Final Report: Platinum-Containing Nanomaterials: Sources, Speciation, and Toxicity in the EnvironmentEPA Grant Number: R833892
Title: Platinum-Containing Nanomaterials: Sources, Speciation, and Toxicity in the Environment
Investigators: Schauer, James J. , Shafer, Martin M. , Toner, Brandy M.
Institution: University of Wisconsin - Madison , University of Minnesota - Twin Cities
EPA Project Officer: Lasat, Mitch
Project Period: February 1, 2009 through January 31, 2013
Project Amount: $399,406
RFA: Exploratory Research: Nanotechnology Research Grants Investigating Fate, Transport, Transformation, and Exposure of Engineered Nanomaterials: A Joint Research Solicitation - EPA, NSF, & DOE (2007) RFA Text | Recipients Lists
Research Category: Nanotechnology , Safer Chemicals
Platinum is the archetypal element where chemical and physical speciation is essential for valid toxicology assessments, yet critical basic information on environmental pools, speciation and reactivity is lacking. Anthropogenic platinum emissions to the environment have dramatically risen over the past 2-3 decades and consumptive use, particularly in nano-catalytic applications, is projected to increase. Nano-particulate species of platinum can represent a substantial fraction of total platinum in primary emissions. Platinum in most primary emissions was thought to be present in relatively benign metal-elemental species; however, evidence is mounting that a portion of the platinum in major sources may exist in more toxic/bioavailable forms and that the speciation of platinum (particularly nano-sized platinum) can change rapidly after release to the environment. Information on environmental levels of the recognized toxic species of platinum (e.g. chloroplatinates) is essentially absent. Our research addressed three major questions:
1. What are the primary sources and environmental receptors of platinum (including nano-sized platinum)?
2. What are the chemical forms of platinum introduced into the environment from current and potential major sources?
3. How does the speciation of platinum change within specific environmental reservoirs after release?
Our focus was on aerosol-mediated emissions, transport and exposure in non-occupational settings. Emissions from on-road vehicles (exhaust catalysts (TWC) are a major source of platinum) was addressed using roadside aerosol and roadway dust sampling. Engine dynamometer experiments were conducted to evaluate particulate platinum emissions from platinum/cerium-based diesel fuel-borne catalysts (FBC). High-volume air samplers were used to collect ambient aerosols in several urban environments. Concentrations and chemical speciation of platinum in particulate (PM) and “soluble” phases of these samples were determined with a suite of analytical tools. Synchrotron XAS (sXAS) was applied to solid phases. “Soluble” species, as defined with physiological relevant fluid extractions, were characterized by magnetic-sector inductively-coupled plasma mass spectrometry (SF-ICPMS), HPLC-ICPMS, HPLC-MS/MS, HPLC-IC-ICPMS, ultra-filtration, and ion chromatography (DEAE and SAX). Several parallel strategies were used to examine the presence of toxic chloroplatinate species in environmental materials. Platinum species transformations are being evaluated in controlled laboratory experiments with both environmental and model samples.
Through our multidisciplinary approach we substantially advanced our understanding of the sources, speciation, transformation, and potential human exposures to platinum materials in the environment. We provide some of the first measurements of the recognized toxic species of platinum in environmental media. Vital information on the concentrations and chemical species of platinum in mobile source emissions and important environmental receptors were developed. Fundamental data on rates of species transformation will be acquired. The chemical speciation and exposure data will enable enhanced assessments of the toxicological relevance of environmental platinum (and nano-sized platinum) species.
A. Synchrotron X-Ray Absorption Studies
- sXAS and chemical speciation results indicate the presence of a large component of oxidized species of platinum (particularly Pt(IV)-oxide) in emissions from diesel engines burning platinum-amended fuel.
- sXAS studies of used automobile three-way-catalysts provides strong support for the presence of oxidized platinum species and HPLC-IC-ICPMS analysis of catalyst extracts indicate the presence of only very low concentrations (<0.1-1 ng/g) of tetrachloroplatinate (Pt2+). Levels of hexachloroplatinate were below detection (<0.1 ng/g).
- Given the low total platinum concentrations in real-world samples and our current understanding of EXAFS spectra, it will be difficult to distinguish with XAS tools between chlorine bonded with platinum as a chloroplatinate and chlorine associated in a more ionic form. Thus chloroplatinate characterization of environmental materials will need to be approached from an extraction and hyphenated SF-ICPMS approach.
