2010 Progress Report: Platinum-Containing Nanomaterials: Sources, Speciation, and Toxicity in the Environment

EPA 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 Period Covered by this Report: February 1, 2010 through January 31,2011
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

Objective:

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 represents a major fraction of total platinum in most 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 (chloroplatinates) is essentially absent. Our research addresses three major questions:
 
(a)  What are the primary sources and environmental receptors of platinum and nano-platinum?
(b)  What are the chemical forms of platinum introduced into the environment from current and potential major sources?
(c)  How does the speciation of platinum change within specific environmental reservoirs after release?
 
Our focus is on aerosol-mediated emissions, transport, and exposure in non-occupational settings. Emissions from vehicles (exhaust catalysts [TWC] are a major source of platinum) are being addressed using roadside aerosol and roadway dust sampling. Engine dynamometer experiments are being conducted to evaluate platinum emissions from fuel-borne catalysts (FBC). High-volume air samplers are being used to collect ambient aerosols in several urban environments. Concentrations and chemical speciation of platinum in particulate (PM) and “soluble” phases of these samples are being determined with a suite of analytical tools. Synchrotron XAS (sXAS) is applied to solid phases. “Soluble” species, as defined with physiological relevant fluid extractions, are characterized by SF-ICPMS, HPLC-ICPMS, HPLC-MS/MS, ultra-filtration, and ion chromatography. Platinum species transformation will be evaluated in controlled laboratory experiments with both environmental and model samples.
 
Through our multidisciplinary approach, we expect to substantially advance our understanding of the sources, speciation, transformation, and potential human exposures to nano-platinum materials in the environment. We expect to 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 will be provided. 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 nano-platinum species.

Progress Summary:

 
1.     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.
2.     sXAS studies of used automobile three-way-catalysts provides strong support for the presence of oxidized platinum species along with preliminary evidence for chloroplatinates at levels of several percent of total platinum.
 
Synchrotron X-Ray Absorption Spectroscopy (sXAS) is a powerful approach that provides key speciation information directly from solids (e.g., engine PM and catalyst materials). In the XANES region of the spectrum, data on the oxidation state of Pt can be obtained, and in the EXAFS region data on nearest neighbor bonding in up to 3 shells are addressed. At beamline 10.3.2. at LBL-ALS, 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 as well as on soot (engine PM) collected from the face of a diesel particulate filter (DPF) on 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 then perform XAS on those spots. We have also now, on three separate occasions (36 shifts), the latest in November 2010, performed bulk (1000 µm) and semi-focused (250 µm) sXAS at ANL-APS on beamline 20-BM. The significantly higher x-ray flux at the APS permits higher quality XANES/EXAFS spectra on the low concentration, relatively homogeneously distributed platinum species in the PM from diesel amended fuel. At both ALS and APS, we have collected high-quality EXAFS spectra on a large number of platinum reference materials – our library of spectra now numbers 18. These reference spectra are used to fit spectra obtained from our source and receptor samples. The spectra obtained in our November 2010 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 platinum (IV) oxide-monohydrate. The quality of the EXAFS spectra obtained will support shell-by-shell fitting and formal EXAFS analysis.
 
 
3.     All primary field sampling activities planned for this study were effectively completed. This included:
a.    Two 3-week long road-side atmospheric aerosol sampling campaigns in the Milwaukee metro area, one in August 2010 and another in March 2011. The contrasting campaigns were designed to evaluate the influence of chloride levels on platinum speciation. In addition to large-mass bulk (TSP) samples (collected on 8" x 10" Teflon sheets), detailed size-fractionated aerosol also was obtained.
b.    Road dust sampling campaigns in Los Angeles, Atlanta, Denver and Milwaukee. 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 (280-720 ng/g), followed by Atlanta (100-380 ng/g), Denver (170-300 ng/g), and Milwaukee (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.
 
 
4.    A significant (2-4%) fraction of the total platinum in the particulate matter from diesel engines burning platinum-amended fuel was extractable with water and a majority of this pool of platinum was present as nano-sized particles. The water solubility of platinum in roadway dusts was significantly lower (0.2 to 2%), though substantially higher solubilities were measured in macrophage vacuole and dilute HCl solutions. A smaller fraction of the filterable platinum in road dust extracts was present as nano-sized particles than that measured in the diesel PM.
 
