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Understanding Lead Activity and Solubility in Contaminated Soils

Explore lead distribution, activity measurements, soil properties, and regression analysis to predict lead activity. Learn about the impact of soil characteristics on lead solubility and bioavailability for risk assessment and research needs.

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Understanding Lead Activity and Solubility 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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