomean concentrations in the inhalable fraction were: palladium (Pd) (152 μg/kg), platinum (Pt) (55 μg/kg), rhodium (Rh) (21 μg/kg) and iridium (Ir) (0.23 μg/kg). Osmium (Os) concentrations were below the limit of detection (LOD). Bioaccessible PGEs (n = 16) using Gambles solution were below LOD for Ir and ruthenium (Ru). For the remainder, the geomean % bioaccessibility was highest for platinum (16%), followed by rhodium (14%) and palladium (3.4%). This study provides evidence that PGE in road dust are bioaccessible in the human lung.

Determination of Metal Impurities in Carbon Nanotubes Sampled Using Surface Wipes

Residual metal impurities in carbon nanotubes (CNTs) provide a means to distinguish CNT from non-CNT sources of elemental carbon in environmental samples. A practical and cost-effective analytical approach is needed to support routine surface monitoring of CNT metal tracers using wipe sampling. Wipe sampling for CNT metal tracers is considered a qualitative indicator of the presence of CNTs, not a quantitative exposure metric. In this study, two digestion approaches (microwave-assisted nitric acid/H2O2 digestion and ultrasonic nitric/HF acid digestion) in conjunction with Inductively Coupled Plasma Mass Spectrometry (ICP-MS) determination were evaluated for their ability to extract metal impurities from CNT particles captured on wipe substrates. Aliquots of different carbon nanotubes (including NIST 2483 single-wall CNT) with and without GhostWipes™ (ASTM E-1792 compliant) were used to compare the performance of the digestion methods. The microwave digestion method accommodated the bulky wipe sample and also eliminated potential ICP-MS signal interferences related to incomplete digestion. Although quantitative recoveries requiring lengthy multistep digestion protocols may be necessary in other applications, the near-total recoveries achieved in the present study for CNT catalyst elements were adequate for identifying surface contamination of CNTs in the workplace using wipe sampling.