A detailed chemical speciation scheme was developed to assess soluble, nano, dissolved, and anionic species of platinum in roadway dusts, roadside soils and PM collected from engines burning Pt-amended fuel.
 

i. Total metal (Pt, Pd, Rh and 45 other elements) content in the particulates is determined by magnetic-sector ICP-MS after complete dissolution of the particulates (microwave assisted acid digestion in Teflon bombs).

ii.   A battery of extraction fluids are used to assess the soluble/mobile fraction of Pt (and other elements) in the particulates:  (a) high purity water (MQ) is used as a reference fluid, (b) two physiologically relevant fluids - Gambles and a surrogate macrophage vacuole solution, simulate availability to biological systems, (c) 1 N HCl is the preferred matrix for extraction/preservation of chloroplatinate species, (d) methanol is applied to selected samples to access binding sites sequestered in the hydrophobic soot matrix. Total extractable Pt (and other metals) in each fluid is determined by magnetic-sector ICP-MS, after filtration at 0.22 micron.

iii.  Colloidal/nano and dissolved species are physically separated by ultrafiltration at 10 kDa in all extraction fluids, and then each fraction is subjected to SF-ICP-MS.

iv.  Anionic species (e.g., chloroplatinates) are physically separated from neutral and cationic species using DEAE Ion Chromatography, and each fraction is then characterized by SF-ICP-MS.

v.    Soluble ions (nitrate, chloride, sulfate, ammonium) and soluble organic carbon are also measured.

 
The kinetics of release of platinum (and other metals) from the particulates is being 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.
 
5.     Significant progress was made in developing a method for speciating chloroplatinates in extracts of source and receptor particulates.
 
The HPLC-SF-ICP-MS chloroplatinate analysis method that we have developed is based on a method published by Nachtigall, et al. (1) 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. 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 have also 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. Currently, the limit of detection of the method is 11 parts-per-trillion (ppt) [0.7 pg injected] in both standard mixtures and in spiked tunnel dust (CRM 723) extracts. To achieve our ultimate effective sensitivity goal of 1 ppt, which will enable us to characterize chloroplatinates at our target level of 0.05% of total platinum, we developed and demonstrated the viability of a turbo-vap volume reduction method that provides another 10-fold effective concentration. Chloroplatinates undergo hydrolysis transformation in aqueous media, and the identification and quantification of these transformation products is important to elucidating the fate and transport of soluble Pt species in the environment. We have also developed a gradient HPLC-SF-ICP-MS method (total run time = 25 min.) to elucidate the identity of these transformation products. Current efforts are focused on validating extraction methods for the chloroplatinates from each of three source materials (diesel engine PM, road dust, and bulk cordierite TWC). We have demonstrated that a 25% MeOH solution with EDTA is effective for retaining the speciation of chloroplatinates in road dust slurries; however, this solution may not be effective for diesel PM. For the diesel PM, we are working on an alternate extraction method that incorporates a long-chain quaternary amine to complex the chloroplatinates. This hydrophobic complex is partitioned into MIBK and analyzed for platinum after volume reduction.

 

Future Activities:

Year 3 (2011-2012) Research Plans
 
All field sampling activities planned for this study were effectively completed in Years 1 and 2; therefore, only targeted limited additional sampling, in support of the Environmental Transformation Studies, XAS catalyst characterization and chloroplatinate sub-studies will be performed.
 
The primary thrust of our Year 3 experimental program will be the Environmental Transformation Studies. These experiments will be conducted at the UW-Madison Biotron, where well-defined environmental conditions can be maintained and exposures conducted. We will simulate three transformation scenarios (a) aerosol in contact with air, (b) soil-sediment system, and (c) aquatic suspension at the Biotron. Real-world samples that we have collected including (1) engine exhaust particulate matter from Pt-FBC-treated diesel fuel, (2) tunnel/road dust, (3) roadside aerosol, and (4) size-resolved particulate matter from urban atmospheres, will be subjected to the exposure scenarios under carefully controlled and step-varied conditions of (i) time, (ii) humidity, (iii) light and (iv) oxidant.
 
A parallel thrust in Year 3, there will be completing and submitting for peer review the manuscripts that are currently in draft form; including (a) Chemical and Physical Speciation of Platinum in PM from Engines Burning Platinum-Amended Diesel Fuel, (b) Chemical and Physical Speciation of Platinum in Road Dusts and Road-side Aerosols, and (c) A Sensitive Method Incorporating HPLC-(IC)-SFICPMS for Determination of Chloroplatinate Species in Environmental Matrices and Source Materials. We will also initiate work on manuscripts that address (a) Environmental Transformations of platinum and (b) occurrence of Chloroplatinate species in the environment.

References:

  1. Nachtigall, D.; Artelt, S.; Wunsch, G. J. Chromatogr. A. 775 (1997) 197-210.
  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.

Journal Articles:

No journal articles submitted with this report: View all 7 publications for this project

Supplemental Keywords:

platinum, speciation, environment, aerosol, chloroplatinate, transformation, Synchrotron XAS, physiological extraction

Progress and Final Reports:

Original Abstract
  • 2009 Progress Report
  • 2011 Progress Report
  • Final Report