Synchrotron X-Ray Absorption Spectroscopy (sXAS) is a powerful approach that can provide chemical speciation information directly from solids (e.g. engine PM and catalyst materials). In the XANES region of the XAS spectrum, data on the oxidation state of Pt can be obtained, and in the EXAFS spectral region, data on nearest neighbor bonding (e.g. O, Cl, S, Pt) can be acquired. On beamline 10.3.2., at LBL-ALS, (Advanced Light Source at Berkeley National Laboratory) we performed focused beam (3 µm) platinum XANES/EXAFS in conjunction with detailed x-ray fluorescence mapping of thin-sections of new and used three-way catalysts. Similarly we conducted XANES/EXAFS studies of engine PM collected from the face of a diesel particulate filter (DPF) from an engine running with platinum-amended fuel. The fine spatial resolution at ALS 10.3.2 enables one to map areas/particles enriched in platinum, identify elemental co-associations, and subsequently perform XAS on those spots. We also, on four separate occasions (48 beamline shifts), the latest in October 2011, performed bulk (1000 µm) and semi-focused (250 µm) sXAS at ANL-APS (Advanced Photon Source at Argonne National Laboratory) on beamline 20-BM. The significantly higher x-ray flux at the APS permits higher quality XANES/EXAFS spectra on the low concentration (< 100 ppm), relatively homogeneously distributed platinum species in the PM from diesel amended fuel. At both ALS and APS we collected high quality EXAFS spectra on a large number of platinum reference materials at both the L2 and L3 edges – our library of platinum reference material XAS spectra now numbers 18. These reference spectra were used to fit spectra obtained from our source and receptor samples. The XAS spectra obtained in our November 2010 and October 2011 shifts at ANL-APS confirm our previous findings that a large fraction of the platinum in PM from diesel engines burning platinum-amended fuel is present in oxidized forms, most likely as a platinum (IV) oxide. During the October 2011 beam time we implemented a series of sample preparation and data acquisition improvements that resulted in the cleanest and highest quality spectra to date of PM from engines burning platinum amended fuel. These data were subjected to both Linear Least Squares Fitting and first principles EXAFS shell-by-shell modeling. Modeling of these EXAFS spectra, from both fresh and aged samples, indicate that all are well-fit by oxide models. Except for one sample, addition of chlorine to the models does not improve fit.
B. All primary field sampling activities planned for this study were effectively completed. These included:
- Two three-week long road-side atmospheric aerosol sampling campaigns in the Milwaukee metro area, one in August 2010 and another in March 2011 were carried-out. The contrasting campaigns were designed to evaluate the influence of chloride levels on platinum speciation. In addition to the high air volume (larger PM mass) bulk (TSP) samples (collected on 8x10” Teflon sheets), detailed size-fractionated aerosol also was obtained using Cascade Impactors.
- Road dust sampling campaigns in Los Angeles, Atlanta, Denver, and Milwaukee were carried-out. The road dust samples were sieved to create TSP fractions and also resuspended in a dilution chamber and PM10 and PM2.5 fractions pulled off.
- Concentrations of TOTAL platinum in the road dusts were highest in Los Angeles (7 sites, 280-720 ng/g), followed by Atlanta (3 sites, 100-380 ng/g), Denver (3 sites, 170-300 ng/g), and Milwaukee (3, sites 70-90 ng/g). In general, the ratios of platinum-group elements (Pt/Pd; Pt/Rh; Pd/Rh) in the road dusts were similar within and between cities, indicating a similar (vehicular) source.
- A relatively small fraction of the total platinum in roadside atmospheric aerosol was present in nano-sized (<0.25 µm) particles (10-20%). Particle-size distributions of roadside aerosol platinum exhibit a mode around 1 µm.
C. Chemical Speciation of Platinum in Roadway Dusts, Catalysts, and Particulate Matter from Diesel Engines Burning Platinum-Amended Fuel.
- The fraction of total platinum in particulate matter from diesel engines burning platinum-amended fuel that was extractable with Water and Gambles Saline ranged from 1-2%. The large majority of this pool of platinum was present as truly dissolved species with anionic character dominating (>80%). Greater fractions of total platinum were extractable in Macrophage Vacuole Fluid (3-4%) and dilute HCl (5-7%), though the speciation was similar.
