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© Sauvé 1997 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
© Sauvé 1997 Soil Lead Contamination l Particulates atmospheric emissions –Fossil fuel, smelting, fuel additive l Car battery recycling l Lead-arsenate pesticides in orchards l Sewage sludge and other soil amendments
© Sauvé 1997 Soil Lead (Pb 2+ ) Activity l Pb minerals have very low solubility l High affinity of Pb for organic matter l Metal toxicity controlled by ion activity in aquatic systems l Need to distinguish between insoluble, soluble and solution free lead(II) in soils and link to ecotoxicology
© Sauvé 1997 Lead Activity Measurements l Anodic stripping voltammetry l Ion exchange resins and charge separation l Donnan dialysis l Competitive chelation l Fluorescence quenching l Ion-selective electrodes
© Sauvé 1997 Soil Samples l Soil from various origins: urban, orchard, industrial, forest l Contamination from different sources: pesticides, sewage sludge, battery recycling or industrial l Represent wide range of soils from Ithaca (NY) and from Montréal (QC)
© Sauvé 1997 Procedures l Soil preparation –Soil is sieved to <5 mm and stored moist –Shake 10 g of soil in 20 mL of 0.01 M KNO 3 for 20 min –Centrifuge 10 min at rpm –Filter to <0.22 µm
© Sauvé 1997 Procedures l Determination of ASV-labile Pb –Calculations for free Pb 2+ l Determination of dissolved Pb by GFAAS in 0.01 M KNO 3 extracts l Soil “Total” by HNO 3 digestions and AAS
© Sauvé 1997 Pb 2+ Calculations Pb 2+ Activity Calculations l ASV measures the labile metal, the amount which is available for reaction at the Hg electrode l The ASV-labile Pb therefore excludes Pb which is complexed with dissolved organic matter i.e. non-reactive at the Hg drop
© Sauvé 1997 Calculations of Pb 2+ l ASV-labile metals easily dissociated inorganic ion-pairs and free metal l Chemical equilibrium allows Pb 2+ to be calculated by partioning the ASV-labile Pb into the various inorganic species: PbOH +, Pb(OH) 2 0, Pb(OH) 3 +, PbHCO 3 -, PbCO 3 0, Pb(CO 3 ) 2 2-, PbNO 3 +
© Sauvé 1997 Equilibrium Constants From Lumsdon et al. (1995) and Smith and Martel (1989).
© Sauvé 1997 Soil Properties l Soil total Pb varies from 10 to mg Pb·kg -1 dry soil l pH from 3.5 to 8.2 l Soil O.M. from 0.45 to 10.8 % l Dissolved Pb from 0.1 to 124 µg Pb·L -1 l ASV-labile Pb from 0.1 to 312 nM
© Sauvé 1997 Regression (Pb 2+ ) l Soil solution free Pb 2+ activity can be predicted using the whole dataset (84 soils):
Pb 2+, pH and Total Pb
© Sauvé 1997 Regression (Dissolved Pb) l Dissolved Pb can be predicted using the whole dataset (84 soils):
Dissolved, pH and Total Pb
© Sauvé 1997 Summary l Free lead represents < 1% of soluble Pb in uncontaminated neutral-pH soils but as much as 40-60% soluble Pb in acidic contaminated soils l 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
© Sauvé 1997 Summary l Soil Pb solubility and speciation will vary according to the total soil Pb loading and the soil pH l Risk assessment should ideally account for the soil characteristics controlling bioavailability
© Sauvé 1997 Research Needs l Need to elucidate the relationships between the speciation and the toxicity of lead on possible plant uptake and soil microbial processes
© Sauvé 1997
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