Overview
This University of Warsaw laboratory study incubated ionic platinum (Pt(II)) and citrate-functionalized platinum nanoparticles (5 nm and 50 nm) in four soil types for 60 days to quantify speciation-dependent mobility and immobilization. Sequential extraction combined ICP-MS (total Pt) with adsorptive stripping voltammetry (AdSV, ionic Pt only) to separate nanoparticulate from ionic forms. The principal findings are that soil organic matter is the dominant control on platinum mobility, that platinum nanoparticles are substantially less mobile than ionic platinum in all soils, and that combined ICP-MS plus AdSV is required because ICP-MS alone cannot distinguish ionic from nanoparticulate platinum. These findings are relevant to the platinum wiki as the soil-to-food-chain pathway context: the degree of ionic platinum mobility in low-organic soils determines what fraction of road-traffic-emitted platinum is available for plant uptake or groundwater migration.
Key numbers
Background platinum concentration: below ICP-MS detection limit (less than 0.2 micrograms per gram dry weight) in all four tested soil types before spiking.
Traffic emission context: platinum emitted from catalytic converters is up to 99% in metallic nanoparticle form; some studies report soluble forms at up to 10% of total city-traffic emission.
Platinum extractability with 0.43 mol/L acetic acid (mobile fraction, Fraction 1S), after 60 days incubation; ICP-MS (total); n=3:
Sandy soil:
- Ionic Pt(II) in water: 9.8 plus or minus 2.4%
- Ionic Pt(II) with citrate: 13.0 plus or minus 1.4%
- 5 nm Pt-NPs in water: 0.85 plus or minus 0.11%
- 5 nm Pt-NPs with citrate: 0.91 plus or minus 0.12%
- 50 nm Pt-NPs: less than 0.2% (both conditions)
Chalk loam:
- Ionic Pt(II) in water: 4.8 plus or minus 0.2%
- Ionic Pt(II) with citrate: 4.4 plus or minus 0.2%
- 5 nm Pt-NPs: 0.58 plus or minus 0.02% (water); 0.55 plus or minus 0.02% (citrate)
- 50 nm Pt-NPs: less than 0.2% (both)
Peat soil (high organic matter):
- Ionic Pt(II) in water: 0.42 plus or minus 0.04%
- Ionic Pt(II) with citrate: 0.77 plus or minus 0.06%
- 5 nm and 50 nm Pt-NPs: less than 0.2% (all conditions)
Zabrze roadside soil (low-organic, anthropogenically transformed):
- Ionic Pt(II) in water: 10.0 plus or minus 1.7%
- Ionic Pt(II) with citrate: 15.1 plus or minus 1.5%
- 5 nm Pt-NPs in water: 0.80 plus or minus 0.15%
- 5 nm Pt-NPs with citrate: 1.20 plus or minus 0.01%
- 50 nm Pt-NPs in water: less than 0.2%; with citrate: 0.22 plus or minus 0.02%
Sequential extraction (Zabrze roadside, 5 nm Pt-NPs; ICP-MS vs AdSV):
- Mobile fraction (Fraction 1S), ICP-MS: 0.80% (water), 1.20% (citrate)
- Mobile fraction (Fraction 1S), AdSV (ionic Pt only): 0.39% (water), 0.37% (citrate)
- Oxidizable fraction (Fraction 2S), ICP-MS: 1.17% (water), 1.44% (citrate)
- Oxidizable fraction (Fraction 2S), AdSV: 0.88% (water), 1.15% (citrate)
- Combined fraction (Fraction 2R), ICP-MS: 1.89% (water), 2.49% (citrate)
- Combined fraction (Fraction 2R), AdSV: 1.61% (water), 1.98% (citrate)
- Ionic fraction in 2R (AdSV as fraction of ICP-MS): approximately 75-80%; i.e., after 60 days approximately three-quarters of mobile nanoparticulate Pt has transformed to ionic form under oxidizing sequential extraction conditions
Peat soil with ionic Pt(II), oxidizable fraction (2R), ICP-MS: 76-114%; essentially complete release under oxidizing conditions, consistent with organic matter binding and release
Methods
- Soil types: sandy (pH 8.1, TOC less than 1%), chalk loam (pH 7.8, clay 10-30%), peat/potting (pH 5.5-6.5, high organic matter and NPK), Zabrze roadside (pH 9.1, TOC less than 1%, near highway A1 motorway)
- Pt-NPs: citrate-functionalized; 5 nm (zeta -42 mV, 3.5 x 10^13 particles/mL) and 50 nm (4.0 x 10^10 particles/mL); purity 99.9%
- Ionic Pt: Pt(II) from K2PtCl4 solution; 50 mg/L spike; conditions with and without 10 micromol/L citrate
- Incubation: 500 mg soil + 2 mL solution + 200 microL spike; 60 days; moistened twice weekly
- Sequential extraction: modified BCR with ultrasound-assisted extraction below 30 degrees C; acetic acid (Fraction 1S), H2O2 then acetic acid (Fraction 2S/2R); filtered 0.45 micron
- ICP-MS: NexION 300D; isotopes 194Pt, 195Pt, 196Pt; aqua regia microwave digestion
- AdSV: micro AUTOLAB TYPE II; HMDE; Ag/AgCl reference; H2SO4 1 M + HCHO 2 mol/L + semicarbazide 15 mmol/L; deposition 60 s at 0.0 V; scan -0.8 to -1.25 V at 20 mV/s; signal at -0.9 V; specific to ionic Pt(II) form
Relevance to this wiki
This paper provides the soil-mobility data underpinning food-chain platinum entry. Sandy and disturbed low-organic soils (including roadside urban soils similar to those near the Sicilian industrial areas in Orecchio & Amorello 2019) allow approximately 10% of ionic platinum to mobilize into the acetic acid-extractable fraction, the fraction most accessible to plant roots and groundwater. Peat and high-organic soils immobilize platinum strongly. The finding that citrate (simulating rhizosphere conditions) increases mobility modestly but does not transform nanoparticles to ionic form clarifies that ICP-MS-measured increases in mobile Pt under citrate reflect reduced NP agglomeration rather than ionic speciation. The methodological demonstration that ICP-MS and AdSV must be combined to characterize platinum speciation applies directly to interpreting total-Pt food measurements.
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