- The Water and Gambles Saline solubility of platinum in roadway dusts was significantly lower (0.2%), though substantially higher solubilities were measured in Macrophage Vacuole and dilute HCl solutions (1-2% of Total). The anionic component of the roadway dust extracts was much smaller (<50%) than measured in diesel PM.
- The Water and Gambles Saline solubility of platinum in used TWC was very low (0.01%); higher solubilities were measured in Macrophage Vacuole (0.04%) and especially dilute HCl solutions (0.3% of Total). The anionic component of the used catalyst extracts was the smallest of all materials examined. A substantial nano-particle component of the TWC extracts was observed.
- Water and Gambles Saline solubility of platinum in roadside aerosol was the highest of all materials examined (10% of total platinum); even higher solubilities were measured in Macrophage Vacuole (20%); however the soluble fraction in dilute HCl solutions was similar to that in water. The anionic component of the roadside aerosol extracts was at least 50% of the dissolved extractable platinum. The nano-particle component of the roadside aerosol extracts was minimal.
A detailed chemical speciation scheme was developed to assess soluble, truly dissolved, anionic and nano-sized species of platinum in roadway dusts, roadside soils and PM collected from engines burning Pt-amended fuel.
- Total metal (Pt, Pd, Rh and 45 other elements) content in the particulates was determined by magnetic-sector ICP-MS after complete dissolution of the particulates (microwave assisted acid digestion in Teflon bombs).
- A battery of extraction fluids were used to assess the soluble/mobile fraction of Pt (and other elements) in the particulates: (a) high purity water (MQ) was used as a reference fluid; (b) two physiologically relevant fluids - Gambles and a surrogate macrophage vacuole solution, to simulate availability to biological systems; (c) 1 M HCl, consistent with many regulatory agencies approach for anthropogenic and/or potentially mobile platinum species; and (d) methanol was applied to selected samples to access binding sites sequestered in the hydrophobic soot matrix. Total Pt (and other metals) in each fluid extract is determined by magnetic-sector ICP-MS, after filtration at 0.22 micron.
- Colloidal/nano and dissolved species are physically separated by ultrafiltration at 10 kDa (~ 2nm) in all extraction fluids, and then each fraction is subjected to SF-ICP-MS.
- Anionic species (e.g. chloroplatinates) are physically separated from neutral and cationic species using DEAE Chromatography (and in many samples, Strong Anion Chromatography – SAX, also was applied), and each fraction is then characterized by SF-ICP-MS.
- Soluble ions (nitrate, chloride, sulfate, ammonium) in the extracts are quantified by ion-chromatography
- Soluble organic carbon in the extracts is determined by UV/Acid oxidation, NDIR detections using a Sievers TOC instrument.
The kinetics of release of platinum (and other metals) from the particulates were examined by time-coarse extractions in each fluid – time points of 2, 6, 24, and 48 hours. Three solid-solution ratios are employed (200, 500, and 2000 mg/L) to ensure that the partitioning of metals to the solids is well characterized.
D. Chloroplatinate Speciation
- We developed a method, using HPLC (IC) – SF-ICPMS for speciating and quantifying chloroplatinates at very low concentrations (ng/L) in extracts of source and receptor particulates. After exhaustive experimentation with a suite of potential extractants, a 5 mM K2EDTA solution was chosen as the best option to move forward with PM extraction for chloroplatinates. While not completely quantitative in extracting chloroplatinates from environmental materials, the EDTA solution does not artifactually produce chloroplatinates and is effective in stabilizing chloroplatinate speciation.
- We applied these extraction and analysis methods for chloroplatinates to many of the source and environmental receptor materials that we collected.
- Measured chloroplatinate concentrations in Road Dust were below the detection limit of 0.04 ng/g for tetrachloroplatinate (Pt2+) and 0.03 ng/g for hexachloroplatinate (Pt4+). Very low levels (0.08 ng/g) of tetrachloroplatinate were measured in Roadside aerosol.
- Hexachloroplatinate levels in Three Way Catalysts (TWC) were all below detection (<0.1 ng/g), however tetrachloroplatinate was quantifiable in new TWC (0.07-0.09 ng/g) and especially in USED TWC (0.27 – 0.75 ng/g).
