PGMs in the environment
The addition of catalytic converters to automobiles has had immense success in reducing the levels of harmful pollutants emitted by cars and other vehicles with internal combustion engines. In use, tiny amounts of PGM within the autocatalyst become detached and are emitted with the exhaust gas into the environment. Although these amounts are extremely small, the high number of vehicles fitted with catalytic converters raised questions about whether it poses a risk to human health or the environment. Whether platinum might be emitted or subsequently converted in the environment into a reactive chloroplatinate compound capable of causing respiratory sensitisation was a principal concern.
To address this question, the PGM industry sponsored more advanced research than had previously been attempted. This demonstrated that the concerns were unfounded. Extensive research by the University of Wisconsin on a variety of new and used catalytic converters, air samples, and soil samples, showed that levels of chloroplatinates are vanishingly low. Chloroplatinates extractable from catalytic converters were at the level of parts per billion, while levels of chloroplatinates in air were at levels of less than one part per quadrillion
Chloroplatinate Method Development and Quantification of Chloroplatinates in New and Used Catalyst Materials and Urban Atmospheric Aerosols
In order to specifically measure levels of chloroplatinates in Vehicle Exhaust Catalysts (VEC) and urban atmospheric aerosols, an extraction and analysis protocol that would enable accurate quantification of major chloroplatinate species was developed and validated by the University of Wisconsin (USA).
The analytical challenges of quantification and speciation of chloroplatinates in catalyst materials and particularly environmental materials at native, background concentrations are many. Though available information was extremely limited (there exists little or no data on environmental concentrations of chloroplatinates), reasonable estimates suggested low ng/g concentrations. Effective extraction, isolation, and quantification at these very low concentrations without altering the chemical form (speciation) of the chloroplatinate target species has, to a large degree, eluded the analytical community. A significant and very detailed method development project was therefore undertaken, which resulted in an acceptable procedure for analysing selected chloroplatinates in extracted solutions from substrate materials.
The method developed was then used to measure concentrations of chloroplatinate within eight different VEC and also atmospheric particulate matter collected from roadside canyons and/or roadside motorways as well as background urban sites from six climatically diverse European cities.
For three-way automotive catalysts (TWC), concentrations of tetrachloroplatinate were nearly identical in marine and Midwest aged catalysts at 2.9 ± 1.2 ng/g. This represents just 1.3% and 1.2% respectively of extractable platinum and a vanishingly small percentage (0.00036 and 0.00023% respectively) of total platinum contained in the catalyst. Concentrations of hexachloroplatinate were greater than tetrachloroplatinate and slightly higher in the marine aged catalyst (5.3 ± 2.9 ng/g) than in the Midwest aged catalyst (3.8 ± 1.4 ng/g). As a percentage of bulk platinum these concentrations represent 0.00066 and 0.0003% of total platinum respectively. For diesel catalysts the results were generally higher than observed in the TWC. As a percentage of total platinum, though, the levels were even lower than in the TWC, typically less than 0.0001% (except around 0.0004% for the Light Duty Diesel aged catalyst).
PM3 (fine particulate matter <3 µm) air concentrations (mass per volume of air) of tetrachloroplatinate averaged 0.022 pg/m3 from roadside aerosol sampling, excluding the higher levels measured at the urban canyon sites of Stockholm (0.051 pg/m3) and Thessaloniki (0.11 pg/m3). PM3 air concentrations of hexachloroplatinate averaged 0.066 pg/m3, excluding the higher levels measured at the urban canyon sites of Stockholm (0.11 pg/m3) and Thessaloniki (0.22 pg/m3). Chloroplatinate levels in the coarse PM fraction were at or below the detection limit (<0.01 pg/m3).
Unidentified anionic platinum species were observed in extracts of most the samples characterized in this study. These species are not parent hexa- or tetra- chloroplatinate structures. Their prevalence is significantly greater in samples with complex ligands; e.g. in natural organic matter such as present in both the surrogate soils and "gulley-pot" matrices examined in this study. It was recommended that future work should address the identification of these species.
A copy of the full University of Wisconsin report can be obtained by contacting IPA (science@ipa-news.com). This research is also being written up for publication in the peer-reviewed scientific literature.
Speciation of Platinum in Vehicle Exhaust Catalysts
Chemical and Physical Forms of Platinum in Three-Way (TWC) and Diesel Oxidation Catalysts (DOC)
In order to better clarify the speciation of platinum within Vehicle Exhaust Catalysts (VEC), IPA sponsored a programme of work at the University of Wisconsin (USA) to determine the concentrations and chemical and physical forms of platinum in extracts of eight VEC, covering both new and aged, gasoline (Three-Way Catalysts) and diesel (Light Duty and Heavy Duty Oxidation Catalysts), catalysts.
Various environmentally or physiologically relevant fluids were used to extract platinum from the VEC, including high purity water, dilute acid (0.07M HCl), two synthetic surrogate lung fluids, and a synthetic surrogate gastric (stomach) fluid. The extracts were analysed using ion-exchange and chromatography techniques, with focus given to the identification and quantification of anionic platinum as this encompasses platinum species likely to be of higher toxicological importance. Anionic tetrachloroplatinate and anionic hexachloroplatinate, which are known to be capable of causing respiratory sensitisation (allergy), were also specifically measured.
