Speciation and Complexation of Cd in Soil Solutions

1
Speciation and Complexation of Cd in Soil
Solutions

• S.Sauvé, W. Norvell, M. McBride and W.
  Hendershot
• Cornell University (Ithaca, NY, USA)
• McGill University (Montréal, QC, Canada)
• QSAR Risk Assessment (Montréal, QC, Canada)
• (emailsebastien.sauve_at_umontreal.ca)

2
Objectives

• Determine the free metal speciation of divalent
  Cd2 in soil solutions
• Identify the physico-chemical characteristics of
  the soil which control Cd solubility and
  speciation
• Quantify the contributions of pH, total metal
  and organic matter
• Propose simple semi-mechanistic regression models
  to estimate Cd solubility and free Cd2
  speciation in contaminated soils

3
Soils

• 2 datasets
• 64 Field-collected soils
• Cd originating from long-term phosphate
  fertilizers, aerial deposition, sewage sludge
  application, diffuse and point source industrial
  contamination
• Ithaca (NY), Montréal (QC), Québec Colorado
• 46 Metal salt spiked soils
• One soil spiked up from 1 to 50 mg Cd kg-1 soil
• pH adjusted with HNO3 and KOH

4
Soil Properties

• Soil pHCaCl2 of 3.5 to 8.9
• Soil organic matter of 8.0 to 108 g C kg-1
• Dissolved organic carbon 1.1 to 140 mg C L-1
• Cd levels
• Soil totals of 0.1 to 56 mg Cd kg-1
• Dissolved Cd of 0.03 to 3500 µg Cd L-1
• Free Cd2 of 10-10 to 10-5 M

5
Methodology

• Totals by HNO3 reflux digestion
• Soil solutions obtained using 12 soil0.01 M
  KNO3 extractions filtered to lt0.22µm
• Dissolved Cd by GFAAS (Zeeman)
• Electrochemically labile Cd by differential pulse
  anodic stripping voltammetry (ASV)
• Free Cd2 speciation by partitioning ASV-labile
  Cd into inorganic ion-pairs

6
Free Cd2 Speciation

• ASV is not sensitive to metals strongly complexed
  with dissolved organic matter
• ASV-labile Cd is mainly inorganic species

7
Soil Solution Metal Distribution

• 20-60 bound to dissolved organic matter
• 20-30 inorganic species
• 10-40 free

8
Dissolved Cd

• Total Cd
• pH
• Field spiked datasets are similar at pHlt8
• KOH effect on DOM at pHgt8

9
Predictive Regressions

• Field-collected dataset
• Field spiked soils (pHlt7)

10
Dissolved Cd - Kd Partitioning(Field-collected
soils only)

• Janssen et al. 1996 Data
• Lee et al. 1996
• Anderson and Christensen 1988

11
Free Cd2

• Total Cd
• pH
• Field spiked datasets are similar
• No obvious effects of KOH induced DOM

12
Predictive Regressions for Free Cd2

• Spiked dataset
• Field spiked soils

13
Conclusions

• Dissolved and free Cd2 in soil solutions can be
  predicted from simple regressions with total
  metal burden and soil solution pH
• Risk Assessment should consider the relative
  impact of soil properties like pH