- Extremely high levels of tetrachloroplatinate were measured in the PM from engines burning platinum-amended fuel. At recommended fuel additive dosage and under normal engine load, 81,000 ng/g of TCP was measured (hexachloroplatinate levels were below 0.5 ng/g). Under conditions of high platinum dose and high engine load we observed significant levels of both TCP (780 ng/g) and hexachloroplatinate (280 ng/g).
The HPLC-SF-ICP-MS chloroplatinate analysis method that we developed is based on a method published by Nachtigall, et al.  and Nischwitz, et al. [2, 3]. The two chloroplatinate species, tetrachloroplatinate (PtCl42-, “Tetra”) and hexachloroplatinate (PtCl62-, “Hexa”) are separated isocratically using a Dionex AG11 guard column containing an alkanol quaternary ammonium stationary phase and a mobile phase consisting of 0.1 M sodium perchlorate/HCl at pH = 1.9, and detected with a magnetic sector ICP-MS (SF-ICPMS). The perchlorate anion acts as a counter-ion to the chloroplatinates for this separation. Since this mobile phase is detrimental to most mass spectrometry interfaces, a switching valve and an automated timing program was developed to minimize the amount of NaClO4 introduced to the ICP-MS. Using this program, only regions of the chromatogram containing the void, Tetra, and Hexa species are allowed into the ICP-MS. We also have now moved to a totally automated HPLC fraction collection protocol, where the regions of the chromatogram noted above are physically collected offline from the SF-ICPMS and analyzed as discrete samples. This gives us the flexibility to run the collected fractions on the SF-ICPMS under further optimized signal/noise conditions. The limit of detection of the instrumental method is 25 parts-per-trillion (ppt, ng/L) [2 pg injected] in both standard mixtures and in spiked tunnel dust (CRM 723) extracts. A 5-fold further improvement in effective sensitivity (0.4 pg injected, which allows us to characterize chloroplatinates at our target level of 0.1% of total platinum in solids) is achieved using a turbo-vap volume reduction pre-concentration method. Extensive experimental efforts were directed at validating extraction methods for the chloroplatinates from each of three source materials (diesel engine PM, road dust, and bulk cordierite TWC). Extractants examined included (a) EDTA alone (5 and 20 mM), (b) methanolic EDTA (5 different ratios: 10/90, 25/75, 50/50, 75/25, 90/10), and (c) dilute HCl (0.05, 0.1, and 0.5 M). For road dust and catalyst materials, a 5 mM K2EDTA solution, while not totally quantitative in extracting chloroplatinates from environmental materials, does not artifactually produce chloroplatinates and is effective in stabilizing chloroplatinate speciation; and thus we moved forward with this extractant. A 50% MeOH solution with EDTA is marginally more effective in both extracting and maintaining chloroplatinate speciation from diesel PM than other solvent systems examined.
E. Environmental Transformation Studies
The primary thrust of our current (and recently completed) experimental program is Environmental Transformation Studies. These experiments are being conducted at the UW-Madison Biotron, where well-defined environmental conditions can be maintained and exposures conducted. We are simulating four transformation scenarios (a) neat sample in contact with air, (b) sample mixed 50+50 with NaCl to evaluate a road-salt exposure scenario, (c) a soil burial scenario where samples are mixed 1+9 with a surrogate soil, and (d) suspension in a simulated roadway runoff/soil leachate. Real-world samples that we collected, including (1) engine exhaust particulate matter from Pt-FBC-treated diesel fuel, (2) size-resolved roadside aerosol, and (3) size-resolved particulate matter from urban atmospheres, were subjected to these exposure scenarios under carefully controlled temperature, light, and humidity conditions. Samples were collected/sacrificed at time points of 2, 6, 12, and 24 weeks, and then subjected to the comprehensive physical and chemical speciation program previously described. The very large chemistry database that resulted is currently undergoing interpretive analysis.
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2. Nischwitz, V.; Michalke, B.; Kettrup, A. J. Chromatogr. A. 1016 (2003) 223.
3. Nischwitz, V.; Michalke, B.; Kettrup,A. Anal. Chim. Acta. 521 (2004) 87-93.