The amount of the platinum present within the VEC that could be extracted using the five extractant fluids was very small – less than 1%. The highest extractions (0.5-0.8%) were achieved using the dilute acid, while only approximately 0.01% was extracted using High Purity Water. Between 0.05% and 0.2% of the platinum present was extracted using the artificial lung fluids.
The speciation analysis showed that, of the total amount of platinum that could be extracted from the aged VEC using the various fluids, typically only 0.05-0.5% was tetrachloroplatinate and hexachloroplatinate. Extractable tetrachloroplatinate and hexachloroplatinate therefore amounted to only about 0.00005-0.0005% of the total platinum in the aged VEC, or about 5-15 ng/g of catalyst.
The total amount of extractable anionic platinum, which represents an upper boundary limit on extractable anionic chloroplatinate levels, was <0.02-0.1% of total platinum in aged catalysts.
A copy of the full University of Wisconsin report can be obtained by contacting IPA (science@ipa-news.com). This research is also being written up for publication in the peer-reviewed scientific literature.
European Aerosol Sampling Program
Platinum concentrations and chemical speciation in size-resolved urban aerosols
The objective of this study, conducted by the University of Wisconsin (USA), was to determine the concentrations and chemical speciation of platinum in size-resolved atmospheric urban aerosols.
Six European cities were selected for study, representing diverse geographic and climatic urban environments. Within the chosen cities, several sites were selected for sampling, with an aim of representing roadside canyons, roadside motorways and background urban locations in each city. Samplers were also placed at rural locations in Northern, Central, and Southern Europe to characterize the regional background speciation of platinum.
For each of the particulate size fractions – PM3 'fine fraction', PM3-7 'coarse fraction', and PM7 'super coarse fraction' – in addition to measuring the total platinum present, soluble platinum1, dissolved platinum2 and anionic platinum were also measured following extraction with various solvents: high-purity Milli-Q water (MQ), Gambles solution, Artificial Lysosomal Fluid (ALF), and 0.07M HCl.
1 Soluble platinum was defined as that which passed through a 0.22 µm filter following solvent extraction
2 Dissolved platinum was defined as that which passed through a 10 kD ultrafilter following solvent extraction
The results presented here and in the original Study Report are in picograms of platinum per cubic metre of air (pg m-3). A picogram is a millionth of a millionth of a gram.
Total platinum concentrations
Total platinum concentrations at urban sites ranged from less than 4 pg m-3 (Stockholm urban background) to over 22 pg m-3 in the urban roadside canyons of Stockholm and Thessaloniki, and peaked at over 45 pg m3 in the urban roadside canyon of London. Total platinum concentrations at the rural background sites ranged from 1.7 pg m-3 in Central Europe to 5.3 pg m-3 in Southern Europe.
The highest concentrations of platinum per gram of particulate were in PM7. However, PM3 represented approximately 74% of the total particulate mass, on average, and approximately 60%, on average, of the total platinum mass was present in this fraction, with the coarse (PM3-7) and super-coarse (>PM7) fractions contributing about equally to the remainder of the total platinum.
Soluble platinum concentrations
With the exception of urban canyon sites in London and Thessaloniki, soluble platinum concentrations in the fine aerosol (PM3) were all below 1 pg m-3. Rural background concentrations were especially low (<0.03 pg m-3 in MQ and 0.1 pg m-3 in the other solvents) and consistent. Urban background concentrations were also quite consistent across sites, averaging 0.1 pg m-3 in MQ, 0.2 pg m-3 in Gambles Solution, and 0.4 pg m-3 in ALF and 0.07M HCl. Concentrations in London and Thessaloniki urban canyon sites were over an order of magnitude greater at 1-6 pg m-3.
Dissolved platinum concentrations
The same trends observed for soluble platinum in the PM3 fraction were also observed for dissolved platinum, but the dissolved platinum concentrations were in most cases significantly lower: typically in the 0.2 to 0.4 pg m-3 range for urban sites, but up to 2.5 pg m-3 in London and 3.8 pg m-3 in Thessaloniki urban canyon sites with the more aggressive, lower pH solvent extractants.
Anionic platinum concentrations
Anionic dissolved platinum concentrations, which represent an upper boundary of the potential chloroplatinate levels, averaged from 0.02 pg m-3 at rural background sites to 0.07 pg m-3 in roadside and canyon sites (excluding London and Thessaloniki) in MQ extracts of the PM3. The average concentration for the London and Thessaloniki urban canyon sites was 1.11 pg m-3.
Anionic platinum concentrations in Total Suspended Particles (TSP, i.e. the sum of the PM3, PM3-7 and PM7 size fractions) in MQ extracts averaged 0.03 pg m-3 at rural background sites, 0.08 pg m-3 at urban background sites, 0.11 pg m-3 at roadside and canyon sites (excluding London and Thessaloniki), and 0.96 pg m-3 at the London and Thessaloniki urban canyon sites. Corresponding average concentrations in the Gambles extracts were 0.06, 0.48, 0.70 and 1.05 pg m-3 respectively. Average concentrations using ALF were lower than those for Gambles, apart from the London and Thessaloniki urban canyon sites where the average concentration was 1.99 pg m-3, the highest average concentration of anionic platinum identified in TSP across the various extracts and sampling site categories.
Various aspects of this research programme have been or will be published in the peer-reviewed scientific literature, including:
- Shafer et al (2016) Oxidative Potential of Size-Fractionated Atmospheric Aerosol in Urban and Rural Sites across Europe. Faraday Discussions, 189:381-405