Ion%20Activity%20and%20Solubility%20of%20Lead%20in%20Contaminated%20Soils

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Ion Activity and Solubility of Lead in
Contaminated Soils

• S. Sauvé1, M.B. McBride1 and
• W.H. Hendershot2

1 Cornell University 2 McGill University-Macdonal
d Campus
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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

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Lead Distribution
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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

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Lead Activity Measurements

• Anodic stripping voltammetry
• Ion exchange resins and charge separation
• Donnan dialysis
• Competitive chelation
• Fluorescence quenching
• Ion-selective electrodes

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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)

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Procedures

• Soil preparation
• Soil is sieved to lt5 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 lt0.22 µm

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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

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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

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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

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

• Soil total Pb varies from 10 to 14900 mg Pbkg-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 PbL-1
• ASV-labile Pb from 0.1 to 312 nM

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Regression (Pb2)

• Soil solution free Pb2 activity can be predicted
  using the whole dataset (84 soils)

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Pb2, pH and Total Pb
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Regression (Dissolved Pb)

• Dissolved Pb can be predicted using the whole
  dataset (84 soils)

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Dissolved, pH and Total Pb
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Summary

• Free lead represents lt 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 410-5 of the
  total soil Pb content in uncontaminated
  neutral-pH soils and no more than 0.05 in acidic
  contaminated soils

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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

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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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