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Ion Activity and Solubility of Lead in Contaminated Soils

Ion Activity and Solubility of Lead in Contaminated Soils. S. Sauvé 1 , M.B. McBride 1 and W.H. Hendershot 2. 1 Cornell University 2 McGill University-Macdonald Campus. Soil Lead Contamination. Particulates atmospheric emissions Fossil fuel, smelting, fuel additive

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Ion Activity and Solubility of Lead in Contaminated Soils

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  1. Ion Activity and Solubility of Lead in Contaminated Soils S. Sauvé1, M.B. McBride1 and W.H. Hendershot2 1 Cornell University 2 McGill University-Macdonald Campus

  2. Soil Lead Contamination • Particulates atmospheric emissions • Fossil fuel, smelting, fuel additive • Car battery recycling • Lead-arsenate pesticides in orchards • Sewage sludge and other soil amendments

  3. Lead Distribution

  4. Soil Lead (Pb2+) Activity • Pb minerals have very low solubility • High affinity of Pb for organic matter • Metal toxicity controlled by ion activity in aquatic systems • Need to distinguish between insoluble, soluble and solution free lead(II) in soils and link to ecotoxicology

  5. Lead Activity Measurements • Anodic stripping voltammetry • Ion exchange resins and charge separation • Donnan dialysis • Competitive chelation • Fluorescence quenching • Ion-selective electrodes

  6. Soil Samples • Soil from various origins: urban, orchard, industrial, forest • Contamination from different sources: pesticides, sewage sludge, battery recycling or industrial • Represent wide range of soils from Ithaca (NY) and from Montréal (QC)

  7. Procedures • Soil preparation • Soil is sieved to <5 mm and stored moist • Shake 10 g of soil in 20 mL of 0.01 M KNO3 for 20 min • Centrifuge 10 min at 10000 rpm • Filter to <0.22 µm

  8. Procedures • Determination of ASV-labile Pb • Calculations for free Pb2+ • Determination of dissolved Pb by GFAAS in 0.01 M KNO3 extracts • Soil “Total” by HNO3 digestions and AAS

  9. Pb2+ Activity Calculations • ASV measures the labile metal, the amount which is available for reaction at the Hg electrode • The ASV-labile Pb therefore excludes Pb which is complexed with dissolved organic matter i.e. non-reactive at the Hg drop

  10. Calculations of Pb2+ • ASV-labile metals easily dissociated inorganic ion-pairs and free metal • Chemical equilibrium allows Pb2+ to be calculated by partioning the ASV-labile Pb into the various inorganic species: PbOH+, Pb(OH)20, Pb(OH)3+, PbHCO3-, PbCO30, Pb(CO3)22- , PbNO3+

  11. Equilibrium Constants From Lumsdon et al. (1995) and Smith and Martel (1989).

  12. Soil Properties • Soil total Pb varies from 10 to 14900 mg Pb·kg-1 dry soil • pH from 3.5 to 8.2 • Soil O.M. from 0.45 to 10.8 % • Dissolved Pb from 0.1 to 124 µg Pb·L-1 • ASV-labile Pb from 0.1 to 312 nM

  13. Regression (Pb2+) • Soil solution free Pb2+ activity can be predicted using the whole dataset (84 soils):

  14. Pb2+, pH and Total Pb

  15. Regression (Dissolved Pb) • Dissolved Pb can be predicted using the whole dataset (84 soils):

  16. Dissolved, pH and Total Pb

  17. Summary • Free lead represents < 1% of soluble Pb in uncontaminated neutral-pH soils but as much as 40-60% soluble Pb in acidic contaminated soils • Dissolved Pb represents as little 4·10-5 % of the total soil Pb content in uncontaminated neutral-pH soils and no more than 0.05% in acidic contaminated soils

  18. Summary • Soil Pb solubility and speciation will vary according to the total soil Pb loading and the soil pH • Risk assessment should ideally account for the soil characteristics controlling bioavailability

  19. Research Needs • Need to elucidate the relationships between the speciation and the toxicity of lead on possible plant uptake and soil microbial processes

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